Scheme 1. Synthesis route of PD 5. Reagents and conditions: (i) Boc anhydride, i-PrOH, DCM, 21 °C, 16 h; (ii) tert-butyl acrylate, i-PrOH, 60 °C, 24 h; (iii) bromomaleic acid, AcOH, reflux, 5 h; (iv) DCC, NHS, THF, 21 °C, 16 h, (v) BCN(endo)–PEG2–NH₂, MeCN, 21 °C, 16 h.