Evans aldol reaction mechanisms

Finally, stereochemical aspects of the mechanism have been extensively studied by Evans and coworkers during their construction of a model to map the diastereoselective induction observed in the course of the aldol reaction. Similarly, mechanistic studies were performed by the Evans group with regards to their bis(oxazolinyl)-pyridine (pybox)-copper(II) complex, a catalyst for the enantioselective Mukaiyama reaction. ... 

The role of substitution in determining the course of saccharinic acid formation has been critically examined recently by Kenner and coworkers. They conclude that an 0-glycosyl or 0-alkyl anion is more readily extruded from the sugar enediol anion of the Isbell mechanism than is a hydroxyl ion. In addition, substitution at certain positions in the sugar molecule may inhibit competing side-reactions. For example, a substituent at C4 of the hexose molecule inhibits cleavage (by reverse aldolization) into two three-carbon fragments and the resultant formation of lactic acid, a result that had been demonstrated earlier by the experiments of Evans and his associates. A combination of the two above effects, then, preferentially... 

Condensation reactions of simple carboxylic acids with imines are of intense interest because of their applications to 3-lactam synthesis. Activation of the carboxylic acid derivative is accomplished by preforming the enolate in situ or by using a silyl ketene acetal derivative with Lewis acid catalysis. The first example of an enolate-imine condensation of this type can be attributed to Gillman and Speeter, who in 1943 reported the synthesis of 3-lactams from Reformatsky reagents and Schiff bases. Subsequently, other workers have investigated the mechanism and syn-anti selectivity of this reaction. A review of these studies by Evans et al. covering work through 1980 has appeared in their review, Stereoselective Aldol Condensations . ... 

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