Aza-polycyclic aromatic compounds

Superacid-promoted dicationic species containing heteroaromatic rings, where positive charge centres migrate through consecutive deprotonation-reprotonation steps, undergo cyclization reactions followed by aromatization and superacid-promoted elimination of benzene (Scheme 10).31 The process leads to the synthesis of aza-polycyclic aromatic compounds in moderate to good yields. Seven examples include pirazole, oxazole, and thiazole heterocycles. 

Scheme 3. Proposed mechanism for aza-polycyclic aromatic compound formation.

Also, synthesis of aza-polycyclic aromatic compounds such as novel phenazines and quinoxalines using TSA was reported by Karami et al. (2011) (Scheme 3.7). This stimulated our interest in further studies on the chemistry of catalysts. 

The gas-phase reactivity of pyridine radical cation was studied and it was shown to combine with molecules of acetylene to provide aza-polycyclic aromatic structures, such as the quinolizium cation. Similar processes were demonstrated for the radical cation of pyrimidine. It is suggested that similar chemical processes are involved in formation of aza-polycyclic aromatic compounds in interstellar clouds and solar nebula. 

Various polycyclic aromatic hydrocarbons, carbonyl arenas, polycyclic nitro-aromatics, aza arenes, N-nitroso compounds, and other carcinogens have been detected in ambient air (Hughes et al, 1980 Kneip et al, 1983 Spiegelhalder and Preussman, 1983). Diesel exhaust particles... 

Thus, the diaddition reaction of bifunctional nucleophiles to alkyl-1,4-diazinium salts followed by the oxidation of cycloadducts formed provides us with a simple and effective route to preparation of polycyclic aza-aromatic compounds. 

Meta-cyclizations are often called meta-bridging reactions, since they yield polycyclic meta-bridged structures (74ACR181). In the series of aza-aromatic compounds, these reactions are mostly found with 3-nitro-substituted azines or with 1,3-diazines, as illustrated by Scheme 54 (77JOC2589). 

Biological activation of polycyclic-hydrocarbon carcinogens proceeds through dihydrodiols and diol epoxides and such pathways are clearly possible for aza-aromatic compounds. To assist in investigating the existence of such routes, a range of dihydrodiols and diol epoxides of benz[c]acridine and benz[a]acridine (R.E. Lehr and S. Kumar, J. org. Chem., 1981, 367 ... 

Results indicated the complex nature of organic materials associated with low Btu coal gasification. Several classes of organic compounds were found, including neutral sulfur and nitrogen heterocycles, phenols, bases including aza arenes and polycyclic aromatic hydrocarbons. Some organic compounds were not... 

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). 

Karami, B., Khodabakhshi, S. and Nikrooz, M. 2011. Synthesis of aza-polycychc compounds Novel phenazines and quinoxalines using molybdate sulfuric acid (MSA). Polycycl. Aromat. Compd 31(2) 97-109. 

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