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.

This article covers case studies in natural product syntheses with comments on various aspects of metathesis reactions and on how the difficulties encountered in these syntheses have led to better understanding of these reactions or to more effective reaction conditions and catalysts. Ring-closingmetathesis leading to small and medium-sized rings is discussed, followed by macrocycle formation, asymmetric ring-closing metathesis, and cross metathesis.