The origin of endo stereoselectivity in the hetero-Diels-Alder reactions of aldehydes with ortho-xylylenes: CH-pi, pi-pi, and steric effects on stereoselectivity.

Theoretical studies of stereoselectivity have been carried out with B3LYP and MP2 calculations. The high endo selectivity of hetero-Diels-Alder reactions of ortho-xylylenes with acetaldehydes is shown to result from attractive CH-pi interactions between alkyl groups of the aldehyde and the aromatic ring in the transition states of the reaction. For the hetero-Diels-Alder reactions of ortho-xylylenes with benzaldehyde, the stereoselectivity is shown to be mainly governed by the attractive pi-pi interactions between the phenyl rings of the benzaldehyde and the ortho-xylylene. MP2 calculations are necessary to reproduce the stabilizing dispersion interactions.