Triterpene glycosides ofAstragalus and their genins. XXI. Circular dichroism of cycloartane ketones

The CD spectra of a number of natural and synthetic cycloartane ketones have been studied. The influence of 6-hydroxy group on the Cotton effect due to a 3-keto function has been found. The Cotton effect has been determined for 6-oxo-and 11-oxocycloartanes.Institute of the Chemistry of Plant Substances of the Uzbek SSR Academy of Sciences, Tashkent. Translated from Khimiya Prirodnykh Soedinenii, No. 5, pp. 614–628, September–October, 1986.     

Synthesis and circular dichroism studies of steroidal schiff bases of biogenic amines

A number of imines (Schiff bases) have been prepared from ketosteroids (estrone, estrone-3-methyl ether, androsterone, epiandrosterone, 5α-cholestan-3-one, 6-ketocholestanol and testosterone) and primary amines (phenethylamine, tyramine and hexanamine). The CD spectra of these compounds have been measured in methanol. Except for the testosterone imines, the azomethine n-π^* band (~ 235 nm) exhibited the same sign (but not as high an ellipticity) as the parent ketone's n - π^* band (~295 nm). The imines of the estrogens (estrone and estrone 3-methyl ether) gave CD spectra characterized by a negative ¹L_b Cotton effect ([θ] ? -1000) near 280 nm and a band near 230 nm ([θ] ? +19,000) which is considered the sum of a positive azomethine n - π^* Cotton effect and positive aromatic ¹L_a Cotton effect(s). The ¹L_a band originating in the amine moiety was prominent in the CD of 5α-cholestan-3-one and 6-ketocholestanol phenethylamine imines. When aromatic Cotton effects originating in the amine moiety were detected, they were of the same sign as the n - π^* Cotton effect.     

Circular dichroism of some chlorine-containing guaianolides

A study of the circular dichroism of chlorine-containing guainolides has shown that a negative Cotton effect in the 260-nm region correlates with the trans-linkage of the lactone ring in acroptilin, hyrcanin, chlorohyssopifolin B, elegin, and biebsanin. It has been established that this nature of the linkage does not change on relactonization and hydrogenation. A chloromethylene substituent at C₄ causes a bathochromic shift of the lactone Cotton effect.     

Circular dichroism of steroids with a lactone ring B. Brassinosteroids and compounds related to them

The circular dichroism spectra of steroids having the lactone group in ring B that is characteristic for brassinosteroids, and their isomers, have been studied. It has been shown that in the spectra of the isomeric B-homo-7-oxa-6-ketosteroids and B-homo-6-oxa-7-ketosteroids differences are observed in the sign of the Cotton effect of the n-* transition of the carboxy group, which can be used for proving their structures.Institute of Bioorganic Chemistry, Belorussian Academy of Sciences, Minsk. Translated from Khimiya Prirodnykh Soedinenii, No. 1, pp. 76–82, January–February, 1992.     

Supramolecular chirogenesis in bis(zinc porphyrin): An absolute configuration probe highly sensitive to guest structure

The role of the ligand's structure and absolute configuration in the supramolecular chirality induction in achiral bis(zinc porphyrin) has been studied. The amines with bulkier substituents resulted in stronger CD signals due to increased helical displacement in the anti conformer. All the amines with an R absolute configuration gave a negative first Cotton effect and positive second Cotton effect, while the ligands with an S absolute configuration produced CD signals with opposite signs due to formation of the left- and right-handed screw diastereomers, respectively.     

Configuration absolue et dichroïsme circulaire du nitroxyde de la tetramethyl-2,2,5,5 hydroxy-3 pyrrolidine. Etude comparee de la pentamethyl-2,2,3,5,5 cyclopentanone

The absolute configuration of R(+) 3-hydroxy-2,2,5,5-tetramethyl-1-pyrrolidinyloxyl has been determined by Horeau's method. The circular dichroism agrees with the octant rule if the molecule has the half-chair conformation found in the crystal. The synthesis of R(+) 2,2,3,5,5-pentamethyl cyclopentanone is described. The sign of the Cotton effect of this ketone (analog of the studied nitroxide) has been also interpreted considering the predominant conformation in which the substituent responsible for the chirality is in a quasi-equatorial site.     

Circular dichroism and exciton effect of syndiotactic methacrylic polymers with chiral diacyl hydrazide side chain. II. Poly(methacrylic acid–Nβ (L-(+)-lactoyl) hydrazide)

Syndiotactic poly(methacrylic acid hydrazide) has been condensed with L -(+)-lactic acid to form optically active side chains which are connected to the main chain by diacyl hydrazide moieties. A model compound was prepared in the same way from pivalic acid hydrazide. The CD spectra of polymer and model compound have been recorded in aqueous solution at different pH. In the acidic and neutral range, a strong Cotton effect is observed at 195 nm for the polymer which is not present for the model. This Cotton effect may be part of an exciton doublet, the second half of which may be located below 190 nm, the limit of the CD spectrometer employed. In the alkaline region, the diacyl hydrazide moiety is deprotonated and the polymer leads in contrast to the model to a well developed exciton effect centered around a crossover point of 241 nm. The influence of the addition of alcohols and salt on the CD spectra is studied, as well as the effect of temperature. For both the acidic to neutral range on one hand, and the alkaline region, on the other hand, ordered conformations are postulated. For the alkaline region a specific helical conformation is discussed.     

Optical rotatory dispersion of indoline alkaloids with keto groups

The optical rotatory dispersion curves of a series of indolines, N(a)-acetyl-indolines and N(a)-methoxycarbonyl-indolines containing keto groups have been measured. It has been shown that the absolute stereochemistry of N(a)-acetyl-indolines containing keto groups can still be determined from the character of the Cotton Effect due to the aromatic chromophore, the ketone Cotton Effect however being more difficult to interpret. The use of N(a)-methoxycarbonyl-indoline and indoline chromophores is dangerous due to the complex nature of the Cotton Effect of the former and the overlapping of that of the latter with the ketone Cotton Effect.     

氨氯地平圆二色谱的理论解析

采用量子化学密度泛函理论(DFT)和含时密度泛函理论(TD-DFT)在B3LYP/6-311++G**水平上对氨氯地平分子的圆二色谱进行理论研究。计算结果表明:振动圆二色谱(VCD)的803、1113、1190、1238、1348、1509、1514、1736、2972、3022和3091 cm-1处存在吸收峰,产生以上吸收峰的振动均涉及到分子手性中心或者所处的基团与分子手性中心形成相关。研究还发现,甲醇溶液使电子圆二色谱(ECD)的吸收峰红移;甲醇溶液中,S-氨氯地平的ECD谱在207、366 nm处存在正性康登效应,223 nm处存在负性康登效应;以上吸收峰均来源于π-π*电子跃迁。这些结论对于深入理解氨氯地平的手征光学性质具有一定的指导意义。     

Optical rotary dispersion of nitrobenzene derivatives—IV : Absolute conformation of twisted nitrobenzen derivatives and its control through conformational transmission

The origin of Cotton effect shown by o-nitrobenzoyl esters is discussed. The Cotton effect observed for 1-(?)-methyl ester of 3-nitrophalic acid and 4-nitrohemimellitic acid (6 and 12), indicated that the favoured twisting direction of the nitrobenzene moiety is determined by the asymmetric menthyl group through the intervening carboxyl groups. This provides a new concept in conformational analysis of aromatic compounds. Extending the concept of conformational transmission to 2,2′-dinitrobipenhyl derivatives with known ab twisted nitrobenzene was related to the sign of the Cotton effect at 330 nm in accordance with the result of X-ray crystal analysis 1-(?)-menthyl 4-bromo-2-nitrobenzoate (1)     

Bicyclic γ-butyrolactones. Relation between conformation of the lactone ring and chiroptical properties

The CD curves of a set of condensed γ-butyrolactones have been investigated. A simple correlation between the sign of the Cotton effect (CE) and the configuration of C can be deduced from the resulting data.     

Optically active monosubstituted cymantrene derivatives. Relationship of chiroptical characteristics with absolute configuration of the compounds

The results of the study of the chiroptical properties (CD, ORD and [α]), of fourteen optically active monosubstituted cymantrene derivatives with a chiral center directly attached to the Cp ring are summarized for the first time. It has been found that the (R)-enantiomers of these compounds exhibit a negative Cotton effect for the “metallocenyl” transition in CD spectra within the range 310–360 nm independent on the nature of the substituent and the solvent.     

The i.r. spectrum of curium-244

The spectrum of curium-244 has been observed on the high resolution Fourier-transform spectrometer at Laboratoire Aimé Cotton. An electrodeless lamp containing 50 μg of CmI₃ was run for 12 hr and 800,000 points were taken. A total of 1743 lines have been ascribed to curium and 87% of the lines have been assigned to transitions between known energy levels.     

Circular dichroism of some chlorine-containing guaianolides

A study of the circular dichroism of chlorine-containing guainolides has shown that a negative Cotton effect in the 260-nm region correlates with the trans-linkage of the lactone ring in acroptilin, hyrcanin, chlorohyssopifolin B, elegin, and biebsanin. It has been established that this nature of the linkage does not change on relactonization and hydrogenation. A chloromethylene substituent at C₄ causes a bathochromic shift of the lactone Cotton effect.Institute of the Chemistry of Plant Substances, Academy of Sciences of the Uzbek SSR, Tashkent. Translated from Khimiya Prirodnykh Soedinenii, No. 5, pp. 650–652, September–October, 1979.     

Phthalimide as a chromophoric tag in the circular dichroism determination of absolute configuration of α-aminoacid amides and dipeptides. A case of a dipeptide isostructurality

Both N-phthaloyl aminoacid amides and methyl amides as well as N-phthaloyl dipeptide methyl esters give Cotton effects of opposite signs at around 220 and 240 nm. The signs of these Cotton effects are directly correlated with the absolute configuration of the N-phthaloyl substituted stereogenic center: for -configuration the Cotton effect at 240 nm is positive and the Cotton effect at 220 is negative. The X-ray analysis shows that the diastereomeric , and , N-phthaloyl alanylvaline methyl esters form isostructural orthorombic crystals due to the conformational flexibility of the molecules and a similar space requirements of the methoxycarbonyl and isopropyl groups.     

Circular dichroism of olefins—IV: A semiquantitative prediction of the intensity of the cotton effect of allyl alcohols and ethers

A semi quantitative prediction of the intensity of the Cotton effect of a few allyl alcohols and ethers in the steroid and diterpene series has been derived. It relies on a calculation of the weight of the various conformations and on the allylic bond polarization model.     

Helical supramolecular aggregates based on ureidopyrimidinone quadruple hydrogen bonding

A series of mono- and bifunctional compounds 2-7, based on the ureido pyrimidinone quadruple hydrogen bonding unit, was prepared to study the mode of aggregation of these compounds in the bulk and in solution. Compounds 2-7 exhibit thermotropic liquid crystalline properties, as evidenced by differential scanning calorimetry and optical polarization microscopy. The presence of an ordered hexagonal discotic (D(ho)) phase of 2 a was confirmed by X-ray diffraction on an aligned sample. In chloroform, the bifunctional compounds form cyclic dimers at millimolar concentrations, and these dimers exist in equilibrium with linear species above a critical concentration, which may be from 6 mM to greater than 260 mM, depending on the structure of the spacer. Circular dichroism measurements in chloroform did not show a Cotton effect. Dodecane solutions of compounds 3, 4 b, and 7 b display a Cotton effect at the absorption band of the phenyl-pyrimidinone unit. Amplification of chirality was observed in mixtures of 7 a and 7 b, but not in mixtures of 4 a and 4 b, indicating that 7 a and 7 b form mixed polymeric aggregates with a helical architecture in dodecane solution, whereas 4 a and 4 b do not. The Cotton effect is lost upon increasing the temperature. Half of the helicity is lost at 25 degrees C for 3 and at 60 degrees C for 4 b, suggesting that 3, bearing the shorter spacer, forms less stable columns than 4 b. Compound 7 b loses half of its helicity at 45 degrees C. Compounds 2 b, 5, and 6 do not exhibit helical organization, as evidenced by the absence of Cotton effects.     

Microscale determination of the absolute configuration of alpha-aryl-substituted alcohols by the CD exciton chirality method

The absolute configurations of a broad spectrum of aryl alcohols 1 have been determined for the first time by the CD exciton chirality method. The configurational assignment is additionally verified by computer modeling and lipase-catalyzed acetylation of the racemic alcohols. The CD-spectroscopic data have revealed that the S enantiomers of the benzoate derivatives 2 display a positive first Cotton effect and the R enantiomers a negative one at around 228 nm. Thus, the sense of the first Cotton effect of the benzoate derivative 2 allows a reliable assignment of the absolute configuration of the corresponding alcohol 1.     

The amide-aromatic-ring system : An inherently dissymmetric chromophore

Two cyclic peptide like compounds, cyclo-anthranoyl-L- prolyl (1) and cyclo-homoanthranoyl-L-prolyl (2) have been synthesized and investigated by UV spectroscopy and measurement of circular dichroism. Compound 1 of entirely rigid conformation with two N-CO groups conjugated to the aromatic ring, Ar-NH-CO left handed helical, Ar-CO-N right handed helical, shows a very strong positive Cotton effect centered around 250 nm and a strong negative one centered around 227 nm. Compound 2, which has only one conjugated skewed electron system, Ar-NH-CO, can exist in two different stable conformations, one of them being left handed, the other one right handed helical. It also shows a strong positive Cotton effect centered around 235 nm. Since from earlier ¹³C NMR data the latter was concluded to be the preferred conformation, a right handed helical sense of the amide bond-aromatic ring system is likely to correlate with a positive Cotton effect of this inherently dissymmetric chromophore.     

Circular Dichroism of Polyacetoxyalkylpyrazolo-(3,4-b) Quinoxalines

The circular dichroism spectra of a series of polyacetoxy-alkylpyrozolo(3,4-b) quinoxalines showed multiple Cotton effects with opposite sign to that of the corresponding polyhyroxylalkyl analogs having the same configuration of the side chain. Polyacetoxyalkyl analogs having the S chirality at the C-1′ in the Fischer projection formula showed a positive Cotton effect centered at the long-wavelength absorption and those possessing the R chirality showed a negative Cotton effect at the same region. The sign of the Cotton effect at the long-wavelength absorption is in accord with the rotation at the sodium D line.