Highly Enantioselective Cyclizations of Conjugated Trienes with Low Catalyst Loadings: A Robust Chiral N-Heterocyclic Carbene Enabled by Acetic Acid Cocatalyst

Densely functionalized cyclopentenones are useful synthetic intermediates. We report herein a new method to synthesize this important class of compounds through a highly enantioselective (98 → 99% ee) triene cyclization that is cocatalyzed by acetic acid and a chiral N-heterocyclic carbene (NHC). We discovered that acetic acid not only could coexist with NHCs but also could greatly stabilize the active catalyst, which enables a long-lived catalyst with high reactivity and selectivity.