5-Oxo-compound 7 and its epimer 8 prepared from epoxidation of methyl arteannu-inate followed by rearrangement with BF_3·Et_2O exhibited a negative and a positive Cotton effect near294 nm in CD,respectively,a cis A/B ring fusion for 7 and a trans A/B ring fusion for 8 wereassigned.It is consistent with the prediction from the octant rule.Therefore the cis A/B ring fusionfor arteannuic acid is established.In a similar way,compounds 13a,b and 14 were obtained fromthe dihydroarteannuinate.Compounds 13a,b showed the same signs as that of 7 and compound14 as that of 8.Thus the cis A/B ring configuration of arteannuic acid is further proved. 5-Oxo-compound 7 and its epimer 8 prepared from epoxidation of methyl arteannu-inate followed by rearrangement with BF_3 · Et_2O exhibited a negative and a positive Cotton effect near 294 nm in CD, respectively, a cis A / B ring fusion for 7 and a trans A / B ring fusion for 8 were assigned. It is consistent with the prediction from the octant rule. Wherefore the A A / B ring fusion for arteannuic acid is established. a similar way, compounds 13a, b and 14 were obtained from the dihydroarteannuinate .Compounds 13a, b showed the same signs as that of 7 and compound14 as that of 8.Thus the cis A / B ring configuration of arteannuic acid is further proven.