Scheme 1. Synthesis of mono- and dirhamnoside-containing BA saponins (3 and 4). Reagents and conditions: (a) 12 (1.2–1.5 equiv.), TMSOTf (0.1 equiv.), 4 Å MS, DCM, 0 °C, 30 min, 76% (for 13), 72% (for 16); (b) NaOMe, MeOH/DCM 2 : 1, rt, 16 h, 97% (for 14), 85% (for 4); (c) 10% Pd/C, H2, EtOAc, 80 °C, 4 h, 86% (for 18), 82% (for 3, two steps from 13); (d) 2,2-DMP (3 equiv.), PTSA (0.1 equiv.), acetone, rt, 2 h, 62%; (e) 80% aq. HOAc, DCM, rt, 16 h, 63%.
