GENERAL-METHOD FOR THE ASYMMETRIC-SYNTHESIS OF ANTIDIASTEREOISOMERS OF BETA-SUBSTITUTED L-2-AMINOBUTANOIC ACIDS VIA CHIRAL NICKEL(II) SCHIFFS BASE COMPLEXES OF DEHYDROAMINOBUTANOIC ACID - X-RAY CRYSTAL AND MOLECULAR-STRUCTURE OF THE NICKEL(II) COMPLEX OF THE SCHIFFS BASE FROM [(BENZYLPROLYL)AMINO]BENZOPHENONE AND DEHYDROAMINOBUTANOIC ACID

An efficient approach to the asymmetric synthesis of (L)-allo-isomers of β-substituted α-aminobutanoic acid is described. The chiral Ni II complex of a Schiff's base derived from (S)-o-[N-(N-benzylprolyl) amino]benzophenone (BBP) and glycine was treated with acetaldehyde in MeOH. The addition proceeds with high diastereoselectivity to give, if catalysed by MeONa, the corresponding complex of (R)-threonine, and, if catalysed by Et 3N, the corresponding complex of (S)-allo-threonine. The (R)-threonine complex was converted into the chiral NiII complex of dehydroaminobutanoic acid, and a X-ray diffraction structural study of its major isomer showed that the dehydroaminobutanoic acid moiety was in the E-configuration. The complex, in turn, entered into Michael addition reactions with nucleophiles, including MeOH, EtOH, PhSH, and PhCH2SH. The reaction proceeded with high diastereoselectivity, producing predominantly complexes of the allo-threonine derivatives (d.e. > 90%). Diastereoisomerically and enantiomerically pure α-amino acids were obtained after chromatographic purification, decomposition of the complexes, and recovery of the initial chiral auxiliary, BBP. The thiol addition reaction is accompanied by a side reaction leading to the formation of sizeable amounts of the vinylglycine complex. An approach to the synthesis of optically active vinylglycine starting with racemic methionine is described.