Conformational enantiomorphism

In previous articles [103,104] only orientations with plus and minus signs were considered (i.e. no zero orientation values). It was argued that small deformations of a planar arrangement of four atoms will result in very similar conformations with no zero orientations. However, this leads to difficulties when considering symmetry For an achiral conformation, enantiomorphic chiral deformations may exist. In order to avoid losing this chiral/achiral property, we decided to allow zero orientations expUc-itly and even to allow a relatively large tolerance for these orientations. 

These results allowed the proposal, at the beginning of the 1980s, of a different molecular model for cholesteric induction 65,66 This model is sketched in Figure 7.15 in the case when both nematic host and chiral guest have a biaryl structure. Nematic molecules exist in chiral enantiomorphic conformations of opposite helicity in fast interconversion. The chiral dopant has a well-defined helicity (M in Figure 7.15) and stabilizes the homochiral conformation of the solvent In this way, the M chirality is transferred from the dopant to the near molecule of the solvent and from this to the next near one and so on. This... 

A solution or melt of a racemic mixture of enantiomers may crystallize either as a racemic phase or as a mixture of two resolved enantiomorphic phases. The molecules in these two enantiomorphic phases will be exact mirror images of one another. However, a given enantiomer, say R, will have different environments in the racemate and in the resolved crystal and will be conformationally different. Correspondingly, the R molecule in the resolved crystal and the S molecule in the racemate will not be exact mirror images. 

Further evidence upon the role of the 3-methyl substituent in reversed ester of pethidine is available from a recent investigation of enantiomorphs of a- and /3-prodine [280] (Table 5.11). It is to be noted that activity is governed by C-4 rather than C-3 geometry since the two more active optical antipodes (dextro a- and /3-prodine) have identical C-4 but different C-3 configurations. This result supports the view that the importance of the 3-methyl substituent lies in its influence upon the conformation of the molecule. Its effect upon activity in (-i-)-a-prodine where it is equational and favours a chair conformation is minor as seen by the similar potencies of (+)-a-prodine and 3-desmethylprodine. Both... 

Chains of identical chirality and conformation are isomorphous. Chains of opposite chirality but equivalent conformation are enantiomorphous. 

Anticlined enantiomorphous chains the conformation of A corresponds to a (TG ) , bond succession (right-handed helix). The conformation of B corresponds to a (G T) . bond succession (left-handed helix). 

Crystal structures of two hexitols, galactitol and D-mannitol, have been published. Galactitol is meso, but the permissible intramolecular center of symmetry is not utilized in the crystal.28 The molecules crystallize as enantiomorphic pairs that, in conformation, are almost centrosymmetric molecules the difference therefrom is of the same order of magnitude as the thermal motion of the atoms. The carbon atoms and terminal oxygen atoms form an approximately planar chain. All of the oxygen atoms are both donors and acceptors for an intricate network of hydrogen bonding. 

A few examples of the Cl (l), that is, the 1C (d) conformation have been described. /3-D-Arabinopyranose and its enantiomorph crystallize in this conformation (Ia2e3e4a),1(i as illustrated in formula 20 for the /3-D compound. The Cl (d) conformer is shown, for comparison, in formula 21. As may be seen, neither form has any large nonbonded interactions. Both forms have one axial hydroxyl group on... 

Chirality in Crystals. When chiral molecules form crystals the space group symmetry must conform with the chirality of the molecules. In the case of racemic mixtures there are two possibilities. By far the commonest is that the racemic mixture persists in each crystal, where there are then pairs of opposite enantiomorphs related by inversion centers or mirror planes. In rare cases, spontaneous resolution occurs and each crystal contains only R or only S molecules. In that event or, obviously, when a resolved optically active compound crystallizes, the space group must be one that has no rotoinversion axis. According to our earlier discussion (page 34) the chiral molecule cannot itself reside on such an axis. Neither can it reside elsewhere in the unit cell unless its enantiomorph is also present. 

Figure 2 (a) Chair and (b) enantiomorphic skew-boat conformations of the [M(P207)] chelate ring... 

In the envelope conformation (A) the peroxide bond and the two carbon atoms are all coplanar (with the C-O-O-C dihedral angle being close to 0°) while the ethereal oxygen atom can be displaced by as much as 0.65 A to either side of this plane. In conformation B the peroxide bond straddles the plane of the remaining three atoms and this dihedral is around 50°. While conformation A is achiral, B has C.y symmetry. Usually ozonides crystallize in chiral space groups however, both enantiomorphic forms of B are usually encountered in the crystal lattice. Furthermore, disorder of the peroxide oxygen atoms over several occupancies is frequent, and in recent analyses, due mostly to improvement in the structure refinement algorithms, this disorder could be taken into account and suitably refined models could be built from the diffraction data. 

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