1,3,4-Thiadiazoles, Regioslective O-Demthyltion on Dehydroactive cyclization of 1-(3,4,5-Trimethoxybenoyl)-4-substituted Thiosemicarbazides with Sulphuric acid

Abstract

Cyclization of 1-(3,4,5-trimethoxybenzoyl)-4-substituted thiosemicarbazides 2a–g with sulphuric acid at ambient temperature afforded the selectively demethylated products 2-(4-hydroxy-3,5-dimethoxyphenyl)- 5-substituted amino-1,3,4-thiadiazoles 4a–g. Meanwhile, dehydrative cyclization of 1-(3,4,5-trimethoxybenzoyl)-4-(benzyl or t-butyl)thiosemicarbazides 2h,i with sulphuric acid yielded 2-(4-hydroxy-3,5-dimethoxyphenyl)- 5-amino-1,3,4-thiadiazole 5. On the other hand, dehydration of 2h,i by heating with phosphorus oxychloride yielded 2-(3,4,5-trimethoxyphenyl)-5-amino-1,3,4-thiadiazole 6.