Attempted Synthesis of Spongidines by a Radical Cascade Terminating onto a Pyridine Ring

Mn(III)-based oxidative free-radical cyclization of an unsaturated beta-keto ester containing a pyridine ring as radical trap has been studied. This intramolecular reaction of nucleophilic carbon-centered radicals with the pyridine ring leads to the stereospecific construction of a tetracyclic compound in which five chiral centers are created in one pot. This synthetic approach represents the first attempt to prepare the anti-inflammatory pyridinium alkaloids spongidine A, B, and D.