beta-Branched alpha-halo carboxylic acid derivatives via stereoselective 1,4 addition of dialkylaluminum chlorides to alpha,beta-unsaturated N-acyloxazolidinones

The stereoselective synthesis of beta-branched alpha-halo carboxylic acids containing two newly formed chiral centers is accomplished by a reaction cascade consisting of the 1,4 addition of dialkylaluminum chlorides to alpha,beta-unsaturated N-acyloxazolidinones and subsequent reaction of the intermediate aluminum enolates with N-halosuccinimide. The most efficient stereocontrol in these tandem processes has been achieved with oxazolidinones derived from glucosamine. Not only aryl substituted but also purely aliphatic beta-branched alpha-halo carboxylic acids can be stereoselectively synthesized by this method. However, the reactions of beta-aryl alpha,beta-unsaturated N-acyloxazolidinones show the highest diastereoselectivity and give one out of four possible diastereomers in high excess.