a compound of the formula : ## str1 ## wherein : n is an integer 2 , 3 , 4 , or 5 ; r 1 is hydrogen or an alkyl group ; and r 2 and r 3 are defined as follows : r 2 is hydrogen or an alkyl group ; r 3 is hydrogen or an alkyl group ; or r 2 and r 3 together form a heterocyclic ring , e . g . piperidine , pyrrolidine . the compounds have high radiosensitizing activity , and act as agents toxic to hypoxic cells in the absence of radiation .