An Unusual Rearrangement and its Dependence on Configuration

In the course of a synthetic study, we encountered an unusual rearrangement that we now report, one that involves a 5‐nitronorbornenyl system having 7‐endo‐ and 7‐exo‐pyridin‐2‐yl groups being treated under Nef‐reaction conditions. The stereoisomers differ in their Nef‐reaction behavior: the isomer with the pyridin‐2‐yl group exo to the NO₂ moiety primarily affords the Nef reaction, however, with formation of a rather unusual rearrangement side‐product. The endo‐pyridyl stereoisomer proceeded exclusively with rearrangement.