aReagents and conditions: (a) (+)-21 (1.0 equiv), 53 (2.0 equiv), BEt3 (1.3 equiv), toluene, −78 °C, 6 h, 77%; (b) TBAF (1.0 equiv), THF, 0 °C, 5 min, 66%; (c) MnO2 (40 equiv), CH2Cl2, 25 °C, 10 h, 39%, 72% brsm; (d) (+)-21 (1.0 equiv), 90 (2.0 equiv), BEt3 (1.3 equiv), toluene, −78 °C, 6 h, 69%; (e) TBAF (1.0 equiv), THF, 0 °C, 5 min, 67%; (f) MnO2 (40 equiv), CH2Cl2, 25 °C, 10 h, 44%, 66% brsm.