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The extraordinary power of high-pressure/solvent-free conditions for the Diels–Alder reaction of thiophene has been established. For example, maleic anhydride (X=O) reacted with thiophene at 0.8 GPa and 100 °C to give the desired exo adduct in almost quantitative yield (see scheme). Further application of this method to maleimide (X=NR) and acrylic dienophiles demonstrated its remarkable potential.
Kumamoto et al. (2004) studied this question.