Biosynthetically Inspired Divergent Syntheses of Merocytochalasans

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.

Bioinspired Synthesis of Spirochensilide A from Lanosterol

A bioinspired synthesis of spirochensilide A from commercially available lanosterol is reported. The synthesis features a directed C-H oxidation, a Wagner-Meerwein-type double methyl migration, a Meinwald rearrangement, and a double-bond isomerization/spiroketal formation cascade. The proposed biosynthetic speculation was modified by this synthetic sequence, which also served as a platform for the synthesis of other lanostanes with migrating methyl groups.

Cytochalasans from the Endophytic Fungus Phomopsis sp. shj2 and Their Antimigratory Activities

Cytochalasans from the endophytic fungi featured structure diversity. Our previous study has disclosed that cytochalasans from the endophytic fungus Phomopsis sp. shj2 exhibited an antimigratory effect. Further chemical investigation on Phomopsis sp. shj2 has led to the discovery of seven new cytochalasans (1–7), together with four known ones. Their structures were elucidated through extensive spectroscopic data interpretation and single-crystal X-ray diffraction analysis. Compounds 1–3 and 8–11 exhibited antimigratory effects against MDA-MB-231 in vitro with IC₅₀ values in the range of 1.01−10.42 μM.

Natural product anticipation through synthesis

Natural product synthesis remains one of the most vibrant and intellectually rewarding areas of chemistry, although the justifications for pursuing it have evolved over time. In the early years, the emphasis lay on structure elucidation and confirmation through synthesis, as exemplified by celebrated studies on cocaine, morphine, strychnine and chlorophyll. This was followed by a phase where the sheer demonstration that highly complex molecules could be recreated in the laboratory in a rational manner was enough to justify the economic expense and intellectual agonies of a synthesis. Since then, syntheses of natural products have served as platforms for the demonstration of elegant strategies, for inventing new methodology 'on the fly' or to demonstrate the usefulness and scope of methods established with simpler molecules. We now add another aspect that we find fascinating, viz. 'natural product anticipation'. In this Review, we survey cases where the synthesis of a compound in the laboratory has preceded its isolation from nature. The focus of our Review lies on examples where this anticipation of a natural product has triggered a successful search or where synthesis and isolation have occurred independently. Finally, we highlight cases where a potential natural product structure has been suggested as a result of synthetic endeavours but not yet confirmed by isolation, inviting further collaborations between synthetic and natural product chemists.

Total Synthesis of (±)-Spiroaxillarone A via a Reversible Sulfa-Michael Addition

A bioinspired strategy is described for the total synthesis of spiroaxillarone A, which exhibited significant antimalarial activity against resistant Plasmodium falciparum (IC₅₀ = 2.32 μM). The key steps include an intermolecular ethanethiol Michael addition, o-quinone Michael addition, and subsequent β-ethanethiol elimination. This synthetic sequence provides a potential biosynthetic pathway of spiroaxillarone A.

Berberine bridge enzyme-like oxidase-catalysed double bond isomerization acts as the pathway switch in cytochalasin synthesis

Cytochalasans (CYTs), as well as their polycyclic (pcCYTs) and polymerized (meCYTs) derivatives, constitute one of the largest families of fungal polyketide-nonribosomal peptide (PK-NRP) hybrid natural products. However, the mechanism of chemical conversion from mono-CYTs (moCYTs) to both pcCYTs and meCYTs remains unknown. Here, we show the first successful example of the reconstitution of the CYT core backbone as well as the whole pathway in a heterologous host. Importantly, we also describe the berberine bridge enzyme (BBE)-like oxidase AspoA, which uses Glu538 as a general acid biocatalyst to catalyse an unusual protonation-driven double bond isomerization reaction and acts as a switch to alter the native (for moCYTs) and nonenzymatic (for pcCYTs and meCYTs) pathways to synthesize aspochalasin family compounds. Our results present an unprecedented function of BBE-like enzymes and highly suggest that the isolated pcCYTs and meCYTs are most likely artificially derived products.

Complex Natural Products Derived from Pyrogallols

Pyrogallols (1,2,3-trihydroxybenzenes) are abundant in Nature, easily oxidized, and are central precursors to important natural products. The rich chemistry of their oxidized derivatives, the hydroxy-o-quinones, has been studied for over a century and still attracts the interest of the scientific community. Only in the last ten years have critical insights of pyrogallol chemistry from the mid-twentieth century been applied to modern natural product synthesis. Historical studies of pyrogallol chemistry, including [5+2], [4+2], and formal [5+5] cycloadditions are discussed here and reactivity guidelines established. The application and remarkable selectivity of these cycloadditions is then showcased in the recent syntheses of several fungal natural products, including dibefurin, epicolactone, the merocytochalasans, and preuisolactone A. The authors hope that this contribution will spark further interest in the fascinating chemistry of pyrogallols and natural products derived from them.

Asperflavipines C–E and aspermichalasine A: three cytochalasan heterotetramers and an unusual cytochalasan monomer from Aspergillus micronesiensis

Asperflavipines C–E, three new cytochalasin heterotetramers, possessing a highly complex tetradecacyclic ring system with continuous bridged ring systems, and aspermichalasine A, possessing an unusual 5/6/5/8 ring system, were isolated.

Bioinspired Network Analysis Enabled Divergent Syntheses and Structure Revision of Pentacyclic Cytochalasans

We accomplished the divergent total syntheses of ten pentacyclic cytochalasans (aspergillin PZ, trichodermone, trichoderones, flavipesines, and flavichalasines) from a common precursor aspochalasin D and revised the structures of trichoderone B, spicochalasin A, flavichalasine C, aspergilluchalasin based on structure network analysis of the cytochalasans biosynthetic pathways and DFT calculations. The key steps of the syntheses include transannular alkene/epoxyalkene and carbonyl-ene cyclizations to establish the C/D ring of pentacyclic aspochalasans. Our bioinspired approach to these pentacyclic cytochalasans validate the proposed biosynthetic speculation from a chemical view and provide a platform for the synthesis of more than 400 valuable cytochalasans bearing different macrocycles and amino-acid residues.

Total syntheses of melodienones by redox isomerization of propargylic alcohols

A remarkable base-promoted methodology for the rapid construction of the (E)- and (Z)-γ-oxo-α,β-alkenoic ester skeletons from readily accessible vinyl propargylic alcohols through modified redox isomerization was uncovered. This approach manifested its high simplicity and efficiency with excellent tolerance of functional substituents, which led to the straightforward structural modifications of various natural products and efficient total syntheses of melodienone, homomelodienone, isomelodienone, and homoisomelodienone within 4 linear steps.

Bisaspochalasins D and E: Two Heterocycle-Fused Cytochalasan Homodimers from an Endophytic Aspergillus flavipes

Two heterocycle-fused cytochalasan homodimers, bisaspochalasins D (1) and E (2), were isolated from an endophytic Aspergillus flavipes. Their chemical structures were elucidated using a combination of HRESIMS, NMR, theoretical calculations, and crystallographic techniques. Bisaspochalasin D (1) is dimerized by the first reported naturally occurring triple heterobridged 3,8-dioxa-6-azabicyclo[3.2.1]octane framework, while bisaspochalasin E (2) employs a pyrrole ring as the linking moiety. Possible dimerization mechanisms of bisaspochalasins D and E were proposed. The bioassay screening revealed that bisaspochalasin D showed cytotoxic activities against five cancer cell lines (HL-60, SMMC-7721, A-549, MCF-7, and SW-480) with IC₅₀ values ranging from 4.45 to 22.99 μM. Additionally, bisaspochalasin D exhibited neurotrophic activities in a PC12 cell-based assay. At a concentration of 10 μM, bisaspochalasin D can promote neurite growth by inducing a differentiation rate of 12.52% for PC12 cells.