Z-enamine transition state

The (Z)-enamine transition state 31 was proposed as the intermediate in the syn-selective reaction of an acyclic ketone donor with an aldehyde acceptor catalyzed by 22. In this type of reaction, formation of the ( )-enamine of the acyclic ketone is disfavored due to steric repulsion between R and substituents (Figure 28.3) [18]. The (F)-enamine formed by an aldehyde donor with the axially chiral catalyst 23 can react with an aldehyde acceptor that is activated by the proximal acidic proton of 23 via transition model 33, thus affording the syn-aldol product [19],... 

The obtained syn diastereoselectivity with acyclic ketone donors could be explained by Z-enamine transition state (Scheme 5.8), whereas the reactions of cyclic ketones such as protected DHA, which are capable only of forming fi-enamine due to ring size constraints, give preferentially fl h-aldol adducts (Scheme 5.8). These models can be applied to other primary-tertiary diamine catalyzed aldol reactions. 

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