DIMETHYLDIOXIRANE OXIDATION OF TITANIUM ENOLATES - DIASTEREOSELECTIVE ALPHA-HYDROXYLATIONS

The oxidation of titanium enolates, derived from a transmetalation reaction of the corresponding lithium enolates with (i-PrO)(3)TiCl, (Et(2)N)(3)TiCl, or Cp(2)TiCl(2), by dimethyldioxirane has been investigated. Furthermore, the diastereoselective hydroxylation of the chiral metal enolates, e.g., derived from camphor (1f), menthone (1g), flavanone (1h), and 2-benzylcyclopentanone (1i), by dimethyldioxirane has been examined. The diastereoselectivity of the oxygen transfer strongly depends on the metal partner coordinated to the enolate. The titanium enolates 4 resulted in much higher diastereoselectivities (up to 96% de) than the corresponding sodium enolates 5 and at least as high if not higher than the silyl enol ethers 6. Moreover, the aldol reaction of ester-derived sodium enolates with acetone, the unavoidable medium for dimethyldioxirane, could be totally suppressed by the use of the chlorotitanocene enolates 4. Thus, the oxidation of chiral titanium enolates by dimethyldioxirane represents a general, convenient, effective, and chemo- and diastereoselective synthesis of alpha-hydroxy carbonyl compounds.