The [2 + 2] Cycloaddition–Retroelectrocyclization and [4 + 2] Hetero-Diels–Alder Reactions of 2‑(Dicyanomethylene)indan-1,3-dione with Electron-Rich Alkynes: Influence of Lewis Acids on Reactivity

The reaction of electrophilic 2-(dicyanomethylene)­indan-1,3-dione (DCID) with substituted, electron-rich alkynes provides two classes of push–pull chromophores with interesting optoelectronic properties. The formal [2 + 2] cycloaddition–retroelectrocyclization reaction at the exocyclic double bond of DCID gives cyanobuta-1,3-dienes, and the formal [4 + 2] hetero-Diels–Alder (HDA) reaction at an enone moiety of DCID generates fused 4H-pyran heterocycles. Both products can be obtained in good yield and excellent selectivity by carefully tuning the reaction conditions; in particular, the use of Lewis acids dramatically enhances formation of the HDA adduct.