Scheme 2. Synthesis of trirhamnoside-containing BA saponin (5). Reagents and conditions: (a) 12 (1.0 equiv.), TMSOTf (0.1 equiv.), 4 Å MS, DCM, 0 °C, 30 min, 63%; (b) 10% Pd/C, H₂, EtOAc, 80 °C, 4 h; (c) NaOMe, MeOH/DCM 2 : 1, rt, 16 h, 68% (two steps).