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Asymmetric induction 1,2-addition, Felkin Model

Normally, additions depicted by model C lead to the highest asymmetric induction. The antiperiplanar effect of OR substituents can be very efficient in the Houk model B ( , , , , ) however it plays no role in model C. Furthermore, the Houk model B must be considered in all cycloaddition-like reactions. The Felkin-Anh model A is operative for nucleophilic additions other than cuprate additions ( ). The epoxidation reactions are unique as they demonstrate the activation of one diastereoface by a hydroxy group which forms a hydrogen bridge to the reagent ( Henbest phenomenon ). The stereochemical outcome may thus be interpreted in terms of the reactive conformations 1 and 2 where the hydroxy function is perpendicular to the olefinic plane and has an optimal activating effect. [Pg.131]

A DFT study of enolborane addition of o -heteroatom-substituted aldehydes has focused on the relevance of the Cornforth and polar Felkin-Anh (PKA) models for asymmetric induction.154 Using chiral substrates, MeCH(X)CHO, polar (X = F, Cl, (g) OMe) and less polar (X = SMe, NMe2, PMe2) substituents have been examined. The former favour Cornforth TS structures, the latter PKA. TS preferences have been correlated with the relative energy of the corresponding rotamer of the uncomplexed aldehyde. An in-depth study of addition of ( > and (Z)-enolborane nucleophiles to 2-methoxypropanal successfully predicts experimentally determined diastereofacial selectivities. [Pg.20]

Let us consider the nucleophilic addition to a carbonyl group adjacent to a stereogenic center. Following Anh-Felkin s modification of Crams s model for asymmetric induction, the reaction can follow either of the pathways shown in... [Pg.196]

This chapter begins with a detailed examination of the evolution of the theory of nucleophilic attack on a chiral aldehyde or ketone, from Cram s original rule of steric control of asymmetric induction to the Felkin-Anh-Heathcock formulation. Then follows a discussion of Cram s simpler rigid model (chelate rule), then carbonyl additions using chiral catalysts and chiral (nonenolate) nucleophiles. The chapter concludes with asymmetric 1,4-additions to conjugated carbonyls and azomethines. [Pg.121]

A study of the stereochemical outcome of the addition of lithium enolates to a-alkoxyaldehydes has shown that the predominant product is not that predicted by application of Cram s cyclic model for asymmetric induction. Assuming the alkoxy-group to be the largest group a- to the aldehyde, the major product is that predicted by Felkin s model (Scheme 59). ... [Pg.97]

Discuss the factors behind selection of the reactive conformation in the Felkin-Ahn model for conversion of 53 to 54. How would Cornforth have rationalized this stereochemical result [For an interesting system where the Cornforth and Felkin-Ahn models predict different stereochemical results see Evans, D. A. Siska, S. J. Cee, V. J. Resurrecting the Cornforth Model for Carbonyl Addition Studies on the Origin of 1,2-Asymmetric Induction in Enolate Additions to Heteroatom-Substituted Aldehydes Andrew. Chem. Int. Ed. 2003, 42, 1761-1765] (CJH-5)... [Pg.471]

Show your understanding of the Cram and Felkin-Ahn models for asymmetric induction in carbonyl addition reactions by predicting (or rationalizing) the stereochemical course of the following reactions. (Calcimycin-3)... [Pg.524]


See other pages where Asymmetric induction 1,2-addition, Felkin Model is mentioned: [Pg.450]    [Pg.1127]    [Pg.182]    [Pg.182]    [Pg.719]    [Pg.270]    [Pg.182]    [Pg.116]    [Pg.511]   
See also in sourсe #XX -- [ Pg.391 , Pg.392 , Pg.393 , Pg.394 , Pg.395 , Pg.396 , Pg.401 , Pg.402 , Pg.403 ]




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