32 THE GINKGOLIDES

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  • 32 イチョウ苦味成分Ginkgolideの構造

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From the root bark of Ginkgo biloba L. ("icho" in Japanese), a unique fossil tree which has remained unchanged for 1-2 million years, four bitter principles designated ginkgolides A, B, C and M (for "minor") (abbreviated to GA, GB, GC and GM) have been isolated. Early work was greatly hindered by purification problems and polymorphism, but the molecular formulae were finally settled by high resolution MS of GA dimethyl ether (Fig. 1). As shown in Fig. 2, a tert-Bu group is present; these are the first plant products to contain such a group. Establishing the number of lactone groups presented difficulty, but was achieved by titration according to Method B (Fig. 3). The partial structure comprising rings A/B was elucidated from the data summarized in Figs. 5 and 6. NMR analysis of the isolated 4 proton system E-H (Fig. 7) was simplified in GC because of an additional 2°-OH group. The bislactone system containing H_J was clarified (Figs. 8-10) by an NMR analysis of GA-triether utilizing extensively the techniques of solvent shifts and decoupling, and comparison with the triether-d_6 spectrum which resembles the GA spectrum (Fig. 13). An attempt to determine the number of lactone rings was made by measuring the M^+ and isotope peaks of GA dimethyl ether (Fig. 11), the lactone rings of which had been cleaved and recyclized in the presence of H_2O^<18> as depicted in Fig.13-Method B. However, the MS indicated that only two lactones had incorporated O^<18>; presumably, one of the lactones had been cleaved by O-alkyl fission and recyclized by expulsion of O^<18>H. Alkali fusion of GA gave two important bisnor products, but these are not discussed in the Abstract because of limitations of space. Part structures shown in Fig. 12 are proposed for the ginkgolides.

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