Synthesis of antiproliferative Cephalotaxus esters and their evaluation against several human hematopoietic and solid tumor cell lines: uncovering differential susceptibilities to multidrug resistance

Anaerobic Hydroxylation of C(sp3)-H Bonds Enabled by the Synergistic Nature of Photoexcited Nitroarenes

A photoexcited-nitroarene-mediated anaerobic C-H hydroxylation of aliphatic systems is reported. The success of this reaction is due to the bifunctional nature of the photoexcited nitroarene, which serves as the C-H bond activator and the oxygen atom source. Compared to previous methods, this approach is cost- and atom-economical due to the commercial availability of the nitroarene, the sole mediator of the reaction. Because of the anaerobic conditions of the transformation, a noteworthy expansion in substrate scope can be obtained compared to prior reports. Mechanistic studies support that the photoexcited nitroarenes engage in successive hydrogen atom transfer and radical recombination events with hydrocarbons, leading to N-arylhydroxylamine ether intermediates. Spontaneous fragmentation of these intermediates leads to the key oxygen atom transfer products.

Efficient synthesis of ethyl 2-(oxazolin-2-yl)alkanoates via ethoxycarbonylketene-induced electrophilic ring expansion of aziridines

Alkyl 2-diazo-3-oxoalkanoates generate alkoxycarbonylketenes, which undergo an electrophilic ring expansion with aziridines to afford alkyl 2-(oxazolin-2-yl)alkanoates in good to excellent yields under microwave heating. The method is a convenient and clean reaction without any activators and catalysts and can be also applied in the synthesis of 2-(oxazolin-2-yl)alkanamides and 1-(oxazolin-2-yl)alkylphosphonates.

Nucleophilic strategies to construct –CF2– linkages using borazine-CF2Ar reagents

Using nucleophilic, boron-based –CF2Ar reagents, we demonstrate three methods to form C–CF bonds: (1) nucleophilic aromatic substitution, (2) palladium catalyzed cross-coupling, and (3) nucleophilic substitution.

An Update on Pharmacological Relevance and Chemical Synthesis of Natural Products and Derivatives with Anti SARS-CoV-2 Activity

Natural products recognized traditionally as a vital source of active constituents in pharmacotherapy. The COVID-19 infection caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is highly transmissible, pathogenic, and considered an ongoing global health emergency. The emergence of COVID-19 globally and the lack of adequate treatment brought attention towards herbal medicines, and scientists across the globe instigated the search for novel drugs from medicinal plants and natural products to tackle this deadly virus. The natural products rich in scaffold diversity and structural complexity are an excellent source for antiviral drug discovery. Recently the investigation of several natural products and their synthetic derivatives resulted in the identification of promising anti SARS-CoV-2 agents. This review article will highlight the pharmacological relevance and chemical synthesis of the recently discovered natural product and their synthetic analogs as SARS-CoV-2 inhibitors. The summarized information will pave the path for the natural product-based drug discovery of safe and potent antiviral agents, particularly against SARS-CoV-2.

Natural STAT3 inhibitors: A mini perspective

Signal transducer and activator of transcription 3 (STAT3) plays pivotal role in several cellular processes such as cell proliferation and survival and has been found to be aberrantly activated in many cancers. STAT3 is largely believed to be one of the key oncogenes and crucial therapeutic targets. Much research has suggested the leading mechanisms for regulating the STAT3 pathway and its role in promoting tumorigenesis. Therefore, intensive efforts have been devoted to develop potent STAT3 inhibitors and several of them are currently undergoing clinical trials. Nevertheless, many natural products were identified as STAT3 inhibitors but attract less attention compared to the small molecule counterpart. In this review, the development of natural STAT3 inhibitors with an emphasis on their biological profile and chemical synthesis are detailed. The current state of STAT3 inhibitors and the future directions and opportunities for STAT3 inhibitor are discussed.

Collective Asymmetric Total Synthesis of C-11 Oxygenated Cephalotaxus Alkaloids

While numerous studies pertaining to the total synthesis of Cephalotaxus alkaloids have been reported, only two strategies have been reported to date for the successful synthesis of the C-11 oxygenated subset, due to the additional synthetic challenge posed by the remote C-11 stereocenter. Herein, we report the collective asymmetric total synthesis of C-11 oxygenated Cephalotaxus alkaloids using a chiral proline both as a starting material and as the only chirality source. A tetracyclic advanced intermediate was synthesized in a highly stereoselective manner from l-proline in 8 steps involving sequential chirality transfer steps such as a diastereoselective N-alkylation, stereospecific Stevens rearrangement and intramolecular Friedel-Crafts reaction via an unusual O-acyloxocarbenium intermediate. From a common intermediate, the asymmetric total synthesis of six C-11 oxygenated Cephalotaxus alkaloids was completed by a series of oxidation state adjustments.

Ethnopharmacology, chemodiversity, and bioactivity of Cephalotaxus medicinal plants

Cephalotaxus is the only genus of Cephalotaxaceae family, and its natural resources are declining due to habitat fragmentation, excessive exploitation and destruction. In many areas of China, folk herbal doctors traditionally use Cephalotaxus plants to treat innominate swollen poison, many of which are cancer. Not only among Han people, but also among minority ethnic groups, Cephalotaxus is used to treat various diseases, e.g., cough, internal bleeding and cancer in Miao medicine, bruises, rheumatism and pain in Yao medicine, and ascariasis, hookworm disease, scrofula in She medicine, etc. Medicinal values of some Cephalotaxus species and compounds are acknowledged officially. However, there is a lack of comprehensive review summarizing the ethnomedicinal knowledge of Cephalotaxus, relevant medicinal phytometabolites and their bioactivities. The research progresses in ethnopharmacology, chemodiversity, and bioactivities of Cephalotaxus medicinal plants are reviewed and commented here. Knowledge gaps are pinpointed and future research directions are suggested. Classic medicinal books, folk medicine books, herbal manuals and ethnomedicinal publications were reviewed for the genus Cephalotaxus (Sanjianshan in Chinese). The relevant data about ethnobotany, phytochemistry, and pharmacology were collected as comprehensively as possible from online databases including Scopus, NCBI PubMed, Bing Scholar, and China National Knowledge Infrastructure (CNKI). "Cephalotaxus", and the respective species name were used as keywords in database search. The obtained articles of the past six decades were collated and analyzed. Four Cephalotaxus species are listed in the official medicinal book in China. They are used as ethnomedicines by many ethnic groups such as Miao, Yao, Dong, She and Han. Inspirations are obtained from traditional applications, and Cephalotaxus phytometabolites are developed into anticancer reagents. Cephalotaxine-type alkaloids, homoerythrina-type alkaloids and homoharringtonine (HHT) are abundant in Cephalotaxus, e.g., C. lanceolata, C. fortunei var. alpina, C. griffithii, and C. hainanensis, etc. New methods of alkaloid analysis and purification are continuously developed and applied. Diterpenoids, sesquiterpenoids, flavonoids, lignans, phenolics, and other components are also identified and isolated in various Cephalotaxus species. Alkaloids such as HHT, terpenoids and other compounds have anticancer activities against multiple types of human cancer. Cephalotaxus extracts and compounds showed anti-inflammatory and antioxidant activities, immunomodulatory activity, antimicrobial activity and nematotoxicity, antihyperglycemic effect, and bone effect, etc. Drug metabolism and pharmacokinetic studies of Cephalotaxus are increasing. We should continue to collect and sort out folk medicinal knowledge of Cephalotaxus and associated organisms, so as to obtain new enlightenment to translate traditional tips into great therapeutic drugs. Transcriptomics, genomics, metabolomics and proteomics studies can contribute massive information for bioactivity and phytochemistry of Cephalotaxus medicinal plants. We should continue to strengthen the application of state-of-the-art technologies in more Cephalotaxus species and for more useful compounds and pharmacological activities.

Turning the Tide: Natural Products and Natural-Product-Inspired Chemicals as Potential Counters to SARS-CoV-2 Infection

The novel and highly pathogenic severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), which causes coronavirus disease 2019 (COVID-19), has become a continued focus of global attention due to the serious threat it poses to public health. There are no specific drugs available to combat SARS-CoV-2 infection. Natural products (carolacton, homoharringtonine, emetine, and cepharanthine) and natural product-inspired small molecules (ivermectin, GS-5734, EIDD-2801, and ebselen) are potential anti-SARS-CoV-2 agents that have attracted significant attention due to their broad-spectrum antiviral activities. Here, we review the research on potential landmark anti-SARS-CoV-2 agents, systematically discussing the importance of natural products and natural-product-inspired small molecules in the research and development of safe and effective antiviral agents.

Can a Ketone Be More Reactive than an Aldehyde? Catalytic Asymmetric Synthesis of Substituted Tetrahydrofurans

O-heterocycles bearing tetrasubstituted stereogenic centers are prepared via catalytic chemo- and enantioselective nucleophilic additions to ketoaldehydes, in which the ketone reacts preferentially over the aldehyde. Five- and six-membered rings with both aromatic and aliphatic substituents, as well as an alkynyl substituent, are obtained. Moreover, 2,2,5-trisubstituted and 2,2,5,5-tetrasubstituted tetrahydrofurans are synthesized with excellent stereoselectivities. Additionally, the synthetic utility of the described method is demonstrated with a three-step synthesis of the side chain of anhydroharringtonine.

Cycloaddition/annulation strategies for the construction of multisubstituted pyrrolidines and their applications in natural product synthesis

Pyrrolidines are privileged substructures of numerous bioactive natural products and drugs.

Cephalotaxus Alkaloids

Cephalotaxus alkaloids represent a family of plant secondary metabolites known for 60 years. Significant activity against leukemia in mice was demonstrated for extracts of Cephalotaxus. Cephalotaxine (CET) (1), the major alkaloid of this series was isolated from Cephalotaxus drupacea species by Paudler in 1963. The subsequent discovery of promising antitumor activity among new Cephalotaxus derivatives reported by Chinese, Japanese, and American teams triggered extensive structure elucidation and biological studies in this family. The structural feature of this cephalotaxane family relies mainly on its tetracyclic alkaloid backbone, which comprises an azaspiranic 1-azaspiro[4.4]nonane unit (rings C and D) and a benzazepine ring system (rings A and B), which is linked by its C3 alcohol function to a chiral oxygenated side chain by a carboxylic function alpha to a tetrasubstituted carbon center. The botanical distribution of these alkaloids is limited to the Cephalotaxus genus (Cephalotaxaceae). The scope of biological activities of the Cephalotaxus alkaloids is mainly centered on the antileukemic activity of homoharringtonine (HHT) (2), which in particular demonstrated marked benefits in the treatment of orphan myeloid leukemia and was approved as soon as 2009 by European Medicine Agency and by US Food and Drug Administration in 2012. Its exact mechanism of action was partly elucidated and it was early recognized that HHT (2) inhibited protein synthesis at the level of the ribosome machinery. Interestingly, after a latency period of two decades, the topic of Cephalotaxus alkaloids reemerged as a prolific source of new natural structures. To date, more than 70 compounds have been identified and characterized. Synthetic studies also regained attention during the past two decades, and numerous methodologies were developed to access the first semisynthetic HHT (2) of high purity suitable for clinical studies, and then high grade enantiomerically pure CET (1), HHT (2), and analogs.

Diastereoselective Synthesis of Cephalotaxus Esters via Asymmetric Mukaiyama Aldol Reaction

We report a protocol for efficient stereoselective installation of the chiral oxygen-containing tetrasubstituted tertiary carbon stereocenter of the side chain of cephalotaxus esters by means of highly diastereoselective Mukaiyama aldol reactions between α-keto esters (2) and a (Z)-α-chloro ketene silyl acetal. This protocol permitted synthesis of cephalotaxus esters including six natural products in good to excellent yields (up to 94%) with high diastereoselectivities (dr up to 97:3) and could be performed on a multigram scale.

New Regio- and Stereoselective Cascades via Unstabilized Azomethine Ylide Cycloadditions for the Synthesis of Highly Substituted Tropane and Indolizidine Frameworks

Multisubstituted tropanes and indolizidines have been prepared with high regio- and stereoselectivity by the [3+2] cycloaddition of unstabilized azomethine ylides generated from readily prepared trimethylsilyl-substituted 1,2-dihydropyridines via protonation or alkylation followed by desilylation. Starting from 1,2-dihydropyridines bearing a ring trimethylsilyl substituent at the 6-position, an intermolecular alkylation/desilylation provides endocyclic unstabilized ylides that successfully undergo cycloaddition with a range of symmetrical and unsymmetrical alkyne and alkene dipolarophiles to afford densely substituted tropanes incorporating quaternary carbons in good yields and with high regio- and stereoselectivity. Additionally, an intramolecular alkylation/desilylation/cycloaddition sequence provides convenient and rapid entry to bridged tricyclic tropane skeletons, allowing for five contiguous carbon stereocenters to be set in a single experimental operation and under mild conditions. Starting from 1,2-dihydropyridines with trimethylsilylmethyl groups on nitrogen, protonation followed by desilylation generates exocyclic unstabilized ylides that undergo cycloaddition with unsymmetrical alkynes to give indolizidines with good regio- and stereoselectivity. N-Trimethylsilylmethyl-1,2-dihydropyridines can also be alkylated and subsequently desilylated to give endocyclic unstabilized ylides that undergo intermolecular cycloadditions with carbonyl compounds to give bicyclic oxazolidine products in good overall yields. Moreover, an intramolecular alkylation/desilylation/cycloaddition sequence with the N-trimethylsilylmethyl-1,2-dihydropyridines affords tricyclic indolizidines that incorporate quaternary carbons and up to five stereocenters with good to excellent regio- and diastereoselectivity.

Anticancer Alkaloids from Trees: Development into Drugs

Trees have made an enormous phytochemical contribution in anticancer drugs' development more than any other life form. The contributions include alkaloids that are biosynthesized in various ways and yield. Lead alkaloids isolated from the trees are taxol and camptothecins that currently have annual sales in billion dollars. Other important alkaloids isolated from these life forms include rohitukine, harringtonine, acronycine, thalicarpine, usambarensine, ellipticine, and matrines. Studies on their mechanism of action and target on the DNA and protein of cancerous cells aided the development of potent hemisynthesized congeners. The molecules and their congeners passed/are passing a long period of historical development before approved as antineoplastic drugs for cancer chemotherapy. Some of them did not find the application as anticancer drugs due to ineffectiveness in clinical trials; others are generating research interest in the antineoplastic activity at the present and have reached clinical trial stages. Potentials in antineoplastic molecules from trees are high and are hoped to be commensurate with cancer types afflicting human society in the future.

Highly α-regioselective neodymium-mediated allylation of diaryl ketones

The first utility of neodymium as a mediating-metal in the Barbier reaction of diaryl ketones with allyl halides is reported in this paper. This one-pot reaction was highly α-regioselective and was carried out under mild conditions.

Synthetic approaches towards alkaloids bearing α-tertiary amines

Alkaloids account for some of the most beautiful and biologically active natural products. Although they are usually classified along biosynthetic criteria, they can also be categorized according to certain structural motifs. Amongst these, the α-tertiary amine (ATA), i.e. a tetrasubstituted carbon atom surrounded by three carbons and one nitrogen, is particularly interesting. A limited number of methods have been described to access this functional group and fewer still are commonly used in synthesis. Herein, we review some approaches to asymmetrically access ATAs and provide an overview of alkaloid total syntheses where those have been employed.

Discovery and resupply of pharmacologically active plant-derived natural products: A review

Medicinal plants have historically proven their value as a source of molecules with therapeutic potential, and nowadays still represent an important pool for the identification of novel drug leads. In the past decades, pharmaceutical industry focused mainly on libraries of synthetic compounds as drug discovery source. They are comparably easy to produce and resupply, and demonstrate good compatibility with established high throughput screening (HTS) platforms. However, at the same time there has been a declining trend in the number of new drugs reaching the market, raising renewed scientific interest in drug discovery from natural sources, despite of its known challenges. In this survey, a brief outline of historical development is provided together with a comprehensive overview of used approaches and recent developments relevant to plant-derived natural product drug discovery. Associated challenges and major strengths of natural product-based drug discovery are critically discussed. A snapshot of the advanced plant-derived natural products that are currently in actively recruiting clinical trials is also presented. Importantly, the transition of a natural compound from a "screening hit" through a "drug lead" to a "marketed drug" is associated with increasingly challenging demands for compound amount, which often cannot be met by re-isolation from the respective plant sources. In this regard, existing alternatives for resupply are also discussed, including different biotechnology approaches and total organic synthesis. While the intrinsic complexity of natural product-based drug discovery necessitates highly integrated interdisciplinary approaches, the reviewed scientific developments, recent technological advances, and research trends clearly indicate that natural products will be among the most important sources of new drugs also in the future.

A Concise and Scalable Formal Synthesis of the Enantiopure Side-Chain Diester of Homoharringtonine

The enantiopure side-chain diester of homoharringtonine (HHT) has been prepared in 38% overall yield, from cheap L-aspartic acid by a linear sequence of 10 chemical operations (mean yield per step: 91%). The experimental operation in each step is convenient, economic and prone to scale up. About 7–10 g of the side-chain diester of HHT is easily prepared within one week from 5 g of L-aspartic acid.

The plant alkaloid and anti-leukemia drug homoharringtonine sensitizes resistant human colorectal carcinoma cells to TRAIL-induced apoptosis via multiple mechanisms

TNF-related apoptosis-inducing ligand (TRAIL) is a pro-apoptotic ligand from the TNF-alpha family that is under consideration, along with agonistic anti-TRAIL receptor antibodies, as a potential anti-tumor agent. However, most primary human tumors are resistant to monotherapy with TRAIL apoptogens, and thus the potential applicability of TRAIL in anti-tumor therapy ultimately depends on its rational combination with drugs targeting these resistances. In our high-throughput screening for novel agents/drugs that could sensitize TRAIL-resistant colorectal cancer cells to TRAIL-induced apoptosis, we found homoharringtonine (HHT), a cephalotaxus alkaloid and tested anti-leukemia drug, to be a very effective, low nanomolar enhancer of TRAIL-mediated apoptosis/growth suppression of these resistant cells. Co-treatment of TRAIL-resistant RKO or HT-29 cells with HHT and TRAIL led to the effective induction of apoptosis and the complete elimination of the treated cells. HHT suppressed the expression of the anti-apoptotic proteins Mcl-1 and cFLIP and enhanced the TRAIL-triggered activation of JNK and p38 kinases. The shRNA-mediated down-regulation of cFLIP or Mcl-1 in HT-29 or RKO cells variably enhanced their TRAIL-induced apoptosis but it did not markedly sensitize them to TRAIL-mediated growth suppression. However, with the notable exception of RKO/sh cFLIP cells, the downregulation of cFLIP or Mcl-1 significantly lowered the effective concentration of HHT in HHT + TRAIL co-treatment. Combined HHT + TRAIL therapy also led to the strong suppression of HT-29 tumors implanted into immunodeficient mice. Thus, HHT represents a very efficient enhancer of TRAIL-induced apoptosis with potential application in TRAIL-based, anti-cancer combination therapy.

Enantiospecific synthesis of the diacid side-chain of deoxyharringtonine and homodeoxyharringtonine

The enantiospecific synthesis of the diacid side-chain of deoxyharringtonine (2) and homodeoxyharringtonine (3) was accomplished from (L)-N-Boc-α-amino alcohol 10 in high yield. A key feature of the synthesis is the construction of the chiral tertiary alcohol by a three-step sequence (i.e., Wittig reaction, Meisenheimer rearrangement, and catalytic hydrogenation).

Unstabilized azomethine ylides for the stereoselective synthesis of substituted piperidines, tropanes, and azabicyclo[3.1.0] systems

Acid treatment of densely substituted 2-silyl-1,2-dihydropyridines provides a new and convenient entry to reactive azomethine ylides that can (1) be protonated and reduced with high stereoselectivity to give piperidines, (2) participate in [3 + 2] dipolar cycloaddition to give tropanes, and (3) undergo a Nazarov-like 6-π electrocyclization that upon reduction give 2-azabicyclo[3.1.0] systems.

Construction of chiral tertiary alcohol stereocenters via the [2,3]-Meisenheimer rearrangement: enantioselective synthesis of the side-chain acids of homoharringtonine and harringtonine

For the first time, the [2,3]-Meisenheimer rearrangement has been developed into a general strategy for the construction of chiral tertiary alcohols. The effectiveness and practicality of this methodology are illustrated by the successful synthesis of (R)-20 and (R)-30, the side chain acids of homoharringtonine and harringtonine, respectively.

Synthesis of novel 5-alkyl/aryl/heteroaryl substituted diethyl 3,4-dihydro-2H-pyrrole-4,4-dicarboxylates by aziridine ring expansion of 2-[(aziridin-1-yl)-1-alkyl/aryl/heteroaryl-methylene]malonic acid diethyl esters

A novel synthetic methodology has been developed for the synthesis of diethyl 5-alkyl/aryl/heteroaryl substituted 3,4-dihydro-2H-pyrrole-4,4-dicarboxylates (also called 2-substituted pyrroline-4,5-dihydro-3,3-dicarboxylic acid diethyl esters) by iodide ion induced ring expansion of 2-[(aziridin-1-yl)-1-alkyl/aryl/heteroaryl-methylene]malonic acid diethyl esters in very good to excellent yields under mild reaction conditions. The electronic and steric impact of the substituents on the kinetics of ring expansion of N-vinyl aziridines to pyrrolines has been studied. Various diversely substituted novel pyrroline derivatives have been synthesized by this methodology and the products can be used as key intermediates in the synthesis of substituted pyrrolines, pyrroles and pyrrolidines.

Cell viability assessment: toward content-rich platforms

IMPORTANCE OF THE FIELD

Monitoring cell viability in vitro is critical in many areas of biomedical research, and the ultimate goal in drug discovery is the ability to predict the in vivo toxicology of drug candidates based on their toxicity profile in vitro. Over the last decade, the contribution of high-throughput screening toward this goal has been tremendous, providing the ability to screen compounds in parallel against multiple cell types. However, the toxic effects of drug candidates uncovered during clinical trials are by far the main reason for their failure. Over the same period, our understanding of programmed cell death has evolved dramatically with the identification of critical control points in the cell death pathways. As a result, cell viability should no longer be characterized solely on the basis of discrete end point measurements such as membrane permeability.

AREAS COVERED IN THIS REVIEW

This review summarizes the traditional viability assays currently commercially available, focusing on methods amenable to high density format. Assays categorized into the following classes are discussed: dye exclusion assays, DNA condensation-based assays and assays monitoring a metabolic function.

WHAT THE READER WILL GAIN

We describe current approaches for assessing cell viability and, using case studies, emphasize their limitations. As an alternative, we propose the use of live, multiplexed readouts to accurately record cell death induction.

TAKE HOME MESSAGE

Current low-content methods based on single parameter readouts are prone to error due to the heterogeneity of cell populations and the multi-faceted nature of cell death. High-content approaches based on continuous, multiplexed readouts are becoming increasingly important for monitoring multiple markers of cell death induction simultaneously on a cell by cell basis. The use of such content-rich platforms is a necessity to predict the toxicology of drug candidates accurately.