青蒿素及其一类物结构和合成的研究——ⅩⅤ.青蒿酸A,B环的构型

STUDIES ON THE STRUCTURE AND SYNTHESIS OF ARTEANNUIN AND RELATED COMPOUNDS——XV.DETERMINATION OF A,B RING CONFIGURATION OF ARTEANNUIC ACID

青蒿酸甲酯4环氧化所得的化合物5和6经三氟化硼重排生成的7及其表异构物8在圆二色谱294nm附近分别出现负的和正的Cotton效应.因而推定青蒿酸的A,B环为顺式稠合.用双氢青蒿酸甲酯也得到相同的CD结果.

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·EtO.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.EtO 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-CD)and reaction mecha-nism,the methyl group at Cin compounds 7,8 and 13a,b were assigned as havingα-configuration.