Stereoisomers stereogenic centers

Thiol (Section 15 13) Compound of the type RSH or ArSH Th reo (Section 7 11) Term applied to the relative configuration of two stereogenic centers within a molecule The threo stereoisomer has like substituents on opposite sides of a Fischer projection... 

Diastereomers include all stereoisomers that are not related as an object and its mirror image. Consider the four structures in Fig. 2.3. These structures represent fee four stereoisomers of 2,3,4-trihydroxybutanal. The configurations of C-2 and C-3 are indicated. Each stereogenic center is designated J or 5 by application of the sequence rule. Each of the four structures is stereoisomeric wife respect to any of fee others. The 2R R and 25,35 isomers are enantiomeric, as are fee 2R, iS and 25,3J pair. The 21 ,35 isomer is diastereomeric wife fee 25,35 and 2R,3R isomers because they are stereoisomers but not enantiomers. Any given structure can have only one enantiomer. All other stereoisomers of feat molecule are diastereomeric. The relative configuration of diastereomeric molecules is fiequently specified using fee terms syn and anti. The molecules are represented as extended chains. Diastereomers wife substituents on the same side of the extended chain are syn stereoisomers, whereas those wife substituents on opposite sides are anti stereoisomers. 

Since chirality is a property of a molecule as a whole, the specific juxtaposition of two or more stereogenic centers in a molecule may result in an achiral molecule. For example, there are three stereoisomers of tartaric acid (2,3-dihydroxybutanedioic acid). Two of these are chiral and optically active but the third is not. 

The reason that the third stereoisomer is achiral is that the substituents on the two asymmetric carbons are located with respect to each other in such a way that a molecular plane of symmetry exists. Compounds that incorporate asymmetric atoms but are nevertheless achiral are called meso forms. This situation occurs whenever pairs of stereogenic centers are disposed in the molecule in such a way as to create a plane of symmetry. A... 

Diels-Alder reaction and. 494-495 El reaction and, 392 E2 reaction and, 387-388 R.S configuration and, 297-300 S 1 reaction and, 374-375 S -2 reactions and, 363-364 Stereogenic center, 292 Stereoisomers, 111 kinds of, 310-311 number of, 302 properties of, 306 Stereospecilic, 228, 494 Stereospecific numbering, sn-glycerol 3-phosphate and, 1132 Steric hindrance, Sjvj2 reaction and, 365-366 Steric strain, 96... 

Unsaturated -lactone 34 adopts a well-defined conformation and provides a suitable platform for the introduction of the stereogenic center at C-24 (monensin numbering). Catalytic hydrogenation of the carbon-carbon double bond in 34 takes place preferentially from the less hindered side of the molecule and provides an 8 1 mixture of stereoisomers in favor of 35 (100% yield). Cleavage of -lactone 35 with concentrated hydriodic acid at 130°C, followed by treatment of the resultant iodide 36 with triphenylphosphine, completes the synthesis of intermediate 19. 

Conduritols and inositols are cyclic polyalcohols with significant biological activity. The presence of four stereogenic centers in the stmcture of conduritols allows the existence of 10 stereoisomers. Enzymatic methods have been reported for the resolution of racemic mixtures or the desymmetrization of meso-conduritols. For example, Mucor miehei lipase (MML) showed enantiomeric discrimination between all-(R) and all-(S) stereoisomers ofconduritol E tetraacetate (Figure 6.52). Alcoholysis resulted in the removal of the four acetyl groups ofthe all-(R) enantiomer whereas the all-(S) enantiomer was recovered [141]. 

Owing to the concerted mechanism, chirality at C(3) [or C(4)] leads to enantiospecific formation of new stereogenic centers formed at C(l) [or C(6)].203 These relationships are illustrated in the example below. Both the configuration of the new stereocenter and the new double bond are those expected on the basis of a chairlike TS. Since there are two stereogenic centers, the double bond and the asymmetric carbon, there are four possible stereoisomers of the product. Only two are formed. The Zs-double bond isomer has the 5-con figuration at C(4) and the Z-isomer has the -configuration. These are the products expected for a chair TS. The stereochemistry of the new double bond is determined by the relative stability of the two chair TSs. TS B is less favorable than A because of the axial placement of the larger phenyl substituent. 

The importance of the three stereogenic centers became evident as two of the stereoisomers [(2S,3S,7S) and (2S,3R,7R)] were recognized early as sex pheromones and that other enantiomers and diastereoisomers were often found to be inhibitory to the attractive response. Recently, the sawfly pheromone field has undergone a major advance with the recognition that several sawfly species synthesize and utilize sex pheromones of different structural types than the 3,7-dimethylpentadecan-2-ols. Shorter and longer chain lengths (undecan-2-ols 2 and tridecan-2-ols 3) and an additional methyl group in position 9 or 11 (4 and 5) characterize these new pheromone discoveries (Fig. 1). With an ad-... 

Lactide (LA), the cyclic diester of lactic acid, has two stereogenic centers and hence exists as three stereoisomers L-lactide (S,S), D-lactide (R,R), and meso-lactide (R,S). In addition, rac-lactide, a commercially available racemic mixture of the (R,R) and (S,S) forms, is also frequently studied. PLA may exhibit several stereoregular architectures (in addition to the non-stereoregular atactic form), namely isotactic, syndiotactic, and heterotactic (Scheme 15). The purely isotactic form may be readily prepared from the ROP of L-LA (or D-LA), assuming that epimerization does not occur during ring opening. The physical properties, and hence medical uses, of the different stereoisomers of PLA and their copolymers vary widely and the reader is directed to several recent reviews for more information.736 740-743... 

The essence of asymmetric synthesis is producing a new stereogenic center in such a manner that the product consists of stereoisomers in unequal amount. In most cases, this can be achieved by the formation of a new sp3 stereocenter. There is also another type of asymmetric reaction in which the employed substrates contain either a stereogenic unit or a pro-stereogenic unit apart from the functional group, and asymmetric synthesis occurs even though the nature of the reaction is not directly related to the newly formed sp3 stereocenter. The Wittig reaction is invoked for the asymmetric synthesis of such molecules.47... 

In the enantioselective synthesis, the asymmetry (i.e., the stereoselectivity) is induced by the external chiral catalyst, while the diastereoselective synthesis does not require a chiral catalyst. The stereogenic center already present in the molecule is able to induce stereoselectivity, assuming that the synthesis starts with a single enantiomer. For instance, imagine that an a,/ -substituted product is formed, and that the reactant already contains a stereogenic carbon at a. If the reaction of (aS) leads, e.g., largely to (aS, / R) and hardly to the (aS, /IS) diastereomer (i.e., stereoisomers that are not mirror-images of each other), the reaction is diastereoselective (Scheme 14.2). 

The tandem Diels-Alder reaction of bisdiene 129 and bisdienophile 128 led to the production of a highly complex molecule as a single stereoisomer. Three new rings and six new stereogenic centers were created during the process [108]. 

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-... 

Reduction of chiral ketoximes results in formation of a new stereogenic center. Although mixtures of stereoisomers are generally obtained, kineticaUy controlled reduction of cyclic oximes (e.g. 86, equation 59 and 87, equation 60) with sodium cyanoborohydride can proceed with high diastereoselectivity Stereoselectivity in these reactions closely resembles that of reduction of ketones with complex hydrides featuring attack from the least hindered side. 

Due to its stereogenic center, lactic acid exists in two enantiomeric forms (d- and L-lactic acid), leading to three different lactide stereoisomers (d-, l- and mesolactide). Depending on the relative amounts of the different stereoisomers in the final polyester, the crystallinity of the resulting PLA is heavily influenced and this way the properties of the polymer can be adjusted to satisfy the needs of different applications [18-20]. 

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