Diastereomers characteristics

Figures 46 and 47 show the effect of successive incorporation of meso-diastereomer on the phase transition characteristic of the ( )-isomer as the concentration of meso-isomer increases, the phase transition surface pressure IT occurs at higher n. The same result was found (Arnett et al., 1988b) for mixtures of ( )- and meso-C-15 6,6 and C-15 9,9, which are not shown here. According to the surface phase rule, this is indicative of...

The ability of a chiral molecule to distinguish between the enantiomers of a second (different) chiral molecule was defined in Sect. II as a diastereomer discrimination. This phenomenon may be observed in a mixed monolayer of two chiral surfactants and may also occur when a chiral substance is dissolved in the aqueous subphase under the monolayer of a second chiral substance. As before, examples of such chiral discrimination would not include those whose difference in monolayer behavior results only from the gross structural differences of diastereomers such as the different force-area characteristics exhibited by mixed monolayers of l-oleoyl-2-stearoyl-3-s -phospha-tidylcholine with epimeric steroids (120). The relevant experiment, that of comparing the monolayer behavior of mixed monolayers of cholesterol with enantiomeric phospholipids, has been reported (121). As might be anticipated from our previous discussion of... 

The inherent difficulty in analyzing enantiomers arises from the well-known fact that apart from their chiroptical characteristics, optical isomers have identical physical and chemical properties in an achiral environment (assuming ideal conditions). Therefore, methods of distinguishing enantiomers must rely on either their chiroptical properties (optical rotation, optical rotatory dispersion, circular dichroism), or must employ a chiral environment via diastereomer formation or interaction. Recently, it has become increasingly clear that such diastereomeric relationships may already exist in nonracemic mixtures of enantiomers via self-association in the absence of a chiral auxiliary (see Section 3.1.4.7.). 

Eq. 52 and 53 demonstrate remarkable characteristics of this [3 + 2]-cycloaddition starting with a pure diastereomer 130, two stereoisomeric cyclopentanes 131 are obtained. This stereorandom outcome is most simply rationalized assuming a stepwise mechanism with a 1,5-zwitterion as an intermediate in the cycloaddition. The vinylcyclopropane 132 only gives five-membered ring products 133 and no cyclo-heptene derivative, which would result from a conceivable [5 + 2]-cycloaddition. Less activated olefins or cyclopropanes do not undergo a similar [3 + 2]-cycloaddition. Due to the specific substitution pattern, the cyclopentane formation from these siloxycyclopropanes is of no preparative value. 

Two alternative postulates for enantioselection may be proposed. In the first all four diastereomers are formed in varying amounts, and the relative amounts are determined by their respective thermodynamic stabilities. Assuming approximately equal amounts of each diastereomer to be present, if one of them has a transition state that is about 2.5 kcal lower in energy than the transition states of the others, more than 90% enantioselection for 9.51 would result. The proposed mechanism for enantioselection is thus similar to that of asymmetric hydrogenation (see Section 9.3.1). In the second postulate only one such diastereomer is produced that is, the thermodynamic stability of one of the diastereomers is higher than that of the others. The stable diastereomer owing to its steric and electronic characteristics is converted to 9.51 in an enantioselec-tive manner. 

Usually the H NMR spectra of the aggregates (135)3-(136)6 are simple, exhibiting for instance only two sharp singlets for the imide protons of the barbiturate (at 13-14 ppm in CDC13). This strong downfield shift is characteristic for the melamine-barbiturate hexamer, while the simple pattern in general is only in agreement with the formation of the chiral assembly with an overall D3 symmetry. However, the simultaneous formation of all three supramolecular diastereomers was detected by H NMR spectroscopy in the case of sterically hindered cyanurates... 

Characteristic of such dienes, a-keto-p,7-unsaturated esters such as (3 see also Table 3 and Table 5) exhibit good thermal reactivity toward simple vinyl ethers in [4 + 2] cycloaddition reactions that proceed with exclusive regiocontrol predominately through an erulo transition state. The endo selectivity increases as the reaction temperature is decreased and both the reaction rate and the endo selectivity increase as the reaction pressure is increased (Figure 3, Table 3). The substantial increase in the diastereoselectivity of the pressure-promoted [4 + 2] cycloaddition reaction of (3) with a cis 1,2-disub-stituted dienophile has been attributed to the additional differences in the volume of activation between the reaction paths leading to the endo and exo diastereomers due to the additional cis C-2 dienophile sub-... 

The four stereoisomers can be divided as shown into two pairs of enantiomers, where the (R )-(S) and (S,S)-(9) stereoisomers are enantiomers of one another, and the (S,R)-(10) and (i ,5)-(ll) stereoisomers are also an enantiomeric pair. The stereoisomers that do not have an enantiomeric relationship to one another, such as (i ,i )-(8) and (JB,S)-(11) are known as diastereomers. Like enantiomers, these molecules are not superimposable on one another, but unlike enantiomers, they do not exhibit the same physical, chemical, and spectral characteristics. Thus, they have different melting/boiling points, lipid solubility. 

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