Racemic and meso diastereomers

The produced dibenzylsilyl(3-methyl-cyclopentadienyl)zirconium dichloride 1 exists in racemic and meso forms. Figure 1 shows the H-NMR spectra of zirconocene 1 with different molar ratios of the racemic and meso-diastereomers. 

It must be kept in mind that the ( )-diastereomer is a mixture of enantiomers that were not separable by reverse phase HPLC. Since the method for separation of (+)- and meso-diastereomers (reverse phase HPLC, C-18 column) is based on the difference in conformation between isomers (related ultimately to their differential attraction to the column), it is unlikely that the two enantiomers of the (+ )-diastereomer have different conformations at the air-water interface. It is not known, however, how the energetics of compression and expansion will differ for films cast from either R,R or S,S enantiomer from those cast from the racemic mixture. 

Another example studied is the isomers of the complex derived from racemic and meso forms of 2,3-diaminobutane. The complex is depicted below. With the racemic addition of Eu[(+)fpc]3 gives the two sets of resonances due to R,R- and 5,5-diastereomers. With meso form, a doubling of each spectral peak was observed. The doubling is due to the presence of a meso plane in the molecule which makes each half of the molecule enantiotropic. 

Turning now to chemical methods used for stereochemical studies of carotenoids the modified Horeau method (32) has been successfully used to confirm the configuration of (2/ )- 3, 3-caroten-2-ol [(33), R=H] (38) and to establish the configuration of aleuriaxanthin (36) (34). However, no conclusive results could be obtained for plectaniaxanthin [(35), R = R =H] (28). In principle the Horeau method is based on partial resolution of racemic and meso a-phenylbutyric anhydride by means of an optically active secondary alcohol, which readts preferentially with one diastereomer. The unreacted anhydride is quantitatively determined by an appropriate procedure (32, 68, 94). 

Words—Jaigon.This chapter is filled with jargon Be certain that you learn the difference between enantiomers and diastereomers. Learn also what stereogenic, chiral, racemic, and meso mean. 

Meso compounds contain chirality centers hut are achiral overall because they have a plane of symmetry. Racemic mixtures, or racemates, are 50 50 mixtures of (-I-) and (-) enantiomers. Racemates and individual diastereomers differ in their physical properties, such as solubility, melting point, and boiling point. 

There is considerable evidence for this type of reaction in organoarsenic chemistry and, indeed dissociation-equilibrium was believed to be the cause of racemization of arsonium halides which, for synthetic reasons, usually contained at least one alkyl substituent (often" the benzyl group) on the arsenic atom. In 1939, fractional crystallization was achieved of the diastereomers of the diarsonium picrate 3 . The individual diastereo-mers of the salt, racemic (R, R ) and meso (R, S ), were stable in boiling ethanol, a fact that should have dispelled concerns of dissociation equilibria in halide-free arsonium salts. In 1940, the tetrahedral structure of [AsPh4]I was established by X-ray crystallography. ... 

Very few examples of asymmetric 1,4-induction are reported in connection with the addition of acidic Ti complexes to chiral y-alkoxycarbonyl compounds. According to the Cram model, the chelation is expected to afford a flexible seven-membered ring intermediate, resulting in less efficient induction (equation 34). An early example of asymmetric 1,4-induction is provided by the reaction of o-phthalal-dehyde (96 equation 35) with 2 equiv. of MeTi(OPr )3 which affords an 83 17 molar mixture of racemic- 97) and meso-(98), whereas Ae analogous reaction with MeMgl leads to a 1 1 mixture of the above diastereomers. 

Figure 1. H-NMR spectroscopic measured cyclopentadienyl protons of the racemic (r) and meso (m) diastereomers of dibenzylsilyI(3-methyl-cyclopentadienyl)zirconium dichloride.

Sable and coworkers also synthesized all four of the predicted racemic forms of cyclopentanetetrol (131-134), and the meso diastereomer (130). The all-cfs, meso isomer is unknown. cis-Hydroxylation of the enediols (136 and 137) is effected with permanganate or with silver chlorate-osmium tetroxide Irans-hydroxylation is caused by peroxybenzoic acid. Ihe epoxy group enters ds to the neighboring, free hydroxyl group in the intermediates (136-A, 136-B, and 137-A), according to Henbest s rule. However, prior reaction of the diol structure in the intermediate (136-A) with acetone caused the epoxide group to enter trana,... 

Substituent effects on the rates of elimination of aryl-substituted 1,2-diphenyl-1,2-dihaloethanes are small, suggesting that double bond formation is advanced in the transition structure and that most of the charge is localized on the departing halide ion. ° The debromination of 2,3-dibromobutane with iodide ion is highly stereospecific, with the meso diastereomer giving almost exclusively trans-2-butene, and the racemic diastereomer producing mostly a s-2-butene (Figure 10.38). The results were taken as evidence for a... 

Products of the Cope rearrangement of racemic (top) and meso (bottom) diastereomers of 3,4-di-methyl-1,5-hexadiene through chair transition structures. 

The barbituric acid derivatives, which have been known as hypnotic sedatives [136], were chosen as the first of the heterocycHc 1,3-dicarbonyl compounds for study because they consist of a cyclic diamide structure. The reaction of barbituric acid with 1,1-diphenylethene was conducted in acetic acid at room temperature in air using a catalytic amount of manganese(III) acetate. Surprisingly, solid products were easily obtained as a diastere-omixture, meso and racemic. Although the diastereomers could not be... 

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