Asymmetric synthesis of phosphorus analogues of dicarboxylic α-amino acids

Abstract

An efficient approach to the asymmetric synthesis of phosphorus analogues of dicarboxylic α-amino acids is described. The method of choice consists in the reaction of the nickel(II) complex (4) of the Schiff's base derived from (S)-o-[(N-benzylprolyl)amino]benzophenone 3 and glycine with the appropriate alkyl halide, substituted with an alkylphosphonate group. The reactions were carried out in MeCN at 25 °C, with solid KOH as a catalyst. Michael-type base-catalysed addition of vinyl-phosphonate and vinylphosphinate to complex 4 in dimethylformamide (DMF) at 50–70 °C could also be employed. Significant diastereoselectivity (90% d.e.) was observed for the alkylation of complex 4. Optically pure (S)-phosphinothrieine, (S)-2-amino-3-phosphonopropanoic acid, (S)-2-amino-4-phosphonobutanoic acid and (S)-2-amino-5-phosphonopentanoic acid were obtained after the alkylated diastereoisomeric complexes had been separated on SiO2 and hydrolysed with aq. HCl. The initial chiral reagent 3 was recovered (60–85%). Novel amino acids 9, having free carboxy groups and esterified phosphonic and phosphinic groups, could also be obtained as intermediates due to the mild conditions of the decomposition of the alkylated diastereoisomeric complexes.