Asymmetric synthesis of allo-threonine and threonine; the use of a chiral pyridoxal–like pyridinophane-zinc complex as an enzyme mimic

allo-Threonine and threonine having 88 and 74% enantiomeric excess (e.e), respectively, were obtained in 1.7:1 ratio, by a biomimetic aldol condensation between acetaldehyde and the zinc chelate of a Schiff base produced from glycine and a chiral, pyridoxal-like pyridinophane derivative (2).