Configuration labels, chirality

Secondly, we have labeled one of the substituted methanes 5-fluorochloro bromomethane and the other f -fluorochlorobromomethane. S (sinister, left) and R (rectus, right) are labels that are useful in designating the absolute configuration of chiral molecules in space. The rules for assigning S stereochemistry in one case and R stereochemistry in the other are somewhat complex but it doesn t matter, so never mind. The point is that we have a way of talking about the two possibilities. 

In Section 4.3 we mentioned two important stereochemical terms, enantiomers and enantiotopic nuclei. Enantiomers are structures related as the left hand is related to the right nonsuperimposable mirror images. Any chiral (dissymmetric) molecule can exist in two (and only two) enantiomeric forms. For example, chiral alcohol 10-11 has two enantiomeric configurations, labeled R and S.10 Enantiotopic nuclei are those related by a plane of symmetry. The methylene hydrogens of benzyl alcohol (10-12) are enantiotopic and are labeled pro-R and pro-S10 ... 

Whereas the 0 effect on P NMR chemical shifts is sufficient for determining the configurations of chirally labeled samples of prochiral phosphorus atoms in a diastereomeric environment, additional use of the quadrupolar effect of O on P NMR resonances is required for configurational analyses of oxygen chiral phosphate monoesters. The basic strategy for the configurational analyses of phosphate monoesters is the same, i.e., the enantiomeric center in the monoester must be converted to a diastereomeric center in a cyclic phosphodiester so that... 

A conceptually similar approach can be used to determine the configurations of chiral phosphate monoesters of chiral alcohols without the necessity for transferring the chiral phosphoryl group to 1,3-butanediol. For example, in a number of stereochemical studies of hydrolysis reactions performed in the author s laboratory, 3 - or 5 -nucleotides were obtained as reaction products. In the case of samples of 5 -AMP, these can be enzymically converted to isotopically labeled cyclic 3, 5 -nucleotides by initial pyrophosphorylation with adenylate kinase and pyruvate kinase, followed by enzymic cyclization with inversion of configuration by a bacterial adenylate cyclase (20) (see Figs. 6 and 7). In the case of samples of 3 -nucleotides or 5 -nucleotides other than 5 -AMP, the cyclization reactions can be accomplished chemically (23). In either case, following chemical con-... 

Other mechanisms must also operate, however, to account tor the fact that 5-10% of the product is formed with retained configuration at the chiral center. Isotopic labeling studies have also demonstrated that the 3-bromo-2-butyl radical undergoes reversible loss of bromine atom to give 2-butene at a rate which is competitive with that of the bromination reaction ... 

The reaction of aldehydes with Wilkinson s catalyst goes through complexes of the form 26 and 27, which have been trapped. The reaction has been shown to give retention of configuration at a chiral and deuterium labeling demonstrates that the reaction is intramolecular RCOD give RD. 

Oae and co-workers (288) were the first to show that nucleophilic displacement at sulfur is accompanied by retention of configuration. They found that chiral 0-labeled alkyl aryl sulfoxides exchange oxygen with dimethylsulfoxide at about 150°C, almost without racemization. To explain the steric course (retention) of this reaction, the formation of a trigonal-bipyramidal intermediate 246 was postulated in which the entering and departing oxygen atoms occupy apical and equatorial positions, respectively. 

A method for designating the stereoisomeric configuration of a chiral carbon atom within a molecular entity. The designation d was arbitrarily assigned to (-F)-glycer-aldehyde, and (-)-glyceraldehyde was assigned the label... 

Reetz and coworkers developed a highly efficient method for screening of enantioselectivity of asymmetrically catalyzed reactions of chiral or prochiral substrates using ESI-MS [60]. This method is based on the use of isotopically labeled substrates in the form of pseudo-enantiomers or pseudo-prochiral compounds. Pseudo-enantiomers are chiral compounds which are characterized by different absolute configurations and one of them is isotopically labeled. With these labeled compounds two different stereochemical processes are possible. The first is a kinetic separation of a racemic mixture, the second the asymmetric conversion of prochiral substrates with enantiotopic groups. The conversion can be monitored by measuring the relative amounts of substrates or products by electrospray mass spectrometry. Since only small amounts of sample are required for this method, reactions are easily carried out in microtiter plates. The combination of MS and the use of pseudo-enantiomers can be used for the investigation of different kinds of asymmetric conversion as shown in Fig. 3 [60]. 

Attempts to produce descriptors similar to cis and trims for stereochemicidly more complicated coordination entities have tailed to achieve generality, and labels such as foe and mer are no longer recommended. Nevertheless, a diastereoisomeric structure may be indicated for any polyhedron using a configuration index as an affix to the name or formula. Finally, the chiralities of enantiomeric structures can be indicated using chirality symbols. 

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