Enantioselective Diels-Alder reaction of α-Acyloxyacroleins catalyzed by chiral 1,1′-binaphthyl-2,2′-diammonium salts

A diammonium salt of chiral 1,1′-binaphthyl-2,2′-diamine (2a) and trifluoromethanesulfonimide (Tf₂NH) shows excellent catalytic activity and enantioselectivity for the Diels-Alder reaction of α-acyloxyacroleins. For example, in the presence of 5 mol% of 2a and 9.5 mol% of Tf₂NH, the Diels-Alder reaction of α- (cyclohexanecarbonyloxy)acrolein with cyclopentadiene proceeded in EtCN at -75 °C to give the adducts in 88% yield with 92% exo and 91 % ee. The electron-donating property of the acyl group of the α-acyloxyacroleins increases the enantioselectivity due to the formation of strong intramolecular hydrogen bonding of the acyl group with a proton of the ammonium group in the transition state. This catalyst can be easily prepared in situ by mixing the commercially available chiral diamine and Tf₂NH.