Synthesis of anti Aldols in Racemic Form

Procter et al. reported a novel anti-selective aldol process using titanium enolates of N-propionylpyrrolidine [61]. The aldol products vere obtained in good yield and anti diastereoselectivity. The aldolates vere produced in racemic form, ho vever. As sho vn in Table 2.30, formation of the titanium enolate of 172 vas achieved by transmetalation of the lithium enolate vith Cp2TiCl2 in THE. The resulting titanium enolate reacted vith a range of aldehydes. Interestingly, the lithium enolate of 172 has stereoselectivity and the corresponding zirconium enolate afforded syn diastereoselectivity. 

Kazmaier reported the synthesis of several racemic a-aminoaldols in which the a-amino group and the j5-hydroxyl group are anti [63]. Reactions of N-benzyloxycarbonyl amino acid ester enoiates with a variety of aldehydes afforded moderate to good yields and good to excellent diastereoselectivity as shown in Table 2.32. Titanium enoiates were generated by deprotonation of 179 with LDA in THF followed by the addition of 2 equiv. Ti(Oi-Pr)3Cl in THF at -78 °C. 

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