With just a nudge (in the form of silica gel, an acidic ion-exchange resin, or heating at about 40 °C), the acetate derivatives of enantiomerically enriched cyclopropenyl alcohols undergo sigmatropic rearrangement to give alkylidenecyclopropanes with high ee values (see scheme). Similarly, the rearrangement of phosphinite derivatives at room temperature leads to phosphine oxide precursors of unusual chiral phosphine ligands. R1, R2=alkyl, aryl.
The straightforward approach: The copper-catalyzed carbomagnesiation reaction of chiral cyclopropenylcarbinol derivatives, obtained through kinetic resolution of secondary allylic alcohols, leads to the preparation of enantiomerically pure alkylidenecyclopropane derivatives. The reaction mechanism is composed of a syn carbometalation followed by a syn elimination reaction.