Cinchona alkaloid catalyzed sulfa-Michael addition reactions leading to enantiopure β-functionalized cysteines

Sulfa-Michael additions to alpha,beta-unsaturated N-acylated oxazolidin-2-ones and related alpha,beta-unsaturated alpha-amino acid derivatives have been enantioselectively catalyzed by Cinchona alkaloids functionalized with a hydrogen bond donating group at the C6' position. The series of Cinchona alkaloids includes known C6' (thio)urea and sulfonamide derivatives and several novel species with a benzimidazole, squaramide or a benzamide group at the C6' position. The sulfonamides were especially suited as bifunctional organocatalysts as they gave the products in very good diastereoselectivity and high enantioselectivity. In particular, the C6' sulfonamides catalyzed the reaction with the alpha,beta-unsaturated alpha-amino acid derivatives to afford the products in a diastereomeric ratio as good as 93:7, with the major isomer being formed in an ee of up to 99%. The products of the organocatalytic sulfa-Michael addition to alpha,beta-unsaturated alpha-amino acid derivatives were subsequently converted in high yields to enantiopure beta-functionalized cysteines suitable for native chemical ligation.