Seven-membered heteroaromatic ring

The aromatic character of the dibenzo derivative 2 has also been calculated. Studies on the behavior of the deep red anion 3, as a potential IOji heteroaromatic system, were described previously <1996CHEC-II(9)268>. H NMR spectra (NMR - nuclear magnetic resonance) of l,3-dithiepin-2-carbodithiolate 4, a ligand obtained from the reaction of carbon disulfide with the lithium salt of 1,3-dithiepin, as well as its palladium salt suggest a good deal of Hiickel aromatic character in the seven-membered dithiepin ring <1991ICA(185)169>. 

Our survey of heteroaromatic tautomerism will be completed in three further chapters, two of which will appear in Volume 77 of our series covering the tautomerism of (i) rings with three, four, and seven or more members and (ii) two or more fused five- or six-membered heterocyclic rings. The final chapter dealing with the tautomerism of six-membered monocyclic ring systems will appear in a subsequent volume. 

Known and Potential Monocyclic Seven-Membered-Ring Heteroaromatics... 

Transition from the tropylium ion to its neutral heteroaromatic counterparts is possible by replacement of a CH+ group by a heteroatom with a vacant p-orbital. The latter effectively accepts tt-electrons, thus providing ring electron delocalization. A typical example is the boron atom in a seven-membered borepine (8) (92AG1267). Correspondingly, this type of heteroatom can be referred to as borepine-like . Other little-known representatives of this family are alumopine (8, Z = A1H) or gallepine (8, Z = GaH). 

Grimmett, M. R., Advances in Imidazole Chemistry, 12, 103 27, 241 Electrophilic Substitution in the Azines, 47, 325 Halogenation of Heterocycles I. Five-member ed Rings, 57, 291 II. Six and Seven-numbered Rings, 58, 271 III. Heterocycles Fused to Other Aromatic and Heteroaromatic Rings, 59, 245. 

Halogenation of heterocycles five-membered rings, 57, 291 fused to other aromatic and heteroaromatic rings, 59, 245 six- and seven-membered rings, 58, 271 of heterocyclic compounds, 7, 1 Hammett equation, applications to... 

Six- and Seven-membered Rings, 58, 271 III. Heterocycles Fused to Other Aromatic and Heteroaromatic Rings, 59, 245. 

SEXTET 71-electron HETEROAROMATICS WITH SEVEN-MEMBERED RINGS... 

Seven-membered rings can be fused onto the cyclopen-tenone using heteroaromatic enyne-starting materials. A high stereoselectivity is observed in some cases (Scheme 264). 

Seven-membered rings are featured in three of the chapters of the present volume, namely, the benzazepines (S. Kasparek), 1,5-benzodiazepines (D. M. G. Lloyd and H. P. Cleghorn), and 2,3-dihydro-l,4-diazepines (D. M. G. Lloyd, H. P. Cleghorn, and D. R. Marshall). Recent advances in oxazole chemistry are described by R. Lakhan and B. Ternai, and H.-J. Timpe surveys the heteroaromatic A-imines. The final chapter is a review of aromaticity (M. J. Cook, A. R. Katritzky, and P. Linda), and it concentrates on the heterocyclic aspects of this controversial subject. 

A useful reaction for the synthesis of unsaturated seven-membered heterocycles is the (2 + 2)-cycloaddition of heteroaromatic compounds, e.g., 1 //-pyrrole, furan, or thiophene derivatives, with acetylenes. In combination with a subsequent intramolecular (2 + 2)-cycloreversion (Section IV,B,2) of the annulated cyclobutene moiety, ring enlargement with two carbon atoms can be achieved. 1-Heterocycloheptatrienes, such as benzol6]azepines,26,27,65,66 benzo[fc]oxepins,67,68 benzo[6]-thiepins,69,70 and thiepins,18,71 have been successfully prepared in this way other routes are either nonexistent or laborious.72 In these compounds the reacting carbon-carbon double bond constitutes part of a (4n + 2)7r-electron system and in the (2 + 2)-cycloaddition the resonance energy of the aromatic nucleus is lost. Just like the nonaromatic heterocycles, heteroaromatic compounds have been reported to undergo (2 + 2)-cycloaddition reactions both with electron-deficient and with electron-rich acetylenes. 

Recently, Lautens engineered a silver-promoted domino Hartwig-Buchwald amination/direct arylation access to polycyclic heteroaromatics [195], From substrate 130, a unique a hetero-pentacycle 131 was assembled with a seven-membered ring as its core. 

Rearrangements of aromatic and heteroaromatic nitrenes can be initiated with either heat or light. The thermal reaction is typically induced by flash vacuum thermolysis, with isolation of the products at low temperatures. Photochemical experiments are conducted either under matrix isolation conditions or in solution at ambient temperature. These rearrangements are usually initiated by ring expansion of the nitrene to a seven-membered ring ketenimine or carbodiimide (i.e., an azacycloheptatetraene). 

In the total synthesis of the marine terpenoid (-)-frondosin B, Trauner and Hughes described an intramolecular palladium-catalyzed alkenylation reaction between a benzofuran and an enol triflate (124 to 125, Scheme 10.41). Although the mechanism to form the key seven-membered ring is still unclear, a reasonable hypothesis would involve oxidative addition of palladium(0) to the C—OTf bond, C3-palladation of the benzofuran and reductive elimination to form the new C—C bond. This work is notable as it was the first example of heteroaromatic C—H activation in a complex molecule setting. 

In condensed heteroaromatic systems with a ring-junction pyrrolic nitrogen atom, -electron density is always shifted from the electron-rich six-membered ring (formally contains seven -electrons) toward the five-membered ring (formally has six -electrons). As a result, electrophiles are directed to carbon atoms of the latter. Thus, imidazo[l,2-tf]pyridines 160 unsubstituted at C(3) almost always react with electrophiles at that position. 

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