Total synthesis of (+)-11,11'-dideoxyverticillin A

Studies on the epipolythiodioxopiperazine alkaloid verticillin D: Scaled production, streamlined purification, and absolute configuration

Verticillins, epipolythiodioxopiperazine alkaloids that were first described over 50 years ago, have undergone extensive cytotoxic and pharmacological evaluations over the last decade. However, of the 27 verticillin analogues in the literature, the chemistry of verticillin D, which has two additional secondary hydroxy moieties, relative to verticillin A, has remained largely unexplored since its discovery in 1999. With the goal of advancing our understanding of verticillin D, there were three main objectives with this study: improving production, streamlining purification, and assigning absolute configuration via X-ray crystallography. To begin, the production of verticillin D was analyzed across seven fungal strains, and the top producer was further assessed under two fermentation conditions. Clonostachys rosea (strain MSX51257) biosynthesized the highest amount of verticillin D, with production peaking between 15 and 25 days on rice media. Interestingly, in contrast to similar studies that yield verticillin A, the biosynthesis of verticillin D was not accompanied by a suite of structurally related verticillin analogues. As such, the purification of verticillin D was more rapid and could be accomplished without the use of HPLC. These materials were used, in part, to determine the absolute configuration of verticillin D via X-ray crystallography, allowing for assignment of the asymmetric centers at both the 13 and 13' positions as R, which has never been accomplished. This is only the third report of an X-ray structure of a verticillin analogue.

Synthetic Approaches to Dimeric and Oligomeric Hexahydropyrrolo[2,3-b]indole Alkaloids

Natural product synthesis has been the prime focus for the development of new chemical transformations and the drug discovery. The dimeric and oligomeric hexahydropyrrolo[2,3-b]indole alkaloids represent a architecturally intriguing class of cyclotryptamine alkaloids. These alkaloids share contiguous stereogenic centers with vicinal all-carbon quaternary stereogenic centers. Members of this class of alkaloids are reported to show interesting biological activities. Therefore, there are tremendous synthetic interests not only because of their striking architecture but also due to their important biological activities. In this feature article we have discussed the detailed biogenetic hypothesis of these alkaloids and various elegant strategies from different groups, including our contributions in this area. Special emphasis is given for the synthetic approaches to dimeric, trimeric and tetrameric hexahydropyrrolo[2,3-b]indole alkaloids.

A versatile entry to unnatural, disulfide-linked amino acids and peptides through the disulfuration of azlactones

Despite the evident demand and promising potential of disulfide-functionalized amino acids and peptides in linker chemistry and peptide drug discovery, those disulfurated specifically at the α-position constitute a unique yet rather highly underexplored chemical space. In this study, we have developed a method for preparing SS-linked amino acid/peptide derivatives through a base-catalyzed disulfuration reaction of azlactones, followed by the ring-opening functionalization. The disulfuration reaction proceeds under mild conditions, yielding disulfurated azlactones in excellent yields across a variety of N-dithiophthalimides and diverse azlactones derived from various amino acids and peptides. Leveraging the ready availability of N-dithiophthalimides from several bilateral disulfurating reagents, this method allows for the modular integration of functional molecules and azlactones into SS-linkage in two-step operations. Furthermore, due to the transformability of the azlactone moiety through ring-opening with various nucleophiles, our method provides a wide variety of functional molecule-tagged amino acids and oligopeptides bearing SS-linkages in a modular and time-efficient manner, serving as a valuable tool for linker chemistry and peptide chemistry.

Concise Synthesis of Cyctetryptomycin A and B Enabled by Zr-Catalyzed Dimerization

A concise synthetic strategy utilizing a Zr catalyst for the construction of cyctetryptomycin A and B is reported. Cyctetryptomycin A and B are recently isolated, complex tetrameric natural products for which total synthesis has not been previously reported. This study presents a practical approach for the construction of two consecutive quaternary carbon centers with a Zr catalyst. Furthermore, the first total synthesis of cyctetryptomycin A and B was achieved by this Zr-catalyzed radical coupling. The radical dimerization reaction mediated by the Zr catalyst required 1,2-bis(diphenylphosphino)ethane (dppe) as an indispensable additive. Through both experimental and theoretical investigations into the mechanism of this Zr-catalyzed reaction, the specific role of dppe was elucidated. In addition, the synthetic approach was extended to enable the practical synthesis of other dimeric natural products, including tetratryptomycin A, dibrevianamide F, and ditryptophenaline. Finally, the synthetic mechanism of cyctetryptomycin A and B, through the oxidative macrocyclization of tetratryptomycin A by CttpC, was newly elucidated by both experimental and docking simulations.

Collective Asymmetric Synthesis of Thiosilvatins

Thiosilvatins are a family of biologically active sulfenylated diketopiperazine natural products. The first members were reported over 40 years ago, but total synthesis of a thiosilvatin has remained elusive. Here, we describe the first, collective, synthesis of the parent epidithiodiketopiperazine (-)-dithiosilvatin and ten related thiosilvatins. Several of the targets are structurally revised. A catalytic asymmetric sulfenylation of triketopiperazines efficiently controls absolute configuration at the thioaminal units. Further synthetic highlights include a diastereoconvergent installation of the requisite cis-orientation of the sulfur atoms and a tandem epidisulfide formation/O-prenylation under mild Mitsunobu conditions. The described methods for late-stage diversification of sensitive bis(methylthio)diketopiperazines offer a blueprint for systematically exploring this interesting 3D-pharmacophore in stereochemically pure form.

N-Halosuccinimide enables cascade oxidative trifluorination and halogenative cyclization of tryptamine-derived isocyanides

Both the pyrroloindoline core and N-CF3 moiety hold significant importance in medicinal chemistry. However, to date, no instances of constructing N-CF3-containing pyrroloindolines have been reported. Herein, we present a robust and operationally simple approach to assembling such intriguing skeletons from tryptamine-derived isocyanides through a cascade sequence, which includes an oxidative trifluorination and a subsequent halogenative cyclization. Key to the success lies in the development of a facile conversion of isocyanides to N-CF3 moiety with commercially available reagents N-halosuccinimide and Et3N·HF. The protocol features mild reaction conditions, broad functional group tolerance, good to excellent yields, and high diastereoselectivities. In addition, we demonstrate that the halide substituent within the products serves as a versatile functional handle for accessing diverse C3-quaternary-substituted N-CF3-pyrroloindolines.

Total Synthesis of (±)-3-Thiaglutamate

The total synthesis of (±)-3-thiaglutamate is reported. Central to our strategy is an thiol addition to an imine to form the thioaminal of the natural product. The resulting thioaminal product is then subjected to triflic acid global deprotection to produce 3-thiaglutamate as a triflate salt. This work constitutes the first total synthesis of 3-thiaglutamate and demonstrates that the hemithioaminal group in 3-thiaglutamate can be stabilized under acidic conditions.

Verticillins: fungal epipolythiodioxopiperazine alkaloids with chemotherapeutic potential

Covering: 1970 through June of 2023Verticillins are epipolythiodioxopiperazine (ETP) alkaloids, many of which possess potent, nanomolar-level cytotoxicity against a variety of cancer cell lines. Over the last decade, their in vivo activity and mode of action have been explored in detail. Notably, recent studies have indicated that these compounds may be selective inhibitors of histone methyltransferases (HMTases) that alter the epigenome and modify targets that play a crucial role in apoptosis, altering immune cell recognition, and generating reactive oxygen species. Verticillin A (1) was the first of 27 analogues reported from fungal cultures since 1970. Subsequent genome sequencing identified the biosynthetic gene cluster responsible for producing verticillins, allowing a putative pathway to be proposed. Further, molecular sequencing played a pivotal role in clarifying the taxonomic characterization of verticillin-producing fungi, suggesting that most producing strains belong to the genus Clonostachys (i.e., Bionectria), Bionectriaceae. Recent studies have explored the total synthesis of these molecules and the generation of analogues via both semisynthetic and precursor-directed biosynthetic approaches. In addition, nanoparticles have been used to deliver these molecules, which, like many natural products, possess challenging solubility profiles. This review summarizes over 50 years of chemical and biological research on this class of fungal metabolites and offers insights and suggestions on future opportunities to push these compounds into pre-clinical and clinical development.

Total Synthesis and Stereochemical Assignment of Enteropeptin A

The total synthesis and structural elucidation of the antimicrobial sactipeptide enteropeptin A is reported. Enteropeptin A contains a thioaminoketal group with an unassigned stereochemical configuration that is embedded in a highly unusual thiomorpholine ring. In this synthesis, a linear peptide containing a dehydroamino acid and a pendant cysteine residue is subjected to Markovnikov hydrothiolation by a dithiophosphoric acid catalyst. This cyclization reaction forms the central thiomorpholine ring found in the enteropeptins. Both diastereomers at the unassigned thioaminoketal stereocenter of enteropeptin A were prepared, and their comparison to an authentic standard allowed for the unambiguous stereochemical assignment of the natural product to be of the D configuration. This inaugural total synthesis of enteropeptin A represents the first total synthesis of a sactipeptide reported to date. Moreover, the strategy disclosed herein serves as a general platform for the synthesis of stereochemically defined thiomorpholine-containing peptides, which may enable the discovery of new cyclic peptide antibiotics.

Regio- and Diastereoselective Radical Dimerization Reactions for the Construction of Benzo[f]isoindole Dimers

This study presents a novel approach for synthesizing benzo[f]isoindole dimers, which involves cascade cyclization and oxidative radical dimerization. Our method allows for the formation of up to five carbon-carbon bonds in a single reaction, exhibiting remarkable diastereoselectivity and regioselectivity. The mechanism and regioselectivity were investigated through a combination of experiments and calculations.

Rapid Synthesis of Chiral Figure-Eight Macrocycles Using a Preorganized Natural Product-Based Scaffold

Chiral D2 -symmetric figure-eight shaped macrocycles are promising scaffolds for amplifying the chiroptical properties of π-conjugated systems. By harnessing the inherent and adaptable conformational dynamics of a chiral C2 -symmetric bispyrrolidinoindoline (BPI) manifold, we developed an enantio-divergent modular synthetic platform to rapidly generate a diverse range of chiral macrocycles, spanning from 14- to 66-membered rings, eliminating the need for optical resolution. Notably, a 32-membered figure-eight macrocycle showed excellent circularly polarized luminescence (CPL: |glum |=1.1×10-2 ) complemented by a robust emission quantum yield (Φfl =0.74), to achieve outstanding CPL brightness (BCPL : ϵ×Φfl ×|glum |/2=480). Using quadruple Sonogashira couplings, this versatile synthetic platform enables precise adjustments of the angle, distance, and length among intersecting π-conjugated chromophores. Our synthetic strategy offers a streamlined and systematic approach to significantly enhance BCPL values for a variety of chiral D2 -symmetric figure-eight macrocycles.

Visible-Light-Mediated Synthesis of N-Acyl-N,O-hemiacetals from Terminal Alkynes: Access to N,N-, N,S-, and N,O-Acetals

A mild electron donor-acceptor complex-mediated approach for the synthesis of N-acyl-N,O-hemiacetals has been reported. The key feature of this protocol is that it allows for direct access to electrophilic N-acylimines at room temperature without prefunctionalization of the hydroxyl group. The in situ generated N-acylimine can react with different nucleophiles, viz., alcohols, thiols, and nitriles, to afford a diverse range of scaffolds such as N,O-, N,S-, and N,N-acetals.

Electrochemical bromocyclization enables 3,5-diversification of heterocyclic indolines

Electrophilic bromocyclization reactions are widely used as key steps in the synthesis of diverse functionalized tetrahydrofuroindolines and hexahydropyrroloindolines. However, the direct dibromination variants of these reactions for the synthesis of 3,5-dibromoindolines remain undeveloped. Here, we report a protonic-acid-promoted electrooxidative protocol for the dearomative C3,C5-dibromocyclizations of tryptophol and tryptamine derivatives. This electrosynthetic approach, which enables direct selective construction of heterocyclic 3a,5a-dibromoindolines with inexpensive, non-hazardous NaBr as both the electrolyte and Br source, provides a convenient, practical method for the late-stage 3,5-diversification of heterocyclic indolines.

Total synthesis of complex 2,5-diketopiperazine alkaloids

The 2,5-diketopiperazine (DKP) motif is present in many biologically relevant, complex natural products. The cyclodipeptide substructure offers structural rigidity and stability to proteolysis that makes these compounds promising candidates for medical applications. Due to their fascinating molecular architecture, synthetic organic chemists have focused significant effort on the total synthesis of these compounds. This review covers many such efforts on the total synthesis of DKP containing complex alkaloid natural products.

Exploration of Verticillins in High-Grade Serous Ovarian Cancer and Evaluation of Multiple Formulations in Preclinical In Vitro and In Vivo Models

Verticillins are epipolythiodioxopiperazine alkaloids isolated from a fungus with nanomolar anti-tumor activity in high-grade serous ovarian cancer (HGSOC). HGSOC is the fifth leading cause of death in women, and natural products continue to be an inspiration for new drug entities to help tackle chemoresistance. Verticillin D was recently found in a new fungal strain and compared to verticillin A. Both compounds exhibited nanomolar cytotoxic activity against OVCAR4 and OVCAR8 HGSOC cell lines, significantly reduced 2D foci and 3D spheroids, and induced apoptosis. In addition, verticillin A and verticillin D reduced tumor burden in vivo using OVCAR8 xenografts in the peritoneal space as a model. Unfortunately, mice treated with verticillin D displayed signs of liver toxicity. Tolerability studies to optimize verticillin A formulation for in vivo delivery were performed and compared to a semi-synthetic succinate version of verticillin A to monitor bioavailability in athymic nude females. Formulation of verticillins achieved tolerable drug delivery. Thus, formulation studies are effective at improving tolerability and demonstrating efficacy for verticillins.

Electrochemically Enabled Total Syntheses of Natural Products

Electrochemical techniques have helped to enable the total synthesis of natural products since the pioneering work of Kolbe in the mid 1800's. The electrochemical toolset grows every day and these new possibilities change the way chemists look at and think about natural products. This review provides a perspective on total syntheses wherein electrochemical techniques enabled the carbon─carbon bond formations in the skeletal assembly of important natural products, discussion of mechanistic details, and representative examples of the bond formations enabled over the last several decades. These bond formations are often distinctly different from those possible with conventional chemistries and allow assemblies complementary to other techniques.

Natural epidithiodiketopiperazine alkaloids as potential anticancer agents: Recent mechanisms of action, structural modification, and synthetic strategies

Cancer has become a grave health crisis that threatens the lives of millions of people worldwide. Because of the drawbacks of the available anticancer drugs, the development of novel and efficient anticancer agents should be encouraged. Epidithiodiketopiperazine (ETP) alkaloids with a 2,5-diketopiperazine (DKP) ring equipped with transannular disulfide or polysulfide bridges or S-methyl moieties constitute a special subclass of fungal natural products. Owing to their privileged sulfur units and intriguing architectural structures, ETP alkaloids exhibit excellent anticancer activities by regulating multiple cancer proteins/signaling pathways, including HIF-1, NF-κB, NOTCH, Wnt, and PI3K/AKT/mTOR, or by inducing cell-cycle arrest, apoptosis, and autophagy. Furthermore, a series of ETP alkaloid derivatives obtained via structural modification showed more potent anticancer activity than natural ETP alkaloids. To solve supply difficulties from natural resources, the total synthetic routes for several ETP alkaloids have been designed. In this review, we summarized several ETP alkaloids with anticancer properties with particular emphasis on their underlying mechanisms of action, structural modifications, and synthetic strategies, which will offer guidance to design and innovate potential anticancer drugs.

Transition-Metal-Catalyzed C-H Bond Activation for the Formation of C-C Bonds in Complex Molecules

Site-predictable and chemoselective C-H bond functionalization reactions offer synthetically powerful strategies for the step-economic diversification of both feedstock and fine chemicals. Many transition-metal-catalyzed methods have emerged for the selective activation and functionalization of C-H bonds. However, challenges of regio- and chemoselectivity have emerged with application to highly complex molecules bearing significant functional group density and diversity. As molecular complexity increases within molecular structures the risks of catalyst intolerance and limited applicability grow with the number of functional groups and potentially Lewis basic heteroatoms. Given the abundance of C-H bonds within highly complex and already diversified molecules such as pharmaceuticals, natural products, and materials, design and selection of reaction conditions and tolerant catalysts has proved critical for successful direct functionalization. As such, innovations within transition-metal-catalyzed C-H bond functionalization for the direct formation of carbon-carbon bonds have been discovered and developed to overcome these challenges and limitations. This review highlights progress made for the direct metal-catalyzed C-C bond forming reactions including alkylation, methylation, arylation, and olefination of C-H bonds within complex targets.

A one-step gram-scale protocol for stereoselective domino dimerization to asperazine A analogs

Here, we present an efficient protocol for stereoselective 4N-based domino dimerization in one single step, establishing a 22-membered library of asperazine A analogs. We describe steps for performing a gram-scale 2N-monomer to access the unsymmetrical 4N-dimer. We detail the synthesis of the desired dimer 3a as a yellow solid in 78% yield. This process demonstrates the 2-(iodomethyl)cyclopropane-1,1-dicarboxylate to be an iodine cation source. The protocol is limited to unprotected aniline of 2N-monomer. For complete details on the use and execution of this protocol, please refer to Bai et al. (2022).1.

Bispyrrolidinoindoline Epi(poly)thiodioxopiperazines (BPI-ETPs) and Simplified Mimetics: Structural Characterization, Bioactivities, and Total Synthesis

Within the 2,5-dioxopiperazine-containing natural products generated by "head-to-tail" cyclization of peptides, those derived from tryptophan allow further structural diversification due to the rich chemical reactivity of the indole heterocycle, which can generate tetracyclic fragments of hexahydropyrrolo[2,3-b]indole or pyrrolidinoindoline skeleton fused to the 2,5-dioxopiperazine. Even more complex are the dimeric bispyrrolidinoindoline epi(poly)thiodioxopiperazines (BPI-ETPs), since they feature transannular (poly)sulfide bridges connecting C3 and C6 of their 2,5-dioxopiperazine rings. Homo- and heterodimers composed of diastereomeric epi(poly)thiodioxopiperazines increase the complexity of the family. Furthermore, putative biogenetically generated downstream metabolites with C11 and C11'-hydroxylated cores, as well as deoxygenated and/or oxidized side chain counterparts, have also been described. The isolation of these complex polycyclic tryptophan-derived alkaloids from the classical sources, their structural characterization, the description of the relevant biological activities and putative biogenetic routes, and the synthetic efforts to generate and confirm their structures and also to prepare and further evaluate structurally simple analogs will be reported.

Interrogating the thionium hydrogen bond as a noncovalent stereocontrolling interaction in chiral phosphate catalysis

CH⋯O bonds are a privileged noncovalent interaction determining the energies and geometries of a large number of structures. In catalytic settings, these are invoked as a decisive feature controlling many asymmetric transformations involving aldehydes. However, little is known about their stereochemical role when the interaction involves other substrate types. We report the results of computations that show for the first time thionium hydrogen bonds to be an important noncovalent interaction in asymmetric catalysis. As a validating case study, we explored an asymmetric Pummerer rearrangement involving thionium intermediates to yield enantioenriched N,S-acetals under BINOL-derived chiral phosphate catalysis. DFT and QM/MM hybrid calculations showed that the lowest energy pathway corresponded to a transition state involving two hydrogen bonding interactions from the thionium intermediate to the catalyst. However, the enantiomer resulting from this process differed from the originally published absolute configuration. Experimental determination of the absolute configuration resolved this conflict in favor of our calculations. The reaction features required for enantioselectivity were further interrogated by statistical modeling analysis that utilized bespoke featurization techniques to enable the translation of enantioselectivity trends from intermolecular reactions to those proceeding intramolecularly. Through this suite of computational modeling techniques, a new model is revealed that provides a different explanation for the product outcome and enabled reassignment of the absolute product configuration.

Biosynthesis of pyrroloindoline-containing natural products

Covering: up to 2022Pyrroloindoline is a privileged tricyclic indoline motif widely present in many biologically active and medicinally valuable natural products. Thus, understanding the biosynthesis of this molecule is critical for developing convenient synthetic routes, which is highly challenging for its chemical synthesis due to the presence of rich chiral centers in this molecule, especially the fully substituted chiral carbon center at the C3-position of its rigid tricyclic structure. In recent years, progress has been made in elucidating the biosynthetic pathways and enzymatic mechanisms of pyrroloindoline-containing natural products (PiNPs). This article reviews the main advances in the past few decades based on the different substitutions on the C3 position of PiNPs, especially the various key enzymatic mechanisms involved in the biosynthesis of different types of PiNPs.

A C-H Activation Approach to the Tricyclic Core of Glionitrin A and B

Synthesis of diketopiperazines has been of long-standing interest in both natural product synthesis and medicinal chemistry. Here, we present an operationally convenient and efficient approach to the fused indoline-diketopiperazine tricyclic core of glionitrin A/B and related structures using a Pd-catalyzed C-H activation reaction to form the indoline five-membered ring. Exploratory work aimed at elaborating the tricyclic structures into the corresponding natural products is discussed.

Strategies for total synthesis of bispyrrolidinoindoline alkaloids

Covering up to 2021Complex cyclotryptamine alkaloids with a bispyrrolidino[2,3-b]indoline (BPI) skeleton are an intriguing family of natural products, exhibiting wide systematic occurrences, large structural diversity, and multiple biological activities. Based on their structural characteristics, BPI alkaloids can be classified into chimonanthine-type BPI alkaloids, BPI diketopiperazines, and BPI epipolythiodiketopiperazines. These intricate molecules have captivated great attention soon after their isolation and identification in the 1960s. Due to the structural complexity, the total synthesis of these cyclotryptamine alkaloids is challenging. Nevertheless, remarkable progress has been achieved in the last six decades; in particular, several methods have been successfully established for the construction of vicinal all-carbon quaternary stereocenters. In this review, the structural diversity and chemical synthesis of these BPI alkaloids were summarized. BPI alkaloids are mainly synthesized by the methods of oxidative dimerization, reductive dimerization, and alkylation of bisoxindole. The purpose of this review is to present overall strategies for assembling the BPI skeleton and efforts towards controlling the stereocenters.

Total synthesis of himastatin

The natural product himastatin has an unusual homodimeric structure that presents a substantial synthetic challenge. We report the concise total synthesis of himastatin from readily accessible precursors, incorporating a final-stage dimerization strategy that was inspired by a detailed consideration of the compound's biogenesis. Combining this approach with a modular synthesis enabled expedient access to more than a dozen designed derivatives of himastatin, including synthetic probes that provide insight into its antibiotic activity.

An organophotocatalytic late-stage N-CH3 oxidation of trialkylamines to N-formamides with O2 in continuous flow

We report an organophotocatalytic, N-CH3-selective oxidation of trialkylamines in continuous flow. Based on the 9,10-dicyanoanthracene (DCA) core, a new catalyst (DCAS) was designed with solubilizing groups for flow processing. This allowed O2 to be harnessed as a sustainable oxidant for late-stage photocatalytic N-CH3 oxidations of complex natural products and active pharmaceutical ingredients bearing functional groups not tolerated by previous methods. The organophotocatalytic gas-liquid flow process affords cleaner reactions than in batch mode, in short residence times of 13.5 min and productivities of up to 0.65 g per day. Spectroscopic and computational mechanistic studies showed that catalyst derivatization not only enhanced solubility of the new catalyst compared to poorly-soluble DCA, but profoundly diverted the photocatalytic mechanism from singlet electron transfer (SET) reductive quenching with amines toward energy transfer (EnT) with O2.

The N-heterocyclic carbene-catalyzed [3 + 2] annulation of isoindigos with enals: the enantioselective construction of three contiguous stereogenic centers

An NHC-catalyzed enantioselective [3 + 2] annulation of enals and isoindigo is introduced as an efficient strategy for the construction of dimeric spirocyclic bisindoline alkaloid derivatives with moderate yields and good enantioselectivities.

Total Synthesis of (-)-Glionitrin A and B Enabled by an Asymmetric Oxidative Sulfenylation of Triketopiperazines

Asymmetric construction of dithiodiketopiperazines on otherwise achiral scaffolds remains a pivotal synthetic challenge encountered in many biologically significant natural products. Herein, we report the first total syntheses of (−)-glionitrin A/B and revise the absolute configurations. Emerging from the study is a novel oxidative sulfenylation of triketopiperazines that enables asymmetric formation of dithiodiketopiperazines on sensitive substrates. The concise route paves the way for further studies on the potent antimicrobial and antitumor activities of glionitrin A and the intriguing ability of glionitrin B to inhibit invasive ability of cancer cells.

The intriguing chemistry and biology of sulfur-containing natural products from marine microorganisms (1987-2020)

Natural products derived from marine microorganisms have received great attention as a potential resource of new compound entities for drug discovery. The unique marine environment brings us a large group of sulfur-containing natural products with abundant biological functionality including antitumor, antibiotic, anti-inflammatory and antiviral activities. We reviewed all the 484 sulfur-containing natural products (non-sulfated) isolated from marine microorganisms, of which 59.9% are thioethers, 29.8% are thiazole/thiazoline-containing compounds and 10.3% are sulfoxides, sulfones, thioesters and many others. A selection of 133 compounds was further discussed on their structure-activity relationships, mechanisms of action, biosynthesis, and druggability. This is the first systematic review on sulfur-containing natural products from marine microorganisms conducted from January 1987, when the first one was reported, to December 2020.

Supplementary Information

The online version contains supplementary material available at 10.1007/s42995-021-00101-2.

Acyclic Twisted Amides

In this contribution, we provide a comprehensive overview of acyclic twisted amides, covering the literature since 1993 (the year of the first recognized report on acyclic twisted amides) through June 2020. The review focuses on classes of acyclic twisted amides and their key structural properties, such as amide bond twist and nitrogen pyramidalization, which are primarily responsible for disrupting nN to π*C═O conjugation. Through discussing acyclic twisted amides in comparison with the classic bridged lactams and conformationally restricted cyclic fused amides, the reader is provided with an overview of amidic distortion that results in novel conformational features of acyclic amides that can be exploited in various fields of chemistry ranging from organic synthesis and polymers to biochemistry and structural chemistry and the current position of acyclic twisted amides in modern chemistry.

Increasing cytochrome P450 enzyme diversity by identification of two distinct cyclodipeptide dimerases

Genome mining revealed the presence of two cdps-p450 operons in Saccharopolyspora antimicrobica. Heterologous expression, biochemical characterisation and structure elucidation proved that the two P450 enzymes catalyse distinct regio- and stereospecific dimerizations of cyclo-(l-Trp-l-Trp), which significantly expands the repertoire of diketopiperazine-tailoring enzymes. TtpB1 connects the monomers via C3-C3', both from the opposite side of H-11/H-11', while TtpB2 is characterised as the first P450 to mainly catalyse the unusual linkage between N1' and C3 from the H-11 side.

Semisynthetic Derivatives of the Verticillin Class of Natural Products through Acylation of the C11 Hydroxy Group

The verticillins, a class of epipolythiodioxopiperazine alkaloids (ETPs) first described 50 years ago with the discovery of verticillin A (1), have gained attention due to their potent activity against cancer cells, noted both in vitro and in vivo. In this study, the complex scaffold afforded through optimized fermentation was used as a feedstock for semisynthetic efforts designed to explore the reactivity of the C11 and C11' hydroxy substituents. Functionality introduced at these positions would be expected to impact not only the potency but also the pharmacokinetic properties of the resulting compound. With this in mind, verticillin H (2) was used as a starting material to generate nine semisynthetic analogues (4-12) containing a variety of ester, carbonate, carbamate, and sulfonate moieties. Likewise, verticillin A succinate (13) was synthesized from 1 to demonstrate the successful application of this strategy to other ETPs. The synthesized compounds and their corresponding starting materials (i.e., 1 and 2) were screened for activity against a panel of melanoma, breast, and ovarian cancer cell lines: MDA-MB-435, MDA-MB-231, and OVCAR3. All analogues retained IC₅₀ values in the nanomolar range, comparable to, and in some cases more potent than, the parent compounds.

Metal-catalyzed C-S bond formation using sulfur surrogates

Sulfur-containing compounds are present in a wide range of biologically important natural products, drugs, catalysts, and ligands and they have wide applications in material chemistry. Transition metal-catalyzed C-S bond-forming reactions have successfully overcome the obstacles associated with traditional organosulfur compound syntheses such as stoichiometric use of metal-catalysts, catalyst-poisoning and harsh reaction conditions. One of the key demands in metal-catalyzed C-S bond-forming reactions is the use of an appropriate sulfur source due to its odor and availability. The unpleasant odor of many organic sulfur sources might be one of the reasons for the metal-catalyzed C-S bond-forming reactions being less explored compared to other metal-catalyzed C-heteroatom bond-forming reactions. Hence, employing an appropriate sulfur surrogate in the synthesis of organosulfur compounds in metal-catalyzed reactions is still of prime interest for chemists. This review explores the recent advances in C-S bond formation using transition metal-catalyzed cross-coupling reactions and C-H bond functionalization using diverse and commercially available sulfur surrogates. Based on the different transition metal-catalysts, this review has been divided into three major classes namely (1) palladium-catalyzed C-S bond formation, (2) copper-catalyzed C-S bond formation, and (3) other metal-catalyzed C-S bond formation. This review is further arranged based on the different sulfur surrogates. Also, this review provides an insight into the growing opportunities in the construction of complex organosulfur scaffolds covering natural product synthesis and functional materials.

Phosphine-Catalyzed Asymmetric Allylic Alkylation of Achiral MBH Carbonates with 3,3'-Bisindolines: Enantioselective Construction of Quaternary Stereogenic Centers

The asymmetric allylic alkylation (AAA) of achiral Morita-Baylis-Hillman (MBH) carbonates with 3,3'-bisindolines under the catalysis of amino-acid-derived bifunctional phosphines was accomplished. With the AAA approach introduced herein, challenging 3,3'-bisindolines bearing an all-carbon quaternary stereocenter (C3a) have been constructed in good yields with good to excellent enantioselectivties. In addition, the synthetic value of this protocol was demonstrated by the facile synthesis of the core skeleton of gliocladin C.

Recent advances in total syntheses of complex dimeric natural products

Dimeric natural products are a collection of molecules with diverse molecular architectures and significant bio-activities. In this tutorial review, total synthesis of complex dimeric natural products accomplished in recent years are summarized and various dimerization strategies are discussed. By highlighting the selected representative examples, this review aims to demonstrate the recent tactics of dimerization which is an important process integrated into the whole synthetic sequences of dimeric natural products, provide insights on structural and chemical properties of monomers and dimers of related natural products, and promote further technological advances in organic synthesis and biological studies of complex dimeric natural products.

Fragment Coupling Reactions in Total Synthesis That Form Carbon-Carbon Bonds via Carbanionic or Free Radical Intermediates

Fragment coupling reactions that form carbon-carbon bonds are valuable transformations in synthetic design. Advances in metal-catalyzed cross-coupling reactions in the early 2000s brought a high level of predictability and reliability to carbon-carbon bond constructions involving the union of unsaturated fragments. By comparison, recent years have witnessed an increase in fragment couplings proceeding via carbanionic and open-shell (free radical) intermediates. The latter has been driven by advances in methods to generate and utilize carbon-centered radicals under mild conditions. In this Review, we survey a selection of recent syntheses that have implemented carbanion- or radical-based fragment couplings to form carbon-carbon bonds. We aim to highlight the strategic value of these disconnections in their respective settings and to identify extensible lessons from each example that might be instructive to students.

Catalytic asymmetric preparation of pyrroloindolines: strategies and applications to total synthesis

Pyrroloindolines are widely present in natural products. In this review, we summarize state-of-the-art of catalytic asymmetric synthesis of pyrroloindolines, as well as related applications to natural products total synthesis.

Divergent Synthesis of Bioactive Dithiodiketopiperazine Natural Products Based on a Double C(sp3 )-H Activation Strategy

This article provides a detailed report of our efforts to synthesize the dithiodiketopiperazine (DTP) natural products (-)-epicoccin G and (-)-rostratin A using a double C(sp³ )-H activation strategy. The strategy's viability was first established on a model system lacking the C8/C8' alcohols. Then, an efficient stereoselective route including an organocatalytic epoxidation was secured to access a key bis-triflate substrate. This bis-triflate served as the functional handles for the key transformation of the synthesis: a double C(sp³ )-H activation. The successful double activation opened access to a common intermediate for both natural products in high overall yield and on a multigram scale. After several unsuccessful attempts, this intermediate was efficiently converted to (-)-epicoccin G and to the more challenging (-)-rostratin A via suitable oxidation/reduction and protecting group sequences, and via a final sulfuration that occurred in good yield and high diastereoselectivity. These efforts culminated in the synthesis of (-)-epicoccin G and (-)-rostratin A in high overall yields (19.6 % over 14 steps and 12.7 % over 17 steps, respectively), with the latter being obtained on a 500 mg scale. Toxicity assessments of these natural products and several analogues (including the newly synthesized epicoccin K) in the leukemia cell line K562 confirmed the importance of the disulfide bridge for activity and identified dianhydrorostratin A as a 20x more potent analogue.

n→π* Interactions Modulate the Disulfide Reduction Potential of Epidithiodiketopiperazines

Epithiodiketopiperazines (ETPs) are a structurally complex class of fungal natural products with potent anticancer activity. In ETPs, the diketopiperazine ring is spanned by a disulfide bond that is constrained in a high-energy eclipsed conformation. We employed computational, synthetic, and spectroscopic methods to investigate the physicochemical attributes of this atypical disulfide bond. We find that the disulfide bond is stabilized by two n→π* interactions, each with large energies (3-5 kcal/mol). The n→π* interactions in ETPs make disulfide reduction much more difficult, endowing stability in physiological environments in a manner that could impact their biological activity. These data reveal a previously unappreciated means to stabilize a disulfide bond and highlight the utility of the n→π* interaction in molecular design.

Synthesis of Polycyclic Fused Indoline Scaffolds through a Substrate-Guided Reactivity Switch

Zn(II)-catalyzed divergent synthesis of functionalized polycyclic indolines through formal [3 + 2] and [4 + 2] cycloadditions of indoles with 1,2-diaza-1,3-dienes (DDs) is reported. The nature and type of substituents of substrates are found to act as a chemical switch to trigger two distinct reaction pathways and to obtain two different types of products upon the influence of the same catalyst. The mechanism of both [4 + 2] and [3 + 2] cycloadditions was investigated and fully rationalized by density functional theory (DFT) calculations.

Discovery and Total Synthesis of a Bis(cyclotryptamine) Alkaloid Bearing the Elusive Piperidinoindoline Scaffold

Bis(cyclotryptamine) alkaloids have been popular topics of study for many decades. Five possible scaffolds for bis(cyclotryptamine) alkaloids were originally postulated in the 1950s, but only four of these scaffolds have been observed in natural products to date. We describe synthetic access to the elusive fifth scaffold, the piperidinoindoline, through syntheses of compounds now termed "dihydropsychotriadine" and "psychotriadine". The latter of these compounds was subsequently identified in extracts of the flower Psychotria colorata. Our synthetic route features a stereospecific solid-state photodecarbonylation reaction to introduce the key vicinal quaternary stereocenters.

Synthesis of Potent Cytotoxic Epidithiodiketopiperazines Designed for Derivatization

We describe our design, synthesis, and chemical study of a set of functional epidithiodiketopiperazines (ETPs) and evaluation of their activity against five human cancer cell lines. Our structure-activity relationship-guided substitution of ETP alkaloids offers versatile derivatization while maintaining potent anticancer activity, offering exciting opportunity for their use as there are no examples of complex and potently anticancer (nM) ETPs being directly used as conjugatable probes or warheads. Our synthetic solutions to strategically designed ETPs with functional linkers required advances in stereoselective late-stage oxidation and thiolation chemistry in complex settings, including the application of novel reagents for dihydroxylation and cis-sulfidation of diketopiperazines. We demonstrate that complex ETPs equipped with a strategically substituted azide functional group are readily derivatized to the corresponding ETP-triazoles without compromising anticancer activity. Our chemical stability studies of ETPs along with cytotoxic evaluation of our designed ETPs against A549, DU 145, HeLa, HCT 116, and MCF7 human cancer cell lines provide insights into the impact of structural features on potency and chemical stability, informing future utility of ETPs in chemical and biological studies.

Iron and cobalt catalysis: new perspectives in synthetic radical chemistry

Iron and cobalt complexes are at the origin of high valuable synthetic pathways involving radical intemediates.