Asymmetric Diels−Alder Reactions of Chiral Cyclopropylidene Imide Dienophiles: Preparation of <i>gem</i>-Dimethyl- and Spirocyclopropane Norbornyl Carboxylic Acids

A highly efficient strategy has been developed for the rapid asymmetric synthesis of gem-dimethyl and spirocyclopropyl norbornyl carboxylic acids. The key transformation involved the unprecedented asymmetric Diels−Alder reaction of highly reactive β,β-cyclopropyl-α,β-unstaturated N-acyloxazolidinones with cyclopentadiene affording the adducts in high yield and de.