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.

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