Scheme 3.
Reagents and conditions: (i) BnNNH2, AcOH, NaCNBH3, THF, rt 14 h, 10 45% and 11 26%; (ii) H2, 5% Pd-C, EtOH, rt, 16 h, 3R 100% and 3S 91%; (iii) tri-O-TBS protected gallic acid, DCC, DMAP, CH2Cl2, rt, 16 h, 3R 83% and 3S 59%; (iv) HF·Py, THF/Py (4:1), 0 °C, 2h, 7 41% and 8 69%.