Leaving group effects in reductively triggered fragmentation of 4-nitrobenzyl carbamates

The rates and extent of release of a series of substituted anilines from 4- nitrobenzyl carbamates, following nitro group reduction by radiolytic, enzy mic and chemical methods, are reported. The yield of released anilines decr eased over the pH range 4-7, but was independent of the basicity of the lea ving aniline. Detailed studies of the fragmentation of one example identifi ed the 4-hydroxylamine as the key intermediate. At pH greater than 5 the re leased aniline 3b condenses with a reactive 4-iminoquinomethane intermediat e 4a to give amine 26, thus depleting the measurable amount of aniline 3b r eleased. At pH less than 5 the release of amine proceeds to completion. The efficiency of reductively triggered release of anilines 7 varied with smal l changes in the leaving group, but this was not uniformly related to anili ne basicity. The competing reaction of the released aniline 3b to form amin e 26 lowers the efficiency of release of 3b. This reaction occurs at the re latively high concentrations (50 mu M) used in the study and indicates the released effector amine should be toxic at concentrations considerably lowe r than 50 mu M. This highlights the need for prodrugs of very potent cytoto xic effectors to be used in tumour-directed nitroreductase enzyme-prodrug t herapy.