ent-Kaurane- and ent-pimarane-type diterpenoids were isolated from the methanol extract of Siegesbeckia pubescens by column chromatography. Their structures were elucidated as ent-16H,17-hydroxy-kauran-19-oic acid (1), ent-4,17-dihydroxy-16-methyl-kauran-19-oic acid (2), ent-16,17-dihydroxy-kauran-19-oic acid (3), kirenol (4) and ent-16,17,18-trihydroxy-kauran-19-oic acid (5) by spectral analysis. The cytotoxicity of these compounds in Caki cells was assayed by a cell counting kit. Only one group treated with kirenol (4), an ent-pimarane-type diterpenoid, showed the inhibition of the cell growth in Caki cells.
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