Thermal intramolecular Diels-Alder reaction of furan; Synthesis of nitrogen tetracycles, isobenzofuran and isobenzothiophene

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 degrees C cleaved epoxy bridge and concomitant aromatisation gave the isobenzo-furan and thiophene (13a-b) in 72-76% yields respectively.