Ring Closing Enyne Metathesis:  Control over Mode Selectivity and Stereoselectivity

Ring closing enyne metathesis to form 10−15-membered rings was achieved by using a tartrate-derived linker to attach ene and yne subunits. The exo/endo selectivity of the ring closure reaction of these substrates was found to be a function of ring size, whereby larger rings (12−15) give endo-products selectively, while smaller rings (5−11) give exo-products. The E/Z selectivity of the resultant macrocyclic 1,3-dienes was not predictable except for 10- and 11-membered rings. However, both the exo/endo-mode selectivity of the ring closure and the E/Z selectivity of the 1,3-dienes were improved by performing these reactions under ethylene atmosphere. The presence of ethylene induces a selective cross metathesis between the alkyne moiety and ethylene to generate an acyclic 1,3-diene which can undergo ring closing diene metathesis between the isolated olefin and the distal monosubstituted double bond of the 1,3-diene to generate exclusively the endo-product with high E-selectivity.