Total Synthesis of VM55599. Utilization of an Intramolecular Diels−Alder Cycloaddition of Potential Biogenetic Relevance

Transition-Metal-Free Base-Promoted Deaminative Coupling of Gramines with Aminomaleimides

The direct utilization of amines for C-C bond formation without prefunctionalization remains a significant challenge. Herein, we report the base-promoted deaminative coupling of gramines with aminomalaimides under redox-neutral conditions. In this operationally simple reaction, a series of indolmethyl-substituted aminomaleimides that emitted fluorescence were synthesized in good-to-excellent yields. Biological evaluation revealed that some products exhibited antiproliferative activity against human cancer cell lines.

π-Facial Stereoselectivity in Acyl Nitroso Cycloadditions to 5,5-Unsymmetrically Substituted Cyclopentadienes: Computational Exploration of Origins of Selectivity and the Role of Substituent Conformations on Selectivity

The π-facial selectivity of Diels-Alder cycloadditions of 5-monosubstituted cyclopentadienes is known experimentally and has been extensively studied computationally. Previous studies on 5-monosubstituted cyclopentadienes by the Burnell and Houk groups showed that facial selectivity arises principally from hyperconjugative aromaticity or antiaromaticity of polar groups that cause distortion of the cyclopentadiene; steric effects of nonpolar groups can also be important. We have now explored the stereoselective cycloaddition of 5,5-unsymmetrically substituted cyclopentadienes to an acyl nitroso dienophile reported by Kan and co-workers. Computational studies with M06-2X/6-311+G(d,p) indicate that the stereoselectivity in the cycloadditions of 5,5-unsymmetrically substituted cyclopentadienes is not just a simple combination of effects found for monosubstituted counterparts. Substituent conformations and diene-dienophile steric and electronic interaction effects all influence stereoselectivity. Predictions are made about several as-yet-unstudied cyclopentadiene cycloadditions.

A unified strategy to reverse-prenylated indole alkaloids: total syntheses of preparaherquamide, premalbrancheamide, and (+)-VM-55599

A full account of our studies toward reverse-prenylated indole alkaloids that contain a bicyclo[2.2.2]core is described. A divergent route is reported which has resulted in the synthesis of preparaherquamide, (+)-VM-55599, and premalbrancheamide. An intramolecular Dieckmann cyclization between an enolate and isocyanate was used to forge the bicyclo[2.2.2]diazaoctane core that is characteristic of these molecules. The pentacyclic indole scaffold was constructed through a one-pot Hofmann rearrangement followed by Fischer indole synthesis. The utilization of our previously reported indole peripheral functionalization strategy also led to natural products including malbrancheamides B, C, stephacidin A, notoamides F, I and R, aspergamide B, and waikialoid A. Ultimately, the divergent route that we devised provided access to a wide range of prenylated indole alkaloids that are differently substituted on the cyclic amine core.

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.

First total synthesis of ent-asperparaline C and assignment of the absolute configuration of asperparaline C

The first asymmetric total synthesis of a member of the asperparaline family was accomplished and the unknown absolute configuration of asperparaline C has been determined to be all-(S).

Total Synthesis of Tryprostatin B: Synthesis and Asymmetric Phase-Transfer-Catalyzed Reaction of Prenylated Gramine Salt

A concise and efficient total synthesis of microtubule inhibitor tryprostatin B (1) is described. The key step is the preparation of a diprenylated gramine salt 9a. In this step, the prenyl group is incorporated at the 2-position of the indole moiety by direct lithiation of the Boc-protected gramine. We also developed and optimized the asymmetric phase-transfer-catalyzed reaction with salt 9a to provide the C2-prenyl tryptophan intermediate 2 resulting in 93% enantiomeric excess (ee) and 65% yield. The total synthesis of 1 is done in six steps with 35% overall yield.

Anti-hepatitis C Virus Natural Product from a Fungus, Penicillium herquei

New diazabicyclo[2.2.2]octane derivatives, peniciherquamides A-C (1-3), and a novel herqueinone derivative, neoherqueinone (5), were isolated from a fungal culture broth of Penicillium herquei. The structures of these novel compounds were determined by interpretation of spectroscopic data (1D/2D NMR, MS, and IR). Four known compounds, preparaherquamide (4), peniciherqueinone (6), and herqueinone/isoherqueinone (7/7a), were also obtained. The isolated compounds were tested for anti-hepatitis C virus (HCV) activity, and peniciherquamide C (3) was found to display an IC50 value of 5.1 μM. To our knowledge, this is the first report of a diazabicyclo[2.2.2]octane derivative with anti-HCV activity.

Comparative analysis of the biosynthetic systems for fungal bicyclo[2.2.2]diazaoctane indole alkaloids: the (+)/(-)-notoamide, paraherquamide and malbrancheamide pathways

The biosynthesis of fungal bicyclo[2.2.2]diazaoctane indole alkaloids with a wide spectrum of biological activities have attracted increasing interest. Their intriguing mode of assembly has long been proposed to feature a non-ribosomal peptide synthetase, a presumed intramolecular Diels-Alderase, a variant number of prenyltransferases, and a series of oxidases responsible for the diverse tailoring modifications of their cyclodipeptide-based structural core. Until recently, the details of these biosynthetic pathways have remained largely unknown due to lack of information on the fungal derived biosynthetic gene clusters. Herein, we report a comparative analysis of four natural product metabolic systems of a select group of bicyclo[2.2.2]diazaoctane indole alkaloids including (+)/(-)-notoamide, paraherquamide and malbrancheamide, in which we propose an enzyme for each step in the biosynthetic pathway based on deep annotation and on-going biochemical studies.

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.

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.

An improved synthesis of alpha,beta-unsaturated nitrones relevant to the stephacidins and analogs thereof

A new method is reported for the synthesis of the alpha,beta-unsaturated nitrone moiety characteristic of the stephacidin/avrainvillamide family of bioactive prenylated indole alkaloids. Application to the synthesis of stephacidin analogs and a potential biological probe are showcased.

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.

Detection of VM55599 and preparaherquamide from Aspergillus japonicus and Penicillium fellutanum: biosynthetic implications

The secondary metabolites VM55599 (4) and preparaherquamide (5) have been identified by LC-MS(n) analysis as natural metabolites in cultures of Penicillium fellutanum, whereas preparaherquamide has been identified only in cultures of Aspergillus japonicus. In accord with a previous proposal, the identification of both metabolites, which have a diastereomeric relationship, provides indirect support for a unified biosynthetic scheme.

Studies Towards Paraherquamides E & F and Related C-labeled Putative Biosynthetic Intermediates: Stereocontrolled Synthesis of the α-Alkyl-β-Methylproline Ring System

A substituted 2R-allyl-3S-methylproline ethyl ester suitable for elaboration to paraherquamides E, F and related (13)C-labelled putative biosynthesis intermediates have been prepared efficiently in six steps and 24% overall yield. The key steps are a 5-exo-trig cyclization of a zinc enolate on an unactivated alkene and a stereocontrolled alkylation of the enolate formed from 3S-methyl-pyrrolidine-1,2R-dicarboxylic acid 1-tert-butyl ester 2-ethyl ester.

Biomimetic total synthesis of malbrancheamide and malbrancheamide B

The biomimetic total syntheses of both malbrancheamide and malbrancheamide B are reported. The synthesis of the two monochloro species enabled the structure of malbrancheamide B to be unambiguously assigned. The syntheses each feature an intramolecular Diels-Alder reaction of a 5-hydroxypyrazin-2(1H)-one to construct the bicyclo[2.2.2]diazaoctane core, which has also been proposed as the biosynthetic route to these compounds.

Improved Biomimetic Total Synthesis of d,l-Stephacidin A

The direct conversion of beta-hydroxyproline derivatives into 5-hydroxypyrazin-2(1H)-ones under Mitsunobu conditions has been discovered to be a general biomimetic protocol generating IMDA intermediates and has been applied to the concise, biomimetic total syntheses of D,L-stephacidin A and D,L-brevianamide B.

Concise, Biomimetic Total Synthesis of d,l-Marcfortine C

A biomimetic total synthesis of the fungal metabolite marcfortine C utilizing an intramolecular Diels-Alder reaction is described. In addition, a key stereoselective oxaziridine-mediated oxidation/pinacol rearrangement of indole 24 was used to complete the total synthesis.

Alpha,beta-unsaturated imines from titanium hydroamination and functionalization by rhodium C-H activation

Titanium pyrrolyl complexes are effective catalysts for the synthesis of alpha,beta-unsaturated imines, which can be further functionalized using Rh-catalyzed alkene or alkyne insertion into the beta-C-H bond.

Total synthesis and biosynthesis of the paraherquamides: an intriguing story of the biological Diels-Alder construction

The syntheses and biosyntheses of the paraherquamide and brevianamide families of prenylated indole-derived alkaloids are reviewed. It has been proposed that the unique bicyclo[2.2.2]diazaoctan ring system that is common to this family of natural products, arises by a biological intramolecular Diels-Alder cycloaddition reaction. Both synthetic approaches and total syntheses of several members of this family of natural products are reviewed. The biosynthesis of these alkaloids has also constituted an active area of research and the current state of knowledge on the biosynthesis of these natural products are reviewed.

Intermolecular and Intramolecular Diels−Alder Cycloadditions of 3-Ylidenepiperazine-2,5-diones and 5-Acyloxy-2(1H)-pyrazinones

The 3-ylidenepiperazine-2,5-diones 16 and 39 and 5-acyloxy-2(1H)-pyrazinones 17 can serve as starting materials for the Diels-Alder reactions of alkenes and alkynes to the piperazine ring, under acidic conditions or in the presence of acetyl chloride, to afford tricyclic piperazine-2,5-diones 19, 20, 23-25, 27, 44, and 45. Intramolecular cycloadditions occur if 3-ylidenepiperazine-2,5-diones 30 and 32 are used as the starting materials. This procedure is a convenient path to bridged bicyclo[2.2.2]diazaoctane ring systems such as 31 and 33, the former being found in biologically active secondary mold metabolites, such as VM55599 (1) or brevianamide A (5), which have been isolated from various fungi. The synthesis of the indole compound 31 provided evidence for the proposed biochemical pathway with a Diels-Alder reaction as key step. Quantum chemical calculations have revealed that piperazinones with a cationic azadiene moiety are the most reactive species in Diels-Alder cycloadditions.