Organocatalytic Enantioselective Acyloin Rearrangement of α-Hydroxy Acetals to α-Alkoxy Ketones

We report an unprecedented organocatalytic enantioselective acyloin rearrangement of alpha,alpha-disubstituted alpha-hydroxy acetals. In the presence of a catalytic amount of chiral binol-derived N-triflyl phosphoramide, alpha-hydroxy acetals rearranged to alpha-alkoxy ketones in good to high yields with high enantioselectivities. Formation of an ion pair between the in situ generated oxocarbenium ion and the chiral phosphoramide anion was proposed to be responsible for the highly efficient transfer of chirality. Conditions for removal of cyclohexyl and cyclopentyl groups from the corresponding alpha-alkoxy ketones were uncovered underpinning their potential general utility as hydroxy protecting groups. Conversion of the rearranged products to the enantioenriched alpha-hydroxy ketone, 1,2-diol, beta-amino alcohol and 1,4-dioxane was also documented.