Enantioselective divergent syntheses of several polyhalogenated Plocamium monoterpenes and evaluation of their selectivity for solid tumors

Antitumor Activity and Mechanism of Terpenoids in Seaweeds Based on Literature Review and Network Pharmacology

Seaweeds are a treasure trove of natural secondary metabolites. Terpenoids extracted from seaweeds are shown to possess a variety of antitumor cellular activities. However, due to the complex and diverse structures of terpenoids, their therapeutic targets and complex mechanisms of action have not been clarified. The present study summarises the research on terpenoids from seaweeds in oncological diseases over the last 20 years. Terpenoids show different degrees of inhibitory effects on different types of tumor cells, suggesting that terpenoids in seaweeds may have potential antitumor disease potential. Terpenoids with potential antitumor activity and their mechanism of action are investigated using network pharmacology. A total of 125 terpenoids and 286 targets are obtained. Proto-oncogene tyrosine-protein kinase Src(SRC), Signal transducer and activator of transcription 3 (STAT3), Mitogen-activated protein kinase (MAPK3, MAPK1), Heat shock protein HSP 90-alpha (HSP90AA1), Phosphatidylinositol-4,5-bisphosphate 3-kinase catalytic subunit alpha (PIK3CA), and RAC-alpha serine/threonine-protein kinase (AKT1) are defined as core targets. According to GO function and Kyoto encyclopedia of genes and genomes(KEGG) enrichment analysis, terpenoids may affect the Phoshatidylinositol 3'-kinase (PI3K)-Akt signaling pathway, Epidermal growth factor receptor (EGFR) tyrosine kinase inhibitor resistance, Prostate cancer, MAPK signaling pathway, and Proteoglycans in cancer. In addition, the molecular docking results show that the selected terpenoids are all able to bind strongly to the active protein. Terpenoids may slow down the progression of cancer by controlling apoptosis, proliferation, and protein and enzyme binding.

Construction of Enantioenriched Quaternary C-Cl Oxindoles through Palladium-Catalyzed Asymmetric Allylic Substitution with Chloroenolates

A palladium-catalyzed asymmetric chloroenolate allylation with vinyl benzoxazinanones under mild reaction conditions has been developed, affording a series of optically active 3,3-disubstituted oxindoles exhibiting a chloro-group and a linear aryl amino side chain in good yields with up to 96% ee. Versatile functional group tolerance on the benzene ring has been demonstrated, and the utility of this method was probed by a scale-up synthesis and highlighted by product derivatizations.

Anti-Inflammatory Halogenated Monoterpenes from the Red Alga Portieria hornemannii

The chemical investigation of a red alga Portieria hornemannii enabled the identification of three new halogenated monoterpenes (1-3) along with two previously identified metabolites (4 and 5). Their structures were determined by spectroscopic analysis and also by utilizing single-crystal diffraction analysis and quantum chemical calculation, as well as by comparison with literature data. Further corrections for dichloro and dibromo carbons using the sorted training set (STS) method were established in this study to significantly improve the accuracy in GIAO 13C NMR calculation of compounds 1-3. To discover the potential bioactive metabolites from P. hornemannii, the anti-inflammatory activities of all compounds were examined. Compounds 1 and 3-5 showed significant anti-inflammatory activity to inhibit the production of pro-inflammatory cytokines in the LPS-stimulated mature dendritic cells.

An α-chloroaldehyde-based formal synthesis of eribulin

Eribulin (Halaven) is the most structurally complex non-peptidic drug made by total synthesis and has challenged preconceptions of synthetic feasibility in drug discovery and development. However, despite decades of research, the synthesis and manufacture of eribulin remains a daunting task. Here, we report syntheses of the most complex fragment of eribulin (C14-C35) used in two distinct industrial routes to this important anticancer drug. Our convergent strategy relies on a doubly diastereoselective Corey-Chaykovsky reaction to affect the union of two tetrahydrofuran-containing subunits. Notably, this process relies exclusively on enantiomerically enriched α-chloroaldehydes as building blocks for constructing the three densely functionalized oxygen heterocycles found in the C14-C35 fragment and all associated stereocenters. Overall, eribulin can now be produced in a total of 52 steps, which is a significant reduction from that reported in both academic and industrial syntheses.

The Rise of Trichlorides Enabling an Improved Chlorine Technology

Chlorine plays a central role for the industrial production of numerous materials with global relevance. More recently, polychlorides have been evolved from an area of academic interest to a research topic with enormous industrial potential. In this minireview, the value of trichlorides for chlorine storage and chlorination reactions are outlined. Particularly, the inexpensive ionic liquid [NEt₃ Me][Cl₃ ] shows a similar and sometimes even advantageous reactivity compared to chlorine gas, while offering a superior safety profile. Used as a chlorine storage, [NEt₃ Me][Cl₃ ] could help to overcome the current limitations of storing and transporting chlorine in larger quantities. Thus, trichlorides could become a key technique for the flexibilization of the chlorine production enabling an exploitation of renewable, yet fluctuating, electrical energy. As the loaded storage, [NEt₃ Me][Cl₃ ], is a proven chlorination reagent, it could directly be employed for downstream processes, paving the path to a more practical and safer chlorine industry.

Naturally Occurring Organohalogen Compounds-A Comprehensive Review

The present volume is the third in a trilogy that documents naturally occurring organohalogen compounds, bringing the total number-from fewer than 25 in 1968-to approximately 8000 compounds to date. Nearly all of these natural products contain chlorine or bromine, with a few containing iodine and, fewer still, fluorine. Produced by ubiquitous marine (algae, sponges, corals, bryozoa, nudibranchs, fungi, bacteria) and terrestrial organisms (plants, fungi, bacteria, insects, higher animals) and universal abiotic processes (volcanos, forest fires, geothermal events), organohalogens pervade the global ecosystem. Newly identified extraterrestrial sources are also documented. In addition to chemical structures, biological activity, biohalogenation, biodegradation, natural function, and future outlook are presented.

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.

Enantiomerically enriched tetrahydropyridine allyl chlorides

Enantiomerically enriched allyl halides are rare due to their configurational lability. Stable piperidine-based allyl chloride enantiomers can be produced via kinetic resolution, and undergo highly enantiospecific catalyst-free substitutions.

Total Synthesis as a Vehicle for Collaboration

"Collaboration" is not the first word most would associate with the field of total synthesis. In fact, the spirit of total synthesis is all-too-often reputed as being more competitive, rather than collaborative, sometimes even within individual laboratories. However, recent studies in total synthesis have inspired a number of collaborative efforts that strategically blend synthetic methodology, biocatalysis, biosynthesis, computational chemistry, and drug discovery with complex molecule synthesis. This Perspective highlights select recent advances in these areas, including collaborative syntheses of chlorolissoclimide, nigelladine A, artemisinin, ingenol, hippolachnin A, communesin A, and citrinalin B. The legendary Woodward-Eschenmoser collaboration that led to the total synthesis of vitamin B₁₂ is also discussed.

A Novel Polyhalogenated Monoterpene Induces Cell Cycle Arrest and Apoptosis in Breast Cancer Cells

Breast cancer is the most common cancer type and a primary cause of cancer mortality among females worldwide. Here, we analyzed the anticancer efficacy of a novel bromochlorinated monoterpene, PPM1, a synthetic analogue of polyhalogenated monoterpenes from Plocamium red algae and structurally similar non-brominated monoterpenes. PPM1, but not the non-brominated monoterpenes, decreased selectively the viability of several triple-negative as well as triple-positive breast cancer cells with different p53 status without significantly affecting normal breast epithelial cells. PPM1 induced accumulation of triple-negative MDA-MB-231 cells with 4N DNA content characterized by decreased histone H3-S10/T3 phosphorylation indicating cell cycle arrest in the G₂ phase. Western immunoblot analysis revealed that PPM1 treatment triggered an initial rapid activation of Aurora kinases A/B/C and p21^Waf1/Cip1 accumulation, which was followed by accumulation of polyploid >4N cells. Flow cytometric analysis showed mitochondrial potential disruption, caspase 3/7 activation, phosphatidylserine externalization, reduction of the amount polyploid cells, and DNA fragmentation consistent with induction of apoptosis. Cell viability was partially restored by the pan-caspase inhibitor Z-VAD-FMK indicating caspase contribution. In vivo, PPM1 inhibited growth, proliferation, and induced apoptosis in MDA-MB-231 xenografted onto the chick chorioallantoic membrane. Hence, Plocamium polyhalogenated monoterpenes and synthetic analogues deserve further exploration as promising anticancer lead compounds.

Seaweed Secondary Metabolites In Vitro and In Vivo Anticancer Activity

Isolation, finding or discovery of novel anticancer agents is very important for cancer treatment, and seaweeds are one of the largest producers of chemically active metabolites with valuable cytotoxic properties, and therefore can be used as new chemotherapeutic agents or source of inspiration to develop new ones. Identification of the more potent and selective anticancer components isolated from brown, green and red seaweeds, as well as studies of their mode of action is very attractive and constitute a small but relevant progress for pharmacological applications. Several researchers have carried out in vitro and in vivo studies in various cell lines and have disclosed the active metabolites among the terpenoids, including carotenoids, polyphenols and alkaloids that can be found in seaweeds. In this review the type of metabolites and their cytotoxic or antiproliferative effects will be discussed additionally their mode of action, structure-activity relationship and selectivity will also be revealed. The diterpene dictyolactone, the sterol cholest-5-en-3β,7α-diol and the halogenated monoterpene halomon are among the reported compounds, the ones that present sub-micromolar cytotoxicity. Additionally, one dimeric sesquiterpene of the cyclolaurane-type, three bromophenols and one halogenated monoterpene should be emphasized because they exhibit half maximal inhibitory concentration (IC₅₀) values between 1–5 µM against several cell lines.

Synthesis facilitates an understanding of the structural basis for translation inhibition by the lissoclimides

The lissoclimides are unusual succinimide-containing labdane diterpenoids that were reported to be potent cytotoxins. Our short semisynthesis and analogue-oriented synthesis approaches provide a series of lissoclimide natural products and analogues that expand the structure-activity relationships (SARs) in this family. The semisynthesis approach yielded significant quantities of chlorolissoclimide (CL) to permit an evaluation against the National Cancer Institute's 60-cell line panel and allowed us to obtain an X-ray co-crystal structure of the synthetic secondary metabolite with the eukaryotic 80S ribosome. Although it shares a binding site with other imide-based natural product translation inhibitors, CL engages in a particularly interesting and novel face-on halogen-π interaction between the ligand's alkyl chloride and a guanine residue. Our analogue-oriented synthesis provides many more lissoclimide compounds, which were tested against aggressive human cancer cell lines and for protein synthesis inhibitory activity. Finally, computational modelling was used to explain the SARs of certain key compounds and set the stage for the structure-guided design of better translation inhibitors.

Integrating Molecular Networking and Biological Assays To Target the Isolation of a Cytotoxic Cyclic Octapeptide, Samoamide A, from an American Samoan Marine Cyanobacterium

Integrating LC-MS/MS molecular networking and bioassay-guided fractionation enabled the targeted isolation of a new and bioactive cyclic octapeptide, samoamide A (1), from a sample of cf. Symploca sp. collected in American Samoa. The structure of 1 was established by detailed 1D and 2D NMR experiments, HRESIMS data, and chemical degradation/chromatographic (e.g., Marfey's analysis) studies. Pure compound 1 was shown to have in vitro cytotoxic activity against several human cancer cell lines in both traditional cell culture and zone inhibition bioassays. Although there was no particular selectivity between the cell lines tested for samoamide A, the most potent activity was observed against H460 human non-small-cell lung cancer cells (IC50 = 1.1 μM). Molecular modeling studies suggested that one possible mechanism of action for 1 is the inhibition of the enzyme dipeptidyl peptidase (CD26, DPP4) at a reported allosteric binding site, which could lead to many downstream pharmacological effects. However, this interaction was moderate when tested in vitro at up to 10 μM and only resulted in about 16% peptidase inhibition. Combining bioassay screening with the cheminformatics strategy of LC-MS/MS molecular networking as a discovery tool expedited the targeted isolation of a natural product possessing both a novel chemical structure and a desired biological activity.

Cytotoxic halogenated monoterpenes from Plocamium cartilagineum

As a result of our efforts to identify bioactive agents from marine algae, we have isolated and identified one new halogenated monoterpene 1 [(-)-(5E,7Z)-348-trichloro-7-dichloromethyl-3-methyl-157-octatriene] in addition to three known compounds (2, 3 and 4) from the red alga Plocamium cartilagineum collected by hand from the eastern coast of South Africa. Compound 1 was found to be active as a cytotoxic agent in human lung cancer (NCI-H460) and mouse neuro-2a cell lines (IC50 4 μg/mL). Two of these compounds (3 and 4) were found to have cytotoxic activity in other cell line assays, especially against human leukaemia and human colon cancers (IC50 1.3 μg/mL). None of these metabolites were active as sodium channel blockers or activators. All structures were determined by spectroscopic methods (UV, IR, LRMS, HRMS, 1D NMR and 2D NMR). 1D and 2D NOE experiments were carried out on these compounds to confirm the geometry of the double bonds.

Natural product analogues: towards a blueprint for analogue-focused synthesis

A review of approaches to natural product analogues leads to the suggestion of new methods for the generation of biologically active natural product-like scaffolds.

Selective bromochlorination of a homoallylic alcohol for the total synthesis of (-)-anverene

The scope of a recently reported method for the catalytic enantioselective bromochlorination of allylic alcohols is expanded to include a specific homoallylic alcohol. Critical factors for optimization of this reaction are highlighted. The utility of the product bromochloride is demonstrated by the first total synthesis of an antibacterial polyhalogenated monoterpene, (-)-anverene.

Stereoselective Halogenation in Natural Product Synthesis

At last count, nearly 5000 halogenated natural products have been discovered. In approximately half of these compounds, the carbon atom to which the halogen is bound is sp(3) -hybridized; therefore, there are an enormous number of natural products for which stereocontrolled halogenation must be a critical component of any synthesis strategy. In this Review, we critically discuss the methods and strategies used for stereoselective introduction of halogen atoms in the context of natural product synthesis. Using the successes of the past, we also attempt to identify gaps in our synthesis technology that would aid the synthesis of halogenated natural products, as well as existing methods that have not yet seen application in complex molecule synthesis. The chemistry described herein demonstrates yet again how natural products continue to provide the inspiration for critical advances in chemical synthesis.

Marine natural products

This review covers the literature published in 2014 for marine natural products (MNPs), with 1116 citations (753 for the period January to December 2014) 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 (1378 in 456 papers for 2014), together with the relevant biological activities, source organisms and country of origin. Reviews, biosynthetic studies, first syntheses, and syntheses that lead to the revision of structures or stereochemistries, have been included.

Asymmetric chlorination of 4-substituted pyrazolones catalyzed by natural cinchona alkaloid

A natural quinidine catalyzed asymmetric chlorination of 4-substituted pyrazolones was revealed in high yield and excellent enantioselectivity.

Highly Selective Synthesis of Halomon, Plocamenone, and Isoplocamenone

Over 160 chiral vicinal bromochlorinated natural products have been identified; however, a lack of synthetic methods for the selective incorporation of halogens into organic molecules has hindered their synthesis. Here we disclose the first total synthesis and structural confirmation of isoplocamenone and plocamenone, as well as the first selective and scalable synthesis of the preclinical anticancer natural product halomon. The synthesis of these inter-halogenated compounds has been enabled by our recently developed chemo-, regio-, and enantioselective dihalogenation reaction.

Catalytic chemo-, regio-, and enantioselective bromochlorination of allylic alcohols

Herein we describe a highly chemo-, regio-, and enantioselective bromochlorination reaction of allylic alcohols, employing readily available halogen sources and a simple Schiff base as the chiral catalyst. The application of this interhalogenation reaction to a variety of substrates, the rapid enantioselective synthesis of a bromochlorinated natural product, and preliminary extension of this chemistry to dibromination and dichlorination are reported.