Betaines, heterocyclic, derivatives

Benzo[/)]thiophene chemistry, recent advances in, 11, 177 29, 171 Benzo[c]thiophenes, 14, 331 1,2,3-Benzotriazines, 19, 215 Benzyne, reactions with heterocyclic compounds, 28, 183 Betaines, heterocyclic derivatives of alternant hydrocarbons,... 

Tliis chapter covers nitrogen-containing fulvalenes that can be obtained by replacement of CH=CH and/or CH, for example, types 1-3 starting from compounds 1-6. Compounds in which nitrogen atoms are arranged on the periphery of the cross-conjugated system as in 15 or 16, as well as derivatives in which the central double bond contains heteroatoms as in 17, are not included. For azoniafulvalenes of type 17 and related heterocyclic betaines see (94AFIC197). 

Benzo[c]phenanthridine alkaloids are widespread in Papaveraceae, Fumariaceae, and Rutaceae. Fagaridine (118), the structure of which had to be revised, is a derivative of the unknown 5-methyl-benzo[c]phenan-thridine-8-olate (119) which is isoconjugate with the 2-methyl-chrysene anion (Scheme 43). Thus, Fagaridine is a member of class 1 of conjugated heterocyclic mesomeric betaines, which are isoconjugate with odd alternant hydrocarbon anions. 

Hydroxypyridine (86, R = H) and its derivatives also belong to the class of heterocyclic enols. In benzene and dioxane, 3-hydroxy-pyridine occurs as the neutral molecule (and not as a betaine).Its reaction with diazomethane, in heterogeneous media, gives a mixture of 3-methoxypyridine (86, R = Me) (10%) and l-methyl-3-hydroxy-pyridinium betaine (87) (30%If tert-butanol is used as a... 

N-Heteroaromatic compounds like pyridine, pyridazine, pyrazine, isoquinoline, and their derivatives42,250 react with diphenyl cyclopropenone in a formal (3+2) cycloaddition mode to the C=N bond of the heterocycle. As expected from the results discussed earlier (p. 67), the reaction is initiated by attack of nitrogen at the cyclopropenone C3 position and followed by stabilization of the intermediate betaine 390 through nucleophilic interaction of the Cl/C3 bond with the activated a-site of the heterocycle, giving rise to derivatives of 2-hydroxy pyrrocoline 391—394). In some cases, e.g. diphenyl cyclopropenone and pyridine42, further interaction with a second cyclopropenone molecule is possible under the basic conditions leading to esters of type 392. 

Other polyaza derivatives of aromatic systems, such as the blue tria-zaphentalene derivative (478), are also adequately described as meso-meric betaines or inner salts. Ohta and Kato, in a section entitled Bridged Heterocyclic Compounds (ref. 9b, page 241), have referred to seven types of compounds and the stereochemically unacceptable structures (479-485) which had been allocated to them. The possibility was considered by Ohta and Kato that some of these structures might have to be replaced by their meso-ionic equivalents. However, we have now established that this is not the case, and the relation between the original structures (479-485) and the correct structures (486-492) is given in Fig. 10. ... 

There are few examples of the preparations of heterocyclic compounds containing two or more heteroatoms which involve cyclization with formation of a bond between two heteroatoms. The best known instances of this type of reaction, all of which are [6 + 0] reactions, are the preparations of benzocinnolines as outlined in equations (l)-(4). A similar type of approach to that outlined in equation (4) has been used for the direct preparation of the di-N-oxide (2) from the dioxime (1 equation 5). The naphthotriazine betaine (4) is obtained as one of the products of the thermal decomposition of the azidoazo compound (3 equation 6). 1,2-Dithiins and their dibenzo derivatives have been prepared by oxidation of appropriate dithiols and related starting materials as outlined in equation (7). All of these reactions are, however, somewhat specialized and there has been essentially no systematic study of the preparation of six-membered heterocycles via formation of a bond between two heteroatoms. 

Examples of Known Heterocyclic Mesomeric Betaines Derived from the... 

Examples of these betaines (293) are the bridged heterocycles 295, which have been made by condensation of l,2,4-triazin-5-one derivatives 294 with aldehydes or ketones.2-2is yj g molecular structure of compound 295 (R = H, R2 = R = Me, R" = SMe) has been determined by X-ray crystallography. Acetylation gives derivatives 295 (R = Ac) which with methyl iodide give the deep red salts 296 in high yield. 

VIL The Chemistry of Heterocyclic Betaines Derived from Even Alternant Hydrocarbon Dianions... 

Volume 26 contains three chapters. C.A. Ramsden reviews heterocyclic betaines derived from six-membered heteroaromatic rings. These compounds have been intensively studied over the past 10 years and have not previously been comprehensively reviewed. O. Meth-Cohn and B. Tamowski summarize the chemistry of the thiocoumarins, a somewhat neglected group of heterocycles. Finally, W. Friedrichsen has reviewed the chemistry of the benzo[c]furans which, unlike the aza and thia analogs, have not previously been considered in this series. Here again we have a group of compounds the chemistry of which has advanced considerably over the last decade. 

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