Reversed stereochemical control in the presence of CeCl3 and TiCl4 in the Lewis acid mediated reduction of alpha-alkyl-beta-keto esters by metal hydrides. A general methodology for the diastereoselective synthesis of syn- and anti-alpha-alkyl-beta-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 natur e of the metal atom. Strongly chelating TiCl4 led to the syn isomer in high diastereomeric excess in noncoordinating solvents (CH2Cl2) at -78 degrees C with BH3. py as reducing agent, while nonchelating CeCl3 gave a high exce ss of the anti isomer in coordinating solvents (THF) at the same temperatur e with lithium triethylborohydride (LiEt3BH) as reducing agent. The methodo logy has been successfully utilized for obtaining important syn- and anti-a lpha-alkyl-beta-hydroxy esters with high diastereoselectivity.