SIMPLE DIASTEREOSELECTIVITY OF THE ALDOL REACTION OF PERSUBSTITUTED ENOLATES - STEREOSELECTIVE CONSTRUCTION OF QUATERNARY CENTERS

Simple diastereoselectivity of several important categories of aldol reactions of persubstituted enolates has been investigated for sterically least biased cyclic and acyclic ketone and aldehyde enolates, 1-4, and has been found to be useful for the stereoselective construction of quaternary carbon centers. The types of reactions examined involve the reaction of lithium, borinate, borate, trialkoxytitanium, trichlorotitanium, and zirconium (Cp2ZrCl) enolates, and the reactions of enol silyl ethers under high pressure, fluoride catalysis, and Lewis acid catalysis. In contrast to the less substituted metal enolates, uncatalyzed reactions of persubstituted metal enolates proceeded in a sense anticipated from the conventional Zimmerman-Traxler chair transition state (TS) model. The fluoride-catalyzed reaction of the cyclic enolate 1a showed stereoselectivity consistent with the open extended TS, while enolates 2a-4a showed anomalous behavior. The selectivity of the Lewis acid mediated aldol reaction of enol silyl ethers was found to be dependent on the Lewis acid used, and the BF3.Et2O-mediated reaction of 2a and 3a showed maximum selectivity in a sense predicted by the chair TS. The stereostructures of the aldols have been determined by single-crystal X-ray analysis or by chemical correlations.