Chiral Synthesis via Organoboranes. 4. Synthetic Utility of Boronic Esters of Essentially 100percent Optical Purity. Synthesis of Homologated Boronic Acids and Esters of Very High Enantiomeric Purities

Article abstract of DOI:10.1021/jo00350a031

2-Alkyl-1,3,2-dioxaborinanes, R^*BO2(CH2)3, of essentially 100percent optical purity, prepared by the asymmetric hydroboration of readily available prochiral olefins with subsequent removal of the chiral auxiliary, can be homologated to α-chloroalkyl derivatives, R^*CHClBO2(CH2)3, of essentially 100percent ee by reaction with LiCHCl2.The intermediates R^*CHClBO2(CH2)3 are smoothly reduced with KIPBH to give the corresponding one-carbon-homologated boronic esters R^*CH2BO2(CH2)3 in very high optical purity.The operation can be repeated to produce R^*CH2CH2BO2(CH2)3 etc.Consequently, it is now possible to synthesize a wide variety of optically active boronic esters, not available by direct asymmetric hydroboration, either (+) or (-), in essentially 100percent ee, and to convert these into synthetically valuable compounds.