Scheme 19.

Assembly of the EFGHI carbon skeleton and formation of the FG bicyclic ketal 115. Reagents and conditions: a) Cy₂BCl (2.4 equiv), i-Pr₂NEt, CH₂Cl₂, −78 °C, then 10 (1.4 equiv), −78 to 0 °C; then H₂O₂, MeOH, pH 7 buffer, 0 °C to RT, dr 60:40; b) HF, H₂O, CH₃CN, 0 °C, 92% (2 steps); c) Dess-Martin periodinane, pyr, CH₂Cl₂, 85%. See ref³¹ for abbreviations.