aReagents and conditions: (a) (+)-21 (1.0 equiv), (−)-22 (2.0 equiv), BEt3 (1.1 equiv), toluene, −78 °C, 6 h, 82%; (b) TBAF (1.0 equiv), AcOH (1.05 equiv), THF, 0 °C, 30 min, 88%; (c) IBX (5.0 equiv), EtOAc, 70 °C, 9 h, 68%; (d) DMP (1.5 equiv), CH2Cl2, 0 → 25 °C, 2 h, 83%; BEt3 = triethyl borane, IBX = 2-iodoxybenzoic acid, DMP = Dess–Martin periodinane.