Crystal and molecular structures of N-benzyl-C-(2-pyridyl) nitrone and its ZnBr2 complex

Lewis acid modulated reactions play a very important role in organic synthesis as they provide versatile intermediates, sometimes dictating the stereochemistry of the products (1). Nitrones are substrates rather sensitive to Lewis-acid modulation as we have amply demonstrated during the last years (2) in nucleophilic additions to chiral nitrones. For instance, it has been demonstrated in our laboratory that by employing either ZnBr2 or Et2AlCl, one can produce hydroxylamines with either syn or anti stereochemistry (with respect to the alfa group), respectively (3). 1,3-Dipolar cycloaddition reactions with nitrones are also susceptible to be influenced by the presence of Lewis acids. In fact, Lewis acids catalyzed nitrone cycloadditions have been extensively studied (4). Kanemasa and Tsuruoka have suggested the participation of nitrone-MgBr2 complexes in some cycloaddition reactions with allylic alcohols (5). The scope and limitations of chiral Mg(II) and Cu(II) complexes on the selectivity of cycloaddition of nitrones with alkenes have been studied by Jorgensen and coworkers (6). Activation of nitrones by chiral Lewis acids towards cycloadditions with electron-rich alkenes have also been reported