Base-Promoted Formal (2+1) Cycloaddition of Benzofuran-Derived Oxadienes with Bromomalonates

Abstract A base-promoted formal (2+1) cycloaddition of benzofuran-derived oxadienes with bromomalonates was established that afforded benzofuranone-based spirocyclopropanes in generally good yields (up to 98%) and high diastereoselectivities (up to 91:9 dr). This reaction is the first highly diastereoselective formal (2+1) cycloaddition of benzofuran-derived oxadienes, and will contribute to the chemistry of these compounds. In addition, this approach provides an atom-economic and useful protocol for constructing benzofuranone-based spiro skeletons.