Exploiting an opening: Lewis acid promoted ring-opening of donor–acceptor cyclopropanes generates a 1,3-zwitterion; cycloaddition with a silyl ynol ether leads to a general synthesis of cyclopentenones (see scheme). Substitution is tolerated on the ynol and on all positions of the cyclopropane to give tri-, tetra-, and penta-substituted cyclopentenones in high yield.
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