Synthesis of bioactive heterocycles: One pot regioselective synthesis of pyrano[3,2- f]benzo[ b]thiophene derivatives

Abstract

Regioselective synthesis of hitherto unreported pyrano[3,2- f]benzo[ b]thiophene derivatives ( 8a-f) in 90–95% yields are reported by the application of a less studied rearrangement of 6-(4-aryloxybut-2-ynylthio)[1]benzopyran-2-ones ( 5a-f) via oxidation with m-chloroperoxybenzoic acid followed by thermal rearrangement and treatment with methanol. Substrates 5a-f are prepared by the reaction of 6-mercaptocoumarin (unstable) with 1-aryloxy-4-chlorobut-2-ynes ( 4) in refluxing acetone in the presence of anhydrous potassium carbonate and sodium iodide. 6-Mercaptocoumarin ( 3) is in turn prepared by the zinc-acid reduction of the disulfide ( 2) obtained by the reaction of the diazotised 6-aminocoumarin ( 1) with potassium ethylxanthate followed by decomposition. Pyrano[3,2- f]-benzo[ b]thiophene derivatives ( 8a-f) are easily converted to 1-acetylpyrano[3,2- f]benzo[ b]thiophene-7-one ( 12) when refluxed in acetic acid in the presence of catalytic amount of conc. sulfuric acid for 4 h.