The Diels-Alder adduct (3aS,7aR,4S)-1,3,3a,-4,5,7a-hexahydro-7-methyl-3-oxoisobenzofuran-4-carboxylic acid

The title compound is a derivative of the major cycloaddition product of dimethyl fumarate and 3-methyl-(E)-2,4-pentadiene-1-ol. The structure determined confirms that the major cycloadduct arises from the expected endo transition state.

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