Migratory Decarboxylative Coupling of Coumarins: Synthetic and Mechanistic Aspects

On the move: Decarboxylative coupling of allyl 4-methyl-3-carboxycoumarins provides the products of γ-allylation of the methyl group rather than the typical regiospecific α-allylation. Mechanistic studies show that intramolecular proton transfer from the 4-methyl group to the 3-carboxylate allows allylation of the remote methyl group. The resulting 4-butenyl-3-carboxyl coumarin undergoes Pd0-catalyzed decarboxylation to provide the observed products (see scheme).