Scheme 5. Synthesis of Serine-Based Dipeptide Nitrile 43 with a Carboxy-Functionalized 1,2,3-Triazolyl Residue at P1.

Reagents and conditions: (a) methyl-2-azidoacetate, CuSO₄·5H₂O, sodium ascorbate, DMSO/H₂O (1:2), 0 °C, 1 h, rt, overnight; (b) iBCF, NMM, NH₃, THF, −15 °C, 10 min, rt, 30 min; (c) TFA/CH₂Cl₂ (1:1), 2 h; (d) Boc-3-methyl-l-phenylalanine, PyBOP, DIPEA, THF, 3 h; (e) 4-fluorobenzoyl chloride, TEA, CH₂Cl₂, 2 h; (f) cyanuric chloride, DMF, 2 h; (g) LiOH, THF/H₂O (5:1), 0 °C, 5 min; procedure following ref (95).