position substituents being selected from certain ester polyether and alkanoyl groups the latter having components which bind to biological substrates these specific derivatives are effective as spin traps for different types of free radicals to form persistent nitroxide adducts of extended half life these adducts can be characterized by esr spectrometry technique and provide information e g concerning the identification of free radicals application_number US 4672793 A description the pyrroline n oxide spin traps function by having a charge distribution and steric configuration between the n oxide and the 2 position favourable for attracting and retaining various free radicals as adducts substituents in the 3 and 5 positions serve the function of bestowing a limited increase in adduct lifetime but have no effect on the selectivity recent tests have indicated that substituents in the 4 position have a further effect on adduct formation and stability as well as on the selectivity in the course of investigating this further effect we have prepared various 4 substituted derivatives the first type of compound investigated was one having a carboxylic acid ester group spaced from the pyrroline c 4 ring carbon by at least two carbon atoms it was found possible to prepare this type by starting with the 4.