Origins of Stereoselectivity of Enamine-Iminium-Activated Nazarov Cyclizations by Vicinal Diamines.

The mechanism and sources of asymmetric induction in Nazarov reactions reported by Tius and co-workers have been determined with quantum chemical calculations. A chiral vicinal diamine forms an enamine-iminium adduct with α-ketoenones, and this undergoes a cationic conrotatory electrocyclization. The chiral diamine imparts stereocontrol in the enamine-iminium complex by forming a six-membered ring that favors one helicity of the electrocyclization transition state.