Electron deficient heteroaromatic ammonioamidates. Part 25. N-(Quinazolin-3-io)amidates. Part 12. The synthesis of n-(6,7-dimethoxy-2-methylquinazolin-3-io)ethoxythioformamidate and of 2-(3,4-dimethoxyphenyl)-8,9-dimethoxy-5-methyl-10bH-[1,3,4]-thiadiazolo [3,2-c] quinazoline, the ring isomer of N-(6,7-dimethoxy-2-methylquinazolin-3-io)-3,4-dimethoxy(thiobenzamidate)

Abstract

The synthesis of N-(6,7-dimethoxy-2-methylquinazolin-3-io)ethoxythioformamidate (7a) has been carried out starting with the thioacylhydrazone (5a)via cyclization of the acetylamino thioacylhydrazone (6a). An analogous attempt to synthesize the related thiobenzamidate (7b) failed and furnished instead the cyclic form of the desired product, the [1,3,4] thiadiazolo [3,2-c] quinazoline (12b). In contrast to the thioacylhydrazones (5a) and (6a) the related thiobenzoylhydrazones (5b) and (6b) are shown to exist as their cyclic isomers, the thiadiazolines (8b) and (9b) respectively, under the conditions studied. Cyclization of (9b) furnished the thiadiazoloquinazoline (12b). The existence of this cyclic form (12b), and the absence of the dimer (17) of the quinazolinio(thioamidate)(7b) are in sharp contrast to the case of the related quinazolinioamidates (1).