Copper-Catalyzed Formal [4+2] Cycloaddition of Enoldiazoimides with Sulfur Ylides

Enoldiazoimides, a new subclass of enoldiazo compounds, generate enol-substituted carbonyl ylides whose reactions with sulfur ylides enable an unprecedented formal [4+2] cycloaddition. The resulting multifunctionalized indolizidinones, which incorporate sulfur, are formed in good yields under mild reaction conditions. The uniqueness of this transformation stems from the role of the silyl-protected enol, since the corresponding acetyldiazoimide failed to provide any cross-products in metal-catalyzed reactions with sulfur ylides. This copper-catalyzed cycloaddition is initiated with the generation of enol-substituted carbonyl ylides and sulfur ylides from enoldiazoimides and sulfonium salts, respectively, and proceeds through stepwise six-membered ring formation, C-O and C-S bond cleavage, and silyl and acetyl group migration.