Isomerism enantiomers

Indeed, the atmospheric input of PCBs into Lake Michigan has been found to exceed that from landfills (Hornbuckle et al. 1995), so that important interphase partitions should be taken into account. Atmospheric deposition and transport of toxaphene into sediments in the Great Lakes are dominant, although during the more recent 10-year period there is some increase in the hepta- and hexachlorinated congeners that suggests a slow transformation by dechlorination with an estimated half-life > 50 years (Pearson et al. 1997). Attention has been drawn in Chapter 2, Sections 2.4.1.3, 2.4.2.1, and 2.5 to analytical problems with toxaphene, and specifically to the presence of isomeric enantiomers. 

Define each term related to optical isomerism enantiomers, chiral, dextrorotatory, levorotatory, racemic mixture. 

FIGURE 7 8 Stereo isomeric 2 3 dihydroxybu tanoic acids Stereoisomers I and II are enantiomers Stereoisomers III and IV are enantiomers All other rela tionships are diastereomeric (see text)... 

FIGURE 7 10 Stereo isomeric 2 3 butanediols shown in their eclipsed con formations for convenience Stereoisomers (a) and (b) are enantiomers of each other Structure (c) is a diastereo mer of (a) and (b) and is achiral It is called meso 2 3 butanediol... 

Excluding enantiomers there are three isomeric cyclopropanedicarboxyhc acids Two of them A and B are constitutional isomers of each other and each forms a cyclic anhydnde on being heated The third diacid C does not form a cyclic anhydride C is a constitutional isomer of A and a stereoisomer of B Identify A B and C Construct molecular models of the cyclic anhy dndes formed on heating A and B Why doesn t C form a cyclic anhydride" ... 

Propylene glycol, dipropylene glycol, and tripropylene glycol all have several isomeric forms. Propylene glycol has one asymmetric carbon and thus there are two enantiomers (R)-I,2-propanediol and (3)-1,2-propanediol. 1,3-Propanediol is a stmctural isomer. Dipropylene glycol exists in three stmctural forms and since each stmctural isomer has two asymmetric carbons there are four possible stereochemical isomers per stmcture or a total of twelve isomers. These twelve consist of four enantiomer pairs and two meso- compounds. Tripropylene glycol has four stmctural isomers and each stmctural isomer has... 

Discrimination between the enantiomers of a racemic mixture is a complex task in analytical sciences. Because enantiomers differ only in their structural orientation, and not in their physico-chemical properties, separation can only be achieved within an environment which is unichiral. Unichiral means that a counterpart of the race-mate to be separated consists of a pure enantiomeric form, or shows at least enrichment in one isomeric form. Discrimination or separation can be performed by a wide variety of adsorption techniques, e.g. chromatography in different modes and electrophoresis. As explained above, the enantioseparation of a racemate requires a non-racemic counterpart, and this can be presented in three different ways ... 

These cases are completely different from the cis-trans isomerism of compounds with one double bond (p. 157). In the latter cases, the four groups are all in one plane, the isomers are not enantiomers, and neither isomer is chiral, while in allenes, the groups are in two perpendicular planes and the isomers are a pair of optically active enantiomers. 

Only the structures of di- and trisulfane have been determined experimentally. For a number of other sulfanes structural information is available from theoretical calculations using either density functional theory or ab initio molecular orbital theory. In all cases the unbranched chain has been confirmed as the most stable structure but these chains can exist as different ro-tamers and, in some cases, as enantiomers. However, by theoretical methods information about the structures and stabilities of additional isomeric sul-fane molecules with branched sulfur chains and cluster-like structures was obtained which were identified as local minima on the potential energy hypersurface (see later). 

To date, direct asymmetric synthesis of optically active chiral-at-metal complexes, which by definition leads to a mixture of enantiomers in unequal amounts thanks to an external chiral auxiUary, has never been achieved. The most studied strategy is currently indirect asymmetric synthesis, which involves (i) the stereoselective formation of the chiral-at-metal complex thanks to a chiral inductor located either on the ligand or on the counterion and then (ii) removal of this internal chiral auxiliary (Fig. 4). Indeed, when the isomerization of the stereogenic metal center is possible in solution, in-... 

BINAP is a versatile ligand the S-enantiomer, complexed with rhodium, is used in the commercial production of 1-menthol (Scheme 4.23). In this case the reaction involves isomerization of diethylgeranylamine to R)-citronellal enamine, which proceeds to approximately 99% ee. 

Of added interest in this study was the finding that the enantiomeric ratios of a-terpineol also differ widely among populations. In southern populations and the population from the northwestern tip of the island, the amount of (+)-a-terpineol was shown to range from 92.3 to 97.0%, while this enantiomer made up 85.6% in the upper-middle population, but only 66.0% in the population from the southeastern tip of the island. It would be of interest to see if the enzymes responsible for the biosynthesis of a-terpineol have different stereochemical requirements in these populations, or whether some isomerization has occurred in the formation, preparation, or analyses of these oil samples. 

In the case of chiral molecules that are biologically active the desired activity almost always resides in only one of the enantiomers. The other enantiomer constitutes isomeric ballast that does not contribute towards the desired activity and may even exhibit unwanted side effects. Hence, there is a marked trend in pharmaceuticals, agrochemicals and flavours and fragrances towards the marketing of products as enantiomerically pure compounds. This, in turn, has generated a demand for economical methods for the synthesis of pure enantiomers (Sheldon, 1993a). 

Bioassay of alternate molecular forms supports the view that the ORs are capable of resolving isomeric distinctions in neutral (non-biological) odourants. Stereochemical pairs of odours were tested for differential sensitivities in the blind subterranean mole rat (Spalax ehrenbergi). The subjects responded to one enantiomer, but not to its stereoisomer. Both sexes were attracted to the odour of R-(-)-carvone but unresponsive to S-(+)-carvone in contrast, males and females were repelled by the odour of (+)-citronellol, but not by (-)-citronellol (Heth et al., 1992). The lack of responsiveness by mole rats could be central due to lack of salience, or peripheral due to hyposmia/anosmia for one isomer. Both carvones have distinct odours for the human nose. 

Cleavage at any of the three cyclopropyl bonds (a, b, or c) could potentially account for the isomerization.178 However, cleavage at a or b, although both yielding cis product, have different stereochemical implications since the products obtained are enantiomers ... 

Still another way to characterize metal surface sites by a chemical reaction is with the unique molecules (+)— and (—)—apopinene (Fig. 1.5).25-28 The apopinenes are an enantiomeric pair of molecules with a double bond steri-cally hindered on one side by a gem-dimethyl group. During hydrogenation, each enantiomer may hydrogenate to the saturated symmetrical apopinane or isomerize to its enantiomer, which will have the same reactivity on a symmetrical surface (Scheme 1.1). 

It was envisioned that hydrindanone 83 and cyclopentene 85 could be used as intermediates in the synthesis of e f-retigeranic acid A (1) and e f-retigeranic acid B (2), respectively. To prepare the building block 90, cyclopentene 85 was reduced with diimide (93 %) in order to prevent isomerization and subsequently deprotected with PPTS to yield hydrindanone 90 (quant.), which could provide access to <77/-retigeranic acid B (2) (Scheme 10.7). Hydrindanone 83 was reduced via an enol triflate and then subjected to Pd-catalyzed reduction to provide cyclopentene 91 (87 % from 83). Upon hydrogenation of 91 with Pd/C and cleavage of the acetal with iodine, protected hydrindanone 92 (95 % from 91) was obtained. The deprotection of 92 provided ent-60, whose enantiomer was used in previous syntheses of retigeranic acid A (1) by Corey [14] and Hudlicky [46, 47]. 

Optical isomerism is exhibited by compounds that are chiral, i.e., are not superimposable on their mirror images. Such a compound and its mirror image are called optical isomers or enantiomers. They have identical physical and chemical properties except when they interact with other chiral molecules. 

Kinetic resolution results of ketone and imine derivatives are indicated in Table 21.19. In the kinetic resolution of cyclic ketones or keto esters, ruthenium atrop-isomeric diphosphine catalysts 25 induced high enantiomer-discriminating ability, and high enantiopurity is realized at near 50% conversion [116, 117]. In the case of a bicyclic keto ester, the presence of hydrogen chloride in methanol served to raise the enantiomer-discriminating ability of the Ru-binap catalyst (entry 1) [116]. 

In general, the same sense of chiral induction is obtained with either geometrical stereoisomer, which facilitates the use of (E/Z)-isomeric mixtures. An exception to this was recently reported by Heller and Bomer [56d]. Remarkably, hydrogenation of methyl (Z)-/3-acetylamino pentenoate with [(S,S)-Et-DuPhosRh (COD)]BF4 at 1 bar gave the (R)-enantiomer of product in 31% ee, whereas the same reaction at 30 bar resulted in an inversion of configuration and the (S)-product in 77% ee. 

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