Enantioselective and Structure-Selective Diels−Alder Reactions of Unsymmetrical Quinones Catalyzed by a Chiral Oxazaborolidinium Cation. Predictive Selection Rules

The chiral oxazaborolidinium cation 1 promotes Diels−Alder reactions between 2-triisopropylsilyloxy-1,3-butadiene and a number of unsymmetrical 1,4-benzoquinones in a highly enantioselective and structurally selective manner. The basis for the enantioselectivity is explained rationally in terms of a preferred type of transition-state assembly. Selection rules have been developed that allow the prediction of the principal reaction product of Diels−Alder reaction between unsymmetrical diene and quinone components.