Diels−Alder Reactions of 3-Substituted Coumarins in Water and under High-Pressure Condition. An Uncatalyzed Route to Tetrahydro-6<i>H</i>-benzo[<i>c</i>]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