21 (-)-アルテミシンの(+)-イソアラントラクトンよりの誘導

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Optically active 3-oxo-8β(H),11α(H)-eudesm-4-en-8,13-olide(20) the racemic modification of which was a key compound in our total synthesis of (±)-artemisin(1) can be expected to become a relay compound linking natural (-)-artemisin and our optically inactive synthetic intermediates. Sodium borohydride reduction of (+)-isoalantolactone(2) afforded (+)-dihydroisoalantolactone(3) whose cis lactone ring could be transformed into the trans form by the following series of conversions. Alkaline hydrolysis opened the lactone ring yielding the hydroxy-acid which was immediately esterified with diazomethane to give the hydroxy-ester(6). Chromic anhydride oxidation in acetone followed by hydrolysis afforded the keto-acid(8). Thermodynamically controlled reduction with sodium in isopropyl alcohol gave the unsaturated acid(10) with 8α-hydroxyl group (equatorial). Lactonization could be achieved by heating in vacuo, and the resulting trans lactone (12) was refluxed in acetic acid with p-toluenesulfonic acid to give the unsaturated trans lactone with endo-cyclic double bond which could be also obtained directly from the hydroxy-acid(10) by refluxing in acetic acid with p-toluenesulfonic acid. Selenium dioxide oxidation in ethanol converted the lactone(13) into the requisite optically active key compound(20), m.p. 140-141°[α]_D+15.5°which was proved identical with the (+)-3-oxo-8β(H),11α(H)-eudesm-4-en-8,13-olide derived from (-)-artemisin. Identity was further confirmed by the comparison with our synthetic racemic compound. By the series of transformations which we had explored in the successful synthesis of (±)-artemisin, this α,β-unsaturated ketone was converted into natural (-)-artemisin.
天然有機化合物討論会の論文
1968-09-20

21 (-)-アルテミシンの(+)-イソアラントラクトンよりの誘導

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