Synthesis of cyclopentenones from cyclopropanes and silyl ynol ethers.

Five-membered carbocyclic rings appear in all classes of organic materials including pharmaceutical agents, polymers, natural products, and catalysts. Accordingly, their preparation has challenged synthetic chemists since the inception of the field. In this regard, [3+2] cycloadditions—both concerted and stepwise—represent convergent strategies for the formation of the cyclopentane nucleus. Dipolar cycloadditions, in particular, have proven especially successful for this construction. Of the various all-carbon dipolar synthons available, donor–acceptor cyclopropanes (1) have proven especially versatile. In the presence of Lewis acids, donor– acceptor cyclopropanes undergo ring-opening to yield 1,3zwitterions. Pursuing a general interest in the reactivity of electron-rich alkynes, we envisioned a cycloaddition between such intermediates and ynol ethers (2, Scheme 1). In analogy to Diels–Alder reactions involving the diene