First intramolecular Diels-Alder reactions using chromone derivatives: Synthesis of chromeno[3,4-: B] xanthones and 2-(benzo [c] chromenyl)chromones

A series of novel (E)-2-(2-propargyloxystyryl)chromones and (E,E)-2-[4-(2-propargyloxyphenyl)buta-1,3-dien-1-yl]chromones were designed and synthesized via aldol condensation of 2-methylchromones with 2-propargyloxy(benzaldehyde and cinnamaldehyde), respectively. Both chromone derivatives were used as substrates in microwave-assisted intramolecular Diels-Alder reactions, affording chromeno[3,4-b]xanthones and 2-(benzo[c]chromenyl)chromones. This is the first report involving chromone derivatives in intramolecular Diels-Alder reactions for the synthesis of new oxygen heterocycles, namely xanthone- and flavone-type compounds.Thanks are due to the University of Aveiro and FCT/MEC for the financial support of the QOPNA research unit (FCTUID/QUI/ 00062/2013) through national funds and, where applicable, co-financed by the FEDER, within the PT2020 Partnership Agreement, and to the Portuguese NMR Network, as well as to the Instituto Politécnico de Bragança. H. M. T. A. is grateful to FCT for his PhD grant (SFRH/BD/86277/2012).