Studies on stable diazoalkanes as potential fluorogenic reagents. I. 7-Substituted 4-diazomethylcoumarins.

As a new type of stable diazoalkane, 4-diazomethylcoumarins (4) bearing various 7-substituents were prepared for potential use in introducing fluorophores into acidic substances. Selenium dioxide oxidation of 4-methylcoumarins to the corresponding coumarin-4-carbaldehydes followed by triethylamine-mediated Bamford-Stevens reaction of their tosylhydrazones readily gave 4 as excellently stable and almost non-fluorescent crystals, exhibiting characteristic diazomethyl 1H- and 13C-nuclear magnetic resonance signals. Labeling reactions of carboxylic acids using 4 in refluxing chloroform in the presence of silica gel catalyst gave excellent yields of fluorescent coumarin-4-ylmethyl esters. In view of their stability, accessibility and fluorogenicity, these compounds (4), especially those bearing 7-methoxy and 7-acetyloxy substituents, should be practically useful fluorogenic reagents for carboxylic acids.