Ring Transformation of α‐Amino‐β‐oxoesters to δ‐Butyrolactams

δ‐Butyrolactams with an exocyclic ester moiety in the δ‐position were accessed by reduction of 1‐azido‐2‐cyclopentanone carboxylates with PBu3 or Zn‐AcOH. The ring transformation proceeded readily under the conditions of this reduction which is an unprecedented process. Primary amines formed by catalytic hydrogenation of α‐azido‐β‐oxoesters with larger ring sizes or with benzannulation do not show this rearrangement under the conditions of the azide reduction. In these cases, the ring transformation to respective lactams was achieved by treatment of the α‐amino‐β‐oxoesters with a stoichiometric amount of lithium diisopropylamide.