Highly Regio-, Enantio-, and Exo-selective Diels–Alder Reactions Enabled by a Bispyrrolidine Diboronate

Catalytic asymmetric Diels−Alder reaction is one of the most powerful reactions in organic chemistry. It is still a challenge to achieve high and general exo-selectivity. A novel bispyrrolidine diboronate compound has been derived from the reaction of two molecules of oxazaborolidine with one molecule of water. Upon activation, it effectively catalyzes (0.4-5 mol % loading) the Diels–Alder reaction between a variety of mono- and di-carbonyl activated dienophiles and dienes with better than 20:1 regioselectivity, up to >99:1 enantioselectivity, and better than 20:1 exo/endo selectivity. Mechanistic studies show a remarkable nonlinearity in enantioselectivity, and a second-order kinetics with respect to the catalyst concentration. A model is proposed to rationalize the above observations. The reaction is expected to find wide applications in organic synthesis.