1-Aryl-2-methyl-4,5-dihydropyrrol-3-carboxylic acid ethyl ester (a cyc
lic enamine) was observed to dehydrogenate to give 1-aryl-2-methyl-pyr
rol-3-carboxylic acid ethyl ester upon irradiation in the presence of
oxygen and in the presence or absence of meso-tetraphenylporphine (TPP
). N-Aryl cyclic amines were shown to be singlet oxygen sensitizers.