Symmetrical nitroxide synthesis: Meso versus d,l diastereomer formation

The syntheses of Ct-symmetrical isoindoline nitroxide 2a and meso-isoindoli ne nitroxide 2b have been achieved by two different routes. The chiral Ct-s ymmetric nitroxide derives from the addition of a Grignard reagent from the least hindered face opposite the large phenyl group in nitrone intermediat e 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 magn esium oxide group of the tight ion pair of iminium 19. The assignment of th ese structures is confirmed by preparation of several derivatives as well a s by X-ray crystallography.