Synthetic studies on tetrahydropyrroloquinoline-containing natural products: Syntheses of discorhabdin C, batzelline C and isobatzelline C

A Divergent Synthesis of Numerous Pyrroloiminoquinone Alkaloids Identifies Promising Antiprotozoal Agents

On the basis of a streamlined route to the pyrroloiminoquinone (PIQ) core, we made 16 natural products spread across four classes of biosynthetically related alkaloid natural products, and multiple structural analogs, all in ≤8 steps longest linear sequence (LLS). The strategy features a Larock indole synthesis as the key operation in a five-step synthesis of a key methoxy-PIQ intermediate. Critically, this compound was readily diverged via selective methylation of either (or both) of the imine-like or pyrrole nitrogens, which then permitted further divergence by either O-demethylation to o-quinone natural products or displacement of the methoxy group with a range of amine nucleophiles. Based on a single, early report of their potential utility against the malaria parasite, we assayed these compounds against several strains of Plasmodium falciparum, as well as two species of the related protozoan parasite Babesia. In combination with evaluations of their human cytotoxicity, we identified several compounds with potent (low-nM IC50) antimalarial and antibabesial activities that are much less toxic toward mammalian cells and are therefore promising lead compounds for antiprotozoal drug discovery.

Divergent total syntheses of pyrroloiminoquinone alkaloids enabled by the development of a Larock/Buchwald-Hartwig annulation/cyclization

Pyrroloiminoquinone alkaloids are a large class of natural products that display a wide range of biological activities. Synthetic approaches to these natural products typically rely on a common late-stage C10-oxygenated pyrroloiminoquinone intermediate, but these strategies often lead to lengthy synthetic sequences that are not amenable to divergent syntheses. We devised an alternative approach aimed at the early introduction of the C10 nitrogen, which we hypothesized would enable late-stage diversification. This strategy hinged upon a Larock/Buchwald-Hartwig annulation/cyclization to quickly access the core of these alkaloids. We report the development of this cascade process, which was facilitated by a dual ligand system in addition to selective functionalization of the key intermediate, to provide efficient syntheses of makaluvamines A, C, and D and isobatzelline B, and the first total synthesis of makaluvamine N.

Total Synthesis of (+)-Discorhabdin V

The discorhabdin natural products are a large subset of pyrroloiminoquinone alkaloids with a myriad of biological activities. Despite garnering much synthetic attention, few members have thus far been completed, particularly those featuring a bridging carbon-nitrogen bond that is found in numerous discorhabdins, including discorhabdin V. Herein we report the first total synthesis and full stereochemical assignment of (+)-discorhabdin V. To access the pyrroloiminoquinone we developed a convergent N-alkylation/oxidative aminocyclization/bromination cascade that joins two key components, which are both made on multigram scale. An intramolecular Heck reaction then forms the quaternary carbon center in an intermediate containing the carbon-nitrogen bridge, and a reductive N,O-acetal cyclization sequence introduces the final piperidine ring. Furthermore, we have established the relative configuration of (+)-discorhabdin V through experimental NOESY data and DP4 NMR probability calculations. The absolute configuration of the natural product has also been determined by circular dichroism and the use of an amino acid derived chiral starting material. Our work represents one of only two reports of a total synthesis of a nitrogen-bridged discorhabdin and paves the way for future biological evaluation of such compounds.

Unified Divergent Total Synthesis of Discorhabdin B, H, K, and Aleutianamine via the Late-Stage Oxidative N,S-Acetal Formation

This study achieved the total syntheses of (+)-discorhabdin B, (-)-discorhabdin H, (+)-discorhabdin K, and (-)-aleutianamine. A phenethylamine fragment bearing a o-pivaloylthio group, corresponding to the D/E/G ring moiety, was prepared from benzothiophen-2-carboxylic acid methyl ester and condensed with a known pyrroloiminoquinone derivative. The adduct was subjected to [bis(trifluoroacetoxy)iodo]benzene (PIFA)-promoted oxidative spirocyclization to furnish the A/B/C/D/E spirocyclohexadienone fused with pyrroloiminoquinone. The total synthesis of (±)-discorhabdin B was completed via the key construction of the highly strained G ring with the N,S-acetal moiety featuring a newly developed CuBr2-mediated oxidative cascade cyclization. The stereocontrolled total synthesis of (+)-discorhabdin B was accomplished by a diastereoselective PIFA-promoted oxidative spirocyclization using a chiral thioester. (-)-Disocrhabdin H and (+)-discorhabdin K were synthesized by the site- and face-selective thia-Michael addition of l-ovothiol A to (+)-N-Ts-discorhabdin B with the concomitant formation of the F ring by forming the C2-N18 bond. The total synthesis of (-)-aleutianamine was achieved via a skeletal rearrangement initiated by the Luche reduction of the dienone moiety of (+)-N-Ts-discorhabdin B.

Pyrroloiminoquinone Alkaloids: Total Synthesis of Makaluvamines A and K

Herein, an efficient, scalable, and concise approach to an advanced pyrroloiminoquinone synthetic intermediate (6b) by way of a Larock indole synthesis is reported. The synthetic utility of this intermediate is demonstrated by its ready conversion to makaluvamines A (1) and K (4).

[Synthetic Studies on Complex Natural Products Based on Development of a Novel Synthetic Method for Heteroaromatic Skeleton]

Among my recent work on the syntheses of complex natural products based on the development of a novel synthetic method for the heteroaromatic skeleton, this article primarily deals with the total syntheses of (+)-CC-1065, isobatzeline A/B, and batzeline A. These syntheses were accomplished via a novel indole synthesis utilizing a ring expansion reaction of benzocyclobutenone oxime sulfonate as the key step. The 1,2-dihydro-3H-pyrrolo[3,2-e]indole segments of (+)-CC-1065 were rapidly constructed via a two-directional double-ring expansion strategy. Highly substituted pyrrolidine-fused common 5-chloro-2-methylthioindoles of isobatzeline A/B and batzeline A were constructed using a ring expansion reaction of benzocyclobutenone oxime sulfonate with NaSMe and a benzyne-mediated cyclization/functionalization reaction.

Marine Pyrrole Alkaloids

Nitrogen heterocycles are essential parts of the chemical machinery of life and often reveal intriguing structures. They are not only widespread in terrestrial habitats but can also frequently be found as natural products in the marine environment. This review highlights the important class of marine pyrrole alkaloids, well-known for their diverse biological activities. A broad overview of the marine pyrrole alkaloids with a focus on their isolation, biological activities, chemical synthesis, and derivatization covering the decade from 2010 to 2020 is provided. With relevant structural subclasses categorized, this review shall provide a clear and timely synopsis of this area.

Anodic Oxidation of Phenols: A Key Step for the Synthesis of Natural Products

Natural products have played a significant role not only in discovery of drugs but also in development of organic chemistry by providing the synthetic challenges. Inspired by biosynthesis where enzymes catalyze a multi-step reaction, we have investigated the natural product synthesis utilizing electrochemical reactions as the key step. Electrochemical organic synthesis, so-called electro-organic synthesis, enables to control the reactivity of substrates simply by tuning electrolysis conditions. In this Personal Account, we overview the recent progress of our research projects about natural product synthesis, in which anodic oxidation of phenol compounds affords the important frameworks such as diaryl ether, spirodienone, and spiroisoxazoline.

Divergent Total Syntheses of Isobatzellines A/B and Batzelline A

Divergent total syntheses of isobatzellines A/B and batzelline A were accomplished. A fully substituted common indole intermediate bearing C-2 methylthio and C-5 chloro groups was constructed via ring expansion of benzocyclobutenone oxime sulfonate with NaSMe and a benzyne-mediated cyclization/functionalization sequence as the key steps. The total synthesis of isobatzelline B was achieved via formation of the iminoquinone structure by the redox-neutral acid-promoted C-5 proto-dechlorination of the common indole intermediate. The total syntheses of isobatzelline A and batzelline A were completed in a divergent manner by oxidation of the common indole intermediate using MnO₂ or Mn(OAc)₃, respectively.

Anodic Oxidation as an Enabling Tool for the Synthesis of Natural Products

Electrochemistry provides a valuable toolbox for organic synthesis and offers an appealing, environmentally benign alternative to the use of stoichiometric quantities of chemical oxidants or reductants. Its potential to control current efficiency along with providing alternative reaction conditions in a classical sense makes electrochemistry a suitable method for large-scale industrial transformations as well as for laboratory applications in the synthesis of complex molecular architectures. Even though research in this field has intensified over the recent decades, many synthetic chemists still hesitate to add electroorganic reactions to their standard repertoire, and hence, the full potential of preparative organic electrochemistry has not yet been unleashed. This short review highlights the versatility of anodic transformations by summarizing their application in natural product synthesis.

1 Introduction

2 Shono-Type Oxidation

3 C–N/N–N Bond Formation

4 Aryl–Alkene/Aryl–Aryl Coupling

5 Cycloadditions Triggered by Oxidation of Electron-Rich Arenes

6 Spirocycles

7 Miscellaneous Transformations

8 Future Prospects

A Convenient C-H Functionalization Platform for Pyrroloiminoquinone Alkaloid Synthesis

Pyrroloiminoquinone alkaloids represent a structurally intriguing class of natural products that display an array of useful biological properties. Here, we present a versatile and scalable platform for the synthesis of this diverse family - and in particular the antitumor discorhabdins - built upon sequential selective C-H functionalization of tryptamine. The utility of this strategy is showcased through short formal syntheses of damirones A-C, makaluvamines D and I, and discorhadbin E. Additionally, we describe efforts to develop the first catalytic asymmetric entry to the discorhabdin subclass.

Electrochemical strategies for C-H functionalization and C-N bond formation

Conventional methods for carrying out carbon-hydrogen functionalization and carbon-nitrogen bond formation are typically conducted at elevated temperatures, and rely on expensive catalysts as well as the use of stoichiometric, and perhaps toxic, oxidants. In this regard, electrochemical synthesis has recently been recognized as a sustainable and scalable strategy for the construction of challenging carbon-carbon and carbon-heteroatom bonds. Here, electrosynthesis has proven to be an environmentally benign, highly effective and versatile platform for achieving a wide range of nonclassical bond disconnections via generation of radical intermediates under mild reaction conditions. This review provides an overview on the use of anodic electrochemical methods for expediting the development of carbon-hydrogen functionalization and carbon-nitrogen bond formation strategies. Emphasis is placed on methodology development and mechanistic insight and aims to provide inspiration for future synthetic applications in the field of electrosynthesis.

Marine pyrroloiminoquinone alkaloids

Recent reports on the synthetic studies of marine pyrroloiminoquinone alakloids and their analogs are reviewed.

Synthesis of the marine pyrroloiminoquinone alkaloids, discorhabdins

Many natural products with biologically interesting structures have been isolated from marine animals and plants such as sponges, corals, worms, etc. Some of them are discorhabdin alkaloids. The discorhabdin alkaloids (discorhabdin A-X), isolated from marine sponges, have a unique structure with azacarbocyclic spirocyclohexanone and pyrroloiminoquinone units. Due to their prominent potent antitumor activity, discorhabdins have attracted considerable attention. Many studies have been reported toward the synthesis of discorhabdins. We have accomplished the first total synthesis of discorhabdin A (1), having the strongest activity in vitro among discorhabdins in 2003. In 2009, we have also accomplished the first total synthesis of prianosin B (2), having the 16,17-dehydropyrroloiminoquinone moiety, by a novel dehydrogenation reaction with a catalytic amount of NaN(3). These synthetic studies, as well as syntheses of the discorhabdins by various chemists to-date, are reviewed here.

Pyrroloiminoquinone and related metabolites from marine sponges

This review presents the structure, biological activity, biosynthetic studies and, where applicable, references to syntheses of 81 marine alkaloids containing either tetra-, hexa- or octa-hydrogenated variants of pyrrolo[4,3,2-de]quinoline, pyrrolo[4,3,2-de]pyrrolo[2,3-h]quinoline and pyrido[2,3-h]pyrrolo[4,3,2-de]quinoline core skeletons. The literature describing the isolation of pyrroloiminoquinones, and related metabolites, from marine sponges is littered with taxonomic inconsistencies and recent efforts to clarify the taxonomy of the sponges that produce this group of metabolites are discussed.

Synthesis of the naphthalene-derived inhibitors against Cdc25A dual-specificity protein phosphatase and their biological activity

The novel naphthalene-type analogues 14 and 18 and the naphthoquinone-type analogues, 8, 9, 15, 16, 19, 21, 22, and 23-28 have been synthesized, and their in vitro Cdc25A phosphatase-inhibitory activity was examined. In assessment of the inhibitory activity, it was revealed that the naphthoquinone core is contributed to the activity, rather than the alkyl side chain.

A Biomimetic Approach to the Discorhabdin Alkaloids: Total Syntheses of Discorhabdins C and E and Dethiadiscorhabdin D

The characteristic spirodienone structure of the discorhabdin alkaloids is readily formed by reaction of the tyramine-substituted indoloquinonimines 26, 35, and 36 with cupric chloride, triethylamine, and oxygen. This cyclization provides a possibly biomimetic route to discorhabdins C and E (41 and 42). The unbrominated spirodienone 40 reacts with hydrogen over Pd/C to give enone 46. Bromination at the alpha position gives a mixture of bromoenones that undergo smooth conversion to dethiadiscorhabdin D (4) upon treatment with basic alumina.

Synthesis of Pyrrolo[4,3,2-de]quinolines from 6,7-Dimethoxy-4-methylquinoline. Formal Total Syntheses of Damirones A and B, Batzelline C, Isobatzelline C, Discorhabdin C, and Makaluvamines A-D

2-Amino-4-nitrophenol and 2-methoxy-5-nitroaniline were converted into the 5-nitroquinolines 6b and 6d, respectively, and then the latter into nitro-acetal 6f. 6,7-Dimethoxy-4-methylquinoline (6g) was nitrated at C-5 and then the methyl substituent converted into aldehyde 6j and then protected giving acetal 6l. Various means, notably a large excess of NiCl(2)/NaBH(4), were used to reduce both nitro group and pyridine ring, forming 1,2,3,4-tetrahydroquinolines such as 7b, 7c, 7d, which under acidic conditions closed to give 1,3,4,5-tetrahydropyrrolo[4,3,2-de]quinolines 9a, 9d, 9c, respectively. In some cases it was unnecessary to protect the aldehyde function, for example quinolinium salt 12c gave 9j and nitro-aldehyde 6j gave 9e (after BOC protection) directly by reaction with NiCl(2)/NaBH(4). Substitution of the indole and aniline nitrogens in the 1,3,4,5-tetrahydropyrrolo[4,3,2-de]quinolines was based on a combination of protection, selective deprotection, and the exploitation of the greater acidity of the indole N-hydrogen. 8-Chlorination of 6h and then conversions, as above, gave chloro-diamine-acetal 7e which on acid treatment produced iminoquinone 11b; formylation of the nitrogens in 7e and then acidic treatment allowed formation of the chlorine-substituted 1,3,4,5-tetrahydropyrrolo[4,3,2-de]quinoline 9m which was then converted into 9p. De-O-methylation and then oxidation of 9b and 9c gave o-quinones 10b and 10a, respectively.