A topochemical rearrangement with multiple inversions of configuration

Crystalline 2-benzyloxypyridine-1-oxide rearranges slowly at room temperatu re to crystalline 1-benzyloxy-2-pyridone. No intermediates are detected whe n the process is followed by solid-state C-13 NMR. The crystal structure of the pyridine-1-oxide strongly suggests that a topochemically controlled in tramolecular process, in which the benzyl group migrates with retention of configuration, is not feasible. On the other hand, although somewhat disfav oured by initial solid-state O . . .C . . .O angles significantly less than the ideal 180 degrees, intermolecular topochemically controlled processes can be envisaged that lead, with multiple inversions of configuration, eith er to net retention of configuration or to net inversion of configuration i n the benzyl group. In contrast to the 50-80% inversion observed in solutio n, in the solid state only inversion is observed experimentally when chiral ly labelled alpha -deuteriobenzyloxypyridine-1-oxide is allowed to rearrang e.