Scheme 4. Synthesis of mono- and dirhamnoside-containing UA saponins (7 and 8). Reagents and conditions: (a) 12 (1.2 equiv.), TMSOTf (0.1 equiv.), 4 Å MS, DCM, 0 °C, 30 min, 75% (for 21), 70% (for 24); (b) NaOMe, MeOH/DCM 2 : 1, rt, 16 h, 98% (for 22), 86% (for 8); (c) 10% Pd/C, H₂, EtOAc, 80 °C, 4 h, 89% (for 26), 75% (for 7, two steps from 21); (d) 2,2-DMP (3 equiv.), PTSA (0.1 equiv.), CH₃COCH₃, rt, 2 h, 74%; (e) 80% aq. HOAc, DCM, rt, 16 h, 83%.