| Abstract
| - Diels−Alder reactions of 3-substituted coumarins 1a−g with methyl-1,3-butadienes 2a−c carried out inwater alone and in CH2Cl2 under 9 kbar pressure are reported. In aqueous medium satisfactory resultswere obtained by operating at 150 °C, whereas under high pressure the cycloadditions were complete at60−70 °C with excellent yields (85−95%). The reactions with isoprene (2b) always resulted in theexclusive formation of para cycloadducts, whereas with (E)-piperylene (2c) only ortho products weredetected. The cycloaddition of 3-phenylsulfonylcoumarin (1a) with (E)-piperylene (2c) allowed the endoadduct to be obtained exclusively, whereas 3-carboxycoumarin (1b) reacted with 2c to give a mixture ofthe corresponding endo/exo adducts in a 58:42 ratio in water and in a 45:55 ratio under high-pressurecondition.
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