Bicyclohumulenone (4), which has recently been isolated from a liverwort, has a stereostructure supposedly derived in vivo from the CC conformer of humulene. On the other hand the marine sesquiterpene, africanol (18) is considered to be biosynthesized from the CT conformer of humulene. Treatment of 9,10-epoxyhumulene (1) with BF_3・OEt_2/Ac_2O/-50°→-20°/1 hr afforded a bicyclohumulerir derivative (2-CC) in 70% yield, through a biogenetic like CC transition state. The product 2-CC was then successfully converted to 4 through 7 steps (2-CC→5→6→7→8→9→10→4) in 41% overall yield. On the contrary cyclization by means of Me_3SiOSO_2CF_3/toluene/-20°→-10°/6 hr led to the formation of tricyclic compounds, 19 and 20, in 80% yield through a CT transition state. Δ^<3,4>-Africene (24), one of the possible biogenetic and chemical synthetic precursors of africanol, was obtained from 20 through 2 steps (20→22→24) in 86% overall yield.
