Claisen-type addition of glycine to pyridoxal in water

The reaction between 5′-deoxypyridoxal and glycine in D₂O buffered at pD 7.0 does not result in significant formation of the expected products of pyridoxal-catalyzed transamination or deuterium exchange of the α-amino protons of glycine, but rather gives a quantitative yield of the two diastereomeric products of the formal Claisen-type addition of glycine to 5′-deoxypyridoxal. The unexpected extensive formation of these products reflects the extraordinary selectivity of the 5′-deoxypyridoxal-stabilized glycine enolate toward addition to the carbonyl group of 5′-deoxypyridoxal in the protic solvent water.