Structure of arnebinone, a novel monoterpenylbenzoquinone with inhibitory effect to prostaglandin biosynthesis

Two compounds possessing inhibitory effect against a prostaglandin synthesizing enzyme system are isolated from the root of Arnebia euchroma. A slightly yellow coloured oil was identified as a mixture of shikonofuran B and C (3), which had been isolated from Lithospermumerythrorhizon. The structure of the other orange compound, named arnebinone (4), was elucidated by spectroscopic studies to be a novel monoterpenylbenzoquinone, in which the monoterpenyl moiety formed a condensed six membered ring to a benzoquinone. The monoterpenyl moiety has a methyl, a vinyl and an isopropiridene groups. The structure of arnebinone (4) indicates that it is presumably biosynthesized via successive migration reactions from geranylhydroquinone (5), the common intermediate of anphthoquinone congeners, arnebinol (1) and shikonofuran (3).