AN EFFICIENT SYNTHESIS OF OPTICALLY PURE (R)-2-(AMINOMETHYL)ALANINE AND (S)-2-(AMINOMETHYL)ALANINE ((R)-AMA AND (S)-AMA) AND (R)-2-(AMINOMETHYL)LEUCINE AND (S)-2-(AMINOMETHYL)LEUCINE ((R)-AML AND (S)-AML)

An efficient synthesis of enantiomerically pure (R)- and (S)-2-(aminomethyl)alanine ((R)- and (S)-Ama) 1a and (R)- and (S)-2-(aminomethyl)leucine ((R)- and (S)-Aml) 1b is described (Schemes 1 and 2). Resolution of the racemic amino acids was achieved using L-phenylalanine cyclohexylamide (2) as chiral auxiliary. The free amino acids 1a, b were converted to the N-alpha-Boc, N-gamma-Z-protected derivatives 11a, b (Scheme 3) ready for incorporation into peptides. Based on the three crystal structures of the diastereoisomeric peptides 8a, 8b, and 9b, the absolute configurations in both series were determined. beta-Turn type-I geometries were observed for structures 8b and 9b, whereas 8a crystallized in an extended backbone conformation.