The ring expansion of 2-ester-2-aryl-azetidine carbamates can be achieved using Brønsted acids to form 6,6-disubstituted 1,3-oxazinan-2-ones. The reaction is rapid at room temperature with Boc or Cbz derivatives, and proceeds with excellent yield (up to 96%) and broad substrate scope. Derivatives of drug compounds and natural products are incorporated. The combina-tion of this ring expansion in a 3-step N–H insertion/cyclization/expansion (NICE) sequence is applied to directly access medicinally relevant scaffolds from acyclic precursors.