Oxepine

Oxepin 1, thiepin 2 and azepine 3 are the parent compounds of the seven-membered heterocycles with one heteroatom. Of the seven-membered ring heterocycles with several heteroatoms, only diazepines e.g. 1,2- and 1,4-diazepines 4 and 5 will be discussed. 

Monocyclic oxepins exist in equilibrium with bicyclic isomeric benzene oxides (1,2-epoxybenzenes), e.g. 1 with 4 [1]  

For this reason, the bond parameters for oxepin and other monocyclic derivatives have not been determined. The spectroscopic data show that oxepins have a polyolefinic structure with localized C=C bonds. Oxepins exist in a nonplanar boat conformation that equilibrates with inversion as in la and lb [2]. This process is slowed down by fusion with benzene rings. 13-Methyltribenzoxepin-l 1-carboxylic acid 6 can be separated into its enantiomers which racemize with an energy barrier AG 20.s 86.9 kJ mol-i [3]. 

The Chemistry of Heterocycles, Second Edition. By Theophil Eicher and Siegfried Hauptnoann Copyright 2003 Wiley-VCH Verlag GmbH Co. KGaA ISBN 3-527-30720-6 

Oxepin 1 and benzene oxide 4 can be distinguished at low temperature by NMR spectroscopy (-130°C) and by their UV absorptions  

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The thiepin system 31 is formed quantitatively by ring expansion of the diazoacetate derivative 30 via carbene formation catalyzed by 7r-allylpalladium chloride and its intramolecular insertion[31], The 4-diazomethyl-4//-pyrane 32 is expanded to the oxepine 33 in quantitative yield with the same catalyst[32]. 

Apparent nucleophilic attack on large, fully unsaturated rings may occur by way of attack on a valence tautomer, such as the reaction of oxepin with azide ion. Attack on the oxanorcaradiene valence tautomer leads to ring opening of the three-membered ring, and formation of 5-azido-6-hydroxy-l,3-cyclohexadiene (Section 5.17.2.2.4). 

Diene moieties, reactive in [2 + 4] additions, can be formed from benzazetines by ring opening to azaxylylenes (Section 5.09.4.2.3). 3,4-Bis(trifluoromethyl)-l,2-dithietene is in equilibrium with hexafluorobutane-2,3-dithione, which adds alkenes to form 2,3-bis-(trifluoromethyl)-l,4-dithiins (Scheme 17 Section 5.15.2.4.6). Systems with more than two conjugated double bonds can react by [6ir + 2ir] processes, which in azepines can compete with the [47t + 27t] reaction (Scheme 18 Section 5.16.3.8.1). Oxepins prefer to react as 47t components, through their oxanorcaradiene isomer, in which the 47r-system is nearly planar (Section 5.17.2.2.5). Thiepins behave similarly (Section 5.17.2.4.4). Nonaromatic heteronins also react in orbital symmetry-controlled [4 + 2] and [8 + 2] cycloadditions (Scheme 19 Section 5.20.3.2.2). 

Oxepin also forms a diene complex with tricarbonyliron (78JHC1057). 

Ring closure by making a C—C bond is a common synthetic method for rings of all sizes. Scheme 7 gives a few examples of displacement reactions. Other examples are found in the preparation of benzazepines (Section 5.16.4.1.2) and oxepins (Section 5.17.3.2.1). 

Benzene monoxide-oxepin and its sulfur analog are treated elsewhere (Chapter 5.1.7) (67AG(E)385). However, we point out here that electron-withdrawing substituents often favor the benzene oxide tautomer. The first study on oxides of the environmentally hazardous polychloro- and polybromo-biphenyls shows that they exist mainly in the benzene oxide form (81JOC3721). Oxides of polynuclear aromatic hydrocarbons (PAH) also exist mainly in the fused-ring oxirane form. 

Oxepin-2-carboxylic acid, methyl ester H NMR, 7, 552 <79JA2470)... 

Dibenz[6,/]oxepin, 10,11-dihydro-applications, 7, 590 Dibenz[c,e]oxepin, dihydro-conformational interchange, 7, 549 Dibenz[6,/]oxepinylpiperazine, 10,11-dihydro-properties, 7, 590 Dibenz[6,g]oxocin synthesis, 7, 669... 

Oxepin, 2-acetoxy-2,3,4,5-tetrahydro-thermal reactions, 7, 559 Oxepin, 3-chloro-synthesis, 3, 725 Oxepin, 2,3-dihydro-cycloaddition reactions, 7, 563 nucleophilic reactions, 7, 562 reduction, 7, 563 Oxepin, 2,5-dihydro-synthesis, 7, 578, 580 Oxepin, 4,5-dihydro-formation, 7, 579 reduction, 7, 563 synthesis, 7, 579 Oxepin, 2,7-dimethyl-NMR, 7, 552... 

Oxepin, 4-ethoxycarbonyl-2,3,6,7-tetrahydro-synthesis, 7, 578 Oxepin, 2-methyl-enthalpy of isomerization, 7, 555 Oxepin, 2,3,4,5-tetrahydro-reduction, 7, 563 synthesis, 7, 578 Oxepin, 2,3,4,7-tetrahydro-synthesis, 7, 578 Oxepin, 2,3,6,7-tetrahydro-oxidation, 7, 563 reduction, 7, 563 Oxepin-2,6-dicarboxylic acid stability, 7, 565 Oxepinium ions synthesis, 7, 559 Oxepins, 7, 547-592 antiaromaticity, 4, 535 applications, 7, 590-591 aromatization, 7, 566 bond lengths and angles, 7, 550, 551 cycloaddition reactions, 7, 27, 569 deoxygenation, 7, 570 dipole moment, 7, 553 disubstituted synthesis, 7, 584... 

H-Pyran, 2-alkoxy-4-methyl-2,3-dihydro-conformation, 3, 630 4H-Pyran, 2-amino-IR spectra, 3, 593 synthesis, 3, 758 4H-Pyran, 4-benzylidene-synthesis, 3, 762 4H-Pyran, 2,3-dihydro-halogenation, 3, 723 hydroboration, 3, 723 oxepines from, 3, 725 oxidation, 3, 724 reactions, with acids, 3, 723 with carbenes, 3, 725 4H-Pyran, 5,6-dihydro-synthesis, 2, 91 4H-Pyran, 2,6-diphenyl-hydrogenation, 3, 777 4H-Pyran, 6-ethyl-3-vinyl-2,3-dihydro-reactions, with acids, 3, 723 4H-Pyran, 2-methoxy-synthesis, 3, 762 4H-Pyran, 2,4,4,6-tetramethyl-IR spectra, 3, 593 4H-Pyran, 2,4,6-triphenyl-IR spectra, 3, 593... 

Tribenzo[6,d/]thiepin synthesis, 7, 587 Tribenzoxepins resolution, 7, 14 Tribenz[6,4/ oxepins synthesis, 7, 581... 

Dibromopropane Dimethylamine 6,11-Dihydrodibenz-(b,e>oxepin-11 -one Triphenyl phosphine Hydrogen bromide Butyl lithium ... 

The benzene layer is removed by decantation and the remaining mixture is rendered basic with 10% sodium hydroxide solution and is extracted with three 1,500 ml portions of benzene. The benzene extracts are washed, then dried with anhydrous sodium sulfate and concentrated in a vacuum leaving a residue of 1,530 grams, gas and thin layer chromatography analysis show this to be a cis/trans mixture (approx. 4 1) of 11-dimethylamino-propylidene-6,11-dihydrodibenz-(b,e)oxepin (90% yield). This mixture has substantially more activity pharmacologically than the cis/trans mixture obtained by the Grignard route disclosed in the Belgian Patent 641,498. This base is then converted to the hydrochloride with HCI. 

Chemical Name 6-(3-Dimethylamino-1 -propylidene)-12H-benzofuro[26-e] benz[b] oxepin fumarate... 

Oxo-benzo[b] benzofurano[26-e] oxepin Sulfuric acid Fumaric acid... 

The mixture was heated under reflux and a solution of 0.2 g of ethyl iodide in 5 ml of dry tetrahydrofuran was allowed to flow into the reaction medium. When the reaction started, a solution of 6.2 g of 7heated under reflux until the complete disappearance of the magnesium turnings. The reaction medium was then cooled in an ice bath, after which there was added thereto a solution in 45 ml of tetrahydrofuran of 7 g of 6-oxo-benzo[b] -benzofurano[2,3-e] oxepin. The reaction mixture was allowed to stand for 20 hours at a temperature of 20°C, and was then poured into a saturated aqueous solution of ammonium chloride maintained at a temperature of 5°C. The mixture was extracted with ether and the organic portion was washed and dried over anhydrous sodium sulfate. After evaporation of the solvent, 9.4 g of crude product were obtained, which after recrystallization from isopropanol, provided 6.7 g of pure 6-(3-dimethylam nopropyl)-8-hydroxybenzo[b] benzofurano-[2,3-e] oxepin, melting point 160°C (yield, 71 %). 

Oxepin and its derivatives have attracted attention for several reasons. Oxepin is closely related to cycloheptatriene and its aza analog azepine and it is a potential antiaromatic system with 871-elcctrons. Oxepin can undergo valence isomerization to benzene oxide, and the isomeric benzene oxide is the first step in the metabolic oxidation of aromatic compounds by the enzyme monooxygenase. 

Three decades ago the preparation of oxepin represented a considerable synthetic challenge. The theoretical impetus for these efforts was the consideration that oxepin can be regarded as an analog of cyclooctatetraene in the same sense that furan is an analog of benzene. The possibility of such an electronic relationship was supported by molecular orbital calculations suggesting that oxepin might possess a certain amount of aromatic character, despite the fact that it appears to violate the [4n + 2] requirement for aromaticity. By analogy with the closely related cycloheptatriene/norcaradiene system, it was also postulated that oxepin represents a valence tautomer of benzene oxide. Other isomers of oxepin are 7-oxanorbornadiene and 3-oxaquadricyclane.1 Both have been shown to isomerize to oxepin and benzene oxide, respectively (see Section 1.1.2.1.). 

The fluorescence spectrum of dibenz[7>,/]oxepin shows that this molecule adopts a planar structure in the excited state whereas the ground state has bent geometry as expected.19 The emission spectrum is similar to that of anthracene. 

An acetyl group in the 2-position favors the monocyclic structure presumably because of the resonance stabilization.12 The same observation was made with oxepin-2,7-dicarbaldehyde, oxepin-2,7-dicarboxylic acid, and oxepin-2,7-dicarbonitrile.23 Substituents in the 4- and 5-positions of the oxepin such as methyl or methoxycarbonyl groups shift the equilibrium towards the epoxide.12 24 Low temperature 1H NMR studies on 7-ethyloxepin-2-carbonitrile and ethyl 7-ethyloxepin-2-carboxylate established a nonplanar boat geometry with a ring-inversion harrier of 6.5 kcal mol-1.25... 

X-ray structures of 2,7-uiphenyloxepin,34 3,7-di-te/7-butyl-5-phenyl-2-(4-tolyl)oxepin,35 and tert-butyl oxepin-2-carboxylate25 and various 3,6-alkanooxepin derivatives36-38 confirmed a nonplanar boat geometry that had already been assumed from spectroscopic data. 

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