Liquid crystals diastereomers

On the basis of 10,ll-dihydro-5//-dibenzo[a,i/]cycloheptene, the Chen group developed helical overcrowded alkenes 30a-d (Scheme 4.7) [32], which are closely related to the above switchable tetrabenzoheptafulvalene derivatives. Compounds 30a-d switch between diastereomers when irradiated with UV light of different wavelengths, and 30d was used as chiral dopant leading to a cholesteric mesophase when mixed with a nematic liquid crystal [33]. 

Figure 9 UV spectra (7= 75°C), temperature dependent degree of anisotropy (77°C = 28 to 67.5) (left), Ae (7= 75°C) and temperature dependent Ae (7/°C = 28 to 67.5) (right) of the cisoid enones 3-methyl-3a,5a,3,3, 4, 5 -tetrahydrobenzo[2,3]cholest-2-en-1 -one [3] (above) and its 3 3-diastereomer [4] (below) in the nematic liquid crystal phase ZLI-1695 (Merck).

Although fractional crystallization has always been the most common method for the separation of diastereomers. When it can be used, binary-phase diagrams for the diastereomeric salts have been used to calculate the efficiency of optical resolution. However, its tediousness and the fact that it is limited to solids prompted a search for other methods. Fractional distillation has given only limited separation, but gas chromatography and preparative liquid chromatography have proved more useful and, in many cases, have supplanted fraetional crystallization, especially where the quantities to be resolved are small. 

When a 1 1 mixture of the solid and the liquid diastereomers of bridged [10] cyclophane nicotinamides 8 was heated, the solid (5p,3, S )-isomer became predominant over the liquid (f p,3 S)-isomer through the fast dynamic (rope-jumping) racemization and the subsequent spontaneous resolution via crystallization [23]. The CD spectra of these cyclophanes exhibit either two positive or two negative Cotton effects at 220 nm and 280 nm, primarily due to the planar chirality (Fig. 2). 

The development of newer separation techniques, in particular preparative gas and liquid chromatography, has broadened the scope of resolution in recent years. An alternative to the acid-base salt separation by crystallization, for example, would be formation of the covalent amide linkage, chromatographic separation of the diastereomers, and then chemical hydrolysis. 

Resolution of racemates by the selective liquid-liquid extraction (Figure 13.8) of diastereomers due to their different solubility in two-phase system is by far a less common technique than crystallization of diastereomers. In this process, a highly discriminative chiral ionic reagent transfers (preferably in one extraction step) a highly enriched enantiomer as an ionic pair from one phase to another, which dissolves selectively the formed diastereomeric associate but not the starting enantiomers. The following back-extraction with an appropriate acid or base produces the desired enantiomer and recovered chiral selector (SO). 

X-ray data showed that the two pyrrole rings are nearly orthogonal with an inter-ring angle equal to 84.5°. Diastereomers were formed by transesterification with optically pure 2-(S)-methylbutyl alcohol. Unfortunately classical crystallization or GC did not separate the diastereomers. Resolution into enantiomers should be attempted by liquid chromatography on a chiral support. 

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