Alanine-dependent reactions of 5′-deoxypyridoxal in water

The non-enzymatic reaction of 5′-deoxypyridoxal (DPL) with l-alanine in water at 25 °C was investigated. DPL reacts with alanine to form an imine, which then undergoes deprotonation at the α-amino carbon of alanine to form a resonance delocalized DPL-stabilized carbanion. At early reaction times the only detectable products are pyruvate and the dimeric species formed by addition of the α-pyridine stabilized carbanion to DPL. No Claisen-type products of addition of the α-amino carbanion to DPL, as was previously reported to form from the reaction between DPL and glycine [K. Toth, T.L. Amyes, J.P. Richard, J.P.G. Malthouse, M.E. Ni{dotless}́Beilliu, J. Am. Chem. Soc. 126 (2004) 10538-10539], are observed. The electrophile reacts instead at the α-pyridyl carbon. This dimer is in chemical equilibrium with reactants. At longer reaction times about 50% of DPL is converted to 5′-deoxypyridoxamine, the thermodynamically favored product of formal transamination of DPL.