Scheme 3. Reagents and conditions: (a) Boc₂O, DMAP, CH₂Cl₂/CH₃CN, 70 °C, 24 h; (b) hydroxylamine hydrochloride, Et₃N, ethanol, 65 °C, 15 h; (c) acetic anhydride, acetic acid, 85 °C, 15 h; (d) tin(ii) chloride, ethanol/ethyl acetate, 70 °C, 6 h; (e) methyl malonyl chloride, K₂CO₃, CH₃CN, 0 °C, 12 h; (f) 4 M HCl in 1,4-dioxane, rt, 15 h; (g) 4,5-dibromo-pyrrole-2-carboxylic acid, oxalyl chloride, CH₂Cl₂, 15 h, then ii) 24, pyridine, CH₂Cl₂, rt, 6 h; (h) hydrazine monohydrate, ethanol/tetrahydrofuran, 80 °C, 15 h; (i) 1,1-carbonyldiimidazole, 1,4-dioxane/dimethylformamide, 100 °C, 15 h; (j) 2 M NaOH, THF, rt, 15 h.