4.1. Synthesis of Tert–Butyl (2S,4S)–4–(((E)–N,N’–Bis(Tert–Butoxycarbonyl)–1H–Pyrazole–1–Carboximidamido)Methyl)–2–((Tert–Butoxycarbonyl)Amino)–7–Hydroxyheptanoate (4)

A solution of tert–butyl (2S,4S)–4–(((E)–N,N’–bis(tert–butoxycarbonyl)–1H–pyrazole–1–carboximidamido)methyl)–2–((tert–butoxycarbonyl)amino)–7–((tetrahydro–2H–pyran–2–yl)oxy)heptanoate 1 (1 g, 1.38 mmol) and PPTS (0.35 g, 1.38 mmol) at 50 °C in 25 mL ethanol for 3 h. Saturated NaHCO₃ (0.14 g, 1.38 mmol) was added, filtered. The solvent was removed under vacuum, and purified by flash column (ethyl acetate/hexane 35/65) to get white solid 4 (0.66 g, 75.1%). ¹H NMR (300 MHz, CDCl₃) δ 7.98 (s, 1 H), 7.72 (s, 1 H), 6.42 (s, 1 H), 5.85 (brs, 1 H), 4.08 (dd, J = 13.8, 7.0 Hz, 1 H), 3.80–3.37 (m, 4 H), 3.26 (s, 1 H), 1.96–1.74 (m, 2 H), 1.63–1.58 (m, 3 H), 1.46 (s, 9 H), 1.38–1.31(m, 18 H), 1.22 (s, 9 H). ¹³C NMR (75 MHz, CDCl₃) δ 172.30, 157.37, 156.31, 152.29, 143.18, 130.56, 109.15, 82.77, 81.24, 79.26, 61.34, 52.48, 34.99, 33.87, 28.90, 28.29, 28.17, 27.45, 26.85, 21.00, 14.15. HRMS calcd for C31H54N5O9+, 640.3916[M + H]⁺; found, 640.3917.

tert–butyl (2S,4S)–4–(((Z)–1,3–bis(tert–butoxycarbonyl)–2–(4–methoxybenzyl)guanidino) methyl)–2–((tert–butoxycarbonyl)amino)–7–hydroxyheptanoate (3)

A solution of 4 (1 g, 1.56 mmol), 4–Methoxybenzylamine (0.42 g, 3.13 mmol) and N,N–Diisopropylethylamine (2 mL) at 50 °C in 30 mL acetonitrile for 3 h. The solvent was removed under vacuum, and purified by flash column (ethyl acetate/hexane 60/40) to get white solid 3 (0.99 g, 89.3%). ¹HNMR (300 MHz, CDCl₃) δ: 9.36 (s, 1 H), 7.25 (d, J = 8.8 Hz, 2 H), 6.89 (d, J = 8.8 Hz, 2 H), 5.05–4.99 (m, 1 H), 4.40–4.31 (m, 2 H), 4.20 (t, J = 8.0 Hz, 1 H), 3.87–3.81 (m, 1 H), 3.79 (s, 1 H), 3.61–3.49 (m, 3 H), 2.81 (s, 1 H), 1.79 (s, 1 H), 1.65–1.61 (m, 6 H), 1.54–1.52 (m, 18 H), 1.48 (s, 9 H), 1.46 (s, 9 H).HRMS calcd for C36H61N4O10+, 709.4382[M+H]⁺; found, 709.4385.