Meso configuration

In contrast to the general case of 1, we now consider molecules with two stereogenic centres, but which carry an identical set of substituents at each carbon. We then analyse the relative configurations of these substituents and the stereochemical consequences. 

As a prototype molecule, tartaric acid (2 2,3-dihydroxybutanedioic acid), shown here without stereochemical specification, is chosen. Tartaric acid is a compound of considerable historical importance (Section 3.6), and derivatives of tartaric acid are still compounds of contemporary research interest (see Seebach et al.1). (2i ,3i )-(+)-Tartaric acid is shown in sawhorse projection in 3. 

With the aid of molecular models verify that the configurations of C(2) and C 3) are correct. 

The enantiomer of 3 is shown in 4 and is (2S, 3S)-(-)-tartaric acid. Of course, in principle any conformation of 3 may be used, with its mirror image, to demonstrate that the compounds in the pair are enantiomers however, use of the eclipsed forms 3 and 4 is visually easier. 

Fischer projections of 3 and 4 are given in 5 and 6, respectively, and the corresponding Newman projections, looking along the C(2)-C(3) bond, are shown in 7 and 8, but now with the rotation of the back groups around the C(2)-C(3) bond such that the molecules are shown in staggered and visually less cramped conformations. 

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An amount of 16 g. of the glycol (m. p. 18°) dissolved in 250 ml. of water was oxidized with 18 g. of silver chlorate and 0.3 g. of osmic acid. The reaction mixture yielded 3 g. of allitol and no D,L-mannitol. We may therefore assign the meso configuration to the divinylglycol melting at 18°, since on hydroxylation it yielded allitol, but not D,L-mannitol. 

Another proof of the configuration of D-mannitol and also of D-manno-n-manno-octitol (XVI), which is likewise dependent on the experimental proof of the equivalent symmetry of D-mannitol is the following. D-Mannose has been converted, by successive cyanohydrin syntheses, first to a mannoheptose and then to a mannooctose which on reduction yielded a mannooctitol whose octaacetyl derivative is optically inactive. (It was not possible to examine the octitol itself because of its very low solubility in water.)87 The meso character of the octaacetate shows that the mannooctitol must possess a meso configuration, with a plane of symmetry between carbon atoms 4 and 5. To write its formula, the hydroxyl at carbon atom 7 is placed on the... 

Horse liver alcohol dehydrogenase (HLADH (E.C. 1.1.1.1), commercially available) is a well-documented enzyme capable of catalyzing the enantioselective oxidation of acyclic and cyclic meso-configurated dimethanol derivatives to chiral lactols and further to the corresponding chiral lactones with high enantioselectivity and in high yield (Table 11) 162 ,69. Incases where the two enantiomeric lactols are formed, a kinetic enantiomer separation can occur in the second oxidation step166. 

Exercise 5-14 From the compounds listed select all those that may have achiral meso configurations and draw the configurations for each of them. 

The fatty acid free derivative, II, is usually optically active and strongly suggests that the two glycerophosphate residues have the same stereochemical configuration. Otherwise this derivative would have had a meso configuration (with no optical activity). The peroxidation of II will yield 2 mol of formaldehyde per mole of phosphorus together with a dialdehyde, III, which on reaction with dimethylhydrazine will yield glycerol-1,3-diphosphoric acid, IV. 

Meso configurations can have enantiomeric conformers in crystals. Ribitol and xylitol have meso configurations and are optically inactive in solution. In the crystalline state, they have the bent-chain enantiomorphic conformations. Xylitol crystallizes in space group P212121, so each crystal contains either only the optically active left-handed or the optically active right-handed conformers. There are, of course, an equal number of left- and right-handed crystals in any one batch, which on dissolution give no optical activity.1... 

Reaction of RuCl3-3H20 with [18]aneSe in DMF/methanol affords [Ru([18]aneS6)] , the single-crystal X-ray structure of which shows (Fig. 76) a meso configuration with hexathia coordination at Ru(II),... 

Pure allitol (meso configuration m. p. 150-151°) was described for the first time by Lespieau and Wiemann, who synthesized it by the hydroxylation of one of the stereoisomeric forms of divinylglycol. Its structure was established rigorously by Steiger and Reichstein when they demonstrated its identity with the product obtained by the reduction of D-allose. 

Fig. 1. Skeletal structure for [Cu o-C6H4(TeMe)2l2] which has been shown by X-ray crystallography to adopt a meso configuration, but one that deviates significantly from ideal tetrahedral geometry.

There are two reports of X-ray diffraction studies involving 1,4-thiazines. Thus the meso configuration of the bis(thiazine) 24 was established by the observation that the molecule possessed a center of symmetry. The crystal structiu-e of the thiazinium chloride 23 has also been described however. 

Amino-acid Complexes. A small, but reproducible, stereoselective effect has been observed in both the free energy and enthalpy changes associated with formation of [Ni(L,L-methioninate)2]. The meso-complex [Ni(o-Met)(L-Met)] is more stable in AH than the optically active NiL2 by 1.0 (0.1) kJ mol" L The stereoselectivity is attributed to terdentate co-ordination and so supports weak co-ordination of the thioether group. Formation constants of nickel(ii) and copper(ii) with N -benzyl-L-histidine and N N -dibenzyl-L-histidine and of the ternary complexes with d- and L-histidine have been measured. Stabilization of ternary complexes is small but significant stereoselectivity is found with ternary nickel complexes, when the meso configuration is preferred in each case with copper, stereoselectivity is small or absent. The i.r. spectra of tra s-[Ni(Gly)2(H20)2] and its 0-, N-, 1- C-, and 2- C-labelled... 

Numerous meso-configured or otherwise prochiral substrates, preferentially containing enantiotopic methoxycarbonyl groups, have been converted by a pig liver esterase- or lipase-catalyzed enantioselective hydrolysis in water to chiral monoesters (see Sect. 11.1.1.1.1., Tables 11.1-1 to 11.1-4 and Sect. 11.1.1.1.5, Tables 11.1-10 to 11.1-12). In nearly all cases investigated thus far the pig liver esterase-catalyzed hydrolysis of the substrate diester S terminates at the stage of the enantiomeric monoesters P and ent-P. In this case, where the products P and ent-P are not transformed further, the irreversible enantiotopos-differentiation may be described by the process depicted in Scheme 11.1-10167 691. 

To a limited extent and with only moderate success, meso-configured glutarates and succinates have been subjected to pig liver esterase-catalyzed hydrolysis with... 

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