Fig. 1. (a) Representative activation modes of α,β-unsaturated carbonyl compounds for organocatalytic asymmetric DA reactions: acylammonium salt- (this work), iminium-, and hydrogen bond-activated dienophiles. (b) Representative example of a DA-mediated, stereodivergent resolution of a racemic diene (e.g. (±)-4) employing in situ generated α,β-unsaturated acylammonium salt from acid chlorides (e.g. 5) and isothiourea catalysts (e.g. (S)-(–)-BTM) (inset, previously reported TS arrangement).
