Benzenoid aromatic compounds

It soon became evident that not only benzene, but the whole class of polycyclic aromatic compounds (benzenoid hydrocarbons in particular), possess a non-unique... 

Ansamacrolides. Antibiotics ia the ansamacroHde family ate also referred to as ansamycias. They are benzenoid or naphthalenoid aromatic compounds ia which nonadjacent positions are bridged by an aliphatic chain to form a cycHc stmcture. One of the aliphatic—aromatic junctions is always an amide bond. Rifampin is a semisyntheticaHy derived member of this family and has clinical importance. It has selective antibacterial activity and inhibits RNA polymerase. 

Substituents on benzene or benzenoid rings in fused pyridazines, i.e. in cinnolines and phthalazines, usually exhibit reactivity which is similar to that found in the correspondingly substituted fused aromatic compounds, such as naphthalene, and is therefore not discussed here. 

In many cases, however, the ortho isomer is the predominant product, and it is the meta para ratio which is close to the statistical value, in reactions both on benzenoid compounds and on pyri-dine. " There has been no satisfactory explanation of this feature of the reaction. One theory, which lacks verification, is that the radical first forms a complex with the aromatic compound at the position of greatest electron density that this is invariably cither the substituent or the position ortho to the substituent, depending on whether the substituent is electron-attracting or -releasing and that when the preliminary complex collapses to the tr-complex, the new bond is most likely to be formed at the ortho position.For heterocyclic compounds such as pyridine it is possible that the phenyl radical complexes with the nitrogen atom and that a simple electronic reorganization forms the tj-complex at the 2-position. 

Compounds of special interest whose preparation is described include 1,2,3-benzothiadiazole 1,1-dioxide (a benzyne precursor under exceptionally mild conditions), bis(l,3-diphenylimida-zolidinylidene-2) (whose chemistry is quite remarkable), 6- di-melhylamino)julvene (a useful intermediate for fused-ring non-benzenoid aromatic compounds), dipkenylcyclopropenone (the synthesis of which is a milestone in theoretical organic chemistry), ketene di(2-melhoxyethyl) acetal (the easiest ketene acetal to prepare), 2-methylcyclopenlane-l,3-dione (a useful intermediate in steroid synthesis), and 2-phenyl-5-oxazolone (an important intermediate in amino acid chemistry). 

Non-benzenoid aromatic compounds containing a hydroxy group also react with arenediazonium ions and form arylazo derivatives. The first case of such an azo coupling process was found by Nozoe (1949) in his classic work on the natural product hinokitiol (12.15, R=CH3 Nozoe, 1959, 1991). Shortly afterwards Nozoe et al. 

The vast majority of aromatic compounds have a closed loop of six electrons in a ring (the aromatic sextet), and we consider these compounds first. It is noted that a formula periodic table for the benzenoid polyaromatic hydrocarbons has been developed. ... 

Lloyd, D. in Carbocyclic Von-benzenoid Aromatic Compounds, p. 24. Amsterdam-London-New York Elsevier Publishing Company 1966. 

Reduction of aromatic compounds to dihydro derivatives by dissolved metals in liquid ammonia (Birch reduction) is one of the fundamental reactions in organic chemistry308. When benzene derivatives are subjected to this reduction, cyclohexa-1,4-dienes are formed. The 1,4-dienes obtained from the reduction isomerize to more useful 1,3-dienes under protic conditions. A number of syntheses of natural products have been devised where the Birch reduction of a benzenoid compound to a cyclohex-1,3-diene and converting this intermediate in Diels-Alder fasion to polycyclic products is involved (equation 186)308f h. 

The heteroaromatic compounds like furans, pyrroles or thiophenes cannot be generally used as dienes in Diels-Alder syntheses, because at the higher temperature required for the addition of less reactive dienophiles, the equilibrium is on the side of the starting materials due to the unfavorable T AS term comparable to the benzenoid aromatic compounds as mentioned. High pressure again shows the two effects already discussed the shift of the equilibrium toward the products and the enhancement of the rate of reaction which allows the temperature of reaction to be lowered. One... 

On the other hand, the anodic oxidation of 1,3,5-cycloheptatrienes is one of the most powerful key tools for the preparation of a variety of non-benzenoid aromatic compounds such as tropylium salts, tropones, tropolones, 2H -cyclohcpta h furan-2-oncs and azulenes14. 

Untch, K. First International Symposium on the Chemistry of Non-benzenoid Aromatic Compounds, Sendai, Japan, August, 1970. See also Jones, W.M. Angew. Chem. Int. Ed. Engl. 1972, 11, 325. 

Non-benzenoid aromatic compounds, flash vacuum pyrolysis for, 21 148 Non-benzenoid quinones, 21 238 Nonbenzidine-based dyes, 9 448 Nonblack fillers, 21 781... 

While benzene was the first aromatic system studied, the formulation of HtickePs rule and the theory behind it created an impetus to prepare non-benzenoid species such as the tropylium cation and cyclopentadienyl anion that also obeyed Huckel s rule to see if these species were also aromatic. This required that the properties of aromatic compounds be defined. 

Non-Benzenoid Aromatic Compounds.—The synthesis of aromatic molecules containing small, medium, and large rings, using the reaction of dicarbonyl compounds and bis-ylides, has been thoroughly reviewed.95... 

R. A. Raphael, in Non-Benzenoid Aromatic Compounds (Ed. D. Ginsburg), Chapter VIII, Interscience Publishers, New York, 1958. 

T. Nozoe, in Non-Benzenoid Aromatic Compounds (D. Gins-burg, ed.) p. 339. Wiley (Interscience), New York and London, 1959. 

The definition of aromaticity conceived by Hiickel strictly applies to monocyclic ring systems, but indole, constructed from the fusion of benzene and pyrrole, behaves as an aromatic compound, like quinoline and isoquinoline. The ring fusion, however, affects the properties of both components. This is reflected in the valence bond description of indole, shown in Scheme 7.1, where one canonical representation shows electron density shared between N-1 and C-3 in the pyrrole unit (implying enamine character). Note that although other canonical forms can be drawn, where the lone-pair electrons are delocalized into the benzenoid ring, their energy content is relatively high and they are of limited importance. 

LG. Laing in Rodd s Chemistry of Carbon Compounds. A Modem Comprehensive Treatise. Vol. 3, Part C (2.) Aromatic Compounds. Nuclear substituted benzenoid Hydrocarbons with more than one nitrogen atom in the substituent group, S. 133 -215, Elsevier, Amsterdam 1973 A.J. Floyd, M. Sainsbury, Azo-Arenes in Supplement to Vol. 3, Part C, Elsevier Scientific Publishing Comp., S. 259-272, Amsterdam Oxford New York 1981. 

Polycyclic benzenoids These aromatic compounds have two or more benzene rings fused together, e.g. naphthalene and anthracene. 

Two or more benzene rings fused together form a number of polycyclic benzenoid aromatic compounds, naphthalene, anthracene and phenanthrene, and their derivatives. All these hydrocarbons are obtained from coal tar. Naphthalene is the most abundant (5%) of all constituents of coal tar. 

Fortunately, there is now a comprehensive body of knowledge on the metabolic reactions that produce reactive (toxic) intermediates, so the drug designer can be aware of what might occur, and take steps to circumvent the possibility. Nelson (1982) has reviewed the classes and structures of drugs whose toxicities have been linked to metabolic activation. Problem classes include aromatic and some heteroaromatic nitro compounds (which may be reduced to a reactive toxin), and aromatic amines and their N-acylated derivatives (which may be oxidized, before or after hydrolysis, to a toxic hydroxylamine or iminoquinone). These are the most common classes, but others are hydrazines and acyl-hydrazines, haloalkanes, thiols and thioureas, quinones, many alkenes and alkynes, benzenoid aromatics, fused polycyclic aromatic compounds, and electron-rich heteroaromatics such as furans, thiophenes and pyrroles. 

AROMATIC COMPOUND An organic compound that incorporates a closed-chain or (ring) nucleus in its structure. This is in contrast with the aliphatic compound, which is comprised of an open-chain structure. The classical example of an aromatic compound is benzene. Aromatic compounds also are sometimes referred to as benzenoids. Some ring-type compounds are not classified as aromatic. These include the cycloparaffins and cycloolefins which are considered to be derivatives of methane. See also Compound (Chemical) and Organic Chemistry. 

There are several compounds that possess some measure of aromatic character typical of benzene, but do not possess a benzenoid ring. Appropriately, they have (4n + 2) it electrons and are classified as nonbenzenoid aromatic compounds (see Section 21-9). An example is azulene, which is isomeric with naphthalene and has a five- and a seven-membered ring fused through adjacent carbons ... 

The excited states arising in the photolysis of aromatic compounds provide a good starting point. It should be noted in this connection that the expectation that benzenoid excited states differ from the parent ground states both in electronic structure and reactivity has been commented on previously in the literature (2.3) however, a general link between quantum mechanical description and experimental photochemistry lias been needed. 

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