Low Catalyst Loading in Ring‐Closing Metathesis Reactions

An efficient procedure is described for ring‐closing metathesis reactions. A conversion of 95 % for diethyl diallylmalonate in dilute solution could be achieved within a few minutes, reaching TOF=4173 min^?1, with very low loading of commercially available Ru catalysts that contained unsaturated NHC ligands. In general, only 50 to 250 ppm of the catalyst is required to achieve near‐quantitative conversion into a broad variety of 5–16‐membered heterocyclic compounds. The practicality of this procedure was illustrated in the synthesis of 5–8‐membered N‐tert‐butoxycarbonyl (N‐Boc)‐ and N‐para‐toluenesulfonyl (N‐Ts)‐protected cyclic amines and 9–16‐membered lactones. The synthesis of macrocyclic proline‐based lactams required slightly higher catalyst loadings. Along with monocyclic products, oligomeric byproducts, mostly cyclodimers, were isolated and characterized.