Influence of alkyl substituent on the asynchronous transition structure of boron-catalyzed Diels-Alder cycloaddition of alpha,beta-unsaturated aldehydes with 1,1-dimethyl-1,3-butadiene derivatives

The Diels-Alder cycloaddition of dienes with alpha,beta-unsaturated aldehydes was studied. The reaction was carried out at low temperatures under the catalysis of BF3.OEt2 or BBr3 and showed good agreement with ab initio calculations in demonstrating the effect of the beta-alkyl substituents in the dienophiles on the reactivity and mode of cycloaddition. The latter was attributed to the steric interactions in the asynchronous transition structure.