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. Virosine A 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 secu′amamine E. 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.
The synthesis of the Securinega alkaloid secu'amamine E (ent-virosine A) has been accomplished for the first time in 12 steps and 8.5% overall yield. In addition, bubbialine has been prepared and characterized. These two alkaloids and bubbialidine, all featuring an azabicyclo[2.2.2]octane core, were rearranged to their azabicyclo[3.2.1]octane congeners, a framework found in many Securinega alkaloids. These experiments suggest that azabicyclo[2.2.2]octane derivatives could serve as intermediates in the biosynthesis of the rearranged azabicyclo[3.2.1]octane products.
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