Optically active seleninate esters: isolation, absolute configuration, racemization mechanism, and transformation into chiral selenoxide

Optically active seleninate esters were obtained for the first time by chromatographic resolution on an optically active column. The absolute configurations of the optically active seleninate esters were determined by comparing their chiroptical properties with those of two analogous sulfinate esters, the absolute configuration of one of which is known and that of the other was determined by X-ray crystallographic analysis. The optically active seleninate esters were found to racemize in solution. Kinetic studies of the racemization, the oxygen exchange reaction with H(2)(18)O, and theoretical studies clarified that the racemization of the optically active seleninate esters in solution proceeded via an achiral hypervalent selenurane intermediate that was formed by the reaction with water. The reaction of the optically active seleninate ester and the sulfinate ester having bulky substituents with Grignard reagents was found to proceed with the retention of stereochemistry to give an optically active selenoxide and sulfoxides, respectively.     

Optically active telluronium imides: Optical resolution,absolute configuration,and mechanism of racemization

Thermodynamically and kinetically stabilized asymmetric diaryltelluronium imides were synthesized and optically resolved into their enantiomers on an optically active column using medium‐pressure column chromatography. The relationship between the absolute configuration and the optical properties of the chiral telluronium imides was clarified. The mechanism of the racemization, which involves the formation of telluroxides by the hydrolysis of the telluronium imides, was proposed. © 2003 Wiley Periodicals, Inc. Heteroatom Chem 14:523–529, 2003; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/hc.10185     

Optically active Phosphine oxides : Synthesis and absolute configuration of (menthoxycarbonylmethyl) phenylvinyl phosphine oxide

The enantiomeric(-)(menthoxycarbonlymethyl)phenylvinylphosphine oxide 1b has been prepared and its absolute configuration rigorously established via chemical correlation. One-step decarbalkoxylation of (-)-ethyl(methoxycarbonylmethyl)phenyphosphine oxide 6 was effected with LiCl-H₂O-DMSO and shown to proceed without concomitant racemization at phosphorus.     

Optically active heteroaromatic compounds IX. Preparation,absolute configuration and optical purity of some chiral 3-alkylindoles

The Fischer and Brunner reactions as well as the Grignard alkylation of isatin have been used to prepare (S)-3-sec-butyl- and (S)-3-(2-methylbutyl)indoles, respectively, starting from (S)-1-chloro-2-methylbutane. The stereospecificity of the synthetic sequences followed has been investigated and the stereochemical relationships of the title compounds have been established.     

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.     

Synthesis,absolute configuration and conformation of optically active 1,2-homoheptafulvalene

An optically active 1,2-homoheptafulvalene was successfully synthesized and subjected to spectroscopic investigation. The cycloaddition of the optically active hydrocarbon with tetracyanoethylene (TCNE) and 4-phenyl-1,2,4-triazoline-3,5-dione(PTAD) gave a [4 + 2] cycloadduct and a mixture of [8 + 2] cycloadducts, respectively, which are both optically active.     

Isolation, absolute configuration, and chiral crystallization of optically active seleninic acid

An optically pure seleninic acid was isolated as stable crystals for the first time by chiral crystallization. The relationship between the absolute configurations and the circular dichroism spectra of the enantiomers could be determined by X-ray crystallographic analysis.     

Optically controlled resonance energy transfer: mechanism and configuration for all-optical switching

In a molecular system of energy donors and acceptors, resonance energy transfer is the primary mechanism by means of which electronic energy is redistributed between molecules, following the excitation of a donor. Given a suitable geometric configuration it is possible to completely inhibit this energy transfer in such a way that it can only be activated by application of an off-resonant laser beam: this is the principle of optically controlled resonance energy transfer, the basis for an all-optical switch. This paper begins with an investigation of optically controlled energy transfer between a single donor and acceptor molecule, identifying the symmetry and structural constraints and analyzing in detail the dependence on molecular energy level positioning. Spatially correlated donor and acceptor arrays with linear, square, and hexagonally structured arrangements are then assessed as potential configurations for all-optical switching. Built on quantum electrodynamical principles the concept of transfer fidelity, a parameter quantifying the efficiency of energy transportation, is introduced and defined. Results are explored by employing numerical simulations and graphical analysis. Finally, a discussion focuses on the advantages of such energy transfer based processes over all-optical switching of other proposed forms.     

Processing of cyclopropylarenes by toluene dioxygenase: isolation and absolute configuration of metabolites

Several arenes possessing a cyclopropyl substituent were subjected to enzymatic oxidation with toluene dioxygenase. The absolute configuration of metabolites was established by chemical means.     

One-pot synthesis of helical aromatics: stereoselectivity, stability against racemization, and assignment of absolute configuration assisted by experimental and theoretical circular dichroism

Helical aromatics (1) were synthesized via one step in good quantity by solvent-free condensation of N,N'-p-phenylenediamine (2) and various carboxylic acids in the presence of a Lewis acid. Microwave irradiation greatly facilitated the condensation reaction to furnish 1 with a 100% diastereo- and a 50% enantioselectivity, when a chiral carboxylic acid was utilized. 1f, derived from 2-methylglutaric acid, was quite stable, no racemization taking place even at 200 degrees C. The assignment of the absolute configurations to the helical aromatics has been achieved by experimental and theoretical CD spectra calculated by time-dependent density functional theory.     

Arylbiphenylene atropisomers: structure, conformation, stereodynamics, and absolute configuration

Anti and syn conformers, due to restricted sp(2)-sp(2) bond rotation, were detected in hindered 1,8-diarylbiphenylenes, the aryl moieties being phenyl groups bearing 0micron-alkyl substituents such as Me, Et, i-Pr, and t-Bu. By means of low-temperature NOE experiments, the corresponding structures were assigned and were found to be in agreement with the results of single-crystal X-ray diffraction. The interconversion barriers of these conformers were determined by line-shape simulation of the variable-temperature NMR spectra and the experimental values were reproduced satisfactorily by DFT calculations. In the case of the bulkiest aryl substituent investigated (i.e., 2-methylnaphthalene), the syn and anti atropisomers were stable enough as to be separated at ambient temperature. The two enantiomers (M,M and P,P) of the isomer anti were also isolated by enantioselective HPLC, and the theoretical interpretation of the corresponding CD spectrum allowed the absolute configuration to be assigned.     

Vanchrobactin: absolute configuration and total synthesis

The stereochemistry of vanchrobactin, a siderophore produced by the bacterial fish pathogenVibrio anguillarum serotype O2, was elucidated by chiral capillary electrophoresis analysis and total synthesis as N-[N′-(2,3-dihydroxybenzoyl)-d-arginyl]-l-serine.     

Optically active bisbibenzyls from Bazzania trilobata: isolation and stereochemical analysis by chromatographic, chiroptical, and computational methods

Bazzanin S, a new chlorinated bisbibenzyl of the isoplagiochin D type, as well as isoplagiochin D itself were isolated from the liverwort Bazzania trilobata. The structure of bazzanin S was elucidated based on extensive NMR spectral evidence and by mass spectrometry. Both macrocyclic compounds, bazzanin S and isoplagiochin D, as well as previously reported bazzanins exhibit optical activity, but are not enantiopure in nature. The enantiomeric ratios of a broad array of different cyclic bisbibenzyls, isoplagiochin D, bazzanins A-C and F-J, and bazzanin S, were determined by HPLC on a chiral phase. Exemplarily for the halogen-free parent compound isoplagiochin D, elucidation of the absolute configuration was achieved by quantum chemical CD calculations.     

Stilbene oligomers from Parthenocissus laetevirens: isolation, biomimetic synthesis, absolute configuration, and implication of antioxidative defense system in the plant

Five new stilbene oligomers, laetevirenol A-E (4-8), were isolated from Parthenocissus laetevirens, together with three known stilbene oligomers (2, 3, and 9). The structures of the new compounds were elucidated by spectroscopic analysis, including 1D and 2D NMR experiments. Afterward the absolute configurations were determined. Biomimetic transformations revealed a possible biogenetic route, where stilbene trimers were enzymatically synthesized for the first time. In addition, their antioxidant activities were evaluated by 1,1-diphenyl-2-picrylhydrazyl (DPPH) assay. The results showed that stilbene oligomers with an unusual phenanthrene moiety exhibited much stronger antioxidant activities. Thus, the photocatalyzed cyclization of stilbenes was supposed to be an antioxidant activity promoting transformation, which was hypothesized to play a role in the antioxidative defense system of the plant.     

Moenuronic acid: synthetic studies and absolute configuration

The absolute configuration of the moenuronic acid moiety of the antibiotic moenomycin A has been established by a partial synthesis starting from D-galactose. The relation between the absolute conformation of α-hydroxy lactones and their chiroptical properties is discussed in detail.