Three
enzymatic routes toward γ-hydroxy-α-amino acids
by tandem aldol addition–transamination one-pot two-step reactions
are reported. The approaches feature an enantioselective aldol addition
of pyruvate to various nonaromatic aldehydes catalyzed by trans-o-hydroxybenzylidene pyruvate hydratase-aldolase
(HBPA) from Pseudomonas putida. This
affords chiral 4-hydroxy-2-oxo acids, which were subsequently enantioselectively
aminated using S-selective transaminases. Three transamination
processes were investigated involving different amine donors and transaminases:
(i) l-Ala as an amine donor with pyruvate recycling, (ii)
a benzylamine donor using benzaldehyde lyase from Pseudomonas
fluorescens Biovar I (BAL) to transform the benzaldehyde
formed into benzoin, minimizing equilibrium limitations, and (iii) l-Glu as an amine donor with a double cascade comprising branched-chain
α-amino acid aminotransferase (BCAT) and aspartate amino transferase
(AspAT), both from E. coli, using l-Asp as a substrate to regenerate l-Glu. The γ-hydroxy-α-amino
acids thus obtained were transformed into chiral α-amino-γ-butyrolactones,
structural motifs found in many biologically active compounds and
valuable intermediates for the synthesis of pharmaceutical agents.