Scheme 2.

Convergent Assembly of Bisabosqual Tetracycle 6b^a

^aReagents and conditions: (a) H₅IO₆, I₂, EtOH, H₂O, r.t., 6 h, 84% (b) 10, DABCO (0.2 equiv), THF, r.t., 80 h, 70%; (c) 9, DIAD, PPh₃, THF, r.t., 6 h, 96%; (d) s-Bu₃B (1M in THF), (TMS)₃SiH, air, CH₂Cl₂, r.t., 45 min, 72%, d.r. = 3:2 (6b:C₇-epi-6b); (e) TBD, THF, r.t., 15 min, then NaHCO₃, r.t., 2 h, 94%, d.r. = 2:1 (6b:C-7-epi-6b).