Photochemical reactions of azidocoumarins

Photochemical reactions of 6-azidocoumarin and 7-azido-4-methylcoumarin in the presence of secondary amines have been investigated for their potential applications in photoaffinity labeling. It was found that the singlet nitrene generated from 6-azidocoumarin isomerized to a dehydroazepine intermediate that reacted with an amine to yield two isomeric adducts. Photolysis of 7-azido-4-methylcoumarin, in contrast, gave a triplet nitrene that abstracted hydrogen atoms from secondary amine molecules to form 7-amino-4-methylcoumarin as the major product. The difference in the intersystem crossing rate between the two compounds originates from the azido position relative to the carbonyl group. Because of its ability to form a covalent linkage with a nucleophile, 6-azidocoumarin is deemed to have a greater potential as a photoaffinity label than 7-azido-4-methylcoumarin.