(3+3)‐Annulation of Carbonyl Ylides with Donor–Acceptor Cyclopropanes: Synergistic Dirhodium(II) and Lewis Acid Catalysis

The first (3+3)‐annulation process of donor–acceptor cyclopropanes using synergistic catalysis is reported. The Rh 2 (OAc) 4 ‐catalyzed decomposition of diazo carbonyl compounds generated carbonyl ylides in situ. These 1,3‐dipoles were converted with donor–acceptor cyclopropanes, activated by Lewis acid catalysis, to afford multiply substituted pyran scaffolds in high yield and diastereoselectivity. Extensive optimization studies enabled access to 9‐oxabicyclo[3.3.1]nonan‐2‐one and 10‐oxabicyclo[4.3.1]decen‐2‐ol cores, exploiting solvent effects on intermediate reactivity.