Zimmerman-Traxler model, Evans aldol

A.iv. The Evans Model. It is known that (Z) enolates are more stereoselective than ( ) enolates even when r1 is not large. The Zimmerman-Traxler model transition states 352-355 do not account for this observation. It has been suggested that the transition states are not chair-like, but skewed, as in 381-384.221 In this representation (Z) enolate 381 leads to the syn aldol. Similarly, (Z) enolate 382 gives the anti aldol, ( ) enolate 383 give the anti aldol and E) enolate 384 is the precursor to the syn aldol. The major steric interactions in this model are those for r1 r3 and r2 - r3. For both (Z) and E) enolates, the r1 r3 interaction favors 381 and 383, respectively. The r2 r3 interaction is more important for the E) enolate and... 

A.V. The Noyori Open-Chain Model. In the Mukaiyama reaction, the Zimmerman-Traxler and Evans models are not satisfactory for predicting diastereoselectivity. Several open (nonchelated) transition states have been considered as useful models. The condensation reaction of carboxylic acid dianions with aldehydes indicated that anti selectivity increased with increasing dissociation of the gegenion (the cation, M+),224 When analyzing an aldol condensation that does not possess the bridging cation required for the Zimmerman-Traxler model, an aldehyde and enolate adapt an eclipsed orientation as they approach. Noyori reported syn selectivity for the reaction of a mixture of (Z)-silyl enol ether 389 and ( )-silyl enol ether 390 with benzaldehyde in the presence of the cationic tris-(diethylamino) sulfonium (TAS).225 xhis reaction is clearly a variation of the Mukaiyama reaction, which does not usually proceed with good diastereoselectivity... 

The boron-mediated aldol reaction of an Evans s acyl oxazolidinone with an aldehyde affords the Evans-syn aldol adduct 9. The process proceeds via formation of the Z enolate that reacts with the aldehyde, presumably through a well ordered six-membered, chair-shaped Zimmerman-Traxler model... 

The observed stereoselectivity in the Evans aldol reaction can be explained by the ZImmerman-Traxler transition state model. There are eight possible transition states, four of which would lead to the anti aldol product. These, however, are disfavored due to the presence of unfavorable 1,3-diaxial interactions (not depicted below). The possible transition states leading to the syn aldol product are shown below. The preferred transition state leading to the product is transition state A, where the dipoles of the enolate oxygen and the carbonyl group are opposed, and there is the least number of unfavored steric interactions. 

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