Quasi-racemates

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 methyl esters of stearoylalanine [1] and stearoylserine [2] were considered as quasi-racemate candidates because of their slight structural differences. No quasi-racemate behavior was observed, however, in their force-area isotherms although clear diastereomeric discrimination was seen for this combination (Verbiar, 1983). We have seen no indication of quasi-racemate behavior for any other mixed chiral monolayers. 

A quasi-racemate is a molecular compound that is related to a true racemic compound by a small structural change in one of the enantiomers (Fredga, 1944). 

A quasi-racemate or pseudo-racemate is a true racemate like molecular compound formed between optical antipode of different (but related) compounds. The quasi-racemate also has a melting point ciin c resembling the curve of a true racemate but with quasi-racemic compounds the curves are unsymmetrical, because the melting points of the components are different as shown in Fig. (9.3). The curve A represents the melting point of a true-racemate formed by mixing (+) mandelic acid XXII and (-) hexahydromandelic acid XXIII while B represents that of a mixture of (+) XXII and (+) hexa hydro-mandelic acid XXIII. 

In B the straight line is inclined and so is different from A. Since according to a general rule, the configurations of compounds forming a quasi-racemate must be opposite, in the above example, the acids with the same sign of rotation have the same configuration. 

Therefore, the conditions essential for quasi-racemic mixtures are that ... 

Quasi racemates are formed by members of a homologous series and also by pairs of compounds which differ in that the hydrogen is replaced by a chlorine, bromine or iodine atom (but the molecule must not be too small). 

Further since XXV formed a quasi racemic compound with (+) methyl succinic acid XXVI, they have opposite configurations and so XXIV and XXVI have the same configuration. 

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)... 

The isotopic quasi-racemate or differential polarimetric method is a kinetic... 

In a variation of this method, Tencer and Stein (1978), mixed the isotopic quasi-racemate to near, but not exactly, zero rotation so that at a certain time, tz, the observed optical rotation of the reaction mixture was zero. The equations for this type of kinetic experiment enable one to calculate the difference between the individual isotopic rate constants from tz and the ratio of rate constants (the KIE) from te and tz provided that the ratio of the initial rotations for the two isotopic substrates is known. Usually it is preferable to... 

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... 

Figure 19. curves for quasi-racemic equimolar mixture of (5)-(+)-2-tetra-cosanyl acetate (Fig. 18) and methyl ester of (R)-(-)-2methylhexacosanoic acid (Fig. 22) at various temperatures. From Lundquist (79). Permission of Stockholm IJniversity library. 

Probably the most interesting and important example of diastere-omer discrimination known to us is part of the body of work conducted by Monica Lundquist, whose enormous contribution to the area of chiral discrimination in monolayers is discussed in Sect. V. In her third paper (79), she demonstrated that Fredga s method of quasi-racemates (33) can be appUed to the two-dimensional organization of chiral surfactants in monolayers. 

Thus, chiral discrimination may be observed that differentiates the force-area curves of the enantiomers of some surfactants from their racemic modifications. Apparent phase changes in the monolayer can be related to parallel behavior in the crystalline state through X-ray diffraction and differential scanning calorimetry. Formation of racemic compounds and quasi-racemates can be observed in some cases. 

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. 

QUASI-EQUILIBRIUM ASSUMPTION QUASI-EQUIVALENCE Quasi-racemic compound,... 

This method is based on the polarimetric measurement of the optical activity induced by the KIE in a reaction mixture containing an isotopic quasi-racemate, i.e. an approximately 50/50 mixture of the (+)-H and (-)-D substrate or vice versa, as one of the reactants. Variants of the method were independently reported by Bergson et al. (1977), Nadvi and Robinson (1978) and Tencer and Stein (1978). Later the method was successfully applied, particularly by Matsson and co-workers (Matsson, 1985 Hussgnius etal., 1989 Hussenius and Matsson, 1990) to determine both primary and secondary KIEs in proton transfer reactions, and by Sinnott and co-workers (Bennet et al., 1985 Ashwell et al., 1992 Zhang et al., 1994) to determine both primary and secondary as well as heavy-atom KIEs for reactions of carbohydrate derivatives. 

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