Decomposition of 2-(1-Hydroxybenzyl)thiamin in Neutral Aqueous Solutions: Benzaldehyde and Thiamin Are Not the Products

It has been reported that in neutral aqueous solutions, the adduct of benzaldehyde and thiamin, 2-(1-hydroxybenzyl)thiamin, HBzT (2), undergoes general base-catalyzed reversion to thiamin and benzaldehyde (E. J. Crane, III, and M. W. Washabaugh (1991) Bioorg. Chem.19, 351). An unusual mechanism had been invoked to explain the kinetic observations. In the present study, it is shown that in solutions of pH < 8, HBzT fragments at the bridge methylene group to 2,5-dimethyl-4-amino-pyrimidine (3) and 2-benzoyl-5-(2-hydroxyethyl)-4-methylthiazole (4), not thiamin and benzaldehyde (which are the products at higher pH) (Scheme 3). Buffer catalysis is not observed where the products are thiamin and benzaldehyde. In neutral solution under conditions in which the products are 3 and 4, the reaction is general base-catalyzed. It is likely that catalysis assists the cleavage of the methylene bridge. Mechanistic proposals based on the elimination of benzaldehyde from HBzT in neutral and acidic solutions should be reconsidered.