Chirality and prochirality

It is a common observation that enzymes deal stereospecifically with their substrates the acceptable substrates must have a particular stereochemistry and the products in turn are formed with a particular stereochemistry (for an illustration of this see particularly Section 5.1). This stereospecificity is associated in many instances with reactions involving chiral centres. 

In addition to the stereospecificity of reaction associated with chiral centres there is further stereospecificity to be found in reactions at prochiral centres. The conversion of ethanol 1.20) into acetaldehyde 1.21) by the enzyme, alcohol dehydrogenase, with NAD as co-enzyme provides a simple illustration. The methylene 

The stereospecific proton removal from 1.20) to give (L21) can be understood simply as follows Imagine that at the active site of the enzyme there is one binding site specific for the ethanol OH and one specific for CH3. This uniquely locates the molecules on the enzyme surface as shown in 1.20). Now imagine that the enzyme/co-enzyme proton removal can only occur physically from above the plane of the paper. This results in unique removal of the pro-R proton. There is no way on this model that the pro-S proton can be removed. A similar argument can be applied to proton addition to 1.21) and to 

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Analogous definitions and designations apply to molecules containing a chiral centre and a prochiral tetrahedral or trigonal centre. The plane containing the chiral and prochiral centres is called a diastereo-zeroplane (Y. Izumi, 1977). 

The protocol of the allylic alkylation, which proceeds most likely via a c-allyl-Fe-intermediate, could be further improved by replacing the phosphine ligand with an M-heterocyclic carbene (NHC) (Scheme 21) [66]. The addition of a ferf-butyl-substituted NHC ligand 86 allowed for full conversion in the exact stoichiometric reaction between allyl carbonate and pronucleophile. Various C-nucleophiles were allylated in good to excellent regioselectivities conserving the 71 bond geometry of enantiomerically enriched ( )- and (Z)-carbonates 87. Even chirality and prochirality transfer was observed (Scheme 21) [67]. 

Very recently (51), nonequivalence has been found in a variety of additional monobasic solutes whose configurational analysis was thought earlier to lie outside the scope of the CSA technique. 2-Butanol, for example, when dissolved in benzene saturated with TFAE, shows nonequivalence in both methyl resonances. A variety of other chiral and prochiral compounds such as 2-propanol, methyl 2-propyl sulfide, 2-aminobutane, and 2-methyl-1-butanol show nonequivalence for their enantiotopic methyl groups under these conditions. The magnitudes of nonequivalence in these instances are small (0.02-0.03 ppm) but, as illustrated in Figure 4 for enriched 2-butanol,... 

Thus, for both chiral and prochiral a-olefins, the isotactic sequence of the stereogenic tertiary carbon atom of the backbone is due to the enantioselectivity of the chiral active sites to the prochiral carbon atom of the monomer. The stereoselectivity (namely the selection, among the enantiomers, of a racemic... 

RDCs are commonly used for the structure elucidation of proteins and nucleic acids nowadays. Only recently the approach was transferred back to also obtain structural information of small- to medium-sized organic molecules. The central application in this case is the determination of relative configurations of distant chiral and prochiral centres, and also conformational studies of biologically active molecules, for example the enantiomeric differentiation of small molecules in chiral alignment media can be achieved. 

A useful review of the stereodynamic possibilities for inversion at chiral and prochiral sulfur (and Se, Te) in metal complexes is available.994... 

The factors which direct the diene to the top (Ca re) or bottom face (Ca si) of dienophile (XXXI) may be of steric and/or stereoelectronic origin. Moreover, the overall stereofacial bias is intrinsically dependent on the conformation of (XXXI). Thus the rotational freedom around the single bonds which link the chiral and prochiral centers in (XXXI) needs to be restricted in a well-defined manner. 

The chemo-, regie- and enantioselectivity of enzymes make them ideal asymmetric catalysts. Most important is the differential recognition of diastereotopic groups of chiral and prochiral substrates. The mild conditions under which most enzymes operate minimize isomerization, epimerization and racemization associated with many other chemical processes.. 8... 

A special notice is given to the chiral and prochiral groups in molecules. Mathematically, chirality can be endowed with a potential by defining the so called chiral volume Vabcd that is the spade product defined as ... 

It is clear that polymerization of MMA via the unimolecular mechanism offers the greatest prospect for both control of polymer MWD, commoner sequence distributions, as well as tacticity. As chiral and prochiral group 4 metallocene complexes are more readily available than their group 3 or lanthanide analogs, we can expect continued effort in this area will eventually lead to a wide range of applications to polymer synthesis. 

E.A. Halevi J. Chem. Research (S), 1985 206. A brief discussion of the application of kernels and co-kernels to the description of substitutional desymmetrization in the context of chirality and prochirality. 

Chirality kI-rol adj [chir- -h -a/] (1894) The property of an organic molecule of not being identical with its mirror image a compound whose molecules are chiral can exist as enantiomers, but non-chiral compounds cannot be enantiomers. All asymmetric molecules are chiral however, not all chiral molecules are asymmetric since some having axes of rotational symmetry are chiral. Chiral and prochiral atoms are sites or potential sites, respectively, of Stereoisomerism. 

The results suggest that a maximum in induced circular dichroism arises from the optimization of sequence lengths of chiral and prochiral units. This seems to originate from the interposition of relatively short NVC sequences (two-four units) in relatively long sequences of the chiral monomer (three or more units). Copolymers with (—)-menthyl vinyl ether have been shown to be block-like in nature and as a consequence a significant amount of short sequences of NVC units is achievable only at very high (—)-menthyl vinyl ether contents. In the case of free radically prepared copolymers of NVC and ( )-menthyl acrylate it is necessary to have an NVC content of approxi-... 

Metal and Lewis acid induced carbonylative ring enlargement of chiral and prochiral cyclohexadienes can give access to bicyclo[3.2.1 ]octanes. ... 

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