Catalytic Asymmetric Protonation of Ketene Disilyl Acetals

Significance

Ooi and co-workers report the ­design of new chiral P-spiro diaminodioxaphosphonium barfates 2 and demonstrate their potential to catalyze enantioselective protonation reactions. In the presence of catalyst 2 (1 mol%) and 3 as stoichiometric proton source, the ketene disilyl ­acetals 1 were protonated in an enantioselective manner to give, after chromatography on silica gel, α-chiral α-amino acids 5 in quantitative yields and high enantioselectivities. The use of 4 (2 mol%) was found to be essential to ensure the reproducibility of high enantiomeric ratios.