Asymmetric Diels-Alder reactions of chiral cyclopropylidene imide dienophiles: preparation of gem-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.