Crystal chiral

Cains, P.W., 1999. Chiral Crystallization. In SPS Crystallization Manual, Volume 5, Part 5.1 Novel Crystallization Techniques, Harwell, UK. 

Of further particular interest was that the crystallographic results on 2,5-DSP and poly-2,5-DSP had pointed out a very important future possibility that an absolute asymmetric synthesis could be achieved if any prochiral molecule, e.g. an unsymmetrical diolefin derivative, could be crystallized into a chiral crystal and if the reaction of the chiral crystal proceeded in the same manner as the 2,5-DSP crystal with retention of the crystal lattice (Wegner, 1972, 1973). Such types of absolute asymmetric synthesis with a high enantiomeric yield have now been performed by topochemical [2+2] photoreaction of unsymmetric diolefin crystals (Addadi etal., 1982 Hasegawa et al., 1990 Chung et al., 1991a,b). 

Along with the guidepost (Wegner, 1972, 1973) based on the crystal-to-crystal transition from 2,5-DSP to poly-2,5-DSP, absolute asymmetric synthesis has been achieved by the topochemical reaction of a chiral crystal of an achiral diolefin compound in the absence of any external chiral reagents. 

X-Ray crystal structural analysis of a (+)-erystal of 96 shows that molecules of 96 are arranged so as to be chiral in the crystalline lattice (Fig. 14). Scheme 9 which is depicted by referring to Fig. 14 indicates the reason why each chiral crystal of 96 gives the corresponding chiral 97. 

The above results are valuable in that an optically active compound is produced in bulk from achiral material. Only a few successful examples of photochemical conversion of achiral into chiral material in the absence of a chiral source have been reported hitherto 49, and in these cases the conversion was carried out on a fragment of a chiral crystal. In our case, chiral crystals are available in bulk, and mass production of the chiral compound is possible. 

Keywords Bundles Chain-folding Chiral crystal polymorphs Mesophases ... 

The mechanism we propose for chiral crystallization of helical polymers appears to be also supported by the analysis of the circumstances under which some chiral polymorphs are obtained [13]. 

We can conclude this section noting that understanding chiral crystallization of helical polymers is still hardly satisfactory, but quite certainly different mechanisms are likely to apply for different polymers and different crystallization conditions. A lot of experimental work still needs to be carried out if we want to progress beyond speculation and generalization of results that may in fact apply only to very specific instances. 

Chiral Crystallization of Polymers with Helical Chain Conformations References... 

Crystallization of polymers in chiral crystals, even in the case of achiral polymers, is quite frequent and strictly related to the occurrence of helical conformations of the chains. The crystallizable polymer consists of a regular sequence of a chemical repeating unit which can be chiral if it presents an asymmetric center or achiral. On the contrary, helical conformations assumed by the polymer chains in the crystalline state are intrinsically chiral, even though the chemical repeat is achiral. Three possible cases can be distinguished ... 

In the first half of the nineteenth century, it was known that certain minerals, the prime example being quartz, formed chiral crystals. Often, it was seen that rocks could be composed of a physical mixture of small but macroscopic right-handed and left-handed crystals. This kind of mixture, composed of macroscopic chiral domains (crystals) occurring in both enantiomeric forms, was termed a conglomerate. 

A young Louis Pasteur observed that many salts of tartaric acid formed chiral crystals (which he knew was related to their ability to rotate the plane of polarization of plane-polarized light). He succeeded in solving the mystery of racemic acid when he found that the sodium ammonium salt of racemic acid could be crystallized to produce a crystal conglomerate. After physical separation of the macroscopic enantiomers with a dissecting needle, Pasteur... 

Addadi, L., A Link Between Macroscopic Phenomena and Molecular Chirality, Crystals as Probes for the Direct Assignment of Absolute Configuration of Chiral Molecules, 16, 1. 

Chiral induction to obtain the /J-lactam 169 by photolysis of the chiral crystal 168 in the solid state is possible (equation 112). 

F. Assignment of the Absolute Configuration of a Chiral Crystal Containing a... 

The formation of hemihedral crystal faces in a chiral crystal as induced by solvent. 

The ambiguity involved in assigning the absolute configuration of a chiral molecule in a chiral crystal is presented in Scheme 1. Scheme la depicts a chiral molecule of, say, configuration S, with individual atomic coordinates — x — y - z, (i = 1,. . . , n, for n atoms) in a crystal axial system a,b,c. Scheme 1 b represents the enantiomeric crystal structure containing a molecule of configuration... 

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