In the isoquinoline ring system of the title mol-ecule, C(22)H(20)N(2)O(5), the N-heterocyclic ring is in a half-boat conformation. The least-squares plane of the dioxa-2-aza-spiro ring [maximum deviation = 0.076 (1) Å] and forms a dihedral angle of 14.54 (4)° with the phenyl ring. In the crystal, mol-ecules are linked via inter-molecular C-H⋯O hydrogen bonds into layers parallel to (100).