Photochemical Reaction of Ethoxycarbonyl-Substituted Quinolines

Article abstract of DOI:10.1246/bcsj.60.2891

The photochemical reactions of the quinoline derivatives substituted by an ethoxycarbonyl group at the 2-, 3-, and 4-positions of a quinoline nucleus have been investigated in several alcohols and cyclohexane.Irradiation of ethyl 4-quinolinecarboxylate (1) yielded ethyl 2-hydroxyalkyl-4-quinolinecarboxylates (4a-c) in alcohols and ethyl 2-cyclohexyl-4-quinolinecarboxylate (4d) in cyclohexane in a good yield, respectively.The photochemical reactions of ethyl 3-quinolinecarboxylate (2) showed remarkable solvent dependency.Irradiation in methanol and cyclohexane afforded a solvent-additive product, ethyl 4-hydroxymethyl-1,4-dihydro-3-quinolinecarboxylate (5a) and ethyl 4-cyclohexyl-1,4-dihydro-3-quinolinecarboxylate (5b), while such photoaddition of the solvent did not proceed in ethanol and 2-propanol but instead ethyl 1,4-dihydro-3-quinolinecarboxylate (6) and dimeric compounds were formed, both of which were unstable and finally reverted to 2 at room temperature in air.In the case of ethyl 2-quinolinecarboxylate two types of the products, ethyl 4-hydroxyalkyl-1,4-dihydro-2-quinolinecarboxylate (7) and ethyl 1,4-dihydro-2-quinolinecarboxylate (8) were obtained in ethanol and 2-propanol but the yields of those products were poor.On the basis of triplet quenching experiments, the photochemical reactions of those ethyl quinolinecarboxylates are suggested to occur through hydrogen abstraction from the solvents by the ring nitrogen in the S₁ state.