Stereochemical Structures Research Articles

Optical Rotatory Silica Materials Prepared via Sol−Gel Processes

A series of transparent, monolithic silica materials doped with 5−20 wt % of (−)-dibenzoyl-l-tartaric acid (DBTA) and (+)-d-glucose have been prepared via the acid-catalyzed sol−gel reactions of tetraethyl orthosilicate in the presence of the organic dopants. The materials show optical rotations, which can be directly measured on a common polarimeter. The optical rotation was found to be proportional to the dopant concentration and to the thickness of the samples. The specific rotations measured at 589 nm at 25 °C are +53° for the d-glucose-doped and −112° for the DBTA-doped silica samples, which are close to those for the same compounds measured in liquid solutions. Both the DBTA- and d-glucose-doped silicas exhibit a simple normal dispersion with negligible concentration effects. The organic compounds retain their stereochemical structures during the sol−gel reactions and, therefore, their optical activities. These materials can be regarded as nanocomposites in which the organic dopants uniformly distribute in the amorphous silica networks.

Absolute configuration of α-pyrones from Cryptocarya latifolia and Syncolostemon densiflorus

The stereochemical structures of two α-pyrones previously isolated from Cryptocarya latifolia have been shown to be 6 R-[2 R,4 S,6 S-(triacetoxy)-heptyl]-5,6-dihydro-2 H-pyran-2-one and 6 R-[2 S,4 S-(diacetoxy)-pentyl]-5,6-dihydro-2 H-pyran-2-one. Syndenolide from Syncolostemon densiflorus is 6 R-[5 S-(acetoxy)-1 R,2 R,3 S-(trihydroxy)-heptyl]-5,6-dihydro-2 H-pyran-2-one.

Direct chiral resolution of phenylalkylamines using a crown ether chiral stationary phase.

A direct, isocratic high-performance liquid chromatographic method is described for the enantiomeric resolution of a number of phenylalkylamines, namely, racemic cathinone, amphetamine, norephedrine, and norphenylephrine, without sample derivatization. The separations were achieved on an S-18-crown-6-ether chiral stationary phase known as CR(+). The chromatographic parameters alpha (separation factor) and Rs (resolution factor) lay within a narrow range for all compounds used in this study except for cathinone, which resulted in high alpha and Rs values. The recognition mechanism for this column involves the interaction of the crown structure with a charged primary ammonium ion. The stereochemical structures of the compounds in this study contribute to the results obtained for the chromatographic parameters, especially in cathinone's case. This paper will discuss the recognition mechanism contributing to the high alpha and Rs, values obtained for cathinone.

Protection by diphenyliodonium against glutamate neurotoxicity due to blocking of N-methyl-D-aspartate receptors.

The protective effect of diphenyliodonium, known as an inhibitor of flavin enzymes including nitric oxide synthases, was examined against the neurotoxicity of excitatory amino acids on cultured spinal neurons of the rat. Diphenyliodonium reduced the neuronal damage induced by 15-min exposure to glutamate or N-methyl-D-aspartate in a dose-dependent manner; half effective concentrations (EC50) were about 3 microM for both. Protection was only observed when diphenyliodonium was added into the exposure medium. Diphenyliodonium showed no effect on the toxicity induced by 24 h exposure to non-N-methyl-D-aspartate receptor agonists. Using a microfluorometry technique with Fura 2, we observed that diphenyliodonium reversibly inhibited the N-methyl-D-aspartate-evoked intracellular Ca2+ elevation. The amount of 45Ca2+ influx induced by N-methyl-D-aspartate was also inhibited by diphenyliodonium in a dose-dependent manner; EC50 was about 3 microM. Furthermore, we examined the effect of diphenyliodonium on an opening activity of the N-methyl-D-aspartate receptors estimated by binding of dizocilpine maleate to membrane fractions from whole brain of adult rat and from cultured spinal neurons. Diphenyliodonium inhibited the binding of dizocilpine maleate dose-dependently; EC50 was 5-8 microM. These results suggest that diphenyliodonium is a new antagonist to the N-methyl-D-aspartate receptors and that diphenyliodonium protects neurons against glutamate toxicity due to a direct blocking of the Ca2+ influx. This conclusion is supported by the similarity of the stereochemical structures predicted by computer between diphenyliodonium and dizocilpine maleate.

Direct Chiral Resolution of Phenylalkylamines Using a Crown Ether Chiral Stationary Phase

A direct, isocratic high-performance liquid chromatographic method is described for the enantiomeric resolution of a number of phenylalkylamines, namely, racemic cathinone, amphetamine, norephedrine, and norphenylephrine, without sample derivatization. The separations were achieved on an S-18-crown-6-ether chiral stationary phase known as CR(+). The chromatographic parameters α (separation factor) and Rs (resolution factor) lay within a narrow range for all compounds used in this study except for cathinone, which resulted in high α and Rs values. The recognition mechanism for this column involves the interaction of the crown structure with a charged primary ammonium ion. The stereochemical structures of the compounds in this study contribute to the results obtained for the chromatographic parameters, especially in cathinone's case. This paper will discuss the recognition mechanism contributing to the high α and Rs values obtained for cathinone. © 1997 by John Wiley & Sons, Ltd.

Plastic atomic-molecular models — A key to stereochemical structures

Plastic atomic-molecular models — A key to stereochemical structures

Cis-clerodane diterpene lactones from Amphiachyris dracunculoides. 3.

Five cis-clerodane lactones, amphiacrolides J (1), K (2), L (3), M (4), and N (5), were isolated from the aerial parts of the composite Amphiachyris dracunculoides. The first four are new compounds, and the fifth was previously reported from another source. All compounds were assigned stereochemical structures based upon spectral and chemical data, including high-field 1D and 2D NMR for complete assignment of 1H- and 13C-NMR spectra. Amphiacrolide N (5) was converted to amphiacrolide J (1) by treatment with MeOH and TFA, and amphiacrolide L (3) was prepared from amphiacrolide B (9) by CrO3 oxidation to ketone 10 followed by its reduction with NaBH4.

Rollinecins A and B: two new bioactive annonaceous acetogenins from Rollinia mucosa.

Two new mono-THF ring acetogenins, rollinecins A (1) and B (2), were isolated from the partitioned ethanolic extracts of the leaves of Rollinia mucosa (Annonaceae) by activity-directed fractionation. 1 and 2 are epimeric at the C-14 carbinol stereocenter. Their absolute stereochemical structures were solved by preparing their respective per-Mosher ester derivatives. 1 and 2 showed equivalent and selective in vitro activities against several human solid tumor cell lines.

Tuning specificities to aliphatic odorants in mouse olfactory receptor neurons and their local distribution

1. Odor responses to two homologous series of n-fatty acids (nFA) and n-aliphatic alcohols (nAA) with a straight chain of three to nine carbons were examined by measuring odor-induced [Ca2+]i increase in mouse olfactory receptor neurons (ORNs) isolated by the tissue-printing method. 2. One-third of the ORNs responsive to nFA and/or nAA were alternately sensitive to either type of odorant. Their sensitivities were usually near maximal for one or two odorants and decreased with differences in the carbon chain length from the tuned odorants. 3. Two-thirds of the ORNs responsive to nFA and/or nAA were sensitive to both types of odorants. Most of them were also tuned to one or two odorants in each series with similar carbon chain lengths and showed a decrease of sensitivity with increasing stereochemical discrepancy, similar to nFA/nAA discriminating ORNs. 4. In 10 of 20 non-nFA/nAA discriminating ORNs, the sensitivity to nFA was > 10 times greater than to nAA, and 80% of them were localized in a central region of olfactory epithelium on the septum wall where ORNs preferentially project to the dorsomedial or centromedial regions of the olfactory bulb. In addition, the sensitivity to three series of n-aliphatic odorants with an added amino group was examined. Sensitivity became higher as the electronegativity of the functional groups increased, suggesting that a hydrogen bond might partly mediate affinity in one type of non-nFA/nAA discriminating ORNs. 5. The diversity in odorant tuning specificity and sensitivity of the individual ORNs indicated that their receptor sites were finely tuned to the stereochemical structures of numerous odorants by changes in the three-dimensional size and intermolecular positions of the hydrophobic domains for hydrophobic bond, as well as the proton-acceptor or donor for the hydrogen bond and the electrical charge for the ionic bond. 6. The subpopulation of ORNs tuned to an individual odorant increased as the length of carbon chain of the odorant increased from three to nine. This tendency was more marked for nFA than for nAA in the case of non-nFA/nAA discriminating ORNs. 7. Data obtained by the in vitro approach using the tissue-printing method suggested that three or more subtypes of ORNs, which were similar in some cases and significantly different in other cases, were located within close proximity to one another.

Stereochemical structures of some polyacrylates studied by one-dimensional and two-dimensional 13C 1H n.m.r. spectroscopy

The stereochemical structures of samples of poly(ethyl acrylate) (PEA), poly(butyl acrylate) (PBuA) and poly(2-ethylhexyl acrylate) (PEHA), prepared by group transfer and anionic polymerization, were studied by 13C and 1H n.m.r. spectroscopy. The main-chain CH 2 spectral range was analysed by the application of the inverse 13C 1H HETCOR two-dimensional n.m.r. technique. A very similar HETCOR pattern was found for all polymers studied. In all cases, main-chain CH 2 lines in the 1H and 13C n.m.r. spectra could be assigned to configurational tetrads; for PEA and PBuA a partial assignment of carbon resonances to configurational hexads was possible. While PEHA prepared both by group transfer and anionic polymerization and PEA prepared by group transfer polymerization can be well approximated by Bernoullian statistics, for PEA and PBuA prepared by anionic polymerixation a first-order Markov model is required to fit the observed intensities at the hexad level.

Stereochemical structures of (8,8-ethylenedioxy-2-oxo-)- and (8,8-ethylenedioxy-2-hydroxy-)-6-phenylsulphonylbicyclo[3·3·0]octanes

Cyclopentannulation of 4-tert-butyldimethylsilyloxy-2-cyclopenten-1-one from a β-metallated dioxolane-type orthoester affords 4-tert-butyldimethylsilyloxy-8,8-ethylenedioxy-6-phenylsuiphonyl-2-oxobicyclo [3·3·0] octanes as a mixture of two diastereoisomers. Their stereochemical structures were established by NMR and x-ray analysis. The minor product possesses a cis ring fusion, the sulphonyl and the tert-butyldimethylsilyloxy groups being endo and exo, respectively. The major isomer also has a cis ring fusion but an exo position for both dimethylsilyloxy and sulphonyl groups. Both cyclopentannulation products result from an anti addition with respect to the 4-tert-butyldimethylsilyloxy group on the enone.

Coding of odor molecules by mitral/tufted cells in rabbit olfactory bulb. I. Aliphatic compounds

1. Recordings of extracellular spike responses were made from single mitral/tufted cells in the main olfactory bulb of urethan-anesthetized rabbits. Olfactory epithelium ipsilateral to the recorded olfactory bulb was stimulated with homologous series of aliphatic compounds using periodic artificial inhalations. 2. In the dorsomedial part of the main olfactory bulb, single mitral/tufted cells were activated by subsets of n-fatty acids with similar hydrocarbon chain lengths. Response selectivities of single mitral/tufted cells were examined in detail using a series of n-fatty acids at five different concentrations. The results indicate that although the range of effective fatty acids is broader at the higher concentrations, the best response at higher concentrations was similar to that determined at lower concentrations. 3. Analysis of single-unit responses to the panel of fatty acids, including those with branched hydrocarbon chains, suggested that the determinants for the response specificities of individual mitral/tufted cells in the dorsomedial region include the overall size of hydrocarbon chains of the odor ligand molecules. 4. Single mitral/tufted cells in the dorsomedial region tended to be activated not only by fatty acids but also by n-aliphatic aldehydes. For a panel of a homologous series of n-aldehydes at five different concentrations, individual mitral/tufted cells showed response selectivity to subsets of aldehydes with similar hydrocarbon chain lengths. 5. In most cases, normal aliphatic alcohols and alkanes were ineffective in activating mitral/tufted cells in the dorsomedial region. This suggests that carbonyl group (--C = O) in the odor molecules plays an important role in determining response specificity of these neurons. 6. Examination with an expanded panel of stimulus odor molecules that included ketones and esters indicated that single mitral/tufted cells sensitive to subsets of fatty acids and n-aliphatic aldehydes were also responsive to subsets of ketones and/or esters having hydrocarbon chain lengths similar to those of the effective fatty acids and aldehydes. 7. The present results show a clear correlation between the tuning specificity of individual mitral/tufted cells and the stereochemical structure of the odor molecules, with respect to 1) length and/or structure of hydrocarbon chain, 2) difference in functional group, and 3) position of the functional group within the molecule. 8. A hypothetical diagram suggesting functional convergence of olfactory nerve input to individual glomeruli is proposed to explain the mechanism for selective activation of individual mitral/tufted cells by a range of odor molecules with similar stereochemical structures.

Rational approach to stereochemical analysis of styrene-methyl methacrylate copolymers through synthesis of appropriate model compounds

As a model for the central styrene unit in alternating styrene-methyl methacrylate copolymers the different 3,7-dimethyl-3,7-bis((methyloxy)carbonyl)-5-phenylnonanes were investigated. As models for central distyryl dyads the different 3,9-dimethyl-3,9-bis([methoxy]carbonyl)-5,7-diphenylundecanes were prepared and investigated. The comonomer sequence and stereochemical structures of these compounds were analyzed using 1 H, 13 C and 1 H- 13 C chemical shift correlation spectroscopy and were compared with those of the copolymers reported earlier

The Stereochemistry of the Sulfur Atom of 4,7-Dimethyl-6-phenyl-5,6,7,8-tetrahydro-4H-1,3,2,6-dioxathiazocine 2-Oxides

Abstract The reaction of N-(2-hydroxypropyl)-N-(2-hydroxy-1-methylethyl)aniline with thionyl chloride afforded four isomers of 4,7-dimethyl-6-phenyl-5,6,7,8-tetrahydro-4H-1,3,2,6-dioxathiazocine 2-oxide. Oxidation of the four isomers by the use of KMnO4 yielded two kinds of sulfates. On the basis of the coupling constants and γ-effects, the stereochemical structures of the dioxathiazocine 2-oxides are discussed.

Inhibition of tyrosine hydroxylase by R and S enantiomers of salsolinol, 1-methyl-6,7-dihydroxy-1,2,3,4-tetrahydroisoquinoline.

Salsolinol is one of the dopamine-derived tetrahydroisoquinolines and is synthesized from pyruvate or acetaldehyde and dopamine. As it cannot cross the blood-brain barrier, salsolinol as the R enantiomer in the brain is considered to be synthesized in situ in dopaminergic neurons. Effects of R and S enantiomers of salsolinol on kinetic properties of tyrosine hydroxylase [tyrosine, tetrahydrobiopterin:oxygen oxidoreductase (3-hydroxylating); EC 1.14.16.2], the rate-limiting enzyme of catecholamine biosynthesis, were examined. The naturally occurring cofactor of tyrosine hydroxylase, L-erythro-5,6,7,8-tetrahydrobiopterin, was found to induce allostery to the enzyme polymers and to change the affinity to the biopterin itself. Using L-erythro-5,6,7,8-tetrahydrobiopterin, tyrosine hydroxylase recognized the stereochemical structures of the salsolinols differently. The asymmetric center of salsolinol at C-1 played an important role in changing the affinity to L-tyrosine. The allostery of tyrosine hydroxylase toward biopterin cofactors disappeared, and at low concentrations of biopterin such as in brain tissue, the affinity to the cofactor changed markedly. A new type of inhibition of tyrosine hydroxylase, by depleting the allosteric effect of the endogenous biopterin, was found. It is suggested that under physiological conditions, such a conformational change may alter the regulation of DOPA biosynthesis in the brain.

Chemical stereographs: An extension of chemical graphs for representing stereochemical and conformational structures

Chemical stereographs are presented as vehicles for representing qualitative three-dimensional features of molecules that put stereochemical and conformational distinctions in a common graph-theoretic formalism. They extend the concept of a chemical graph by adding “tetrads”, each qualitatively characterizing the three-dimensional arrangement of four atoms with respect to its clinicity and handedness components. The characterization is sufficiently precise to distinguish synperiplanar, synclinical (gauche), anticlinal and antiperiplanar relationships between vicinal atoms of various conformers. Collectively, the tetrads constitute the embedding graph which presents new possibilities in displaying the stereochemical and conformational features of a molecule. A chemical graph and one of its possible embedding graphs constitute a chemical stereograph. Potential applications of chemical stereographs in the areas of structural representations, molecular symmetry analysis, and stereo-specific substructure searching are discussed.

Keto-Enol-Tautomerism and Configurational Isomerism of 2,6-Disubstituted 4-Piperidone-3,5-dicarboxylates

The dialkyl 2,6-dialkylsubstituted 4-piperidone-3,5-dicarboxylates were synthesized by a Mannich procedure. Depending on the substitution at the nitrogen keto-enol-tautomerism and a configurational isomerism at C 2 is observed. The structure of the N-substituted piperidone 24 E (C18H29NO5) has been determined by X-ray analysis: it is characterized by an enol structure of the β-ketoester and an axial position of the alkyl group at C 2 and an equatorial one of the alkyl group at C 6. The O–H···O hydrogen bond shows characteristic values of a strong hydrogen bond. The N-unsubstituted piperidones adopt a ketone structure with the allequatorial position of all substituents. This stereochemistry is confirmed for other enolic and ketone analogues by NMR spectroscopic methods. To work out the reason for the different thermodynamic stabilities of the different stereochemical structures of N-substituted and N-unsubstituted piperidones, various semiempirical calculations were done for series of simple pairs of carbonyl/enol tautomers, substituted acetoacetates, oxoglutarate as well as of systematically varied substituted cyclohexanone, 4-oxacyclohexanone and 4-piperidone derivatives.

The stereochemical investigation of 8‐membered 1,3,6‐dioxazocines viaNMR

AbstractThe stereochemical properties of 2‐, 4‐, 7‐, and 8‐methyl substituted 5,6,7,8‐tetrahydro‐1,3,6‐dioxazocines have been investigated by their pmr and cmr spectroscopy. On the basis of the coupling constant and γ‐effects, the stereochemical structures are discussed.

New β‐dihydroagarofuran sesquiterpenoids from Celastrus paniculatus

AbstractTwo new β‐dihydroagarofuran sesquiterpene polyesters were isolated from Celastrus paniculatus. The structures were deduced using various 2D NMR techniques (HH COSY, CH COSY, COLOC, NOESY), NOE difference spectra and mass spectra. The positions of the ester moieties were located by 2D heteronuclear correlated spectra via long‐range coupling, or combined with NOE experiments. Their stereochemical structures were studied and the complete assignments of the 1H and 13C chemical shifts were made.

ChemInform Abstract: NMR Nuclear Shielding and the Electronic Structures of Macromolecules

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.