A new synthesis of thiophenes and thiapyrans - Part I. A new synthesis of thionaphthene, its scope and limitations
Cyclization of phenyl ω-dimethoxyethyl sulphide (I) under different conditions was studied and an optimum yield of 37% of thionaphthene was obtained by treatment with a mixture of phosphorus pentoxide and phosphoric acid under closely defined conditions. Phenyl ω-diethoxyethyl sulphide (II) also gave thionaphthene in 32% yield under similar conditions.
The new synthesis of thionaphthene, which has been extended to substituted thionaphthenes and to other thiophenes and thiapyrans reported in subsequent communications consists in the cyclization of aryl ω-dimethoxyethyl sulphides Ar [SCH2CH(OMe)2]n. The scope and limitations of the new synthesis are discussed.