Anh-Felkin Modification of Crams Model for Asymmetric Synthesis

2 Anh-Felkin Modification of Cram s Model for Asymmetric Synthesis 

Even if one ignores the difficulty related to the angle of attack, the first two factors which destabilize the TS were serious liabilities of the Cram s model and needed to be addressed. Anh and Felkin considered a different reactant conformer in which 

S = small group, M = medium group, L = large group 

Let us consider the reaction of (S)-3-phenyl-2-butanone, 1. The two conformers of the reactant are la (Eq. 6) and lb (Eq. 7), following the Anh-Felkin modification of Cram s model. The conformer la will predominate over lb because the latter suffers from steric interactions between the two methyl groups that are gauche to each other. The conformers la and lb will generate the products (S,S)-2n and (R, S)-2b, respectively, on reaction with ethyl magnesium iodide. Since la is the predominant conformer, (S,S)-2a is formed as the major product, and (R,S)-2b constitutes the minor product. 

For (R)-3-phenyl-2-butanone, 3, the two reacting Anh-Felkin conformers are 3a and 3b. These conformers are expected to generate predominantly the products (S, 

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