CD curves

The absolute configuration of naturally occurring 5(-)-azetidine-2-carboxylic acid has been established (73CL5), and the DL form has been resolved (69JHC993). ORD and CD curves have been determined for 2-methylazetidine and an octant rule has been proposed for the N-chloro- and N-cyano-2-methylazetidines (74T39). 

Fig. 2.1. UV absorption, ORD, and CD curves of ethyl methyl p-tolyl sulfonium tetrafluoroborate. [Reproduced with permission firm J Og. Chem. 41 3099 (1976).]...

Figure A Diagrammatic representation of the Cotton effect (actually po-sitivc Cotton effect. The negative effect oceur.s when the CD curve. shows a minimum and the ORD curve is the reverse of the above).

Surprisingly, in contrast to a- and y9-peptides, CD spectra of y-peptides gave only a very hmited amount of stmctural information. Experiments conducted on heh-cal y" -hexapeptides did not reveal any characteristic CD signature (no Cotton effect) [200, 201]. Similarly, y -peptides built from 2,4-disubstituted y-amino acids of like configuration and shown to adopt a more stable 2.6-helical structure, do not display typical CD curves either [201]. However, CD spectra of the 2.6-helical -peptide 147 and its Boc-protected derivative recorded in MeOH and CD3CN present an intense maximum around 215 nm with a shoulder at ca. 200 nm [207]. 

Figure 1. Comparison of H2 generation from platinized hexagonal CdS (curve A) and cubic CdS (curve B) In Naflon 117. The Insert shows x-ray powder diffraction spectra of CdS In these films.

In the case of a positive chirality, a Cotton effect with positive first and negative second is observed, whereas a Cotton effect with negative first is found for negative chirality. As this method is based on theoretical calculations, the absolute configuration of organic compounds can be deduced unambiguously from their corresponding CD curves. 

The ORD and CD curves of chiral sulfinamides derived from aromatic sulfinic acids show a strong Cotton effect centered at 240 to 256 nm (83). The position of this Cotton effect, its dependence on the solvent used, and the amplitude reveal a complete analogy with that observed in alkyl aryl sulfoxides and aromatic menthyl sulfin-ates. Thus, the positive sign of the Cotton effect observed in the ORD curves of dextrorotatory sulfinamides is indicative of the (5)-configuration around sulfur in these compounds. 

The ORD and CD curves of optically active polymers containing chromo-phoric groups show that the chromophores can be asymmetrically perturbed by the chirality of the substituents and of the main chain conformation. This is the case with poly( ec-butyl vinyl ketone) (377), which presents a Cotton effect at 292 nm, its intensity being greater in the prevalently isotactic polymer than in the atactic polymer. 

In 1990, Isono et al. reported the isolation of RK-286C (318) from the culture filtrate and the mycelium extract of Streptomyces sp. RK-286 (303). In nature, this isolate was obtained in its optically active form [a]p -I- 45.3 (c 0.22, EtOAc). Based on similar CD curves, the absolute configuration of this isolate was assigned to be similar to that of (-E)-staurosporine (295) (see Scheme 2.74) (284). RK-286C has shown PKC inhibitory activity and platelet aggregation in vitro, as well as weak antifungal activity (303). 

A series of 14 linear and cyclic peptides have been studied 135 by CD, NMR, and MD simulations. Quantitative NOE measurements and MD were used to determine the fractions of type I and II (3-turns in each peptide. The CD spectra for these same peptides were analyzed by convex constraint analysis 136 to derive a set of CD curves for the two types of 13-turns (Figure 6). These basis spectra gave fractional (3-turn contents in good agreement with NMR measurements. Two basis spectra (components 1 and 4) were obtained for the type I... 

Figure 6 The Four Component CD Curves from Convex Constraint Analysis of CD Spectra of Cyclic (S-Tum Models11351"1 ...

The recent stage for the study of the spectroscopy and dynamics of the superexcited molecules was comprehensively discussed in Ref. [5], and let us stress that the formation of the superexcited molecules and their decay processes produce a wide range of interesting structures in the c, c, and Cd curves as a function of the incident photon energy. 

In special cases of nonracemic compounds, both relative and absolute configuration can be determined by means of circular dichroism. Thus, the relative and absolute configurations of acyclic 1.2-, 1,2,3-, 1,2,3,4-, and 1,2,3,4,5-polyols can be determined by the bichromophoric exciton chirality method after a two-step derivatization [primary and secondary hydroxy groups are selectively esterified with 9-anthroyl chloride (2) and ( )-3-(4-methoxyphenyl)-2-propenoyl chloride (4), respectively] and comparison of the CD curves with references curves265. 

The pairwise additivity principle when applied to bichromophoric exciton-coupled systems, such as those composed of 4-bromobenzoate (BB) and 4-methoxycinnamate (MC) derivatives of cyclic polyols, yields more information from the CD spectra of complex molecules173. In such systems both degenerate ( homo") interactions, i.e., BB/BB and MC/MC, and nondegenerate ("hetero") interactions, i.e., BB/MC, contribute to the exciton CD spectra. As the homo exciton Cotton effects arc well separated (BB 236/253 nm, MC 287/322 nm) a unique pattern of CD curves is obtained for each derivative of cyclic tri- or tetraol. This method is particularly useful in identification of glycopyranosides174, a procedure essential for microscale structure determination of oligosaccharides by the exciton chirality method175. 

The close similarity of the CD-curves of methano[10]azaanulene (107) itself, its isomeric methylderivatives (e.g. 109) and the 3-bromo-product 113 on the one hand and of the methylesters 110 ox 111 on the other (typical CD-spectra are shown in Fig. 5) permitted the correlation of relative configurations within each of these groups compounds with the same sign of rotation seem to have the same chiralities 122). 

Both the bisoxido- and bisimino-(cis)bridged dibromo[14]anulenes 115 and 116 with C2-symmetry could be quantitatively separated into their enantiomers by chromatography on triacetylcellulose in ethanol at 1.7 bar 124). The dextrorotatory enantiomers with [a]546 values of 1700° 115) and 1500° 116) in ethanol were eluted first. They exhibit very similar CD-curves with strong Cotton effects around 260... 

The absolute configurations of helicenes have been firmly established by calculations 95 98), by X-ray analysis 99) and by synthesis l00).6 The dextrorotatory enantiomers of hexahelicene and a thiahexahelicene have been shown to possess the right-handed (P) helicity. From the correspondence of the ORD- and CD-curves of these hexahelicenes and higher helicenes positive rotations correspond in all cases with P-helicenes. 

The absolute stereochemistry of broussonetine L (8) was determined by the combination of the benzoate chirality method and the Mosher s method [35-37]. A carbamate (8a) was prepared from broussonetine F (4) by reaction with phenyl chloroformate in tetrahydrofuran-H20 (7 3), and a diacetate (8b) was prepared from 8a with acetic anhydride in pyridine. Finally, a dibenzoate (8c) was obtained by benzoylation of 8b. The CD curve of 8c showed a negative Cotton effect (Ae237 -30.9) and a positive effect (Ae223 +15.9) to confirm a counter-clockwise chirality between two benzoyl groups, Fig. (3) [20]. 

---