Chiroptical properties of optically

IV.3 Preparation and Chiroptical Properties of Optically Active Betweenanenes... 

Chiral molecules may be studied by a great many techniques. Without optical resolution, chiral structures can be detected by the magnetic nonequivalence of diastereotopic groups in NMR spectroscopy. Diaste-reoisomeric pairs of enantiomers, with and without separation, as well as resolved optically active compounds can be used for the investigation of stereochemical problems. Although stereochemical information can be obtained in many ways, the chiroptical properties of optically active compounds constitute an additional handle" for assignment and correlation of configuration that is not available to optically inactive probes. 

The chiroptical properties of optically active thiazoUdines derived from aldoses and natural mercapto aminoacids was studied [92]. PMR parameters for thiazoHdine-4(R)-carboxylate derivatives were obtained by computer-assisted analysis of their spectra. The polyacetoxy-alkyl side chains have planar zig-zag conformations. The configurations at C-2 in the di-astereoisomers were ascertained on the basis of the Jnh.ch coupling constants [93]. The conformation and stereochemistry of diastereomeric sulfoxides of methyl 3-acetyl-5,5-dimethyl-2-(D-galactopentaacetoxypentyl)-1,3-thiazolidine-4-carboxylate 1-oxides were performed by H- and C-NMR spectral analysis [94]. 

Wulff, G. Dhal, P.K. Chirality of polyvinyl compounds. 6. Unusual influences of the comonomer structures on the chiroptical properties of optically active vinyl copolymers... 

L. Angiolini, D. Caretti, L. Giorgini, E. Salatelli, Synthesis and chiroptical properties of optically active methacrylic polymers bearing the (S)- and/or (R)-2-hydroxysuccinimide moiety linked to the tra/is-azobenzene group in the side chain. Macromol. (Them. Phys. 201, 533-542 (2000)... 

Another interesting water-soluble polythiophene is 22, bearing a chiral amino acid side chain, which was prepared by Hjertberg, Inganas and co-workers by oxidative polymerization with FeClg. The chiroptical and sol-vatochromic properties of this optically active polymer are described in the later section entitled Chiroptical properties of optically active poly thiophenes. The synthesis of other, organic solvent-soluble chiral polythiophenes is discussed directly below. 

Chiroptical properties of optically active poly thiophenes... 

The stereogenic sulfur atom in sulfoxides is usually configurationally stable at room temperature thus, sulfoxides may be chiral based on this property alone1. In fact, there are many examples of optically active sulfoxides of both synthetic and natural origin. This chapter reviews the important methods for obtaining optically active sulfoxides, and discusses some reactions at sulfur which either leave the coordination number at three or increase it to four, generally with preservation of optical activity. It also describes briefly some recent studies on the conformational analysis and chiroptical properties of sulfoxides. 

McGrath et al. have also thoroughly studied the chiroptical properties of dendrimers such as 40. They compared the optical activities of the series of 1st-, 2nd- and 3rd-generation compounds of type 40, considering the molar rotation per chiral unit ([ ]D/n) [75]. A big difference of the values was found between the generations which could possibly indicate chiral conformations inside the dendrimers, that enhance the optical rotation values per unit when... 

The influence of the solvent on chiroptical properties of synthetic polymers is dramatically illustrated in the case of poly (propylene oxide). Price and Osgan had already shown, in their first article, that this polymer presents optical activity of opposite sign when dissolved in CHCI3 or in benzene (78). The hypothesis of a conformational transition similar to the helix-coil transition of polypeptides was rejected because the optical activity varies linearly with the content of the two components in the mixture of solvents. Chiellini observed that the ORD curves in several solvents show a maximum around 235 nm, which should not be attributed to a Cotton effect and which was interpreted by a two-term Drude equation. He emphasized the influence of solvation on the position of the conformational equilibrium (383). In turn, Furakawa, as the result of an investigation in 35 different solvents, focused on the polarizability change of methyl and methylene groups in the polymer due to the formation of a contact complex with aromatic solvents (384). 

From the optical and chiroptical properties of [2.2]metacyclophanes, compared with those of [10]paracyclophanes40), it was concluded that in the former an inherently achiral aromatic chromophore perturbed by vibrations is presumably responsible... 

As far as the metal center is concerned, the stereospecificity of the S02 insertion was demonstrated with diastereoisomeric Fe compounds that contain chiral centers on the metal and either in the substituted cyclopentadienyl ring21) or in the alkyl side-chain21,22). The stereochemistry of the S02 insertion with respect to the metal atom was studied with the optically active iron compounds 8a and 8b23 Liquid S02 inserts into the Fe—C bond of 8a and 8b with retention of configuration23) [Eq. (7)]. The similarity in the chiroptical properties of 8a and 9a as well as 8b and 9b was used for the assignment of relative configurations23). 

The first dendrimers with chiral cores for studies on the influence of the stereo-genic centres of a core unit on the chiroptical properties of the overall molecule were presented by Seebach s group [18]. These workers first synthesised dendrimers based on a chiral tris(hydroxymethyl)methane core unit. To these were attached zeroth- to second-generation Frechet dendrons, either directly or separated from the core by an aliphatic (n-propyl) or an aromatic spacer (p-xylylene) (Fig. 4.62). Remarkably, the dendrimers with aliphatic spacer showed no significant optical activity. This loss of chiral information was attributed to a dilution effecf, resulting from linkage of the achiral dendron to the chiral core unit,... 

These considerations, thus, lay the groundwork for tests among several semi-empirical approaches to the estimation of optical rotation of bond systems regarded as helices. Should it be necessary to use Eq. (lb) rather than (la), then a sweeping reassessment of the use of the helical conductor model will be required. However that test turns out, a test between that model and the simple conformational dissymmetry model becomes possible on the basis of the material shown in Table 1. At this point it should be said that our calculations on twistane 16> support the helical conductor model but that the results obtained by Pino and his co-workers 17 18> on the chiroptical properties of isotactic polymers prepared from chiral a-olefins support the conformational dissymmetry model. [We are not able, at present anyhow, to account for their results with the helical conductor model]. 

The number of theoreticians at the University of Calgary doubled in 1970 with the arrival of Arvi Rauk and Rod Truax. Rauk has made many important contributions to theoretical organic chemistry. Perhaps his most significant contribution arises from his long-standing interest in the chiroptical properties of molecules, that is, properties connected with the handedness and optical... 

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