Autoxidation of phorbol esters under normal storage conditions.

The polyfunctional diterpene phorbol and its esters are highly susceptible to autoxidation (2, 3). The widespread use, partially in long-term experiments, of the standard tumor promotor TPA1 (Chart 1) and other phorbolor 4a-phorbol-12,13-diesters in cancer and cell research and, in particular, in studies on their metabolism and mechanism of action (4 6) prompted us to investigate in detail the various products arising from TPA and 4«-phorbol-12,13-diacetate by autoxidation under different conditions of storage. It was found that, in addition to traces of polar products not isolated, the aldehyde 2, the 0-epoxide 3, the hydroperoxide 4_,the hydroxylated A5-6 isomer 5, the ketone 6, and the 6,7-seco hemiacetall are being formed from TPA (Charts 1 and 2). The structures of the compounds shown in the formulas (except that of the hydroperoxide 4, which was not isolated) were rationalized by their infrared, mass, nuclear magnetic resonance, and UV spectra and will be discussed elsewhere. Also the results of experiments on the potential irritant and cocarcinogenic activities of 2, 3, and 6 to mouse skin will be published separately. TLC is an efficient means of checking impure samples of TPA for any of the possible compounds formed by autoxi dation, as may be seen from Table 1.