Synthesis of High-Order and High-Oxidation State Securinega Alkaloids

Synthesis and Structure Revision of Securingine E

Chemical synthesis plays a crucial role in confirming and revising the structures of natural products. Through meticulous synthetic efforts, NMR spectroscopic and single-crystal X-ray diffraction analyses, DFT calculations, and mass spectrometric investigations, we revised the structure of securingine E. The revised structure of securingine E was unambiguously confirmed by its chemical synthesis.

Natural product anticipation via chemical synthesis: Discovery of two new securinega alkaloids

The isolation of a natural product conventionally precedes its chemical synthesis. Often, the isolation and structure determination of a natural product present in minute quantities in its natural source pose formidable challenges, akin to finding "a needle in a haystack." On the other hand, leveraging plausible biosynthetic insights and biomimetic synthetic expertise would allow for the prior synthesis of presumed natural products, followed by their verification in natural sources. In this study, we unveil two novel securinega alkaloids, securingines H and I, employing the natural product anticipation through synthesis approach. Structural analysis of securingines H and I suggests that they are biosynthetic derivatives of secu'amamine E and securinol A, respectively. We posit that this "synthesis first" strategy represents a valuable approach to the discovery of new natural products.

Total synthesis of (-)-securingine G

In this study, we present the first total synthesis of (-)-securingine G. Diverging from the proposed biosynthetic pathway, our approach involves the addition of nucleophilic pyridine anion species to the electrophilic menisdaurilide congener. Crucially, incorporating a weakly basic yet nucleophilic tri(2-pyridinyl)lanthanum complex proved essential to circumvent undesired base-mediated pathways during the key coupling reaction. Notably, we introduce n-Bu3La·5LiCl as a new exchange reagent, facilitating efficient halide/lanthanum exchange of (hetero)aryl halides.

Interaction of Norsecurinine-Type Oligomeric Alkaloids with α-Tubulin: A Molecular Docking Study

The medicinal plant Securinega virosa (Roxb ex. Willd) Baill., also known as Flueggea virosa (Roxb. ex Willd.) Royle, is commonly used in traditional medicine in Africa and Asia for the management of diverse pathologies, such as parasite infections, diabetes, and gastrointestinal diseases. Numerous alkaloids have been isolated from the twigs and leaves of the plant, notably a variety of oligomeric indolizidine alkaloids derived from the monomers securinine and norsecurinine which both display anticancer properties. The recent discovery that securinine can bind to tubulin and inhibit microtubule assembly prompted us to investigate the potential binding of two series of alkaloids, fluevirosines A-H and fluevirosinine A-J, with the tubulin dimer by means of molecular modeling. These natural products are rare high-order alkaloids with tri-, tetra-, and pentameric norsecurinine motifs. Despite their large size (up to 2500 Å3), these alkaloids can bind easily to the large drug-binding cavity (about 4800 Å3) on α-tubulin facing the β-tubulin unit. The molecular docking analysis suggests that these hydrophobic macro-alkaloids can form stable complexes with α/β-tubulin. The tubulin-binding capacity varies depending on the alkaloid size and structure. Structure-binding relationships are discussed. The docking analysis identifies the trimer fluevirosine D, tetramer fluevirosinine D, and pentamer fluevirosinine H as the most interesting tubulin ligands in the series. This study is the first to propose a molecular target for these atypical oligomeric Securinega alkaloids.

Transformation of (allo)securinine to (allo)norsecurinine via a molecular editing strategy

Securinega alkaloids have intrigued chemists since the isolation of securinine in 1956. This family of natural products comprises a securinane subfamily with a piperidine substructure and norsecurinane alkaloids featuring a pyrrolidine core. From a biosynthetic perspective, the piperidine moiety in securinane alkaloids derives from lysine, whereas the pyrrolidine moiety in norsecurinane natural products originates from ornithine, marking an early biogenetic divergence. Herein, we introduce a single-atom deletion strategy that enables the late-stage conversion of securinane to norsecurinane alkaloids. Notably, for the first time, this method enabled the transformation of piperidine-based (allo)securinine into pyrrolidine-based (allo)norsecurinine. Straightforward access to norsecurinine from securinine, which can be readily extracted from the plant Flueggea suffruticosa, abundant across the Korean peninsula, holds promise for synthetic studies of norsecurinine-based oligomeric securinega alkaloids.

Synthesis of Bridged Cycloisoxazoline Scaffolds via Rhodium-Catalyzed Coupling of Nitrones with Cyclic Carbonate

Bridged isoxazolidines were synthesized via Rh(III)-catalyzed C-H allylation of α-aryl nitrones with 5-methylene-1,3-dioxan-2-one. The nitrone group serves as a directing group and 1,3-dipole in the C-H activation/[3 + 2] cycloaddition cascade, exhibiting excellent chemo- and stereoselectivity along with good functional group compatibility. The resulting skeletal structure was conveniently modified to produce a range of important chemical frameworks, and the protocol was applied to biologically active molecules.

Synthesis of Suffranidine B

Efficiently generating intricate molecular complexity is a coveted goal in organic synthesis. This can be realized through the implementation of inventive and audacious strategies coupled with the exploration and advancement of novel molecular reactivity pathways. Herein, we present a concise two-step synthesis of a high-oxidation state heterotrimeric securinega alkaloid, suffranidine B, from 2,3-dehydroallosecurinine and the vinylogous ketoaldehyde compound derived from kojic acid. Key to the success was the astute selection of appropriate acids during both the heterotrimerization and the desymmetrizing cyclization steps. This study underscores the value of biomimicry in the synthesis of complex natural products.