Improved Biomimetic Total Synthesis of d,l-Stephacidin A

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.

Inverse Electron-Demanding Diels-Alder Reactions in the Chemical Synthesis of Prenylated Indole Alkaloids Containing a Bicycle[2.2.2]diazaoctane Moiety: A Theoretical Study

The Diels-Alder reaction is believed to be a key step in the biosynthesis of prenylated indole alkaloids containing a bicycle[2.2.2]diazaoctane moiety. Many chemical syntheses of bicyclic structures by Diels-Alder reactions have been reported, but the reaction mechanism remains underexplored. We have carried out DFT calculations on both acid- and base-promoted Diels-Alder reactions in these syntheses and reveal that the reactions occur through an inverse-electron demand mechanism. We hope that the new mechanism is helpful for the mechanistic understanding of the biosynthesis of this class of important 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).

Synthesis of 6,6- and 7,7-difluoro-1-acetamidopyrrolizidines and their oxidation catalyzed by the nonheme Fe oxygenase LolO

LolO, a 2-oxoglutarate-dependent nonheme Fe oxygenase, catalyzes both the hydroxylation and cycloetherification of 1- exo -acetamidopyrrolizidine (AcAP), a pathway intermediate in the biosynthesis of the loline alkaloids. We have prepared fluorinated AcAP analogs to aid in continued mechanistic investigation of the unusual LolO-catalyzed cycloetherification step. LolO was able to first hydroxylate and then cycloetherify 6,6-difluoro-AcAP (prepared from N , O -protected 4-oxoproline), forming a difluorinated analog of N -acetylnorloline (NANL) and providing evidence for a cycloetherification mechanism involving a C(7) radical as opposed to a C(7) carbocation. By contrast, LolO was able to hydroxylate 7,7-difluoro-AcAP (prepared from 3-oxoproline) but failed to cycloetherify it, forming (1 R , 2 R , 8 S )-7,7-difluoro-2-hydroxy-AcAP as the sole product. Because it completely blocks the cycloetherification step, 7,7-difluoro-AcAP has the potential to become an important tool for accumulating and characterizing the LolO intermediate responsible for catalyzing cycloetherification of 2-hydroxy-AcAP.

Natural product anticipation through synthesis

Natural product synthesis remains one of the most vibrant and intellectually rewarding areas of chemistry, although the justifications for pursuing it have evolved over time. In the early years, the emphasis lay on structure elucidation and confirmation through synthesis, as exemplified by celebrated studies on cocaine, morphine, strychnine and chlorophyll. This was followed by a phase where the sheer demonstration that highly complex molecules could be recreated in the laboratory in a rational manner was enough to justify the economic expense and intellectual agonies of a synthesis. Since then, syntheses of natural products have served as platforms for the demonstration of elegant strategies, for inventing new methodology 'on the fly' or to demonstrate the usefulness and scope of methods established with simpler molecules. We now add another aspect that we find fascinating, viz. 'natural product anticipation'. In this Review, we survey cases where the synthesis of a compound in the laboratory has preceded its isolation from nature. The focus of our Review lies on examples where this anticipation of a natural product has triggered a successful search or where synthesis and isolation have occurred independently. Finally, we highlight cases where a potential natural product structure has been suggested as a result of synthetic endeavours but not yet confirmed by isolation, inviting further collaborations between synthetic and natural product chemists.

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.

Progress in recent development of stereoselective synthesis of β2-amino acid derivatives from β-nitroacrylate derivatives

The synthesis and properties of β-amino acids have drawn considerable attention owing to their ubiquitous presence in naturally occurring products of biological importance. While β3-amino acids can be readily prepared from α-amino acids via one-carbon homologation, the synthesis of β2-amino acids generally requires multistep efforts. This review focuses on the recent advances in the synthesis of β2-amino acids and their derivatives from substituted β-nitropropionate analogues obtained from the stereoselective transformation of β-nitroacrylates.

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.

Solvent-Controlled, Tunable Domino Reaction of 3-Ylideneoxindoles with in Situ-Generated α-Aryldiazomethanes: A Facile Access to 3-Spirocyclopropyl-2-oxindole and Pyrazoloquinazolinone Scaffolds

A mild and efficient solvent-controlled, metal-free switchable 1,3-dipolar cycloaddition/ring contraction or ring expansion domino reaction of 3-ylideneoxindoles with in situ-generated α-aryldiazomethanes has been developed. This domino reaction provided a series of aryl-substituted 3-spirocyclopropyl-2-oxindoles and pyrazoloquinazolinones with excellent regio- and diastereoselectivity from common substrates under varying solvent conditions.

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.

Nine-step total synthesis of (−)-strychnofoline

The asymmetric synthesis of an anti-cancer spirooxindole alkaloid, (−)-strychnofoline, has, for the first time, been accomplished in only nine steps.

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.

Penicimutamides D–E: two new prenylated indole alkaloids from a mutant of the marine-derived Penicillium purpurogenum G59

Two new prenylated indole alkaloids, penicimutamides D–E (1–2), were discovered via activating silent pathways in a marine-derived fungus.

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.

A concise total synthesis of sespenine, a structurally unusual indole terpenoid from Streptomyces

A ten-step (the longest linear sequence) total synthesis of sespenine was accomplished.

Synthesis of models of the BC ring systems of MPC1001 and MPC1001F

Piperazinedione and piperazinetrione systems representing the BC rings of several members of the MPC1001 family of fungal metabolites were synthesized.

Penicimutamides A–C: rare carbamate-containing alkaloids from a mutant of the marine-derived Penicillium purpurogenum G59

Three new and rare carbamate-containing penicimutamides A–C (1–3) were discovered via activating silent pathways in a marine-derived fungus.

Rapid access to spirocyclic oxindole alkaloids: application of the asymmetric palladium-catalyzed [3 + 2] trimethylenemethane cycloaddition

The marcfortines are complex secondary metabolites that show potent anthelmintic activity and are characterized by the presence of a bicyclo[2.2.2]diazaoctane fused to a spirooxindole. Herein, we report the synthesis of two members of this family. The synthesis of marcfortine B utilizes a carboxylative TMM cycloaddition to establish the spirocyclic core, followed by an intramolecular Michael addition and oxidative radical cyclization to access the strained bicyclic ring system. In addition, the first asymmetric synthesis of (−)-marcfortine C is described. The key step involves a cyano-substituted TMM cycloaddition, which proceeds in nearly quantitative yield with high diastereo- and enantioselectivity. The resulting chiral center was used to establish all remaining stereocenters in the natural product.

Bioinspired total synthesis of sespenine

The first total synthesis of sespenine, a rare indole sesquiterpenoid from a mangrove endophyte, has been accomplished. A bioinspired aza-Prins/Friedel-Crafts/retro Friedel-Crafts cascade reaction assembles the bridged tetrahydroquinoline core. Further investigations on the aza-Prins cyclization imply that the C3 configuration of the hydroxyindolenine intermediate is crucial to the biosynthesis of sespenine and its congener xiamycin A.

The cascade radical cyclisation approach to prenylated alkaloids: synthesis of stephacidin A and notoamide B

A strategy for the synthesis of members of the prenylated indole alkaloid family is described, which involves a radical cascade process of an appropriately substituted diketopiperazine (DKP) core structure. Several approaches to the generation of the initial radical were explored, with the most successful involving treatment of a sulfenyl substituted DKP under classical reductive conditions by heating with Bu3SnH and a radical initiator. The required, fully substituted, radical precursor DKP structures were prepared using regio- and stereocontrolled enolate chemistry of simpler proline-tryptophan derived DKPs. The new approach allowed rapid access to a key polycyclic indoline structure, which was converted into either of the natural products stephacidin A or notoamide B.

Synthesis of functionalized indolizidines through Pauson-Khand cycloaddition of 2-allylpyrrolidines

A concise entry to functionalized indolizidine scaffolds through a domino 2-aza-Cope-[3 + 2] dipolar cycloaddition and Pauson-Khand [2 + 2 + 1] cyclization has been accomplished. The process was conducted under mild conditions to afford diverse indolizidine systems as single diastereomers in good overall yields.

Fungal origins of the bicyclo[2.2.2]diazaoctane ring system of prenylated indole alkaloids

Over eight different families of natural products consisting of nearly 70 secondary metabolites that contain the bicyclo[2.2.2]diazaoctane ring system have been isolated from various Aspergillus, Penicillium, and Malbranchea species. Since 1968, these secondary metabolites have been the focus of numerous biogenetic, synthetic, taxonomic, and biological studies and, as such, have made a lasting impact across multiple scientific disciplines. This review covers the isolation, biosynthesis, and biological activity of these unique secondary metabolites containing the bridging bicyclo[2.2.2]diazaoctane ring system. Furthermore, the diverse fungal origin of these natural products is closely examined and, in many cases, updated to reflect the currently accepted fungal taxonomy.

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 Stephacidin and Notoamide Natural Products: A Stereochemical and Genetic Conundrum

The stephacidin and notoamide natural products belong to a group of prenylated indole alkaloids containing a bicyclo[2.2.2]diazaoctane core. Biosynthetically, this bicyclic core is believed to be the product of an intermolecular Diels- Alder (IMDA) cycloaddition of an achiral azadiene. Since all of the natural products in this family have been isolated in enantiomerically pure form to date, it is believed that an elusive Diels-Alderase enzyme mediates the IMDA reaction. Adding further intrigue to this biosynthetic puzzle is the fact that several related Aspergillus fungi produce a number of metabolites with the opposite absolute configuration, implying that these fungi have evolved enantiomerically distinct Diels-Alderases. We have undertaken a program to identify every step in the biogenesis of the stephacidins and notoamides, and by combining the techniques of chemical synthesis and biochemical analysis we have been able to identify the two prenyltransferases involved in the early stages of the stephacidin and notoamide biosyntheses. This has allowed us to propose a modified biosynthesis for stephacidin A, and has brought us closer to our goal of finding evidence for, or against, the presence of a Diels-Alderase in this biosynthetic pathway.

Indole prenylation in alkaloid synthesis

Important biologically active indole alkaloids are decorated with prenyl (3,3-dimethylallyl) and tert-prenyl (1,1-dimethylallyl) groups. Covering the literature until the end of 2010, this review article comprehensively summarises and discusses the currently available technologies of prenylation and tert-prenylation of indoles, which have been applied in natural products total syntheses or could be applied there in the near future. We focus on those procedures which introduce the C(5) units in one step, organised according to the indole position to be functionalised. Key strategies include electrophilic and nucleophilic prenylation and tert-prenylation, prenyl and tert-prenyl rearrangements, transition metal-mediated reactions and enzymatic methods.

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.