Tricyclic isotaxane and taxane derivatives have been synthesized by a very efficient cascade ring-closing dienyne metathesis (RCDEYM) reaction, which formed the A and B rings in one operation. When the alkyne is present at C13 (with no neighboring gem-dimethyl group), the RCEDYM reaction leads to 14,15-isotaxanes 16 a,b and 18 b with the gem-dimethyl group on the A ring. If the alkyne is at the C11 position (and thus flanked by a gem-dimethyl group), RCEDYM reaction only proceeds in the presence of a trisubstituted olefin at C13, which disfavors the competing diene ring-closing metathesis reaction, to give the tricyclic core of Taxol 44.
Ma, C., Letort, A., Aouzal, R., Wilkes, A., Maiti, G., Farrugia, L. J., … Prunet, J. (2016). Cascade Metathesis Reactions for the Synthesis of Taxane and Isotaxane Derivatives. Chemistry - A European Journal, 22(20), 6891–6898. https://doi.org/10.1002/chem.201600592