Thermal Intramolecular Diels–Alder Reaction of Furan; Synthesis of Nitrogen Tetracycles, Isobenzofuran and Isobenzothiophene

Abstract

Thermal intramolecular Diels–Alder (IMDA) reaction of furan cored compounds has been further investigated; a series of key precursors to the IMDA reaction of furan diene (9a–c) have been prepared via facile alkylation and protection. While the cycloaddition process for (10a–c) was afforded in hot toluene, a commercial microwave (2450 MHz) was used for the synthesis of (12a–b). Treatment of fused oxy- and thio-heterotricycles (12a–b) with borontrifluoride-etherate in dichloromethane at −78°C cleaved epoxy bridge and concomitant aromatisation gave the isobenzo-furan and thiophene (13a–b) in 72–76% yields respectively.