Eschenmoser state model

The high levels of diastereoselectivity were rationalized through our usual transition-state models for enolate derivatization. X-ray analysis of the product obtained on reaction of the anti enolate with benzotriazole derivative 23 confirmed that the sense of induced stereoselectivity for the major product isomer was as predicted from the transition-state model, and is shown below, 26. From the high levels of product diastereoselectivities observed when employing benzotriazole derivatives 23 and 24, we reasoned that addition of 1 molar equivalent of benzotriazole to a stirred solution of Eschenmoser s salt at room temperature prior to reaction... 

The originally proposed stereochemical model by Hajos and Parrish" was rejected by M.E. Jung and A. Eschenmoser. They proposed a one-proline aldolase-type mechanism involving a side chain enamine. The most widely accepted transition state model to account for the observed stereochemistry was proposed by C. Agami et al. suggesting the involvement of two (S)-(-)-proline molecules. " " Recently, K.N. Houk and co-workers reexamined the mechanism of the intra- and intermolecular (S)-(-)-proline catalyzed aldol reactions. Their theoretical studies, kinetic, stereochemical and dilution experiments support a one-proline mechanism where the reaction goes through a six-membered chairlike transition state. 

FIGURE 2.1. Working transition state models for the electrophilic attack to the enamine intermediate, (a) List-Houk model, (b) Steric model, (c) Seebach-Eschenmoser model. 

Figure 17.7 (a) Syn-face addition in Houk-List models (b) onti-face addition in Seebach-Eschenmoser transition state models. 

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