Enantioselectivity through H bonding: An unprecedented [4+3] annulation of enals with o-quinone methides catalyzed by N-heterocyclic carbenes (NHCs) to give benzo-ε-lactones is described. High to excellent enantioselectivity was achieved by using a chiral triazolium NHC having a free OH group, which participates in a hydrogen-bonding interaction with the substrate.
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