General preparative route to benzo[g]quinolines (1-azaanthracenes)

Article abstract of DOI:10.1002/jhet.5570360218

A convenient synthetic pathway to benzo[g]quinolines (1-azaanthracenes) has been developed. The nickel catalyzed coupling of methyl 2- chloronicotinate (3a) with benzylic organo zinc reagents 2a-e led to the methyl 2-benzylic substituted nicotinates 4a-e. Treatment of methyl 2- chloro-6-methylnicotinate (3b)with 2a in a similar manner led to methyl 2- benzyl-6-methylnicotinate (4f). The coupling of 2-chloro-3-acetylpyridine (5) with benzyl zinc bromide (2a) led to 2-benzyl-3-acetylpyridine (4g). The coupling of the 2,5-dichlorobenzylic organic zinc reagent (2f) with methyl 2- choronicotinate (3a) was unselective but readily coupled with methyl 2- bromonicotinate (6) to yield methyl 2-(2,5-dichlorobenzyl)nicotinate (4h). The esters 4a-f,h on reduction with lithium aluminum hydride led to the corresponding alcohols 7a-f,h which were subsequently oxidized with manganese dioxide to the respective 2-benzylic substituted pyridine-3-carboxaldehydes 8a-f,h. In one case the coupling of benzy] zinc bromide (2a) with 2- chloropyridine-3-carboxaldehyde (9) led directly to 2-benzylpyridine-3- carboxaldehyde (8a), but in poor yield. Cyclizations of the aldehydes 8a- d,f,h or the ketone 4g with polyphosphoric acid afforded the benzo[g]quinolines 10a-d,f-h in high yields. Aldehyde 8e was cyclized to 10e using a solution of sulfuric acid in methanol. Several of the benzo[g]quinolines 10c,d could be readly converted into the benzo[q]quinoline-5,10-diones 11c,d on treatment with ammonium ceric nitrate.