N-Heterocyclic Carbene Catalyzed Reaction of Enals and 1,2-Dicarbonyl Compounds: Stereoselective Synthesis of Spiro γ-Butyrolactones

Nucleophilic heterocyclic carbene (NHC) catalyzed annulation of enals and cyclic 1,2-dicarbonyl compounds, opening a route to γ-spirolactones,
has been observed for the first time. The strategy works well with isatins, leading to spiroannulated oxindole derivatives. It is conceivable that
the spiroannulation protocol reported herein will be applicable to the synthesis of important natural products that are endowed with a spiro
γ-butyrolactone motif, especially oxindoles and norsesquiterpenoids.