Differentiability enantiomorphs

Let us now differentiate between structures which are asymmetric and dissymmetric. The word asymmetric conveys the idea that the molecule is completely devoid of the elements of symmetry. Dissymmetric on the other hand means not completely devoid of elements of symmetry but possessing so few elements of symmetry that on the whole it will posses two structures which will be the mirror images of each other. Therefore to avoid confusion the term asymmetric is used to cover examples which rotate the plane polarized light. The two forms of an optically active compound are called enantiometers or enantiomorphs or optical antipodes. They are also said to have enantiomeric relationship to each other. 

A clear three-dimensional visualization of the means by which a receptor could differentiate between enantiomers was provided by Easson and Stedman in 1933 [25]. They proposed that three (b, c, d) of the four groups (a, b, c, d) linked to a chiral carbon atom were concerned in the process (either by normal valence forces, or by adsorptive or other forces). The receptor possessed three groups b, c and d for maximum physiological effect, the drug molecule must become attached to the receptor in such a manner that the groups b, c and d in the drug coincide respectively with b, c and d in the receptor. Such coincidence can only occur with one of the enantiomorphs and this consequently represents the more active form of the drug . The interaction (5) and non-interaction (6) were illustrated as follows ... 

Some examples of enantioselective syntheses, carried out in chiral media, have already been given at the end of Section 3.2, which deals with chiral solvents cf. also Table A-2 (Appendix). In general, asymmetric inductions as a result of chiral solvents or chiral cosolvents are disappointingly low [700], The reason for this is that the differential solvation by the chiral solvent of the two enantiomorphic activated complexes which lead to either the (/ )- or (S)-product is not sufficient. That is, the difference in Gibbs energy of activation, AAG = AG — AG -, is not large enough to favour only one of the two enantiomeric products. It should be remembered that a difference of AAG = 10.8 kJ/mol (2.6 kcal/mol) at 20 °C would be sufficient to get an product ratio of 99 1 i.e. an enantiomeric excess ee = 98%). 

As the preceding discussion suggests, conditions may exist under which even the most powerful measuring device available will be incapable of detecting a significant difference between samples of different enantiomeric composition above the noise level of stochastic achirality. We call such a system cryptochiral, because the model demands an excess of one enantiomer over the other in the time domain of observation, while the chirality phenomenon to be observed falls below the threshold of the operational null and thus is undetectable. Note that a cryptochiral substance is operationally indistinguishable from a stochastically achiral one because, at and below the operational null, enantiomorphous systems can be neither differentiated from each other nor distinguished from achiral ones. 

The kinetics of hydrolysis of lactoylcholine—CH3CH(OH)COO(CH2)2 N+(0113)3—and its isomers were studied by Sastrey and White (S3). The rationale for this study stemmed from the possible formation of lactoylcholine from propionylcholine—CH3CH2COO(CH2)2N+ (0113)3—or acrylylcholine—OHLj = CHOOO(OH2)2N+(0113)3—both of which have been isolated from animal tissues (B3, W24). DL-Lactoylcholine was found to be hydrolyzed by cholinesterase at a rate three to four times faster than acetylcholine, but in contrast to acetylcholinesterase, the plasma enzyme did not differentiate between the n- and L-enantiomorphs of lactoylcholine. 

Enclathration is not infrequent and inclusion compounds are becoming a large family of well characterized chemical spedes. TOT ranks among the most attractive host molecules owing to the ability of differentiating between enantiomeric guest molecules in two different enantiomorphous host lattices. Besides, the imusually high propensity of TOT tq form stable clathrates is further illustrated by no less than seven different achiral crystalline forms. 

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