Six-membered ring system

PART I Systems containing Nitrogen by S. D. Carter and G. W. H. Cheeseman 

The format of the previous year has been retained. The reader s attention is drawn to the terminal classified reference list. This has been compiled from references which it has not been possible to include in the text. 

In the past year, reviews on 1,8-naphthyridines, perimidines, polyazaphen-anthrenes, 3-azabicyclo[3.3.1]nonanes, and 1,2- and 2,1-benzothiazines have appeared. Reviews on specialist aspects of pyridine chemistry are devoted to the reactions of newly available pyridines, a,a -disubstituted pyridines, the reactions of pyridines with nucleophiles, the electrochemistry of IjT-disubstituted 4,4 -bipyridinium ions (the viologens such as paraquat), dihydropyridines, and 4-aryl-dihydropyridines (a new class of calcium antagonists). Reviews have been published on the cyclization of oximes and amides to quinolines and isoquinolines, quinoline- and isoquinoline-diones, benzo[fl ]- and benzo[c]-quinolizinium ions, azachrysene preparation, quinazolines with plant-growth-regulating and biocidal activities, quinazolines in pharmaceutical research, isotopic hydrogen exchange in 

This Report follows the pattern established by the previous Reporter in Volume 1. The section dealing with fused 5,6-systems is chiefly concerned with the chemistry of purine and purine analogues and that on fused 6,6-systems with the chemistry of pteridines and flavins. Within the discussion of a given ring system, reference is made to synthetic methods before physical properties and reactivity. Where appropriate, fully aromatic compounds are discussed before those of an increased level of saturation. Because of the limits of space imposed on the Senior Reporters of this volume, it has been necessary to omit reference to much interesting work. 

The past year has seen the publication of Comprehensive Organic Chemistry, one volume of which contains much information on the six-membered ring systems to be reviewed in this article a monograph on the chemistry of condensed pyrazines has also appeared. Reviews on 1,4-thiazines, l,3-benzothiazines, pyridazines, benzo[c]cinnolines, quinazolines, purines, pyrrolo[3,2-c]quino-lines, 1,10-phenanthroline and its complexes, polyaza-phenanthrenes, and 1,9- and 1,10-diaza-anthracenes have been published. Other specialist reviews are devoted to catalytic methods of obtaining pyridine bases pyridine N-oxides the stereochemistry of quinolizines, indolizines, and pyrrolizines benzothiazinone dioxides 2-quinazolones and their cyclic homologues (e.g. 

Lazdin sh and A. A. Avots, Chem. Heterocycl. Compd. (Engl. TransL), 1979,15, 823. 

The reaction of benzyne with pyridine has been studied only at 690°C (benzyne generated from phthalic anhydride).96 The main condensable products in order of decreasing abundance were naphthalene, phenyl-pyridines (three isomers), bipyridyls, and quinoline. It is necessary to consider two modes of 1,4-cycloaddition, followed by rearomatization, to account for the formation of naphthalene (Eq. 4) and quinoline (Eq. 5), respectively. 

The latter pathway is relatively disfavored, as the requisite diene system includes a terminal nitrogen atom. The enthalpy term for the rearomatization of the intermediate adduct (166) will favor extrusion of hydrogen cyanide rather than acetylene, so that isoquinoline is formed in only trace amounts, if at all. Fields and Meyerson also consider mechanisms involving 1,2-cycloaddition of benzyne to pyridine to account for formation of the same 

Phenazine is similarly unreactive toward benzyne (by oxidation of 9), which gave only biphenylene and small amounts of unidentified highly colored products.81 On the other hand, the benzo[h]quinolizinium salt 175 reacts like anthracene with benzyne (generated by aprotic diazotization of anthranilic acid in refluxing acetonitrile) to give the Diels-Alder adduct 176 (78%).100 Aromatization of 176 to 9-(2-pyridyl)anthracene (177) is accomplished in refluxing acetic anhydride in the presence of sodium acetate. 

Alternatively, reduction of 176 with sodium borohydride (probably to a dihydropyridine, although this intermediate was not characterized) followed by thermolysis in acetic acid or acetic anhydride affords anthracene (93%) and pyridine or hydropyridines. These reaction sequences have been employed for the synthesis of overcrowded anthracene derivatives containing substituents with unfavorable peri interactions100 of particular interest is the conversion of 178 via its 1 2 adduct with benzyne into the pentaphenes 179 and 180. 

Although the heterodiene system N=C—C=N is usually unreactive toward the addition of benzyne (see also Section VI, A), compounds containing the system C=N—N=C will react with benzyne in a 1,4-cycloaddition, which is followed by extrusion of the azo bridge as nitrogen. This pattern of 

In spite of the ready formation of triptycene from anthracene and benzyne, the azatriptycene 167 cannot be obtained directly from acridine and benzyne. Instead, only biphenylene (17) was obtained (benzyne generated from 9), or 2-fluorobiphenyl (using o-fluorophenyllithium), or the products were 

4-phenylacridine (2%), N-phenylacridone (4%), and biacridyl derivatives 168-170 (together 50%), (benzyne generated from o-FCgH MgBr), or 171 (benzyne generated from 4 in dichloromethane). The formation of 168 does not involve benzyne at all but reductive dimerization of acridine via radicals 173 hence formation of 169 and 170 was attributed to free radical 

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A large amount of data are available on the C spectra of saturated six-membered ring systems. The subject has been reviewed in detail by Eliel and Pietrusziewicz (79MI20101). 

Phosphorus heterocyclic compounds, 1, 493-538 five-membered ring systems, 1, 513-523 nomenclature, 1, 496 six-membered ring systems, 1, 497-513 Photoaromatization oxirenes from, 7, 125-126 Photobleaching chromenes in, 3, 880 Photochemical reactions heterocyclic compound synthesis from, 5, 159 reviews, 1, 56 heterocyclic compounds reviews, 1, 71, 72... 

Polyaza six-membered ring systems, 3, 1039-1086 Poly-e-caprolactone production, 7, 589 Polycyclic compounds nomenclature, 1, 14-28 Polyfuroxans, 6, 426 Polygermacyclopentanes, 1, 609... 

Polythia six-membered ring systems, 3, 1039-1086 Polythiazides as diuretic, 1, 174 Polythiiranes, 7, 182 Polytopal rearrangements phosphoranes, 1, 529-530 rules, 1, 530 Polyurethanes curing agents... 

Reaction type 3 (equation 10), where the complete hetero-l,3-diene skeleton is incorporated into the newly formed ring system, occurs with compounds having both a nucleophilic center and an electrophilic center If these two functionalities are in positions 1 and 2, various types of six-membered ring systems become accessible 4,4-Bis(trifluoromethyl)-I,3-diaza-1,3-butadienes require only room temperature to react with acetyl cyanide to yield l,4,5,6-tetrahydropynmidin-6-ones [96] Likewise, certain open-chain 1,3-diketones (acetylacetone and acetoacetates) and the heterodiene form six-membered nng systems [97] (equation 19)... 

IV. Transmission of Substituent Effects through Heterocyclic Systems A. Six-Membered Ring Systems... 

All the reactions discussed in this review are aromatic nucleophilic substitutions in the ordinary sense. These reactions are briefiy described in the following sections with respect to their general kinetic features and mainly involve aza-activated six-membered ring systems, although a few studies of other heteroaromatic compounds are also available. 

Volume 81 of Advances in Heterocyclic Chemistry consists of four chapters. G. Rauhut (University of Stuttgart, Germany) provides a contemporary overview of methods and results for the computation of heteroatom-rich five- and six-membered ring systems, covering many advances in MO theory made over the last decade. The review will be useful to a wide range of heterocyclic chemists. 

The catalytic enantioselective cycloaddition reaction of carbonyl compounds with conjugated dienes has been in intensive development in recent years with the main focus on synthetic aspects the number of mechanistic studies has been limited. This chapter will focus on the development and understanding of cycloaddition reactions of carbonyl compounds with chiral Lewis acid catalysts for the preparation of optically active six-membered ring systems. 

To account for the course of this reaction theoretical calculations of the coordination of ketomalonate 37 to copper(II) and zinc(II) have revealed that the six-membered ring system is slightly more stable than the five-membered ring system (Scheme 4.30). The coordination of 37 to catalyst (l )-39 shows that the six-membered intermediate is C2-symmetric with no obvious face-shielding of the carbonyl functionality (top), while for the five-membered intermediate (bottom) the carbonyl is shielded by the phenyl substituent. Calculations of the transition-state energy for the reaction of the two intermediates with 1,3-cyclohexadiene leads to the lowest energy for the five-membered intermediate this approach is in agreement with the experimental results [45]. 

Six-Membered Ring Systems Pyridine and Benzo Derivatives... 

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