Asymmetric synthesis of S-alkyl-substituted (R)-cysteines via a chiral Ni-II complex of the Schiffs base of dehydroalanine with (S)-2-N-(N- benzylprolyl)aminobenzophenone

An efficient procedure was developed for the asymmetric synthesis of S-alky l derivatives of (R)-cysteine by nucleophilic addition of alkanethiols ((Bu SH)-S-n, (BuSH)-S-t. or tert-C5H11SH) to the C=C bond of the dehydroalanine fragment in the Ni-11 complex of the Schiffs base of Delta-Ala with (S)-2- N-(N-benzylprolyl)aminobenzophenone [(S)-BPB-Delta-Ala]Ni-11. Under conditi ons of thermodynamic control of the reaction, the diastereomeric excess of the complexes with the (S, R)-configuration was 88-96%. After decomposition of the complexes, ( R)-S-butylcysteine, (R)-S-tert-butylcysteine. and (R)- S-tert-pentylcysteine were isolated with an enantiomeric purity of >97%.