Homoallylboration and Homocrotylboration of Aldehydes

A simple method for addition of homoallylic fragments to aldehydes is described. Cydopropanated allylboration reagents react with aldehydes in the presence of PhBCl2 to give high yields of bishomoallyl alcohols. Cyclopropanated cis- and trans-crotyl reagents afford the corresponding 1,3-anti- and 1,3-syn-methyl-substituted homocrotylated alcohols with high selectivity, consistent with a Zimmerman-Traxler transition state. Accordingly, the optically active alpha-substituted reactant affords the E-substituted product in 97:3 er.