Asymmetric Epoxidation Using Phase Transfer Catalysis (PTC)

Allylic alcohols can be converted directly to a-b-epoxyketones using sodium hypochlorite in the presence of an asymmetric PTC based on a cinchona alkaloid (Lygo B et al, Chem Comm, 2002, 2360). At least 10 mol % of catalyst is required otherwise conversion stops at 60% (5 mol %) or 10% (2 mol %). The enantiomeric excess is also dependent on the mol % of catalyst. The reaction initially proceeds with poor enantioselectivity (< 60% ee) but this increases significantly once al the allylic alcohol has been consumed. This is in agreement with earlier observations from Lygo’s group that polar additives reduce the ee in the epoxidation step. Slow addition of the allylic alcohol leads to not only an increase in ee, but also an increase in rate.