Evans aldol reactions, oxazolidone

As for the characteristic controlling of stereochemistry during aldol reaction, the Evans aldol reaction has been extensively modified through the application of Lewis acids, chiral auxiliaries (e.g., oxazolidone, oxadiazinones, thiazolidinethione " ), chelating metals, etc. 

As shown in Scheme 9.31, the (S)-enolate (100a), from Evans reagent 100. reacts on its Re face if the metal is not coordinated to the oxazolidone carbonyl group at the time of electrophilic attack, which is the normal situation in an uncatalysed boron enolate aldol reaction (see Scheme 9.14) and on the Si face if the metal is coordinated to the oxazolidone carbonyl group (lOOb), which is the normal situation in enolate alkylation (see 9.3.2). 

Independent work on asymmetric aldol reactions by Evans et al. led to the development of alternative chiral boron enolates that exhibit >100 1 facial selectivity. A pair of chiral N-acyl-2-oxazolidones (35)... 

Evans et al. utilized the chiral oxazolidones to prepare optically pure 3-hydroxy-a-amino acids, - important constituents of peptides and 3-lactams. As shown in Scheme 31, an asymmetric aldol reaction using the boron enolate derived from the V-(a-haloacyl)oxazolidone (68) provides the jyn-3-hydroxy-ot-halocarbonyl derivative (69), which is converted to the ann-3-hydroxy-a-azidocarbonyl derivative (70)... 

Chiral 2-oxazolidones are useful recyclable auxiliaries for carboxylic acids in highly enantioselective aldol type reactions via the boron enolates derived from N-propionyl-2-oxazolidones (D.A. Evans, 1981). Two reagents exhibiting opposite enantioselectivity ate prepared from (S)-valinol and from (lS,2R)-norephedrine by cyclization with COClj or diethyl carbonate and subsequent lithiation and acylation with propionyl chloride at — 78°C. En-olization with dibutylboryl triflate forms the (Z)-enolates (>99% Z) which react with aldehydes at low temperature. The pure (2S,3R) and (2R,3S) acids or methyl esters are isolated in a 70% yield after mild solvolysis. 

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