Mild and efficient boronic acid catalysis of Diels–Alder cycloadditions to 2-alkynoic acids

The concept of boronic acid catalysis (BAC) for the activation of unsaturated carboxylic acids is applied to the Diels–Alder cycloaddition between 2-alkynoic acids as dienophiles and various dienes. These [4+2] cycloadditions produce cyclohexadienyl carboxylic acids, which can be oxidized in situ to produce polysubstituted aromatic carboxylic acids. The boronic acid catalyst is suspected to provide activation by a LUMO-lowering effect of the unsaturated carboxylic acid likely via a covalent, monoacylated hemiboronic ester intermediate.