A series of known and new B,B-dihaloterpenylboranes, readily synthesized from the corresponding terpenes, were examined for the diastereo-and enantioselective synthesis of syn-aldols. Thus, IpcBCl 2, EapBCl2, LgfBCl2, cis-MyrBCl2, 2-IcrBC12, 4-IcrBCl2, and 2-IcrBBr 2 in the presence of i-Pr2NEt, convert 3-pentanone to >_99% Zenolates, converted by aldehydes to pure syn-aldols in 7-74% enantiomeric excess (ee). The most efficient of these reagents, B,B-dibromo-2-isocaranylborane, was tested for the enolboration of 3-pentanone, followed by aldolization with a series of aldehydes, t~) 1997 Elsevier Science Ltd
The diastereoselective syntheses of cross aldols via enolboration of ketones have been essentially mastered over the past two decades. 1 Although most of the literature reports on enolborations utilize R2BX (X=CI, Br, I, OTf, etc.) reagents, 1 there have been only scanty reports involving the corresponding RBX2 reagents. 2 As part of our ongoing projects on enolboration, we recently reported the utilization of B,B-dihalomonoalkylboranes as reagents for the stereoselective enolization of ethyl ketones, followed by syn-selective aldolization with a representative series of aldehydes (Eq. 1). 3 All of the reagents tested provided the syn-aldols selectively.