Reactions of Thiols with Phenylglyoxal to Give Thiomandelic S-Esters Formation of Hemithioacetals and Their Rearrangement

Article abstract of DOI:10.1246/bcsj.55.1493

Equilibrium constants K_h for the addition of 2-mercaptoethanol and glutathione to phenylglyoxal to form hemithioacetals were determined spectrophotometrically over the pH range 7-10.The observed K_h values decrease sigmoidally with pH as the thiol ionizes.Rearrangement of hemithioacetals formed from phenylglyoxal and various thiols was kinetically investigated.The rates increase with thiol concentration following a saturation curve to give K_h identical with the spectrophotometric value.The rearrangement is subject to general base catalysis.The solvent isotope effect on the rate of the rearrangement, k^H2O/k^d2O, is nearly 1.0; that on the equilibrium, K^H2O_h/K^D2O_h, is 0.38.The results strongly support the mechanism involving proton transfers through an enediol intermediate.