Naming enantiomers

Aldotetroses are four-carbon sugars with two chirality centers and an aldehyde carbonyl group. Thus, there are 22 = 4 possible stereoisomeric aldotetroses, or two d,l pairs of enantiomers named erythrose and threose. 

In marked contrast, nature s reducing agent, reduced nicotinamide adenine dinucleotide (NADH), delivers hydride in a stereospecific manner because it is a cofactor in an enzyme-catalysed reaction. For example, reduction of pyruvic acid to lactic acid in vertebrate muscle occurs via attack of hydride to produce just one enantiomer, namely (5)-lactic acid. 

Terashima et al. 231) reported an asymmetric halolactonization reaction. This highly stereoselective reaction permits the synthesis of intermediates for the preparation of chiral a,a-disubstituted a-hydroxycarboxylic acids (227)231c), a-hydroxyketones (228) 231c), functionalized epoxides (229) 231d,e) and natural products 231h,j). Only amino acids have so far been used as a source of the chiral information in the asymmetric halolactonization reaction. Again, the best results have been obtained by using cyclic imino acid enantiomers, namely proline. 

Tetrahedral complexes du not exhibit geometrical isomerism. However, they are potentially chiral just as is tetrahedral carbon. The simple form of optical isomerism exhibited by most organic enantiomers, namely four different substituents, is rarely observed because substituents in tetrahedral complexes are usually too labile10 for the complex to be resolved, i.e., they racemize rapidly. However, an interesting series of cyclopentadienyliron phosphine carbonyl compounds (see Chapter 15 for further... 

Based on the identified homology of the cefD and cefDZ proteins with known eukaryotic enzymes, a mechanism for the A. chrysogenum two-component epimerization system which is different from the epimerization found in prokaryotes has been established <2002JBC46216>. Therefore, it was suggested that the cephalosporin biosynthesis pathway begins with the activation of the substrate isopenicillin N to its CoA, followed by an epimerization to the D-enantiomer, namely penicillinyl-CoA. Next, the required hydrolysis of the CoA-thioesters seems to occur in a nonstereoselective manner by different thioesterases. The resulting product, penicillin N, is the direct precursor of all cephalosporins and cephamycins. 

Nantradol was separated into two diastereoisomers and the more potent of the two was then resolved into its two optically active enantiomers. The laevorotatory enantiomer, named levonantradol (9), was found to be at least 100-times more potent than the dextrorotatory enantiomer. 

Sparteine [(— )-34] is an alkaloid frequently occurring in Fabiaceae. It is commercially available, mostly as the sulfate, and comparatively inexpensive, so there is no need for synthesis. The other enantiomer, named (- j-pachycarpine [(+ )-34], can be obtained by partial synthesis from racemic lupanine, involving resolution with camphorsulfonic acid and lithium aluminum hydride reduction28. 

Capillary columns having a chiral stationary phase are used for the separation of optically active isomers or enantiomers, namely, species that have the same... 

Accordingly, there exist two enantiomers, namely RRRR— and SSSS—, for the isotactic polyketone (Figures 6(c) and 6(d)). 

The four stereoisomers obtained from a single enantiomer of the auxiliary and the two enantiomers (named A and B) of the alcohol are represented in Figure 79 and classified according to their origin as AA, AB, BB, and BA. 

Originally, we attempted to separate two pairs of enantiomers, namely, S,R-( )-ibuprofen and 5,/ -( )-2-phenylpropionic acid [1]. The two samples were dissolved in 70% ethanol and then the respective solutions were spotted with the aid of an automatic sampler on to the adsorbent layer. Thin layer chromatographic conditions that are best suited for separation of the APA enantiomers involve silica gel impregnated with L-arginine, which is kept in the cationic form, due to a properly fixed pH value (<4.8). The mobile phases used were the ternary mixtures of acetonitrile (ACN), methanol (MeOH), and H2O (plus several drops... 

Although Pasteur was unable to provide a structural explanation—that had to wait for van t Hoff and Le Bel a quarter of a century later—he correctly deduced that the enantiomeric quality of the crystals was the result of enantiomeric molecules The rare form of tartanc acid was optically inactive because it contained equal amounts of (+) tartaric acid and (—) tartaric acid It had earlier been called racemic acid (from Latin racemus meaning a bunch of grapes ) a name that subsequently gave rise to our pres ent term for an equal mixture of enantiomers... 

Ibuprofen is an analgesic sold under various names including Advil, Motrin, and Nuprin. The material is sole as a racemic mixture, but only one enantiomer acts as ar analgesic. The other enantiomer is inactive. Assign R oi S forms to the two enantiomers of ibuprofen. 

The two enantiomers of limonene have completely different tastes. One has the taste of lemon (as the name implies) and the other of orange. Assign R or S forms tc the two enantiomers of limonene. 

Divalent sulfur compounds are achiral, but trivalent sulfur compounds called sulfonium stilts (R3S+) can be chiral. Like phosphines, sulfonium salts undergo relatively slow inversion, so chiral sulfonium salts are configurationally stable and can be isolated. The best known example is the coenzyme 5-adenosylmethionine, the so-called biological methyl donor, which is involved in many metabolic pathways as a source of CH3 groups. (The S" in the name S-adenosylmethionine stands for sulfur and means that the adeno-syl group is attached to the sulfur atom of methionine.) The molecule has S stereochemistry at sulfur ana is configurationally stable for several days at room temperature. Jts R enantiomer is also known but has no biological activity. 

Optical isomers (Section 9.4) An alternative name for enantiomers. Optical isomers are isomers that have a mirror-image relationship. 

In the 1990s, a large number of so-called chiral drugs containing a single enantiomer came on the market. One of the first of these was naproxen (trade name Aleve). 

The name enantiomer comes from the Greek words meaning both parts. ... 

In the last two cases the trivial name refers specifically to the D enantiomer. (See also the alphabetical listing of trivial names in the Appendix.)... 

An unprecedented stereoselective procedure to obtain enantiomerically pure transition cluster M3Q4 complexes consists of the direct excision of the M3Q7X4 n polymers using chiral diphosphanes, namely (+)-l,2-bis[(2J ,5R)-2,5-(dimethylphospholano)]ethane [(R,R)-Me-BPE] and its respective enantiomer [(S,S)-Me-BPE] to afford the trinuclear complexes (P)-[Mo3S4Cl3(J ,J -Me-BPE)3] and (Af)-[Mo3S4Cl3(S,S-Me-BPE)3] , respectively [30]. The structures of both enantiomers are shown in Fig. 7.3. The symbols (P) and (M) refer to the rotation of the chlorine atoms around the C3 axis, with the capping sulfur pointing towards the viewer. 

All sixteen of the racemic carba-sugars predicted are known, as well as fifteen of the enantiomers. The most accessible starting-material for the synthesis of racemic carba-sugars is the Diels-Alder adduct of furan and acrylic acid, namely, e i o7-oxabicyclo[2.2.1]hept-5-ene-2-carboxylicacid (29). Furthermore, adduct 29 is readily resolved into the antipodes, (—)-29 and (+)-29, by use of optically active a-methylbenzylamine as the resolution agent. The antipodes were used for the synthesis of enantiomeric carba-sugars by reactions analogous to those adopted in the preparation of the racemates. 

Two types of sulfoximinocarboxylates (analogous to sulfinylcarboxylates 16), namely 5 -aryl-5 -methoxycarbonylmethyl-A(-methyl sulfoximine 36 and -methyl-5 -phenyl-A(-ethoxycarbonyl sulfoximine 37, were subjected to hydrolysis in the presence of PLE in a phosphate buffer. As a result of a kinetic resolution, both the enantiomerically enriched recovered substrates and the products of hydrolysis and subsequent decarboxylation 38 and 39, respectively, were obtained with moderate to good ees (Equations 20 and 21). Interestingly, in each case the enantiomers of the substrates, having opposite spatial arrangement of the analogous substituents, were preferentially hydrolysed. This was explained in terms of the Jones PLE active site model. ... 

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