Symmetrical nitroxide synthesis:: Meso versus d,l diastereomer formation

The syntheses of Ct-symmetrical isoindoline nitroxide 2a and meso-isoindoline nitroxide 2b have been achieved by two different routes. The chiral Ct-symmetric nitroxide derives from the addition of a Grignard reagent from the least hindered face opposite the large phenyl group in nitrone intermediate 21, whereas the isoindoline 3b precursor to the meso nitroxide is formed by the addition of a Grignard reagent from the face opposite the large magnesium oxide group of the tight ion pair of iminium 19. The assignment of these structures is confirmed by preparation of several derivatives as well as by X-ray crystallography.