- General and facile syntheses of thieno|3,2-e|[1,2,4|triazolo|,5-c|-pyrimidin-5(6H)-one (6a) and its 2-substituted derivatives (6b-j) produced by instantaneous isomerization of their |4,3-c| compounds (7a-j), which were prepared by condensation of 4-hydrazinothieno|2,3-d(pyrimidin-2(1H)-one (10) with appropriate triethyl orthoesters or by oxidative cyclization of 4-(benzylidenehydrazino)thieno|2,3-d(pyrimidin-2(1H)-ones (llc-j), are described as novel ring systems and as a new class of potential xanthine oxidase inhibitors. The | 1,5-c| isomers (6a-c) were further prepared by condensation of 3-amino-4-imino-2-oxo-1,2,3,4-tetrahydrothieno|2,3-d(pyrimidine (14) with appropriate triethyl orthoesters as a synthetic method for a reliable structure of the tricyclic ring systems.