ASYMMETRIC-SYNTHESIS USING THE BIS(2,4-DIMETHYL-3-PENTYL)-(L)-TARTRATE ESTERS OF SUBSTITUTED ALLYLBORONIC ACIDS - SYNTHESIS OF GAMMA,BETA-DISUBSTITUTED TETRAHYDROPYRANS AND TETRAHYDROFURANS

A highly useful application of allylboration for the synthesis of optically pure alpha,beta-disubstituted tetrahydropyrans and tetrahydrofurans is described. The allylboronate esters 4, easily prepared by esterification of allylboronic acids 3 with bis(2,4-dimethyl-3-pentyl)-(L)-tartrate, readily undergo smooth reaction with representative aldehydes at -100-degrees-C to afford the corresponding halohydrins 5. These are readily cyclized with sodium hydride in the presence of dimethyl sulfoxide to give the cyclic ethers 6 in good yields (70-80%) and in excellent enantiomeric excesses (83-98%).