Highly selective asymmetric synthesis of 2-hydroxy fatty acid methyl esters through chiral oxazolidinone carboximides

Abstract

Highly selective asymmetric synthesis of 2-hydroxy fatty acid methyl esters has been accomplished through chiral imide enolates. Five chiral oleic acid imides were prepared by reaction of oleic acid with pivaloyl chloride followed by reaction with five different lithiated chiral oxazolidinones including (R)-(+)-4-benzyl-2-, (S)-(-)-4-benzyl-2-, (4R, 5S)-(+)-4-methyl-5-phenyl-2-, (4S, SR)-(-)-4-methyl-5-phenyl-2-, and (R)-(+)-4-isopropyl-2-oxazolidinones in 88-92% yields. The chiral imides were reacted with NaN(Me3Si)2 at -78°C to give enolates, which subsequently reacted with 2-(phenylsulfonyl)-3-phenyloxaziridine to give hydroxylated products in 78-83% yields. Methanolysis of the hydroxylated products with magnesium methoxide gave methyl 2-hydroxyoleate. Enantiomeric excesses (ee) of the products were determined to be very high (98-99% ee) by 1H nuclear magnetic resonance study after esterification of the hydroxy group with (S)-(+)-O-acetylmandelic acid. Enantioselective hydroxylation of other fatty acids including elaidic, petroselinic, vaccenic, and linoleic was evaluated under the similar conditions using (4R, 5S)-(+)-4-methyl-5-phenyl-2-oxazolidinone as a chiral auxiliary to give 98% ee values for all cases.