ASYMMETRIC ALDOL REACTIONS - USE OF THE TITANIUM ENOLATE OF A CHIRAL N-ACYLOXAZOLIDINONE TO REVERSE DIASTEREOFACIAL SELECTIVITIES

Aldol reactions of the titanium enolate of (S)-N-propionyl-4-isopropyl-2-oxazolidinone (readily derived from L-valine) with representative aldehydes give high diastereofacial selectivities for the syn aldol adducts expected from chelation control. This represents a remarkable reversal in selectivity compared with the corresponding boron enolate, thus permitting either enantiomeric form of beta-hydroxy-alpha-methyl carboxylic acids to be made from a single, readily available oxazolidinone simply by changing the metal. A lithium interference effect is shown to be easily prevented by use of excess titanium. Use of diethyl ether as solvent rather than THF significantly enhances the stereoselectivity. Mechanistically, the observed stereochemical reversal constitutes very strong evidence that chelation is operative with titanium, presumably through a chelated chairlike transition structure. In this transition structure, the conformation would be rigidly locked by chelation and the titanium would be at least hexacoordinate, resulting in a ''superaxial'' ligand, thus nicely explaining the high stereocontrol.