Oxidation of the secondary hydroxyl in phorbol-13,20-diacetate with chromium-trioxide/pyridine yields the diketone phorbol-12-on-13,20-diacetate. The newly introduced carbonyl group is sterically hindered thus allowing for selective reduction of the α„β-unsaturated ketone to give neophorbol-13,20-diacetate, a positional isomer of phorbol-13,20-diacetate in respect to the carbonyl group. By addition of the circular dichroismus of phorbol-13,20-diacetate and neophorbol-13,20-diacetate the dichroism of phorbol-12-on-13,20-diacetate was calculated, indicating that the electronic system of the two carbonyl functions in the latter do not interact. This excludes the existence of the structural unit of an 1,2-diketone in phorbol-12-on-13,20-diacetate. UV-absorption and circular dichroic spectra of derivatives of phorbol esterified or etherified in the 4-position suggest strongly the presence of a tertiary 1,2-ketol group in phorbol. By investigations of the UV-absorption below 200 nm of tetrahydro-phorbol derivatives, the cyclopropane component in phorbol is confirmed. Circular dichroism and UV-absorption of certain derivatives of phorbol-12-on suggest the presence of an β-cyclopropyl ketone group.
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