A concise synthesis of d,l-brevianamide B via a biomimetically-inspired IMDA construction

Pd-Catalyzed Asymmetric Allylation Reaction of 2-Aryl-3H-indol-3-ones with Allyltrimethylsilane

An efficient method for the first ene-reaction of 2-aryl-3H-indol-3-ones with allyltrimethylsilane has been developed for the first time. The reaction proceeded under the catalysis of Pd(OAc)2 and chiral phosphoric ligand L11 in the presence of Cu(CF3COO)2·XH2O, PivOH, and 5 Å molecular sieves in DMSO at 60 °C. The present methodology can avoid the impact of amine products generated by the reaction on the catalyst, and at the same time, the high catalytic activity of classical palladium catalysts still has catalytic ability for low electrophilic keto-imines. The desired products were furnished in excellent yields with good enantioselectivity.

An efficient method to access spiro pseudoindoxyl ketones: evaluation of indoxyl and their N-benzylated derivatives for inhibition of the activity of monoamine oxidases

A simple, metal-free approach was developed to obtain novel pseudoindoxyl derivatives. The reaction was mediated by tBuOK on tetrahydrocarbazole 8 in dimethyl sulfoxide (DMSO) at room temperature through the hydroxylation of the indole double bond and a subsequent pinacol-type rearrangement. Spiro pseudoindoxyl compounds and their N-benzylated derivatives were assessed for their inhibitory activities against monoamine oxidase (MAO) enzymes. Based on half-maximal inhibitory concentration (IC50) values, 13 compounds were found to have higher inhibitory activity against MAO-B than against MAO-A. With regard to MAO-B inhibition, 11f showed the best inhibitory activity, with an IC50 value of 1.44 μM, followed by 11h (IC50 = 1.60 μM), 11j (IC50 = 2.78 μM), 11d (IC50 = 2.81 μM), and 11i (IC50 = 3.02 μM). Compound 11f was a competitive inhibitor with a Ki value of 0.51 ± 0.023 μM. In a reversibility experiment using dialysis, 11f showed effective recovery of MAO-B inhibition similar to that of safinamide. These experiments suggested that 11f was a potent, reversible, and competitive inhibitor of MAO-B activity.

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.

Expedited Total Synthesis of (±)-Brevianamide A via the Strategic Use of Gold(I) Catalysis

Two concise and complementary routes to the polycyclic alkaloid (±)-brevianamide A from readily available amino acid building blocks are presented. Key to the synthesis is the strategic use of a gold(I)-catalyzed cascade process that quickly assembles the characteristic pseudoindoxyl motif of the natural product along with the two adjacent quaternary centers in a single step. This sequence, which exemplifies the structural complexity that can be achieved with gold catalysis, allowed for the shortest and highest-yield synthesis of (±)-brevianamide A to date (four steps LLS, 14% overall yield).

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.

Unified total synthesis of the brevianamide alkaloids enabled by chemical investigations into their biosynthesis

The bicyclo[2.2.2]diazaoctane alkaloids are a vast group of natural products which have been the focus of attention from the scientific community for several decades. This interest stems from their broad range of biological activities, their diverse biosynthetic origins, and their topologically complex structures, which combined make them enticing targets for chemical synthesis. In this article, full details of our synthetic studies into the chemical feasibility of a proposed network of biosynthetic pathways towards the brevianamide family of bicyclo[2.2.2]diazaoctane alkaloids are disclosed. Insights into issues of reactivity and selectivity in the biosynthesis of these structures have aided the development of a unified biomimetic synthetic strategy, which has resulted in the total synthesis of all known bicyclo[2.2.2]diazaoctane brevianamides and the anticipation of an as-yet-undiscovered congener.

Total synthesis of the pseudoindoxyl class of natural products

The pseudoindoxyl sub-structural motif, amongst the large set of the indole class of alkaloids, represents a unique subset of the oxygenated indole class of the alkaloid family. A majority of this class of natural products contains complex bridged/polycyclic scaffolds with interesting biological profiles. They are thus attractive synthetic targets. Starting from 1963, twenty-eight natural products having the pseudoindoxyl scaffold have been isolated, among which the synthesis of 13 natural products has been accomplished. In this review, we highlight the completed as well as the formal total synthesis of the natural products with a spiro-pseudoindoxyl ring, with a focus on their development. The challenges and the future perspective based on the recent developments in the field will also be discussed. We strongly believe that this review will not only update but also attract the attention of researchers in dealing with the synthesis of pseudoindoxyl compounds.

A Review of the Synthetic Strategies toward Dihydropyrrolo[1,2-a]Pyrazinones

Dihydropyrrolo[1,2-a]pyrazinone rings are a class of heterocycles present in a wide range of bioactive natural products and analogues thereof. As a direct result of their bioactivity, the synthesis of this privileged class of compounds has been extensively studied. This review provides an overview of these synthetic pathways. The literature is covered up until 2020 and is organized according to the specific strategies used to construct the scaffold: fusing a pyrazinone to an existing pyrrole, employing a pyrazinone-first strategy, an array of multicomponent reactions and some miscellaneous reactions.

Enantioselective, Visible Light Mediated Aza Paternò-Büchi Reactions of Quinoxalinones

3-Substituted quinoxalin-2(1H)-ones and various aryl-substituted or tethered olefins underwent an enantioselective, inter- or intramolecular aza Paternò-Büchi reaction upon irradiation at λ=420 nm in the presence of a chiral sensitizer (10 mol %). For the intermolecular reaction with 1-arylethenes as olefin components, the scope of the reaction was studied (14 examples, 50-99 % yield, 86-98 % ee). The absolute and relative configuration of the products were elucidated by single-crystal X-ray crystallography. The reaction is suggested to occur by triplet energy transfer in a hydrogen-bonded 1:1 complex between the imine substrate and the catalyst. The intramolecular cycloaddition, consecutive reactions of the product azetidines, and an alternative reaction mode of quinoxalinones were investigated in preliminary experiments.

Tryptophan derived natural marine alkaloids and synthetic derivatives as promising antimicrobial agents

Antimicrobial resistance has become a major threat to public health worldwide, as pathogenic microorganisms are finding ways to evade all known antimicrobials. Therefore, the demand for new and effective antimicrobial agents is also increasing. Natural products have always played an important role in drug discovery, either by themselves or as inspiration for synthetic compounds. The marine environment is a rich source of bioactive metabolites, and among them, tryptophan-derived alkaloids stand out for their abundance and by displaying a variety of biological activities, with antimicrobial properties being among the most significant. This review aims to reveal the potential of marine alkaloids derived from tryptophan as antimicrobial agents. Relevant examples of these compounds and their synthetic analogues reported in the last decades are presented and discussed in detail, with their mechanism of action and synthetic approaches whenever relevant. Several tryptophan-derived marine alkaloids have shown potent and promising antimicrobial activities, whether against bacteria, fungi, or virus. Synthetic approaches to many of the compounds have been developed and recent methodologies are proving to be efficient. Even though most of the studies regarding the antimicrobial activity are still preliminary, this class of compounds has proven to be worth of further investigation and may provide useful lead compounds for the development of antimicrobial agents. Overall, marine alkaloids derived from tryptophan are revealed as a valuable class of antimicrobials and molecular modifications in order to reduce the toxicity of these compounds and additional studies regarding their mechanism of action are interesting topics to explore in the future.

Flavin-Dependent Monooxygenases NotI and NotI' Mediate Spiro-Oxindole Formation in Biosynthesis of the Notoamides

The fungal indole alkaloids are a unique class of complex molecules that have a characteristic bicyclo[2.2.2]diazaoctane ring and frequently contain a spiro-oxindole moiety. While various strains produce these compounds, an intriguing case involves the formation of individual antipodes by two unique species of fungi in the generation of the potent anticancer agents (+)- and (-)-notoamide A. NotI and NotI' have been characterized as flavin-dependent monooxygenases that catalyze epoxidation and semi-pinacol rearrangement to form the spiro-oxindole center within these molecules. This work elucidates a key step in the biosynthesis of the notoamides and provides an evolutionary hypothesis regarding a common ancestor for production of enantiopure notoamides.

Fungal-derived brevianamide assembly by a stereoselective semipinacolase

Fungal bicyclo[2.2.2]diazaoctane indole alkaloids represent an important family of natural products with a wide-spectrum of biological activities. Although biomimetic total syntheses of representative compounds have been reported, the details of their biogenesis, especially the mechanisms for assembly of diastereomerically distinct and enantiomerically antipodal metabolites, have remained largely uncharacterized. Brevianamide A represents a basic form of the sub-family bearing a dioxopiperazine core and a rare 3-spiro-ψ-indoxyl skeleton. Here, we identified the Brevianamide A biosynthetic gene cluster from Penicillium brevicompactum NRRL 864 and elucidated the metabolic pathway. BvnE was revealed to be an essential isomerase/semi-pinacolase that specifies selective production of the natural product. Structural elucidation, molecular modeling, and mutational analysis of BvnE, and quantum chemical calculations provided mechanistic insights into the diastereoselective formation of the 3-spiro-ψ-indoxyl moiety in Brevianamide A. This occurs through a BvnE-controlled semi-pinacol rearrangement and a subsequent spontaneous intramolecular [4+2] hetero-Diels-Alder cycloaddition.

Structural and stereochemical diversity in prenylated indole alkaloids containing the bicyclo[2.2.2]diazaoctane ring system from marine and terrestrial fungi

Covering: up to February 2017 Various fungi of the genera Aspergillus, Penicillium, and Malbranchea produce prenylated indole alkaloids possessing a bicyclo[2.2.2]diazaoctane ring system. After the discovery of distinct enantiomers of the natural alkaloids stephacidin A and notoamide B, from A. protuberus MF297-2 and A. amoenus NRRL 35660, another fungi, A. taichungensis, was found to produce their diastereomers, 6-epi-stephacidin A and versicolamide B, as major metabolites. Distinct enantiomers of stephacidin A and 6-epi-stephacidin A may be derived from a common precursor, notoamide S, by enzymes that form a bicyclo[2.2.2]diazaoctane core via a putative intramolecular hetero-Diels-Alder cycloaddition. This review provides our current understanding of the structural and stereochemical homologies and disparities of these alkaloids. Through the deployment of biomimetic syntheses, whole-genome sequencing, and biochemical studies, a unified biogenesis of both the dioxopiperazine and the monooxopiperazine families of prenylated indole alkaloids constituted of bicyclo[2.2.2]diazaoctane ring systems is presented.

A facile approach to 2-alkoxyindolin-3-one and its application to the synthesis of N-benzyl matemone

2-Alkoxycarbonylindolin-3-one is synthesized from a methoxyglycine derivative via a 1,2-aza-Brook rearrangement followed by cyclization with bis(trimethylsilyl)aluminum chloride. A short-step synthesis of N-benzyl matemone is successfully carried out using the present indolin-3-one synthesis.

A short-step synthesis of N-benzyl matemone has been successfully carried out using a newly discovered 2-ethoxycarbonylindolin-3-one synthesis.

Pot-Economy Autooxidative Condensation of 2-Aryl-2-lithio-1,3-dithianes

The autoxidative condensation of 2-aryl-2-lithio-1,3-dithianes is here reported. Treatment of 2-aryl-1,3-dithianes with n-BuLi in the absence of any electrophile leads to condensation of three molecules of 1,3-dithianes and formation of highly functionalized α-thioether ketones orthothioesters in 51-89% yields upon air exposure. The method was further expanded to benzaldehyde dithioacetals, affording corresponding orthothioesters and α-thioether ketones in 48-97% yields. The experimental results combined with density functional theory studies support a mechanism triggered by the autoxidation of 2-aryl-2-lithio-1,3-dithianes to yield a highly reactive thioester that undergoes condensation with two other molecules of 2-aryl-2-lithio-1,3-dithiane.

Further Investigation of the Intermolecular Diels-Alder Cycloaddition for the Synthesis of Bicyclo[2.2.2]diazaoctane Alkaloids

The convergent synthesis of bicyclo[2.2.2]diazaoctane structures using an intermolecular Diels-Alder cycloaddition between a pyrazinone and commercially available fumarate or maleate precursors is reported. High reactivity and stereoselection is observed with both dienophile substrates. Structure validation was achieved by conversion of cycloadducts into known [2.2.2]diazabicyclic compounds or into crystalline derivatives suitable for X-ray analysis. The cycloadduct derived from reaction of pyrazinone and maleic anhydride underwent selective anhydride ring opening and intersected an established precursor in the synthesis of brevianamide B.

Asymmetric amination of 2-substituted indolin-3-ones catalyzed by natural cinchona alkaloids

The natural quinine-catalyzed enantioselective amination of 2-substituted indolin-3-ones with azodicarboxylates has been developed.

Intermolecular Diels-Alder Cycloaddition for the Construction of Bicyclo[2.2.2]diazaoctane Structures: Formal Synthesis of Brevianamide B and Premalbrancheamide

A stereoselective intermolecular Diels-Alder cycloaddition of an intermediate pyrazinone with both achiral and chiral acrylate-derived dienophiles provides rapid access to the bicyclo[2.2.2]diazaoctane core shared among several prenylated indole alkaloids. The product derived from cycloaddition with 2-nitroacrylate required an additional five to six synthetic operations to intercept established precursors to premalbrancheamide and brevianamide B. The chemistry detailed in this manuscript constitutes a formal total synthesis (12 steps each) of these [2.2.2]diazabicyclic natural products from proline methyl ester.

Metal-free regioselective construction of indolin-3-ones via hypervalent iodine oxidation of N-substituted indoles

A metal-free method for regioselective construction of 2-acetoxy indolin-3-ones from N-substituted indoles using PhI(OAc)₂ was developed under mild reaction conditions.

Modular synthesis of cell-permeating 2-ketoglutarate esters

Cell-permeating esters of 2-ketoglutarate (2-KG) have been synthesized through a convergent sequence from two modules in two and three steps, respectively. This route provides access to a full series of mono- and disubstituted 2-KG esters, enabling us to define the effect of regioisomeric masking on metabolite release and antihypoxic activity in cell-based assays. In addition to providing insight into the biological activity of cell permeable 2-KG esters, the straightforward and modular nature of this synthetic route may prove useful for the development of next-generation 2-KG analogues for diagnostic and therapeutic applications.

A cycloisomerization/Friedel-Crafts alkylation strategy for the synthesis of pyrano[3,4-b]indoles

The synthesis of pyrano[3,4-b]indoles is described. The reaction sequence involves Sonogashira coupling of dihydropyran propargyl ether scaffolds with iodoanilines to afford intermediate indoles. Lewis acid-catalyzed ionization of the dihydropyrans, followed by intramolecular C3 alkylation of the indole, provides the title compounds.

Biosynthetic studies of the notoamides: isotopic synthesis of stephacidin A and incorporation into notoamide B and sclerotiamide

The advanced natural product stephacidin A is proposed as a biosynthetic precursor to notoamide B in various Aspergillus species. Doubly (13)C-labeled racemic stephacidin A was synthesized and fed to cultures of the terrestrial-derived fungus, Aspergillus versicolor NRRL 35600, and the marine-derived fungus, Aspergillus sp. MF297-2. Analysis of the metabolites revealed enantiospecific incorporation of intact (-)-stephacidin A into (+)-notoamide B in Aspergillus versicolor and (+)-stephacidin A into (-)-notoamide B in Aspergillus sp. MF297-2. (13)C-Labeled sclerotiamide was also isolated from both fungal cultures.

Natural products synthesis: enabling tools to penetrate Nature's secrets of biogenesis and biomechanism

Selected examples from our laboratory of how synthetic technology platforms developed for the total synthesis of several disparate families of natural products was harnessed to penetrate biomechanistic and/or biosynthetic queries is discussed. Unexpected discoveries of biomechanistic reactivity and/or penetrating the biogenesis of naturally occurring substances were made possible through access to substances available only through chemical synthesis. Hypothesis-driven total synthesis programs are emerging as very useful conceptual templates for penetrating and exploiting the inherent reactivity of biologically active natural substances. In many instances, new enabling synthetic technologies were required to be developed. The examples demonstrate the often untapped richness of complex molecule synthesis to provide powerful tools to understand, manipulate and exploit Nature's vast and creative palette of secondary metabolites.

Asymmetric total syntheses of (+)- and (-)-versicolamide B and biosynthetic implications

The Diels-Alder reaction is one of the most well-studied, synthetically useful organic transformations. While a significant number of naturally occurring substances are postulated to arise by biosynthetic Diels-Alder reactions, rigorous confirmation of a mechanistically distinct natural Diels-Alderase enzyme remains elusive. Within this context, several related fungi within the Aspergillus genus produce a number of metabolites of opposite absolute configuration including (+)- or (-)-versicolamide B. These alkaloids are hypothesized to arise via biosynthetic Diels-Alder reactions implying that each Aspergillus species possesses enantiomerically distinct Diels-Alderases. Herein, experimental validation of these biosynthetic proposals via deployment of the IMDA reaction as a key step in the asymmetric total syntheses of (+)- and (-)-versicolamide B is described. Laboratory validation of the proposed biosynthetic Diels-Alder construction, coupled with the secondary metabolite profile of the producing fungi, reveals that each Aspergillus species has evolved enantiomerically distinct indole oxidases, as well as enantiomerically distinct Diels-Alderases.

STUDIES ON THE BIOSYNTHESIS OF THE STEPHACIDINS AND NOTOAMIDES. TOTAL SYNTHESIS OF NOTOAMIDE S

Notoamide S has been suggested to be the final common precursor between two different Aspergillus sp. fungal strains before diverging to form enantiomerically opposite natural products (+)- and (-)-stephacidin A and (+)- and (-)-notoamide B. The synthesis of notoamide S comes from the coupling of N-Fmoc proline with a 6-hydroxy-7-prenyl-2-reverse prenyl tryptophan derivative that was synthesized via a late stage Claisen rearrangement from a 6-propargyl-2-reverse prenylated indole.

Synthetic approaches to the bicyclo[2.2.2]diazaoctane ring system common to the paraherquamides, stephacidins and related prenylated indole alkaloids

The bicyclo[2.2.2]diazaoctane ring system is common to a number of highly biologically active secondary metabolites isolated from numerous species of fungi. In this tutorial review, we describe the varied synthetic approaches that have been employed to construct this ring system in the course of recent total synthesis endeavors, and this review should be of interest to synthetic organic chemists and natural product chemists. Detailed herein are a number of synthetic disconnections including intramolecular S(N)2' cyclizations, biomimetic Diels-Alder reactions, radical cyclizations, and cationic cascade reactions.

Studies on Paraherquamide Biosynthesis: Synthesis of Deuterium-Labeled 7-Hydroxy-Pre-Paraherquamide, a Putative Precursor of Paraherquamides A, E & F

The stereocontrolled, asymmetric synthesis of triply deuterium-labeled 7-hydroxy-pre-paraherquamide (27) was accomplished, employing a diastereoselective intramolecular S(N)2' cyclization strategy. The deuterium-labeled substrate was interrogated in a precursor incorporation experiment in the paraherquamide-producing organism Penicillium fellutanum. The isolated sample of paraherquamide A revealed incorporation of one of the two geminal deuterons of the CD(2)-group at C-12 exclusively. The lack of signals for the second deuteron of the CD(2)-group at C-12 and for the CH(2)D-group (C-22/C-23) suggests that this substrate suffered an unexpectedly selective catabolic degradation and metabolic re-incorporation of deuterium thus casting doubt on the proposed biosynthetic intermediacy of 27. Consideration of alternative biosynthetic pathways, including oxidation of the indole C-6 position prior to hydroxylation at C-7 or oxidative spiro-contraction of pre-paraherquamide prior to construction of the dioxepin is discussed. The synthesis of 27 also provides for a concise, asymmetric stereocontrolled synthesis of an advanced intermediate that will be potentially useful in the synthesis of paraherquamide E & F.

Calmodulin inhibitory activity of the malbrancheamides and various analogs

The preparation and biological activity of various structural analogs of the malbrancheamides are disclosed. The impact of indole chlorination, C-12a relative stereochemistry, and bicyclo[2.2.2]diazaoctane core oxidation state on the ability of these analogs to inhibit calmodulin dependent phosphodiesterase (PDE1) was studied, and a number of potent compounds were identified.

Premalbrancheamide: synthesis, isotopic labeling, biosynthetic incorporation, and detection in cultures of Malbranchea aurantiaca

An advanced metabolite, named premalbrancheamide, involved in the biosynthesis of malbrancheamide (1) and malbrancheamide B (2) has been synthesized in double (13)C-labeled form and was incorporated into the indole alkaloid 2 by Malbranchea aurantiaca. In addition, premalbrancheamide has been detected as a natural metabolite in cultures of M. aurantiaca. The biosynthetic implications of these experiments are discussed.