Palladium-Catalyzed C–H bond functionalization for the synthesis of tetrahydroisoquinolines

Abstract

The synthesis of new bioactive compounds is always a challenge for chemists, particularly in areas where the appearance of resistant strains to specific drugs is becoming important. Even with the technology and knowledge available, drug discovery is not trivial and hit rates tend to be unfavourable. For this reason, the discovery of privileged bioactive scaffolds that allow multiple structural variations seems to be an appropriate strategy to tackle these issues. In this sense, tetrahydroisoquinoline, a heterocycle that is present in many natural sources, is part of a family of compounds with multiple biological activities that fits with these features. One way to access these heterocycles is by metal-catalyzed cyclization of 2-phenylethylamine derivatives by a C–H activation process. However, the use of unprotected primary amines as a cyclization precursors was almost unprecedented: the elevated reactivity and strong coordination of primary amines to metals difficults their use in metal-catalyzed processes. In this context, to avoid the aforementioned coordination to the metal, the addition of steric hindrance next to the nitrogen was suggested and tested. The results showed that hindered primary amines can be used in this type of cyclization with Pd(II) as a catalyst. Thus, a general synthetic route for the obtention of tetrahydroisoquinoline derivatives coming from primary amines is reported in this project where the additional carbon atom necessary to form the THIQ is introduced through a conjugated addition of a C–H activated carbon to a phenyl vinyl sulfone. The synthetized compounds have a common THIQ skeleton and vary their aromatic substituents and the steric hindrance around the amine. In this work, two different methods for the preparation of the starting amines are reported as well as its cyclization to THIQs.