Stereodivergency of Triazolium and Imidazolium-Derived NHCs for Catalytic, Enantioselective Cyclopentane Synthesis.

[摘要]：Chiral triazolium- and imidazolium-derived N-heterocyclic carbene catalysts promote the direct annulation of a,b-unsatd. aldehydes R1CH:CHCHO (R1 = Ph, 4-MeOC6H4, 1-naphthyl, 2-furyl, etc.) and achiral a-hydroxy enones R2CH:CHCOCMe2OH (R2 = MeO2C, EtO2C, 4-BrC6H4, 4-F3CC6H4) to afford cyclopentane-fused lactones, e.g. I and II, with high enantioselectivity. Remarkably, otherwise structurally identical imidazolium and triazolium precatalysts afford different major products. These studies provide both an efficient entry to valuable chiral structures and a dramatic demonstration of stereodivergency of chiral imidazolium vs. triazolium-derived N-heterocyclic carbene catalysts.

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