42 甘い配糖体オスラジンの構造決定と全合成(口頭発表の部)

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In 1971, Jizba et. al. reported the isolation and the determination of planar structure of osladin, a sweet principle in rhizome of Polypodium vulgare (Polypodiaceae). Although the stereochemistry of the aglycone was proposed by Havel and Cerny in 1975 based on the chemical correlation with solasodin, that of glycosidic part was assumed to be α-rhamnoside without any firm evidence. Thus, structure 2 has generally been accepted as the structure of osladin. As we have been interested in synthesis of sweet glycosides, we have undertaken the total synthesis of this intensely sweet substance. However the compound 2 synthesized to this end, exhibited no sweet taste at all. Since the structure 2 has been widely accepted by reviews, books, and dictionaries, and many saponins seem to follow this sort of casual assignment, we decided to carry out the structural reinvestigation of osladin. As neither the sample of osladin or the spectra were available from original investigators, we have isolated it from the rhizome collected in southern part of Germany by courtesy of Professors Asakawa and Becker, and found that the structure should be revised to 1 on the basis of single crystal X-ray diffraction study. Thus the stereochemical assignments at C-22, 25 and 26 are corrected from S, R, S to R, S, R. Total synthesis of the sweet osladin (1) was accomplished from aldehyde 7 as follows. Inversion of the C-22S stereochemistry of the major alkylation product 8 was achieved by SN2 type replacement with KO_2 via mesylate 10 to give 9. The alcohol 9 was transformed to lactone 13 by ozonolysis, oxidation of hemiacetal, methylation, and solvolysis sequence. 2'-Discriminated and (3-selective glucosylation of 13 with 14 was achieved by using catalytic amount of TfOH to give 15. Rhamnose residue was introduced by our original thermal glycosylation procedure in α-specific manner affording 17. Although the C-25 methyl group of lactone 17 did not equilibrated under basic condition, but the hemiacetal obtained by DIBALH reduction was easily isomerized to give single equatorial isomer 18. The second rhamnosylation into hemiacetal hydroxyl group was accomplished by Koenigs-Knorr procedure using AgOTf to yield trisaccharide 19. Hydroboration, PDC oxidation, and Pd(OH)_2 catalyzed deprotection gave osladin (1). It was sweet.
天然有機化合物討論会の論文
1992-09-10

42 甘い配糖体オスラジンの構造決定と全合成(口頭発表の部)

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