aReagents and conditions: (a) (+)-21 (1.0 equiv), 87 (3.0 equiv), BEt3 (1.0 equiv), toluene, −78 °C, 6 h, 83%; (b) TBAF (1.1 equiv), AcOH (1.1 equiv), THF, 0 °C, 15 min, 87%; (c) DMP (2.0 equiv), CH2Cl2, 0 → 25 °C, 3 h, (−)-18 (46%), (+)-19 (11%, one C7′ epimer); (d) IBX (5.0 equiv), EtOAc, 70 °C, 7 h, 83%; (e) DMP (6.0 equiv), CH2Cl2, 25°C, 6 h, 58%.