Polycyclic aromatic heterocycles

Polycyclic aromatic heterocycles that contain pyridine rings fused with benzene rings include quinoline and isoquinoline. Quinine, used to treat malaria, is an example of a naturally occurring quinoline. 

Polycyclic aromatic heterocycles in which the 2- and 3-positions of a pyrrole are fused to a benzene ring are called indoles. The indole ring occurs in many medicinally important natural products, such as the neurotransmitter serotonin. 

Much research effort has been devoted to the study of tandem hetero-IMDA-cycloelimination reactions for the synthesis of fused polycyclic aromatic heterocycles. Nitrobenzene was found to be the best solvent for the tandem IMDA reaction/cycloelimination of hydrogen cyanide from 2- 2-[(trimethylsilyl)ethynyl]phenoxy -l,3-diazine (X = O)71,72. 

For example, as illustrated in Scheme 3.22, dibenzo[f,h]pyrido[2,3-b]quinoxa-line was prepared in refluxing ethanol as a novel aza-heterocyclic compound. This approach shows an interesting synthesis of the new polycyclic aromatic heterocycles, which was the main purpose of this work (Scheme 3.22). 

FIGURE 32.8 Chemical class separation scheme for synthetic fuel products. HCs, hydrocarbons PACs, polycychc aromatic compounds N-PACs, nitrogen polycychc aromatic compounds 2 -PANHs, secondary nitrogen polycyclic aromatic heterocycles APAHs, amino polycyclic aromatic hydrocarbons 3 -PANHs, tertiary nitrogen polycyclic aromatic heterocycles. 

The pyridine ring can be fused with benzene rings to produce polycyclic aromatic heterocycles. The most important examples are quinoline and isoquinoline, analogs of naphthalene (Sec. 4.13) but with N in place of CH at C-1 or C-2. 

On the basis of the reaction of alkyl radicals with a number of polycyclic aromatics, Szwarc and Binks calculated the relative selectivities of several radicals methyl, 1 (by definition) ethyl, 1.0 n-propyl, 1.0 trichloromethyl, 1.8. The relative reactivities of the three alkyl radicals toward aromatics therefore appears to be the same. On the other hand, quinoline (the only heterocyclic compound so far examined in reactions with alkyl radicals other than methyl) shows a steady increase in its reactivity toward methyl, ethyl, and n-propyl radicals. This would suggest that the nucleophilic character of the alkyl radicals increases in the order Me < Et < n-Pr, and that the selectivity of the radical as defined by Szwarc is not necessarily a measure of its polar character. 

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]. 

Warshawsky D, T Cody, M Radike, R Reilman, B Schumann, K LaDow, J Schneider (1995) Biotransformation of benzo[a]pyrene and other polycyclic aromatic hydrocarbons and heterocyclic analogues by several green algae and other algal species under gold and white light. Chem-Biol Interact 97 131-148. 

Safinowski M, C Griebler, RU Meckenstock (2006) Anaerobic cometabolic transformation of polycyclic and heterocyclic aromatic hydrocarbons evidence from laboratory and field studies. Environ Sci Technol 40 4165-4173. 

Heterocyclic Ring Systems (Unsaturated) and Polycyclic Aromatic Systems... 

Table 5.5 Chemical shifts and couplings in some common heterocyclic and polycyclic aromatic systems.

IARC, International Agency for Research on Cancer (2010) IARC monographs on the evaluation of carcinogenic risks to humans, vol 92. Some non-heterocyclic polycyclic aromatic hydrocarbons and some related exposures. http //monographs.iarc.lr/ENG/Monographs/vol92... 

Acridine is a three-ring nitrogen heterocycle, classified as polycyclic aromatic hydrocarbons (PAHs), some of which are proposed to be human carcinogens based on the evidence of carcinogenicity in experimental animals. Acridine is present in crude oil and tar pitch, as well as in emissions resulting from their combustion. 

Monograph on the Evaluation of Carcinogenic Risks of the Chemical to Man Certain Polycyclic Aromatic Hydrocarbons and Heterocyclic Compounds", Internat. Agency Res. Cancer, World Health Organization, 1973 Vol. 3. 

Petroleum products contain many fluorescing compounds, e.g., aromatic hydrocarbons, polycyclic aromatic hydrocarbons, and various heterocyclic... 

A wide variety of different classes of fluorescent molecules has been investigated in the peroxyoxalate chemiluminescent systems. Among those screened were fluorescent dyes such as rhodamines and fluoresceins, heterocyclic compounds such as benzoxazoles and benzothiazoles, and a number of polycyclic aromatic hydrocarbons such as anthracenes, tetracenes, and perylenes. The polycyclic aromatic hydrocarbons and some of their amino derivatives appear to be the best acceptors as they combine high fluorescence efficiency with high excitation efficiency in the chemiluminescent reaction [28],... 

Gimeno R.A. et al., 2002. Determination of polycyclic aromatic hydrocarbons and polycyclic aromatic sulfur heterocycles by high-performance liquid chromatography with fluorescence and atmospheric pressure chemical ionization mass spectrometry detection in seawater and sediment samples. J Chromatogr A 958 141. 

Certain Polycyclic Aromatic Hydrocarbons and Heterocyclic Compounds, International Agency for Research on Cancer Monographs on the Evaluation of the Carcinogenic Risks to Humans LARC Lyon, France, 1972 Vol. 3. 

Thomas etal. [72] used pyrolysis gas chromatography-mass spectrometry as a fast economic screening technique for polyaromatic hydrocarbons. Thomas used reverse-phase liquid chromatography with atmospheric pressure chemical ionization mass spectrometry/mass spectrometry for the determination of polycyclic aromatic sulphur heterocycles in sediments. 

Another isomerization reaction of arene oxides is equilibrium with oxe-pins [5], Here, the fused six-membered carbocycle and three-membered oxirane merge to form a seven-membered heterocycle, as shown in Fig. 10.2. An extensive computational and experimental study involving 75 epoxides of monocyclic, bicyclic, and polycyclic aromatic hydrocarbons has revealed much information on the structural factors that influence the reaction rate and position of equilibrium [11], Thus, some compounds were stable as oxepins (e.g., naphthalene 2,3-oxide), while others exhibited a balanced equilibrium... 

Check which ionization method was used and examine the general appearance of the mass spectrum. Is the molecular ion peak intensive (as with aromatic, heterocyclic, polycyclic compounds) or weak (as with aliphatic and multifunctional compounds) Are there typical impurities (solvent, grease, plasticizers) or background signals (residual air, column bleed in GC-MS) ... 

WHO. Environmental Health Criteria 202 Selected Non-heterocyclic Polycyclic Aromatic Hydrocarbons (Geneva World Health Organization), 1998. 

Gradually it was recognized that nucleophilic aromatic photosubstitution is a fairly general reaction (Havinga et al., 1967 Havinga and Kronenberg, 1968). It can be realized also with polycyclic and heterocyclic aromatic systems. Various solvents (water, alcohols. 

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. 

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