Hemisynthesis of two marine cheilanthane sesterterpenes from (-)-sclareol: first enantioselective synthesis of petrosaspongiolide R.

The synthesis of two marine sponge metabolites 5 and 8 from naturally occurring (-)-sclareol is described here. The sesterterpenolide (5) is synthetised for the first time, establishing the absolute configuration of this compound. The key intermediate, aldehyde (10), was obtained from (-)-sclareol in good overall yields. The use of Katsumura's Wittig reagent and subsequent photochemical oxidation delivered the sesterterpenolide (8), which was chemoselectively epoxidized on exocyclic terminal olefin using the oxaziridinium salt (14) and transformed in four steps to carboxylic acid (5).