Beckmann rearrangement

Activated oximes rearrange to amides, with migration of the carbon substituent anti to the leaving group. A cyclic ketoxime can therefore expand a ring to a lactam.

Fragmentation of an activated oxime by cleavage of a carbon–carbon bond anti to its N–O leaving group, yielding a nitrile and a carbon electrophile or a product derived from it. Stabilization of a developing carbocation can favour fragmentation over migration. It differs from the usual Beckmann rearrangement, which inserts nitrogen into the migrating carbon–carbon bond and yields an amide after hydrolysis.

A carbon substituent opposite the activated N–O leaving group migrates onto nitrogen as the N–O bond cleaves. The oxime geometry therefore determines which bond is replaced by a carbon–nitrogen bond.