Towards a Series of Chiral Primary Amines Bearing α-Amino Acid and Benzo[d]imidazole Pendants, and Their Application in Asymmetric Aldol Reactions

Abstract

A straightforward reaction path towards chiral primary amines bearing α-amino acid and benzo[d]imidazole pendants has been developed. Six starting essential α-amino acids were converted into the target chiral amines in a four-step synthesis. The prepared amines were screened as organocatalysts in the direct asymmetric aldol reaction between 4-nitrobenzaldehyde or isatin and acetone or cyclohexanone. The aldol adducts were obtained in good chemical yields, diastereoselectivity up to 97:3 (reactions with cyclohexanone), and enantioselectivity up to 71% ee. Trifluoroacetic acid and benzoic acid proved to be the best cocatalysts for the aldol process on 4-nitrobenzaldehyde and isatin, respectively.