Formation of Five- and Six-Membered α,β-Unsaturated Lactones through Ring- Closing Metathesis of Functionalized Acrylates. Applications to Synthesis of Natural Products

α,β-Unsaturated γ- and δ-lactones are important classes of five- and six-membered heterocycles largely present in biologically active compounds of natural and unnatural origin, for which the ring-closing metathesis approach has disclosed simplified synthetic strategies en route or in the final step toward the target molecules. Following from early examples of formation of macrolide rings, this account describes the RCM reactions of allyl and homoallyl acrylic esters promoted by different ruthenium carbene catalysts, and shows how the evolution of catalyst design and performance has accompanied the growth of the RCM application profile in the synthesis of butenolides and pentenolides from increasingly challenging substrates. In addition, selected recent examples of various classes of natural products featuring the unsaturated lactone rings and nowadays accessible via the RCM of acyclic diene precursors are presented, which highlight the impact of the process in the synthesis of biomolecules.