GENERAL STEPS: D-Biotin (2.95 g, 12.00 mmol) was dissolved in N,N-dimethylformamide (DMF, 10 mL) to which pentafluorophenol (Pfp-OH, 2.80 g, 15.20 mmol) was subsequently added. A DMF solution (15 mL) of 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride (EDC-HCl, 5.70 g, 29.70 mmol) was slowly added dropwise to the above mixture over 30 min at 0 °C. After the dropwise addition, the reaction mixture was stirred overnight at room temperature under nitrogen atmosphere. Upon completion of the reaction, the product was washed with dichloromethane (DCM) to afford the target compound 5-((3aS,4S,6aR)-2-oxohexahydro-1H-thieno[3,4-d]imidazol-4-yl)pentanoic acid perfluorophenyl ester in white powder form in 90% yield (4.50 g). The obtained product can be directly used in the subsequent reaction without further purification.1H NMR (D2O) data were as follows: δ 1.41-1.69 (m, 6H, CH2), 2.57 (d, 1H, CH2), 2.77-2.85 (m, 3H, CH2), 3.11-3.12 (m, 1H, CH), 4.14 (t, 1H, CH), 4.30 (t, 1H, CH), 6.37 (d, 2H, NH).
