The bottom part of ring B was constructed by nucleophilic addition to the aldehyde of 2.1 (scheme 2) with dibenzyl acetal of 2-bromobenzaldehyde 2.2 as its aryllithium. This step is much in common with the B ring synthesis in the Nicolaou Taxol total synthesis except that the aldehyde group is located at ring A and not ring B. The diol in 2.3 was protected as the boronic ester 2.4 preparing the molecule for upper part ring closure with tin tetrachloride to tricycle 2.5 in a Grob fragmentation-like reaction.
After deprotection (pinacol) to diol 2.6, DIBAL reduction to triol 2.7 and TBS reprotection (TBSOtf, lutidine) to alcohol 2.8 it was possible to remove the phenylsulfide group in with a tributyltin hydride and AIBN(see Barton-McCombie deoxygenation) to alcohol 2.9. Palladium on carbon hydrogenation removed the benzyl protecting group allowing the Swern oxidation of 2.10 to ketone 2.11