Racemic diastereoisomers

Products containing as ligands PPh(l-Np)2 have two centers of chirality and two racemic diastereoisomers have been separated. The reaction of optically active PMePh 1 -Np leads to a racemic mixture since the phosphine is racemized under UV irradiation ... 

In the case of asymmetric germanium, six racemic diastereoisomers are formed, but until now none of them could be isplated by crystalli ation. 

In analogy with iron-catalyzed Barbier-type reactions with Sml2 (cf. Scheme 8.5), intramolecular nucleophilic acyl substitutions (SNt) can be used to prepare cyclic ketones from esters [50]. An illustrative example is shown in Scheme 8.13 [51], Again, tris(l,3-diphenyl-l,3-propanedionato)iron(III) [Fe(dbm)3] is used as the catalyst. Compound 40 is obtained as one racemic diastereoisomer. 

The symbols (2RS,3RS) and (2RS,3SR) can also be used for the racemic diastereoisomers of compounds with two chiral centres though CAS does not use this system. Where the absolute configuration appears to be unknown, asterisked symbols, e.g. (2R, 3R ) may be used. 

A synthesis of the racemic diastereoisomers of evoninic acid has been reported by Pailer and Pfleger 64). 2-Ethyl nicotinate was brominated to form 78, and the... 

Detailed analyses of intramolecular structures are possible. Comparison of NMR and fluorescence data shows meso- and racemic diastereoisomers are found from 2,4-di(2-pyrenyl)pentane 24 jhe polarization of monomer and excimer of 4,9, disubstituted pyrenes have been measured in nematic liquid crystals 25 Quenching of pyrene fluorescence by alcohols in cyclodextrin inclusion complexes has also been studied in detail 26 Solvent effects on the photophysical properties of pyrene-3-carboxylic acid has been used to measure the pJJ, in different solvents 27 Geminate recombination in excited state proton transfer reactions has been studied with... 

The first model study to show a substantial difference in the fluoresecence spectrum of the meso and the racemic diastereoisomer was reported in 1966 by Bovey and Longworth . Based on H-nmr data information concerning the conformational distribution within each... 

The molecular dynamics of the racemic diastereoisomer upon excitation are however more complicated than that of the meso dlastereolsomers due to the presence of two ground state conformations. Since the decay laws used for meso diastereisomers can also be applied to the racemic diastereoisomer, all the rate constants describing the equilibrium between the ground state conformations must be large compared to the rate constant of excimer formation. The question however still remains from which conformation does excimer formation take place. It is not likely that it occurs directly from the GG conformation. This would mean a simultaneous rotation around two bonds and would be associated with a relatively high activation barrier. One possibility is that excimer formation takes place from the TT conformation. This possibility is depicted in scheme 3. 

I The results obtained for the racemic diastereoisomers lj,c and indicate that even when the rate of conformational change between the different chain conformations is much faster than the rate of excimer formation, resulting in fluorescence decays similar to those of the meso diastereoisomers it is possible to extract some information on the conformational distribution of the chain from the obtained data. 

Fission of the 1,4-biradical affords acetophenone in competition with cycliz-ation. The products are obtained as racemic diastereoisomers. The other ether derivatives (41, n = 2) are also reactive and undergo formation of 1,5-biradicals on irradiation. The selectivity of the reactions was investigated in the presence of chiral hosts. The best yields were obtained for the series (41, R = H, n = 2) using the host molecule (44) in toluene at — 45°C. This gave a 70% yield of products... 

Therefore, it seems unnecessary to present here another classification, and we will only report some examples of reactions illustrating stereoselective synthetic approaches relevant to the topic of this chapter, i.e., the preparation of enantiomerically enriched compounds. Stereoselective reactions leading to unequal amounts of racemic diastereoisomers will not be considered. 

The total syntheses of (+)-muscarine chloride, its optically active or racemic diastereoisomers, and analogs have been carried out independently in several laboratories. In most cases carbohydrates or, rarely, amino acids were used as chiral starting materials, enabling stereoselective synthesis. The different synthetic approaches as well as other aspects of muscarine chemistry, occurrence, and pharmacology were discussed in an excellent review by Wang and Joullie (18) in this treatise and earlier by Wilkinson (23). 

There is a simplification intrinsic in this, in that G-5 actually has two centers of asymmetry (the a-carbon atom on the amphetamine chain, and the norbornyl area itself) and so it is really a mixture of two racemic diastereoisomers. G-6 would still be a racemate, but it would be only a single compound, as are all the other substituted amphetamine derivatives. 

The example of this reaction earUer in the chapter (p. 856) was this type of reaction. The starting alcohol was racemic and the product was just one racemic diastereoisomer—the all cis compound. But if the starting material had been enantiomerically pure, so would the product. One enantiomer gives one enantiomer of the product the other enantiomer of the alcohol... 

In the following sequence one racemic diastereoisomer of F is formed. Give a structure for F and predict its relative stereochemistry from the subsequent transformations give a structure for G. 

Assuming chelate ring conformations, which are analogous to those predicted to be the most strain free for trans-Co(cyclam)X2 " there are 8 meso and 32 racemic diastereoisomers for the Ni(CTH) system. These arise from the asymmetry of the four nitrogen donor atoms and from the asymmetry of two carbon atoms present in the macrocyclic ligand. The asymmetric carbons occur at positions 7 and 14 in 1,7-CTH (Structure II), and at positions 5 and 14 in 1,4-CTH (Structure I). Consideration of absolute and relative configurations is useful in the identification of various... 

N-meso diastereoisomers and one N-racemic diastereoisomer can occur. Carbon-13 NMR measurements suggest that the complex is a mixture of the possible stereoisomers. Base hydrolysis of [Co([15]aneN5)Cl] is quite rapid with fcoH = 2.45 x 10" M s at 30 C and / = 0.1 M, a result which can be rationalized in terms of the mcr-N3 donor set. " Acid aquation with feaq = 1.66 X lO"" s at 50°C is, as expected, quite slow. 

Reaction between NiCl26H20 and anti-113 followed by the addition of HBF4-etherate, gave [anf/-fll3)Ni](BF4)2. A metathesis of this latter complex with an excess of NaSCN resulted in the formation of the racemic diastereoisomer, anti-11 [106]. 

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