Chiral compounds meso form

Although four is the maximum possible number of isomers when the compound has two chiral centers (chiral compounds without a chiral carbon, or with one chiral carbon and another type of chiral center, also follow the rules described here), some compounds have fewer. When the three groups on one chiral atom are the same as those on the other, one of the isomers (called a meso form) has a plane of symmetry, and hence is optically inactive, even though it has two chiral carbons. Tartaric acid is a typical case. There are only three isomers of tartaric acid a pair of enantiomers and an inactive meso form. For compounds that have two chiral atoms, meso forms are found only where the four groups on one of the chiral atoms are the same as those on the other chiral atom. 

If the problem were to partition a set of carbon compounds into two equivalence classes, of which one contains only chiral molecules and the other one only achiral ones, it could not be solved with the criterion of asymmetric C-atoms. In the first case, one would assign meso-forms like 9 and compounds with pseudo-asymmetric 22> C-atoms, such as 11, to the chiral equivalence class, and in the second, chiral molecules like 12 would remain in the achiral subset. However, the latter class would be devoid of chiral molecules, if the compounds under consideration have been confined to molecules with free rotation about all C—C bonds. 

Buta-1,3-diene (10.101, Fig. 10.24) is a gaseous chemical used heavily in the rubber and plastics industry, the presence of which in the atmosphere is also a concern. Butadiene is suspected of increasing the risks of hematopoietic cancers, and it is classified as a probable human carcinogen. Butadiene must undergo metabolic activation to become toxic the metabolites butadiene monoepoxide (10.102, a chiral compound) and diepoxybutane (10.103, which exists in two enantiomeric and one meso-form) react with nucleic acids and glutathione [160 - 163], as does a further metabolite, 3,4-epoxybutane-l,2-diol (10.105). Interestingly, butadiene monoepoxide is at least tenfold more reactive than diepoxybutane toward nucleic acids or H20. Conjugation between the C=C bond and the oxirane may account for this enhanced reactivity. 

Again, applying the principle of a summarized chirality as governing the sequence of elution of the 5 peaks of G1 leads to the distribution of components shown in Fig. 28 extending the information from Fig. 27 by addition of mirror planes (dashed lines), indicating equivalent identities (vertical triple lines) and by enclosing meso forms and racemates in boxes to symbolize the relative molar ratios of the various forms. The compounds within one box relate to those in the other boxes as diastereoisomers. 

Asymmetric synthesis starts with a prochiral compound. This is a compound which is not chiral, but can be converted into a chiral compound by a chiral (bio) catalyst. Subsequently, two types of prochiral compounds can be distinguished The first one has a stereoheterotopic face (which usually is a double bond) to which an addition reaction takes place. An example is the conversion of the prochiral compound propene into 1,2-epoxypropane (which has two enantiomers, of which one may be preferentially formed using an enantioselective catalyst). The second type of prochiral compound has two so-called enantiotopic atoms or groups. If one of these is converted, the compound becomes chiral. Meso-compounds belong to this class. Figure 10.5 and 10.6 show some examples of the different types of asymmetric catalysis with prochiral compounds. 

Similarily, the 4,14-dicarboxylic acid 56 with C2-symmetry could also be resolved via its 1-phenylethylamine salts and its configuration unambiguously correlated with the monocarboxylic acid 55 through the monobromo derivative 5878). Accordingly 55 and 56 with the same sign of optical rotation have the same chirality. Many racemic and optically active homo- and heterodisubstituted 4,12- and 4,14-disubstituted [2.2]metacyclophanes have been prepared and chemically correlated 78,79) mainly to study their chiroptical properties78). Whereas 4,12-homodisubstituted compounds have a center of inversion ( -symmetry) and are therefore achiral meso-forms , the corresponding 4,14-isomers are chiral with C2-symmetry. All heterodisubstituted products are chiral (Q-symmetry see also Section 2.9.4 for the discussion of their chiroptical properties and their use as models for the application of the theory of chirality functions). 

The idea that for every n chiral centers there can be 2M different configurations will be true only if none of the configurations has sufficient symmetry to be identical with its mirror image. For every meso form there will be one less pair of enantiomers and one less total number of possible configurations than is theoretically possible according to the number of chiral centers. At most, one meso compound is possible for structures with two chiral centers, whereas two are possible for structures with four chiral centers. An example is offered by the meso forms of tetrahydroxyhexanedioic acid which, with four chiral atoms, have configurations 28 and 29 ... 

C is correct. Compound E is achiral, and is not the meso compound because D Is (see question 26). Answer A is meso. Answer choice B could not have been formed because there is a change in relative configuration about the original chiral compound. Answer D is chiral. Only answer choice C is left. 

With your models, construct a pair of enantiomers. From each of the models, remove the same common element (e.g., the white component) and the connecting links (bonds). Reconnect the two central carbons by a bond. What you have constructed is the meso form of a molecule, such as meso-tartaric acid. How many chiral carbons are there in this compound (5a) ... 

Diastereoisomerism is encountered in a number of cases such as achiral molecules without asymmetric atoms, chiral molecules with several centers of chirality, and achiral molecules with several centers of chirality (meso forms). Such cases can be encountered in acyclic and cyclic molecules alike, but for the sake of clarity these two classes of compounds will be considered separately. 

The reaction of 13, in which the (3-pyridyl)carbonyloxy groups can be either in syn or in anti position, with spiro[cyclopropene-3,9 -fluorene] creates two new stereogenic centers.35 Thus two diastereomers are possible for each the syn- and the anri-isomer which form a pair of C2-symmetrical enantiomers (R,R/S,S) and a Cs- or Cj-symmetrical meso form (R,S) n The resulting calixarenes 14, bearing dihydroindolizine units, were studied as chromogenic compounds ( calixo-chromes ) in quenching experiments not related to their chirality. 

The 2R,3S and 2S,3R structures are identical because the molecule has a plane of symmetry and is therefore achiral. The symmetry plane cuts through the C2-C3 bond, making one half of the molecule a mirror image of the other half (Figure 9.11). Because of the [ lane of symmetry, the molecule is achiral, despite the fact that it has two chirality centers. Compounds that are achiral, yet contain chirality centers, are called meso (me-zo) compounds. Thus, tartaric acid exists in three stereoisomeric forms two enantiomers and one meso form. 

An interesting synthetic and structural investigation deals with the chiral configuration of cyclotriphosphazenes carrying macrocyclic substituents. Two configurations of compound (83) could be isolated from a reaction mixture of (82) and piperazine, viz. a meso- and racemate-form as proven by X-ray analysis. Further aminolysis yielded two meso-forms of (84), one with a plane of symmetry, the other with a center of symmetry. The results are consistent with inversion of configuration at a P(OR)Cl center for each substitution step going from (82) to (84). The patterns of the P NMR spectra are consistent with the X-ray structures. ... 

More elaborate molecules can also have a plane of symmetry. For example, there are only three stereoisomers of tartaric acid (2,3-dihydroxybutanedioic acid). Two of these are chiral but the third is achiral. In the achiral stereoisomer, the substituents are located with respect to each other in such a way as to generate a plane of symmetry. Compounds that contain two or more stereogenic centers but have a plane of symmetry are called meso forms. Because they are achiral, they do not rotate plane polarized light. Note that the Fischer projection structure of meio-tartaric acid reveals the plane of symmetry. 

The reactions of 24 with ammonia or hydrazine are most unusual. Although the correspondingly substituted disilene does not react, the diene participates in spontaneous 1,2-additions even at room temperature to furnish the 1,4-diamino- (34) or 1,4-dihydrazinotetrasilane (35) in almost quantitative yields. On account of the presence of two stereogenic centers in these molecules the existence of a diastereomeric meso form in addition to the enantiomeric R,R and S,S form is possible. The X-ray crystallographic analyses of both compounds revealed the existence of conglomerates of enantiomerically pure substances [18, 19], Although the individual molecules of the tetrachlorotetrasilane 36 are also chiral, this compound crystallizes as a racemate [18]. 

Inositols are cyclohexanehexols that naturally occur as free, methylated, or phosphorylated forms. Of the nine possible isomers (Figure 2A), three are synthetic epi-, alio-, and ds-inositol), seven are optically inactive as meso forms, and two are enantio-morphs (d- and L-chiro inositol). However, phosphorylation or methylation of the inositol isomer most widely distributed in the nature, myo-inositol, at one of the hydroxyl groups 1, 3, 4, or 6 lead to chiral compounds, as shown in Figure 2B. Inositol... 

The reason for the lack of chirality of the third stereoisomer is that the two asymmetric carbons are located with respect to each other in such a way that a molecular plane of symmetry exists. Compounds that contain asymmetric carbons hut are nevertheless achiral are called meso forms. This situation occurs whenever pairs of asymmetric centers are arranged in the molecule in such a way that a plane of symmetry exists. 

One of the most useful organic reactions discovered in the last several decades is the titanium-catalyzed asymmetric epoxidation of primary aUyUc alcohols developed by Professor Barry Sharpless, then at Stanford University. The reagent consists of tert-butyl hydroperoxide, titanium tetraisopropoxide [Ti(0-iPr)J, and diethyl tartrate. Recall from Section 3.4B that tartaric add has two chiral centers and exists as three stereoisomers a pair of enantiomers and a meso compound. The form of tartaric add used in the Sharpless epoxidation is either pure (+)-diethyl tartrate or its enantiomer, (-)-diethyl tartrate. The fert-butyl hydroperoxide is the oxidizing agent and must be present in molar... 

In Problem 7.6(b), we noted that CM-l,2-dibromocyclobutane was not a chiral compound, because it has a plane of symmetry. However, it does have two asymmetric carbon atoms, each substituted by four different groups, Br, H, CHj, and CHBr. There are many such compounds, called meso-compounds. If we consider 2,3-dibromobutane, 7.56, there are two chiral centers, and thus, there should, in principle, be 2, that is, 4, stereoisomers. However, the mirror images, 7.57, are actually identical, and the plane of symmetry should be obvious. Note that for this type of molecule, it is easiest to see if a compound is meso when it is drawn in an eclipsed sawhorse form. 7.58 and ent-7.58 are true enantiomers and not superimposable. Thus, for this compound, there are three, not four, distinguishable stereoisomers. 

Compounds containing several chiral centers can exist in different stereoi-someric forms. Various stereoisomers usually possess different physicochemical properties, except for isomers that are enantiomers (the remaining isomers with different properties are diastereoisomers). If a molecule contains two identical chiral centers (as in, e.g., 2,3-butanediol, 2,3-dichlorobutane, or tartaric acid), it can occur in the meso form that is not optically active, or in the form of two optically active enantiomers. If a molecule contains two different chiral centers, then it exists as two enantiomeric pairs that are diastereoisomeric with respect to each other (Figure 2.12). 

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