Convenient preparation of chiral phase-transfer catalysts with conformationally fixed biphenyl core for catalytic asymmetric synthesis of α-alkyl- and α,α-dialkyl-α-amino acids: application to the short asymmetric synthesis of BIRT-377

Abstract

Chiral phase-transfer catalysts ( S)- 1a, ( S)- 1b, and ( S)- 2 with conformationally fixed biphenyl cores were conveniently prepared from the known, easily available ( S)-6,6′-dimethylbiphenyl-2,2′-diol 3 and ( S)-4,5,6,4′,5′,6′-hexamethoxybiphenyl-2,2′-dicarboxylic acid 14, respectively, in five steps. The catalysts, ( S)- 1a and ( S)- 1b are readily applicable to asymmetric alkylation of N-(diphenylmethylene)glycine tert-butyl ester with excellent enantioselectivity. In particular, catalyst ( S)- 1b was found to exhibit the unique temperature effect on the enantioselectivity, and asymmetric alkylation of glycine derivatives at room temperature gave higher enantiomeric excess than that at 0 °C. In addition, the catalyst ( S)- 2 exhibited the high catalytic performance (0.01–1 mol %) in the asymmetric alkylation of N-(diphenylmethylene)glycine tert-butyl ester and N-( p-chlorophenylmethylene)alanine tert-butyl ester compared to the existing chiral phase-transfer catalysts, thereby allowing to realize a general and useful procedure for highly practical enantioselective synthesis of structurally diverse natural and unnatural α-alkyl-α-amino acids as well as α,α-dialkyl-α-amino acids. This approach is successfully applied to the short asymmetric synthesis of cell adhesion BIRT-377.