Quasi-racemate method

The quasi racemate method proved very useful for the steric correlation of these compounds with the corresponding benzene deriva-... 

In addition, the steric configuration can be obtained by Raney nickel desulfurization to optically active aliphatic acids of known con-figuration. 2 2 Combined with the quasi racemate method this... 

CD and ORD measurements are a useful tool in studying configuration and conformation. The configuration of optically active 4,4 -dicarboxy-2,2, 5,5 -tetramethyl-3,3 -biselenienyl (1, X = Se) relative to its thiophene analog (1, X = S), was determined by reducing both to the corresponding 4,4 -dihydroxymethyl derivatives (2), which could be related to each other by the quasi-racemate method and by circular dichroism studies.15 Compounds 1 have also been related to 3,3, 6,6 -tetramethyl-2,2 -diphenic acid (3), and it was found that the levorotatory form of 1 (X = Se, S) and the dextrorotatory form of 3 have the -configuration.16... 

The quasi-racemate method is most interesting in the sense that it may be applied to compounds that can not be correlated by direct chemical reactions. 

METHODS BASED ON MEASURING OPTICAL ACTIVITY The isotopic quasi-racemate method (IQRM)... 

Another polarimetric method for the accurate determination of KIEs bears a strong resemblance to the isotopic quasi-racemate method, described above. In this method, Bach and co-workers (1991) utilized what they called isotopically engendered chirality to determine the primary deuterium KIE for an elimination reaction. In theory, the method can be used for any reaction where a substrate with a plane of symmetry yields, under normal conditions, a racemic mixture. For instance, if the plane of symmetry in the unlabelled... 

In addition to other methods, the quasi-racemate method was used (98) to determine the absolute configuration of iV-tosyl methyl-p-tolylsufimide 91. 

The oxidative conversion of the (-)-(5 )-A-tosyl methyl-p-tolyl-sulfimide 91 to the corresponding (-)-(/ )-sulfoximide 151 is of considerable interest because the absolute configurations of the substrate and product were established by the quasi-racemate method and by X-ray analysis (98), respectively. Therefore, the stereochemical result of this experiment indicates unequivocally that oxidation of chiral tricoordinate sulfur compounds proceeds with retention of configuration. 

The quasi racemate method proved very useful for the steric correlation of these compounds with the corresponding benzene derivatives,as geometrically the differences between these groups are so small that the limits of isomorphic exchar eability are not often exceeded/ This confirms the view that the largest similarity between the thiophenes and benzenes is related to such physical properties as shape, size, and molecular weight, which, for instance, are reflected in the melting and boiling points. 

Table 3.3 Kinetic isotope effects for hydrolysis of various glycosyl derivatives. The isotopic quasi-racemate method was used for all measurements except those for AMP, where the remote label method was used.

Hakansson et al. (75CS131) reported the determination of the absolute configurations of 4,4 -dicarboxy-2,2, 5,5 -tetraalkyl-3,3 -bithienyl (69 and 70), and 4,4 -dicarboxy-2,2, 5,5 -tetramethyl-3,3 -biselenienyl (71). Compound 69 was related by chemical methods to hexamethyl-3,3 -bithie-nyl, the absolute configuration of which was known. Compounds 69 and 70 were related by the quasi-racemate method. Biselenienyl 71 and bithienyl 69 had previously been related by the same method. They conclude that the levorotatory forms of 69,70, and 71 have the (R)-configuration. CD studies were carried out using for comparison 3,3, 6,6 -tetramethyl-2,2 -diphenic acid. 

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