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

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 °C, whereas under high pressure the cycloadditions were complete at 60-70 °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.