Concise Total Synthesis of Ivorenolide B.

An expeditious route to the potential immunosuppressive lead compound ivorenolide B (1) is described, which relies on the formation of the distinctive 1,3-diyne subunit embedded into the 17-membered framework of this target by ring-closing alkyne metathesis (RCAM). This key transformation was accomplished with the aid of the molybdenum alkylidyne complex 7, which turned out to be compatible with the acid sensitive propargylic alcohol substituents as well as the terminal alkyne unit present in the cyclization precursor. As the presence of such functionality had been detrimental for alkyne metathesis until very recently, this example illustrates the excellent application profile of this new catalyst as well as the rapidly increasing scope of the transformation. Its structural outreach can be further increased by subjecting cyclo-1,3-diynes to appropriate post-metathetic transformations, most notably with the help of alkynophilic gold or palladium catalysts. This aspect is illustrated by the conversion of the model compound 4 into various cyclophane products.