The title compound, C(19)H(21)N(3)O(3)S, was synthesized via an aza-Wittig reaction of a functionalized imino-phospho-rane with phenyl isocyanate under mild conditions. In the mol-ecule, the fused thienopyrimidine ring system makes a dihedral angle of 66.30 (11)° with the phenyl ring. An intra-molecular C-H⋯O hydrogen bond occurs. The terminal -OCH(2)CH(3) group is disordered over two sites with refined occupancies of 0.537 (13) and 0.463 (13). The crystal packing is stabilized by inter-molecular C-H⋯O and N-H⋯O hydrogen bonds.