Diels-Alder reactions of cyclic isoimidium salts.

Diels−Alder reactions of cyclic isoimidium salts are described. The corresponding cycloadducts are obtained with high regio- and stereoselectivity. The use of homochiral cyclic isoimidium salts delivers cycloadducts with excellent diastereoselectivity (>99:1) that can be efficiently converted to enantiomerically pure lactones.