Scaffolds of thiophene and benzothiophene are the important class of bioactive compounds found abundant in nature. The Diels–Alder reactions of 2-(1′-cycloalkenyl)­thiophenes and 2-(1′-cycloalkenyl)­benzo­[b]­thiophenes having the alkene groups present in five-, six-, seven-, eight-, and twelve-membered rings with substituted N-phenylmaleimides are characterized. The size of the cycloalkene rings plays a critical role in dictating the product distributions of expected and isomerized Diels–Alder adducts. 2D NMR studies indicate that the isolated isomers for 2-(1′-cycloalkenyl)­thiophenes having five-, six-, and seven-membered rings are aromatized benzothiophene products, whereas eight- and twelve-membered rings are un-rearranged adducts. In addition, the product of subsequent ene-reaction with the N-phenylmaleimide is isolated for the five- and six-membered ring cases. Interestingly, in the 2-(1′-cycloalkenyl)­benzo­[b]­thiophene having five-, six-, seven-, eight-, and twelve-membered rings, the un-rearranged dibenzothiophene Diels–Alder adduct is isolated in every instance. Molecular mechanics and density functional theory (M06-2X and PBE0-D3) calculations are performed to understand the differential reactivity of the various dienes for both the initial Diels–Alder reaction and a possible, subsequent ene reaction.

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