Dimers of Podocarpic Acid Derivatives

Treatment of methyl 12-hydroxypodocarpa-8,11,13-trien-19-oate (5) with a commercial sample of thallium(III) trifluoroacetate gave a low yield (5%) of a non-symmetric ether-linked dimer (15). Use of freshly prepared thallium(III) trifluoroacetate gave a 28% yield of the symmetrical biaryl dimer (16). The dimethyl ether (17) was prepared in low yield (10%) by treatment of methyl 12-methoxypodocarpa-8,11,13-trien-19-oate (4) with the latter reagent.