The first stereoselective synthesis of orostanal isolated from a marine sponge Stelletta hiwasaensis

The synthesis of orostanal, containing novel 6-5-6-5 fused rings and exhibiting meaningful bioactivity against HL-60 cells, has been achieved via 12 steps in 18% total yield from a readily accessible intermediate 7. The key steps for the synthesis entailed an alumina-mediated intramolecular aldol cyclization and asymmetric crotylation to construct the chirality of carbon-25.