Reactions of acceptor substituted thiophene-1,1-dioxides with cyclopentadiene: control of selectivity by substitution

Diels-Alder reactions of thiophene-1,1-dioxides with strong electron withdrawing groups (EWG) were studied experimentally and theoretically. Thiophene-1,1-dioxides with two strong EWG behave as dienophiles and regio- and stereoselectively react with cyclopentadiene to give 2+4] cycloadducts 2a-c, which are derivatives of benzothiophene. In contrast, thiophene-1,1-dioxides with one EWG behave as dienes in the inverse electron demand Diels-Alder reaction yielding dihydro-1H-indenes derivatives. Cope 3,3]-sigmatropic rearrangement of adducts 2a-c was also demonstrated. MP2 calculations successfully rationalize the contrasting regioselectivities of these cycloaddition reactions.