Operationally Convenient and Scalable Asymmetric Synthesis of (2S)‐ and (2R)‐α‐(Methyl)cysteine Derivatives through Alkylation of Chiral Alanine Schiff Base NiII Complexes

Abstract

This research demonstrates that the methylation of N‐benzyl cysteine Schiff base derived NiII complexes leads to the formation of the corresponding dehydroalanine‐containing products and cannot be used for preparation of the target α‐(methyl)cysteine. In sharp contrast, the alternative strategy that involves thiomethylation of NiII complexes of alanine Schiff bases is a viable and practically attractive approach to afford the desired α‐(methyl)cysteine containing derivatives. This work also reveals a significant, and rather unexpected, difference in the stereochemical performance of proline and 3,5‐dihydro‐4H‐dinaphth[2,1‐c:1′,2′‐e]azepine derived chiral ligands, the former of which shows superior performance in terms of chemical yields and diastereoselectivity of the α‐(methyl)cysteine products formed.