Amino Acid-Protecting Groups

Albert Isidro-Llobet, Mercedes Álvarez,* and Fernando Albericio* Institute for Research in Biomedicine, Barcelona Science Park, Baldiri Reixac 10, 08028 Barcelona, Spain; CIBER-BBN, Networking Centre on Bioengineering, Biomaterials and Nanomedicine, Barcelona Science Park, Baldiri Reixac 10, 08028 Barcelona, Spain; Laboratory of Organic Chemistry, Faculty of Pharmacy, University of Barcelona, 08028 Barcelona, Spain; and Department of Organic Chemistry, University of Barcelona, Martı ́ i Franqués 1, 08028 Barcelona, Spain Received April 28, 2008


Introduction
Synthetic organic chemistry is based on the concourse of reagents and catalysts to achieve the clean formation of new bonds, and appropriate protecting groups are required to prevent the formation of undesired bonds and side reactions. 1,2 Thus, a promising synthetic strategy can be jeopardized if the corresponding protecting groups are not properly chosen.
Emil Fischer was possibly the first to recognize the need to temporally mask a functional group to allow regioselective bond formation in the synthesis of carbohydrates. 3 However, the first "modern" protecting group was the benzylozycarbonyl (Z) developed by Bergmann and Zervas. 4 Z fits with the main characteristics associated with a protecting group: (i) it is easily introduced into the functional group; (ii) it is stable to a broad range of reaction conditions; and (iii) it is safely removed at the end of the synthetic process or when the functional group requires manipulation. Another cornerstone in this field was when Barany et al. 5,6 described the concept of orthogonality, in the sense that the two or more protecting groups belong to independent classes and are removed by distinct mechanisms. The groups can be removed,therefore,in any order and in the presence of the rest.  (1981)(1982)(1983)(1984), he returned to Barcelona as Associate Professor. During the 1992-1994 period, he was Director of Peptide Research with Milligen/Biosearch in Boston. He rejoined the University of Barcelona, where he was promoted to professor in 1995. Nowadays, he is holding various appointments: General Director of the Barcelona Science Park, Professor at the University of Barcelona, and Group Leader at the Institute for Research in Biomedicine. Professor Albericio's major research interests cover practically all aspects of peptide synthesis and combinatorial chemistry methodologies, as well as synthesis of peptides and small molecules with therapeutic activities. He has published over 500 papers, several review articles, and 22 patents, and he is coauthor of 3 books. He is editor of several scientific journals and acts in the editorial board of several others. In addition, professor Albericio is deeply involved in the development of the third mission of the University, the transference of knowledge and technology to society. He has founded several biotech companies and is acting in the board of directors of several foundations and companies. Furthermore, he is consultant for several companies in the chemical and pharmaceutical areas. Orthogonal protection schemes are usually milder because selective deprotection is governed by alternative cleavage mechanisms rather than by reaction rates. Since the pioneernig work of Bergmann and Zervas, the development of new protecting groups has been deeply tied to peptide chemistry. Protection is totally mandatory for the construction of these polyfunctional molecules, which contain up to eight distinct functional groups in addition to indole and imidazole rings, which should also be protected. Only the carbonyl function is absent from the natural amino acids, because even phosphate-protecting groups have been developed for the synthesis of phosphopeptides. Thus, the protecting groups first developed for peptide synthesis have been rapidily adapted for the protection of building blocks used for the contruction of nonpeptide molecules. 1,2 Herein, we provide a concise but deep analysis of the protection of amino acids. The review is divided into sections depending on the amino acid funcionalities protected. For each case, methods for the introduction of the protecting groups as well as for their removal are discussed. In each section, protecting groups are classified based on the following criteria: (i) the most used in a Boc/Bn strategy; (ii) the most used in a Fmoc/ t Bu strategy; (iii) decreased order of lability; and (iv) the most recently described, for which, in most cases, their potential has not yet been explored. In all cases, families of protecting groups are classified together. The compatibility of each protecting group with regard to the others is indicated in the column "stability to the removal of", which shows which of the following R-amino-protecting groups (Boc, Fmoc, Z, Trt, Alloc, and pNZ) can be removed without affecting a particular protector.
Special attention has been given to new protecting groups described in [2000][2001][2002][2003][2004][2005][2006][2007][2008]. Those described in the literature earlier and those that not have found a broad use have been omitted from this review.

General
Protection of the R-amino functionality of amino acids is one of the most important issues in peptide chemistry and is mandatory to prevent polymerization of the amino acid once it is activated.
Because most peptide syntheses, both in solution and on solid phase, are carried out in the C to N direction, R-aminoprotecting groups (temporary protecting groups) are removed several times during the synthesis, and therefore, removal must be done in mild conditions that do not affect the remaining protecting groups (permanent, usually removed in the last step of the synthetic process, and semipermanent, usually at the C-terminus, removed in the presence of all other protecting groups, when the peptide is to be coupled at its C-terminus) or even the peptidic chain.
The R-amino-protecting group should confer solubility in the most common solvents and prevent or minimize epimerization during the coupling, and its removal should be fast, efficient, and free of side reactions and should render easily eliminated byproducts. Other desired characteristics of R-amino-protected amino acids are that they are crystalline solids, thereby facilitating manipulation, and stable enough.
For solution synthesis, other R-amino-protecting groups used are the Z, the Nps (2-nitrophenylsulfenyl), and the Bpoc [2-(4-biphenyl)isopropoxycarbonyl] in combination with tButype side-chain protection, or the Boc group in combination with Bn-type side-chain protection.

Introduction of the Protecting Groups
Because there are several types of R-amino-protecting groups, there is a wide range of protection methodologies. Most of these are based on the reaction of the free amino acids (side-chain-protected if necessary; see ω-amino protection part for selective Lys and Orn side-chain protection), with a haloformate 7 or dicarbonate 8,9 of the protecting group under Schotten Baumann conditions (use of biphasic system: organic solvent-aqueous basic conditions) 10 or with the corresponding halide in organic solvents. 11 Nevertheless, in some cases, the presence of the free R-carboxylic acid can interfere in the reaction and lead, for instance, to the formation of dipeptides ( Figure 1). [12][13][14][15][16][17][18][19] The methodologies used to overcome this problem can be divided into two types: those that involve a carboxylic acid-protecting group that is removed upon amino protection and those that involve less-reactive electrophiles on the reagent used to introduce the protecting group. An example of the former is the use of trimethylsilyl esters of amino acids prepared in situ, 19,21 while an illustration of the latter is the use of N-hydroxysuccinimido (HOSu) derivative or the corresponding azide, as in the case of the introduction of Fmoc where Fmoc-OSu or Fmoc-N 3 are used instead of Fmoc-Cl. However, the use of Fmoc-OSu can lead to the formation of tiny amounts of Fmoc--Ala-OH or even of Fmoc--Ala-AA-OH (Figure 2), which can jeopardize the preparation of Fmoc-amino acids for the production of peptide-based active pharmaceutical ingredients (API). 20,22 (TMS-Cl) phenol in DCM, 27 4 M HCl in dioxane, and 2 M MeSO 3 H in dioxane, 28 have been successfully used for solution and solid-phase synthetic strategies. The Boc group is stable to bases and nucleophiles as well as to catalytic hydrogenation.
Trityl (Trt). 29,30 It is removed with 1% TFA in DCM or 0.1 M HOBt in 2,2,2-trifluoroethanol (TFE) in solution. It can be removed in even milder conditions such as 0.2% TFA, 1% H 2 O in DCM, 31 or 3% trichloroacetic acid (TCA) in DCM, 32 which are compatible with the TFA labile 3-(4hydroxymethylphenoxy)propionic acid (AB) linker or even with the more acid labile Riniker handle, 33 as well as with the synthesis of oligonucleotide-peptide conjugates. Coupling yields of Trt-amino acids are lower than those of carbamate-protected amino acids. An important application of the Trt group is for the protection of the second C-terminal amino acid in order to prevent diketopiperazines (DKPs) formation in a similar way as for the Boc strategy. 34,35 This procedure involves the coupling of the third amino acid with in situ neutralization after the removal of the Trt group. 31 Incorporation of Trt-amino acids is more difficult than that of carbamate-protected amino acids, which implies the use of more powerful activating conditions. However, the bulkiness of the Trt group protects the R-proton from the base abstraction and, therefore, makes Trt-AA-OH more difficult to racemize. 36 r,r-Dimethyl-3,5-dimethoxybenzyloxycarbonyl (Ddz). 37 Although Ddz is more acid-stable than the Bpoc and the Trt groups, its removal with 1-5% TFA in DCM makes it compatible with t Bu-type side-chain protection. 38 It can also be removed by photolysis at wavelengths above 280 nm, 37 which makes it potentially very useful for SPS libraryscreening procedures. It has been used to prevent DKP formation in the backbone amide linker (BAL) strategy in a similar way as the Trt group. 39 However, an advantage of Ddz-over Trt-amino acids is that their incorporation is easier, which is a crucial factor when the corresponding amino acids are to be incorporated on hindered amines. 39 2-(4-Biphenyl)isopropoxycarbonyl (Bpoc). 40 It is a highly acid-sensitive carbamate-type protecting group, which is removed with 0.2-0.5% of TFA except when used in poly(ethylene glycol)-based resins, in which more TFA is required because some of the acid is used to protonate the oxymethyl moieties. 41 This is a common characteristic of several acid labile-protecting groups. 42 Most Bpoc-amino acids are oils and are unstable because the free R-carboxylic acid is acidic enough to remove the Bpoc group. Thus, these amino acids are usually stored either as DCHA salts or as pentafluorophenyl esters. 43 In the early stages of SPPS, before the introduction of the Fmoc group, Bpoc-amino acids have been used in combination with t Bu-type sid-chain protection. 41 Currently, Bpoc-amino acids are used mostly for peptide derivatives containing phosphate groups such as phosphopeptides or peptide-oligonucleotide conjugates. 44,45 2-Nitrophenylsulfenyl (Nps). 46 It is removed most conveniently with diluted solutions of HCl in AcOH. 47 It is resistant to bases but can be removed by nucleophiles such as 2-mercaptopyridine in combination with AcOH in MeOH, DMF, or DCM. 48 Removal using a Ni Raney column and organic solvents, such as DMF, has also been described. 49 Nps has been applied in both solution and SPS. Its high acid lability requires similar precautions to the Bpoc group in the presence of the free R-carboxylic acid.
Benzyloxycarbonyl (Z). See section on "other protecting groups". (Table 2) 9-Fluorenylmethoxycarbonyl (Fmoc). 50,51 It is removed by bases (mainly secondary amines, because they are better at capturing the dibenzofulvene generated during the removal) and is stable to acids. It is not completely stable to the catalytic hydrogenolysis treatment required to remove benzyl esters when Pd/C or PtO 2 are used as catalysts. The most selective catalyst is Pd/BaSO 4 . 52 Solution removal is done by liquid NH 3 (10 h) and morpholine or piperidine (within minutes), 10% diethylamine (DEA), dimethylacetamide (DMA) (2 h), 53 and polymeric (silica gel or polystyrene) secondary amines (i.e., piperazine, piperidine) in organic solvents. 54,55 This was applied for the first time for SPPS by two different laboratories independently. 56,57 Since then, several optimized removal conditions for SPS have been described, with the most relevant being 20% piperidine in DMF, 56 which is the most common; 1-5% DBU in DMF; 58,59 morpholine-DMF (1:1) 60 or 2% HOBt; 2% hexamethyleneimine; and 25% N-methylpyrrolidine in DMSO-NMP (1:1), 61 with the latter method leaving thioesters intact. The addition of a relatively small amount of HOBt to the piperidine solution [0.1 M HOBt in piperidine-DMF (2:8)] reduces the formation of aspartimide in the sequences prone to this side reaction. 62,63 Fmoc R-amino protection has been used for the SPS of several relevant peptides using the so-called Fmoc/ t Bu strategy, with the production in Tm scale of the T20 peptide being one of the most important examples. 64 Nevertheless, the low solubility of some Fmoc derivatives in the most commonly used solvents for SPPS has stimulated the search for new base-labile protecting groups.

Protecting Groups Removed by Base
2-(4-Nitrophenylsulfonyl)ethoxycarbonyl (Nsc). 65 This is considered the most promising alternative to the Fmoc group. [66][67][68][69] Nsc-amino acids are crystalline solids, more soluble in common solvents than Fmoc amino acids, and can be deprotected with 20% of piperidine or 1% DBU in DMF or preferably in DMF-dioxane (1:1). 65,67 Nevertheless, the use of DBU accelerates aspartimide formation and other side reactions. 70 Nsc is 3-10 times more base-stable than the Fmoc group, 67 thereby preventing its undesired removal under slightly basic conditions. This is particularly relevant in the synthesis of polyproline peptides in which the use of the Fmoc group leads to deletions caused by premature Fmoc removal by the secondary amine of Pro, whereas no Pro insertions are observed when Nsc is used. 68 Nsc is also important in automated SPS, where amino acid solutions are stored for a long time. Further advantages of the Nsc group versus the Fmoc group are that the formation of the olefin-amine adduct after removal is irreversible and faster for Nsc 67 and Nsc protection reduces racemization compared to Fmoc protection, 68 which is particularly important in C-terminal Ser, Cys, and His.
(1,1-Dioxobenzo[b]thiophene-2-yl)methyloxycarbonyl (Bsmoc). 71 It is the most important of a series of protecting groups that are removed via a Michael addition. Other protecting groups from the same family are the Bspoc (2-tert-butylsulfonyl-2-propenoxycarbonyl) 72 and the Mspoc (2-methylsulfonyl-3phenyl-1-prop-2-enyloxycarbonyl) 73 and the Mspoc groups. The Michael addition removal mechanism has several advantages over the -elimination removal mechanism of Fmoc and Nsc: (i) back-alkylation by the -elimination byproduct is prevented because the deblocking event is also a scavenging event; 71 (ii) base-catalyzed side reactions, such as aspartimide formation, are minimized as a result of lower concentrations of secondary amines; 71,74 and (iii) the method can be applied to the rapid, continuous solution-synthesis technique. 74,75 Bsmoc-amino acids have been used to synthesize several model peptides in which the Bsmoc group was removed with 2-5% piperidine in DMF 71 and have shown better performance than Fmoc-amino acids in difficult couplings such as Aib-Aib. 42 Furthermore, the Bsmoc group can be selectively removed with 2% of tris(2-aminoethyl)amine (TAEA) in DCM in the presence of Fm esters. 71 (1,1-Dioxonaphtho[1,2-b]thiophene-2-yl)methyloxycarbonyl (r-Nsmoc). 76 It is a novel alternative to the Bsmoc group and is removed in the same way but slightly faster. R-Nsmoc-amino acids are crystalline solids; thus, they are a good alternative to Bsmoc in the cases where Bsmoc-amino acids are oils.
Tetrachlorophthaloyl (TCP). 83 It is a relatively new protecting group proposed for SPPS. It is removed with hydrazine in DMF (15% of hydrazine, at 40°C, 1 h for repetitive deprotection) but stable to piperidine and to Boc removal conditions. It is also used for side-chain amino protection.
2-[Phenyl(methyl)sulfonio]ethyloxycarbonyl tetrafluoroborate (Pms). 84 Pms-amino acids are water-soluble. They have been developed relatively recently and allow SPPS in water. Pms is removed with 5% aqueous NaHCO 3 , 2 × 3 min and 1 × 30 min for SPS. 84,85 Nevertheless, since Pms is an onium salt, it is rather unstable compared to conventional protecting groups. 86 Ethanesulfonylethoxycarbonyl (Esc). 86 It is another relatively new protecting group for peptide synthesis in water. The derivatives of Esc are more stable than those of Pms. It is removed either by 0.025 M NaOH in H 2 O-EtOH (1:1) or 0.05 M TBAF in DMF.
2-(4-Sulfophenylsulfonyl)ethoxycarbonyl (Sps). 87 Developed parallel to Esc at almost the same time, it is also a protecting group for SPS in water. It is removed with 5% aqueous Na 2 CO 3 . Sps-amino acids have a similar stability to Esc ones, but with the advantage that they absorb in the UV. (Table 3) Benzyloxycarbonyl (Z). 4 It is one of the most widely used R-amino-protecting groups for peptide synthesis in solution because of (i) the easy preparation of Z-protected amino acids; (ii) the high stability of protected amino acids and peptides, which are stable to base and mild acid treatments (stability to Boc removal); (iii) the versatile removal conditions: by catalytic hydrogenolysis during chain elongation or by strong acids (HBr in acetic acid, 88 TFA at high temperatures, 89 TFA-thioanisole, 90 liquid HF, 91 BBr 3 ) 92 in the final deprotection of the peptide; and (iv) the supression of racemization during peptide-bond formation. 93 Allyloxycarbonyl (Alloc). [94][95][96][97][98] It is removed by a palladium-catalyzed (usually Pd(PPh 3 ) 4 ) transfer of the allyl unit to various nucleophiles/scavengers (preferably H 3 N · BH 3 , Me 2 NH · BH 3 , or PhSiH 3 ) 99,100 in the presence of a proton source. The use of scavengers is mandatory to prevent allylation of the free amine upon Alloc removal. If removed on solid phase, washings with sodium N,N-diethyldithiocarbamate (0.02 M in DMF, 3 × 15 min) are carried out in order to remove Pd. Alloc-amino acids are oils but can be stored as DCHA salts or pentafluorophenyl esters, both of which are crystalline solids. 101 The use of Alloc group is compatible with the Boc/Bn and Fmoc/ t Bu strategies and allows tandem removal-acylation reactions when the palladium-catalyzed amino deblocking is performed in the presence of acylating agents. 102 This strategy has been used to prevent DKP formation. 103 Alloc has recently been applied as an R-amino-protecting group for a convergent synthesis of the antitumoral peptide Kahalalide F. 104 o-Nitrobenzenesulfonyl (oNBS) and p-nitrobenzenesulfonyl (pNBS). 105 The most used is oNBS. It is removed by a nucleophilic aromatic substitution mechanism using -mer-captoethanol and DBU when it is protecting N-alkyl derivatives, but the deblocking of primary amines fails under these conditions and the cocktail used is 5% thiophenol in DMF containing 2 equiv of K 2 CO 3 . The main advantage of oNBS-versus Fmocamino acids is that the former do not form oxazolones and thus oNBS-amino acyl chlorides can be used in difficult couplings with less risk of racemization. 106 oNBS R-amino-protection is also used for site-specific alkylation of amino acids on solid phase, 107,108 making these groups unique for the preparation of N-Me peptides ( Figure 3).  2,4-Dinitrobenzenesulfonyl (dNBS). 109 It is removed by treatment with HSCH 2 CO 2 H (1.2 equiv) and TEA (3 equiv) in DCM for 30 min, leaving oNBS unaltered.

Other Protecting Groups
Benzothiazole-2-sulfonyl (Bts). 106,110 This is used in solution in a similar way to NBS groups. It is removed using thiophenol and base (K 2 CO 3 , DIPEA, or t BuOK) in both primary and secondary amines, NaBH 4 in EtOH 42 or HS(CH 2 ) 2 CO 2 H, Na 2 CO 3 in DMF 111 for secondary amines, and other reducing agents, such as Zn, H 3 PO 2 , Al/Hg, 106 which can be used for primary and secondary amines. However, in the latter case, the reaction is slower and highly concentration-dependent. Bts has been used for the synthesis of the cyclosporin 8-11 tetrapeptide subunit, which contains three N-methylamino acids, 110 and more recently for the synthesis of macrocyclic antagonists of the Human Motilin Receptor. 112 2-Nitrophenylsulfanyl (Nps). See the section on protecting groups removed by acid.
2,2,2-Trichloroethyloxycarbonyl (Troc). 113 It is a classical protecting group that can be removed selectively in the presence of Z, Boc, Fmoc, and Alloc groups via a Grob fragmentation using Zn dust in 90% aqueous AcOH or other reducing agents. 113,114 It is not stable to catalytic hydrogenolysis.
Dithiasuccinoyl (Dts). 115 It is removed with mild thiolysis using 0.5 M -mercaptoethanol and 0.5 M DIPEA in DCM or 0.5 M N-methylmercaptoacetamide (NMM) in DCM. 116 It was used for R-amino protection in the first threedimensional orthogonal protection scheme suitable for the preparation of fully and partially protected peptides, which also involved tert-butyl type groups for side-chain protection and an o-nitrobenzyl ester linkage. 6 Although Dts is not commonly used for the synthesis of peptides, it has proved useful for the synthesis of peptide nucleic acids (PNA) 117 and O-glycopeptides by protecting the 2-amino substituent in the corresponding glycosyl donors. 118 p-Nitrobenzyloxycarbonyl (pNZ). 119 It is a classical protecting group that has recently found further applicability for the synthesis of complex peptides as well as for minimizing side reactions. 120 It is much more stable to strong acids than the Z group and is removed by reduction with tin(II) chloride in nearly neutral conditions (1.6 mM HCl (dioxane) ) in solid-phase and in solution synthesis, 120,104 as well as by catalytic hydrogenolysis or Na 2 S 2 O 4 121 for solution synthesis. pNZ is orthogonal to the three most important amino-protecting groups, Boc, Fmoc, and Alloc, thereby making it highly suitable for the synthesis of cyclic complex peptides such as oxathiocoraline. 122 If the second C-terminal amino acid in SPPS is introduced as a pNZ derivative and the pNZ group is removed using SnCl 2 and catalytic ammounts of HCl, the formation of DKP is prevented. The formation of aspartimides is also prevented using pNZ-amino acids from the Asp residue to the N-terminus. 120 r-Azido Carboxylic Acids. 122, 123 Although not widely used because of the instability of azides, there are examples of their successful application in SPPS. 125,126 The azide is reduced to amine using trimethylphosphine in dioxane. R-Azido carboxylic acids can be coupled as acyl chlorides without oxazolone formation.
Propargyloxycarbonyl (Poc). 127,128 It is removed by ultrasonic irradiation in the presence of tetrathiomolybdate complexes such as [(PhCH 2 NEt 3 ) 2 MoS 4 ] in AcCN. It is a relatively new and still not widely used protecting group for solution-phase peptide synthesis. It is stable to Boc removal conditions and has been used to protect amino acid chlorides to be used in couplings on hindered amines without racemization.
o-Nitrobenzyloxycarbonyl (oNZ) and 6-Nitroveratryloxycarbonyl (NVOC). 129 They are removed by photolysis at wavelengths greater than 320 nm in the presence of additives such as N 2 H 4 , NH 2 OH · HCl, or semicarbazide hydrochloride for several hours, with oNZ being the most easily removed. NVOC has been used for combinatorial library production using the Affymax methodology. 130 Research effort is being made to develop more easily removable photolabile protecting groups.
2-(2-Nitrophenyl)propyloxycarbonyl (NPPOC). 131 It is a photolabile amino-protecting group that is removed by UV light (λ ) 365 nm) about twice as fast as the classical NVOC group. 132 It is removed faster than the NPPOC and has been developed recently by the same research group.

2-(3,4-Methylenedioxy-6-nitrophenyl)propyloxycarbonyl (MNPPOC).
Ninhydrin (Nin). See the section on Cys protection. 9-(4-Bromophenyl)-9-fluorenyl (BrPhF). 133 It is a recently proposed safety-catch amino-protecting group and has been tested only for solution synthesis. It prevents epimerization and is more acid-stable than the Trt group because of the antiaromatic nature of the fluorenyl group. t Bu esters can be selectively cleaved in its presence by using ZnBr 2 in DCM or trichloroacetic acid. 134,133 BrPhF is removed by Pdcatalyzed aminolysis with morpholine, followed by treatment of the resulting acid-labile morpholine adduct with DCA and triethylsilane (TES) in DCM.
Azidomethoxycarbonyl (Azoc). 135 It is a novel protecting group proposed for solution and solid-phase synthesis. It is removed by reduction of the azide with phosphines. The removal is rapid when PMe 3 or PBu 3 (5 min on solid phase) are used and slower with polymer-bound PPh 3 (30 min). Azoc is orthogonal to Fmoc and Mtt.
Bidentate Protecting Groups. 136 Another possibility is the use of bidentate reagents such as N-carboxyanhydrides (NCA) and the oxazolidinones derived from hexafluoroacetone (HFA) or formaldehyde, which undergo heterocyclization with the amino and the R-carboxylic groups. In the heterocycle, the carboxylic group is electrophilic, and a carboxy-derivatization is accompanied by N-deprotection ( Figure 4).

General
The protection of the side chains of lysine (Lys) and ornithine (Orn) as well as diaminopropionc acid (Dap) and diaminobutyric acid (Dab) ( Figure 5) is essential in peptide synthesis to prevent their acylation, which would lead to the formation of undesired branched peptides.
Several groups used for the R-amino funcionality have found application for amino side-chain protection. It is worth  commenting that ω-amino protection is more difficult to remove than R-amino protection because of the higher basicity of the former. Thus, for instance, in the case of trityltype protection of the R-amino, the Trt group is used, whereas for the ω-amino, the more electron-rich 4-methyltrityl (Mtt) is preferred.
The most used permanent protecting groups for Orn and Lys side chains are the 2-chlorobenzyloxycarbonyl (Cl-Z) and Z groups in the Boc/Bn strategy, as well as the Boc group in the Fmoc/ t Bu strategy. In the synthesis of branched or cyclic peptides, there are several protecting groups orthogonal to Boc and Fmoc, with Alloc being among the most popular.
The N R -Fmoc protecting group can be prematurely removed by a primary amine of sufficient basicity, such as the ε-amino group of Lys and to a lesser extent the δ-amino of Orn and the γ-amino of Dab, present in the peptide. 137,138 This side reaction is not promoted by either the -amino side chain of the Dap residue or the R-amino group. These results are consistent with the pK a values of these amino functions in the model compounds shown in Table 4. Thus, while the pK a values of the side amino functions of Lys, Orn, and Dab are very close, the pK a of Dap is lower by one unit, making this amino function less basic than the other derivatives. The same explanation applies for the R-amino function.
These pK a values must be taken into consideration when the ω-amino-protecting group of Lys, Orn, or Dab is removed in the presence of an R-amino protected by the Fmoc group. An alternative is a change of strategy, e.g., use of Alloc or Mtt for R-amino protection and Fmoc for ω-amino protection, use of Mtt for ω-amino protection and a coupling/ neutralization protocol similar to that used to prevent DKP formation after Mtt removal, or use of Alloc and a tandem deprotection-coupling reaction. 137

Introduction of the Protecting Groups
For blocking the R-amino function, a safe method is copper(II) complexation where CuSO 4 · 5H 2 O acts as a complexating agent with the R-amino and R-carboxylate groups, thereby allowing the selective protection of the ω-amino funcionality. [139][140][141] Another alternative also based on complexation is the formation of boron complexes using B(Et) 3 as the complexating agent. 142 In some cases (e.g., Z), side-chain protection can be achieved by protecting both the R-amino and the ω-amino  functionalities and then selectively deprotecting the former, taking advantage of their higher lability. 143 (Table 5) 2-Chlorobenzyloxycarbonyl (Cl-Z). It is removed with HF or TFMSA and is preferentially used in the Boc/Bzl solid-phase strategy over the Z group because Cl-Z shows major resistance to the repetitive TFA treatments to remove Boc group. 144 Both Z and Cl-Z are stable to bases and can be removed by hydrogenolysis in solution.

Protecting Groups Removed by Acid
tert-Butyloxycarbonyl (Boc). It is removed with 25-50% TFA. 145 It is used in the Fmoc/ t Bu solid-phase strategy and is resistant to bases and catalytic hydrogenation.

4-Methyltrityl (Mtt).
It can be used for temporary sidechain protection in the Fmoc strategy and is a better option than Boc in the presence of sensitive amino acids such as Tyr, Met, and Trp because it prevents side reactions during TFA cleavage because of the low electrophilicity of the bulky trityl cation. As expected, ω-amino protection with Trt-type groups is more stable than R-amino protection. Removal of Mtt (4-methyltrityl) is performed selectively in the presence of Boc using 1% TFA in DCM for 30 min or with AcOH-TFE-DCM (1:2:7) for 1 h. 146 More acid-labile derivatives, like monomethoxytrityl (Mmt) and dimethoxytrityl (Dmt), are more convenient when hydrophilic resins (e.g., TentaGel) are used. 147 (Table 6) 9-Fluorenylmethoxycarbonyl (Fmoc). 142 For additional information, see also the R-amino section. Fmoc is usually removed with 20% of piperidine in DMF or 1-5% DBU in DMF; its stability to acids makes it useful for the synthesis of cyclic and branched peptides using the Boc/Bn strategy. It is not completely stable to catalytic hydrogenation.

1-(4,4-Dimethyl-2,6-dioxocyclohex-1-ylidene)-3-methylbutyl (ivDde)
. 148 It is useful as a temporary protecting group in the synthesis of cyclic and branched peptides. 149 ivDde is an improved derivative of Dde (1-(4,4-dimethyl-2,6-dioxocyclohex-1-ylidene)-3-ethyl), [150][151][152] which is considerably less base-labile and, therefore, stable to Fmoc removal conditions and can be removed by hydrazinolysis. An additional advantage of ivDde is that its steric hindrance makes it less prone to migrate to free Lys or Orn side chains ( Figure 6). 148 To prevent the reduction of the allyl group by hydrazine, allyl alcohol should be used when ivDde is removed in the presence of allyl-type protecting groups. 153 Trifluoroacetyl (tfa). 154 It is removed by alkali treatment (0.2 N NaOH in 10 min), 155 aqueous piperidine, [156][157][158] or sodium borhydride. 159 It is stable to strong acids and, therefore, compatible with the Boc strategy. The basic conditions used for its removal may promote aspartimide formation if aspartic residues are present or pyroglutamyl formation in the case of N-terminal glutamine residues.
2-(Methylsulfonyl)ethoxycarbonyl (Msc). 160 It is removed with 0.025-0.5 M Ba(OH) 2 or the 4N NaOH (aq) -dioxane-MeOH (0.25:7.5:0.25). It is highly stable to acids (TFA, room temperature (rt), and long reaction times; HF, 0°C, and 30 min; HCl conc, 40°C, and 1 h) 161 and hydrogenolysis. This reactivity allowed the use of ω-protection with Msc in combination with Boc and Z R-protection. 162 Tetrachlorophthaloyl (TCP). 163 It is a relatively new protecting group proposed for SPPS and also used for R-amino protection. TCP side-chain protection is removed with ethylenediamine-DMF (1:200) at 40°C, 1 h, in repetitive deprotections. Nevertheless, hydrazine-based removal used for R-amino deprotection leads to a complex mixture of compounds. 163 TCP is stable to Fmoc, Boc, and Alloc removal conditions. (Table 7) Allyloxycarbonyl (Alloc). 164,165,103 It is removed using a palladium catalyst in the presence of a scavenger to capture the generated carbocation. It is compatible with the Boc/Bn and Fmoc/ t Bu strategies. See also the section on R-amino protection.

2-Chlorobenzyloxycarbonyl (Cl-Z).
See the section on protecting groups removed by acid.
p-Nitrobenzyloxycarbonyl (pNZ). See also the R-amino protection section for removal details and references. pNZ protection of the side chains of Lys and Orn prevents the undesired removal of the R-Fmoc group after side-chain deprotection. 166,167 2-Nitrobenzyloxycarbonyl (oNZ). See the section on R-amino protection.
Phenyldisulfanylethyloxycarbonyl (Phdec) and 2-Pyridyldisulfanylethyloxycarbonyl (Pydec). These are recently developed protecting groups that have been used either for solution or solid-phase synthesis. 169 Both are removed by mild thiolysis using dithiothreitol (DTT) or -mercaptoethanol in Tris · HCl buffer (pH 8.5-9.0) for deprotection in water or by treatment with -mercaptoethanol and DBU in NMP for deprotection in an organic medium.
o-Nitrobenzenesulfonyl (oNBS). It is widely used for the R-N-methylation of amino acids. Because of its high stability to acids and bases, oNBS has found application in the sidechain protection of secondary amines derived from Lys and Orn. It is removed from secondary amines by mercaptoethanol in the presence of DBU. 170,171 4. r-Carboxylic Acid

General
The protection of the C-terminal carboxylic acid is different in SPS to in solution synthesis. In the former, the C-terminal is usually linked to the solid support, and therefore, the linker/ handle acts as a protecting group. There are excellent reviews covering the linkers/handles used in SPPS, and therefore, they are out of the scope of the present review. Nevertheless, in some synthetic strategies where the peptide is linked to the resin by the backbone by an amino acid side chain, and also in the lessfrequent synthesis in the reverse N-C direction, 39,172,173 C-terminal protection is required. Figure 6. Dde N f N′ migration. This side reaction is prevented using ivDde.
In the case of solution synthesis, C-terminal protection is not needed to form the peptide bond. However, in other cases, C-terminal protection is mandatory.

Introduction of the Protecting Groups 174
Protection of the R-carboxylic acid can be performed mainly by the following methods: (i) reaction of an R-aminofree amino acid with an alcohol in acidic conditions (HCl and p-TosOH are the most used acids); 175 (ii) tert-butyl protection by reaction of an R-amino-free or protected amino acid with isobutene in acidic conditions (usually p-TosOH or H 2 SO 4 ); 176,177 (iii) reaction of an R-amino-protected amino acid in the presence of base or as a cessium salt with the corresponding halide (usually bromide); 178,179 and (iv) reaction of an R-amino-protected amino acid with a condensating agent such as DCC in the presence of DMAP and the alcohol derivative of the protecting group. 180 For the particular case of aspartic (Asp) and glutamic (Glu) acids R-carboxyl protection, two main strategies are possible: • Protection of the R-carboxylic acid after selective protection of the side chain of H-Asp-OH or H-Glu-OH either via acidcatalyzed esterification or in the presence of a copper chelate (see the section on protection of side chain of Asp and Glu). Side-chain deprotection renders the desired protected derivative. 181,182,142 • Selective protection of the R-carboxylic acid via formation of an intramolecular anhydride between the two carboxylic acids and reaction with the corresponding alcohol or via reaction with a halide in the presence of base. In both cases, N-protected Asp or Glu acid are used as starting materials. In the first case, selective R-protection is achieved as a result of the major electrophilicity of the R-carboxylic acid, whereas in the second, the selective protection is due to the major acidity of the R-carboxylic acid. 183,184 (Table 8) tert-Butyl ( t Bu). 177 It is used in both solution and solidphase synthesis. It is removed with high concentrations of TFA (solid phase and solution) or HCl in organic solvents (solution). In the latter case, it is effectively used along with Bpoc N Rprotection and Trt side-chain protection or with Z group as N Rprotection. It is stable to base-catalyzed hydrolysis, and its bulkiness generally prevents DKP formation. 185 Table 6. Lys-, Orn-, Dap-, and Dab-Protecting Groups Removed by Base a Except catalytic hydrogenation removal. b Catalytic hydrogenation removal.

Benzyl (Bn).
See the section on other protecting groups. 2-Chlorotrityl (2-Cl-Trt). 186 It is removed with 1% TFA in DCM and is used as a semipermanent protecting group for the synthesis of large peptides using a convergent approach.
2,4-Dimethoxybenzyl (Dmb). 187 It is removed with 1% TFA in DCM (6 × 5 min). Because of its high acid lability, it can be removed in the presence of t Bu-type protecting groups and also on Wang and PAL/Rink resins. It is used for Fmoc/ t Bu SPS of "head-to-tail" cyclic peptides.
2-Phenylisopropyl (2-Ph i Pr). 188 It is removed with 4% TFA in DCM for 15 min (Boc group is stable to these conditions).
5-Phenyl-3,4-ethylenedioxythenyl Derivatives (Phenyl-EDOT n ). 189 They have been recently developed and are removed using very small concentrations of TFA (0.01-0.5%), with the most acid-labile derivative being the 5-(3,4dimethoxyphenyl)-3,4-ethylenedioxythenyl. (Table 9) 9-Fluorenylmethyl (Fm). 190,191 It is removed with secondary amines such as piperidine and DEA in DCM or DMF, as well as by catalytic hydrogenation in solution. 191 Used for SPS in the reverse N-C direction, 172 as well as for the preparation of "head-to-tail" cyclic peptides. 192 193 It is removed by 2% of hydrazine · H 2 O-DMF (1:1) within minutes. It is stable to piperidine.

4-(N-[1-(4,4-dimethyl-2,6-dioxocyclohexylidene)-3methylbutyl]amino)benzyl (Dmab).
Methyl (Me) and Ethyl (Et). 194 Methyl esters are removed by saponification (usually with LiOH), which can lead to epimerization and degradation of Ser, Cys, and Thr. Nevertheless, they have been used extensively in classical peptide synthesis in solution. They are also a reasonable choice to obtain peptide amides by reaction of the methyl ester with ammonia. Ethyl esters have a similar behavior to methyl esters but are more base-stable and, therefore, more prone to base-catalyzed side reactions. 185 Carbamoylmethyl (Cam). 195,196 It is used for solution synthesis. It is removed by saponification with NaOH or Na 2 CO 3 in DMF. It is removed selectively in the presence of Boc and Z. Nevertheless, it cannot be selectively removed in the presence of side-chain Bn-protected DTT in the presence of DIPEA in H 2 O-AcCN to the exchange-labile Co(II) form. It has not been widely used since Asp. (Table 10) Allyl (Al). 164 It is removed using Pd(PPh 3 ) 4 (0.1 equiv) and PhSiH 3 (10 equiv) as scavenger in DCM within minutes or Pd(PPh 3 ) 4 and morpholine as nucleophile in THF-DMSO-0.5 M HCl (2:2:1), both on solid phase and in solution. 197 If removed on solid phase, washings with sodium N,N-diethyldithiocarbamate (0.02 M in DMF, 3 × 15 min) are carried out in order to remove Pd. Allyl C-terminal protection has been used for the synthesis of C-terminal modified peptides using the backbone linker (BAL) strat-egy, 39 and recently for the synthesis of peptide analogues where R-carboxyl protection is necessary both in solution and on solid phase, such as the synthesis of cyclic peptides via head-to-tail cyclization, among others. [198][199][200][201][202] In these cases, when the Al group from the carboxyl group and the Fmoc from the amino group need to be removed, it is preferable to first remove the Al and then the Fmoc. Removal of the Fmoc group first could increase the risk of allylation of the amino function during the removal of the Al. 201,203 Benzyl (Bn). It is used mostly in solution synthesis. It is usually removed by catalytic hydrogenolysis. It can also be removed by saponification or hydrazinolysis to give the corresponding C-terminal hydrazide. Acidolytic removal is also possible, but harsh conditions are required. It is used in combination with the following N R -protecting groups: Boc, Ddz, Bpoc, and Troc. 185 Phenacyl (Pac). 204 It is used for synthesis in solution and removed by nucleophiles such as sodium thiophenoxyde or by treatment with Zn in AcOH. 204,205 It is degraded and only partially removed by catalytic hydrogenation. It is more electrophilic than the methyl ester, thereby making Pac-protected amino acids prone to racemization during coupling because of a reversible cyclization mechanism (Figure 7). p-Nitrobenzyl (pNB). It is highly resistant to acids and removed using a variety of reducing agents such as Na 2 S,  [206][207][208][209] Solidphase removal is performed by treatment with 8 M SnCl 2 in DMF containing 1.6 mM AcOH and 0.2% phenol for 5 h at 25°C or three treatments of 30 min at 60°C. 210 Washings with DMF, MeOH, and DMF, treatment with 8 M benzenesulfinic acid in DMF for 30 min at 25°C, and further washings with DMF and MeOH are performed to eliminate the quinonimine methide formed during the removal. 211 Use of a less concentrated and more easy to handle 6 M SnCl 2 in DMF solution, substitution of AcOH by HCl in dioxane, and alternative washings (DMF, DMF/H 2 O, THF/H 2 O, DMF, and DCM, 3 × 30 s each) have been described in the case of Glu side-chain protection. 166 These conditions should be easily adapted to the removal of the C-terminal protecting group. Removal with TBAF in solution has also been proposed as an alternative to the reductive removal. 212 2-Trimethylsilylethyl (TMSE). 213 It is removed with a quaternary ammonium fluoride such as TBAF or tetraethylammonium fluoride (TEAF) in DMF. It is stable to hydrogenolysis but unstable to anhydrous TFA. Nevertheless, Boc group can be removed selectively in its presence when HCl solutions in organic solvents are used. 2-(Trimethylsilyl)isopropyl (Tmsi). 216 It is used for peptide synthesis in solution. It is removed with TBAF (8 equiv) in THF in 1-1.5 h. It significantly reduces DKP formation in comparison with TMSE.

Other Protecting Groups
2,2,2-Trichloroethyl (Tce). 217 It is used mainly for solution synthesis. It is removed with Zn dust in AcOH in similar conditions as Troc and, therefore, can be removed in the presence of Z, Boc, Alloc, and Fmoc. Tce is stable even at pH 1, and therefore, Boc can be removed selectively in its presence. It is not completely stable to hydrogenolysis.
p-Hydroxyphenacyl (pHP). 218 It is removed by photolysis (λ ) 337 nm) and used as a new phototrigger. It is stable to Boc removal. 4,5-Dimethoxy-2-nitrobenzyl (Dmnb). 219 It is a photolabile protecting group analogous to the NVOC group. It has been used for the synthesis of misacylated tRNAs and recently for the synthesis of caged peptides. 220 1,1-Dimethylallyl (Dma). 221 It is removed by treatment with Pd(PPh 3 ) 4 (10 mol %) in THF at room temperature, followed by dropwise addition of NMM (3 equiv) under nitrogen. PhSiH 3 , potassium 2-ethyl hexanoate, or p-toluene sulfonic acid sodium salt can be used instead of NMM. It is orthogonal to the Fmoc group and can be removed in the presence of Bn-and t Bu-type groups, but it is not stable to their acidolytic removal.
Pentaamine Cobalt(III). 222 It was proposed as a Cterminal-protecting group for the synthesis of side chain to side chain bicyclic peptides. It is described as orthogonal to Fmoc and Boc and is removed in solution by mild reduction with then.

General
The side-chain carboxylic groups of Asp and Glu ( Figure  8) must be protected in order to prevent their activation during peptide synthesis, which would lead to undesired branched peptides.
Furthermore, in the case of Asp acid, the protecting groups used must also prevent or at least minimize the formation of aspartimide. Hydrolysis of the aspartimide during peptide synthesis renders two products: the R-peptide, which is the desired product, and the -peptide, which is usally the major compound. Aminolysis of aspartimide by piperidine gives the corresponding Rand -piperidides ( Figure 9).
The same kind of intramolecular cyclization can also take place in the case of Glu, thereby leading to pyroglutamic formation. 223 However, in the case of Glu, the reaction is much less severe than with Asp.
Currently, the most used protecting groups are t Bu for the Fmoc/ t Bu strategy and, in the Boc/Bn strategy, the cyclohexyl (cHx) group, which is replacing the classical Bn group because it is more effective at preventing the formation of aspartimide.

Introduction of the Protecting Groups
The protection of the side-chain carboxylic acid can be achieved using several methods. The simplest one is the acid-catalyzed esterification of the free amino acid, where protonation of the amino group makes the R-carboxylic acid less reactive, thereby allowing the selective protection of the side chain. 224,225 Copper(II) and boron chelates used for the selective protection of the side chains of Lys and Orn are also applied for the selective protection of the side chains of Asp and Glu. After chelation and reaction with the appropiate protecting-group halide, the chelate is removed in the usual way. 181,182,142 Another alternative is the formation of an intramolecular anhydride between the two carboxylic acids, which leads to selective R-protection thanks to the major electrophilicity of the R-carboxylic acid. This allows the protection of the side chain with a distinct protecting group, followed by the removal of the R-carboxylic acid protection. 175,176 (Table 11) Benzyl (Bn). 185 It is the classical protecting group in Boc/ Bn chemistry and is removed with HF or TFMSA. However, it is more prone to acid-catalyzed aspartimide formation than the cyclohexyl group. Other possible removal conditions are listed in the table.

Protecting Groups Removed by Acid
Cyclohexyl (cHx). It is removed with HF or TFMSA. 226,227 It is widely used in the Boc/Bn solid-phase strategy. It is superior to the benzyl group at preventing acid-catalyzed aspartimide formation because of its major steric hindrance. 228 In addition, it is more resistant to acids than benzyl, thus making it more suitable for the synthesis of long peptides using the Boc/Bn strategy.
tert-Butyl ( t Bu). It is removed with 90% TFA in DCM (solid phase and solution) or 4 M HCl in dioxane (solution). It is the most used protecting group in Fmoc/ t Bu chemistry, which is highly prone to aspartimide formation because of the reiterative use of piperidine. The t Bu group simply minimizes aspartimide formation because of its steric hindrance compared to other less-hindered protecting groups such as allyl. However, although the t Bu group is considered hindered in organic chemistry, it does not prevent aspartimide formation in those sequences prone to it. 229 See also the section on R-amino protection.
-Menthyl (Men). 230 It is removed with HF or TFMSA and is resistant to TFA. It leads to less base-catalyzed aspartimides than the cyclohexyl group but is not widely   used. Sometimes diphenyl sulfide should be added as a scavenger to facilitate Men removal. 231 -3-Methylpent-3-yl (Mpe). 232 It is removed with 95% TFA and is more sterically hindered than the t Bu group and, therefore, less prone to aspartimide formation.

2-Phenylisopropyl (2-Ph i Pr).
It is removed with 1-2% TFA. 233,188 It is used in the Fmoc/ t Bu strategy mostly for the protection of Glu but also of Asp. 234 It can be removed in the presence of t Bu-type protecting groups, and therefore, it is useful for the preparation of cyclic peptides. 235 4-(3,6,9-Trioxadecyl)oxybenzyl (TEGBz or TEGBn). 236 It is a recently developed protecting group that is removed with TFA-DCM. It has been used for the solid-phase synthesis of "difficult" peptide sequences (those very prone to aggregate) because it minimizes chain aggregation during the synthesis. (Table 12) 9-Fluorenylmethyl (Fm). 142,237,238 It is removed with secondary amines such as diethylamine or piperidine in DMF. It is stable to HBr in AcOH and TFA/thioanisole, nonstable to catalytic hydrogenation, and not completely stable to HF even at 0°C. It is used for the Boc/Bn strategy when orthogonal protection of the side chains is required.

Other Protecting Groups
Allyl (Al). 164,165,241 It is removed with palladium and stable to TFA and bases. See also the section on R-carboxylic acid protection.
p-Nitrobenzyl (pNB). 242 It promotes aspartimide formation when used to protect Asp. See also the section on R-carboxylic acid protection.

General
The NH backbone is usually unprotected in peptide synthesis. However, at least three undesired interactions involving the NH backbone have been described.
First of all, peptide chains can aggregate during the synthesis as a result of intra-and intermolecular interactions, thereby significantly reducing coupling and deprotection yields. [245][246][247] Backbone protection ( Figure 10) minimizes these aggregation phenomena by preventing the formation of hydrogen bonds and also because of steric hindrance. Thus, SPS of long peptidic sequences prone to aggregation is improved by protecting some amides of the peptide. [248][249][250][251] Second, nucleophilic attack of the amide NH of the amino acid before an Asp residue (usually Gly, Ser, or Thr) 70,227,63,[252][253][254][255] to the -carboxyl group of Asp renders aspartimide and the subsequent formation of -peptide and other side products. (See the section on Asp and Glu sidechain protection.) Aspartimide formation is more severe in the Fmoc/ t Bu strategy and with the Asp-Gly sequence, but it can occur in many other cases. Finally, although less frequent, internal DKP formation involving the NH and the activated carboxylic acid of the previous amino acid has recently been described during fragment coupling ( Figure  11) 122 The most used backbone protectors for the Fmoc/ t Bu strategy are pseudoprolines (Figure 12), 256,257,251 2-hydroxy-4-methoxybenzyl (Hmb), 258 2,4-dimethoxybenzyl (Dmb), and more recently 3,4-ethylenedioxy-2-thenyl (EDOT n ) and 1-methyl-3-indolylmethyl (MIM) 259 The pseudoproline concept is valid only for -hydroxy or thio amino acids such as Ser/Thr or Cys. Although the rest of protecting groups can be used for all amino acids, practically they are only used for Gly because of the difficulty of elongation of the peptide chain due to steric hindrance 260

Introduction of the Protecting Groups
Because of the steric hindrance of the protected amino acid, it is incorporated usually through the corresponding derivatives. Thus, pseudoproline dipeptides are prepared by reaction of Fmoc-AA-Ser or Fmoc-AA-Thr with 2,2dimethoxypropane. 261 Most of the other backbone protectors are introduced by reductive amination of the aldehyde of the protecting group with the amine of the corresponding amino acid, followed by either R-amino protection or dipeptide formation. 262,258,259

Protecting Groups Removed by Acid (Table 14)
Pseudoprolines (ΨPro). The most used are dimethyloxazolidines (Ψ Me,Me Pro) because of their major acid lability (removed by TFA within minutes) 261 Pseudoproline derivatives have been extensively applied to the synthesis of difficult peptides. 257,263 However, they are limited to Ser and Thr. Dimethylthiazolidines (Cys pseudoprolines) have also been described, but they are not so widely used because of their major acid stability (removed with TFA within hours).

2-Hydroxy-4-methoxybenzyl (Hmb)
. 258 It is used mainly as Fmoc-(FmocHmb)AA1-OH 264 or as Fmoc-AA2-(Hmb)-AA1-OH but also as Fmoc(Hmb)AA1-OH. 265 It is removed with TFA. The main advantage of the Hmb group compared with other backbone protectors such as Dmb is that the coupling on Hmb-amino acids is easier. Thus, Hmb is not restricted to Gly, and derivatives of more hindered amino acids can be used. However, the presence of a free hydroxyl group can be a problem in depsipeptide synthesis or in postsynthetic phosphorylations.

Other Protecting Groups
(6-Hydroxy-3-oxido-1,3-benz[d]oxathiol-5-yl)methyl. 271,272 and 2-hydroxy-4-methoxy-5-(methylsulfinyl)benzyl. 273 These are safety-catch backbone protectors that become unstable to TFA after reduction of the sulfoxide to sulfide. (6-Hydroxy-3-oxido-1,3-benzoxathiol-5-yl)methyl is removed with 20 equiv each of NH 4 I and (CH 3 ) 2 S in TFA at 0°C    -Nmec-Hmb). 274 It is a recently developed protecting group. It has been used for solid-phase synthesis. After the removal of the Boc group with TFA during the cleavage of the peptide from the resin, the Nmec moiety is removed via an intramolecular cyclization in basic conditions (N-methylmorpholine (10 equiv) in DMF/H2O (3:7), 4-8 h), leading to the Hmb-protected peptide. Then, Hmb is removed with 95% TFA and scavengers. The main advantage of the Boc-Nmec-Hmb group is that, after Boc removal, a cationic peptide is obtained that increases the solubility of insoluble peptides, making their purification easier.

General
Asn and Gln ( Figure 13) are often used without side-chain protection.
Nevertheless, unprotected derivatives show poor solubility and, therefore, slow coupling rates. In addition, their free primary amides can undergo two main side reactions: (1) Dehydratation during the coupling (Figure 14), which is a base-catalyzed side reaction and, therefore, more favored in those coupling protocols that involve the use of base. It can be minimized using the corresponding N R -protected pentafluorophenyl esters or    carbodiimide-mediated couplings in the presence of HOBt. 275,276 Dehydratation is more important in the Fmoc/ t Bu strategy than in the Boc/Bn one; in the latter, the use of HF apparently reverts the reaction, whereas in the former, TFA is not acidic enough to revert to the amide. 276 (2) Pyroglutamyl ( Figure 15) formation is a weak acidcatalyzed side reaction that occurs on N-terminal Gln that leads to truncated peptidic chains. Being an acidcatalyzed reaction, it has more importance in the Boc/ Bn strategy and can be minimized by reducing exposure to weak acids. 277 Adequate protection of Asn and Gln side chains prevents both side reactions. As for dehydratation, it is not necessary for the protecting group of choice to be stable during the whole peptide synthesis, but only during the coupling step. Furthermore, protection of Asn and Gln side chains also increases coupling yields by confering more solubility to the corresponding Asn and Gln derivatives and probably reducing the formation of hydrogen bonds that stabilize secondary structures.
Removal of the protecting groups is usually easier in Gln than in Asn, being particularly difficult in N-terminal Asn because of the proximity of the free and therefore protonated R-amino group. [278][279][280] Currently, the most used protecting groups are Xan (9xanthenyl) and Trt, which are compatible with both Boc/Bn and Fmoc/ t Bu strategies. In the case of the former, the Xan group protects Asn and Gln side chains only during the coupling and is removed during TFA treatments for Boc removal.

Introduction of the Protecting Groups
Protection is usually performed via acid-catalyzed reaction of the corresponding alcohol with Z-Gln or Z-Asn, followed by catalytic hydrogenolysis to eliminate the Z group and Fmoc or Boc N R protection. 280,281 In the case of 9-xanthenyl, the direct protection of the Fmoc-Asn and Fmoc-Gln has also been described. 282 (Table 16) 9-Xanthenyl (Xan). 281 It is removed by 90% TFA and scavengers. In contrast to Trt, no extra reaction time is required when the R-amino of Asn is free. 278 Xan is used in both the Boc/Bn and Fmoc/ t Bu strategies. 281,282 In the case of the Boc strategy, Xan is eliminated during TFA treatments to remove the Boc group; however, Asn or Gln residues can undergo dehydratation only during the coupling, and thus, Xan elimination after it is a minor problem. 282 Trityl (Trt). 279, 280 It is removed with TFA-H 2 O-EDT (90:5:5) and used in both the Boc/Bn and Fmoc/ t Bu strategies. The time required for removal increases from 10 Table 16. Asn-and Gln-Protecting Groups Removed by Acid min to more than 4 h when the R-amino of Asn is free. Scavengers must be used to prevent Trp alkylation. It is stable to bases and catalytic hydrogenolysis.

4-Methyltrityl (Mtt). 283,279
It is a more acid-labile alternative to the Trt group (95% TFA, 20 min) and is particularly useful when the R-amino of Asn is free.
Cyclopropyldimethylcarbinyl (Cpd). 284,285 It is removed with TFA-thioanisole-EDT-anisole (90:5:3:2), being another more acid-labile alternative to Trt, especially when the R-amino of Asn is free. It is more soluble and coupling rates are better than with the Trt group.

4,4′-Dimethoxybenzhydryl (Mbh). 286,287
It is used mainly in the Boc/Bn strategy but also in the Fmoc/ t Bu one. Its removal using TFA is slow and requires scavengers to prevent alkylation of Trp. 275 2,4,6-Trimethoxybenzyl (Tmob). 288 It is removed with 95% TFA and scavengers. It is more acid-labile, more soluble, and gives less side reactions during coupling than Mbh-protected derivatives. However, it is not currently widely used because it can cause alkylation of Trp and is reported to give worse results than the Trt group. 289,280

General
Protection of the guanidino group of Arg ( Figure 16) is required to prevent deguanidination, which renders Orn ( Figure 17) 290 and δ-lactam formation ( Figure 18) as a result of the nucleophilicity of the guanidino group. Arg side-chain protection remains unsolved in peptide synthesis because of the difficulty to remove the protecting groups.
Since the guanidino group is basic (pK a ) 12.5), it remains protonated in most of the conditions used for peptide synthesis. 291,292 To prevent deprotonation in Fmoc/ t Bu SPS, washings with 0.25 M HOBt are carried out between Fmoc removal and the next coupling. 293 However, if deprotonation takes place, deguanidination occurs after acylation of the neutral guanidino group. This drawback stimulated research into protecting groups for Arg.
Arg derivatives tend to be worse acylating reagents compared with other amino acid derivatives, mainly because of the formation of the δ-lactam from the activated species (Figure 18). In a solid-phase mode, the presence of the δ-lactam does not translate into an impurity in the crude reaction, because it is not reactive but it is translated in a less active species to be coupled.
In principle, protection of all the nitrogens of the guanidino group is required to fully mask its nucleophilicity. However, diprotection and monoprotection are easier to achieve and to minimize side reactions when bulky and electronwithdrawing protecting groups are used.
The most used protecting strategy is sulfonyl protection of the ω-amino function. For the Boc/Bn strategy, the most used group is Tos, while for the Fmoc/ t Bu strategy, the most popular protecting groups are Pbf (pentamethyl-2,3-dihydrobenzofuran-5-sulfonyl) and Pmc (2,2,5,7,8-pentamethylchroman-6-sulfonyl). However, both, but particularly Pmc, are too acid-stable and their removal in peptides with multiple Args is especially problematic.

Introduction of the Protecting Groups
It depends on the nature of the protecting group. In the case of sulfonyl-protecting groups, which are the most used ones, they are usually introduced by reaction of the corresponding sulfonyl chloride with Z-Arg-OH in H 2 O-acetone using NaOH as a base. To obtain the corresponding Fmoc/ Boc derivative, the Z group is removed by catalytic hydrogenolysis and the Fmoc/Boc group is incorporated under regular conditions. 294 (Table 17) 8.3.1.1. Arylsulfonyl ω-Protection. Although this kind of protection does not fully prevent δ-lactam formation, this process can be minimized by using carbodiimides in the presence of HOBt derivatives to decrease the activity of the active O-acylisourea. 185 Tosyl(Tos).ItisremovedwithHF,TFMSA-TFA-thioanisole, or Na/NH 3 . 295 It is the most used protecting group in the Boc/Bn solid-phase strategy. 296 2,2,5,7,8-Pentamethylchroman-6-sulfonyl (Pmc). 294 It is widely used in the Fmoc/ t Bu solid-phase strategy. It is removed by TFA scavengers. Currently, it is being replaced by the Pbf group.

Protecting Groups Removed by Acid
2,2,4,6,7-Pentamethyl-2,3-dihydrobenzofuran-5-sulfonyl (Pbf). 297 It is removed by TFA scavengers and is more acidlabile than the Pmc group. Currently, it is the best Argprotecting group for the Fmoc/ t Bu solid-phase strategy, although it is still too acid-stable in peptides with multiple Arg-containing peptides where long reaction times are required.
Mesityl-2-sulfonyl (Mts) 298,299 It is removed with TFMSA-TFA-thioanisole. It is used in the Boc/Bn solid-phase strategy and is more acid-labile than the tosyl group.   Methoxy-2,3,6-trimethylphenylsulfonyl (Mtr). 300 It is removed by TFA-thioanisole. Although it is still used, it has been mostly replaced by the more acid-labile Pmc or Pbf in Fmoc/ t Bu chemistry.

4-
1,2-Dimethylindole-3-sulfonyl (MIS). 301 It is a recently developed protecting group, which is much more TFA-labile than Pbf. It is completely removed with 50% TFA in 30 min, even in multiple Arg-containing peptides. Butyloxycarbonyl (bis-Boc). 302 It is removed with 90-95% TFA in the presence of scavengers and prevents deguanidination but does not completely prevent δ-lactam formation. 303,304 The coupling rates of bis-Boc-protected Arg are low.
ω-Nitro (NO 2 ). 306 It prevents δ-lactam formation and deguanidination in most cases. It can be removed with HF (SPS) or catalytic hydrogenolysis. In both cases, long reaction times are required, which is an inconvenience in the case of sensitive peptides. For instance, in the case of hydrogenolysis, partial hydrogenation of Trp or even Phe can occur. 307 Because of the clean removal of the nitro group by hydrogenolysis and its low cost, nitro protection is still used for large-scale solution synthesis of peptides 308,309 and even for SPS, where the nitro group is removed by hydrogenolysis after the cleavage from the resin. 310 (Table 18) Trifluoroacetyl (tfa). It has been applied recently for the protection of guanidines used in solution Boc peptide synthesis and Fmoc/ t Bu SPPS. However, although there are references of tfa-protected Arg derivatives, [311][312][313] to date it has not been implemented for Arg protection in peptide synthesis. (Table 19) Nitro (NO 2 ). See the section on protecting groups removed by acid.

Other Protecting Groups
ω,ω′-bis-Benzyloxycarbonyl (bis-Z). 314 Its removal by catalytic hydrogenation requires long reaction times. It is used mostly in Boc/Bn chemistry but also in the Fmoc/ t Bu strategy.
ω,ω′-bis-Allyloxycarbonyl (Alloc). 165 It is removed with Pd(PPh 3 ) 4 and scavengers (dimethylbarbituric acid) 315 and is compatible with the Boc/Bn solid-phase strategy. The base treatment required to remove the Fmoc group also eliminates one of the Alloc groups.

General
Protection of the side chain of Cys ( Figure 19) is mandatory in peptide synthesis because the nucleophilic thiol can otherwise be acylated, alkylated, or oxidized to disulfide by air.
Nevertheless, even protected Cys can undergo several side reactions. The most relevant are listed here: • Oxidation and alkylation of the thioether. Although less critical than in the case of Met, it can also occur. [316][317][318] Oxidation of the Cys residues during global deprotection can be minimized using 10% of H 2 O as scavenger. 122 • -Elimination ( Figure 20) occurs when protected Cys is exposed to strong bases, such as sodium in liquid ammonia (required to remove the Benzyl group), alkaline conditions, or hydrazylnolysis, or exposed to strong acids such as HF. This side reaction is particularly critical in the case of C-terminal Cys, which in the Fmoc/ t Bu strategy undergoes -elimination followed by piperidine addition to give piperidylalanine residue. The extent of -elimination also depends strongly on the protecting group used, with S t Bu being the worst case followed by Acm and Trt. 319,320 The Bn group can also produce -elimination.
• Reaction with carbocations resulting from the elimination of protecting groups: after its deprotection, Cys can react with the cations generated in acidic conditions. For instance, S-tert-butylated Cys has been observed after the removal of the Boc group or after global deprotection in a Fmoc/ t Bu strategy. 321 • Reattachment to the resin: resin-bound carbocations generated in the acidolytic cleavage from resins can react with both protected and unprotected Cys, thus causing reattachment of the peptide to the resin. 322 -Transfer of Acm (acetamidomethyl) group to Ser, Thr, Gln, and Tyr during Acm removal. [323][324][325] -Formation of thiazolidines of N-terminal Cys ( Figure 21) can take place if His-protecting groups such as Bom (benzyloxymethyl) or Bum (tert-butyloxymethyl), which generate formaldehyde when removed, are present. It can be minimized using Cys as scavenger. 326,327 -Racemization: Cys is highly prone to racemize during the anchoring to the solid support or during the couplings. 328,329 The extent of the racemization also depends on the Sprotecting groups (S t Bu > Trt > Acm > MeBn > t Bu) [330][331][332][333][334] and coupling methods used (favored if preactivation in the presence of base is performed and in the coupling methods involving the use of base). Epimerization of the Cys linked to a hydroxyl resin can even take place during the synthesis as a result of the repetitive base treatments to remove the Fmoc group, with 2-chlorotrityl resin being the least prone to this process. 330,335 The most used protecting groups for the Fmoc/ t Bu strategy are the Acm or Trt groups, when the desired product is the disulfide, and the Trt group, when the desired product is the free thiol. For the Boc/Bn strategy, the most used are Bn and Meb (p-methylbenzyl) to obtain the free thiol and Acm to obtain disulfides.

Introduction of the Protecting Groups
The Cys thiol shows high nucleophilicity; therefore, Cys thiol protection is usually carried out using fully unprotected Cys as starting material. The S-protecting agents used can  be alkyl halides or tosylates, under acidic or basic conditions, or alcohols, which are dehydrated under acidic conditions. Benzyl-type protection can also be performed via reduction of the thiazolidine formed with the corresponding benzaldehyde. 336

Removal
The classification of the protecting group of Cys is particularly complex because most of the protecting groups used can be removed either by oxidation to the disulfide bridge or by other mechanisms. The following classification has drawn up taking into account these other mechanisms but also indicating the conditions for the oxidative removal in each particular case.     (Table 20) p-Methylbenzyl (Meb). 337 More acid-labile than the Bn, it is removed with HF and scavengers at low temperatures. 338,339 It is gradually replacing the Bn in the Boc/Bn solid-phase strategy. It can also be removed with Tl(III) trifluoroacetate or with MeSiCl 3 in the presence of diphenylsulfoxide to yield disulfide bridges. However, p-methoxybenzyl (Mob) is usually a cleaner option. 340 p-Methoxybenzyl (Mob). 337 It is more acid-labile than Meb and is also used in the Boc/Bn solid-phase strategy. However, it is partially removed in the repetitive treatments to remove the Boc group when long peptide sequences are synthesized. 337 It is completely removed by HF at 0°C and scavengers, TFMSA/TFA 341 and Hg (II) acetate or trifluoroacetate in TFA or AcOH, respectively. 342 It can be selectively removed in the presence of Meb using Ag(I) trifluoromethanesulfonate in TFA. 343 An intramolecular disulfide bridge between two Cys(Mob)-protected residues can be formed by removing the Mob group with MeSiCl 3 or SiCl 4 in TFA in the presence of diphenyl sulfoxide at 4°C in 30 min. 344 In addition, oxidative removal with Tl(III) trifluoroacetate also leads to the formation of a disulfide bridge by reaction with a free Cys side chain.

Protecting Groups Removed by Acid
Trityl (Trt). 345 It is removed with TFA and scavengers, such as triisopropylsilane (TIS) to prevent retritylation, or AgNO 3 . 346 It is used for the Fmoc/ t Bu strategy, although Fmoc-Cys(Trt) can undergo racemization in basic carboxyl activation conditions. 331 It can also be removed by oxidation with iodine, thereby leading to a dilulfide bridge by reaction with a free Cys side chain. Other oxidative removals are listed in Table 20. 347 Monomethoxytrityl (Mmt). 348 It is removed with diluted TFA and scavengers. It is considerably more acid-labile than the S-trityl group and can be removed selectively in its presence as well as in the presence of t Bu-protecting groups. Oxidative removal is similar to the case of the Trt group.
Trimethoxybenzyl (Tmob). 349 It is another more acidlabile alternative to the Trt group for the Fmoc/ t Bu strategy. It is removed with diluted TFA (5-30%) and scavengers; however, the trimethoxybenzyl cation resulting from its cleavage can alkylate Trp residues. 9-Xanthenyl (Xan). 331 It has similar stability features to Mmt; thus, it can also be removed selectively in the presence of S-trityl and t Bu-protecting groups or Rink and PAL handles.
Benzyl (Bn). 351 It is removed with HF at 25°C or Na in liquid ammonia. However, although still used, it is being replaced by other benzyl derivatives that do not require such harsh conditions for their removal.
tert-Butyl ( t Bu) and 1-Adamantyl (1-Ada). 352 Both are fully stable to TFA and can, therefore, be used in the Boc/ Bn strategy. They are also quite stable to HF at low temperatures but cleaved at higher temperatures in the presence of scavengers. 334 They are also stable to Ag(I) trifluoromethanesulfonate in TFA, 343 which quantitatively removes the S-Mmt group, as well as to iodine oxidation. Other possible cleavage conditions are listed in Table 20. 342 (Table 21) 9-Fluorenylmethyl (Fm). 353 It is removed with base (i.e., 50% piperidine-DMF for 2 h or 10% piperidine-DMF overnight) 354 and is very stable to strong acids such as HF. It is used in the Boc/Bn solid-phase strategy. It can be removed on solid phase or in solution, thereby yielding a disulfide because of air oxidation unless reducing thiols are employed. It is resistant to oxidative cleavage with iodine or Tl(TFA) 3 of other Cys-protecting groups. 334 2-(2,4-Dinitrophenyl)ethyl (Dnpe). 355 It is removed with bases such as piperidine-DMF (1:1) in 30-60 min, thereby yielding the disulfide bridge, or in the presence of -mercaptoethanol to give the free thiol. It is a less sterically hindered alternative to the Fm group for the Boc/Bn strategy (specially suited to facilitate the cleavage of peptides with C-terminal Cys), stable to strong acids such as HF and oxidative conditions to form disulfide bridges with Acm (I 2 or Tl(TFA) 3 in TFA).

Protecting Groups Removed by Base
Benzyl (Bn). See the section on protecting groups removed by acid.
9-Fluorenylmethoxycarbonyl (Fmoc). 356 Only preliminary solution studies are available for Cys thiol protection with Fmoc. It seems to be more base-labile than the Fm group. It is removed with TEA in the presence of I 2 or benzenethiol in DCM to yield the corresponding disulfide. These removal conditions do not affect the N R -Fmoc group. (Table 22) Acetamidomethyl (Acm). 357,358 Removed by oxidative treatment with I 2 or Tl(TFA) 3 to form disulfide bonds or with Hg(II) and Ag(TFMSO) 343 to obtain the free thiol. It is compatible with both the Boc/Bn and Fmoc/ t Bu strategies. Nevertheless, it is partially removed with HF or even TFA depending on the scavengers used. 359,325 In the latter case, absence of water and use of TIS minimizes the removal. 360 Phenylacetamidomethyl (PhAcm). 361 It is an analogue of Acm that can be removed in similar conditions and also by treatment with the enzyme penicillin aminohydrolase.

Other Protecting Groups
tert-Butylmercapto (S t Bu). 362 It is removed with thiols (benzenethiol, -mercaptoethanol, or dithiothreitol), 363 Na 2 SO 3 in AcOH, 364 or phosphines (PBu 3 or PPh 3 in CF 3 CH 2 OH). 365 It is compatible with the Boc and Fmoc strategies. It is partially removed with HF but completely stable to TFA and to bases like piperidine. 366 3-Nitro-2-pyridinesulfenyl (Npys). It is removed by reducing thiols and phosphines to render the free thiol. 366 It is stable to TFA and HF, but it is not stable to the low-high cleavage protocol or to bases. 368 It is used in the Boc/Bn strategy mainly to obtain disulfide bonds by nucleophilic displacement by the thiol of a free Cys. 369 2-Pyridinesulfenyl (S-Pyr). 370 It is used in the Boc/Bn strategy and is useful when orthogonal protection of unprotected fragments is required. Ligation of a free thiocarboxylic peptide with an S-Pyr-protected N-terminal Cys occurs at pH 2, the subsequent S-to-N migration occurs at pH 7, and final treatment with DTT renders the final ligated peptide with free Cys. S-Pyr is stable to 1 M TFMSA in TFA-anisole (10:1) at 0°C for 2 h (cleavage conditions for the MBHA resin).  Allyloxycarbonyl (Alloc). 164 It is removed with tributyltin hydride catalyzed by Pd(0) (usually Pd(PPh 3 ) 4 ). Because of its base lability, it is used only in the Boc/Bn solid-phase strategy. 371 It is an allyl-type protecting group that can be removed by palladium to render the free thiol both in solution and on solid phase, and it is the only Cys-protecting group that allows a selective and easy release of the thiol on solid phase. It is completely stable to TFA and piperidine and can also be removed by iodine oxidation to render a disulfide bridge.

N-Allyloxycarbonyl-N-[2,3,5,6-tetrafluoro-4-(phenylthio)phenyl]aminomethyl (Fsam).
o-Nitrobenzyl (oNB). 372, 373 It is a protecting group removed by photolysis (λ ) 300-400 nm) and is used mainly in the synthesis of caged peptides. 4-Picolyl. 374 It is removed in solution with Zn dust in AcOH to render the free thiol. It was initially proposed for the Boc/Bn strategy but more recently has been successfully applied to the Fmoc/ t Bu synthesis of dihydrooxytocin, which was further oxidized to oxytocin.
Ninhydrin (Nin). 375 It has been proposed as a protecting group for N-terminal Cys. It protects both the amino and the thiol groups by forming a thiazolidine. Stable to HF and TFA, it is removed with 1 M Cys-OMe, 1 M DIPEA in DMF for 30 min (solid phase), 10% TFA in H 2 O and Zn dust (solution), and reducing thiols such as Cys in combination with tris-carboxymethylphosphine (TCEP) (solution). It is coupled to amines linked to the solid phase without using further protection at the amino group. Its main applications are in ligation and its combination with His(Bom) in the Boc/ Bn strategy, which prevents thiazolidine formation after Bom removal (see His protection).
The mercaptopropionic acid (des-amino Cys), which acts as an N-terminal capping in some peptides of therapeutic interest, can be introduced as a dimer. The free thiol is obtained after reduction with -mercaptoethanol or Bu 3 P. 376

General
The thioether funcionality of Met ( Figure 22) can undergo two side reactions, oxidation to sulfoxide and S-alkylation.
The latter can lead to the formation of homoserine lactone in C-terminal Met (Figure 23). 377 These side reactions are favored in acidic conditions.
In the Fmoc/ t Bu strategy, Met is used unprotected in most of the cases. To prevent oxidation during amino acid sidechain deprotection and cleavage from the resin, ethylmethylsulfide or thioanisole are used. 378,379 In contrast, in the Boc/Bn strategy, free Met may not be the best option because of the strong acidic conditions applied mainly in the cleavage from the resin but also in the removal of the Boc group. Therefore, very frequently, N R -Bocprotected Met sulfoxide is directly used and is reduced at the end of the synthesis.

Introduction of the Protecting Groups
The sulfoxide derivatives of Met are commercially available and can be prepared via oxidation with H 2 O 2 . 380

Removal: Sulfoxide Reduction
In the case of SPS, the reduction of Met(O) can be performed either during the cleavage or after it. In the latter    case, the sulfoxide functionality confers extra polarity to protected peptides, which facilitates its purification; however, it must be taken into consideration that sulfoxides are chiral, and therefore, different diastereomers will be observed.
Several reduction methods have been used: (1) Reduction during the low-high HF or TFMSA cleavage in the Boc/Bn strategy. DMS and p-thiocresol or anisole should be used as scavengers to prevent S-alkylation. (2) N-Methylmercaptoacetamide in 10% aquous acetic acid. [381][382][383] It requires long reaction times, and disulfide bridges may be reduced. (3) TFA-NH 4 I-DMS. [384][385][386] This method of reduction does not affect disulfide bridges, and if there are free Cys residues, a disulfide bridge is formed during the reduction of the Met sulfoxide. tert-Butyl-type groups are removed during the reduction. Dimerization of Trp (see Trp section) can occur in the case of long reaction times as a result of overexposure to acidic conditions. (4) TiCl 4 (3equiv)-NaI(6equiv)inMeOH-acetonitrile-DMF (5:5:4). 387 Although a very fast reduction method, it can also lead to reduction of disulfide bridges or oxidation of Trp, with the latter caused by the I 2 generated in the sulfoxyde reduction. (5) TFA-TMSBr-EDT. 388,389 In this method, the reduction is carried out by addition of TMSBr and EDT at the end of the cleavage step. It appears to be compatible with Trp-containing peptides. The peptide is isolated by precipitation in diethylether. (6) Bu 4 NBr in TFA. It is an alternative to method 5 in which the reduction is also carried out during the cleavage step. 390 (7) Sulfurtrioxide(5equiv),EDT(5equiv)inpyridine-DMF (2:8). 391 In this method, protection of hydroxyl groups is required to prevent sulfonylation.

Met des-tert-butylation
If tert-butylation occurs during the global deprotection step, reversion to the free Met residue is accomplished by heating a solution of the peptide in 4% AcOH (aq) at 60-65°C . 392,393 11. Histidine (His)

General
The imidazole ring of His ( Figure 24) has two nucleophilic points, the πand τ-nitrogens. 394 Unprotected His is highly prone to racemization during the coupling ( Figure 25) and acylation during peptide synthesis followed by N τ to R-amino migration ( Figure  26). 395,396 The basic and nucleophilic π-nitrogen is the one involved in racemization mechanisms and can be masked in two ways: (i) direct protection and (ii) τ-nitrogen protection with bulky or electron-withdrawing protecting groups, which reduce the basicity of the π-nitrogen.
Although a large number of protecting groups have been tested for His side-chain protection, either in the πor τ-nitrogen, the problem has still not been fully resolved, with the situation being more critical in the case of the Boc/Bn solid-phase strategy.

Introduction of the Protecting Groups 394
Protection of the imidazole ring of His requires R-amino and carboxylic acid protection with orthogonal protecting groups. In cases such as Trt, the R-amino group can be used unprotected and at the end of the synthesis the N R -trityl is removed, thereby leaving the N im -trityl unalterated. Generally, the reaction of the imidazole ring of His with the corresponding active species (halides in general) gives the N τ -protected imidazole as a majority and sometimes single product. Nevertheless, N π protection is preferred because, as previously mentioned, the N π is the one directly involved in His racemization. Thus, when possible, N π protection is performed by masking the τ-nitrogen with an orthogonal protecting group, which is removed at the end of the synthesis of the derivative. (Table 23) 11.3.1.1. N τ -Tosyl (Tos). 397 It is removed with HF. It minimizes racemization by reducing the basicity of the N π by inductive effect and also because of steric hindrance. Although it is still quite commonly used in the Boc/Bn solidphase strategy, it is unstable in the presence of N R groups and HOBt. 398,395 N τ -Trityl (Trt). It is the usual protecting group for the Fmoc/ t Bu strategy. 21,399 It is removed with 95% TFA but is much less acid-labile than the N R -trityl group and cannot be selectively removed in the presence of t Bu groups. 400 Using N τ protection, the free N π can still catalyze racemization. However, the bulkiness of the Trt group minimizes this side  These are more acid-labile derivatives of the Trt group; they are removed with 15% and 5% TFA in DCM in 1 h. 400 N τ -tert-Butyloxycarbonyl (Boc). It is only useful for the synthesis of short sequences via Fmoc chemistry because of its instability to prolonged piperidine treatments. 399 Its slightly greater acid stability compared with Trt makes it highly suitable for the preparation of His-containing protected peptides using a ClTrtCl resin. N τ -2,4-Dimethylpent-3-yloxycarbonyl (Doc). 401 It is removed with liquid HF and is used in the Boc/Bn sol-d phase strategy. In contrast to other proposed carbamate-type Hisprotecting groups, it is very resistant to nucleophiles because of its bulkiness, thereby preventing N im to N R transfer. It is not stable to 2% hydrazine in DMF but is more stable to piperidine than the 2,4-dinitrophenyl (Dnp) group (see section on other protecting groups) (its half-life in 20% piperidine in DMF is 84 h). 11.3.1.2. N π -Protection. N π -Benzyloxymethyl (Bom). It is removed by HF, TFMSA, or hydrogenolysis and is completely stable to bases and nucleophiles. It has been extensively used for the Boc/Bn solid-phase strategy. Because formaldehyde is released during Bom cleavage, appropiate scavengers should be used to prevent formylation, methylation, or the formation of thiazolidines when an N-terminal Cys is present. 326,327 In addition, a recent report shows that R-amino Boc removal of Bom-protected His requires harsher conditions than those commonly used. 402 N π -tert-Butoxymethyl (Bum). 403,404 It is removed by TFA and resistant to hydrogenolysis. Formylation during its removal can be prevented using appropiate scavengers in the same way as for Bom. It prevents racemization of His in the Fmoc/ t Bu strategy; however, it is not widely used because of the difficult synthesis of Fmoc-His(π-Bum)-OH. (Table 24) N τ -9-Fluorenylmethoxycarbonyl (Fmoc) 405 It is removed with piperidine-DMF (2:8) and has been used for the synthesis of peptide-oligonucleotide conjugates. 234 N τ -2,6-Dimethoxybenzoyl (Dmbz) 406 It is a relatively recently developed protecting group for the Fmoc/ t Bu strategy, and therefore, it has not been widely used. Removed with ammonia solutions and stable to the removal of tert-butyl type groups, it minimizes His racemization during the coupling to the same extent as Trt and also reduces acyl migration. (Table 25) N τ -2,4-Dinitrophenyl (Dnp) 407 It is removed by thiolysis 408,409 and is stable to HF. It is also commonly used in the Boc/Bn solid-phase strategy. However, it also has some drawbacks: incomplete removal can occur in sequences rich in His and it is labile to nucleophiles. These features makes it incompatible with Lys(Fmoc) because after Fmoc removal the Dnp group can migrate to the free amino of the Lys. 410 In addition, it must be removed before eliminating the last R-Boc group. 411

General
Amino acids containing unprotected hydroxyl functionalities such as Ser, Thr, and Hyp ( Figure 27) can undergo side reactions such as dehydratation or O-acylation followed by O-N migration after amino deprotection ( Figure 28).
Although the protected derivatives are the safest way to incorporate Ser, Thr, or Hyp into the peptide sequence, they can also be used with the free hydroxyl functionality. Protection is more necessary in SPS, because an excess of acylating agents is used, and for Ser, whose primary alcohol is more prone to acylation than the secondary alcohols of Thr and Hyp, which have been successfully used without protection in several syntheses, including solid phase. 412,413 Nevertheless, there are also some reports of the successful use of unprotected Ser in solution-phase synthesis, but care must be taken when choosing the activating agents. 414,415 In peptide synthesis, hydroxyl functionalities are protected as ethers, which are more stable than the corresponding carbamates and esters. The most used protecting groups for the Boc/Bn and Fmoc/ t Bu strategies are Bn (benzyl) and t Bu (tert-butyl), respectively.

Introduction of the Protecting Groups
Distinct protection methods are used depending on the kind of protecting group. t Bu protection is carried out via addition of isobutylene in acidic conditions. 416 Bn protection is performed using benzyl bromide in basic conditions in the case of Ser, 417,418 and reaction with benzyl alcohol in acidic medium in the case of Thr. 419 Bn and t Bu protections can also be achieved via formation of 2,2-difluoro-1,3,2-oxazaborolidin-5-ones by reaction of the lithium salt of Ser or the sodium salt of Thr with BF 3 . Treatment with isobutylene ( t Bu protection) or benzyl 2,2,2-trichloroacetimidate (Bn protection) followed by a base treatment to destroy the 2,2-difluoro-1,3,2-oxazaborolidin-5-one generates the desired protected derivatives. 420 Trt and alkylsilane protection are achieved using the respective chlorides in the presence of a base. 421,422 (Table 26) Benzyl (Bn). 423 It is removed with HF in the presence of scavengers and is the most used protecting group for Ser and Thr in the Boc/Bn solid-phase strategy. When many benzyl ethers are present, appropriate scavengers should be used to avoid benzylation of free amino acid side chains.

Protecting Groups Removed by Acid
Cyclohexyl (cHx). 424 It is an alternative to the benzyl group for the protection of Ser in the Boc/Bn solid-phase strategy. It is more stable to acids and completely stable to catalytic hydrogenation. However, it has not been widely used.
tert-Butyl ( t Bu). 416 It is removed with TFA and used mainly in the Fmoc/ t Bu solid-phase strategy. t Bu ethers are less acid-labile than the Boc group, and some reports indicate that they can be used even as temporary protecting groups in the Boc/Bn solid-phase strategy. 425 Trityl (Trt). 421 It is removed with 1% TFA. It has been shown that the same peptide with all the hydroxyl groups protected by Trt or t Bu is obtained with better purity in the case of the former. 426 tert-Butyldimethylsilyl (TBDMS). 422 It is more acid-labile than the t Bu group and can be removed selectively in the presence of this group using AcOH-THF-H 2 O (3:1:1) or TBAF.
Pseudoprolines. See the section on amide backbone protection. (Table 27) tert-Butyldimethylsilyl (TBDMS). See the section on protecting groups removed by acid.

Other Protecting Groups
tert-Butyldiphenylsilyl (TBDPS). 427,428 It is typically removed by TBAF but also by 2 M NaOH (aq) -EtOH (1:1). It is more acid-stable than TBDMS and stable to the removal of N-Trt, O-Trt, O-TBDMS, and Boc. 4,5-Dimethoxy-2-nitrobenzyloxycarbonyl (Dmnb). 429 It is a photolabile protecting group analogous to the corresponding Dmnb ester. Ser(Dmnb) has been used recently to control protein phosporyltion. 430 Propargyloxycarbonyl (Poc). 431 It is removed with [(PhCH 2 NEt 3 ) 2 MoS 4 ] in AcCN, 1 h, rt. These removal conditions do not affect Boc, Z, methyl, or benzyl esters. It has recently been applied to the protection of Ser and Thr for peptide synthesis in solution.

General
Use of unprotected Tyr ( Figure 29) can lead to acylation of the phenol group because of the nucleophilicity of the phenolate ion under basic conditions. In addition, the electron-rich aromatic ring can undergo alkylation at the ortho position.
The acidity of the phenol group makes alkyl-type protecting groups less stable than in the case of Ser, Thr, and Hyp.   The most used Tyr-protecting groups for the Boc/Bn and Fmoc/ t Bu solid-phase strategies are Bn and t Bu group, respectively.

Introduction of the Protecting Groups
To protect the phenolic function of Tyr, 345 both the amino and carboxylic groups must be protected by either forming a copper(II) chelate or using orthogonal protecting groups.
t Bu-protected Tyr is obtained using isobutylene in acidic medium, 416 whereas with the other Tyr-protected derivatives, the corresponding alkyl halide is used as the protecting agent. 432,433 (Table 28) Benzyl (Bn). It is removed with HF but can lead to benzylation of the aromatic ring of Tyr, and it is not stable enough to the repetitive treatments with 50% TFA in DCM to remove the Boc group. 434 Milder removal conditions for the Boc group allow the synthesis of long peptides using benzyl protection. 27 In solution synthesis, it is usually removed by catalytic hydrogenation.

Protecting Groups Removed by Acid
tert-Butyl ( t Bu). It is removed with TFA and is the most used protecting group for the Fmoc/ t Bu strategy solid-phase strategy. It is more stable than the tert-butyl ethers of Ser, Thr, and Hyp. It is also stable to fluoride ions (TBAF).
2,6-Dichlorobenzyl (Dcb). 432 It is removed with HF, and because of its major acid stability, it is an alternative to the Benzyl group for the Boc/Bn solid-phase strategy.
2-Bromobenzyl (BrBn). 435 It is another more acid-stable derivative of the benzyl group; however, it has not found as wide application as Dcb.
Benzyloxycarbonyl (Z). 337 It is removed with HF and protects the phenol functionality by forming a carbonate. Although still used, it is too acid-labile to withstand repetitive treatments with 50% TFA to remove the Boc group.

2-Bromobenzyloxycarbonyl (BrZ). 432,433
It protects the phenol functionality by forming a carbonate, but unlike with other carbonates, only minor amounts of O-to-N transfer are observed. In contrast to the above-mentioned Z group, BrZ is very stable to acidic conditions (removed with HF) and widely used for the SPS of long peptides using the Boc/Bn solid-phase strategy. 432,433 It cannot be used in the Fmoc/ t Bu strategy because, being a carbonate, it is very sensitive to bases and nucleophiles. 436 3-Pentyl (Pen). 437 It is a relatively new protecting group, stable to 50% TFA, bases, and catalytic hydrogenation, and readily removed with HF.
tert-Butyloxycarbonyl (Boc). 438 This carbonate has been used occasionally for Tyr side-chain protection in the Boc/ Bn solid-phase strategy but only protects the phenol during the coupling and is removed with TFA along to N R -Boc.
Trityl (Trt) and 2-Chlorotrityl (2-Cl-Trt). They are very acid-labile and have the advantage of the low electrophilicity of trityl cations. Thus, they are a better alternative to t Bu for the synthesis of peptides containing residues prone to alkylation such as Trp and Met. 421,439,426 Removal is carried out with 2% TFA in DCM. 293 tert-Butyldimethylsilyl (TBDMS). 422 Unlike the t Bu ethers, the TBDMS ether of Tyr is more acid-labile than the corresponding t Bu ethers; however, it can be removed selectively with TBAF.  (Table 29) Benzyl (Bn). See section on protecting groups removed by acid.

Other Protecting Groups
tert-Butyldimethylsilyl (TBDMS). See section on protecting groups removed by acid.
Allyl (Al). 440,98, 164 Removed with Pd(0), it is strictly orthogonal to the most common protecting groups. It is used in both solution strategies and SPS. o-Nitrobenzyl (oNB). 441 A photolabile protecting group, it has the same properties as the oNB ester. It has been used for the synthesis of Tyr caged peptides. 442 Propargyloxycarbonyl (Poc). 431 It is removed with [(PhCH 2 NEt 3 ) 2 MoS 4 ] in AcCN, 1 h, rt. These removal conditions do not affect Boc, Z, methyl, or benzyl esters. It has recently been applied to the protection of Tyr for peptide synthesis in solution.

General
The indole group of Trp ( Figure 30) can undergo oxidation and alkylation if it is not protected. 444 Alkylation during acid treatments can be done by carbocations from released protecting groups or from the resin, with the latter leading to irreversible bonding of the peptide to the support ( Figure 31). 445 Dimerization of Trp caused by alkylation by another protonated Trp has also been observed ( Figure 32). 446,447 In the Boc/Bn strategy, the higher risk of oxidation and alkylation in acidic media makes the protection of Trp necessary. In addition, care must be taken when chosing the scavengers in the final cleavage. For instance, thioanisole should be avoided because thioanisole cation adducts can alkylate Trp, and TIS, which is mainly used in the Fmoc/ t Bu strategy, should be used instead of TES to prevent reduction of the indole ring of Trp to indoline. 448 The most used protecting group for the Boc/Bn strategy is For (formyl).
In contrast, in the Fmoc/ t Bu strategy, unprotected Trp is often used. However, in many cases, protection is necessary. A critical example is when the peptidic sequences contain Arg protected by either Mtr, Pmc, or Pbf groups, which after removal can react with the indole ring in the 2 position. 449,450 The most used protecting group for the Fmoc/ t Bu strategy is Boc.

Introduction of the Protecting Groups
Carbamate protection of the tert-butyl, benzyl, or phenacyl esters of N R Boc or Z-Trp is easily carried out using di-tertbutyldicarbonate or an appropiate chloroformate in the presence of a tertiaty base. After that, the carboxylic acid and/or amino-protecting groups are removed and N R derivatization yields the Boc and Fmoc derivatives of the protected Trp. [451][452][453][454][455] The formyl group is introduced using an excess of formic acid. 456 (Table 30) Formyl (For). 457 Removal with HF may be slow, and the use of thiols (i.e., EDT) as scavengers makes it faster 458 In the case of base cleavage, care must be taken with the reaction conditions in order to avoid free amine formylation. 459,460 tert-Butyloxycarbonyl (Boc). 453,454 It is removed with high concentrations of TFA and is the protecting group of choice for the Fmoc/ t Bu solid phase strategy. It is more stable than Boc R-amino protection, which can be removed in the presence of protected Trp if care is taken with the reaction conditions, but not as a routine procedure. Boc protection avoids Trp alkylation during the removal of Mtr, Pmc, and Pbf from the Arg side chain. 461,462 The N-carboxylated compound can be detected after tert-butyl removal but later becomes unstable, thereby giving the free indole. The stability of this carbamic acid makes Boc-protected Trp less prone to electrophilic additions during the final cleavage. 453,454 Cyclohexyloxycarbonyl (Hoc). 452 It is an alternative to the formyl group for the Boc/Bn strategy. Its high resistance to bases makes it useful for the synthesis of protected peptides on base-labile resins. 463 Although it is generally removed with HF in the presence of p-cresol, Trp alkylation by p-cresol can occur during the removal. A proposed solution for this problem is the use of Fmoc-Leu or butanedithiol as scavengers. 464 Mesityl-2-sulfonyl (Mts). 465 Another alternative for the Boc/Bn strategy, Mts is removed by 1 M CF 3 SO 3 H/TFA or MeSO 3 H but not by HF. Although it has not been widely applied, there are reports of its use. 466

Protecting Groups Removed by Base
Formyl (For). See the section on protecting groups removed by acid. (Table 31) Allyloxycarbonyl (Alloc). 455 Removed with Pd(0), its orthogonality to Boc and Fmoc (when removed with DBU but not when removed with piperidine) makes it potentially useful for both the Boc/Bn and the Fmoc/ t Bu solid-phase strategies.

Acknowledgments
This work was partially supported by CICYT (CTQ2006-03794/BQU), the "Generalitat de Catalunya" (2005SGR 00662), the Institute for Research in Biomedicine, and the Barcelona Science Park. A.I.-L. thanks the DURSI, Generalitat de Catalunya, and European Social Funds for a predoctoral fellowship.