Novel and Facile Synthesis of β‐(4‐Azuleno[1,2‐b]thienyl)‐ and β‐(4‐Azuleno[2,1‐b]thienyl)‐α,β‐unsaturated Ketones by Intramolecular Tropylium Ion Mediated Furan Ring Opening Reaction.

Abstract

The novel and efficient methods for the synthesis of β-(4-azuleno[1,2-b]thienyl)-α,β-unsaturated ketones (1) and β-(4-azuleno[2,1-b]thienyl)-α,β-unsaturated ketones (2) have been described. Refluxing the dichloromethane solutions of 2-tropylio-3-(2-furyl)thiophene tetrafluoroborates (3) or 3-tropylio-2-(2-furyl)thiophene tetrafluoroborates (4) afforded (1) and (2), respectively, in moderate yields. The reaction involves an intramolecular tropylium ion-mediated furan ring-opening reaction.