Studies on the structures and syntheses of arteannuin and related compounds: XV. Determination of A/B ring configuration of arteannuic acid

5‐Oxo‐compound 7 and its epimer 8 prepared from epoxidation of methyl arteannuinate followed by rearrangement with BF 3 · Et 2 O 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. Therefore the cis A/B ring fusion for arteannuic acid is established. In 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 compound 14 as that of 8. Thus the cis A/B ring configuration of arteannuic acid is further proved.