Similarly, aldol-type addition of enolates to aldehydes can be accomplished with excellent selectivity. For examples, the chiral cyclpentadienyltitanium complex derived from CpTiCl3 and TADDOL has been used for highly enantio- and diastereoselective aldol rections (Scheme 4). The products are obtained by transmetalating the Li-enolate with the Ti-TADDOL complex.
 |
Scheme 4 |
The addition of diketene to aldehydes can be accomplished using chiral titanium(IV) Schiff base complex to afford δ -hydroxy- β -ketoesters with reasonable enantioselectivity (Scheme 5). This reaction is too most probably proceeding via a Ti-enolate formed in situ from diketone and titanium(IV)-Schiff base complex. Catalytic version of this reaction can be carried out using Ti-BINOL complex. In this case a Mukaiyama-type aldol reaction between aldehydes and ketene silyl-acetal leads to provide silylated aldols with high optical purity (Scheme 6).
 |
Scheme 5 |
 |
Scheme 6 |