Asymmetric Aldol Reactions via Zirconium Enolates

Zirconium enolates of chiral amido derivatives (6) have been were employed to achieve an asymmetric aldol reaction. Hydrolysis of the aldol products (7) gave /3-hydroxycarboxylic acids (8) with high enantioselectivity (Eq. 3). 

Zr(0-f-Bu)4 is a mild reagent that can be used in cross-aldol reactions and intramolecular aldol reactions without the basic treatment of the starting ketones, whereas the use of CpaZrCl enolate gave unsatisfactory results. For example treatment of bromo-ketone (9) with Zr(0-f-Bu)4 followed by the addition of aldehyde (10) gave a-bromo-/8-hydroxyketone (11) in 56 % yield (Eq. 4) [4], 

---