Direct Catalytic Asymmetric Aldol Additions of Methyl Ynones. Spontaneous Reversal in the Sense of Enantioinduction

In this Communication, we report the direct, catalytic, asymmetric aldol addition of methyl ynones using our dinuclear zinc catalyst. A spontaneous reversal in the sense of enantioinduction was observed for these reactions; formation of the (S)-enantiomer is favored in the early stages (69% ee after 5 min), whereas the (R)-enantiomer is isolated as the major product after prolonged reaction times (97% ee after 22 h). It could be shown that this reversal in enantioselectivity is due to formation of a new catalytic species which incorporates the aldol product.