Organocatalytic, Asymmetric Synthesis of Aza-Quaternary Center of Izidine Alkaloids: Synthesis of (−)-Tricyclic Skeleton of Cylindricine

We report a highly efficient, enantioselective, organocatalytic method for the synthesis of izidinone alkaloids containing a bridgehead aza-quaternary center using ketone-derived N,O-hemiaminal with a tethered acetal. Alkyl-, aryl-, and alkenyl-substituted substrates cyclize to the respective products with excellent enantioselectivities. Five- and seven-membered ring formation has also been established. Furthermore, synthesis of the tricyclic skeleton of cylindricine alkaloids has been achieved in high yield.