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

Regioselective synthesis of hitherto unreported pyrano[3,2-f]benzo[b]thioph ene derivatives (8a-f) in 90-95 % yields are reported by the application of a less studied rearrangement of 6-(4-aryloxybut-2-ynylthio)[1]benzo-pyran- 2-ones (5a-f) via oxidation with m-chloroperoxybenzoic acid followed by the rmal rearrangement and treatment with methanol. Substrates 5a-f are prepare d by the reaction of 6-mercaptocoumarin (unstable) with 1-aryloxy-4-chlorob ut-2ynes (4) in refluxing acetone in the presence of anhydrous potassium ca rbonate and sodium iodide. 6-Mercaptocoumarin (3) is in turn prepared by th e zinc-acid reduction of the disulfide (2) obtained by the reaction of the diazotised 6-aminocoumarin (1) with potassium ethylxanthate followed by dec omposition. Pyrano[3,2-f]benzo[b]thiophene derivatives (8a-f) are easily co nverted 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 f or 4 h. (C) 1999 Elsevier Science Ltd. All rights reserved.