(2S)-1-(tert-butoxycarbonyl)-4-oxopyrrolidine-2-carboxylic acid → (2S)-1-((tert-butoxy)carbonyl)-4-methylidenepyrrolidine-2-carboxylic acid

Participants

Reaction Conditions

Reaction Steps

To a suspension of methyltriphenylphosphonium bromide (2.2 eq.) in 2-methyl tetrahydrofuran (3 vol) was added rapidly solid potassium tert-butoxide (2.3 eq.) maintaining the temperature around 0° C. The temperature was kept at +20° C. for 2 hours (a suspension remained) and re-cooled to 0° C. Keeping the temperature below 6° C., (S)-1-(tert-butoxycarbonyl)-4-oxopyrrolidine-2-carboxylic acid (1 eq.) was added over 40 minutes. The reaction was warmed to room temperature and stirred for 16 h and then cooled to 0° C. The reaction was quenched with saturated NaHCO3 (5 vol) and water (2 vol) and the aqueous layer was separated. The organic layer was extracted with saturated NaHCO3/water (1.8 vol/1.8 vol) and the combined aqueous layers were filtered through Celite®. The aqueous layer was acidified with 6 N HCl (2.6 vol) at ambient temperature and extracted twice with isopropyl acetate (16 vol, then 8 vol). The organic phase was dried (MgSO4) and the solvent removed. The crude product was dissolved in isopropyl acetate (10 vol) and extracted with 0.5 M NaOH (10 vol, then 1 vol). The combined aqueous layers were acidified at ambient temperature with 6 N HCl to pH=3, and extracted twice with ethyl acetate (10 vol, then 8 vol). The combined extracts were dried (Na2SO4), the solvent removed and the crude product was recrystallized from cyclohexane (5 vol) to afford the title compound. 1H-NMR (400 MHz, DMSO): δ 12.9, (broad, 1H); 5.00 (m, 2H); 4.24 (dt, J=1.9H, J=7.3 Hz, 1H), 3.91 (m, 2H); 2.98 (m, 1H); ≈2.50 (m, 1H, coincides with DMSO); 1.41 and 1.36 (2 s rotamers, 9H).

Provenance & Evidence