The synthesis of cardenolide and bufadienolide aglycones, and related steroids bearing a heterocyclic subunit

Cytochrome P450 Mining for Bufadienolide Diversification

Bufadienolides are a class of steroids with a distinctive α-pyrone ring at C17, mostly produced by toads and consisting of over 100 orthologues. They exhibit potent cardiotonic and antitumor activities and are active ingredients of the traditional Chinese medicine Chansu and Cinobufacini. Direct extraction from toads is costly, and chemical synthesis is difficult, limiting the accessibility of active bufadienolides with diverse modifications and trace content. In this work, based on the transcriptome and genome analyses, using a yeast-based screening platform, we obtained eight cytochrome P450 (CYP) enzymes from toads, which catalyze the hydroxylation of bufalin and resibufogenin at different sites. Moreover, a reported fungal CYP enzyme Sth10 was found functioning in the modification of bufalin and resibufogenin at multiple sites. A total of 15 bufadienolides were produced and structurally identified, of which six were first discovered. All of the compounds were effective in inhibiting the proliferation of tumor cells, especially 19-hydroxy-bufalin (2) and 1β-hydroxy-bufalin (3), which were generated from bufalin hydroxylation catalyzed by CYP46A35. The catalytic efficiency of CYP46A35 was improved about six times and its substrate diversity was expanded to progesterone and testosterone, the common precursors for steroid drugs, achieving their efficient and site-specific hydroxylation. These findings elucidate the key modification process in the synthesis of bufadienolides by toads and provide an effective way for the synthesis of unavailable bufadienolides with site-specific modification and active potentials.

Synthesis of Bufadienolide Cinobufagin via Late-Stage Singlet Oxygen Oxidation/Rearrangement Approach

This manuscript describes a concise synthesis of cinobufagin, a natural steroid of the bufadienolide family, from readily available dehydroepiandrosterone (DHEA), as well as its α5-epimer derived from 3-epi-andosterone. This synthesis features expedient installation of the 17β-pyrone moiety with the 14β,15β-epoxide and the 16β-acetoxy group using a photochemical regioselective singlet oxygen [4 + 2] cycloaddition followed by CoTPP-promoted in situ endoperoxide rearrangement to provide a 14β,16β-bis-epoxide in 64% yield with a 1.6:1 d.r. This β,β-bis-epoxide intermediate was subsequently subjected to a regioselective scandium(III) trifluoromethanesulfonate catalyzed House-Meinwald rearrangement to establish the 17β-configuration. The synthesis of cinobufagin is achieved in 12 steps (LLS) and 7.6% overall yield, and we demonstrate that it could be used as a platform for the subsequent medicinal chemistry exploration of cinobufagin analogs such as cinobufagin 5α-epimer.

Formal Total Synthesis of Batrachotoxin Enabled by Radical and Weix Coupling Reactions

Batrachotoxin (1), originally isolated from a Columbian poison-dart frog, is a steroidal alkaloid. Its 6/6/6/5-membered carbocycle (ABCD-ring) contains two double bonds, one nitrogen, and five oxygen functionalities. We developed a radical-based convergent strategy and realized the total synthesis of 1 in 28 steps. The AB-ring and D-ring fragments were efficiently synthesized and linked by exploiting a powerful Et3B/O2-mediated radical coupling reaction. Vinyl triflate and vinyl bromide were then utilized for a Pd/Ni-promoted Weix coupling reaction to cyclize the C-ring. A hydroxy group of the C-ring was stereoselectively installed by a decarboxylative hydroxylation reaction to prepare an advanced intermediate of our previous total synthesis of 1.

Total Synthesis of (-)-Batrachotoxin Enabled by a Pd/Ag-Promoted Suzuki-Miyaura Coupling Reaction

Batrachotoxin is an extremely potent cardio- and neurotoxic steroidal alkaloid found in certain species of frogs, birds, and beetles. The steroidal 6/6/6/5-membered carbocycle (ABCD-ring) is U-shaped and functionalized with two double bonds, a six-membered C3-hemiacetal across the AB-ring, a seven-membered oxazepane on the CD-ring, and a dimethylpyrrolecarboxy group at the D-ring carbon chain. These structural features present an unusual and formidable synthetic challenge. Herein we report a total synthesis of batrachotoxin based on a newly devised convergent strategy through a 22-step sequence. Enantiopure AB-ring and D-ring fragments were prepared and subjected to a crucial C(sp2 )-C(sp2 ) coupling reaction. Although both C(sp2 ) centers were sterically encumbered by proximal tetrasubstituted carbon atoms, Ag2 O strongly promoted the Pd(PPh3 )4 -catalyzed Suzuki-Miyaura coupling reaction at room temperature, thereby connecting the two fragments without damaging their preexisting functionalities. Subsequent treatment with t-BuOK induced Dieckmann condensation to cyclize the C-ring. The judiciously optimized functionalizations realized oxazepane formation, carbon chain extension, and pyrrole carboxylic acid condensation to deliver batrachotoxin.

Bufadienolides originated from toad source and their anti-inflammatory activity

Bufadienolide, an essential member of the C-24 steroid family, is characterized by an α-pyrone positioned at C-17. As the predominantly active constituent in traditional Chinese medicine of Chansu, bufadienolide has been prescribed in the treatment of numerous ailments. It is a specifically potent inhibitor of Na+/K+ ATPase with excellent anti-inflammatory activity. However, the severe side effects triggered by unbiased inhibition of the whole-body cells distributed α1-subtype of Na+/K+ ATPase, restrict its future applicability. Thus, researchers have paved the road for the structural alteration of desirable bufadienolide derivatives with minimal adverse effects via biotransformation. In this review, we give priority to the present evidence for structural diversity, MS fragmentation principles, anti-inflammatory efficacy, and structure modification of bufadienolides derived from toads to offer a scientific foundation for future in-depth investigations and views.

Toward the Asymmetric Synthesis of Cardenolides and Related Steroidal Systems: syn-SN2' of Organometallics with C14-C17 Vinylepoxides

In a program aimed at establishing a common sequence of C-C bond-forming reactions for asymmetric construction of tetracyclic triterpenoid natural products and related synthetic systems, effort has been directed toward introducing C17β-substitution by late-stage functionalization of stereodefined "steroidal" D-ring vinylepoxides (spanning C14-C17). It has been found that cyanocuprates participate in syn-SN2' reactions that result in products bearing various C17β-substituents and containing a β-OH at C14.

Chemistry and the Potential Antiviral, Anticancer, and Anti-Inflammatory Activities of Cardiotonic Steroids Derived from Toads

Cardiotonic steroids (CTS) were first documented by ancient Egyptians more than 3000 years ago. Cardiotonic steroids are a group of steroid hormones that circulate in the blood of amphibians and toads and can also be extracted from natural products such as plants, herbs, and marines. It is well known that cardiotonic steroids reveal effects against congestive heart failure and atrial fibrillation; therefore, the term "cardiotonic" has been coined. Cardiotonic steroids are divided into two distinct groups: cardenolides (plant-derived) and bufadienolides (mainly of animal origin). Cardenolides have an unsaturated five-membered lactone ring attached to the steroid nucleus at position 17; bufadienolides have a doubly unsaturated six-membered lactone ring. Cancer is a leading cause of mortality in humans all over the world. In 2040, the global cancer load is expected to be 28.4 million cases, which would be a 47% increase from 2020. Moreover, viruses and inflammations also have a very nebative impact on human health and lead to mortality. In the current review, we focus on the chemistry, antiviral and anti-cancer activities of cardiotonic steroids from the naturally derived (toads) venom to combat these chronic devastating health problems. The databases of different research engines (Google Scholar, PubMed, Science Direct, and Sci-Finder) were screened using different combinations of the following terms: “cardiotonic steroids”, “anti-inflammatory”, “antiviral”, “anticancer”, “toad venom”, “bufadienolides”, and “poison chemical composition”. Various cardiotonic steroids were isolated from diverse toad species and exhibited superior anti-inflammatory, anticancer, and antiviral activities in in vivo and in vitro models such as marinobufagenin, gammabufotalin, resibufogenin, and bufalin. These steroids are especially difficult to identify. However, several compounds and their bioactivities were identified by using different molecular and biotechnological techniques. Biotechnology is a new tool to fully or partially generate upscaled quantities of natural products, which are otherwise only available at trace amounts in organisms.

Recent advances on synthesis and biological activities of C-17 aza-heterocycle derived steroids

Steroids modification for improving their biological activities is one of the most efficient and fruitful methods to develop novel medicines. Steroids with aza-heterocycles attaching to the C-17 owing various biological activities have received great attentions and some of the compounds are developed successfully as drugs. In this review, the research of the syntheses and biological activities of steroids bearing various aza-heterocycles published in the last 8 years is assembled, and some important structure-activity relationships (SARs) of active compounds are presented. According to the analysis of the literatures and our experiences in this field, the potential of aza-heterocyclic steroids as medicinal drugs is proposed.

Multicomponent Approach to the Synthesis of 4-(1H-indol-3-yl)-5-(4-methoxyphenyl)furan-2(5H)-one

A simple one-pot approach was developed for the synthesis of furan-2(5H)-one derivative containing indole fragments. This method includes the telescoped multicomponent reaction of indole, 4-methoxyphenylglyoxal, and Meldrum’s acid. The synthetic utility of the prepared furan-2(5H)-one was demonstrated by condensation with 4-methoxybenzaldehyde. The advantages of this method include the employment of readily accessible starting materials, atom economy, process simplicity, and the easy isolation of the target products. The structure of the synthesized furanones was confirmed by 1H and 13C-NMR spectroscopy and high-resolution mass spectrometry with electrospray ionization (ESI-HRMS).

Enantioselective Synthesis of Spiropyrazolone-Fused Cyclopenta[c]chromen-4-ones Bearing Five Contiguous Stereocenters via (3+2) Cycloaddition

An enantioselective synthesis of spiropyrazolone-fused cyclopenta[c]chromen-4-ones is demonstrated via a (3+2) cycloaddition reaction. The reactions of 3-homoacylcoumarins and α,β-unsaturated pyrazolones in the presence of the cinchona-alkaloid derived hydrogen-bonding catalyst provide aforementioned spiropyrazolone-chromenone adducts bearing five contiguous stereocenters, of which one is the spiro all-carbon quaternary stereocenter in high yields (up to 98%) with good to excellent stereoselectivities (>25:1 dr and up to 99% ee). This one-pot methodology could also be practically demonstrated on a gram-scale with similar efficacy.

Synthesis of Cardiotonic Steroids Oleandrigenin and Rhodexin B

This article describes a concise synthesis of cardiotonic steroids oleandrigenin (7) and its subsequent elaboration into the natural product rhodexin B (2) from the readily available intermediate (8) that could be derived from the commercially available steroids testosterone or DHEA via three-step sequences. These studies feature an expedient installation of the β16-oxidation based on β14-hydroxyl-directed epoxidation and subsequent epoxide rearrangement. The following singlet oxygen oxidation of the C17 furan moiety provides access to oleandrigenin (7) in 12 steps (LLS) and a 3.1% overall yield from 8. The synthetic oleandrigenin (7) was successfully glycosylated with l-rhamnopyranoside-based donor 28 using a Pd(II)-catalyst, and the subsequent deprotection under acidic conditions provided cytotoxic natural product rhodexin B (2) in a 66% yield (two steps).

Construction of cyclopentane-fused coumarins via DBU-catalyzed [3+2] cycloaddition of 3-homoacyl coumarins with cyclic 1-azadienes

The metal-free DBU catalyzed [3+2] cycloaddition of 3-homoacyl coumarins with cyclic 1-azadienes proceeded smoothly to furnish the corresponding highly functionalized cyclopentane-fused coumarins with excellent diastereoselectivity and complete chemoselectivity and in good yields under mild conditions.

A metal-free DBU catalyzed [3+2] cycloaddition of 3-homoacyl coumarins with cyclic 1-azadienes has been developed for the synthesis of cyclopentane-fused coumarins with excellent diastereoselectivity and complete chemoselectivity.

Asymmetric Total Synthesis of Dankasterones A and B and Periconiastone A Through Radical Cyclization

We describe herein the assembly of the cis-decalin framework through radical cyclization initiated by metal-catalyzed hydrogen atom transfer (MHAT), further applied it in the asymmetric synthesis of dankasterones A and B and periconiastone A. Position-selective C-H oxygenation allowed for installation of the necessary functionality. A radical rearrangement was adopted to create 13(14→8)abeo-8-ergostane skeleton. Interconversion of dankasterone B and periconiastone A was realized through biomimetic intramolecular aldol and retro-aldol reactions. The MHAT-based approach, serves as a new dissection means, is complementary to the conventional ways to establish cis-decalin framework.

Total Synthesis of the Cardiotonic Steroid (+)-Cannogenol

The total synthesis of (+)-cannogenol, an aglycon common to various biologically important cardiotonic glycosides, has been achieved. Synthesis of the versatile intermediate involves Mizoroki-Heck and intramolecular Diels-Alder reactions from the enantiomerically pure CD-ring segment, newly prepared in a multidecagram scale this time. Total synthesis by the site-selective transformations of the versatile intermediate demonstrated the applicability of our synthetic approach.

Unified Total Synthesis of Five Bufadienolides

We report a unified total synthesis of five bufadienolides: bufalin (1), bufogenin B (2), bufotalin (3), vulgarobufotoxin (4), and 3-(N-succinyl argininyl) bufotalin (5). After the steroidal ABCD ring 8 was produced, the D ring was cross-coupled with a 2-pyrone moiety and stereoselectively epoxidized to generate 6. TMSOTf promoted a stereospecific 1,2-hydride shift from 6 to establish the β-oriented 2-pyrone of 19. Functional group manipulations from 19 furnished 1-5, which potently inhibited cancer cell growth.

Molecular mechanisms of bufadienolides and their novel strategies for cancer treatment

Bufadienolides are cardioactive C24 steroids with an α-pyrone ring at position C17. In the last ten years, accumulating studies have revealed the anticancer activities of bufadienolides and their underlying mechanisms, such as induction of autophagy and apoptosis, cell cycle disruption, inhibition of angiogenesis, epithelial-mesenchymal transition (EMT) and stemness, and multidrug resistance reversal. As Na⁺/K⁺-ATPase inhibitors, bufadienolides have inevitable cardiotoxicity. Short half-lives, poor stability, low plasma concentration and oral bioavailability in vivo are obstacles for their applications as drugs. To improve the drug potency of bufadienolides and reduce their side effects, prodrug strategies and drug delivery systems such as liposomes and nanoparticles have been applied. Therefore, systematic and recapitulated information about the antitumor activity of bufadienolides, with special emphasis on the molecular or cellular mechanisms, prodrug strategies and drug delivery systems, is of high interest. Here, we systematically review the anticancer effects of bufadienolides and the molecular or cellular mechanisms of action. Research advancements regarding bufadienolide prodrugs and their tumor-targeting delivery strategies are critically summarized. This work highlights recent scientific advances regarding bufadienolides as effective anticancer agents from 2011 to 2019, which will help researchers to understand the molecular pathways involving bufadienolides, resulting in a selective and safe new lead compound or therapeutic strategy with improved therapeutic applications of bufadienolides for cancer therapy.

Implications for glycosylated compounds and their anti-cancer effects

Glycosylated compounds are major secondary metabolites of plants, which have various therapeutic effects on human diseases, by acting as anti-cancer, antioxidant, and anti-inflammatory agents. Glycosylation increases stability, bioactivity, and solubility of compounds and improves their pharmacological properties. Two well-known examples of glycosylated compounds include cardiac and flavonoid, the anti-tumor activities of which have been emphasized by several studies. However, little is known about their role in the treatment or prevention of cancer. In this review, recent studies on anti-tumor properties of cardiac and flavonoid glycosides, and their mechanisms of action, have been investigated. More specifically, this review is aimed at focusing on the multifactorial properties of cardiac and flavonoid compounds as well as their correlation with signaling pathways in the treatment of cancer.

Synthesis and evaluation of Na+/K+-ATP-ase inhibiting and cytotoxic in vitro activities of oleandrigenin and its selected 17β-(butenolidyl)- and 17β-(3-furyl)- analogues

Natural cardiac-active principles built upon the 14,16β-dihydroxy-5β,14β-androstane core and bearing a heterocyclic substituent at 17β, in particular, a cardenolide - oleandrin and a bufadienolide - bufotalin, are receiving a great deal of attention as potential anticancer drugs. The densely substituted and sterically shielded ring D is the particular structural feature of these compounds. The first synthesis of oleandrigenin from easily available steroid starting material is reported here. Furthermore, selected 17β-(4-butenolidyl)- and 17β-(3-furyl)-14,16β-dihydroxy-androstane derivatives were en route synthesized and examined for their Na⁺/K⁺-ATP-ase inhibitory properties as well as cytotoxic activities in normal and cancer cell lines. It was found that the furyl-analogue of oleandrigenin/bufatalin (7) and some related 17-(3-furyl)- derivatives (19, 21) show remarkably high Na⁺/K⁺-ATP-ase inhibitory activity as well as significant cytotoxicity in vitro. In addition, oleandrigenin 2 compared to derivatives 21 and 25 induced strong apoptosis in human cervical carcinoma HeLa cells after 24 h of treatment.

Total synthesis, chemical modification and structure-activity relationship of bufadienolides

Bufadienolides are a type of natural cardiac steroids and originally isolated from the Traditional Chinese Medicine Chan'Su, they have been used for the treatment of heart disease in traditional remedies as well as in modern medicinal therapy with potent anti-tumor activities. Due to their unique molecular structures with unsaturated six-membered lactones attached to the steroid core, bufadienolides have received great attention in the synthetic organic community. This review presents total synthetic efforts to some representative bufadienolides, chemical modification of bufadienolides will also be given to discuss their structure-activity relationship in anti-tumor.

Towards the synthesis of calotropin and related cardenolides from 3-epiandrosterone: A-ring related modifications

Calotropin and related cardiac glycosides isolated from plants such as calotropis gigantea represent an interesting target for biological investigations and are based on a cardiac steroid that is doubly connected to a sugar moiety.

Recent progress in the synthesis of limonoids and limonoid-like natural products

Recent progress in syntheses of limonoids and limonoid-like natural products is reviewed. The current “state-of-art” advance on novel synthetic strategy are summarized and future outlook will be presented.

Synthesis of Oxy-Functionalized Steroidal Skeletons via Mizoroki-Heck and Intramolecular Diels-Alder Reactions

Estrogenic and cardiotonic steroidal skeletons were concisely constructed via Mizoroki-Heck and intramolecular Diels-Alder (IMDA) reactions. Simple modification of the dienophile unsaturation of the IMDA precursor enabled representative AB-ring systems of both steroid classes to be accessed from the same intermediate. The diastereoselectivity of the IMDA reaction used to access the cardiotonic steroidal skeleton was found to be significantly enhanced by performing the reaction in water.

Regioselective single pot C3-glycosylation of strophanthidol using methylboronic acid as a transient protecting group

This manuscript describes a single pot protocol for the selective introduction of unprotected sugars to the C3 position of the cardiotonic steroid strophanthidol. These reactions proceed with high levels of regiocontrol (>20:1 rr) in the presence of three other hydroxyl functionalities including the C19 primary hydroxyl group and could be applied to different sugars to provide the deprotected cardiac glycosides upon work up (5 examples, 77-69% yield per single operation). The selective glycosylation of the less reactive C3 position is accomplished by the use of traceless protection with methylboronic acid that blocks the C5 and C19 hydroxyls by forming a cyclic boronic ester, followed by in situ glycosylation and a work up with ammonia in methanol to remove the boronic ester and the carbohydrate ester protecting groups.

Modular Total Synthesis and Cell-Based Anticancer Activity Evaluation of Ouabagenin and Other Cardiotonic Steroids with Varying Degrees of Oxygenation

A Cu(II)-catalyzed diastereoselective Michael/aldol cascade approach is used to accomplish concise total syntheses of cardiotonic steroids with varying degrees of oxygenation including cardenolides ouabagenin, sarmentologenin, 19-hydroxysarmentogenin, and 5- epi-panogenin. These syntheses enabled the subsequent structure activity relationship (SAR) studies on 37 synthetic and natural steroids to elucidate the effect of oxygenation, stereochemistry, C3-glycosylation, and C17-heterocyclic ring. Based on this parallel evaluation of synthetic and natural steroids and their derivatives, glycosylated steroids cannogenol-l-α-rhamnoside (79a), strophanthidol-l-α-rhamnoside (92), and digitoxigenin-l-α-rhamnoside (97) were identified as the most potent steroids demonstrating broad anticancer activity at 10-100 nM concentrations and selectivity (nontoxic at 3 μM against NIH-3T3, MEF, and developing fish embryos). Further analyses indicate that these molecules show a general mode of anticancer activity involving DNA-damage upregulation that subsequently induces apoptosis.

Asymmetric synthesis of the tetracyclic core of bufogargarizin C by an intramolecular [5 + 2] cycloaddition

A concise asymmetric synthesis of the synthetically challenging 7/5/6/5-tetracyclic core of bufogargarizin C by a unique intramolecular [5 + 2] cycloaddition was reported.

Discovery and characterization of a prevalent human gut bacterial enzyme sufficient for the inactivation of a family of plant toxins

Although the human gut microbiome plays a prominent role in xenobiotic transformation, most of the genes and enzymes responsible for this metabolism are unknown. Recently, we linked the two-gene 'cardiac glycoside reductase' (cgr) operon encoded by the gut Actinobacterium Eggerthella lenta to inactivation of the cardiac medication and plant natural product digoxin. Here, we compared the genomes of 25 E. lenta strains and close relatives, revealing an expanded 8-gene cgr-associated gene cluster present in all digoxin metabolizers and absent in non-metabolizers. Using heterologous expression and in vitro biochemical characterization, we discovered that a single flavin- and [4Fe-4S] cluster-dependent reductase, Cgr2, is sufficient for digoxin inactivation. Unexpectedly, Cgr2 displayed strict specificity for digoxin and other cardenolides. Quantification of cgr2 in gut microbiomes revealed that this gene is widespread and conserved in the human population. Together, these results demonstrate that human-associated gut bacteria maintain specialized enzymes that protect against ingested plant toxins.

A new transformation of coumarins via direct C–H bond activation utilizing an iron–organic framework as a recyclable catalyst

A new mixed-linker iron-based MOF VNU-20 [Fe₃(BTC)(NDC)₂·6.65H₂O] was solvothermally synthesized, and utilized as catalyst for the coupling transformation of coumarins with N,N-dimethylanilines.

Enantioselective Total Synthesis of Cannogenol-3-O-α-l-rhamnoside via Sequential Cu(II)-Catalyzed Michael Addition/Intramolecular Aldol Cyclization Reactions

A concise and scalable enantioselective total synthesis of the natural cardenolides cannogenol and cannogenol-3-O-α-l-rhamnoside has been achieved in 18 linear steps. The synthesis features a Cu(II)-catalyzed enantioselective and diastereoselective Michael reaction/tandem aldol cyclization and a one-pot reduction/transposition, which resulted in a rapid (6 linear steps) assembly of a functionalized intermediate containing C19 oxygenation that could be elaborated to cardenolide cannogenol. In addition, a strategy for achieving regio- and stereoselective glycosylation at the C3 position of synthetic cannogenol was developed and applied to the preparation of cannogenol-3-O-α-l-rhamnoside.

Synthesis of the Tetracyclic Structure of Batrachotoxin Enabled by Bridgehead Radical Coupling and Pd/Ni-Promoted Ullmann Reaction

The steroidal ABCD-ring system of the potent neurotoxin batrachotoxin was efficiently assembled in a convergent fashion. Bridgehead radical coupling between the simple AB-ring and D-ring fragments (3 and 4) formed the sterically congested linkage at the C9-oxygen-attached tetrasubstituted carbon. The C-ring was then cyclized by the Pd/Ni-promoted Ullmann reaction of the vinyl triflate and vinyl bromide of 19, giving rise to tetracyclic structure 1.

De Novo Synthesis of Possible Candidates for the Inagami-Tamura Endogenous Digitalis-like Factor

De novo synthesis of possible candidates for the Inagami-Tamura endogenous digitalis-like factor (EDLF) was achieved to validate a previously proposed structure. Our synthetic approach involves a highly regio- and diastereoselective Mizoroki-Heck reaction and a Friedel-Crafts-type cyclodehydration to construct steroidal tetracycle 14 as a versatile common intermediate leading to seven 2,14β-dihydroxyestradiol analogues 1a-c, 2a-c, and 3 as possible candidates. By comparing the potency of inhibitory activity against Na⁺/K⁺-ATPase between the synthesized candidates and the EDLF, it was found that the proposed structure is not likely to be a true structure of the Inagami-Tamura EDLF.