Synthesis and stereochemistry of the spin adducts of a new chiral spin trap, 3,5-diphenyl-5-methylpyrroline-1-oxide

Abstract

The synthesized spin trap 3,5-diphenyl-5-methylpyrroline-1-oxide (3), its derivatives and spin adducts were studied using NMR, ESR and ENDOR spectroscopic methods. The structure of 3 was determined using x-ray analysis. The nitrone 3 was found to have a rigid structure. Two species were observed in the EPR and ENDOR spectra of the spin adducts of 3, but not in the NMR spectra of its diamagnetic derivatives. These two species were attributed to those formed when the addends attack carbon-2 from both the re and si sides, resulting in trans (I) and cis (II) adducts in relation to the phenyl substituent in position 3. Therefore, these two species differ only through the substitution on carbon-2, whereby there are different steric interactions of the substituents within both species. The lack of observation of these two species in NMR spectra could be interpreted in terms of unfavourable relaxation times on the NMR time-scale. These species are observed in EPR and ENDOR spectra mainly because of their different β-C—H dihedral angles, which do not play a role in NMR spectroscopy. Both species have the ability to trap stereoselectively carbon- and oxygen-centred radicals, which occupy pseudo-equatorial and pseudo-axial positions, respectively. The occupation of oxygen-centred addends in the pseudo-axial positions is stabilized by the ‘anomeric effects.’ This nitrone 3 was found to have the ability to scavenge short-lived free radicals in both aqueous and non-aqueous solutions. © 1998 John Wiley & Sons, Ltd.