Acetic Anhydride → tert-butyl (1R,3R,4S)-3-acetamido-4-((S)-3-(benzyloxycarbonylamino)-2-oxopyrrolidin-1-yl)cyclohexylcarbamate

Participants

Reaction Conditions

Reaction Steps

The reaction was run in three equal portions and combined for aqueous workup. A 5 L, 3-necked round bottom flask was charged with (1R,2S,5R)-2-((S)-3-(benzyloxycarbonylamino)-2-oxopyrrolidin-1-yl)-5-(tert-butoxycarbonylamino)cyclohexanecarboxamide (25.3 g, 53 mmol), acetonitrile (1.9 L), and 2.6 L of water/ice. The mixture was stirred and cooled to 0° C. Iodobenzene diacetate (25.77 g, 80 mmol) was added and the reaction was stirred for 2 h; another 0.5 eq of iodobenzene diacetate was added. The reaction was stirred for 9 h (reaction temp<10° C.). The mixture was charged with 8 eq N,N-diisopropylethylamine and 2 eq acetic anhydride. Over the next thirty minutes, 4 eq N,N-diisopropylethylamine and 2 eq acetic anhydride were added every ten minutes, until the reaction had proceeded to completion (HPLC). The acetonitrile was removed in vacuo; some solid separated from the residue, and this was collected by filtration. The remaining residue was extracted with dichloromethane (3 L, then 1 L). The organic phase was washed sequentially with water, sat. NaHCO3, and brine. The collected solids were added to the organic phase, along with activated carbon (15 g). The mixture was stirred for 30 minutes at 40° C. before being filtered and concentrated in vacuo. The residue was dissolved in EtOAc (1 L), and the resultant solution was stirred at 75° C. for 1 h before being allowed to cool to room temperature. A solid separated and was collected by filtration. This solid was purified further by recrystallization: it was first dissolved in 0.5 L CH2Cl2, then concentrated in vacuo, then re-crystallized from 1 L EtOAc; this was repeated three times. The solids obtained from the mother liquors of the above were recrystallized three times using the same method. The combined solids were recrystallized twice more from acetonitrile (0.7 L) to provide 66 g (84%) of tert-butyl (1R,3R,4S)-3-acetamido-4-((S)-3-(benzyloxycarbonylamino)-2-oxopyrrolidin-1-yl)cyclohexylcarbamate (purity>99.5% by HPLC). LC/MS for primary peak: [M+H]+=489.4; [M-tBu+H]+=433.3. 1H-NMR (400 MHz, d4-MeOH): δ 7.3-7.4 (m, 5H), 5.11 (s, 2H), 4.35 (m, 1H), 4.15 (m, 1H), 4.04 (m, 1H), 3.8 (m, 1H), 3.6 (m, 2H), 2.44 (m, 1H), 2.12 (m, 1H), 1.87-2.05 (m, 4H), 1.87 (s, 3H), 1.55-1.7 (m, 2H), 1.46 (s, 9H). The stereochemical fidelity of the Hofmann rearrangement was confirmed through X-ray crystal structure analysis of this compound, as shown in FIG. 1.

Provenance & Evidence