Reactivity of 3‐Styrylchromones as Dienes in Diels–Alder Reactions under Microwave Irradiation: A New Synthesis of Xanthones

Abstract

AbstractMicrowave irradiation under solvent‐free conditions induced 3‐styrylchromones to undergo Diels–Alder cycloaddition reactions with N‐methyl‐ and N‐phenylmaleimide as well as with other dienophiles. In some of these reactions the cycloadducts underwent in situ oxidation to give xanthones, whereas in other cases it was necessary to add DDQ as an oxidant. The reactions of 3‐styrylchromones with N‐methyl‐ and N‐phenylmaleimide gave the cycloadducts 4‐aryl‐1,3‐dioxo‐3a,4,11a,11b‐tetrahydropyrrolo[3,4‐c]xanthones, which were oxidised to 4‐aryl‐1,3‐dioxopyrrolo[3,4‐c]xanthones with DDQ. The reactions of 3‐styrylchromones with 2‐(2‐nitrovinyl)thiophene gave 2‐aryl‐3‐(2‐thienyl)xanthones directly through a Diels–Alder reaction followed by HNO2 elimination and oxidation. The cycloaddition reactions of 3‐styrylchromones described herein are stereoselective. The reactions of (Z)‐ and (E)‐3‐styrylchromones with N‐methylmaleimide gave, respectively, the endo and exo cycloadducts, while reactions of (Z)‐3‐styrylchromones with the less reactive dienophile N‐phenylmaleimide gave a mixture of compounds. This mixture contained the expected endo cycloadduct and another compound that was identified as the exo adduct of the reaction between the (E)‐3‐styrylchromone — obtained in situ from the isomerisation of its (Z) isomer — and N‐phenylmaleimide. The regio‐ and stereoselectivity of these cycloaddition reactions have been studied by ab initio calculations and can be understood in terms of a thermodynamically controlled reaction. Theoretical calculations are in complete agreement with the experimental results. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)