title: Diels-Alder reactions of 3-substituted coumarins in water and under high-pressure condition. An uncatalyzed route to tetrahydro-6H-benzo[c]chromen-6-ones.
abstract: [reaction: see text] Diels-Alder reactions of 3-substituted coumarins 1a-g with methyl-1,3-butadienes 2a-c carried out in water alone and in CH2Cl2 under 9 kbar pressure are reported. In aqueous medium satisfactory results were obtained by operating at 150 degrees C, whereas under high pressure the cycloadditions were complete at 60-70 degrees C with excellent yields (85-95%). The reactions with isoprene (2b) always resulted in the exclusive formation of para cycloadducts, whereas with (E)-piperylene (2c) only ortho products were detected. The cycloaddition of 3-phenylsulfonylcoumarin (1a) with (E)-piperylene (2c) allowed the endo adduct to be obtained exclusively, whereas 3-carboxycoumarin (1b) reacted with 2c to give a mixture of the corresponding endo/exo adducts in a 58:42 ratio in water and in a 45:55 ratio under high-pressure condition.
authors.affiliation: Centro di Eccellenza Materiali Innovativi Nanostrutturati, and Dipartimento di Chimica, Università degli Studi di Perugia, via Elce di Sotto 8, 06123 Perugia, Italy.
authors.forename: Rugiada
authors.initials: R
authors.lastname: Girotti
authors.forename: Assunta
authors.initials: A
authors.lastname: Marrocchi
authors.forename: Lucio
authors.initials: L
authors.lastname: Minuti
authors.forename: Oriana
authors.initials: O
authors.lastname: Piermatti
authors.forename: Ferdinando
authors.initials: F
authors.lastname: Pizzo
authors.forename: Luigi
authors.initials: L
authors.lastname: Vaccaro