Enyne Metathesis of Norbornene Derivatives: A Facile Approach to Polycyclic Heterocycles

Norbornene derivatives bearing propargyloxy substituents in the 5- and 6-positions undergo a cascade series of enyne metatheses when treated with Grubbs' catalyst and a terminal alkene, leading to heterocyclic dienes which undergo a stereoselective DielsAlder reaction, giving access to a range of polycyclic heterocycles. The process is compatible with both terminal and internal alkynes, and depending upon the structure of the alkene can give symmetrical or unsymmetrical products. Mechanistic studies have shown that the initial metathesis reaction is between the strained norbornene and Grubbs' catalyst, rather than between the alkyne and Grubbs' catalyst.