Reaction of 3-Methylindole with singlet oxygen

The photosensitized oxidation of indoles is of interest because of the involvement of tryptophyl residues in the photodynamic inactivation of some enzymes1 and in the photo-yellowing of wool and silk, sensitized by fluorescent brightening agents ( F B A ' s ) .~~~ There are many data which indicate that electronically excited singletstate oxygen is the reactive intermediate in the dye-sensitized photooxidation of many organic compounds.Much of these data consists of comparisons of the photooxidation product distributions with those obtained by reaction with singlet oxygen generated chemically by the reaction of hydrogen peroxide with sodium hypochlorite.4Since singlet oxygen has been suggested as an intermediate in the dye-sensitized photodegradation of cotton and other fibres,536 it seemed possible that singlet oxygen was the reactive intermediate in the FBA-sensitized photo-yellowing of wool and silk.As part of some studies to investigate this possibility, we needed to know the products from the reaction of singlet oxygen with tryptophyl residues in proteins and whether these products were formed in the FBA-photosensitized reaction.This communication reports the reaction of singlet oxygen with a model for the tryptophyl residues in proteins, 3-methylindole.This compound was chosen rather than tryptophan or a tryptophyl peptide because of the comparative ease of separation and identification of the reaction products and because amino acid residues introduce complications not normally encountered in proteins.Reaction of 3-methylindole with chemically generated singlet oxygen yielded two identifiable products, o-formamidoacetophenone (1) and o-aminoacetophenone ( 2 ) , in yields of 16% and 7% respectively.Similar yields of these compounds were obtained from the eosin-sensitized photooxidation of 3-methylindole, providing evidence that singlet oxygen is an intermediate in this photooxidation.No evidence could be obtained for the presence of the known 3-methylindole oxidation products, 3-methyloxindole and 3-methyldioxindole.The latter was expected because dioxindolylalanine has been claimed as a product of the dye-sensitized photooxidation of tryptophan.'