The biosynthesis of naphthoquinones (10, 12, 14 etc.) with an inverted isoprenyl group and anthraquinones (11 and 17) of Streptocarpus dunnii was studied using precursors labeled with stable isotopes and cell suspension culture of the plant. The administration of [2'-carboxy-^<13>C]-4-(2'-carboxyphenyl)-4-oxobutanoic acid (OSB) to the cell cultures revealed that all these quinones are biosynthesized via OSB, and that anthraquinones are not formed via prenylation on the position corresponding to C-3 of OSB as in Rubiaceous plants, but formed via prenylation on the position corresponding to C-2. Feeding experiments with [7-^2H]-lawsone (19) and its prenyl ether (20) further demonstrated that naphthoquinones are formed via these compounds. On administration of 20, the overflow production of [7-^2H] -2-hydroxy-3-(1,1-dimethylallyl)-1,4-naphthoquinone (22), the Claisen type rearrangement product, was observed, which was caused by the abundant exogeneous precursor (20). Furthermore, on administration of 19 or 20, tectoquinone (23) was also isolated as an overflow product owing to the feedback control. On the basis of these results the biosynthetic pathway of the quinones in S. dunnii cell cultures were presumed to be shown in Scheme 1.
