Highly Stereoselective Olefinating Reagents

Mukaiyama and Michida have reported that E/Z mixtures of ketene acetals 1 and 2 react with various kinds of aldehydes in the presence of a catalytic amount of acetate salts to give α,β-unsaturated esters in high yields with excellent stereoselectivities.¹⁾ This procedure has some advantages over conventional methods such as the Peterson reaction and the Horner-Wadsworth-Emmons (HWE) reaction. For example:
1) Catalytic reaction (with no need of equimolar amount of base); 2) Co-product (TMS₂O) can be removed easily by evaporation; 3) High stereoselectivity is obtained by the proper choice of the counter cation of the catalyst.

It is notable that Z-isomers are obtained with excellent stereoselectivity by using 2 in the synthesis of α-fluoro-α,β-unsaturated esters. On the other hand, E-isomers are the major products in the case of the fluorinated-HWE reagent.²⁾ Therefore, the complementary use for the stereoselective synthesis of α-fluoro-α,β-unsaturated esters has been realized.