Enantioselective, Facially Selective Carbomagnesation of Cyclopropenes

Described is the facially selective and enantioselective addition of carbon nucleophiles to prochiral 3-hydroxymethylcyclopropenes. The process creates up to four stereocenters in a tandem addition/capture sequence that combines three simple materials to give complex and diverse products. The asymmetry is induced by the inexpensive and recoverable ligand (S)-N-methylprolinol. The enantioselectivity (90−98% ee with MeMgCl) is high for a range of cyclopropenes and electrophiles. Importantly, the diastereoselectivity is complementary to that obtained by enantioselective cyclopropanation with aryldiazoacetates. High enantioselectivities are obtained only when methoxide is included in the reaction. Evidence is provided that at least two chiral ligands are involved in the enantioselectivity-determining step.