Geometry and energy of amidoenethiolate conformers formed by ring-opening of several thiazolium ions in base: a theoretical study using density functional theory

An interesting aspect of the ring-opening of thiamine and certain other thiazolium ions is the biphasic kinetics of the ring reclosure process. It has previously been proposed that the two amide rotamers of the ring-opened amidoenethiolates are responsible for the two kinetic processes. We have carried out Hartree-Fock and density functional theory calculations on a series of amidoenethiolates and their corresponding tetrahedral intermediates. The corrected gas phase energy difference of 14.6 kJ/mol between 1' c and 2' c supports the notion that the two rotamers can exist in solution in sufficient quantities to give rise to two kinetic processes. The major enethiolate conformer should cyclize more rapidly (after protonation) than the minor conformer in agreement with previous observations. The most stable tetrahedral intermediate has a geometry that suggests stabilization by the anomeric effect.