Transition structures metal enolate formation

The key idea of the Zimmerman-Traxler model is that aldol additions proceed via six-membered ring transition state structures. In these transition states, the metal (a magnesium cation in the case of the Ivanov reaction) coordinates both to the enolate oxygen and to the O atom of the carbonyl compound. By way of this coordination, the metal ion guides the approach of the electrophilic carbonyl carbon to the nucleophilic enolate carbon. The approach of the carbonyl and enolate carbons occurs in a transition state structure with chair conformation. C—C bond formation is fastest in the transition state with the maximum number of quasi-equatorially oriented and therefore sterically unhindered substituents. 

There are numerous noteworthy structural aspects of N-acyl oxazolidinones that give them a central role as auxiliaries for a large array of asymmetric transformations. Although the enolization reaction of esters and ketones can lead to mixtures of cis- and trans-enolates, the oxazolidinone imi-des exclusively form the corresponding cis-enolates. This observation has been attributed to the pronounced destabilization of the trans-enolate and the transition state structure leading to its formation as a consequence of A, 3 steric interactions (cf 124, Scheme 3.19) [15]. A second important feature of the oxazolidinone enolates relates to the ability of the auxiliary carbonyl functionality to form a chelate with coordinatively unsaturated metal centers (cf 118,121, or 125). This organizing feature provides rigidity to the en-... 

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