Zinc-Catalyzed Tandem Diels-Alder Reactions of Enynals with Alkenes: Generation and Trapping of Cyclic o-Quinodimethanes (o-QDMs)

A systematic investigation of ZnCl2-catalyzed reactions of enynals with alkenes has been undertaken. Structurally unique propeller-like products could be obtained under mild conditions. Cyclic o-quinodimethanes (o-QDMs) are generated through [4+2] cycloaddition between enynals and alkenes. Both electron-poor and electron-rich dienophiles could be used to trap the active intermediate through [4+2] cycloaddition. But [1,5]-H shift products could also be observed when electron-rich alkenes were used as dienophile. DFT calculations were performed to understand the reaction mechanism. A competition between the [4+2] cycloaddition and [1,5]-H shift was proposed for the transformation of cyclic o-QDMs. The selectivity could be affected by the properties of the substrates.