Catalysis of a Diels-Alder Reaction by Amidinium Ions.
Amidines and guanidines are important functional groups in molecular recognition and host-guest chemistry. Here it is shown that lipophilic amidinium ions catalyze a cycloaddition reaction representing the key step of the Quinkert-Dane estrone synthesis. Hydrogen-bond-mediated association with the organic cation leads to an electrophilic activation of the dienophile and to enhanced rates of the Diels-Alder reaction. The observed effects are similar to those expected from mild Lewis acids. In competition experiments, amidinium catalysis favors the reaction of the less electron deficient dienophile.
Schuster
T
Institut für Organische Chemie der J. W. Goethe-Universität Frankfurt, Marie-Curie-Strasse 11, D-60439 Frankfurt am Main, Germany.
Kurz
M
Göbel
MW