The various types of conformationally restricted peptides obtained by short-range cyclizations, from residue i to residue i + 1, are presented. Relevant examples of N in equilibrium C alpha, C' in equilibrium C alpha, N in equilibrium C', C alpha in equilibrium C alpha, C' in equilibrium C', and N in equilibrium N cyclizations are reported and the pertinent literature listed. In the discussion emphasis is place on the conformational consequences for peptides from the incorporation of such ring structures.

Conformationally restricted peptides through short-range cyclizations.

Cyclobutanones and Cyclobutenones in Nature and in Synthesis [New Synthetic Methods(71)]

Publisher Summary This chapter reviews the chemistry of hydantoins. The hydantoins can be synthesized using amino acids, cyanate or thiocyanate salts, alkyl or aryl isocyanates and isothiocyanates, ureas, α-dicarbonyl compounds, Bucherer–Bergs and Read reactions, and cycloaddition reactions with heterocumulenes. The chapter also illustrates physicochemical studies on hydantoins. H-NMR, IR, and UV techniques are widely used to study ionization and tautomerism in hydantoins. Hydantoins are weak acids which owe their acidic character to dissociation of the proton bonded to the 3-nitrogen atom as this allows for maximum delocalization of charge in anion. The chapter also presents physicochemical studies on hydantoins using ultraviolet spectroscopy, mass spectrometry, infrared spectroscopy, nuclear magnetic resonance spectroscopy, crystal structure determinations, and quantum mechanical calculations. The reactivity of hydantoins and their derivatives are described further in the chapter. Hydantoins and thiohydantoins can react with nucleophilic and electrophilic as well as with other types of reagents. On protonation in a strongly acidic solution, hydantoin cations are formed. Hydantoins can easily be alkylated in the N-3 position by treatment with alkyl halides in an alkaline solution in protic or aprotic solvents. Other alkylating agents include dimethyl sulfate and diazomethane.

The Chemistry of Hydantoins

Although the antibiotics nigericin and lasalocid as first representatives of the naturally occurring ionophores were isolated as early as 1951 from Streptomyces cultures 1), it was only in the late sixties that the function of these membranes affecting compounds as complexing and transporting agents for alkali metal ions was established 2–4). Once their outstanding properties as selective alkali cation carriers had been understood, however, they attracted the attention of numerous investigators, and many details have been learnt about their structures, complex formation and physiological activity. Furthermore, a considerable number of model compounds were synthesized, above all the so-called polyether ligands which did not only help to understand the mechanism of action of the naturally occurring ionophores but also led to the rapid development of phase-transfer techniques in organic chemistry, to mention only one application.

Structural chemistry of natural and synthetic ionophores and their complexes with cations.

Recent X-ray and 13C NMR results on macrocyclic compounds containing oxyethylene units (1,4 dioxa groupings) are reviewed. When such “saturated” crown ethers and related compounds form complexes with cations and uncharged protonacidic molecules, they adopt conformations which contain chiefly ag± a units or g± g± a units (genuine corners) and are thus of the types encountered, or conceived for corresponding cycloalkanes. Uncomplexed or incompletely complexed crown ethers, on the other hand, adopt as a rule unusual conformations never encountered in cycloalkanes. These latter crown ethers contain g± g± a units (pseudo corners) characterized by close 1,5 CH … O interactions. Such conformational units are also found in uncomplexed cryptands. In open chains, there is no evidence of this type of chain folding, except in the gas phase for 1,2-dimethoxyethane. It is concluded that pseudo-corner formation is resorted to only when solvation of ether oxygen is excluded, such as in the gas phase or in the inaccessible interior of macrocyclic compounds.

The Conformation of Free and Complexed Oligoethers

(C 10H2005)2 • BaBr2.2H20, monoclinic, P 2 J n , a -15.895 (4), b = 20.844 (7), c = 9.195 (3) A, fl = 98.31 (2) ° , Z = 4, D x = 1.626 Mg m -3. The crystal is formed by (15-crown-5)2Ba 2+ cations with a sandwich structure in which the BaO~0 core has Dsa symmetry and by [Br4(H20)4] 4complex anions where 0567-7408/79/051217-04501.00 the bromide ions are connected by water molecules through hydrogen bonding. Introduetlon. Ce travail fait suite ~ la d&ermination de structure du complexe 1:1 C10H200 5.CuBr 2 .2H20 (Arte, Feneau-Dupont, Declercq, Germain & Van © 1979 International Union of Crystallography 1218 (C 10H2005)2 . BaBr 2. 2H20 Meerssche, 1979). Le complexe form6 par BaBr 2 est un complexe 2:1 dont la structure est totalement diff+rente de celle du compos6 cuivrique. Les cristaux sont des hydrates ; ils proviennent d 'une cristallisation darts le m6thanol. Les conditions exp6rimentales d 'observat ion du spectre de diffraction sont r6sum6es au Tableau 1. Les intensit6s mesur6es n 'ont pas 6t6 corrig6es pour l 'absorpt ion bien que le coefficient d 'absorp t ion lin6aire (/t -4,02 m m -1) et les dimensions du cristal pr ismatique (0,33 x 0,22 x 0,48 mm) fassent pr6voir des coefficients de t ransmission compris dans l 'intervalle 0 ,85-0 ,91 . O1\" Fig. 1. Conformation de i'ion [Ba(C~oH2oOs)2] 2+ et esquisse de la num6rotation des atomes. Les oxyg6nes sont marqu6s d'un point noir. Tableau 1. Conditions exp&imentales Instrument: diffractom6tre automatique fi quatre cercles Syntex P2~ Source: Mo Ka, 2 = 0,7107 A; monochromateur graphite Balayage ~o-20; 20ma x = 48 ° Nombre de r6flexions ind6pendantes mesur6es: 4489 Nombre de r6flexions observ~es: 2789 Condition de rejet: I < 2,50(/) Tableau 2. Coordonndes atomiques ( x l O 4 pour les atomes de Ba et Br, x 10 3 pour les autres)

Complexe 2:1 pentaoxa-1,4,7,10,13 cyclopentadécane–bromure de baryum hydrate

Vicinal alkylation of olefins, regio- and stereoselective addition of [CM + CN] units to cyclopentadiene.

Complexe 1:1 pentaoxa-1,4,7,10,13 cyclopentadécane–bromure de cuivre(II) hydraté

The structure of the title compound has been investigated in order to determine the configuration of the oxime group. The molecules appear to be in the E configuration and are H-bonded to form dimers in the crystal.



From partial structural informations, p-nitrobenzamidoxim seems to adopt the Z configuration.

Determination de la Structure de la N,N-Diethylbenzamide-Oxime

The crystal structures of the title compounds have been determined using X-Ray diffractometry in order to study the effects of capto-dative substitution on molecular geometry. The carbon-carbon central bond is significantly stretched. 2, 3-Bis (ethylthio) -2, 3-diphenyl-1, 4-butanedinitrile (compound A) is triclinic, P1, with a = 15.703 (9), b = 14. 902(5), c = 8.655(4)A, δ = 97.52(3), β = 89.87(4)°, γ = 75.59(4)°, V = 1943.3 A3, Z = 4 and Dx = 1.20 Mg m−3. 2, 3-Bis (ethylthio)-2, 3-bis (p-tolyl)-1, 4-butanedinitrile (compound B) is monoclinic, P2/n with a = 16-872(8), b = 12, O82(5), c = 10.329(5)A, β = 94.16(4)°, V = 2100.0 A3, Z = 4 and Dx = 1.21 Mg m−3.

Etudes structurales en relation avec le concept de substitution "capto-dative". I. Structures aux rayons X du bis(ethylthio)-2,3 diphényl-2,3 butanedinitrile-1,1 et du bis(éthylthio)-2,3 bis(p-tolyl)-2,3 butanedinitrile-1,4

Handle everyday research tasks with reliable, citation-backed results

Your personal Research Agent to handle research tasks with citation-backed results

Popular Tasks used by Researchers

How can I help with your research?

Meet SciSpace

Get more enhanced response by uploading the PDFs you want me to reference.

No relevant PDFs in your library

SciSpace is the AI research assistant for academics. Run systematic literature reviews on 280M+ papers, and write papers with cited sources. Trusted by 1M+ students, PhDs & researchers.

SciSpace | AI for Research

Analyze PDFs

Code & Manuscripts

Funding & Grants

Literature & Patents

Medical & Clinical Data

Systematic Review

Visualize & Present

Web & Data

Build a Google Scholar-like website for your research.

Build a website

Create charts and images for your research

Create a Chart

Write a paper for submission to a journal

Draft a manuscript

Patent Search

Design eye-catching scientific posters in minutes.

Scientific Poster Generation

Systematic Literature Review

One task is running at the moment. Your messages will be shown right after.

Drag and drop or click here to browse

Loved by <highlight>1 million+</highlight> researchers

Extract a list of specific topics and their sources from unstructured text

Topics

Compare and analyze relevant papers that matches with your search

Papers

Get insights from PDFs and bookmarked papers from your library

My library

Recent searches

Try searching for:

Catch AI-generated content in scholarly and non-scholarly content

{ai} Detector

Ai Writer

Get PDF Summaries, highlighted text explanations 

Chat with PDF

Effortlessly create in-text citations and bibliographies in APA and 2,500 other formats

Citation generator

Get explanations, summaries, and answers on academic papers

Ease up your research workflow with {scispace}'s cohort of exciting AI tools

Elevate your academic writing skills and convey your ideas the way you want

Paraphraser

Explore our range of reading and writing tools

Your file is being prepared and should be ready in a few minutes. If it's a large file, it might take a bit longer. You can close this window, and we'll email you the file when it's done.

You have reached a maximum limit of <strong>{limit}</strong> columns in the table. Remove at least <strong>1</strong> column to add or create another one.

E. Arte

Author Tools

Chat about Author