Diastereoselective Diels−Alder Reaction of 2-(α,β-Unsaturated)acyl-3-phenyl-l-menthopyrazoles

The reaction of 2-(α,β-unsaturated)acyl-3-phenyl- l -menthopyrazoles ( 9 ) with dienes gave Diels−Alder adducts in good yield. The addition of MgBr 2 ·OEt 2 or ZnCl 2 accelerated these reactions through the formation of chelating bonds such as N···Mg···O C or N···Zn···O C. The structural fixation by these catalysts also promoted the endo and diastereoselectivities of the Diels−Alder addition on the Re- face of the dienophiles. These results were supported by PM3 calculations, in which the heats of formation of the transition states anticipated the remarkable differences between the Re- and Si- facial attacks of the dienes.