15. The C20 methyl group was introduced as methyl iodide in a nucleophilic substitution with a strong base (LHMDS in HMPA) to bromide 16. Then in preparation to ring-closure the TBS ether was deprotected (HCl/THF) to an alcohol which was converted to the aldehyde 17 in a Swern oxidation. The ring-closing reaction was a Reformatskii reaction with Samarium(II) iodide and acetic acid to acetate 18. The stereochemistry of this particular step was of no consequence because the acetate group is dehydrated to the alkene 19 with DBU in benzene. Synthesis B ring The C5 fragment 24 required for the synthesis of the C ring (scheme 3) was prepared from 2,3-dibromopropene (20) by reaction with ethyl acetate (21), n-butyllithium and a copper salt, followed by organic reduction of acetate 22 to alcohol 23 (lithium aluminium hydride) and its TES silylation. Michael addition of 24 with the cyclooctane 19 to 25 with t-BuLi was catalyzed by copper cyanide. After removal of the TES group (HCl, THF), the alcohol 26 was oxidized to aldehyde 27 (TPAP, NMO)which enabled the intramolecular Aldol reaction to bicycle 28. Synthesis A ring Ring A synthesis (scheme 4) started with reduction of the C9 ketone group in 28 to diol 29 with alane in toluene followed by diol protection in 30 as a dimethyl carbonate. This allowed selective oxidation of the C1 alcohol with DDQ after deprotection to ketone 31. This compound was alkylated to 32 at the C1 ketone group with the Grignard homoallyl magnesium bromide (C4 fragment completing the carbon framework) and deprotected at C11 (TBAF) to diol 33. By reaction with cyclohexylmethylsilyldichloride both alcohol groups participated in a cyclic silyl ether (34) which was again cleaved by reaction with methyl lithium exposing the C11 alcohol in 35. The A ring closure required two ketone groups for a pinacol coupling which were realized by oxidation of the C11 alcohol (TPAP, NMO) to ketone 36 and Wacker oxidation of the allyl group to diketone 37. After formation of the pinacol product 38 the benzyl groups (sodium, ammonia) and the trialkylsilyl groups (TBAF) were removed to form pentaol 39. The pentaol 39 was protected twice: two bottom hydroxyl groups as a carbonate ester (bis(trichloromethyl)carbonate, pyridine) and the C10 hydroxyl group as the acetate forming 40. The acetonide group was removed (HCl, THF), the C7 hydroxyl group protected as a TES silyl ether and the C11 OH group oxidized