Stereoisomers epimers

Aspects of chemical methods used in the structural elucidation of polysaccharides and complex carbohydrates have been reviewed. In a critical examination of the use of g.l.c.-m.s. in the identification of TMS ethers of monosaccharides, a standardized method, which uses a medium resolution mass spectrometer and short chromatographic columns, has been proposed. TMS Ethers of monosaccharides have been characterized by g.l.c.-chemical ionization m.s. with ammonia as reagent gas. Molecular weights were determined, and fragment ions were produced in a quantity high enough to differentiate between stereoisomers (epimers and anomers). Disaccharides have been determined by permethylation followed by g.l.c. The method has been used in the detection of carbohydrate intolerance secondary to intestinal disaccharidase deficiency. 

Some molecules have more than one chirality center. Enantiomers have opposite configuration at all chirality centers, whereas diastereomers have the same configuration in at least one center but opposite configurations at the others. Epimers are diastereomers that differ in configuration at only one chirality center. A compound with n chirality centers can have a maximum of 2n stereoisomers. 

The issue of stereochemistry, on the other hand, is more ambiguous. A priori, an aldol condensation between compounds 3 and 4 could proceed with little or no selectivity for a particular aldol dia-stereoisomer. For the desired C-7 epimer (compound 2) to be produced preferentially, the crucial aldol condensation between compounds 3 and 4 would have to exhibit Cram-Felkin-Anh selectivity22 23 (see 3 + 4 - 2, Scheme 9). In light of observations made during the course of Kishi s lasalocid A synthesis,12 there was good reason to believe that the preferred stereochemical course for the projected aldol reaction between intermediates 3 and 4 would be consistent with a Cram-Felkin-Anh model. Thus, on the basis of the lasalocid A precedent, it was anticipated that compound 2 would emerge as the major product from an aldol coupling of intermediates 3 and 4. 

An important stereochemical issue presents itself here. A priori, an aldol condensation between intermediates 2 and 3 could result in the formation of a mixture of diastereomeric aldol adducts, epi-meric at C-7, with little or no preference for a particular stereoisomer. Cram s rule2,4 predicts the formation of aldol adduct 43. This intermediate possesses the correct absolute configuration at C-7, and it should be noted that Kishi et al. had demonstrated during the course of their monensin synthesis that a similar aldol condensation produced the desired C-7 epimer as the major product.12... 

Only one example, showing high stereoselectivity, is known in this class of reactions. On treatment of the acyclic glycine cation equivalent 1 (see Appendix), containing the ( + )-cam-phor-10-sulfonamide ester as a chiral auxiliary, with boron trifluoridc and anisole at 0"C a mixture of aromatic substitution products is obtained in essentially quantitative yield 55. Besides 11 % of cuV/io-substitution product, the mixture contains (R,S)-2 and its (/ ,/ )-epimer in a ratio >96 4 (NMR). The same stereoisomer 2 predominates when the reaction is conducted in sulfuric acid/acetic acid 1 9, although the selectivity is slightly lower (91 9 besides 25% of ortho substitution). 

The typical isocyclic ring E present in chlorophylls is susceptible to a number of different modifications such as epimerization, which produces stereoisomers by inversion of the configuration at C-13 of their parent pigments. These 13 -epichlorophylls, known as chlorophylls a and b, are minor pigments. They are considered artifacts produced in the course of handling plant extracts and sometimes are also found in small amounts in heated and deep-frozen vegetables, hi the old Fischer systan of nomenclature that can still be found in some literature, these epimers were named 10-epichlorophylls. 

When the lactone silyl ketene acetal 18-1 is heated to 135° C a mixture of four stereoisomers is obtained. Although the maj or one is the expected [3,3] -sigmatropic rearrangement product, lesser amounts of other possible C(4a) and C(5) epimers are also formed. When the reaction mixture is heated to 100° C, partial conversion to the same mixture of stereoisomers is observed, but most of the product at this temperature is an acyclic triene ester. Suggest a structure for the triene ester and show how it can be formed. Discuss the significance of the observation of the triene ester for the lack of complete stereospecificity in the rearrangement. 

Figure 9.3 Stereoisomers of the D-aldoses. D-Ribose and D-arabinose differ only in their configuration about a single carbon atom (carbon 2) and are examples of epimers. Diastereoisomers are stereoisomers which are not enantiomers of each other but are chemically distinct forms, the eight D-hexoses being examples. Some, however, are also epimers of each other, for example D-allose and D-altrose. The number of aldoses in the L series is equal to that of the d series and each compound is an enantiomer of one in the other series.

The P-atom in sarin (9.84), soman (9.85), and tabun (9.87) is a stereogen-ic center, allowing for stereoselective enzymatic hydrolysis [162], This aspect has been extensively investigated for soman, which exists as four stereoisomers by virtue of the presence of a second stereogenic center (C-atom). These stereoisomers are usually designated as C(+)P(-), C(-)P(+), C(+)P(+), and C(-)P(-), where C(+/-) refers to the 1,2,2-trimethylpropyl moiety and P(+/ ) to the P-atom. Such a nomenclature may be convenient but has no implication for the absolute configuration. The C(+)P( ) and Cf-)P(-) epimers are the more active toward acetylcholinesterase and, hence, the more toxic ones. In contrast, the C(+)P(+) and C(-)P(+) epimers are preferentially hydrolyzed... 

A simple epoxy alkanol to begin with is 1,2-epoxyhexan-3-ol (10.47), which has been postulated as a metabolite of the air pollutant hex-l-ene (10.46). This compound was found to be a good substrate for rat liver microsomal EH, yielding hexane-1,2,3-triol (10.48) [127], 1,2-Epoxyhexan-3-ol contains two stereogenic centers and exists as four stereoisomers that were hydrated at different rates, in the order (2S,3R)-erythro > (2S,3S)-threo > ( 2R,3S)-erythro > (2R,3R)-threo. In other words, the metabolic hydrolysis of this substrate is not influenced by the configuration at C(3), but clearly by that at C(2), with the (25)-epimers being better substrates than the (2/7)-cpi-... 

We can also nse the term epimer to describe the relationship between isomers, where the difference is in the confignration at jnst one centre (see Section 3.4.4). This is shown for the four epimers of o-(- -)-glucose. An interesting observation with the 16 stereoisomers is that optical activity of a particular isomer does not appear to relate to the confignration at any particnlar chiral centre. 

Stereoisomers with more than one chiral center and which are not mirror images of each other hence, stereoisomers that are not enantiomers of each other. For example, L-threonine and D-threonine are an enantiomeric pair whereas L-threonine and D-allothreonine are a diastereomeric pair (as is L-threonine and L-allothreo-nine). Diastereomers will have similar physical, chemical, and spectral properties but those properties will not be identical. If n is the number of chiral centers, then the maximum number of stereoisomers will be equal to 2. However, the actual number for a given set of isomers may be less than 2 due to the presence of meso forms. See Enantiomer Epimer Meso Form... 

Only 15 newly reported simple dihydroflavonols are listed in Table 15.9. Several of the compounds are just simply the C-3 epimers of known (2i ,3i )-dihydroflavonols, e.g., (2R,3S)-3-hydroxy-5,7-dimethoxyflavanone (329) from the fern Woodsia scopulina (Dryopterida-ceae), (2i ,3S)-3,5,3, 4 -tetrahydroxy-7-methoxyflavanone (3-epipadmatin, 333) from Inula graveolens (Asteraceae),(2i ,3S)-3,5,3 -trihydroxy-7,4 -dimethoxyflavanone (337) from Lannea coromandelica (Anacardiaceae), and (2i ,3S)-3,5,7,3, 4, 5 -hexahydroxyflavanone (hovenitin III, 338) from the seeds and fruits of Hovenia dulcis (Rhamnaceae). The previously known (2i ,3i )-stereoisomer of hovenitin III has the trivial name ampelopsin. The same plant source has yielded the (27 ,37 )- and (27 ,3S)-epimers of 3,5,7,4, 5 -pentahydroxy-3 -methoxyflavanone, which are known as hovenitins I (340) and II (341), respectively. ... 

The reductive cyclization of a mixture of epimers of compound 52 gave a 76 24 mixture of stereoisomers 53 and 54 in 50% overall yield. The exclusive formation of the cis ring junction was attributed to ready isomerization of the hemiaminal. A similar mixture was also prepared by cyclization of acetal 55 (Scheme 11) <1998TL2315>. 

The construction of the C2-C3 bond of the 1-hydroxyethylene moiety by the addition of a two-carbon fragment to aminoalkyl epoxides, amino ketones, p-amino-a-hydroxy aldehydes, or 4-amino-3-oxo phosphonates was also studied. As summarized in Scheme 14, Evans et al.[28] added a malonate to an aminoalkyl epoxide and the product spontaneously cyclized, leading to a lactone.— Then, the second side chain was incorporated into the a-carbon of the lactone, providing a mixture of C2 epimers. The final compound was obtained by hydrolysis and decarboxylation. The lack of diastereoselectivity of this method is offset by the small number of steps and the accessibility to all eight possible stereoisomers. Biihlmayer et al.[30] also used a similar method, but they transformed the epoxide into the apparently more reactive iodide. Then, the iodide compound was treated with an enolate. 

Irradiation of dehydroergosterol (Formula 328) gives photodehydro-ergosterol (Formula 329) (140,141). This photisomerization is stereospecific and cannot involve complete fission of the 9,10-bond because irradiation of the C-10 epimer of dehydroergosterol gives a stereoisomer of photodehydroergosterol (141). 

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