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

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 degrees 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 alpha-alkyl-alpha-amino acids as well as alpha, alpha-dialkyl-alpha-amino acids. This approach is successfully applied to the short asymmetric synthesis of cell adhesion BIRT-377. (c) 2007 Elsevier Ltd. All rights reserved.