Securinega Alkaloids: Complex Structures, Potent Bioactivities, and Efficient Total Syntheses

The Securinega alkaloids feature a compact tetracyclic structural framework and can be divided into four subclasses characterized by either a bridged [2.2.2]- or a [3.2.1]-bicyclic core with two homologous series in each subclass. In the last two decades, many innovative strategies to chemically access the Securinega alkaloids have been developed. This Focus Review discusses the selected structures and syntheses of representative members of the Securinega alkaloids. Ring-closing metathesis has enabled the syntheses of securinine and norsecurinine, and different cycloaddition approaches were key to the syntheses of nirurine and virosaines A and B. VirosineA was accessed through a Vilsmeier-Haack/Mannich reaction cascade. A bio-inspired vinylogous Mannich reaction has enabled the synthesis of allosecurinine and this strategy has been extended by an intramolecular 1,6-addition to obtain bubbialidine and secuamamineE. A rearrangement process of the latter two alkaloids has furnished allonorsecurinine and allosecurinine, respectively. Finally, an expanded model for the biogenesis of the Securinega alkaloid subclasses is discussed.