A Chlorine-Atom-Controlled Terminal-Epoxide-Initiated Bicyclization Cascade Enables a Synthesis of the Potent Cytotoxins Haterumaimides J and K

Synthesis of Differentially Halogenated Lissoclimide Analogues To Probe Ribosome E-Site Binding

Halogenated natural products from marine sources often demonstrate potent activity against microorganisms and cancer cell lines. During the last three decades, the lissoclimide class of chlorinated labdane diterpenoids has been characterized with respect to structure and cytotoxic activity. Recently, our laboratories have developed different strategies to produce a broad range of naturally occurring lissoclimides and designed synthetic analogues. This work led to the discovery of a novel halogen-π dispersion interaction between the C2 chloride of chlorolissoclimide and guanine residues in the tRNA exit (E) site of the ribosome. In this study, we aimed to synthesize lissoclimide analogues bearing different substituents in place of the chloride to investigate the importance of the halogen identity for binding, translation inhibition, and cytotoxicity. With previous access to the protio and chloro compounds (haterumaimide Q and chlorolissoclimide), we synthesized two more halogenated variants, fluorolissoclimide and bromolissoclimide, as well as a methylated analogue, methyllissoclimide, to complete a panel of chemical probes for functional and structural studies. Using an integrative approach, we explored the effects of these analogues on the eukaryotic translational machinery in vivo and in vitro. X-ray cocrystal structures with the eukaryotic ribosome were solved for each probe molecule, and the effects on ribosomal thermal stability and FRET-derived ribosome binding constants were determined. Together, these data provide a detailed understanding of the different modes of binding of lissoclimides and insight into their relative activities, which vary according to the substitutions that interact with the eukaryote-specific ribosomal protein eL42. Ultimately, we learned that the presence of a lissoclimide C2-halogen atom─offering a potentially stabilizing halogen-π interaction─appears to facilitate or to synergize with a hydrogen-bonding interaction between the C7-hydroxyl group and the backbone of the ribosomal protein eL42, leading to stronger translation inhibition. We therefore conclude that the C2-halogen and C7-hydroxyl groups are critical contributors to potency, and this idea is borne out in the observations of reduced biological activities in the absence of either group.

Harnessing natural inhibitors of protein synthesis for cancer therapy: A comprehensive review

Cancer treatment remains a formidable challenge in modern medicine, necessitating a nuanced understanding of its molecular underpinnings and the identification of novel therapeutic modalities. Among the intricate web of cellular pathways implicated in oncogenesis, protein synthesis has emerged as a fundamental process warranting meticulous investigation. This review elucidates the multifaceted role of protein synthesis pathways in tumor initiation and progression, highlighting the potential of targeting key nodes within these pathways as viable therapeutic strategies. Natural products have long served as a source of bioactive compounds with therapeutic potential owing to their structural diversity and evolutionary honing. Within this framework, we provide a thorough examination of natural inhibitors of protein synthesis as promising candidates for cancer therapy, drawing upon recent advancements and mechanistic insights. By synthesizing current evidence and elucidating key challenges and opportunities, this review aims to galvanize further research into the development of natural product-based anticancer therapeutics, thereby advancing the clinical armamentarium against malignancies.

Enantioselective synthesis of the aglycone of burnettramic acid A

Enantioselective total synthesis of the aglycone of burnettramic acid A, an antifungal pyrrolizidinedione with a terminally mannosylated long acyl chain produced by Aspergillus fungi, has been achieved from a known carboxylic acid by a 14-step sequence. The key steps include 2 types of asymmetric alkylation, coupling of an acetylide intermediate with (S)-epichlorohydrin to provide an acetylenic epoxide in 1 pot, and the Birch reduction to effect desulfonylation, semi-reduction of triple bond, and debenzylation in a concurrent manner. Good agreement of the synthetic aglycone with naturally occurring one in 1H and 13C nuclear magnetic resonance (NMR) spectra, coupled with previously reported unambiguous stereochemical assignment of the sugar moiety, has confirmed the structure of burnettramic acid A.

Biomimetic chlorine-induced polyene cyclizations harnessing hypervalent chloroiodane-HFIP assemblies

While bromo- and iodocyclizations have recently been successfully implemented, the challenging chlorocyclizations have been scantly investigated. We present a selective and generally applicable concept of chlorination-induced polyene cyclization by utilizing HFIP-chloroiodane networks mimicking terpene cyclases. A manifold of different alkenes was converted with excellent selectivities (up to d.r. >95 : 5). The cyclization platform was even extended to several structurally challenging terpenes and terpenoid carbon frameworks.

A Practical and Robust Zwitterionic Cooperative Lewis Acid/Acetate/Benzimidazolium Catalyst for Direct 1,4-Additions

A catalyst type is disclosed allowing for exceptional efficiency in direct 1,4-additions. The catalyst is a zwitterionic entity, in which acetate binds to Cu^II , which is formally negatively charged and serving as counterion for benzimidazolium. All 3 functionalities are involved in the catalytic activation. For maleimides productivity was increased by a factor >300 compared to literature (TONs up to 6700). High stereoselectivity and productivity was attained for a broad range of other Michael acceptors as well. The polyfunctional catalyst is accessible in only 4 steps from N-Ph-benzimidazole with an overall yield of 96 % and robust during catalysis. This allowed to reuse the same catalyst multiple times with nearly constant efficiency. Mechanistic studies, in particular by DFT, give a detailed picture how the catalyst operates. The benzimidazolium unit stabilizes the coordinated enolate nucleophile and prevents that acetate/acetic acid dissociate from the catalyst.

The Recurring Roles of Chlorine in Synthetic and Biological Studies of the Lissoclimides

Halogenated natural products number in the thousands, but only in rare cases are the evolutionary advantages conferred by the halogens understood. We set out to investigate the lissoclimide family of cytotoxins, which includes several chlorinated members, because of our long-standing interest in the synthesis of chlorinated secondary metabolites.Our initial success in this endeavor was a semisynthesis of chlorolissoclimide (CL) from the commercially available sesquiterpenoid sclareolide. Featuring a highly selective and efficient-and plausibly biomimetic-C-H chlorination, we were able to access enough CL for collaborative studies, including X-ray cocrystallography with the eukaryotic ribosome. Through this experiment, we learned that CL's chlorine atom engages in a novel halogen-π dispersion interaction with a neighboring nucleobase in the ribosome E-site.Owing to the limitations of our semisynthesis approach, we established an analogue-oriented approach to access numerous lissoclimide compounds to both improve our understanding of structure-activity relationships and to learn more about the halogen-π interaction. In the course of these studies, we made over a dozen lissoclimide-like compounds, the most interesting of which contained chlorine-bearing carbons with unnatural configurations. Rationalizing the retained potency of these compounds that appeared to be a poor fit for the lissoclimide binding pocket, we came to realize that the chlorine atoms would engage in these same halogen-π interactions even at the expense of a chair to twist-boat conformational change, which also permitted the compounds to fit in the binding site.Finally, because neither of the first two approaches could easily access the most potent natural lissoclimides, we designed a synthesis that took advantage of rarely used terminal epoxides to initiate polyene cyclizations. In this case, the chlorine atom was incorporated early and helped control the stereochemical outcome of the key step.Over the course of this project, three different synthesis approaches were designed and executed, and our ability to access numerous lissoclimides fueled a range of collaborative biological studies. Further, chlorine played impactful roles throughout various aspects of both synthesis and biology. We remain inspired to learn more about the mechanism of action of these compounds and to deeply investigate the potentially valuable halogen-π dispersion interaction in the context of small molecule/nucleic acid binding. In that context, our work offers an instance wherein we might have gained a rudimentary understanding of the evolutionary importance of the halogen in a halogenated natural product.

Stereocontrolled Radical Bicyclizations of Oxygenated Precursors Enable Short Syntheses of Oxidized Abietane Diterpenoids

The power of cation-initiated cyclizations of polyenes for the synthesis of polycyclic terpenoids cannot be overstated. However, a major limitation is the intolerance of many relevant reaction conditions toward the inclusion in the substrate of polar functionality, particularly in unprotected form. Radical polycyclizations are important alternatives to bioinspired cationic variants, in part owing to the range of possible initiation strategies, and in part for the functional group tolerance of radical reactions. In this article, we demonstrate that Co-catalyzed MHAT-initiated radical bicyclizations are not only tolerant of oxidation at virtually every position in the substrate, oftentimes in unprotected form, but these functional groups can also contribute to high levels of stereochemical control in these complexity-generating transformations. Specifically, we show the effects of protected or unprotected hydroxy groups at six different positions and their impact on stereoselectivity. Further, we show how multiply oxidized substrates perform in these reactions, and finally, we document the utility of these reactions in the synthesis of three aromatic abietane diterpenoids.

Marine natural products

This review covers the literature published in 2019 for marine natural products (MNPs), with 719 citations (701 for the period January to December 2019) referring to compounds isolated from marine microorganisms and phytoplankton, green, brown and red algae, sponges, cnidarians, bryozoans, molluscs, tunicates, echinoderms, mangroves and other intertidal plants and microorganisms. The emphasis is on new compounds (1490 in 440 papers for 2019), together with the relevant biological activities, source organisms and country of origin. Pertinent reviews, biosynthetic studies, first syntheses, and syntheses that led to the revision of structures or stereochemistries, have been included. Methods used to study marine fungi and their chemical diversity have also been discussed.

Cobalt-Catalyzed Hydrogen-Atom Transfer Induces Bicyclizations that Tolerate Electron-Rich and Electron-Deficient Intermediate Alkenes

A novel CoII -catalyzed polyene cyclization was developed that is uniquely effective when performed in hexafluoroisopropanol as the solvent. The process is presumably initiated by metal-catalyzed hydrogen-atom transfer (MHAT) to 1,1-disubstituted or monosubstituted alkenes, and the reaction is remarkable for its tolerance of internal alkenes bearing either electron-rich methyl or electron-deficient nitrile substituents. Electron-rich aromatic terminators are required in both cases. Terpenoid scaffolds with different substitution patterns are obtained with excellent diastereoselectivities, and the bioactive C20-oxidized abietane diterpenoid carnosaldehyde was made to showcase the utility of the nitrile-bearing products. Also provided are the results of several mechanistic experiments that suggest the process features an MHAT-induced radical bicyclization with late-stage oxidation to regenerate the aromatic terminator.

Nature-driven approaches to non-natural terpene analogues

The reactions catalysed by terpene synthases belong to the most complex and fascinating cascade-type transformations in Nature.