Stereoselective ring contraction diverts the Mitsunobu reaction of a 6-hydroxy-1,4-diazepan-2-one

Attempted Mitsunobu inversion at C-6 of a 6-hydroxy-1,4-diazepan-2-one led instead to a ring-contracted 1,4-piperazine-2-one by way of transannular participation by N-4. The structure and unusual half-boat conformation of the product were determined by NMR analysis.