The preparation of thiazole-4- and -5-carboxylates, and an infrared study of their rotational isomers

Convenient general procedures have been developed for preparing series of thiazole-4- and -5-carboxylates containing alkyl and halogeno substituents. While both series of esters show i.r. carbonyl doublets caused by rotational isomerism, the more intense absorptions of the 4-carboxylates are the lower wavenumber components, whereas those of the 5-carboxylates are the higher wavenumber components. In both series the stronger bands arise from the thermochemically more stable forms; identification of these forms as the carbonyl O,S-syn-s-trans rotamers is more certain with the 4-carboxylates than with the 5-carboxylates.