Enynes as Substrates

A very common point for most of these processes is the presence of triarylpho-sphine bonded to gold in the catalytic system employed. This seems to be a crucial factor for achieving high selectivity in enyne metathesis. Thus, gold is the most active metal for catalyzing these processes using the mildest reaction conditions [147]. 

Methylene cyclohexenes are other products obtained by cyclopropanation of the carbene gold intermediate over a second olefin unit [148]. 

If the substrate contains additional double bonds, bi- or even tri-cyclic products can be obtained [149]. Furthermore, Echavarren et al. synthesized several gold(I) cationic derivatives to avoid having to use silver salts that could lead to unwanted side reactions [150] or the hygroscopic character of AuCl3 [151]. 

Further research on this subject was recently reported, in relation to the use of dienynes as substrates for intramolecular cycloaddition. While thermal intramolecular [4+2] cycloadditions of enynes with alkenes only took place at high temperatures, the gold(I) catalyzed transformations provided bi- or tri-cyclic ring systems under mild conditions [152]. 

Double cyclization was observed with siloxy enynes when a new cycloisomerization mechanism was used that involved a cascade of 1,2- alkyl shifts [154]. 

---