Polycyclic aromatics synthesis

Unbumed Hydrocarbons Various unburned hydrocarbon species may be emitted from hydrocarbon flames. In general, there are two classes of unburned hydrocarbons (1) small molecules that are the intermediate products of combustion (for example, formaldehyde) and (2) larger molecules that are formed by pyro-synthesis in hot, fuel-rich zones within flames, e.g., benzene, toluene, xylene, and various polycyclic aromatic hydrocarbons (PAHs). Many of these species are listed as Hazardous Air Pollutants (HAPs) in Title III of the Clean Air Act Amendment of 1990 and are therefore of particular concern. In a well-adjusted combustion system, emission or HAPs is extremely low (typically, parts per trillion to parts per billion). However, emission of certain HAPs may be of concern in poorly designed or maladjusted systems. 

Epoxides are often encountered in nature, both as intermediates in key biosynthetic pathways and as secondary metabolites. The selective epoxidation of squa-lene, resulting in 2,3-squalene oxide, for example, is the prelude to the remarkable olefin oligomerization cascade that creates the steroid nucleus [7]. Tetrahydrodiols, the ultimate products of metabolism of polycyclic aromatic hydrocarbons, bind to the nucleic acids of mammalian cells and are implicated in carcinogenesis [8], In organic synthesis, epoxides are invaluable building blocks for introduction of diverse functionality into the hydrocarbon backbone in a 1,2-fashion. It is therefore not surprising that chemistry of epoxides has received much attention [9]. 

Dihydro-1-vinylnaphthalene (67) as well as 3,4-dihydro-2-vinylnaphtha-lene (68) are more reactive than the corresponding aromatic dienes. Therefore they may also undergo cycloaddition reactions with low reactive dienophiles, thus showing a wider range of applications in organic synthesis. The cycloadditions of dienes 67 and 68 and of the 6-methoxy-2,4-dihydro-1-vinylnaphthalene 69 have been used extensively in the synthesis of steroids, heterocyclic compounds and polycyclic aromatic compounds. Some of the reactions of dienes 67-69 are summarized in Schemes 2.24, 2.25 and 2.26. In order to synthesize indeno[c]phenanthrenones, the cycloaddition of diene 67 with 3-bromoindan-l-one, which is a precursor of inden-l-one, was studied. Bromoindanone was prepared by treating commercially available indanone with NBS [64]. 

Dihydrovinylphenanthrenes are more reactive than the corresponding vinyl phenanthrenes and undergo Diels-Alder reactions easily. They have been used in the synthesis of polycyclic aromatic compounds and helicenes. Examples of cycloaddition reactions of the 3,4-dihydro-1-vinylphenanthrene (70), [61] 3,4-dihydro-2-vinylphenanthrene (71) [68] and l,2-dihydro-4-vinylphenanthrene (72) [69] are reported in Equation 2.22 and Schemes 2.27 and 2.28. 

The synthesis of toxic organic compounds by humans, and their release into the natural environment began to assume significant proportions during the 20th century, especially after the Second World War. Prior to 1900, the chemical industry was relatively small, and the largest chemical impact of humans on the environment was probably dne to the release of hydrocarbons, especially polycyclic aromatic hydrocarbons (PAHs), with the combnstion of coal and other fuels. 

Methods for the synthesis of the biologically active dihydrodiol and diol epoxide metabolites of both carcinogenic and noncarcinogenic polycyclic aromatic hydrocarbons are reviewed. Four general synthetic routes to the trans-dihydrodiol precursors of the bay region anti and syn diol epoxide derivatives have been developed. Syntheses of the oxidized metabolites of the following hydrocarbons via these methods are described benzo(a)pyrene, benz(a)anthracene, benzo-(e)pyrene, dibenz(a,h)anthracene, triphenylene, phen-anthrene, anthracene, chrysene, benzo(c)phenanthrene, dibenzo(a,i)pyrene, dibenzo(a,h)pyrene, 7-methyl-benz(a)anthracene, 7,12-dimethylbenz(a)anthracene, 3-methylcholanthrene, 5-methylchrysene, fluoranthene, benzo(b)fluoranthene, benzo(j)fluoranthene, benzo(k)-fluoranthene, and dibenzo(a,e)fluoranthene. 

By the end of the nineteenth century around 600 fluorescent compounds had been identified [3], including fluorescein (A. von Baeyer, 1871), eosine (H. Garo, 1874), and polycyclic aromatic hydrocarbons (C. Liebermann, 1880) [5], Although it is generally accepted that fluorescence markers are relatively new analytical benefits, it is surprising to note that their chemical synthesis is rather old, such as the fluorescein reported by Baeyer, the 2,5-diphenyloxazole by Fisher in 1896, and the fluorene by Berthelot in 1867 [18],... 

Removal of the amide function is much easier if the reaction is intramolecular, and —CONEt2 amides (sometimes even —CONPr-i2 amides) may be converted to lactones, lactams and other heterocycles in this way . Addition of an aldehyde or ketone as an electrophile generates a hydroxyl group (in some cases, atroposelectively, as it happens —though this is usually irrelevant to the stereochemistry of the product) which cyclizes to give a lactone via a benzylic cation in acid. This reaction has found wide use in the synthesis of polycyclic aromatics, particularly alkaloids. 

Polycyclic aromatic compounds, synthesis by photocyclization of stilbenes, 30, 1... 

All 60 C-atoms of Cjq are incorporated in the CgoHjo polycyclic aromatic hydrocarbon (PAH) 6, for which an efficient synthesis was developed [153], Laser irradiation of 6 at 337 nm induces hydrogen loss and the formation of CgQ, as detected by mass spectrometry (Scheme 1.6). Control experiments with C-labeled material and with the C48H24 homologue of 6 verified that the C50 is formed by a molecular transformation directly from the C50H30 PAH and not by fragmentation and recombination in the gas phase. 

The syntheses from [4+1] atom fragments, in which the Group 16 heteroatom is introduced between two nitrogen atoms, are the most widely applicable and versatile methods available for construction of the 1,2,5-thiadiazole ring system. These methods have been applied to the synthesis of monocyclic and polycyclic aromatic forms of these ring systems in addition to the direct formation of 1-oxides and 1,1-dioxides, 2-oxides, quaternary salts, and reduced forms. The earliest use of the [4+ 1] synthesis dates back to 1889 when Hinsburg prepared 2,1,3-benzothiadiazole (I) from o-phenylenediamine and sodium bisulfite. 

We earlier recalled that one can build a huge number of molecules by simply assembling CeH rings, a scheme that was summarized in Table 1.4, and that a solid such as graphite can be obtained in this gedanken synthesis. Let us briefly discuss the case of linear acenes, which are linear polycyclic aromatic hydrocarbons (PAHs) composed of laterally fused CeHe rings. 

When 2D or 3D PAHs are considered then the choice becomes much larger. An exhaustive review on the advances in the synthesis of polycyclic aromatic compounds can be found in Harvey, 2004. The most used strategies are flash vacuum pyrolysis, cross-coupling, oxidative photocyclization, Diels-Adler cycloaddition, etc. 

Ruehle, P. H., L. C. Bosch, and W. P. Duncan, Synthesis of Nitrated Polycyclic Aromatic Hydrocarbons, in Nitrated Polycyclic Aromatic Hydrocarbons (C. M. White, Ed.), pp. 169-235, Hiithig, Heidelberg, 1985. 

M.L. Tedjamulia. Y. Tominaga, M. Sugiura, H. Kudo, M.L. Lee u. R.N. Castle, Polynucl. Aromal. Hydrocarbons, Int. Symp., 7th, 1161-1172 (1982) The Synthesis of Potentially Mutagenic Polycyclic Aromatic Amines". 

In the aromatic-ring-annelated oxepin series the resonance effect is clearly the major influence dominating other factors (e.g. temperature, solvent, etc.) which affect the oxepin-arene oxide equilibrium. It is however very difficult to exclude the presence of a minor (spectroscopically undetectable) contribution from either tautomer at equilibrium. This problem has been investigated by the synthesis of chiral arene oxides from polycyclic aromatic hydrocarbons (PAHs). The presence of oxepin (26) in equilibrium with naphthalene 1,2-oxide has been excluded by the synthesis of the optically active arene oxide which showed no evidence of racemization in solution at ambient temperature via the achiral oxepin (26) <79JCS(Pl)2437>. 

Environmental chemicals and pollutants are also capable of inducing P450 enzymes. As previously noted, exposure to benzo[a]pyrene and other polycyclic aromatic hydrocarbons, which are present in tobacco smoke, charcoal-broiled meat, and other organic pyrolysis products, is known to induce CYP1A enzymes and to alter the rates of drug metabolism. Other environmental chemicals known to induce specific P450s include the polychlorinated biphenyls (PCBs), which were once used widely in industry as insulating materials and plasticizers, and 2,3,7,8-tetrachlorodibenzo-p-dioxin (dioxin, TCDD), a trace byproduct of the chemical synthesis of the defoliant 2,4,5-T (see Chapter 56). 

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