Zimmerman and Traxler

From a historical standpoint, the Zimmerman-Traxler study on the stereochemical aspects of the Ivanov reaction is of considerable significance (22). Their investigation revealed that the magnesium enediolate 19a, upon condensation with benzaldehyde (Et2 O, reflux 5 hr), afforded the erythro and threo acids 20E and 20T, respectively, in a 24 76 ratio (eq. [13]). In the analysis of plausible transition states, both chair A and boat B geometries were considered. Zimmerman and Traxler concluded that the major diastereomer 20T could... 

The simple diastereoselectivity of aldol reactions was first studied in detail for the Ivanov reaction (Figure 13.45). The Ivanov reaction consists of the addition of a carboxylate enolate to an aldehyde. In the example of Figure 13.45, the diastereomer of the /1-hydroxycarboxylic acid product that is referred to as the and-diastereomer is formed in a threefold excess in comparison to the. vy/j-diastereoisomer. Zimmerman and Traxler suggested a transition state model to explain this selectivity, and their transition state model now is referred to as the Zimmer-man-Traxler model (Figure 13.46). This model has been applied ever since with good success to explain the simple diastereoselectivities of a great variety of aldol reactions. 

The Ivanov reaction is the preparation of a 3-hydroxy acid by reaction of the magnesium dianion of a carboxylic acid with an aldehyde or ketone." In a seminal paper, Zimmerman and Traxler investigated the Ivanov reaction of phenylacetic acid and benzaldehyde and obtained anti and syn 3-hydroxy acids (127) and (128) in 69% and 22% yields, respectively (equation 85). The observed stereochemistry was rationalized with a cyclic, chair-like transition state in which a magnesium cation is chelated by one oxygen each of the carboxylate enolate and the aldehyde (the Zimmerman-Traxler transition state ). 

Blagoy et al. examined the effect of the counterion on the stereochemistry of this reaction." With the bis(bromomagnesium) salt, a (127) (128) ratio of about 60 40, similar to that observed by Zimmerman and Traxler, is obtained. With the disodium salt, the sole isomer obtained (in 40% yield) is (127). 

The six-membered ring transition state for the aldol reaction was proposed by Zimmerman and Traxler and is sometimes called the Zimmerman-Traxler transition state. 

On the basis of investigation of the stereochemical outcome of the reaction of doubly deprotonated phenyl acetic acid to benzaldehyde (Eq. (19)), Zimmerman and Traxler proposed in a seminal paper a transition state model that involves a six-membered chair-like assembly of the reactants... 

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