Cuprous Chloride Accelerated Stille Reactions. A General and Effective Coupling System for Sterically Congested Substrates and for Enantioselective Synthesis

Entry into Lithium Ynolates from α,α,α-Tribromomethyl Ketones: Synthesis of Cyclobutenes via the [2 + 2] Cycloaddition with α,β-Unsaturated Carbonyls

This study reports the synthesis of cyclobutene derivatives in good yields via the [2 + 2] cycloaddition between lithium ynolates and α,β-unsaturated carbonyls. The ynolates are generated from α,α,α-tribromomethyl ketones and tert-butyl lithium via a simple and novel method, which does not produce any harmful byproducts, such as lithium alkoxide, which induces a polymerization reaction with α,β-unsaturated carbonyls.

Convergent and Efficient Total Synthesis of (+)-Heilonine Enabled by C-H Functionalizations

We report a convergent and efficient total synthesis of the C-nor D-homo steroidal alkaloid (+)-heilonine with a hexacyclic ring system, nine stereocenters, and a trans-hydrindane moiety. Our synthesis features four selective C-H functionalizations to form key C-C bonds and stereocenters, a Stille carbonylative cross-coupling to connect the AB ring system with the DEF ring system, and a Nazarov cyclization to construct the five-membered C ring. These enabling transformations significantly reduced functional group manipulations and delivered (+)-heilonine in 11 or 13 longest linear sequence (LLS) steps.

Bioinspired Functionalization of Carbonyl Compounds Enabled by Metal Chelated Bifunctional Ligands

In Nature, enzymatic reactions proceed through exceptionally ordered transition states giving rise to extraordinary levels of stereoselection. In those reactions, the active site of the enzyme plays crucial roles - through one position, it holds the substrate in the proximity to the reaction epicentre that facilitates both the reactivity and stereoselectivity of the chemical process. Inspired by this natural phenomenon, synthetic chemists have designed bifunctional ligands that not only coordinate to a metal centre but also preassociate with an organic substrate, for example aldehyde and ketone, and exerts stereodirecting influence to accelerate the attack of the incoming reacting partner from a particular enantiotopic face. The chief goal of the current review is to give an overview of the recently developed approaches enabled by privileged bio-inspired bifunctional ligands that not only bind to the metal catalyst but also activates carbonyl substrates via organocatalysis, thereby easing in the new bond forming step. As carbonyl α-functionalizations are dominated by enamine and enolate chemistry, the current review primarily focusses on enamine- and enolate-metal catalysis by bifunctional ligands. Thus, developments based on traditional cooperative catalysis occurring through two directly coupled but independent catalytic cycles of an organocatalyst and a metal catalyst are not covered.

Unified Total Synthesis of the Limonoid Alkaloids: Strategies for the De Novo Synthesis of Highly Substituted Pyridine Scaffolds

Highly substituted pyridine scaffolds are found in many biologically active natural products and therapeutics. Accordingly, numerous complementary de novo approaches to obtain differentially substituted pyridines have been disclosed. This article delineates the evolution of the synthetic strategies designed to assemble the demanding tetrasubstituted pyridine core present in the limonoid alkaloids isolated from Xylocarpus granatum, including xylogranatopyridine B, granatumine A and related congeners. In addition, NMR calculations suggested structural misassignment of several limonoid alkaloids, and predicted their C3-epimers as the correct structures, which was further validated unequivocally through chemical synthesis. The materials produced in this study were evaluated for cytotoxicity, anti-oxidant effects, anti-inflammatory action, PTP1B and Nlrp3 inflammasome inhibition, which led to compelling anti-inflammatory activity and anti-oxidant effects being discovered.

Synthesis of rearranged indole diterpenes of the paxilline type

Covering: up to 2021Rearranged indole diterpenes of the paxilline type comprise a large group of fungal metabolites that possess diverse structural features and potentially useful biological effects. The unique indoloterpenoid motif, which is common to all congeners, was first confirmed by crystallographic studies of paxilline. This family of natural products has fascinated organic chemists for the past four decades and has inspired numerous syntheses and synthetic approaches. The present review highlights efforts that have laid the foundation and introduced new directions to this field of natural product synthesis. The introduction includes a summary of biosynthetic considerations and biological activities, the main body of the manuscript provides a detailed discussion of selected syntheses, and the review concludes with a brief outlook on the future of the field.

Taming Shapeshifting Anions: Total Synthesis of Ocellatusone C

Guided by a synthetic design aimed at late-stage diversification, we report the preparation of unusual shapeshifting anions and their subsequent application to the total synthesis of the polyketide natural product ocellatusone C. Site-selective core functionalization of a readily accessible bicyclo[3.3.1]nonane architecture sets the stage for shape-selective side chain installation via a nonfluxional π-allyl Pd-complex derived from a barbaralyl-type anion. Several interesting chemical findings, including substituent-dependent bridged bicycloisomerism and the isolation of a stabilized, 3° carbon-bound Pd-ketone enolate complex, are reported.

Total Synthesis of Resiniferatoxin Enabled by Photocatalytic Decarboxylative Radical Cyclization

Resiniferatoxin (1) is a complex daphnane diterpenoid with a highly oxygenated 5/7/6-membered ABC-ring system. Here we report a new synthetic route to 1 that requires 27 steps from a starting d-ribose derivative. The carbon spacer and A-ring are sequentially attached to the C-ring by radical allylation and Stille coupling reactions, respectively. An Ir(III)-catalyzed photoinduced decarboxylative radical reaction then forged the sterically hindered bond between the tetra- and trisubstituted carbons to cyclize the central seven-membered B-ring.

Asymmetric Total Synthesis of Indole Diterpenes Paspalicine, Paspalinine, and Paspalinine-13-ene

Paspaline-derived indole diterpenes (IDTs) are structurally complex mycotoxins with unique tremorgenic activity. Reported are asymmetric total syntheses of three paspaline-derived IDTs paspalicine, paspalinine and paspalinine-13-ene. Our synthesis features a green Achmatowicz rearrangement/bicycloketalization for the efficient construction of FG rings (75 % yield) and a cascade ring-closing metathesis of dienyne for highly regioselective formation of CD rings (72 % yield). Other highlights include four palladium-mediated reactions (Stille, aza-Wacker, Suzuki, and Heck) to forge the BE rings and the installation of two continuous all-carbon quaternary stereocenters via reductive ring-opening of cyclopropane and α-methylation of the conjugate ester. Our new synthetic strategy is expected to be applicable to the chemical synthesis of other paspaline-derived IDTs and will facilitate the bioactivity studies of these agriculturally and pharmacologically important IDTs.

Total Synthesis of Shearinines D and G: A Convergent Approach to Indole Diterpenoids

The first total syntheses of the indole diterpenoids (+)‐shearinine G and D are disclosed. The successful routes rely on late‐stage coupling of two complex fragments. Formation of the challenging trans‐hydrindane motif was accomplished by diastereoselective, intramolecular cyclopropanation. A one‐pot sequence consisting of Sharpless dihydroxylation/Achmatowicz reaction was developed to install the dioxabicyclo[3.2.1]octane motif. The indenone subunit was accessed by Prins cyclization. Tuning the electronic nature of the substituents on the parent arylcarboxaldehyde allowed access to divergent products that were further transformed into shearinines G and D. Riley‐type oxidation of a bicyclic enone yielded a surprising stereochemical outcome.

Synthesis of bent-shaped π-extended thienoacenes from 2,5-distannylated 3,4-dialkynethiophene

Bent-shaped thienoacenes show promise as next-generation organic semiconductors. Here we present the synthesis of an air-stable, pure and easily scalable thiophene precursor, 2,5-distannylated-3,4-dialkyne thiophene, starting from 3,4-dialkyne thiophene in quantitative yields. This precursor has been used for the synthesis of a versatile class of syn-thienoacenes comprising up to 13 fused rings, helical acenes and donor-acceptor acenes.

The 2-Pyridyl Problem: Challenging Nucleophiles in Cross-Coupling Arylations

Azine-containing biaryls are ubiquitous scaffolds in many areas of chemistry, and efficient methods for their synthesis are continually desired. Pyridine rings are prominent amongst these motifs. Transition-metal-catalysed cross-coupling reactions have been widely used for their synthesis and functionalisation as they often provide a swift and tuneable route to related biaryl scaffolds. However, 2-pyridine organometallics are capricious coupling partners and 2-pyridyl boron reagents in particular are notorious for their instability and poor reactivity in Suzuki-Miyaura cross-coupling reactions. The synthesis of pyridine-containing biaryls is therefore limited, and methods for the formation of unsymmetrical 2,2'-bis-pyridines are scarce. This Review focuses on the methods developed for the challenging coupling of 2-pyridine nucleophiles with (hetero)aryl electrophiles, and ranges from traditional cross-coupling processes to alternative nucleophilic reagents and novel main group approaches.

Recent advances in the total syntheses of indole diterpenoids

Indole diterpenoids constitute a large family of natural products that are characterized by a hybrid molecular architecture consisting of an indole nucleus and diterpenoid moiety. Their pharmacologically and agriculturally important biological properties as well as intriguing molecular architectures have attracted much attention from many synthetic organic chemists. In 2012, we succeeded in the concise total synthesis of a paspalane-type indole diterpenoid, namely paspalinine, by developing a highly efficient indole ring formation protocol. After the report of this total synthesis, 4 research groups achieved the total syntheses of other paspalane- and nodulisporane-type indole diterpenoids using current state-of-the-art methods. This review summarizes the total syntheses of the paspalane- and nodulisporane-type indole diterpenoids that were described in the last 10 years.

Natural polyenic macrolactams and polycyclic derivatives generated by transannular pericyclic reactions: optimized biogenesis challenging chemical synthesis

Covering from 1992 to the end of 2020-11-20.Genetically-encoded polyenic macrolactams, which are constructed by Nature using hybrid polyketide synthase/nonribosomal peptide synthase (PKSs/NRPSs) assembly lines, are part of the large collection of natural products isolated from bacteria. Activation of cryptic (i.e., silent) gene clusters in these microorganisms has more recently allowed to generate and eventually isolate additional members of the family. Having two unsaturated fragments separated by short saturated chains, the primary macrolactam is posited to undergo transannular reactions and further rearrangements thus leading to the generation of a structurally diverse collection of polycyclic (natural) products and oxidized derivatives. The review will cover the challenges that scientists face on the isolation of these unstable compounds from the cultures of the producing microorganisms, their structural characterization, biological activities, optimized biogenetic routes, as well as the skeletal rearrangements of the primary structures of the natural macrolactams derived from pericyclic reactions of the polyenic fragments. The efforts of the synthetic chemists to emulate Nature on the successful generation and structural confirmation of these natural products will also be reported.

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.

Late-Stage Conversion of a Metabolically Labile Aryl Methyl Ether-Containing Natural Product to Fluoroalkyl Analogues

We report the conversion of aryl methyl ethers and phenols into six fluoroalkyl analogues through late-stage functionalization of a natural product-derived FDA-approved therapeutic. This series of short synthetic sequences exploits a combination of both modern and traditional methods and demonstrates that some recently reported methods do not always work as well as desired on a natural product-like scaffold. Nonetheless, reaction optimization can deliver sufficient quantities of each target analogue for medicinal chemistry purposes. In some cases, classical reactions and synthetic sequences still outcompete modern organofluorine transformations, which should encourage the continued search for improved reactions. Overall, the project provides a valuable synthetic roadmap for medicinal chemists to access a range of fluorinated therapeutic candidates with distinct physicochemical properties relative to the original O-based analogue.

Application of copper(i) salt and fluoride promoted Stille coupling reactions in the synthesis of bioactive molecules

The Stille coupling between organostannanes and organohalides is an effective catalytic method for organic synthesis. Despite the ample amount of published results in this area, finding the optimal conditions for this transformation is often not straightforward. It was observed that this reaction could be accelerated with improved efficiency by the addition of a Cu(i) salt and fluoride. This review summarises the application of this simple protocol in the synthesis of natural products, their analogues and other biologically active molecules, from 2004 to 2018.

Synthetic Approaches to Zetekitoxin AB, a Potent Voltage-Gated Sodium Channel Inhibitor

Voltage-gated sodium channels (Na_Vs) are membrane proteins that are involved in the generation and propagation of action potentials in neurons. Recently, the structure of a complex made of a tetrodotoxin-sensitive (TTX-s) Na_V subtype with saxitoxin (STX), a shellfish toxin, was determined. STX potently inhibits TTX-s Na_V, and is used as a biological tool to investigate the function of Na_Vs. More than 50 analogs of STX have been isolated from nature. Among them, zetekitoxin AB (ZTX) has a distinctive chemical structure, and is the most potent inhibitor of Na_Vs, including tetrodotoxin-resistant (TTX-r) Na_V. Despite intensive synthetic studies, total synthesis of ZTX has not yet been achieved. Here, we review recent efforts directed toward the total synthesis of ZTX, including syntheses of 11-saxitoxinethanoic acid (SEA), which is considered a useful synthetic model for ZTX, since it contains a key carbon-carbon bond at the C11 position.

Synthetic Studies on FNC-RED and Its Analogues Containing an All syn-Cyclopentanetetrol Moiety

FNC-RED exhibits innate immune receptor Toll-like receptor 4 (TLR4)/myeloid differentiation factor-2 (MD2) stimulatory activity. We have developed a divergent synthetic route to FNC-RED derivatives containing various alkyl side chains. Key features of the synthetic study include stepwise palladium catalyzed cross-coupling reactions and the construction of an all syn-cyclopentanetetrol moiety.

Catalytic Synthesis of Potassium Acyltrifluoroborates (KATs) through Chemoselective Cross-Coupling with a Bifunctional Reagent

Potassium acyltrifluoroborates (KATs) are increasingly important functional groups, and general methods for their preparation are of great current interest. We report a bifunctional iminium reagent bearing both a tin nucleophile and a trifluoroborate, which was applied in chemoselective Pd⁰ -catalyzed Migita-Kosugi-Stille cross-coupling reactions owith aryl and vinyl halides. This method gives access to previously inaccessible aromatic and α,β-unsaturated acyltrifluoroborates, including precursors to amino-acid derived KATs.

Synthesis of Naphthocyclobutenes from α-Naphthyl Acrylates by Visible-Light Energy-Transfer Catalysis

Methyl (α-naphthyl) acrylates bearing an ortho-substituent with a hydrogen atom produce naphthocyclobutenes upon Ir(Fppy)₃-mediated photosensitization. This reaction can be described as a carbon analogue of the Norrish-Yang reaction: upon triplet excitation of the acrylate a 1,5-HAT results in a 1,4-diradical which forms the cyclobutene. Diastereoselectivities up to >20:1 were observed for the ring-closure reaction.

Divergent Synthesis of Natural Derivatives of (+)-Saxitoxin Including 11-Saxitoxinethanoic Acid

The bis-guanidinium toxins are a collection of natural products that display nanomolar potency against select isoforms of eukaryotic voltage-gated Na+ ion channels. We describe a synthetic strategy that enables access to four of these poisons, namely 11-saxitoxinethanoic acid, C13-acetoxy saxitoxin, decarbamoyl saxitoxin, and saxitoxin. Highlights of this work include an unusual Mislow-Evans rearrangement and a late-stage Stille ketene acetal coupling. The IC50 value of 11-saxitoxinethanoic acid was measured against rat NaV 1.4, and found to be 17.0 nm, similar to those of the sulfated toxins gonyautoxin II and III.

Synthetic Studies Toward Pactamycin Highlighting Oxidative C-H and Alkene Amination Technologies

A strategy enabled by C-H and alkene amination technologies for synthesizing the aminocyclitol natural product, pactamycin, is disclosed. This work features two disparate approaches for assembling the five-membered ring core of the target, the first of which utilizes acyl anion catalysis and a second involving β-ketoester aerobic hydroxylation. Installation of the C3-N bond, one of three contiguous nitrogen centers, is made possible through Rh-catalyzed allylic C-H amination of a sulfamate ester. Subsequent efforts are presented to introduce the C1,C2 cis-diamino moiety en route to pactamycin, including carbamate-mediated alkene aziridination. In the course of these studies, assembly of the core of C2- epi-pactamycate, which bears the carbon skeleton and all of the requisite nitrogen and oxygen functional groups found in the natural product, has been achieved.

Effect of protic additives in Cu-catalysed asymmetric Diels-Alder cycloadditions of doubly activated dienophiles: towards the synthesis of magellanine-type Lycopodium alkaloids

The pronounced beneficial effect of a precise amount of protic additive in an enantioselective Cu-catalysed Diels-Alder reaction is reported. This reaction, which employs a cyclic alkylidene β-ketoester as a dienophile, represents one of the first examples of a transformation where these extremely versatile, though highly unstable reaction partners participate effectively in catalytic asymmetric cycloaddition with a functionalised diene. The cycloadduct was used as an intermediate towards the synthesis of magellanine-type Lycopodium alkaloids featuring a Stille cross-coupling of a highly congested enol triflate and a unique Meinwald rearrangement/cyclopropanation sequence.

Total synthesis of architecturally complex indole terpenoids: strategic and tactical evolution

Indole terpenes have attracted the interests of synthetic chemists due to their complex architectures and potent biological activities. Examples of total syntheses of several indole terpenes were reviewed in this article to honor Professor KC Nicolaou.

The chemistry and biology of mycolactones

Mycolactones are a group of macrolides excreted by the human pathogen Mycobacterium ulcerans, which exhibit cytotoxic, immunosuppressive and analgesic properties. As the virulence factor of M. ulcerans, mycolactones are central to the pathogenesis of the neglected disease Buruli ulcer, a chronic and debilitating medical condition characterized by necrotic skin ulcers. Due to their complex structure and fascinating biology, mycolactones have inspired various total synthesis endeavors and structure-activity relationship studies. Although this review intends to cover all synthesis efforts in the field, special emphasis is given to the comparison of conceptually different approaches and to the discussion of more recent contributions. Furthermore, a detailed discussion of molecular targets and structure-activity relationships is provided.

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

Covering: early studies through to March 2016Cardenolides and bufadienolides constitute an attractive class of biologically active steroid derivatives which have been used for the treatment of heart disease in traditional remedies as well as in modern medicinal therapy. Due to their application as therapeutic agents and their unique molecular structures, bearing unsaturated 5- or 6-membered lactones (or other heterocycles) attached to the steroid core, cardio-active steroids have received great attention, which has intensified during the last decade, in the synthetic organic community. Advances in the field of cross-coupling reactions have provided a powerful tool for the attachment of lactone subunits to the steroid core. This current review covers a methodological analysis of synthetic efforts to cardenolide and bufadienolide aglycones. Special emphasis is given to cross-coupling reactions applied for the attachment of lactone subunits at sterically very hindered positions of the steroid core. The carefully selected partial and total syntheses of representative cardio-active steroids will also be presented to exemplify recent achievements (improvements) in the field.

Total syntheses of both enantiomers of amphirionin 4: A chemoenzymatic based strategy for functionalized tetrahydrofurans

The total syntheses of (-)-amphirionin-4 and (+)-amphirionin-4 have been achieved in a convergent and enantioselective manner. The tetrahydrofuranol cores of amphirionin-4 were constructed in optically active form by enzymatic resolution of racemic cis-3-hydroxy-5-methyldihydrofuran-2(3H)-one. The polyene side chain was efficiently synthesized using Stille coupling. The remote C8-stereocenter was constructed using the Nozaki-Hiyama-Kishi coupling reaction. A detailed ¹H-NMR studies of Mosher esters of (-)-amphirionin-4 and (+)-amphirionin-4 were carried out to support the assignment of the absolute configurations of C-4 and C-8 asymmetric centers of amphirionin-4.

A regioselective double Stille coupling reaction of bicyclic stannolanes

A regioselective double Stille coupling reaction was explored using bicyclic stannolanes that were easily prepared from the radical cascade reaction of β-amino-α-methylene esters. Various 1-bromo-2-iodoarenes underwent the double coupling reaction to afford benzoisoindole derivatives in a regioselective manner, where the carbon attached to the iodine selectively coupled with the vinylic carbon, and then the carbon attached to bromine coupled with the alkyl carbon. The combination of intra- and intermolecular coupling reactions provided hexahydroindeno[1,2-b]pyrrole derivatives in good yields. The yields were further improved in the presence of excess amounts of CsF. An attempt to identify the reaction intermediate was made wherein the decomposition of the stannolanes with aqueous HCl and HBr afforded trigonal bipyramidal (TBP) pentacoordinated tin complexes, as confirmed by microanalyses and (119)Sn NMR. Using DCl for the decomposition selectively introduced a deuterium to the E-position of the exomethylene unit. The complexes smoothly underwent the intramolecular Stille coupling reaction in the presence of both a palladium catalyst and DABCO, affording hexahydroindeno[1,2-b]pyrroles in good yields. These results suggest that the double coupling reaction progresses through a TBP tin complex, promoting the second intramolecular coupling reaction between the aryl halide and Csp(3)-tin bond.

Total Syntheses of Nannocystins A and A0, Two Elongation Factor 1 Inhibitors

Asymmetric total syntheses of nannocystins A and A0 were achieved in a convergent route starting from simple materials. Nannocystin family natural products bear potent anticancer activity as elongation factor 1 inhibitors. In this synthesis, the challenging tertiary amide bond was constructed by peptide coupling between an acyl chloride and a secondary amine. A late-stage ring-closing metathesis reaction successfully rendered the macrocycle. This efficient synthetic strategy should be applicable to other nannocystins and analogues and therefore should benefit future structure-activity relationship studies.

Recent Advances of Total Syntheses of Diterpenoids Starting from Carvone

This review focuses on total syntheses of diterpenoids starting from carvone (1) since 2006 in the alphabetical order of compound names.

Enantioselective Total Synthesis of (+)-Amphirionin-4

An enantioselective total synthesis of (+)-amphirionin-4 has been accomplished in a convergent manner. The synthesis features an efficient enzymatic lipase resolution to access the tetrahydrofuranol core in optically active form. The functionalized tetrahydrofuran derivative was synthesized via an oxocarbenium ion-mediated highly diastereoselective syn-allylation reaction. The polyene side chain was synthesized using Stille coupling reactions. Nozaki-Hiyama-Kishi coupling was utilized to construct the C-8 stereocenter and complete the synthesis of (+)-amphirionin-4.

Three-Step Synthesis of Fluoranthenes through Pd-Catalyzed Inter- and Intramolecular C-H Arylation

A three-step synthetic method for the preparation of fluoranthenes, involving Miura's intermolecular C-H arylation, nonaflation, and intramolecular C-H arylation, has been developed. Various 1-naphthols and haloarenes were successfully used as substrates. Reaction conditions that afford high site selectivity have been developed for the intramolecular C-H arylation step.