Carboxylicacids and their corresponding carboxylate anions are generally utilized as Brønsted acids/bases and oxygen nucleophiles in organic synthesis. However, a few asymmetric reactions have used carboxylicacids as electrophiles. Although chiral thioureas bearing both arylboronic acid and tertiary amine were found to promote the aza-Michael addition of BnONH2 to α,β-unsaturatedcarboxylic acids
The guanidine-catalyzed 6-exo-trig-type intramolecular asymmetric oxa-Michael addition of α,β-unsaturated esters with a 2-hydroxyaryl moiety at the C-5 carbon has been examined for the construction of chromane skeletons with a quaternarycarbon chiral center. The bulkiness of the alkyl group and the E/Z geometry of the α,β-unsaturated ester function played important roles in the asymmetric induction