Oxidative Ring Closure Research Articles

Understanding Electrochromic Processes Initiated by Dithienylcyclopentene Cation-Radicals

Simple photochromic dithienylethylenes with either a perfluoro or a perhydro cyclopentene ring, and a variety of substituents (chlorine, iodine, trimethylsilyl, phenylthio, aldehyde, carboxylic acid, and ethynylanisyl), have been prepared and their electrochemical behavior was explored by cyclic voltammetry. All dithienylethylenes present two-electron irreversible oxidation waves in their open form, but the cation-radical of the open isomers can follow two different reaction pathways: dimerization or ring closure, whereas the halogen derivatives follow a dimerization mechanism, the presence of donor groups, such as the phenylthio-substituted compound, promote an efficient oxidative ring closure following an ECE/DISP mechanism. Electrochromic properties are also found in the corresponding ring-closed isomers. Depending on the substituents on the thiophene ring, and the perfluro or perhydro cyclopentene ring, open isomers can be obtained from oxidation (chemical or electrochemical) of the corresponding ring-closed isomers via an EC mechanism. This reaction pathway is favored by the presence of electron-withdrawing groups in the molecule. For all these compounds, closed or open, the oxidation lies between 0.8 and 1.5 V vs SCE, and provokes a permanent modification of the color, even after an oxidation-reduction cycle. This could be qualified as "electrochromism with memory". On the other hand, the ring-closed electron-rich isomers (E degrees < 0.8 V), which show reversible waves at the cation-radical or even dication level, give rise to "true electrochromism", for which no structural changes are observed. The experimental study was completed by theoretical calculations at the DFT level, using B3LYP density functional, which gave information on the total energy, the geometry, and the electronic structures of several representative compounds, either in the neutral form or in the cation-radical state. These results are important for the potential design of photochromic systems, such as three-state conjugated systems and photoelectrical molecular switching devices.

Site-directed mutagenesis and spectroscopic studies of the iron-binding site of ( S)-2-hydroxypropylphosphonic acid epoxidase

( S)-2-Hydroxylpropanylphosphonic acid epoxidase (HppE) is a novel type of mononuclear non-heme iron-dependent enzyme that catalyzes the O 2 coupled, oxidative epoxide ring closure of HPP to form fosfomycin, which is a clinically useful antibiotic. Sequence alignment of the only two known HppE sequences led to the speculation that the conserved residues His138, Glu142, and His180 are the metal binding ligands of the Streptomyces wedmorensis enzyme. Substitution of these residues with alanine resulted in significant reduction of metal binding affinity, as indicated by EPR analysis of the enzyme–Fe(II)-substrate-nitrosyl complex and the spectral properties of the Cu(II)-reconstituted mutant proteins. The catalytic activities for both epoxidation and self-hydroxylation were also either eliminated or diminished in proportion to the iron content in these mutants. The complete loss of enzymatic activity for the E142A and H180A mutants in vivo and in vitro is consistent with the postulated roles of the altered residues in metal binding. The H138A mutant is also inactive in vivo, but in vitro it retains 27% of the active site iron and nearly 20% of the wild-type activity. Thus, it cannot be unequivocally stated whether H138 is an iron ligand or simply facilitates iron binding due to proximity. The results reported herein provide initial evidence implicating an unusual histidine/carboxylate iron ligation in HppE. By analogy with other well-characterized enzymes from the 2-His-1-carboxylate family, this type of iron core is consistent with a mechanism in which both oxygen and HPP bind to the iron as a first step in the in the conversion of HPP to fosfomycin.

A Catalytic Asymmetric Bioorganic Route to Enantioenriched Tetrahydro‐ and Dihydropyranones.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Oxidative cyclization of aldehyde thiosemicarbazones induced by potassium ferricyanide and by tris(p-bromophenyl)amino hexachloroantimoniate. A joint experimental and computational study

The oxidative ring closure reaction of some aryl-substituted thiosemicarbazones induced by bona fide one-electron abstracting agents was investigated, by means of both experimental and computational techniques. The corresponding 1,2,4-triazole derivatives were the only cyclization products observed. The occurrence of two slightly different mechanistic pathways for the reaction is discussed.

A Catalytic Asymmetric Bioorganic Route to Enantioenriched Tetrahydro- and Dihydropyranones

A conceptually novel approach to hetero Diels-Alder adducts of carbonyl compounds is described using as the key steps an antibody-mediated kinetic resolution of hydroxyenones followed by a ring-closure process. Various beta-hydroxyenones proved to be very good substrates for antibodies 84G3- and 93F3-catalyzed retro-aldol reactions, allowing the preparation of highly enantiomerically enriched (up to 99% ee) precursors of pyranones. An attractive feature of this methodology is the possibility to convert these acyclic-enantioenriched beta-hydroxyenones into tetrahydropyranones by a conventional Michael-type addition procedure or into the corresponding dihydropyranones using an alternative palladium-catalyzed oxidative ring closure. For the palladium-mediated cyclization, a biphasic system has been implemented that allows the direct preparation of enantiopure dihydropyranones from the corresponding racemic aldol precursors using a sequential antibody-resolution/palladium-cyclization strategy, without isolation of the intermediate enantioenriched hydroxyenones. This bioorganic route is best applied to the preparation of hetero Diels-Alder adducts otherwise derived from less nucleophilic dienes and unactivated dienophiles.

Total Synthesis of Madindolines, Potent Selective Inhibitors of Interleukine 6, Novel Bioactive Microbial Metabolites

The first total synthesis of madindolines A and B, potent selective inhibitors of interleukin 6 (IL-6), has been achieved by stereoselective aldol reaction, ring-closing metathesis, reductive amination, followed by an asymmetric oxidative ring closure of indole, and resulted in determination of their absolute stereochemistries. We have further developed a more efficient second-generation synthesis, which is suitable for gram-scale preparation of these compounds, by stereoselective acylation of ester 34, followed by an intramolecular acylation of ester 32 with allylsilane (19% yield over 9 steps). Synthetic [ 3 H]-madindoline A bound to gp-130, selectively. Furthermore, synthetic madindoline A markedly inhibited osteoclastogenesis in vitro and bone resorption in ovariectomized mice in vivo.

Unexpected Migration and Oxidative Cyclization of Substituted 2‐Acetophenone Triflates under Basic Conditions: Synthetic and Mechanistic Insights.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Pyrene‐Fused Porphyrins: Annulation Reactions of meso‐Pyrenylporphyrins.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Pyrene-Fused Porphyrins: Annulation Reactions of meso-Pyrenylporphyrins

Abstract We prepared novel pyrene-fused porphyrins via the oxidative intramolecular ring closure reactions of meso-pyrenylporphyrin. The excitation energies and the oxidation potentials of the isolated dark-colored pyrene-fused porphyrins were significantly low compared with those of the corresponding precursors.

Unexpected migration and oxidative cyclization of substituted 2-acetophenone triflates under basic conditions: synthetic and mechanistic insights.

Oxidative ring closure of alkyl-substituted 2-hydroxyacetophenone trifluoromethanesulfonate esters (triflates) occurs upon exposure to base in anaerobic DMF at 20-90 degrees C. Alkyl substitution is required for ring closure. A migrated enol triflate product forms at lower temperature in high yield via migration of the trifluoromethanesulfonate in the unsubstituted and monoalkyl-substituted cases. The alkyl-substituted enol triflates also enter into the benzofuran-3-one ring-forming process under thermal cyclization conditions. Potential mechanistic pathways are evaluated.

Complexes of Schiff bases and intermediates in the copper-catalyzed oxidative heterocyclization by atmospheric oxygen.

After complexation with copper(II) ions, Schiff bases 1a-d may undergo an oxidative ring closure using atmospheric oxygen to give a number of imidazo[1,5-a]pyridines 2, an imidazo[1,5-a]imidazole 3, and an imidazo[5,1-a]isochinoline 4. This ligand oxidation can be performed with catalytic amounts of copper ions in the reaction. A catalytic cycle for the copper-catalyzed oxidative heterocyclization will be presented together with isolated copper complexes of Schiff bases 1a,b and intermediates 5 and 8 that were found by X-ray structure analyses which confirm this reaction scheme.

Spectroscopic studies of substrate interactions with clavaminate synthase 2, a multifunctional alpha-KG-dependent non-heme iron enzyme: correlation with mechanisms and reactivities.

Using a single ferrous active site, clavaminate synthase 2 (CS2) activates O(2) and catalyzes the hydroxylation of deoxyguanidinoproclavaminic acid (DGPC), the oxidative ring closure of proclavaminic acid (PC), and the desaturation of dihydroclavaminic acid (and a substrate analogue, deoxyproclavaminic acid (DPC)), each coupled to the oxidative decarboxylation of cosubstrate, alpha-ketoglutarate (alpha-KG). CS2 can also catalyze an uncoupled decarboxylation of alpha-KG both in the absence and in the presence of substrate, which results in enzyme deactivation. Resting CS2/Fe(II) has a six-coordinate Fe(II) site, and alpha-KG binds to the iron in a bidentate mode. The active site becomes five-coordinate only when both substrate and alpha-KG are bound, the latter still in a bidentate mode. Absorption, CD, MCD, and VTVH MCD studies of the interaction of CS2 with DGPC, PC, and DPC provide significant molecular level insight into the structure/function correlations of this multifunctional enzyme. There are varying amounts of six-coordinate ferrous species in the substrate complexes, which correlate to the uncoupled reaction. Five-coordinate ferrous species with similar geometric and electronic structures are present for all three substrate/alpha-KG complexes. Coordinative unsaturation of the Fe(II) in the presence of both cosubstrate and substrate appears to be critical for the coupling of the oxidative decarboxylation of alpha-KG to the different substrate oxidation reactions. In addition to the substrate orientation relative to the open coordination position on the iron site, it is hypothesized that the enzyme can affect the nature of the reactivity by further regulating the binding energy of the water to the ferrous species in the enzyme/succinate/product complex.

Organometallic dehydro[14] annulenes containing Vollhardt's cyclobutadiene: are CpCo-complexed cyclobutadienes more aromatic then benzene?

Pd-catalyzed coupling of 1,2-diethynyl-3,4-(bistrimethylsilyl)cyclobutadienecyclopentadienylcobalt to a series of 1-iodo-2-(trimethylsilylethynyl)benzenes and 1-chloro-4-trimethylsilylbut-1-ene-3-yne is followed by desilylation with potassium carbonate. Cu(OAc)(2)-promoted oxidative ring closure leads to dehydro[14]annulenes and dehydro[14]benzoannulenes fused to a cyclobutadiene(cyclopentadienylcobalt) complex. Five of these fused dehydroannulenes were structurally characterized. (1)H NMR spectroscopy of the organometallic dehydro[14]annulenes incorporating the (bistrimethylsilyl)cyclobutadiene(cyclopentadienylcobalt) unit suggested that the aromaticity of the fused cyclobutadiene complex might be stronger than that of benzene according to the ring-current criterion.

Directly Linked and Fused Oligoporphyrin Arrays from Oxidation of Metalloporphyrins

The AgI-promoted oxidative meso-meso coupling reaction of 5,15-diaryl ZnII-porphyrins is advantageous in light of its high regioselectivity as well as its easy extension to large porphyrin arrays. Linear meso-meso linked porphyrin 128-mer and three-dimensionally arranged grid porphyrin 48-mer were isolated in a discrete form by repetitive oxidation reaction and subsequent gel-permeation chromatography (GPC)-HPLC. 5,15-Diaryl CuII-, NiII-, and PdII-porphyrins were converted to meso-β doubly-linked diporphyrins by oxidation with(p-BrC6H4)3NSbCl6. End-aryl-capped meso-meso linked CuII-, NiII-, and PdII- diporphyrins were converted to completely fused meso-meso β-β β-βtriply-linked diporphyrins through the oxidative ring closure (ODRC) reaction with (p-BrC6H4)3NSbCl6. Finally, we found that ScIII-catalyzed oxidation with DDQ gave a very efficient ODRC reaction and hence allowed the synthesis of triply-linked oligoporphyrins up to 12-mer.

Carbopeptoids: peptides and diketopiperazines incorporating the anomeric centre of mannopyranose

An approach to the synthesis of D-mannopyranose derivatives incorporating an anomeric α-amino acid component is described. An N-acylated bicyclic [2.2.2] lactone, formed via an oxidative ring closure, provides access to a new class of glycopeptide analogues of D-mannopyranose and determines the anomeric configuration of compounds derived therefrom. The mannopyranose diketopiperazine may be equilibrated to the more stable furanose form under basic conditions; in general, mannopyranose derivatives containing an α-amino acid moiety at the anomeric position are less stable than the mannofuranose isomers.

ChemInform Abstract: A Facile Access to 2‐Methylthio/Alkoxy/Amino‐3‐acylimidazo[1,2‐a]pyridines Based on Cupric Chloride Promoted Oxidative Ring Closure of α‐Oxoketene N,S‐, N,O‐, and N,N‐Acetals.

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.

A facile access to 2-methylthio/alkoxy/amino-3-acylimidazo[1, 2-a]pyridines based on cupric chloride promoted oxidative ring closure of alpha-oxoketene N,S-, N,O-, and N,N-acetals.

A facile access to 2-methylthio/alkoxy/amino-3-acylimidazo[1, 2-a]pyridines based on cupric chloride promoted oxidative ring closure of alpha-oxoketene N,S-, N,O-, and N,N-acetals.

Synthesis of [28]Heptaphyrin(1.0.0.1.0.0.0) and [32]Octaphyrin(1.0.0.0.1.0.0.0) via a Directed Oxidative Ring Closure: The First Expanded Porphyrins Containing a Quaterpyrrole Subunit

ADVERTISEMENT RETURN TO ISSUEPREVCommunicationNEXTSynthesis of [28]Heptaphyrin(1.0.0.1.0.0.0) and [32]Octaphyrin(1.0.0.0.1.0.0.0) via a Directed Oxidative Ring Closure: The First Expanded Porphyrins Containing a Quaterpyrrole SubunitJonathan L. Sessler, Daniel Seidel, and Vincent LynchView Author Information Department of Chemistry and Biochemistry University of Texas at Austin, Austin, Texas 78712-1167 Cite this: J. Am. Chem. Soc. 1999, 121, 48, 11257–11258Publication Date (Web):November 18, 1999Publication History Received13 August 1999Published online18 November 1999Published inissue 1 December 1999https://doi.org/10.1021/ja992934xCopyright © 1999 American Chemical SocietyRIGHTS & PERMISSIONSArticle Views1048Altmetric-Citations94LEARN 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 Read OnlinePDF (65 KB) Get e-AlertsSupporting Info (3)»Supporting Information Supporting Information SUBJECTS:Anions,Chemical structure,Macrocycles,Nuclear magnetic resonance spectroscopy,Pyrroles Get e-Alerts

Pyridines from allyl cobaloximes: a new decomposition pathway

Thermochemical and mass spectrometric studies on allyl cobaloxime indicate that it thermally decomposes in a stepwise manner. In comparison with the behaviour of benzyl cobaloximes, which are also precursors to stabilised carbon radicals, there are superficial similarities but also significant differences. Both show a final highly exothermic transition and both produce a heterocyclic ring during thermolysis. Whereas benzyl cobaloximes produce the corresponding 3,4-dimethyl-5-arylisoxazole in moderate to good yields, allyl cobaloximes provide low yields of the corresponding 2,3-dimethylpyridines. Each route involves the cyclisation of an analogous monoxime. The allyl series favours cyclisation through dehydration whereas the benzyl series follows an oxidative ring closure. It is proposed that the low yields of pyridines are due to the relatively ineffective capture of allyl radicals by the dimethylglyoxime monoanion.

Novel base-catalysed formation of benzo[b]furano[60]- and -[70]fullerenes

Reaction of C60Cl6 with phenol in the presence of aq. KOH and under nitrogen, produces benzo[b]furano[2′,3′∶1,2][60]fullerene arising from substitution/elimination occurring across the 1,2-positions of the hexachloro precursor. Use of either 4-fluorophenol or 4-methoxyphenol produces the corresponding fluoro- and methoxy-substituted derivatives. Reaction with an excess of phenolic reagent results in nucleophilic replacement of some of the other chloro addends in the precursor by phenoxy groups, which may also participate in oxidative ring closure. C70Cl10 reacts with phenol in the presence of aq. KOH to give benzo[b]furano[2′,3′∶7,8][70]fullerene. The corresponding reaction of aniline with C60Cl6 produces derivatives of benzo[b]pyrrolo[2′,3′∶1,2][60]fullerene, but thiophenol does not react. Removal of eclipsing steric hindrance accounts for the ready ring-closure formation of the cycloadducts across bonds containing adjacent chlorines.