Reversed Stereochemical Control in the Presence of CeCl₃and TiCl₄in the Lewis Acid Mediated Reduction of α-Alkyl-β-keto Esters by Metal Hydrides. A General Methodology for the Diastereoselective Synthesis ofsyn- andanti-α-Alkyl-β-hydroxy Esters

The Lewis acid-mediated reduction of alpha-alkyl-beta-keto esters has been shown to proceed by different stereochemical control depending on the nature of the metal atom. Strongly chelating TiCl(4) led to the syn isomer in high diastereomeric excess in noncoordinating solvents (CH(2)Cl(2)) at -78 degrees C with BH(3).py as reducing agent, while nonchelating CeCl(3) gave a high excess of the anti isomer in coordinating solvents (THF) at the same temperature with lithium triethylborohydride (LiEt(3)BH) as reducing agent. The methodology has been successfully utilized for obtaining important syn- and anti-alpha-alkyl-beta-hydroxy esters with high diastereoselectivity.