Regioselective and asymmetric Diels-Alder reaction of 1-and 2-substituted cyclopentadienes catalyzed by a bronsted acid activated chiral oxazaborolidine

Cationic oxazaborolidine 2 affords Diels-Alder adducts of ethyl acrylate and 2-substituted cyclopentadienes, derived from the corresponding mixture of regioisomers, as single isomers in excellent yields and enantioselectivities. Furthermore, Diels-Alder adducts of 1-substituted cyclopentadienes are obtained through a one-pot procedure whereby ethyl acrylate is initially employed to consume all 2-substituted cyclopentadiene. Subsequently, various 2,5-disubstituted benzoquinones are added to react with remaining 1-substituted cyclopentadiene. Remarkably, reaction occurs selectively at the double bond coordinated anti to catalyst 2 to provide adducts containing adjacent all-carbon quaternary stereocenters in high yields and excellent enantioselectivities.