5-Bromo-2H-3,1-benzoxazine-2,4(1H)-dione (I) and L-proline (II) are heated in DMF to form (S)-6-bromo-1,2,3,11a-tetrahydro-5H-pyrrolo[2,1-c][1,4]benzodiazepine-5,11(10H)-dione (III). This lactam is then reacted with t-BuOK and diphenyl chlorophosphate, and subsequently with the anion of tert-butyl isocyanoacetate.