Conformational Analysis and CD Calculations of Methyl-Substituted 13-Tridecano-13-lactones

Conformational aspects in the studies of organic compounds by electronic circular dichroism

The electronic circular dichroism (ECD) spectra of flexible molecules include the contributions of all conformers populated at the working temperature. ECD spectra of chiral substrates depend on their stereochemistry in terms of both absolute configuration, as reflected in the sign of the spectrum, and molecular conformation, which dictates the overall spectral shape (possibly including the sign) in a very sensitive manner. The unique high sensitivity of ECD towards conformation, as well as of other chiroptical spectroscopies, renders these techniques a useful alternative or complement to standard spectroscopic tools for conformational investigations, such as NMR. This tutorial review provides first a brief discussion of the main principles of ECD spectroscopy and related methods for interpretation of spectra, with special reference to conformational aspects. The review focuses on the common problems encountered in the application of ECD for assignments of absolute configuration of flexible molecules. These problems can be handled either by taking into account the whole conformational ensemble or by considering rigid derivatives prepared ad hoc. Finally, the review presents the relatively less common but very interesting application of ECD spectroscopy for conformational analyses of organic compounds.

Ten-membered lactones from the marine-derived fungus Curvularia sp

Investigation of the secondary metabolites of the marine-derived fungus Curvularia sp. yielded four new 10-membered lactones (1-4), along with the known modiolide A (5). The structures of 1-4 were characterized on the basis of spectroscopic and MS data and resemble known 10-membered lactones, but feature modified oxidation patterns around their macrocycles.

The boat-shaped polyketide resistoflavin results from re-facial central hydroxylation of the discoid metabolite resistomycin

Resistoflavin (1) is a rare boat-shaped pentacyclic polyketide metabolite of Streptomyces resistomycificus with marked antibacterial activity. By a series of experiments we have disclosed that the optically active molecule is derived from the discoid polyketide resistomycin (2) by an unusual, enantioface-differentiating hydroxylation, which leads to the capped pentacyclic ring system. In vivo and in vitro experiments unequivocally demonstrate that this reaction is catalyzed by RemO, an FAD-dependent monooxygenase. In addition, we were able to establish the absolute configuration of 1 and thus the stereochemical course of this rare enzymatic reaction by extensive computational methods. Comparison of the experimental CD spectrum with those quantum chemically calculated for (R)-1 and (S)-1 revealed the R-configuration of 1. Consequently, the enzyme-catalyzed hydroxylation takes place from the Re-face of 2 with loss of aromaticity in favor of a chiral carbinol center. While other oxygenases involved in polyketide tailoring functionalize the periphery of polyphenols, RemO is unique in its ability to catalyze a central, nonperipheral hydroxylation of a fused ring system.

Spontaneous Resolution of Aromatic Sulfonamides: Effective Screening Method and Discrimination of Absolute Structure

An effective screening method combining parallel synthesis and solid-state CD measurements was established to identify achiral aromatic sulfonamides that show spontaneous resolution with rapidity. We found that 4 of the 12 achiral sulfonamides crystallized as chiral crystals through this method. The chirality of each sulfonamide was discriminated by solid-state CD spectra and Flack parameter in an X-ray analysis. Correspondence between the observed Cotton effect and the absolute configuration could be confirmed by time-dependent DFT calculations. [structure: see text]

An attempt to determine the absolute configuration of two ascolactone stereoisomers with time-dependent density functional theory

Two stereoisomers of ascolactone (A, B), natural products with two asymmetric carbon atoms, are isolated from the marine-derived fungus Ascochyta salicorniae. Although these compounds show virtually opposite CD spectra and [alpha]D, 1H- and 13C-NMR data exclude the presence of enantiomers and suggest ascolactone A and B to be epimeric lactones. By comparing the experimental CD spectra with those calculated employing time-dependent density functional theory (TDDFT), we elucidate the configuration at one of the asymmetric carbon atoms.

Polyketides from the marine-derived fungus Ascochyta salicorniae and their potential to inhibit protein phosphatases

Chemical investigation of the marine fungus Ascochyta salicorniae led to the isolation of two new epimeric compounds, ascolactones A (1) and B (2), in addition to the structurally-related polyketides hyalopyrone (3), ascochitine (4), ascochital (5) and ascosalipyrone (6). The absolute configurations of the epimeric compounds 1 and 2 were assigned as (1R,9R) and (1S,9R), respectively, through simulation of the chiroptical properties using quantum-chemical CD calculations, and chiral GC-MS subsequent to oxidative cleavage (Baeyer-Villiger oxidation) of the side chain. In silico screening using the PASS software identified some of the A. salicorniae compounds (1-6) as potential inhibitors of protein phosphatases. Compound was found to inhibit the enzymatic activity of MPtpB with an IC(50) value of 11.5 microM.