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

Abstract

The rates and extent of release of a series of substituted anilines from 4-nitrobenzyl carbamates, following nitro group reduction by radiolytic, enzymic and chemical methods, are reported. The yield of released anilines decreased over the pH range 4–7, but was independent of the basicity of the leaving aniline. Detailed studies of the fragmentation of one example identified the 4-hydroxylamine as the key intermediate. At pH greater than 5 the released aniline 3b condenses with a reactive 4-iminoquinomethane intermediate 4a to give amine 26, thus depleting the measurable amount of aniline 3b released. At pH less than 5 the release of amine proceeds to completion. The efficiency of reductively triggered release of anilines 7 varied with small changes in the leaving group, but this was not uniformly related to aniline basicity. The competing reaction of the released aniline 3b to form amine 26 lowers the efficiency of release of 3b. This reaction occurs at the relatively high concentrations (50 μM) used in the study and indicates the released effector amine should be toxic at concentrations considerably lower than 50 μM. This highlights the need for prodrugs of very potent cytotoxic effectors to be used in tumour-directed nitroreductase enzyme-prodrug therapy.