Synthesis And Fluorescent Property Of Triazolylcoumarins

Much attention has been fixed on coumarin and its derivatives for their bioactivity,optical property,and ease to be synthesized.While it is known that 3-or 4-substituent on coumarins affects signifcantly fluorescent properties of coumarins, there were relatively few reports on 3-substituted coumarins by far.Moreover,most of the synthesis of 3-substituted coumarins involves chiral catalyst or heavy metal catalyst,which limits applications of 3-substituted coumarins.Click chemistry provided a way to synthesize 1,2,3-triazole compounds efficiently under mild conditions.So a series of 3-1,2,3-triazolylcoumarins were synthesized by click chemistry in this dissertation in order to obtain new coumarins with intense fluorescence and potent bioactivity.3-Azidocoumarins with or without 7-substituent,the most important intermediates for synthesis of 3-1,2,3-triazolylcoumarins,were prepared by two different synthetic routes.7-Substituted-3-azidocoumarins were synthesized starting from 2,4-dihydroxybenzaldehyde and N-acetylglycine.7-Acetyloxy-3-acetylamino-coumarin was produced and then treated with a base followed by acidification to yield 3-amino-7-hydroxycoumarin.Azidation of the aminocoumarin followed by nucleophilic substitution with sodium azide afforded 3-azido-7-hydroxycoumarin. Methylation and acetylation of 7-hydroxy group gave 3-azido-7-methoxycoumarin and 3-azido-7-acetyloxycoumarin,respectively.3-Azidocoumarin was prepared from salicylaldehyde and diethyl malonate.Coumarin-3-carboxylic acid was produced and then transformed to 3-aminocoumarin.Azidation followed by nucleophilic substitution with azide afforded 3-azidocoumarin.The[3+2]dipolar cycloaddition between 3-azidocoumarins with or without 7-substituent and end alkynes in the presence of sodum ascorbate and copper(â…¡)sulfate under mild conditions afforded 3-(1,2,3-triazol-1-yl)coumarins in excellent yields.Totally 16 3-(1,2,3-triazol-1-yl)coumarin compounds were synthesized by click chemistry,12 of which were new compounds.The chemical structures of the 3-1,2,3- triazolylcoumarins were characterized by ¹H NMR,¹³C NMR,FT-IR,and MS.UV absorption spectra and fluorescence emission spectra of the 3-(1,2,3-triazol-1-yl)coumarins were measured to investigate their fluorecent property.The fluorescence quantum yields of the 3-(1,2,3-triazol-1-yl)coumarins were calculated based on their fluorescence emission spectra.All the 3-(1,2,3-triazol-1-yl)-coumarins showed a maximum UV absorption at 340-350nm wavelength,shifting to much longer wavelength compared to coumarin(320nm).Their maximum fluorescence emission wavelength were also similar at about 420nm.The 3-(1,2,3-triazol-1-yl)coumarins showed intense fluorescence and very high fluorescence quantum yields(0.600～0.992),much higher than those of stilbenylcoumarins and other coumarins,probably due to the electron-withdrawing ability,the conjugation system,and the planar rigid structure of 1,2,3-triazole ring.Comparing the influence of different 7-substituent on the fluorescent property of the 3-(1,2,3-triazol-1-yl)coumarins,we found that 7-hydroxy-3-(1,2,3-triazol-1-yl)coumarins exhibited the highest fluorescence quantum yields,while 7-methoxyl-3-(1,2,3-triazol-1-yl)-coumarins showed the lowest fluorescence quantum yields.This is perhaps because the modification of 7-hydroxy group with an electron-withdrawing acetyl group brought about unfavorable effect on the 7-hydroxy-3-(1,2,3-triazol-1-yl)coumarins push-pull(D-A)system while the strong electron-donating ability of 7-methoxyl induced twisted intramolecular electron tarnsfer(TICT)leading to decrease in the fluorescence quantum yields.