Exo-Selective Diels−Alder Reactions of Vinylazepines. Origin of Divergent Stereoselectivity in Diels−Alder Reactions of Vinylazepines, Vinylpiperideines, and Vinylcycloalkenes

Diels−Alder reactions of vinylazepines with N-phenylmaleimide afforded exclusively the exo
cycloadduct, while high endo stereoselectivity was observed, as previously reported, in analogous
reactions of vinylpiperideines. This curious contrast was confirmed by X-ray analysis of cycloadducts
not susceptible to epimerization. The stereoselectivity of Diels−Alder reactions of vinylazepines,
vinylpiperideines, and vinylcycloalkenes exhibits surprising divergence depending on the detailed
diene structure, and DFT calculations (Becke3LYP) were undertaken to shed light on these
observations. The model calculations correctly predict the major stereoisomers in these reactions,
though they tend to significantly underestimate the stereoselectivity. The results suggest some
general considerations in predicting or controlling the stereochemistry of this class of Diels−Alder
reactions.