Nortriterpenoids from Schisandra chinensis and their absolute configurational assignments by electronic circular dichroism study

Twelve new nortriterpenoids, namely wuweizidilactones J–P (1–7), schindilactones I–K (8–10), preschisanartanin N (11), and schisdilactone J (12), as well as 23 known analogues, were isolated from the leaves and stems of Schisandra chinensis. The absolute configurations of 1 and 11 were established by calculated electronic circular dichroism (ECD) spectra and that of 7 was confirmed by single-crystal X-ray diffraction. The absolute configurations of 2–6, 8, and 9 were determined by an empirical comparison of their experimental ECD spectra with that of 1. It is the first time that the ECD spectra of 18(13→14)-abeo-schiartane-, 18-norschiartane-, and schisanartane-type nortriterpenoids possessing an α,β-unsaturated-γ-lactone moiety are presented, this method providing an alternative means of obtaining absolute configurational assignments for these types of nortriterpenoids. All isolates were evaluated for their anti-acetylcholinesterase and anti-butyrylcholinesterase activities, and compounds 8, 13, 23, and 31 show anti-acetylcholinesterase activity at concentrations of 50 μM, with 12.7, 10.7, 16.6, and 32.1% inhibition, respectively.     

Optical rotatory dispersion and circular dichroism. LXXV. Circular dichroism of some aryl-amino acids

CD. curves have been recorded for α-aryl-α-amino acids, esters and amides related to α-phenylglycine, α-phenyl-alanine and their N-dimethyl derivatives, and for the corresponding α-cyclohexyl-α-amino acids and esters. Compounds with the (S)-configuration at the single asymmetric carbon atom give strong positive Cotton effects near 220 nm. The conformations of the acids are discussed and compared with those of other α-amino acids.     

Circular and linear dichroism of proteins

Circular dichroism (CD) is an important technique in the structural characterisation of proteins, and especially for secondary structure determination. The CD of proteins can be calculated from first principles using the so-called matrix method, with an accuracy which is almost quantitative for helical proteins. Thus, for proteins of unknown structure, CD calculations and experimental data can be used in conjunction to aid structure analysis. Linear dichroism (LD) can be calculated using analogous methodology and has been used to establish the relative orientations of subunits in proteins and protein orientation in an environment such as a membrane. However, simple analysis of LD data is not possible, due to overlapping transitions. So coupling the calculations and experiment is an important strategy. In this paper, the use of LD for the determination of protein orientation and how these data can be interpreted with the aid of calculations, are discussed. We review methods for the calculation of CD spectra, focusing on semiempirical and ab initio parameter sets used in the matrix method. Lastly, a new web interface for online CD and LD calculation is presented.     

Circular dichroism of some carotanes

The circular dichroism method has been used to study the stereochemistry of the carotanes. An analysis of the results obtained has shown that an -oriented ester group at C₆ corresponds to a positive Cotton effect (CE) in the 240–260-nm region. A link has been established between the position of the carbonyl group in the ring B and the sign of a CE in the 340-nm region. Hydrolysis of an ester group at C₆ takes place more readily in an alkaline medium than in an acid medium, and in the case of carbonyl-containing carotanes dehydration takes place as well as hydrolysis.Institute of the Chemistry of Plant Substances, Academy of Sciences of the Uzbek SSR, Tashkent. Translated from Khimiya Prirodnykh Soedinenii, No. 1, pp. 45–48, January–February, 1984.     

Circular dichroism of α-deuteriophenylethanes

CD measurements on a series of α-deuteriophenylethanes (1) which owe their chirality solely to deuterium substitution show that all compounds of related configuration display positive Cotton effects with pronounced fine structure for the ¹L_b band of benzene (240–270 nm), as do the configurationally related chiral 1-substituted phenylethanes (2) where the α-substituent is N, O or halogen. From a discussion of conformational preferences and a quadrant sector rule, the sign of the Cotton effects permits conclusions to be drawn regarding the confomational equilibrium in α-deuteriophenylethanes.     

Circular dichroism of large molecules

The circular dichroism of molecules, which are large compared to the wavelength of light, is considered. Explicit expressions are obtained for the circular dichroism and absorption of an exciton dimer and of a free particle on a helix. The dimensions are described for which the dipole approximation for the optical properties fails.     

The stereochemical assignments of the four 8-azaestrone configurational isomers

Complete correlation has now been accomplished for the four possible isomers in the 8-azacstrone scries, synthesized by two different synthetic routes.     

Circular dichroism of veralosidine and its derivatives

Summary On the basis of their CD spectra, the alkaloids veralosidine, veralosine, veralosinine, and veralosidinine have been assigned to the 25S series.Institute of the Chemistry of Plant Substances, Academy of Sciences of the Uzbek SSR, Tashkent. Translated from Khimiya Prirodnykh Soedinenii, No. 5, pp. 623–625, September–October, 1976.     

Circular dichroism of A-nor-steroids and related derivatives

The Circular Dichroism (CD.) of several saturated and conjugated keto-A-nor-steroids follows the known rules. Homochiral α,β-epoxy-cyclopentanones and -cyclohexanones give the same sign of the Cotton effect within the R-band, whereas the K-band CD. of the former has opposite sign to that at 300 nm. More examples for the application of the rule for the determination of the CD. of non-coplanar conjugated lactones as well as α,β-epoxy-lactones are presented.     

Circular dichroism of veralosidine and its derivatives

On the basis of their CD spectra, the alkaloids veralosidine, veralosine, veralosinine, and veralosidinine have been assigned to the 25S series.     

Stereochemical assignments of 22-isoxazolinylsteroids using circular dichroism

Circular dichroism (CD) spectra of 22-hydroxy- and 22-acetoxy-22-isoxazolinylsteroids were studied. The configuration of the C-5′ center of the heterocycle was established from the sign of the n-π*-transition band of the azomethine chromophore. The band molecular ellipticity was shown to depend on the mutual placement of the isoxazoline ring and the steroid skeleton of the studied compounds. Translated from Khimiya Prirodnykh Soedinenii, No. 5, pp. 495–499, September-October, 2008.     

Circular dichroism of acetylated methyl glycosides

The circular dichroism (CD) spectra of a number of acetylated methyl glycosides, from mono-to tetra-O-acetates, have been obtained. The signs of the CD maxima of the n-* transitions (210–215 nm) of the O-acetate groups have been predicted, and these predictions coincide with the figures obtained experimentally.Pacific Ocean Institute of Bioorganic Chemistry of the Far Eastern Scientific Center, Academy of Sciences of the USSR, Vladivostok. Translated from Khimiya Prirodnykh Soedinenii, No. 1, pp. 54–56, January–February, 1980.     

Circular dichroism spectra of trans-chalcone epoxides

The electronic absorption and CD spectra of (−)-trans-chalcone epoxide and its derivatives with methyl and alkoxy substituents at the ortho-positions of the aromatic rings have been measured. The spectra have been assigned with help of the energies, oscillatory strengths, and rotatory strengths of the singlet transitions obtained from DFT calculations. The features of the CD spectra, indicative of the absolute configuration, are the carbonyl n–π¹ band and two further strong bands assigned to the overlapping signals of π–π¹ and n_epoxy–π¹ excitations.     

Circular dichroism of some steroid alkaloids

Summary 1. The trans-linkage of rings A/B in compounds (I-III) and of rings B/C and E/F in veralodine and the equatorial orientation of the OH group at C₃ in isogermine has been confirmed by CD spectroscopy.2. A relationship has been established between the nature of the substituents and the configurations at C₂₀ and the sign of the CE in the 200 nm region for compounds (I-III).3. The 22R configuration has been established for veralomidine.Institute of the Chemistry of Plant Substances, Academy of Sciences of the Uzbek SSR, Tashkent. Translated from Khimiya Prirodnykh Soedinenii, No. 5, pp. 630–633, September–October, 1976.