Reagents and conditions: a) Triethylsilane, BF3·Et2O, CH2Cl2, 0 °C, 1 h, 11: 72%, 12: 5%, 13: 2% ; b) TfOH, 5 Å molecular sieves, CH2Cl2, 0 °C, 15 min (for →15)/45 min (for →18), 15: 46%, 18: 71%; c) Pd-C (10%), H2 (1 atm), MeOH, r.t., 3 h (for →16)/4 h (for →19); then 0.1 M NaOMe, MeOH, r.t., 14 h (for →16)/20 h (for →19) ; then 0.1 M aq. NaOH, 0 °C → r.t., 3 h, 16: 99%, 19: 90%.