An oxidative photocyclization approach to the synthesis of Securiflustra securifrons alkaloids

Securines and securamines are cytotoxic alkaloids that contain reactive alkene and heterocyclic residues embedded in skeletons comprising four to six oxidized rings. This structural complexity imparts a rich chemistry to the isolates but has impeded synthetic access to the structures in the nearly three decades since their isolation. We present a flexible route to eight isolates that exemplify the three skeletal classes of metabolites. The route proceeds by the modular assembly of the advanced azides 38 and 49 (13 steps, 6 to 10% yield), sequential oxidative photocyclizations, and late-stage functional group manipulations. With this approach, the targets were obtained in 17 to 19 steps, 12 to 13 purifications, and 0.5 to 3.5% overall yield. The structure of an advanced intermediate was elucidated by microcrystal electron diffraction (MicroED) analysis. The route will support structure-function and target identification studies of the securamines.