Enantioselective , Protecting Group-Free Synthesis of 1 S-Ethyl-4-Substituted Quinolizidines

A practical enantioselective protecting group-free four-step route to the key quinolizidinone 6 from phenylglycinol-derived bicyclic lactam 1 is reported. The organometallic addition reaction upon 6 takes place stereoselectively to give 1-ethyl-4-substituted quinolizidines 4-epi-207I and 7-9. Following a similar synthetic sequence, 9a-epi-6 is also accessed. However, the addition of Grignard reagents upon 9a-epi-6 proceeds in a non-stereoselective manner. In order to gain insight into the different 10


Introduction
The extracts from the skin of certain poisonous frogs and toads contain alkaloids showing promising biomedical activities.So far, none of these alkaloids has been reported from any other natural source.Most of them contain as a common structural feature an azabicyclic "izidine" nucleus, e. g. disubstituted pyrrolizidines, diand trisubstituted indolizidines, or disubstituted quinolizidines. 1 Among them, 1,4-disubstituted quinolizidines 2 are a relatively new class of alkaloids that have been isolated in minute quantities.Their structures have been partially elucidated based on the GC-MS and GC-FTIR spectra, the latter showing significant Bohlmann bands 3 indicating that the hydrogens at positions 4 and 9a are cis.
The relative configuration at position 1 is only tentative and the absolute configuration of the stereocenters is unknown.As total synthesis is required for structural proof, it would be highly desirable to develop general asymmetric methodologies to easily access these biologically interesting compounds.
There are currently about 20 compounds assigned to this particular structural family of amphibian alkaloids 4 including seven 1-ethylquinolizidines representatives (Figure 1).Among them, six show a 1,4-trans relative configuration while alkaloid (-)-207I is unique, with substituents being 1,4-cis.The relative stereochemistry of natural quinolizidine (-)-207I 40 was determined in 1997 by Momose's group by comparison of the GC-MS and GC-FIT spectra of the 1-epi-207I isomer synthesized by them and the natural alkaloid. 5In 2003, Toyooka and coworkers published the first enantioselective synthesis of (+)-207I, the enantiomer of the alkaloid, thus determining the absolute selectivity observed between both subtypes.Comparing the latter compounds with 1-epi-207I led the authors to conclude that the α7 subtype selectivity of 1,4-disubstituted quinolizidines is remarkably dependent on the structure of the C4 side chain.Increasing the length of the 4-moiety beyond three carbons appears to markedly reduce potency and selectivity at the α7 receptor.
In this paper we disclose a general protocol for the enantioselective construction of these biologically attractive structural motifs using chiral bicyclic lactams as enantiomeric scaffolds. 9Taking into account the aforementioned biological studies, we planned to attach a 1 to 3 carbon chain at the C4 position of the azabicyclic nucleus.

Retrosynthetic analysis
Our retrosynthetic analysis is briefly outlined in Scheme 1.We envisioned that the alkyl group at C-4 could be installed by a stereoselective addition of an organometallic reagent to the carbonyl amide group of a 1-ethyl quinolizidine derivative.This bicyclic lactam was surmised to be constructed by a ring-closing metathesis reaction of a monocyclic diallylated derivative.In turn, the required trisubstituted 2-piperidone would be obtained by alkylation of (5S,6S)-6-allyl-5-ethyl-2-piperidone, whose enantiomer had been previously synthesized by our group by an α-amidoalkylation reaction 10

Preparation of (5S,6S)-6-allyl-5-ethyl-2-piperidone (3)
As previously reported by our group in the enantiomeric series, the TiCl4-promoted addition of allyltrimethylsilane to 1 occurs stereoselectively, with inversion of the configuration at the C8a to afford a 9:1 mixture of cis-6-allyl-5-ethyl disubstituted lactam 2 and its C6 epimer, 6-epi-2.The absolute stereochemistry of 2 was unambiguously confirmed by X-Ray crystallographic analysis (Figure 2).The configuration of the 5 and 6 stereocenters remains in the final product since both of them are configurationally stable in the subsequent synthetic transformations (Scheme 2).followed by alkylation with allyl bromide.More conveniently, we developed a one-pot two-step process from 2 to 4 involving removal of the 2-phenylethanol moiety and alkylation in the same vessel in basic media by sequential treatment with O2 and allyl bromide (69% overall yield). 12With all the carbon atoms installed 65 in compound 4 a ring-closing metathesis reaction could be performed and the required six-membered piperidine ring accessed.A second-generation ruthenium Grubbs catalyst mediated this reaction from 4 to generate 5 (87%) under mild conditions. 13Chemoselective reduction of 5 employing catalytic hydrogenation was accomplished to give 6, which is a valuable synthetic intermediate for the formation of the targeted diversely 4-substituted 1-ethylquinolizidines.

Addition of Grignard reagents to azabicyclic compound 6
With bicyclic lactam 6 in hand, the stage was set to execute the installation of an alkyl substituent at carbon 4. To this end, a onepot procedure, involving the reaction of 6 with Grignard reagents followed by dehydration to the corresponding iminium salts and reduction to the final products, was studied.
In order to access alkaloid (-)-207I we first considered the addition of allylmagnesium bromide.Interestingly, while there are several reports dealing with the addition of organometallic reagents to the 2-quinolizidinone nucleus, 14 to the best of our knowledge, the addition of an allylmetallic reagent is unprecedented. 15Only after much experimentation did we find that the reaction indeed occurs using an excess of Grignard reagent (4 equiv) in the presence of anhydrous CeCl3. 16eing aware that the reduction step would be responsible for the stereochemistry of the final product, different reducing agents were evaluated.While reduction with NaBH3CN or NaBH(AcO)3 gave inseparable mixtures of (-)-207I and its C4-epimer, 4-epi-207I, (34:66 and 23:77, respectively), NaBH4 and DIBAL-H furnished 4-epi-207I with a high degree of diastereoselectivity (3:97 and 4:96, respectively) (Scheme 3). 17In order to access C-4 analogues of the alkaloid (-)-207I, the addition of different Grignard reagents was studied.In all cases, the alkyl chain introduced was limited to a length of up to three carbon atoms, taking into account previous biological studies. 7,8To this end, an excess of (2-methylallyl)magnesium bromide was added to 6, followed by treatment with NaBH4 to give 7 and 4-epi-7 (96:4) in 56% yield.Following similar experimental conditions, a propyl and a methyl chain were also introduced to stereoselectively furnish 8 and 9 in 64 and 69% yield, respectively (Scheme 4).

Structural elucidation of the final bicyclic amines 50
The absolute configuration of the C-4 stereogenic center in (-)-207I and 4-epi-207I was assigned by correlation of the NMR data of these two compounds with those reported for the previously synthesized enantiomer, (+)-207I. 6orthy of note, in 4-epi-207I the peaks corresponding to C-6 55 and C-9a are about 5 and 3 ppm more shielded, respectively, than in (-)-207I (Table 1).This shielding probably reflects the γ-gauche effect due to an axial disposition of the allyl substituent in 4-epi-207I.Comparison of the 13 C NMR and 1 H NMR (Table 2) of compounds 7-9 with that of 4-epi-207I led to the depicted 60 configuration being assigned to the C-4 stereocenter of these new products (Scheme 4). 18ble 1.Significant 13 C NMR data for bicyclic amines.
65 carbon (-)-207I [a]   In fact, full geometry optimization of 9 and its hypothetical C4 epimer (4-epi-9), performed with the B3LYP density functional method using the 6-31G(d) basis set, revealed that while both compounds were in a chair-chair conformation, only 9 displayed 75 its 4-methyl group in an axial valence (Figure 3), while the methyl group in 4-epi-9 was in an equatorial disposition (Figure 4).Thus, theoretical calculations are in concordance with the 13 C-NMR observations.

Synthesis of 9a-epi derivatives of 4-substituted 2ethylquinolizidines
With compounds 6-epi-2 in hand (Scheme 2), we decided to go a 10 step further and apply the developed procedure to prepare 9a-epi-6 to study the stereochemical outcome of the Grignard addition reactions in compounds with a C9a R configuration.Thus, the introduction of an allyl group on the piperidone nitrogen of 6-epi-2 gave compound 6-epi-4 in 56% yield.Subsequent treatment with Grubbs second-generation catalyst furnished 9a-epi-5, which was hydrogenated to give 9a-epi-6 with yields comparable to those obtained in the previous series.The addition reaction of allylmagnesium bromide in the presence of anhydrous CeCl3 to 9a-epi-6 followed by NaBH4 reduction occurs in a non steroselective manner to give a 1:1 mixture of 10 and 9a-epi-207I.Similar results were observed in the reaction of 9a-epi-6 with methylmagnesium bromide, yielding an equimolecular mixture of the two possible products.These results are in striking contrast with the stereochemical behaviour of the additions previously studied in 6, which occurred with very high 35 stereoselectivity.
The structural assignation of these quinolizidines was done by comparison with the spectroscopic data of compound 10 whose enantiomer had been previously described in the literature. 5,19ing adopts a half-chair conformation with the C3, C4, N and C9a atoms in the same plane, with the ethyl group either in an equatorial, AI, or axial disposition, AII (Figures 5 and 6), AI being more stable by 3.1 Kcal/mol.This fact could be ascribed to a 1,3diaxial destabilizing interaction in AII between the axial ethyl 65 substituent and the axial protons at positions 3 and 9. On the basis of these findings, the stereochemical outcome of the addition reaction can be explained considering an axial attack of the hydride, under stereoelectronic control, 20 at the electrophilic carbon of the lowest-energy iminium ion intermediate, AI.This 70 directed nucleophile attack dictates an S configuration in the newly created stereocenter, as depicted in Figure 5.In a similar fashion, in the two possible conformations of the iminium salt B, the non-substituted ring adopts a chair conformation, whereas the substituted ring shows a half-chair 75 conformation with the C3, C4, N and C9a atoms in the same plane.However, in the iminium salt B both possible conformations, BI, with the ethyl group in an equatorial disposition, and BII, with the ethyl group in an axial disposition, are energetically similar (∆E < 1 Kcal/mol) (Figures 7 and 8).The smaller energy gap between 80 both conformations may be indicative of a relative stabilization of the conformation BII in comparison with AII because only one 1,3-diaxial destabilizing interaction (ax-Et/ax-H3) is found in the former.Consequently, as both conformations are of similar energies, the stereoelectronic controlled addition of the hydride upon the iminium salt can occur on both BI and BII, yielding compounds 11 and 4-epi-11 in nearly equal amounts.

Conclusions
In conclusion, we have developed a straightforward enantioselective synthesis of potentially biologically interesting 1ethyl-4-substituted quinolizidines without using protecting groups.The stereochemistry of the stereocenters is defined by the use of 30 (S)-phenylglycinol as the source of chirality and by two stereocontrolled reactions, an α-amidoalkylation and an organometallic addition.Compounds in the 9a-epi series have been efficiently obtained following the synthetic sequence developed in the original series.However, the final organometallic addition 35 reaction proved to be not stereoselective.In order to rationalize the different stereochemical outcome in the Grignard addition reactions in the two series, theoretical calculations on the iminium salts were performed indicating that the addition of the hydride occurs in a stereoelectronic controlled fashion.

Experimental Section
General Methods NMR spectra were recorded in CDCl3 at 300 or 400 MHz ( 1 H) and 75.4 or 100.6 MHz ( 13 C), and chemical shifts are reported in δ values downfield from TMS or relative to residual chloroform 45 (7.26 ppm, 77.0 ppm) as an internal standard.Data are reported in the following manner: chemical shift, multiplicity, coupling constant (J) in hertz (Hz), integrated intensity.Multiplicities are reported using the following abbreviations: s, singlet; d, doublet; dd, doublet of doublets; t, triplet; m, multiplet; br s, broad signal, 50 app, apparent.Evaporation of solvents was accomplished with a rotatory evaporator.Melting points were determined in a capillary tube and are uncorrected.Thin-layer chromatography was done on SiO2 (silica gel 60 F254), and the spots were located by UV, 1% aqueous KMnO4 or iodoplatinate (for tertiary amines).55   Chromatography refers to flash column chromatography and was carried out on SiO2 (silica gel 60, SDS, 230-400 mesh) or Al2O3 (Aluminium oxide 90 active basic, Merck).Mass spectra were recorded on a LTQ spectrometer using electrospray (ES + ) ionization techniques.TiCl4 (20 mL, 182.12 mmol) was slowly added to a cooled (0 ºC) solution of 1 (11.17g, 45.53 mmol) in anhyd CH2Cl2 (90 mL) and the mixture was stirred for 15 min.Allyltrimethylsilane (14.5 mL, 91.06 mmol) was added in 3 portions and the resulting mixture was warmed at rt and stirred for 16 h.The mixture was poured onto ice and the aqueous layer was extracted with CH2Cl2.The combined organic extracts were dried, filtered and concentrated to give a 90:10 ( 1 H NMR) mixture of 2 and 6-epi-2, which was purified by column chromatography (Biotage Si 40M 2197-1, CH2Cl2-MeOH 99.5:0.5 to 97:3) to yield 2 (10.02 g, 77%) and C6-epi-2 (0.87 g,
Method B. In a round-bottomed flask equipped with a 1 gallon gas bag of O2, an excess of freshly ground NaOH (2.78 g, 69.6 mmol) was added to a solution of 2 (2.00 g, 6.96 mmol) in MTBE (20 70 mL).The mixture was heated at 40 ºC and stirred slowly at this temperature for 24 h.The progress of reaction was monitored by TLC and, when 2 was consumed, the gas bag was disconnected and the reaction mixture was cooled to rt.Allyl bromide (0.75 mL, 8.70 mmoL) was slowly added and stirring was continued for 75 additional 19 h at rt.The solvent was removed, the residue was partitioned between H2O and CH2Cl2, and the aqueous layer was extracted with CH2Cl2.The organic extracts were washed with saturated NH4Cl solution, dried and concentrated to give a residue, which was purified by column chromatography (hexane-EtOAc  (5S,6R)-1,6-Diallyl-5-ethyl-2-piperidone, (6-epi-4).
100 Anhydrous CeCl3 21 (2.0 equiv) was added to a solution of 6 (1 equiv) in anhyd THF.The resulting suspension was vigorously stirred at rt for 1 h.Grignard reagent was added dropwise (4.0-8.0 equiv) over 30 min and the mixture was stirred for additional 18 h.MeOH was added to quench the reaction and the mixture was 105 cooled at -78 ºC.NaBH4 (1.25 equiv) and AcOH (0.3 mL) were added and the mixture was stirred at -78 ºC for 30 min.The mixture was concentrated and the residue was partitioned between Et2O and 1N HCl solution.Then, the acidic aqueous phase was washed with Et2O and basified with 4N NaOH solution (pH=12-110 14).The resulting suspension was centrifuged (800 g for 30 min at rt) and the supernatant was extracted with CH2Cl2. 22The combined organic extracts were dried, filtered, concentrated and analysed by GC-MS.The crude product was purified by flash chromatography (Al2O3).Following the general procedure, from lactam 6 (300 mg, 1.66 mmol), CeCl3 (1.63 g, 6.62 mmol), allylmagnesium bromide (7.0 mL, 7.0 mmol, 1.0 M solution in Et2O), THF (7 mL), and NaBH4 (78 mg, 2.7 mmol), a 97:3 diastereomeric mixture of 4-epi-207I and (-)-207I (GC-MS) was obtained.Traces of diallylated quinolizidine were also detected.Initial geometries were obtained using the PCMODEL program. 23urther geometry optimizations were carried out using the Gaussian 03 suite of programs on an Compaq HPC320 computer, 24 at the Hartree-Fock (HF) level, 25 and at the Becke's three-40 parameter hybrid functional with the Lee, Yang and Parr correlation functional (B3LYP) level, 26 using the 6-31G(d) basis set. 27Analytical energy second derivatives were calculated at all optimized structures to confirm that these were minima.

Figure 3
Figure 3 The most stable conformation of compound 9 (γ-gauche effects are indicated).

Figure 5
Figure 5 Two views of the most stable conformation of the 10

Figure 6
Figure 6 Two views of the most stable conformation of the iminium salt intermediate AII.1,3-Diaxial destabilizing 15

Figure 7
Figure 7 Two views of the most stable conformation of the iminium salt intermediate BI and indication of the hydride stereocontrolled addition.

Figure 8
Figure 8 Two views of the most stable conformation of the iminium salt intermediate BII and indication of the hydride stereocontrolled addition.1,3-Diaxial destabilizing interaction is indicated.25