1973 Volume 46 Issue 1 Pages 9-14
The triketone (6-oxohelvolic acid (IIa) or its tetrahydro-derivative (IIb)) was readily oxidized in air into the anhydride (IIIa or b) in an alkaline solution; it spontaneously turned red or deep green and then gradually lost its color. In this reaction process, the presence of a relatively stable intermediate radical was confirmed by the ESR measurement. As a result of an analysis of the well-resolved proton hyperfine structure of this radical, it was explained that the structure of helvolic acid is of the protostane type: that is, it is a distinctive structure of the trans-fusion of the A/B ring juncture and a boat form of the B-ring. A reasonable oxidation mechanism including this intermediate radical was presented.
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