SYNTHESIS AND SPIN-TRAPPING STEREOCHEMISTRY OF THE CHIRAL SPIN TRAP, 5,5-DIMETHYL-3-PHENYLPYRROLINE-1-OXIDE

A chiral spin trap, 5,5-dimethyl-3-phenylpyrroline-1-oxide (5), and se veral hydroxylamines were synthesized, The structures and conformation s of these compounds were investigated mainly by H-1 NMR spectroscopy. This spin trap was used to trap carbon- and oxygen-centered radicals, The corresponding hydrogen spin adduct was prepared by oxidizing the hydroxylamine to the nitroxide radical. By the combination of H-1 NMR and EPR results, it was shown that the conformation with the phenyl gr oup of C-3 at the equatorial position is exclusively populated and its lifetime is long compared with the EPR time-scale, Addition of carbon -centered radicals to 5 leads to trans adducts whereas oxygen-centered radicals formed cis isomers, These could be confirmed by the investig ation of the monovalent oxidation products of the hydroxylamines. The preference for these conformers is explained by the competition betwee n steric and stereoelectronic effects, Despite the racemic nature of 5 and the formation of a new chiral center(s) in the spin adducts, the presence of different diastereomers would not be observed by EPR where as H-1 NMR studies of some of the nitrones showed clear evidence of di astereomeric mixtures, However, the spin adduct of sec-hydroxybutyl ra dical showed some linewidth effects,which could be attributed to the p resence of two groups of diastereomers that were resolved by ENDOR spe ctroscopy, In general, the spin adducts of 5 are closely related to th ose of the well known DMPO, but the presence of a phenyl substituent a t the 3-position results in a variation of the beta-H coupling constan ts, In contrast to DMPO, 5 can scavenge short-lived radicals in aqueou s and non-aqueous solutions.