Thiadiazoles and thiadiazolines. Part 4. Acetylation, hydrolysis, and cyclocondensations of Δ2-1,3,4-thiadiazoline-α-carboxamidines

Abstract

The thiadiazolinecarboxamidine (1a) forms a stable acetate in glacial acetic acid but is diacetylated to give the thiadiazoline (3) using acetic anhydride and mineral acid. Similar treatment of (1c) and (1d) gives, at 60 °C or below, the monoacetyl derivatives (10) and (6) respectively. The acetylthiadiazolinecarboxamidine (6) decomposes at 60–140 °C yielding the 4-acetylthiadiazoline (7), which may also be obtained by acetylation of an oil produced by basic hydrolysis of (1a). Neutral hydrolysis in aqueous methanol converts (1a) and (1b) into the carbamoylthiadiazoline (11); the trimethylthiadiazolinecarboxamidine (1e) into the N-methylcarbamoylthiadiazoline (12), and the diacetylthiadiazolinecarboxamidine (3) into the N-acetylcarbamoylthiadiazoline (5). The thiadiazolinecarboxamidine (1a) reacts with 1,3-dicarbonyl compounds to give 4-pyrimidin-2-ylthiadiazolines (15)–(17), and decomposes in refluxing mesitylene to give, amongst other products, trans-stilbene and 2,5-diphenyl-1,3,4-thiadiazole.