Direct chiroptical detection

D2 and D3 vitamins (ergocalciferol and cholecalci-ferol) has not been equally successfulJ Vitamin D extracted from natural sources has a single conformational stereochemistry that is one of several isomers produced in synthetic preparations. To certify that the natural form is present in a synthetic product, where it can be accurately assayed in the presence of the other isomers, is a formidable analytical task. Whether direct CD detection can satisfactorily solve it is currently unknown. A prior non-selective derivati-zation reaction might be required on all isomers. The A and E vitamins are achiral and not subject to chiroptical detection unless first derivatized by reaction with a chiral host. 

The seminal work on steroid analyses using chiroptical detection was done by Djerrasi by the determination of hecogenin acetate in the presence of tigonenin acetate.Every steroid is chiral and therefore amenable to polarimetric detection after chromatographic separation. Chromophores are fairly uncommon, and analysis by ORD or CD is therefore less suitable. The only unsaturation in the cholesterol molecule, for example, is the isolated A -double bond, which has an absorbance maximum at 205 nm. Unsaturation coupled with chirality provides some selectivity, as ably demonstrated by the work of Potapov for analogs of progesterone Even simpler than that is the direct discrimination between the ketosteroids testosterone and dihydrotestosterone, which have opposite signs in methylene chloride solution (Fig. 6). 

Usually, the chiroptical properties of highly cross-linked polymers cannot be measured. The asymmetry of the empty cavities can be deduced from their excellent racemate resolution ability, but under special conditions it can also be directly detected by optical activity measurements [41]. For this, the polymer is suspended in a solvent that has the same refractive index as the polymer, a technique which was developed for other types of insoluble polymers. The values of molar optical rotation thus determined are shown in Table 4.3. 

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