Stereochemical Relations Research Articles

Probing helix propensity of monomers within a helical oligomer

A simple strategy is proposed to assess the propensity of a given monomer to follow or not follow a particular helical scheme and to study helix reversal phenomena within helical oligomers. It consists of placing a monomer having a low helix propensity between two conformationally stable helical segments. Helix reversion then occurs preferentially at the site of this monomer, leading to the formation of isomers having P (right-handed) or M (left-handed) helicities at each of the two helical segments. The proportion between the P-P/M-M and P-M isomers is indicative of the stereochemical relations between the inserted monomer and the helical frame. Thus, xylylene or carboxylic acid anhydride spacers have been introduced between two helical oligoamides of 8-amino-2-quinolinecarboxylic acid. Both these spacers presumably lack some of the structural features that confer quinoline units with a high helix propensity. Only one species is observed in solution in the case of an anhydride spacer. This species was shown by x-ray crystallography to be a racemic mixture of P-P and M-M helices. Unexpectedly, the anhydride is consistently incorporated within helical oligoamides. For the xylylene spacer, the P-P/M-M racemate and P-M meso compound are in equal proportions in chloroform, showing that this spacer does not have a propensity to adopt any helical conformation in this solvent. However, the equilibria between the various isomers are shifted in toluene, where one species largely prevails. This species was shown by x-ray crystallography to be the P-P/M-M racemate. Molecular dynamics simulations are consistent with these solution data.

Uncovering stereochemical relations in a compound with a stereogenic N-O axis: methyl 2-(4-methyl-2-thioxo-2,3-dihydrothiazol-3-yloxy)propanoate.

The geometry of racemic methyl 2-(4-methyl-2-thioxo-2,3-dihydrothiazol-3-yloxy)propanoate, C(8)H(11)NO(3)S(2), (I), is characterized by a distorted heterocyclic five-membered ring and an enantiomorphic N-alkoxy substituent, which is inclined at an angle of -68.8 degrees to the thiazolethione plane in (M)-(I). The unit cell consists of a 1:1 ratio of R,P- and S,M-configured molecules of (I). The combination of a P configuration at the N-O axis and an R configuration at the asymmetric propanoate C(beta) atom on one side, and an S,M configuration on the other side, is considered to originate from steric interactions. The largest substituent at the asymmetric propanoate C(beta) atom, i.e. the methoxycarbonyl group, resides above the methyl substituent; the medium-sized propanoate gamma-methyl substituent points in the opposite direction with respect to the N-O bond, whereas the H atom is located above the C=S double bond of the thiazolethione subunit.

Epitaxy of CdS and PbS on fatty acid monolayer: lattice matching and stereochemical relations

Epitaxy of CdS and PbS on fatty acid monolayer: lattice matching and stereochemical relations

A facile C-C bond cleavage in the epoxides and its use for the synthesis of oxygenated heterocycles by a ring expansion strategy.

The bicyclic epoxy alcohols when treated with DAST gave a new class of rearranged organofluorine compounds, by a ring expansion via C-C bond cleavage of the oxirane ring. The outcome of this reaction with respect to ring size and stereochemical relation between the functionalities is presented here.

Stereochemistry of Hydrodenitrogenation: The Mechanism of Elimination of the Amino Group from Cyclohexylamines over Sulfided Ni–Mo/γ-Al2O3 Catalysts

The hydrodenitrogenation of cyclohexylamine and of the diastereomers of 2-methylcyclohexylamine and 2,6-dimethylcyclo-hexylamine was studied at 200 to 350°C and 50 bar pressure over a sulfided Ni–Mo/γ-Al2O3 catalyst. The removal of the amino group from cyclohexylamines occurs primarily by means of a β elimination mechanism. Elimination of a β hydrogen atom attached to a tertiary carbon atom is faster than that of a β hydrogen atom attached to a secondary carbon atom. The rate of elimination also depended on the stereochemical configuration of the amino group with respect to the β hydrogen atoms. Elimination was more rapid when the configuration of the amino group was anti periplanar to the β hydrogen atom (in a chair conformation) than when it was syn periplanar (in a boat conformation). This is consistent with an E2 elimination mechanism. The relative elimination rates of all the diastereomers of the substituted cyclohexylamines were rationalized in terms of the stereochemical relation between the amino group and a β hydrogen atom, the number and type of β hydrogen atoms, and the energy required to attain the required chair/boat conformation(s). In contrast to previous proposals for anti elimination of H2O from alcohols, it was shown that an anti elimination mechanism is possible on the surface of heterogeneous catalysts if SH or OH groups protrude from the catalyst surface.

Stereochemische aspekte in zusammenhang mit der synthese von 2,6-anhydro-1-desoxy-1-nitroalditolen

2,6-Anhydro-1-deoxy-1-nitroalditols were prepared in good yields by known procedures via addition of nitromethane to d-glucose, d-mannose, d-galactose, d-xylose, d-lyxose and l-arabinose, followed by a cyclization step in boiling water. In the case of d-ribose, a mixture of pyranoid and furanoid anhydroalditols was isolated by different methods in relatively poor yields, at variance with recent reports by other authors. Establishment of the stereochemical relations of the products from 1H-n.m.r. spectra of their acetates gave further insight into the diastereo-selectivity of the cyclization step.

Methamphetamine from Ephedrine: I. Chloroephedrines and Aziridines

Abstract Illicit methamphetamine clandestinely synthesized from ephedrine via reduction of chloroephedrine is discussed. The stereochemistry, mechanism, synthetic impurities, and analysis of clandestine methamphetamine samples is addressed. Stereochemical relation of (+)methamphetamine to its initial precursor (−)ephedrine or (+)pseudoephedrine is achieved by detection of (+)chloropseudoephedrine or cis-1,2-dimethyl-3-phenylaziridine, in the case of the first, and (-)chloroephedrine or trans-1,2-dimethyl-3-phenylaziridine in the case of the latter.

Prostaglandins and cancer: Further stereochemical studies on the molecule of phospholipase A 2

(1) This paper continues the stereochemical analysis of the molecule of phospholipase A 2 as revealed by X-ray crystallography. (2) Previously its stereochemical relations to tumor-promoting phorbol esters and to PGE 1 have been reported. (3) Here the stereochemical relations with a series of anti-tumor retinoids and diterpinoids are described. (4) These stereochemical relationships lead to a series of testable hypotheses and experiments concerning the mechanism of action of these agents.

Insertion de SO 2 dans la liaison titane—carbone : III. Stereochimie de la reaction au niveau de l'atome de titane et synthese de complexes presentant trois atomes chiraux (C, Ti, S)

The stereochemistry at the titanium atom has been studied in insertion reactions of sulfur dioxide with the two diastereoisomers of η 5-C 5H 5-η 5-C 5H 4-CHMePhTi(C 6F 5)CH 3. This reaction is regioselective and stereospecific. A stereochemical relation between the starting complexes and the insertion products is proposed on systematic correlation of the relative positions of the NMR signals for various diastereoisomeric pairs η 5-C 5H 5-η 5-C 5H 4CHMePhTiAB. This relation shows that the insertion reaction involves retention at the titanium atom.

Prostaglandins and cancer

The experimental demonstration that prostaglandins (PG's) are tumor promoting agents (skin) confirms the prediction made on stereochemical grounds in this journal in 1975. This present paper explores more fully the stereochemical relations between the tumor promoting/cathartic group of drugs (such as phorbol esters, anthralin, colocynth and podophyllum), a group of anti-tumor agents (such as other phorbol esters, datiscoside, bruceantin, decumbrin and tetandrine) and two simple protein structures. (i) One is composed to two parallel strands of peptide chains some 13–14 Å apart. One chain has the sequence — pSer-X-Ile-X-His-X-pSer—and the other has the same sequence in reverse. Experiments with CPK molecular models indicate that this structure could bind two PGs in co-operation with two Ca 2+ ions chelated between the phosphoserine moieties. Many drugs of the tumor promoting/cathartic class have molecular structures that enable them to bind to this model receptor in such a way as to allow Ca 2+ binding and orienting of both phosphoserines at each end correctly. Certain anti-tumor drugs have molecular structures that allow them to bind to the receptor, and to distort these key phosphoserine moieties so that they can no longer chelate a Ca 2+ ion between each phosphoserine pair. This model PG (tp) receptor (2 PG molecules) also provides a structure that is to a high degree complementary to spermine (1 molecule). (ii) The second site is an allosteric site on phospholipase A 2 adjacent to the active site as revealed by X-ray crystallography. This is complementary to lysophosphatidylethanolamine, which stimulates the enzyme and the active tumor promoting phorbol esters such as TPA. The hypothesis is therefore presented in three stages: (1) Some tumor promoters act at a PG receptor whose normal function is to chelate two Ca 2+ ions per two molecules of PG per receptor. (2) This binding may release a molecule of spermine normally bound here. (3) Other tumor promoters (e.g. TPA) act at an earlier stage, i.e. the rate controlling step in PG synthesis by phospholipase A 2. Other data linking prostaglandins, polyamines, phospholipase A 2 and cancer are reviewed. Experiments to test these hypotheses are suggested.

Identification of chiral 1,2‐diacylglycerols in fresh milk fat

AbstractTrityl chloride, which is known to attack primary hydroxyl groups selectively, was used for the isolation of 1,2‐diacylglycerols from a mixture of diacylglycerols, hydroxyacylglycerols and sterols obtained from fresh milk. 1,2‐diacyl‐3‐tritylglycerols could be isolated, which were transformed into 1,2‐diacetyl‐3‐tritylglycerol. The stereochemical relation of this compound with respect to model compounds synthesized from serine showed that the predominating enantiomers of the 1,2‐diacylglycerols present in fresh milk have the sn‐1,2‐configuration. This provides additional evidence that these compounds are biosynthetic intermediates rather than lipolysis products.

Stereochemistry of hydrogen–deuterium exchange α to the sulphinyl group in 2H-naphtho[1,8-bc]thiophen derivatives

The rate of deuterium exchange in MeONa—MeOD of α-protons having an equal stereochemical relation to a sulphinyl group was found to follow an opposite trend in 2H-naphtho[1,8-bc]thiophen derivatives (6)–(8) to that previously observed in 2-thiabicyclo[2.2.1]heptane derivatives (1) and (2), for which tetrahedral α-sulphinyl carbanions of different stability had been proposed as intermediates in the exchange process. The experimental results were rationalized by assiging to tetrahedral and planar α-sulphinyl carbanions a separate role in determining, according to their relative stability, the rate of proton abstraction or the stereochemistry of the exchange, or both.

EFFECT OF CRYSTALLINE SEQUENCE LENGTH AND ITS DISTRIBUTION ON MELTING OF TWO COMPONENT COPOLYMERS

The behaviors of melting in copolymers may be affected by the copolymer compositions, the crystalline sequence length and its distribution, the stereochemical relations among the repeating structural units and the conditions of crystallization, etc.. Therefore, it may be difficult to deduce explicit expressions on melting in copolymers.In this work, the effect of crystalline sequence length and its distribution on the melting behaviors of copolymers composed of units of type A, capable of crystallizing and other units B incapable of crystallizing, is discussed in particular from a standpoint differing from that of our preceding paper.Derived equation relating to the crystallinity against the absolute temperature for random copolymer was in accord with the relation obtained in the preceding paper. If considered the Gibbs free energy difference between the crystalline regions and the amorphous regions as a variable quantity together with the amorphous fraction and the absolute temperature, the derived equation may be valid for fair explanations of the experimental relations between the crystallinity and the absolute temperature.

Structural characterization of an anhydropenicillin and its stereochemical relationship to penicillins.

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTStructural characterization of an anhydropenicillin and its stereochemical relation to penicillinsGary L. Simon, Robert B. Morin, and Lawrence F. DahlCite this: J. Am. Chem. Soc. 1972, 94, 24, 8557–8563Publication Date (Print):November 1, 1972Publication History Published online1 May 2002Published inissue 1 November 1972https://doi.org/10.1021/ja00779a044RIGHTS & PERMISSIONSArticle Views40Altmetric-Citations14LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (798 KB) Get e-AlertsSupporting Info (1)»Supporting Information Supporting Information Get e-Alerts

Organometallic pnicogen complexes. VI. Synthesis, structure, and bonding of a new iron-antimony cluster complex, {[Fe(h5-C5H5)(CO)2]3SbCl}2[FeCl4].CH2Cl2; geometry of the tetrachloroferrate(II) anion and its stereochemical relation with other tetrachlorometalate anions

Organometallic pnicogen complexes. VI. Synthesis, structure, and bonding of a new iron-antimony cluster complex, {[Fe(h5-C5H5)(CO)2]3SbCl}2[FeCl4].CH2Cl2; geometry of the tetrachloroferrate(II) anion and its stereochemical relation with other tetrachlorometalate anions

Conformational equilibria and stereochemical relationships among carboxylic acids.

Conformational equilibria and stereochemical relationships among carboxylic acids.

Stereochemical relations in the bornane series : An example of preferred exo attack

The four α-hydroxybornanones have been obtained as their crystalline p-chlorobenzoates; the two endo ketoesters via photochemical reaction of camphorquinone and p-chlorobenzaldehyde and the two exo-isomers from cis, exo-bornanediol. cis-Diols were produced in every case by reaction of these ketoersters with p-methylbenzyl magnesium chloride, the endo-ketoesters affording cis, endo-diols via exo attack of the Grignard reagent on the bornane system. Two of the possible trans-diols were also synthesized.

Studies on monoterpene glucosides—IX: Chemical correlation between asperuloside and loganin

The absolute structure of loganin was established by chemical conversion of asperuloside to loganin penta-acetate. The stereochemical relations of a series of the intermediates are also discussed.

Structure and optical activity of the DNA-aminoacridine complex.

AbstractThe electronic absorption and circular dichroism spectra of the DNA‐acridine orange complex have been measured over a range of ionic strength, pH, and DNA phosphate to dye (P/D) ratios. Three circular dichroism bands associated with the long wavelength absorption band of acridine orange are induced on complex formation with DNA. Two of the dichroism bands, due mainly to dimeric dye molecules, are favored by low ionic strength, low pH (3.2), and a low P/D ratio (∼3), while the third, deriving primarily from monomeric dye, is optimum at high ionic strength, neutral pH, and a larger P/D ratio (9). The data suggest that monomeric acridine orange binds to DNA in the form of a left‐handed helical array with four dye molecules per turn, while the bound dimer has a skewed sandwich conformation which is itself dissymmetric. The stereochemical relations between the bound monomer dye and the DNA are consistent with a modified intercalation model for the DNA‐acridine complex.

Stereochemical relations between lycorine, caranine, pluviine, and lycorenine

Treatment of α-dihydrocaranine (VI) with ethanolic potassium hydroxide followed by hydrolysis of the resulting ethoxymethylether (VIII) and methylation with diazomethane afforded α-dihydropluviine (XII). The product, XVIII, obtained by the von Braun reaction of α-dihydrocaranine, was converted into α-deoxydihydrolycorenine (XXIII) without affecting asymmetric centres of the respective compounds. Thus the configurational relationship between lycorine, caranine, pluviine, and lycorenine have been clarified.