Electrophilic ring-closure reactions thiophene

Electrophilic Ring-closure Reactions. - In a series of interesting papers, several thienospirans have been described, prepared by cyclization onto thiophen. Thus (50) and (51) were converted into (52) and (53), respectively,... 

Three phenalenothiophens (521)—(523) have been synthesized by unambiguous methods from the corresponding phenalenothiophenones (524)— (526), which in turn were prepared by electrophilic ring-closure of the corresponding 3-(a-naphthyl)thiophen-2-carboxylic acid, 2-(a-naphthyl)-thiophen-3-carboxylic acid, and 3-(a-naphthyl)thiophen-4-carboxylic acid. The reaction of (527) with glycerol, sulphuric acid, and iron leads to the... 

Ring closures which depend on the conversion of the heteroatom into an electrophile are mostly associated with the formation of thiophene, selenophene and tellurophene rings and some illustrative examples are shown in Scheme 17. The last example which concerns the conversion of reaction with isocyanides is of particular interest since it appears to entail the attack of an electrophilic nitrogen species on the aryl ring. 

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