A solution of 506a (2.27 g, 4.2 mmol) in dry dichloromethane (50 ml) was treated with 30% hydrobromic acid in acetic acid (1.84 ml, 9.2 mmol, 2.2 equiv) at 0° C., under nitrogen. After 10 min stirring at 0° C. the reaction was complete and a white solid crystallised in the medium. The solid was filtered and washed with ethylacetate and diethylether to afford 2.20 g (100%) of [3S(1S,9S)] 5-bromo-3-(9-benzoylamino-6,10-dioxo-1,2,3,4,7,8,9,10-octahydro-6H-pyridazino[1,2-a][1,2]diazepine-1-carboxamido)-4-oxopentanoic acid which was used without further purification: 1H NMR (D6-DMSO) δ 8.87 (1H, d, J=7.3), 8.63 (1H, d, J=7.6), 7.91-7.87 (2H, m), 7.60-7.44 (3H, m), 6.92 (1H, bs), 5.14-5.09 (1H, m), 4.92-4.65 (2H, m), 4.43 (2H, AB), 4.41-4.35 (1H, m), 3.33-3.22 (1H, m), 2.98-2.90 (1H, m), 2.89-2.57 (2H, m), 2.35-2.15 (3H, m), 1.99-1.91 (2H, m), 1.75-1.60 (2H, m). A solution of the bromoketone (535 mg, 1 mmol) in dry DMF (10 ml) was treated with potassium fluoride (150 mg, 2.5 mmol, 2.5 equiv), under nitrogen. After 5 min stirring at room temperature, 2-mercaptothiazole (140 mg, 1.2 mmol, 1.2 equiv) was added. After overnight reaction ethylacetate (150 ml) was added and the organic solution was washed with brine, dried over magnesium sulphate and reduced in vacuo. The residue was crystallised in diethyl ether, filtered and purified on silica gel using a gradient of MeOH (0% to 5%) in dichloromethane. Evaporation afforded 344 mg (60%) of a white solid: mp. 90-95° C. (decomp.); [α]D20 −82° (c 0.2, CH2Cl2)); IR (KBr) 3328, 2941, 1745, 1659, 1535, 1422, 1276, 1255, 1223, 1072; 1H NMR (D6-DMSO)δ 8.92 (1H, d, J=7.6), 8.68 (1H, d, J=7.6), 7.98-7.90 (2H, m), 7.75-7.67 (1H, m), 7.64-7.50 (4H, m), 5.22-5.18 (1H, m), 4.95-4.74 (2H, m), 4.58-4.38 (3H, m), 3.52-3.19 (1H, m), 3.05-2.65 (4H, m), 2.40-1.50 (6H, m). Anal. Calcd for C25H27N5O4S2.H2O: C, 50.75; H, 4.94 N, 11.84. Found: C, 51.34; H, 4.70; N, 11.58. MS (ES+) 572.
