Selectivity in cycloadditions—VII: Cycloadditions of nitrile oxides to thiophene and benzo[b]thlophene, regiochemistry

Thiophene adds benzo- and mesitonitrile oxide yielding mainly cycloadducts 1 or products derived from them. With benzothiophene the regioselectivity of the cycloadditions of the two nitrile oxides is lower and both the cycloadducts 10 and 11 could be isolated in a 78:22 and 26:74 ratio, respectively.

Frontier orbital considerations, using EH and MINDO/3 calculations, and a comparison with the regioselectivities of related systems allowed elucidation of the observed regiochemistry. Some differences in orientation between cycloadditions and electrophilic substitution reactions of benzothiophene are pointed out and discussed.