Diastereoselective reactions of enolates
Triphenylglycol-derived esters 2 and 8a have been applied in asymmetric aldol reactions. The homochiral propionates 7 and 8a,b react with imines in a stereodivergent manner: Doubly deprotonated 7 delivers anti-fklactams 14 whereas the lithium enolates of %ah afford ciS-f)-lactams 15 in 87-9796 e.e. Diastereoselective carbon silylation occurs when deprotonated 8a is treated with chlorotrimethylsilane. Approaches towards the construction of quaternary carbon centers are based on diastereoselective carboxalkylations of enolates with menthylchloroformate 17 and on a novel tandem reaction (16 - 20). The generation of preformed endates discovered three decades ago turned out to be extremely fruitful with respect to the formation of carbon-carbon bonds in a diastereoselective andor enanticselective manner (1). Thus, the versatility of the aldd reaction has been enhanced substantially due to preformed enolates (2). A couple of years ago, we reported on (R)- and (S)-2-hydroxy-l,2,2-triphenylethyl acetate 2 ("HYTRA"), a chid reagent which offers a solution for the long-standing problem of the stereoselective aldol addition of a-unsubstituted enolates (3). HYTRA 2 is readily available from mandelic acid via triphenylglycol 1. As shown in the illustrative procedure given below the crude mixture of adducts (3: predominant diastereomer) delivers khydroxycarboxylic acid 4 upon alkaline hydrolysis. Thereby, the chiral auxiliary reagent 1 is recovered and can be reused (4). The HYTRA-aldd method has been widely applied in syntheses of natural products and drugs (5).