Traxler

Zimmerman-Traxler Transition State Ivanov condensation JACS 1957, 7.9, 1920. 

The Zimmerman-Traxler like transition state model can involve either a chair or boat geometry. 

R. N. Traxler, Its Composition, Properties and Uses, Reinhold Publishing Co., New York, 1961, Chapt. 6. 

The diastereoselectivity observed on carbonyl addition of ( )-alkenyltitanium derivatives is in accordance with the reaction proceeding via a chair-like six-membered Zimmerman Traxler transition state. The following facts are diagnostic ... 

The most widely accepted transition state hypothesis for aldol reactions is the Zimmerman Traxler model29 which involves a six-membered chair-like assembly of the rcaetants. This... 

The fact that (Z)-lithium enolatcs generally display a higher simple diastereoselectivity giving. vyn-aldols compared to (E)-enolates affording nn/i-aldols is a challenge to the Zimmer-man-Traxler model, and has become the source of extended speculation. 

A Zimmerman-Traxler transition state model is postulated in order to rationalize the ul topicity of this aldol addition [i.e., the (S)-enolate preferentially attacks the 7 e-face of the aldehyde]33. In the two alternative transition states 3a [ul topicity (S)jRe] and 3b [Ik topicity (S)/Si, the substituents at the stereogenic center of the enolatc are oriented in such a way that... 

Although the chiral propanoates I and 5 are similar and the reaction conditions are almost identical, the stereochemical outcomes arc explained by completely different transition state models. Predominant attack of the ketene acetal 2 to the Ai-face of 2-methylpropanal is interpreted by assuming a Zimmerman-Traxler like model, which minimizes steric hindrance in a plausible way65. 

The preference of the (5, .S )-boron cnolatc to attack almost exclusively the Si-face of an aldehyde is rationalized by assuming the Zimmerman-Traxler transition state model. It is postulated that the methyl group of the propyl residue directs the 3-elhylpenlane-3-thiol group towards the borolane moiety, the chirality of which is thus effectively transferred34. 

The major aldol product 4 is consistent with the intermediacy of a Zimmerman-Traxler cyclic transition state such as A. Diastereoselectivities are moderate for aldehydes which are less sterically demanding than 2,2-dimethylpropanal. 

The configurational course depends on the enolatc configuration and the metal ion which determines whether a cyclic (e.g., Zimmerman-Traxler type) or an acyclic transition state is traversed. At present the following transition state models have been proposed. 

Parekh VR, RW Traxler, JM Sobek (1977) -alkane oxidation enzymes of a pseudomonad. Appl Environ Microbiol 33 881-884. 

TS, which is usually based on the chair (Zimmerman-Traxler) model. This pattern is particularly prevalent for the allylic borane reagents, where the Lewis acidity of boron promotes a tight cyclic TS, but at the same time limits the possibility of additional chelation. The dominant factors in these cases are the E- or Z-configuration of the allylic reagent and the conformational preferences of the reacting aldehyde (e.g., a Felkin-type preference.)... 

The selectivity of the aldol addition can be rationalized in terms of a Zimmer -man-Traxler transition-state model with TS-2-50 having the lowest energy and leading to dr-values of >95 5 for 2-51 and 2-52 [18]. The chiral copper complex, responsible for the enantioselective 1,4-addition of the dialkyl zinc derivative in the first anionic transformation, seems to have no influence on the aldol addition. To facilitate the ee-determination of the domino Michael/aldol products and to show that 2-51 and 2-52 are l -epimers, the mixture of the two compounds was oxidized to the corresponding diketones 2-53. 

A proposed simplified mechanism for the conjugate addition/aldol cyclization, as depicted in Scheme 2.25, is based on detailed mechanistic studies performed on related Rh-catalyzed enone conjugate additions [45]. A model accounting for the observed relative stereochemistry invokes the intermediacy of a (Z)-enolate and a Zimmerman-Traxler-type transition state as shown in 2-110 to give 2-111. 

Buchdunger E, Mett H, Trinks U, Regenass U, Muller M, Meyer T, Beilstein P, Wirz B, Schneider P, Traxler P (1995) Clin Cancer Res 1 813... 

Lydon NB, Mett H, Mueller M, Becker M, Cozens RM, Stover D, Daniels D,Traxler P, Buchdunger E (1999) Int J Cancer 81 669... 

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