A solution of 6-amino-4,7-dimethyl-2,10-dihydro-9-oxa-1,2,4a-triaza-phenanthren-3-one ((Example #54, Step E, 0.060 g, 0.24 mmol) and 3-oxo-azetidine-1-carboxylic acid tert-butyl ester (0.083 g, 0.41 mmol) in MeOH (4.5 mL) and acetic acid (0.5 mL) was stirred at ambient temperature for 1 h. Sodium cyanoborohydride (0.031 g, 0.41 mmol) was added and the resulting dark solution was stirred at ambient temperature for 14 h. The solvent was removed in vacuo to give the crude product, which was purified by preparative TLC (eluting with 33% EtOAc in petroleum ether) to give 3-(4,7-dimethyl-3-oxo-2,3,4,10-tetrahydro-9-oxa-1,2,4a-triaza-phenanthren-6-ylamino)-azetidine-1-carboxylic acid tert-butyl ester (0.042 g, 44%) as an off-white solid. LC/MS (Table 1, Method 2) Rt=1.065 min.; MS m/z: 424 [M+23]+.
