Chiroptical Phenomena

Examples of zig- zag coupling paths Substituent X is separated from the carbonyl groups by 1, 2, 3, 4, and 5 C—C bonds, respectively, in structures (19)—(23). Structure (24) illustrates a coupling path which extends into a front octant region 

An empirical analysis of c.d. data for compounds containing a chiral cyclo-pentanone ring (e.g. ring D in steroids) has led to the proposal that the observed 

data for a highly purified sample of 17/3-hydroxy-4,4-dimethyl-5a-oestran-3-one ° differ only slightly from a previous report. 

Although the acetyl side-chain in 3a,17 -dihydroxy-4,4-14a-trimethyl-19-nor-10a-pregn-5-ene-ll,20-dione (9) has the normal conformational preference in the crystal, the carbonyl oxygen atom almost eclipsing C-16, c.d. and other 

data for 18 highly substituted steroid and triterpenoid olefins include some additional examples which for the most part follow the dissignate sector rule with respect to the lowest-energy transition and a consignate rule for the next transition of higher energy.  

curves for the imines (Schiff bases) formed from some keto-steroids and 2-phenylethylamine, tyramine, or hexanamine show n- n bands generally of the same sign as those of the corresponding ketones, but near 235 nm.80 Additional 

Tamura, and T. Matsumoto, Nihon Daigaku KogakubaKiyo Bunrui A, 1980, 21,223. 72 T. Akiyama, S. Takagi, U. Sankawa, S. Inari, and H. Saito, Biochemistry, 1980, 19, 1904. 

3 p-Acetoxyeti-5-enic esters have been used to resolve enantiomers of perhydro-anthracenol (18)81 and perhydrophenanthren-4-ol (19)82 as tricyclic systems for study of the c.d. characteristics of the corresponding ketones. 

-visible spectra are reported for solutions of the antitumour triterpenoid tingenone in aqueous sodium deoxycholate,83 and c.d. curves have been recorded for morphine and related alkaloids in cholesteric liquid crystals.84 

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It must be stressed, however, that the methyl alkyl sulfoxide rule is not valid for alkyl benzyl and alkyl allyl sulfoxides (222-224), where the electronic, steric, and solvent effects exert influences on the chiroptical phenomena in a way that is difficult to rationalize. This rule was found to be satisfactory and was used for the assignment of absolute configurations of steroidal (200,201,225), penicillin (226), and amino acid (227-230) sulfoxides. 

The most commonly encountered manifestations of chiroptical phenomena are circular birefringence (also known as optical rotation), optical rotatory dispersion (ORD), and circular dichroism (CD). An explanation as to the nature of circularly and linearly polarized light is provided, and the origins of the various chiroptical effects are discussed. In each instance, a concise summary of the calculations used by workers in the field to report the results of their investigations is provided. 

Snatzke, G. (1979), Circular Dichroism and Absolute Conformation—Application of Qualitative MO Theory to Chiroptical Phenomena", Angew. Chem. Int. Ed. Engl. 18, 363. 

In the presence of intense electromagnetic radiation from a laser, the induced moments may show a nonlinear dependence on the incident field strength. Nonlinear optical activity as well as new chiroptical phenomena may now be observed. 

New and promising fields are the chiroptical analyses of chiral crystals, chiral liquid crystal phases, chiral membranes, Langmuir-Blodgett films, chiral surfaces, etc. Their chirality is caused by the molecular chirality and the suprastructural chirality of the phase. Because the spatial extension of the structures of the suprastructural chirality is often about three orders of magnitude larger than molecular diameters, the spectral regions of chiroptical phenomena for both levels of chirality are different. 

A plot of the ellipticity and optical rotation of the iodine adduct as a function of the DP, shows a maximum for DP of between 70 and 100 [51]. The amplitude of the Cotton effect for a DP of about 20 is very small, and oligomers with DP <12 display no colour reaction and no induced Cotton effect. Thus the chiroptical phenomena in the long wave region are not based on the asymmetric carbon atoms of the base units they are induced by the secondary structure and come from the chiral exterior of the iodine chain, although this is not bound by covalent bonds or salt formation. The induced Cotton effects therefore show that iodine-amylose must be present in the form of a helical channel inclusion compound even in diluted aqueous solution. 

Finally, optical activity of non-regular polyelectrolytes reflects well the chemical modifications through local (monomeric units) conformation changes and alterations of chromophores either by a chemical reaction ionization, protonation) or by a more labile effect such as solvation. Another general feature is the non-linear variation of chiroptical phenomena with the degree of reaction. Before commenting on the latter, another example where chiroptical phenomena appear as a good means to detect and characterize weak interactions will be described. 

Chiroptical Phenomena to Study Interactions Between Polyamides and Mineral Salts... 

It is noteworthy that, up to now, chiroptical phenomena have been the only means to determine association constants between salt and nylon-type polyamides in highly concentrated alcoholic salt solutions. Under these conditions, UV absorption spectroscopy is unsuitable because no suitably large changes of absorption in tlie bands between 185 and 220 nm were observed. Potentiometric determination of bound and unbound Ca failed because Ca " specific electrodes dissolve in the medium. Conductometric measurements were impossible because polymer needs high salt concentrations to dissolve and only very small differences would have to be measured due to com-plexation. Nevertheless, the complexation being now identified, it seems that it will be possible to confirm the Kc values using H NMR. Indeed, CaCl2 acts as a chemical... 

On the other hand, a non-linear variation of the chiroptical phenomena as a function of the degree of reaction has been found as a quasi-general feature of these polymers. Using the example of a series of random copolymers of a chiral acrylamide and acrylic acid of different compositions, it has been shown that the non-linear variation of their optical activity with composition can be explained by neighboring effects between monomeric units without considering any contribution from ordered macromolecular... 

Electronic chiroptical phenomena are among the oldest to be noted in chemistry. Understanding of the phenomena required first the introduction of quantum mechanics, and then the advent of computers sufficiently powerful to predict chiroptical properties for real molecules. The availability of excellent commercial instrumentation and pervasive applications in the sphere of biochemistry have maintained the field of ECD research as a mature but healthy one. 

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