Abstract An improved preparation of 2-oxo-6-thioxo-1, 2, 3, 6-hexahydropyrimidine-4- carboxylic acid 3, a potent inhibitor of dihydroorotase is presented. Trans-5-alkyl-2-oxo-6- thioxohexahydropyrimidine-4-carboxylic acids 12a-c were synthesised via the thiation of the p-methoxybenzyl esters of 5-alkyldihydroorotic acids with Lawesson's reagent followed by subsequent de-protection. The corresponding cisisomers were prepared by reduction of 5 ...