Step 1: Synthesis of methyl 3-(tert-butoxycarbonyl(methyl)amino)propionate: To a solution of N-Boc-3-(methylamino)propionic acid (7.0 g, 0.032 mol) in DMF (100 mL) was added K2CO3 (13.3 g, 0.096 mol) and iodomethane (9.0 g, 0.064 mol) at room temperature. The reaction mixture was stirred at room temperature for 3 hours. After completion of the reaction, the reaction was quenched with ice water and extracted with ethyl acetate (2 x 100 mL). The organic layers were combined, dried with anhydrous Na2SO4 and concentrated under reduced pressure. The residue was purified by silica gel column chromatography (gradient elution 0%~10% EtOAc/petroleum ether) to afford methyl 3-(tert-butoxycarbonyl(methyl)amino)propionate (6.9 g, 92%) as a white solid.ESI-MS (EI+, m/z): 118 [M-100+H]+.
Step 2: Synthesis of tert-butyl 3-hydrazino-3-oxopropyl(methyl)carbamate: methyl 3-(tert-butoxycarbonyl(methyl)amino)propionate (6.9 g, 0.0317 mol) was dissolved in EtOH (15 mL) and hydrazine monohydrate (7.7 mL, 0.158 mol) was added at room temperature. The reaction mixture was heated to 80 °C and stirred overnight. After the reaction was completed, the reaction mixture was concentrated and DCM (300 mL) was added. The organic layer was washed with water and saturated sodium chloride solution, dried with anhydrous Na2SO4, and concentrated to give tert-butyl 3-hydrazino-3-oxopropyl(methyl)carbamate (6.8 g, 98%) as a viscous oil, which was used directly in the next step of the reaction.ESI-MS (EI+, m/z): 118 [M-100+H]+.
Step 3: Synthesis of tert-butyl 3-(2-((2S,5R)-6-(benzyloxy)-7-oxo-1,6-diazabicyclo[3.2.1]octane-2-carbonyl)hydrazinyl)-3-oxopropyl(methyl)carbamate: tert-butyl 3-hydrazinyl-3-oxopropyl(methyl)carbamate was synthesized over the medium of 3-hydrazinyl-3-oxopropyl(methyl)carbamate (4.3 g, 19.8 mmol) and (2S,5R )-6-(benzyloxy)-7-oxo-1,6-diazabicyclo[3.2.1]octane-2-carboxylic acid (4.9 g, 0.0178 mol) were added to a solution of HATU (8.2 g, 0.0218 mol) in DMF (100 mL), followed by a slow dropwise addition of DIPEA (9.6 mL, 0.0594 mol). The mixture was stirred at 0 °C for 1 h and then quenched with ice water. The aqueous layer was extracted with EtOAc (2 x 100 mL). The organic layers were combined, dried with anhydrous Na2SO4, filtered and concentrated. Purification of the residue by silica gel column chromatography (gradient elution 20%-60% EtOAc/petroleum ether) afforded tert-butyl 3-(2-((2S,5R)-6-(benzyloxy)-7-oxo-1,6-diazabicyclo[3.2.1]octane-2-carbonyl)hydrazinyl)-3-oxopropyl(methyl)carbamate (6.7 g, 78%) as a white solid.ESI -MS (EI+, m/z): 476 [M+H]+.
Step 4: Synthesis of tert-butyl 2-(5-((2S,5R)-6-(benzyloxy)-7-oxo-1,6-diazabicyclo[3.2.1]octan-2-yl)-1,3,4-oxadiazol-2-yl)ethyl(methyl)carbamate:
Method A: To tert-butyl 3-(2-((2S,5R)-6-(benzyloxy)-7-oxo-1,6-diazabicyclo[3.2.1]octane-2-carbonyl)hydrazinyl)-3-oxopropyl(methyl)carbamate (6.7 g, 14.1 mmol) and pyridine (9.1 mL, 0.113 mol) were added to a -78 °C anhydrous DCM (110 mL ) solution was slowly added Tf2O (7.2 mL, 0.0423 mol) dropwise. The reaction mixture was warmed to 0 °C and then stirred at 0 °C for 3 hours. Aqueous NaHCO3 solution was added slowly at 0 °C. The organic layer was separated and the aqueous layer was washed with DCM (3 x 50 mL). The organic layers were combined, dried with anhydrous Na2SO4 and concentrated. Purification of the crude product by reversed-phase Biotage (water/acetonitrile gradient) afforded tert-butyl 2-(2-((5-((2S,5R)-6-(benzyloxy)-7-oxo-1,6-diazabicyclo[3.2.1]octan-2-yl)-1,3,4-oxadiazol-2-yl)ethyl (methyl)carbamate (3.3 g, 51%) as a light yellow solid.ESI-MS ( EI+, m/z): 458.0 [M+H]+. 1H NMR (500MHz, CDCl3): δ7.45-7.36 (m, 5H), 5.09 (d, J=11.5Hz, 1H), 4.94 (d, J=11.5Hz, 1H), 4.71 (t, J=4.5Hz, 1H), 3.67-3.59 (m 2H), 3.37 (s, 1H), 3.31 (t, J=6.5Hz, 2H), 2.94-2.86 (m, 2H), 2.82 (s, 3H), 2.32-2.28 (m, 2H), 2.15-2.12 (m, 1H), 2.00-1.95 (m, 1H), 1.45 (s, 9H).
Method B: I2 (5.2 g, 20.0 mmol) was added to a solution of PPh3 (5.3 g, 20.0 mmol) in CH2Cl2 (250 mL) at room temperature. after complete dissolution of I2, the solution was cooled to 0 °C and TEA (7.0 mL, 50.0 mmol) was added. Tert-butyl 3-(2-((2S,5R)-6-(benzyloxy)-7-oxo-1,6-diazabicyclo[3.2.1]octane-2-carbonyl)hydrazinyl)-3-oxopropyl(methyl)carbamate (4.8 g, 10.0 mmol) was added, and the mixture was stirred for 1 h at room temperature. The mixture was concentrated, EtOAc (250 mL) was added and the POPI3 was removed by filtration. the filtrate was concentrated and the residue was purified by silica gel column chromatography (gradient elution 30%~50% EtOAc/petroleum ether) to afford 2-(5-((2S,5R)-6-(benzyloxy)-7-oxo-1,6-diazabicyclo[3.2.1]octan-2-yl)-1,3,4- (oxadiazol-2-yl)ethyl(methyl)carbamic acid tert-butyl ester (4.1g, 85%), as white solid.ESI-MS (EI+, m/z): 458 [M+H]+.
Steps 5-7: In accordance with Steps 3-5 in Example 4, tert-butyl (2-(5-((2S,5R)-6-(benzyloxy)-7-oxo-1,6-diazabicyclo[3.2.1]octan-2-yl)-1,3,4-oxadiazol-2-yl)ethyl) urethane was replaced by tert-butyl (2-(5-((2S,5R)-6- (benzyloxy)-7-oxo-1,6 oxo-1,6-diazabicyclo[3.2.1]octan-2-yl)-1,3,4-oxadiazol-2-yl)ethyl(methyl)carbamate for the reaction.
