A straightforward synthesis of some fused aza-arenes via nucleophilic displacement of a ring hydrogen atom in nitroarenes by aromatic hydrazone anions

摘要

6-Nitroquinoline 6 undergoes direct cyclocondensation with aromatic aldehyde hydrazones 9 in the presence ofnsodium hydride in DMF at low temperature, giving the corresponding 3-aryl-1H-pyrazolo[3,4-f ]quinolines 10 and/orn3-aryl[1,2,4]triazino[6,5-f ]quinolines 11 in low to moderate yield. With aromatic keto hydrazones 7, 3,3-disubstitutedn2,3-dihydro[1,2,4]triazino[6,5-f ]quinoline-4-oxides 8 are obtained in moderate to good yield. The mode of cyclocondensationnis considerably dependent on the electronic nature of a ring substituent of the aromatic hydrazones;nelectron-donating substituents favor the formation of 11, while electron-withdrawing substituents work favorably for thenformation of 10. Monocyclic nitroarenes 15 react similarly with 4-nitrobenzaldehyde hydrazone 9a to give anotherntype of cyclocondensation product, 3-aryl-1H-indazoles 16, in moderate yield. In contrast, nucleophilic substitutionnof a ring hydrogen atom takes place with 4-methylbenzaldehyde hydrazone 9f to yield N-arylated hydrazone 22b,nwhich, however, fails to cyclize to 16 under the conditions employed. The reaction has been suggested to proceednthrough the initial attack of a hydrazone anion on the position adjacent to the nitro group, followed by migration ofnan ipso hydrogen atom to the nitro group in the Meisenheimer intermediate 18. The resulting N2-arylated hydrazonenanion would undergo ring closure via either addition to the nitroso group or displacement of this moiety, eventuallynleading to the fused aza-arenes 8, 10/11, 13 or 16.