Progress in the Chemistry of Cytochalasans

Structure and Biosynthesis of Perochalasins A-C, Open-Chain Merocytochalasans Produced by the Marine-Derived Fungus Peroneutypa sp. M16

Novel open-chain merocytochalasans, perochalasins A-C (1-3), containing an unusual N-O six-membered heterocyclic moiety, were isolated from cultures of the marine-derived Peroneutypa sp. M16 fungus, along with cytochalasin Z27 (4), cytochalasin Z28 (5), [12]-cytochalasin (6), and phenochalasin B (7). The structures of compounds 1-3 were established by analysis of the spectroscopic data. Full genome sequencing of Peroneutypa sp. M16 enabled the identification of a cytochalasan biosynthetic gene cluster and a proposal for the biosynthetic assembly of perochalasins. The proposal is supported by the nonenzymatic conversion of phenochalasin B (7) into 1-3, based on isotope-labeled hydroxylamine (15NH2OH and ND2OD) feeding studies in vivo and in vitro. In contrast to other merocytochalasans, these are the first cytochalasans confirmed to arise via nucleophilic addition and at a distinct location from the reactive macrocycle olefin, potentially expanding further the range of merocytochalasans to be discovered or engineered. Cytochalasin Z27 (4) exhibited antiplasmodial activities in the low micromolar range against the chloroquine-sensitive Plasmodium falciparum 3D7 strain as well as against resistant strains of the parasite (Dd2, TM90C6B, and 3D7r_MMV848).

Evolution-Based Discovery of Polyketide Acylated Valine from a Cytochalasin-Like Gene Cluster in Simplicillium lamelliciola HDN13430

Utilizing a gene evolution-oriented approach for gene cluster mining, a cryptic cytochalasin-like gene cluster (sla) in Antarctic-derived Simplicillium lamelliciola HDN13430 was identified. Compared with the canonical cytochalasin biosynthetic gene clusters (BGCs), the sla gene cluster lacks the key α,β-hydrolase gene. Heterologous expression of the sla gene cluster led to the discovery of a new compound, slamysin (1), characterized by an N-acylated amino acid structure and demonstrating weak anti-Bacillus cereus activity. These findings underscore the potential of genetic evolution in uncovering novel compounds and indicating specific adaptive evolution within specialized habitats.

Boerelasins A-D, Four Unprecedented Cytochalasins from the Endophytic Fungus Boeremia Exigua

Four undescribed cytochalasins (1-4) were isolated from the endophytic fungus Boeremia exigua. Structurally, boerelasin A (1) represents the first example of a cytochalasin with a rare 5/5 bicyclic carbon core. Boerelasin B (2) possesses an unprecedented 5/6/5/6/8 pentacyclic ring system. Boerelasin C (3), a derivative from the common biosynthetic intermediate to 1, is a macrocyclic ring-opening cytochalasin, and boerelasin D (4) contains an uncommon six-carbon alkyl acid side chain. The structures were elucidated based on spectroscopic methods, electronic circular dichroism, spin-spin coupling constants, and calculated nuclear magnetic resonance with DP4+ analysis. These compounds exhibited significant cytotoxicity against the tumor cells.

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.

Asporychalasin, a bioactive cytochalasan with an unprecedented 6/6/11 skeleton from the Red Sea sediment Aspergillus oryzae

The cytochalasan asporychalasin (1) was obtained from the marine fungus Aspergillus oryzae, isolated from the Red Sea sediments collected off Jeddah, Saudi Arabia. The chemical structure of 1 was elucidated by extensive spectroscopic analysis and quantum-mechanical calculations of ¹³C NMR resonances and ECD to possess an unprecedented 6/6/11-fused tricyclic skeleton, including an isoquinolindione ring in place of the typical isoindolone. Asporychalasin exhibited moderate antiproliferative activity against three human cancer cell lines, lung carcinoma (A549), liver carcinoma (HepG2), and breast carcinoma (MCF7), and no toxicity on zebrafish embryos.

Biomimetic Synthesis of Natural Products: A Journey To Learn, To Mimic, and To Be Better

Total synthesis of natural products has been one of the most exciting and dynamic areas in synthetic organic chemistry. Nowadays, the major challenge in this field is not whether a given target of interest can be synthesized but how to make it with commendable efficiency and practicality. To meet this grand challenge, a wise way is to learn from Mother Nature who is recognized for her superb capability of forging complicated and sometimes beyond-imagination molecules in her own delicate way. Indeed, since Sir Robert Robinson published his groundbreaking synthesis of tropinone in 1917, biomimetic synthesis of natural products, a process of imitating nature's way to make molecules, has evolved into one of the most popular research directions in organic synthesis.Our group has been engaging in biomimetic synthesis of natural products in the past decade. During this time, we have come to realize that the successful implementation of a biomimetic synthesis entails the orchestrated combination of bioinspiration and rational design. On the one hand, we prefer to utilize some elegant bioinspired transformations (e.g., Diels-Alder dimerization, 6π-electrocyclization, and [2 + 2]-photocycloaddition) as the key steps of our synthesis, which enable rapid construction of the core skeletons of the chased targets with high efficiency; on the other hand, various powerful reactions (e.g., dyotropic rearrangement of β-lactone, tandem aldol condensation/Grob fragmentation reaction, and organocatalytic asymmetric Mukaiyama-Michael addition) are rationally designed by us, which allow for facile access to the requisite precursors for attempting biomimetic transformations. In some cases, the proposed biomimetic transformation may fail to give a satisfactory result in practice, and thus we opt to develop creative tactics (e.g., hydrogen atom transfer-triggered vinyl cyclobutane ring opening/oxygen insertion/cyclization cascade) that can meet the challenge. Guided by this synthesis concept, we have achieved the total syntheses of multiple families of natural products of great importance in both chemistry and biology, representatives of which include xanthanolides, cytochalasans, and plakortin-type polyketides. Of note, most of these targets could be accessed in a concise, efficient, and scalable manner, which paves the way for further exploration of their biological functions and medicinal potential. Moreover, owing to their biomimetic nature, our syntheses provide valuable information for deciphering the underlying biosynthetic pathways of the chased targets, which could not be attained by other synthetic modes.

New Cytotoxic Cytochalasans from a Plant-Associated Fungus Chaetomium globosum kz-19

Four new cytochalasans, phychaetoglobins A-D (1-4), together with twelve known cytochalasans (5-16), were isolated from a mangrove-associated fungus Chaetomium globosum kz-19. The new structures were elucidated on the basis of extensive 1D and 2D NMR, HR ESIMS spectroscopic analyses, and electronic circular dichroism (ECD) calculations. The absolute configuration of 2 was established by application of Mosher's method. Compounds 4-8 exhibited moderate cytotoxicities against A549 and HeLa cell lines with the IC50 values less than 20 μM.