Conformation Of Six-membered Ring Research Articles

Mechanisms of Stereodirecting Participation and Ester Migration from Near and Far in Glycosylation and Related Reactions.

This review is the counterpart of a 2018 Chemical Reviews article (Adero, P. O.; Amarasekara, H.; Wen, P.; Bohé, L.; Crich, D. Chem. Rev. 2018, 118, 8242-8284) that examined the mechanisms of chemical glycosylation in the absence of stereodirecting participation. Attention is now turned to a critical review of the evidence in support of stereodirecting participation in glycosylation reactions by esters from either the vicinal or more remote positions. As participation by esters is often accompanied by ester migration, the mechanism(s) of migration are also reviewed. Esters are central to the entire review, which accordingly opens with an overview of their structure and their influence on the conformations of six-membered rings. Next the structure and relative energetics of dioxacarbeniun ions are covered with emphasis on the influence of ring size. The existing kinetic evidence for participation is then presented followed by an overview of the various intermediates either isolated or characterized spectroscopically. The evidence supporting participation from remote or distal positions is critically examined, and alternative hypotheses for the stereodirecting effect of such esters are presented. The mechanisms of ester migration are first examined from the perspective of glycosylation reactions and then more broadly in the context of partially acylated polyols.

Pickett angles and Cremer–Pople coordinates as collective variables for the enhanced sampling of six-membered ring conformations

Although completely equivalent for the description of puckered ring conformers, the two popular coordinates sets of Strauss–Pickett dihedral angles and Cremer–Pople spherical coordinates are shown to have contrasting features when employed as collective variables in free-energy calculations with accelerated sampling techniques. Results from a 100 ns molecular dynamics simulation at conformational equilibrium and from combined metadynamics/umbrella sampling calculations of glucose are exploited to elucidate these differences.

ChemInform Abstract: Pentacoordinated Molecules. Part 85. Influence of Hydrogen Bonding on the Formation of Boat and Chair Conformations of Six-Membered Rings in Spirocyclic Tetraoxyphosphoranes.

ChemInform 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.

Keto–enol tautomerization of cyanuric acid in the gas phase and in water and methanol

The keto–enol tautomerism of cyanuric acid, both in the gas phase and in water and methanol, has been studied at the B3LYP/aug-cc-pvDZ level of theory in this paper. The harmonic frequencies of all the structures are calculated. The results show that the transition states of the tautomerism are four-membered ring conformations in gas phase, whereas six-membered ring conformations in water and methanol. In the first proton transfer, the activation energy Δ E # is 69.9 and 62.0 kJ/mol for water and methanol, respectively, which is much lower than that in gas phase (189.7 kJ/mol). Water and methanol molecules produce an important catalytic effect in the tautomerism, especially for methanol system. The AIM and NBO analyses show that there exists an H-bonding complex between the cyanuric acid and the water (methanol). By viewpoints of the reaction enthalpy and reaction rate, keto–enol tautomerism in water and methanol systems is easier than that in gas phase. The keto–enol tautomerisms are endothermic both in gas phase and in water and methanol, so the enol forms are less stable than the keto ones.

Quantitative description of six-membered ring conformations following the IUPAC conformational nomenclature

Although several methods of quantitative conformational characterization exist in the literature, all these methods use a spherical polar coordinate representation which is in contrast to the qualitative description based on the IUPAC nomenclature. To bridge this gap this paper introduces a method to characterize six-membered ring conformations as a linear combination of ideal basic conformations. The linear combination coefficients are derived by projection of the vector of torsion angles onto those of ideal basic conformations. As the IUPAC nomenclature uses subscripts and superscripts to indicate atoms below and above the reference plane, the linear combination coefficients combined with the IUPAC name provide an instant visual image of the conformation. The method introduced here is based on endocyclic dihedral angles and requires only three dihedral angles for a full characterization, which is often available by NMR measurements for rigid conformations. We provide a table of equations to determine the missing dihedral angles based on redundancy conditions. The relationship between linear combinations and spherical representation similar to the well-known Cremer–Pople parameters is presented. In deriving the spherical conformational parameters we solved an inconsistency of previous definitions for spherical representation, namely that none of previous definitions place the intermediate halfchair or twistboat conformations exactly halfway between the pole (chair) and the equator (boat and twistboat) of the sphere as expected based on intuitive stereochemistry. To make our method generally available we provide an interface on the Internet that carries out all calculations described in the paper and allows the user to visualize, rotate and manipulate the ring (http://www.nrc.ca/ibs/6ring.html). By simplifying both the concepts and the access to carry out the calculations more experimental chemists can benefit from the description of ring conformation.

Complete Assignment of1H and13C NMR Spectra and Conformational Analysis of Thioamide Cannabinoids

The complete assignment of the 1H and 13C NMR spectra of two carbothioamide- substituted meroterpenes is presented. Resonance assignments were achieved by the use of one- and two- dimensional NMR, NOED, selective decoupling measurements and the deuterium isotope effect on the 13C chemical shifts. Six-membered ring conformations were determined by analysis of proton spin coupling constants with the aid of spectral simulations in conjunction with molecular mechanics calculations and equations correlating coupling constants and dihedral angles. The anisotropy effect of benzene rings was used for structural assignments.

NMR and x-ray evidence for the chair conformation of six-membered rings attached diequatorially to five-coordinate phosphorus. Implications for reported transition-state analogs of nucleoside cyclic 3',5'-monophosphate hydrolysis

A series of phosphoranes, 5, 6a, 6b, and 7a, has been prepared and structurally characterized by 1 H NMR spectroscopy and/or X-ray crystallography. In the crystalline state, 5a, 6a, and 6b feature five-coordinate phosphorus bonded in a somewhat distorted, trigonal bipyramidal fashion. The phosphorus-containing six-membered ring is attached to phosphorus diequatoially and is in the chair conformation. 1 H NMR coupling constants reveal that for all four phosphoranes a chair-form ring, rather than a boat or twist conformation, also is populated in solution

The conformation of six-membered rings described by puckering coordinates derived from endocyclic torsion angles

The conformation of six-membered rings described by puckering coordinates derived from endocyclic torsion angles

Pentacoordinated molecules. 85. Influence of hydrogen bonding on the formation of boat and chair conformations of six-membered rings in spirocyclic tetraoxyphosphoranes

X-ray analysis revealed phosphoranes in a trigonal-bipyramidal framework with the rings positioned in axial-equatorial sites. Hydrogen bonding between molecular units gave rise to helical chain form 1 and dimeric forms 2 and 3. The phosphorinane rings were present in a twist-chair conformation for 1 and in both twist-chair and twist-boat conformations for 2 and 3. Variable-temperature solution-state 1 H and 13 C NMR supported retention of the solid-state structures and the presence of hydrogen bonding

Zur strukturellen flexibilität der “antiaromatischen” 1,4- dihydropyrazine. Kristall- und molekülstrukturen metallorganischer derivate

Molecular and crystal structures of the cyclic conjugated 8-π-electron systems 1,4-bis(trimethylgermyl)- and 1,4-bis(trimethylsilyl)-1,4-dihydropyrazine (I and II) and of 2,3,5,6-tetramethyl-1,4-bis(trimethylsilyl)-1,4-dihydropyrazine (III) have been determined by X-ray crystallography at low temperatures. Whereas the first two species exhibit virtually planar six-membered ring conformations and very little pyramidalization at the nitrogen centers, the sterically more crowded tetramethyl derivative III has a pronounced boat conformation with a dihedral angle of about 140° between the ring halves. These structural results are in excellent agreement with theoretical calculations as well as spectroscopic data and illustrate the exceptional structural flexibility of the “antiaromatic” 1,4-dihydropyrazine system.

Crystal and Molecular Structure of 3,8-Di-t-butyl-5,6-diphenyl-1-thioxo-2,9-dithia-1-phosphabicyclo[4.3.0]nona-3,7-diene

Abstract The crystal and molecular structures of the title compound were determined by means of X-ray diffraction. Crystal data: C26H31S3P, Mr=470.68, orthorhombic, a=17.116(4), b=19.798(4), c=14.898(3) Å, V=5048.4 Å3 (21 °C), space group Pbca, Dm=1.23, Dx=1.239 g cm−3, Z=8, μ=3.58 cm−1 (Mo Kα). The structure was solved by the direct method and refined by the full-matrix least-squares method to R=0.083 for 3318 observed reflections. The bicyclo skeleton is so highly strained that abnormal bond distances [P–C of 1.895(5) and C–C of 1.594(6) Å] were observed. Conformation of six-membered ring is a boat-form.

The conformation of six-membered rings

Although an indefinite number of conformations exists for six-mem- bered rings, and although there is general consensus about the six classical forms (Bucourt and Hainaut, 1965; Cano et al., 1977; Romers et al., t974), there is no generally accepted nomenclature or symbolic formalism to distinguish among them. A quantitative basis for a classification exists in the literature (Cremer and Pople, 1975), and the purpose of this communication is to propose a logical nomenclature adapted to the classification. The analysis starts out from crystallographic fractional coordinates and involves transformation first to a set of Cartesian coordinates, and eventually to Cartesian coordinates of pucker for the ring system under consideration. From these coordinates, a set of three parameters of pucker in the form of polar coordinates (Q, O, ~) is obtained. These coordinates map out the conformation of the ring on the surface of a sphere, radius Q, and with poles at O = 0, 180% as suggested by Hendrickson (1967). Figure 1 depicts the surface of this sphere in two-dimensional polar projection. Each of the hexagons represents a canonical conformation in terms of the six classical forms. The symbols on the sides of the hexagons indicate the signs of the endocyclic torsion angles: the torsion angle definition of Klyne and Prelog (1960) is used. The established terminology for chair, boat, and half-chair forms is retained. Of the variant forms twist-boat/skew-boat and envelope/half- boat/sofa, the first form is preferred in each case. For the 1,3-diplanar form, the name screw-boat is proposed. This scheme provides a set of names with six different initials which can conveniently be used to represent the various types. For a unique description of ring conformation, a well-defined atomic numbering scheme is needed next. Whenever a single atom in a ring (e.g., 317

Cyclitols. Part XXXIV. X-Ray crystal and molecular structure of 1,2:5,6-Di-O-isopropylidene-3,4-di-O-tosyl-L-chiro-inostol, and its conformation in solution by nuclear magnetic resonance

The crystal structure of the title compound was determined from X-ray diffractometer data by direct methods and refined by least-squares techniques to R 0·047 for 1317 observed reflections. Crystals are tetragonal, space group P41212, a= 9·304 ± 0·005, c= 32·008 ± 0·017 Å, Z= 4. The cyclohexane ring was found to be in a flattened skew conformation, with four substituents in axial orientation. By contrast, n.m.r. spectroscopy indicates that in solution the title compound adopts predominantly a distorted chair conformation. The conformations of six-membered rings fused to five-membered rings are discussed.

Ring inversion of perfluoro-p-dithiane

Interconversion of chair conformations of six-membered rings containing two identical heteroatoms has been much studied by the nmr method, with the exception of the 1,4-dithiane system. The results so far obtained for 1,2- and 1,3-dithianes in comparison with those of dioxanes and the corresponding diaza systems suggest that this barrier should be readily measureable by the nmr method. We are concerned here with the ring inversion of octafluoro-1,4-dithiane (1).

A Direct, Qualitative Determination of Nonchair and Distorted-Chair Conformations of Six-Membered Rings

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTA Direct, Qualitative Determination of Nonchair and Distorted-Chair Conformations of Six-Membered RingsJoseph B. LambertCite this: J. Am. Chem. Soc. 1967, 89, 8, 1836–1840Publication Date (Print):April 1, 1967Publication History Published online1 May 2002Published inissue 1 April 1967https://doi.org/10.1021/ja00984a015RIGHTS & PERMISSIONSArticle Views277Altmetric-Citations146LEARN 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 (598 KB) Get e-Alerts Get e-Alerts

The stereochemistry of the diels-alder reaction of heterodienophiles. The conformation of six-membered rings: (1,2)

The stereochemistry of the diels-alder reaction of heterodienophiles. The conformation of six-membered rings: (1,2)

The conformation of six-membered rings involving a planar group

The conformation of six-membered rings involving a planar group