Catalytic performance of a new Brønsted acidic oligo(ionic liquid) in efficient synthesis of pyrano[3,2-c]quinolines and pyrano[2,3-d]pyrimidines

An acidic liquid 1,3-ionene was synthesized from co-oligomerization of epichlorohydrin and imidazole followed by subjecting the resulting light oligomer to anion exchange with concentrated sulfuric acid and subsequent sulfonylation with chlorosulfonic acid. The 1H and 13C NMR spectra of the synthesized oligo(ionic liquid) displayed significant chemical shifts on transferring the oligomer from D2O to DMSO. This observation was interpreted as resulting from a conformational change in the multicationic oligomer. Application of this oligo(ionic liquid) as both solvent and acidic promoter to the synthesis of pyrano[3,2-c]quinolines and pyrano[2,3-d]pyrimidines gave fairly high yields of the products in short reaction times. The oligo(ionic liquid) is virtually stable, as could be stored for several months at room temperature and can be easily recycled several times in the synthesis of pyrano[3,2-c]quinolines and pyrano[2,3-d]pyrimidines at 80 °C without appreciable loss of activity.