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.