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

Abstract

This paper deals with the determination of the A,B ring configuration of artean- nuic acid(1)using circular dichroism(CD).The 5-oxo-compound 7 was prepared from the methyl arteannuinate(4)through epoxidation with m-CPBA followed by treatment with BF_3·Et_2O.Epimerization of 7 with alkali produced its epimer 8.Since 7 exhibited a negative Cotton effect near 294nm,8 exhibited a positive Cotton effect near 292 nm,a cis A,B ring fusion for 7 and a trans A,B ring fusion for 8 might be assigned respectively.The prediction from the octant rule is in consistent with the experimental result. Therefore the cis A,B ring fusion for arteannuic acid was establi- shed.In a similar way,compounds 13a,b and 14 were obtained from methyl dihydroa- rteannuinate 11.Compound 13a,b showed the same sign as that of 7 and compound 14 that of 8.Thus the cis A,B ring configuration of arteannuic acid was confirmed. β-Epoxide 6 or 15 was treated with BF_3.Et_2O to give a mixture of 5-oxo-compound 9 or 16 and an aldehyde 10 or 17,which was difficult to be separated.The formation of 18 or 17 was postulated to be the result of a Wagner-Meerwein type rearrangement. Epimerization of 9 or 16 with alkali produced the trans A,B ring fused isomer 8 or 14. On the basis of benzene induced solvent shift △(CDC1_3-C_6D_6)and reaction mecha- nism,the methyl group at C_4 in compounds 7,8 and 13a,b were assigned as having α- configuration.