Stereochemical recognition

Moss and his coworkers [115] have however successfully developed micelles of diastereomeric peptide-like surfactants and used them in deacylation, with, in some cases, considerable selectivity. 

There is an interesting difference between the evidence for enantiomeric and diastereomeric recognition. The stereoselective deacylation of enantiomers with a chiral histidine surfactant seems to depend wholly upon transition-state interactions 

Micellized enantiomers and diastereomers can give a degree of stereochemical recognition, but in addition the stereochemical course of reaction can be influenced by carrying it out in a micellar pseudophase. 

Deamination of optically active primary amines in water generally proceeds with extensive racemization, but some inversion of configuration. However, if the amine is sufficiently hydrophobic for its salt to be micellized net retention of configuration is observed, because micellization encourages water molecules to attack the carbocat-ionic intermediates preferentially from the front rather than from the rear [116]. 

In much the same way the stereochemistry of S l reactions of chiral alkyl arenesulfonates can be controlled by carrying out solvolyses in the presence of micelles, or by using self-micellizing substrates [117,118], and the general features of these reactions are as outlined by Moss et al. [116] for deamination of micellized amines. 

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Kinetic resolutions of allylic esters have also been conducted. As noted in Chapter 14, in most cases, catalysts that are selective for kinetic resolution of substrates containing one type of functional group are also selective for reactions of meso substrates and vice versa.The enantioselective reaction of one acetate of a meso substrate involves similar stereochemical recognition to reaction of two enantiomers of a racemic mixture of allylic esters. In one illustrative example, an allylic acetate underwent reaction with a pivalate nucleophile to form the allylic pivalate product, which is less reactive than the starting compound (Equation 20.54). Reaction of one enantiomer of the racemic allylic acetate occurs preferentially, and the pivalate product is formed enantioselec-tively. The product of the resolution in Equation 20.54 was carried forward to form (+)-cyclophellitol. 

Many biological mineralization processes involve oriented nucleation strictly controlled by organized organic interfaces that interact with the nascent crystals by virtue of structural and stereochemical recognition processes [57,91,92,117],... 

How about stereochemical recognition So far only few micelles of optically active surfactants have been used as catalysts in a number of reactions with chiral substrates, but in general the effects are small. Two examples will be given here where chiral micelles could stereoselectively catalyze the hydrolysis of chiral esters. 

Templeton, W., Kowlessur, D., Thomas, E. W., Topham, C. M. Brocklehurst, K. (1990). A re-appraisal of the structural basis of stereochemical recognition in papain. Biochemical Journal, 266, 645-51. 

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