Benzofused nitrogen heterocycles

Scheme 4.43 Synthesis of benzofused nitrogen heterocycles by rhodium-catalyzed cyclohy-drocarbonylation in acidic media.

Fustero S, Moscardo J, Jimenez D, Perez-Carrion MD, Sanchez-Rosello M, del Pozo C (2008) Organocatalytic Approach to Benzofused Nitrogen-Containing Heterocycles Enantioselective Total Synthesis of (-t-)-Angustureine. Chem Eur J 14 9868... 

Heterocyclic aromatic compounds contain C and H atoms other than carbon and hydrogen (Fig. 6). For the monocyclic molecules Hiickel s rule is applicable. For example both pyridine and pyrrole contain six ti electrons. Unlike the former the lone pair of the latter is delocalized. Armit and Robinson have shown a connection between the electronic sextet and the heteroaromaticity. Due to the electronegativity difference between carbon and nitrogen the bonds in pyridine are not of equal length and the delocalization is not perfect. Five membered heteroaromatics with oxygen and sulfur are furan and thiophene respectively. Pyrazole/imidazole, triazoles and tetrazoles are five membered heteroaromatics with two, three and four nitrogen atoms respectively. Three important aromatic six membered heterocyclic molecules are pyrimidine, pyrazine and pyridazine. Benzofused... 

While oxopyrazines, oxopyridazines, and their benzofused derivatives are susceptible to protonation at aza-nitrogen, the corresponding pyrimidones are not, since in the 0x0 or thiono tautomers their nitrogen atoms give rise to an amidine structure whose conjugated system would be dismpted by further protonation. Other heterocyclic systems have yet to be explored. 

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