Catalysis of a Diels-Alder reaction by amidinium ions

Amidines and guanidines are important functional groups in molecular recogn ition and host-guest chemistry. Here it is shown that lipophilic amidinium ions catalyze a cycloaddition reaction representing the key step of the Qui nkert-Dane estrone synthesis. Hydrogen-bond-mediated association with the o rganic cation leads to an electrophilic activation of the dienophile and to enhanced rates of the Diels-Alder reaction. The observed effects are simil ar to those expected from mild Lewis acids. In competition experiments, ami dinium catalysis favors the reaction of the less electron deficient dienoph ile.