Mulberry Diels-Alder adducts: synthesis of chalcomoracin and mulberrofuran C methyl ethers.

The synthesis of each of the heptamethyl ethers of the mulberry Diels-Alder adducts chalcomoracin (1) and mulberrofuran J (2) is described. The key steps in each approach involved a biomimetic intermolecular [4+2]-cycloaddition between a dehydroprenylphenol diene derived from an isoprenoid-substituted phenolic compound and an alpha,beta-unsaturated alkene of a chalcone as the dienophile. Critical to the success of the Diels-Alder reaction was the presence of the free phenol in the 2'-hydroxychalcone.