Concise Synthesis of a Pateamine A Analogue with In Vivo Anticancer Activity Based on an Iron‐Catalyzed Pyrone Ring Opening/Cross‐Coupling

Immune Modulatory Profile of the Pateamines PatA and Des-Methyl Des-Amino PatA

Pateamines act as inhibitors of the RNA helicase eIF4A and exhibit antiviral and anticancer properties. Recently, we observed that inhibition of eIF4A by rocaglates affects the immune response. To investigate whether the observed immunomodulatory effects are specific to rocaglates or the inhibition of eIF4A, a comprehensive study was conducted on the influence of pateamines that exhibit the same inhibitory mode of action as rocaglates on various immune cells. The effects of pateamine A (PatA) and des-methyl des-amino pateamine A (DMDA) on the expression of surface markers, release of cytokines, cell proliferation, inflammatory mediators and metabolic activity in primary human monocyte-derived macrophages (MdM), T cells and B cells were assessed. Additionally, safety and bioavailability profiles were determined. DMDA revealed almost no immunomodulatory effects within the tested concentration range of 0.5-5 nM. PatA reduced B cell activation, as shown by reduced immune globulin release and decreased chemokine release from macrophages, while T cell function remained unaffected. Both DMDA and PatA showed low permeability in Caco-2 and Calu-3 cell barrier assays and no mutagenic potential. However, 10 nM PatA exhibited genotoxic potential, as shown by the micronucleus assay. In conclusion, DMDA had a good safety profile but exhibited low permeability, whereas PatA had a poor safety profile and also low permeability. The observed immunomodulatory effects of elF4A inhibitors on B cells appear to be target-specific.

DMDA-PatA mediates RNA sequence-selective translation repression by anchoring eIF4A and DDX3 to GNG motifs

Small-molecule compounds that elicit mRNA-selective translation repression have attracted interest due to their potential for expansion of druggable space. However, only a limited number of examples have been reported to date. Here, we show that desmethyl desamino pateamine A (DMDA-PatA) represses translation in an mRNA-selective manner by clamping eIF4A, a DEAD-box RNA-binding protein, onto GNG motifs. By systematically comparing multiple eIF4A inhibitors by ribosome profiling, we found that DMDA-PatA has unique mRNA selectivity for translation repression. Unbiased Bind-n-Seq reveals that DMDA-PatA-targeted eIF4A exhibits a preference for GNG motifs in an ATP-independent manner. This unusual RNA binding sterically hinders scanning by 40S ribosomes. A combination of classical molecular dynamics simulations and quantum chemical calculations, and the subsequent development of an inactive DMDA-PatA derivative reveals that the positive charge of the tertiary amine on the trienyl arm induces G selectivity. Moreover, we identified that DDX3, another DEAD-box protein, is an alternative DMDA-PatA target with the same effects on eIF4A. Our results provide an example of the sequence-selective anchoring of RNA-binding proteins and the mRNA-selective inhibition of protein synthesis by small-molecule compounds.

Total Syntheses of Nominal and Actual Prorocentin

The dinoflagellate-derived polyether prorocentin is a co-metabolite of the archetypical serine/threonine phosphatase inhibitor okadaic acid. Whereas a structural relationship cannot be missed and a biosynthetic link was proposed, it is currently unknown whether there is any parallel in the bioactivity profile of these natural products. However, it was insinuated in the past that the structure assigned to prorocentin might need to be revised. Indeed, re-examination of the published spectra cast doubts as to the constitution of the fused/spirotricyclic BCD-ring system in the core. To clarify this issue, a flexible synthesis blueprint was devised that allowed us to obtain the originally proposed structure as well as the most plausible amended structure. The key to success was late-stage gold-catalyzed spirocyclization reactions that furnished the isomeric central segments with excellent selectivity. The lexicon of catalytic transformations used to make the required cyclization precursors comprised a titanium-mediated ester methylenation/metathesis cascade, a rare example of a gold-catalyzed allylic substitution, and chain extensions via organocatalytic asymmetric aldehyde propargylation. A wing sector to be attached to the isomeric cores was obtained by Krische allylation, followed by a superbly selective cobalt-catalyzed oxidative cyclization of the resulting di-unsaturated alcohol with the formation of a 2,5-trans-disubstituted tetrahydrofuran; the remaining terminal alkene was elaborated into an appropriate handle for fragment coupling by platinum-catalyzed asymmetric diboration/oxidation. The assembly of the different building blocks to the envisaged isomeric target compounds proved that the structure of prorocentin needs to be revised as disclosed herein.

A formal [3 + 3] cycloaddition of allenyl imide and activated ketones for the synthesis of tetrasubstituted 2-pyrones

CsOH·H₂O-catalyzed formal [3 + 3] cycloadditions of allenyl imide with β-ketoesters, 1,3-diketones or β-ketonitriles for the synthesis of tetrasubstituted 2-pyrone derivatives have been demonstrated. The allenyl imide was utilized as a C3-synthon, and a ketenyl intermediate was proposed via the process of 1,4-addition of carbon anion to allene followed by elimination of the 2-oxazolidinyl group.

CsOH·H₂O-catalyzed formal [3 + 3] cycloadditions of allenyl imide with β-ketoesters, 1,3-diketones or β-ketonitriles for the synthesis of tetrasubstituted 2-pyrone derivatives were reported.

Lessons from Natural Product Total Synthesis: Macrocyclization and Postcyclization Strategies

Macrocyclic natural products are plentiful in the bacteria, archaea, and eukaryote domains of life. For the significant advantages that they provide to the producing organisms, evolution has learned how to implement various types of macrocyclization reactions into the different biosynthetic pathways and how to effect them with remarkable ease. Mankind greatly benefits from nature’s pool, not least because naturally occurring macrocycles or derivatives thereof serve as important drugs for the treatment of many serious ailments.

In stark contrast, macrocyclization reactions are usually perceived as difficult to accomplish by purely chemical means. While it is true that ring closure necessarily entails an entropic loss and may result in the buildup of (considerable) ring strain that must be compensated for in one way or the other, it is also fair to note tremendous methodological advances during the last decades that greatly alleviated this traditional “macrocycle challenge”. It is therefore increasingly possible to explore the advantages provided by large as well as medium-size ring systems in a more systematic manner. This venture also holds the promise of increasing the “chemical space” amenable to drug development to a considerable extent.

In consideration of this and other important long-term perspectives, it is appropriate to revisit the current state of the art. To this end, a number of vignettes are presented, each of which summarizes a total synthesis project targeting macrocyclic natural products of greatly different chemotypes using a variety of transformations to reach these goals. Although we were occasionally facing “dead ends”, which are also delineated for the sake of a complete picture, these case studies illustrate the notion that the formation of a certain macrocyclic perimeter is (usually) no longer seriously limiting. In addition to substantial progress in the “classical” repertoire (macrolactonization and macrolactamization (pateamine A, spirastrellolide, and belizentrin)), various metal-catalyzed reactions have arguably led to the greatest leaps forward. Among them, palladium-catalyzed C–C bond formation (roseophilin and nominal xestocyclamine A) and, in particular, alkene and alkyne metathesis stand out (iejimalide, spirastrellolide, enigmazole, ingenamine, and sinulariadiolide). In some cases, different methods were pursued in parallel, thus allowing for a critical assessment and comparison.

To the extent that the macrocyclic challenge is vanishing, the opportunity arises to focus attention on the postmacrocyclization phase. One may stipulate that a well-designed cyclization precursor does not only ensure efficient ring closure but also fosters and streamlines the steps that come after the event. One way to do so is dual (multiple) use in that the functional groups serving the actual cyclization reaction also find productive applications downstream from it rather than being subject to simple defunctionalization. In this context, better insight into the conformational peculiarities of large rings and the growing confidence in their accessibility in a stereochemically well defined format rejuvenate the implementation of transannular reactions or reaction cascades that can lead to rapid and substantial increases in molecular complexity. The examples summarized herein showcase such possibilities, with special emphasis on tranannular gold catalysis and the emerging ruthenium-catalyzed trans-hydrometalation chemistry for the selective functionalization of alkynes.

Marine Natural Products with High Anticancer Activities

This review covers recent literature from 2012-2019 concerning 170 marine natural products and their semisynthetic analogues with strong anticancer biological activities. Reports that shed light on cellular and molecular mechanisms and biological functions of these compounds, thus advancing the understanding in cancer biology are also included. Biosynthetic studies and total syntheses, which have provided access to derivatives and have contributed to the proper structure or stereochemistry elucidation or revision are mentioned. The natural compounds isolated from marine organisms are divided into nine groups, namely: alkaloids, sterols and steroids, glycosides, terpenes and terpenoids, macrolides, polypeptides, quinones, phenols and polyphenols, and miscellaneous products. An emphasis is placed on several drugs originating from marine natural products that have already been marketed or are currently in clinical trials.

Construction of sulfur-containing moieties in the total synthesis of natural products

This review surveys the total syntheses of sulfur-containing natural products where sulfur atoms are introduced with different sulfurization agents to construct related sulfur-containing moieties.

Iron catalyzed stereoselective alkene synthesis: a sustainable pathway

Replacing expensive or toxic transition metals with iron has become an important trend. This article summarises the recent progresses of a wide range of Fe-catalyzed reactions for accessing various stereodefined alkenes.

Iron-Catalyzed Reactions of 2-Pyridone Derivatives: 1,6-Addition and Formal Ring Opening/Cross Coupling

In the presence of simple iron salts, 2‐pyridone derivatives react with Grignard reagents under mild conditions to give the corresponding 1,6‐addition products; if the reaction medium is supplemented with an aprotic dipolar cosolvent after the actual addition step, the intermediates primarily formed succumb to ring opening, giving rise to non‐thermodynamic Z,E‐configured dienoic acid amide derivatives which are difficult to make otherwise. Control experiments as well as the isolation and crystallographic characterization of a (tricarbonyl)iron pyridone complex suggest that the active iron catalyst generated in situ exhibits high affinity to the polarized diene system embedded into the heterocyclic ring system of the substrates, which likely serves as the actual recognition element.

Palladium-Catalyzed Cascade Intramolecular Cyclization and Allylation of Enynoates with Allylic Alcohols

A Pd(II)-catalyzed mild and highly regioselective 6- endo cyclization/allylation reaction of enynoates with simple allylic alcohols has been developed. Under mild reaction conditions, the vinyl palladium species generated in situ after cyclization could insert C-C double bond of allylic alcohol through cross-coupling reaction and lead to the formation of allyl pyrone via β-OH elimination. This cascade cross-coupling reaction represents a direct and atom economic methodology for the construction of novel allyl pyrones in moderate to good yields.

Iron- or Palladium-Catalyzed Reaction Cascades Merging Cycloisomerization and Cross-Coupling Chemistry

A conceptually novel reaction cascade is presented, which allows readily available enynes to be converted into functionalized 1,3-dienes comprising a stereodefined tetrasubstituted alkene unit; such compounds are difficult to make by conventional means. The overall transformation is thought to commence with formation of a metallacyclic intermediate that evolves via cleavage of an unstrained C-X bond in its backbone. This non-canonical cycloisomerization process is followed by a cross-coupling step, such that reductive C-C bond formation regenerates the necessary low-valent metal fragment and hence closes an intricate catalytic cycle. The cascade entails the formation of two new C-C bonds at the expense of the constitutional C-X entity of the substrate: importantly, the extruded group X must not be a heteroelement (X=O, NR), since activated β-C-C bonds can also be broken. This concern was reduced to practice in two largely complementary formats: one procedure relies on the use of alkyl-Grignard reagents in combination with catalytic amounts of Fe(acac)_3, whereas the second method amalgamates cycloisomerization with Suzuki coupling by recourse to arylboronic acids and phosphine-ligated palladium catalysts.

Total Synthesis of Putative Chagosensine

The marine macrolide chagosensine is the only natural product known to date that embodies a Z,Z-configured chloro-1,3-diene unit. This distinguishing substructure was prepared by a sequence of palladium-catalyzed 1,2-distannation of an alkyne precursor, regioselective Stille cross-coupling at the terminus of the resulting bisstannyl alkene with an elaborated alkenyl iodide, followed by chloro-destannation of the remaining internal site. The preparation of the required substrates centered on cobalt-catalyzed oxidative cyclization reactions of hydroxylated olefin precursors, which allowed the 2,5-trans-disubstituted tetrahydrofuran rings, embedded into each building block, to be formed with excellent selectivity. The highly strained macrolactone could ultimately be closed under forcing Yamaguchi conditions. Comparison of the spectral data of the synthetic sample with those of authentic chagosensine methyl ester confirmed that the structure of this intriguing compound has been mis-assigned by the isolation team.

Catalysis-Based Total Syntheses of Pateamine A and DMDA-Pat A

The marine natural product pateamine A (1) and its somewhat simplified designer analogue DMDA-Pat A (2) (DMDA = desmethyl-desamino) are potently cytotoxic compounds; most notably, 2 had previously been found to exhibit a promising differential in vivo activity in xenograft melanoma models, even though the ubiquitous eukaryotic initiation factor 4A (eIF4A) constitutes its primary biological target. In addition, 1 had also been identified as a possible lead in the quest for medication against cachexia, an often lethal muscle wasting syndrome affecting many immunocompromised or cancer patients. The short supply of these macrodiolides, however, rendered a more detailed biological assessment difficult. Therefore, a new synthetic approach to 1 and 2 has been devised, which centers on an unorthodox strategy for the formation of the highly isomerization-prone but essential Z, E-configured dienoate substructure embedded into the macrocyclic core. This motif was encoded in the form of a 2-pyrone ring and unveiled only immediately before macrocyclization by an unconventional iron-catalyzed ring opening/cross-coupling reaction, in which the enol ester entity of the pyrone gains the role of a leaving group. Since the required precursor was readily available by gold catalysis, this strategy rendered the overall sequence short, robust, and scalable. A surprisingly easy protecting group management together with a much improved end game for the formation of the trienyl side chain via a modern Stille coupling protocol also helped to make the chosen route practical. Change of a single building block allowed the synthesis to be redirected from the natural lead compound 1 toward its almost equipotent analogue 2. Isolation and reactivity profiling of pyrone tricarbonyliron complexes provide mechanistic information as well as insights into the likely origins of the observed chemoselectivity.

Iron-Catalyzed Cross-Couplings in the Synthesis of Pharmaceuticals: In Pursuit of Sustainability

The scarcity of precious metals has led to the development of sustainable strategies for metal-catalyzed cross-coupling reactions. The establishment of new catalytic methods using iron is attractive owing to the low cost, abundance, ready availability, and very low toxicity of iron. In the last few years, sustainable methods for iron-catalyzed cross-couplings have entered the critical area of pharmaceutical research. Most notably, iron is one of the very few metals that have been successfully field-tested as highly effective base-metal catalysts in practical, kilogram-scale industrial cross-couplings. In this Minireview, we critically discuss the strategic benefits of using iron catalysts as green and sustainable alternatives to precious metals in cross-coupling applications for the synthesis of pharmaceuticals. The Minireview provides an essential introduction to the fundamental aspects of practical iron catalysis, highlights areas for improvement, and identifies new fields to be explored.

Domino Ring Expansion: Regioselective Access to 9-Membered Lactones with a Fused Indole Unit from 2-Nitrophenyl-1,3-cyclohexanediones

The domino anionic fragmentation of 2-nitrophenyl-1,3-cyclohexanediones containing an electrophilic appendage such as aldehyde and epoxide is disclosed. This reaction, initiated by a series of nucleophiles, involves the generation of an intermediate hydroxylate followed by the regioselective formation and fragmentation of an intermediate lactolate into enolate. This strategy, devoid of any protecting group, enlarges the initial ring and provides an original access to decorated 9-membered lactones with a fused indole unit.

Iron-Catalyzed C-O Bond Activation: Opportunity for Sustainable Catalysis

Oxygen-based electrophiles have emerged as some of the most valuable cross-coupling partners in organic synthesis due to several major strategic and environmental benefits, such as abundance and potential to avoid toxic halide waste. In this context, iron-catalyzed C-O activation/cross-coupling holds particular promise to achieve sustainable catalytic protocols due to its natural abundance, inherent low toxicity, and excellent economic and ecological profile. Recently, tremendous progress has been achieved in the development of new methods for functional-group-tolerant iron-catalyzed cross-coupling reactions by selective C-O cleavage. These methods establish highly attractive alternatives to traditional cross-coupling reactions by using halides as electrophilic partners. In particular, new easily accessible oxygen-based electrophiles have emerged as substrates in iron-catalyzed cross-coupling reactions, which significantly broaden the scope of this catalysis platform. New mechanistic manifolds involving iron catalysis have been established; thus opening up vistas for the development of a wide range of unprecedented reactions. The synthetic potential of this sustainable mode of reactivity has been highlighted by the development of new strategies in the construction of complex motifs, including in target synthesis. The most recent advances in sustainable iron-catalyzed cross-coupling of C-O-based electrophiles are reviewed, with a focus on both mechanistic aspects and synthetic utility. It should be noted that this catalytic manifold provides access to motifs that are often not easily available by other methods, such as the assembly of stereodefined dienes or C(sp² )-C(sp³ ) cross-couplings, thus emphasizing the synthetic importance of this mode of reactivity.

Gold Catalysis for Heterocyclic Chemistry: A Representative Case Study on Pyrone Natural Products

2-Pyrones and 4-pyrones are common structural motifs in bioactive natural products. However, traditional methods for their synthesis, which try to emulate the biosynthetic pathway of cyclization of a 1,3,5-tricarbonyl precursor, are often harsh and, therefore, not particularly suitable for applications to polyfunctionalized and/or sensitive target compounds. π-Acid catalysis, in contrast, has proved to be better for a systematic exploration of the pyrone estate. To this end, alkynes are used as stable ketone surrogates, which can be activated under exceedingly mild conditions due to the pronounced carbophilicity of [LAu]⁺ fragments (L=two electron donor ligand); attack of a tethered ester carbonyl group onto the transient alkyne-gold complex then forges the pyrone ring in a fully regiocontrolled manner.

Iron Catalysis in Organic Synthesis: A Critical Assessment of What It Takes To Make This Base Metal a Multitasking Champion

The current status of homogeneous iron catalysis in organic chemistry is contemplated, as are the reasons why this particular research area only recently starts challenging the enduring dominance of the late and mostly noble metals over the field. Centered in the middle of the d-block and able to support formal oxidation states ranging from -II to +VI, iron catalysts hold the promise of being able to encompass organic synthesis at large. They are expected to serve reductive as well as oxidative regimes, can emulate "noble tasks", but are also able to adopt "early" transition metal character. Since a comprehensive coverage of this multidimensional agenda is beyond the scope of an Outlook anyway, emphasis is laid in this article on the analysis of the factors that perhaps allow one to control the multifarious chemical nature of this earth-abundant metal. The challenges are significant, not least at the analytical frontier; their mastery mandates a mindset that differs from the routines that most organic chemists interested in (noble metal) catalysis tend to cultivate. This aspect notwithstanding, it is safe to predict that homogeneous iron catalysis bears the chance to enable a responsible paradigm for chemical synthesis and a sustained catalyst economy, while potentially providing substantial economic advantages. This promise will spur the systematic and in-depth investigations that it takes to upgrade this research area to strategy-level status in organic chemistry and beyond.