By means of a combination of partition and chromatographic methods six irritant constituents were isolated from the fresh latex ofEuphorbia fortissima. Compounds A‐D were di‐esters of the common parent diterpene 12‐deoxyphorbol, and compounds E and F were mono‐esters of the same diterpene. The fresh latex had an irritant dose 50 (ID50) on mice of 0.64 μg μ1−1. Compounds A‐D are short‐acting irritants reaching a maximum activity within 4 h of application to the skin, whilst the monoesters maintained potent irritant effects for up to 24 h. Selective hydrolysis of the di‐esters at the C–20 primary ester group also produced mono‐esters of greater potency after 24 h. An increase in the length of the fatty acid located at C–13 produced greater biological activity in both the mo
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