Synthesis of Complex Condensed Heterocycles by Diels-Alder Reactions

Significance

(E)-3-(2-Alkoxyvinyl)-benzofurans, -furopyridines and -indoles undergo rapid [4+2] cycloaddition reactions with even poorly reactive dienophiles under thermal and pressurized conditions to afford condensed dibenzofurans, dibenzofuro[2,3-c]pyridines, carbazoles, 2-azacyclopenta[c]fluorenes, furo[2,3-c]isochromenes, among other complex ring systems. Interesting temperature and pressure dependence on stereoselectivity was observed: endo products are derived by pressure-mediated reactions while exo isomers result under thermal conditions. For successful cycloaddition to give the [4.4.0] condensed isochromene and isoquinolinone systems, activated dienophiles such as maleic anhydride were required.