Single chiral

Three-component coupling with vinylstannane. norbornene (80). and bro-mobenzene affords the product 91 via oxidative addition, insertion, transme-tallation, and reductive elimination[85]. Asymmetric multipoint control in the formation of 94 and 95 in a ratio of 10 1 was achieved by diastereo-differ-entiative assembly of norbornene (80), the (5 )-(Z)-3-siloxyvinyl iodide 92 and the alkyne 93, showing that the control of four chiralities in 94 is possible by use of the single chirality of the iodide 92. The double bond in 92 should be Z no selectivity was observed with E form[86]. 

Section 7 9 A chemical reaction can convert an achiral substance to a chiral one If the product contains a single chirality center it is formed as a racemic mixture Optically active products can be formed from optically inactive... 

Each of the molecules below (carvone, ibuprofen anc limonene) incorporates a single chiral center. Identify ii and draw both R and S forms of each. 

The PECD measurement clearly takes the form of a cosine function with an amplitude given entirely in terms of the single chiral parameter, b. It therefore provides exactly the same information content as the y asymmetry factor dehned above [Eq. (8)]. Experimental advantages of examining the PECD rather than the single angular distribution /p(0) are likely to include some cancellation of purely instrumental asymmetries (e.g., varying detection efficiency in the forward-backward directions) and consequent improvements in sensitivity. 

Studies also show that by using catalyst 76a or 78 derived from one single chiral tetraol with the same absolute configuration, both enantiomers of the Diels-Alder reaction product can be obtained. For example, reaction of 2-methyl-... 

TABLE 1. Rotational contributions to the Lb Cotton effects for a chiral benzene compound with a single chiral substituent giving a negative vibronic contribution to the Lb Cotton effects... 

Elements of this synthesis were used as the foundation for a clever synthesis of both enantiomers from a single chiral precursor 49, with a Diels-Alder reaction comprising the key step [64]. The critical concept was the recognition that the Diels-Alder reaction of 49 could proceed through two different transition states (Scheme 7), with the product from one transition state (50a) being the tricyclic alkene 44 which had been readily converted to the desired product 48 in the synthesis of the racemate (see above). The second transition state (50b) would provide intermediate 51 that, with slightly different synthetic manipulations, could be converted to the other enantiomer of 48. The full synthesis has been described in Vol. 1, Chap. 1, and so will not be reiterated here. 

Compounds that have a single chiral centre and form non-superimposable mirror images of each other. 

We Umit this section to a discussion of stereochemical studies that sought to demonstrate discriminating enantiomeric interactions in monolayers of simple surfactants having one hydrophobic chain of methylenes and, generally, a single chiral center. Work in this area includes derivatives of long chain fatty acids, alcohols, or esters whose chiral center is included in the methylene chain. 

It is important to perform both the Birch reduction of 5 and the alkylation of enolate 6 at —78 °C. Enolate 6 obtained directly from 5 at low temperatures is considered to be a kinetic enolate . A thermodynamic enolate obtained from 6 by equilibration techniques has been shown to give an opposite sense of stereoselection on alkylation. Although a comprehensive study of this modification has not been carried out, diastereoselectivities for formation of 8 were found to be greater than 99 1 for alkylations with Mel, EtI, and PhCH2Br. Thus, it should be possible to obtain both enantiomers of a target structure by utilization of a single chiral benzamide. SE... 

Zheng M, Semke ED (2007) Enrichment of single chirality carbon nanotubes. J Am Chem Soc 129 6084-6085... 

In chirality transfer reactions leading to products with a single chiral unit, the absolute configuration has to be determined. Examples to fall into this category. 

Epimers differ in configuration about a single chiral center in molecules with more than one chiral center. Anomers are epimers in which the chiral site was formerly a carbonyl C. 

The more active enantiomer at one type of receptor site may not be more active at another receptor type, eg, a type that may be responsible for some other effect. For example, carvedilol, a drug that interacts with adrenoceptors, has a single chiral center and thus two enantiomers (Figure 1-2, Table 1-1). One of these enantiomers, the (S) -) isomer, is a potent B-receptor blocker. The (R)(+) isomer is 100-fold weaker at the receptor. However, the isomers are approximately equipotent as -receptor blockers. Ketamine is an intravenous anesthetic. The (+) enantiomer is a more potent anesthetic and is less toxic than the (-) enantiomer. Unfortunately, the drug is still used as the racemic mixture. 

Another naming system for stereoisomers, the D and L system, is described in Chapter 3. A molecule with a single chiral center (glyceraldehydes, for example) can be named unambiguously by either system. 

D-Apiose can be isolated from parsley and is a component of the cell wall polysaccharide of various marine plants. Among its novel structural features is the presence of only a single chirality center. L-Vancosamine is but one portion of vancomycin, a powerful antibiotic that has emerged as one of only a few antibiotics that are effective against drug-resistant bacteria. L-Vancosamine is not only a branched-chain carbohydrate, it is a deoxy sugar and an amino sugar as well. 

The srtuciurcs 10 and 11 denoted above contain a single chiral center A, the atom to which the ligands are attached. If two different such centers, A and B, are in a molecule the number of optical isomers is increased to four ... 

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