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

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.

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.

Heterocyclic Iminoquinones and Quinones from the National Cancer Institute (NCI, USA) COMPARE Analysis

This review uses the National Cancer Institute (NCI) COMPARE program to establish an extensive list of heterocyclic iminoquinones and quinones with similarities in differential growth inhibition patterns across the 60-cell line panel of the NCI Developmental Therapeutics Program (DTP). Many natural products and synthetic analogues are revealed as potential NAD(P)H:quinone oxidoreductase 1 (NQO1) substrates, through correlations to dipyridoimidazo[5,4-f]benzimidazoleiminoquinone (DPIQ), and as potential thioredoxin reductase (TrxR) inhibitors, through correlations to benzo[1,2,4]triazin-7-ones and pleurotin. The strong correlation to NQO1 infers the enzyme has a major influence on the amount of the active compound with benzo[e]perimidines, phenoxazinones, benz[f]pyrido[1,2-a]indole-6,11-quinones, seriniquinones, kalasinamide, indolequinones, and furano[2,3-b]naphthoquinones, hypothesised as prodrugs. Compounds with very strong correlations to known TrxR inhibitors had inverse correlations to the expression of both reductase enzymes, NQO1 and TrxR, including naphtho[2,3-b][1,4]oxazepane-6,11-diones, benzo[a]carbazole-1,4-diones, pyranonaphthoquinones (including kalafungin, nanaomycin A, and analogues of griseusin A), and discorhabdin C. Quinoline-5,8-dione scaffolds based on streptonigrin and lavendamycin can correlate to either reductase. Inhibitors of TrxR are not necessarily (imino)quinones, e.g., parthenolides, while oxidising moieties are essential for correlations to NQO1, as with the mitosenes. Herein, an overview of synthetic methods and biological activity of each family of heterocyclic imino(quinone) is provided.

Stereoselective Routes to Hexahydropyrroloindoles and Tetrahydropyrroloquinolines from Activated Aziridines and Electron Deficient 3H-Indoles

An unprecedented and novel synthetic route to hexahydropyrrolo[2,3-b]indoles bearing cis-contiguous stereocenters with excellent stereoselectivities (ee of >99%, dr of ≤99:1) has been disclosed that proceeds through the ring opening of activated aziridines with electron deficient 4-substituted indoles followed by a novel cyclization in a domino fashion, thereby obviating the use of 3-substituted indoles as the prerequisite nucleophile. Another efficient synthetic route to tetrahydropyrrolo[4,3,2-de]quinolines in excellent yields (≤93%) and excellent enantioselectivity (ee of >99%) has been established via ring opening of activated aziridines with 4-bromo-1-methyl-1H-indole at relatively higher temperatures followed by Cu(I)-catalyzed intramolecular C-N cyclization in the same pot. The stability and the formation of products at different temperatures are explained by computational studies.

[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.

Construction of N-Heterocycles Fused with a Highly Substituted Benzene Ring by a Benzyne-Mediated Cyclization/Functionalization Cascade Reaction and Its Application to the Total Synthesis of Marine Natural Products

This account summarizes the development of a benzyne-mediated cyclization/functionalization protocol for the versatile construction of highly substituted benzene derivatives fused with an N-heterocyclic ring such as indolines, indoles, and related nitrogen-containing heterocycles. The protocol comprises sequential reactions initiated by generating a benzyne species and subsequent cyclization via addition of magnesium amide to the benzyne, followed by trapping of the resultant magnesium compound in situ with various electrophiles. The substituent scope was expanded by conducting a transmetalation on a copper species to introduce alkyl, aryl, and alkenyl substituents. The utility of the sequential reaction was demonstrated in the synthesis of a carbazole natural product (heptaphylline), pyrrolo[4,3,2-de]quinoline alkaloids (batzellines), and pyrrolo[2,3-c]carbazole alkaloids (dictyodendrines).

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.

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.

Computationally Assisted Discovery and Assignment of a Highly Strained and PANC-1 Selective Alkaloid from Alaska's Deep Ocean

We report here the orchestration of molecular ion networking and a set of computationally assisted structural elucidation approaches in the discovery of a new class of pyrroloiminoquinone alkaloids that possess selective bioactivity against pancreatic cancer cell lines. Aleutianamine represents the first in a new class of pyrroloiminoquinone alkaloids possessing a highly strained multibridged ring system, discovered from Latrunculia ( Latrunculia) austini Samaai, Kelly & Gibbons, 2006 (class Demospongiae, order Poecilosclerida, family Latrunculiidae) recovered during a NOAA deep-water exploration of the Aleutian Islands. The molecule was identified with the guidance of mass spectrometry, nuclear magnetic resonance, and molecular ion networking (MoIN) analysis. The structure of aleutianamine was determined using extensive spectroscopic analysis in conjunction with computationally assisted quantifiable structure elucidation tools. Aleutianamine exhibited potent and selective cytotoxicity toward solid tumor cell lines including pancreatic cancer (PANC-1) with an IC50 of 25 nM and colon cancer (HCT-116) with an IC50 of 1 μM, and represents a potent and selective candidate for advanced preclinical studies.

Recent advances in isolation, synthesis, and evaluation of bioactivities of bispyrroloquinone alkaloids of marine origin

The ocean continues to provide a plethora of unique scaffolds capable of remarkable biological applications. A large number of pyrroloiminoquinone alkaloids, including discorhabdins, epinardins, batzellines, makaluvamines, and veiutamine, have been isolated from various marine organisms. A class of pyrroloiminoquinone-related alkaloids, known as bispyrroloquinones, is the focus of this review article. This family of marine alkaloids, which contain an aryl substituted bispyrroloquinone ring system, includes three subclasses of alkaloids namely, wakayin, tsitsikammamines A-B, and zyzzyanones A-D. Both wakayin and the tsitsikammamines contain a tetracyclic fused bispyrroloiminoquinone ring system, while zyzzyanones contain a fused tricyclic bispyrroloquinone ring system. The unique chemical structures of these marine natural products and their diverse biological properties, including antifungal and antimicrobial activity, as well as the potent, albeit generally nonspecific and universal cytotoxicities, have attracted great interest of synthetic chemists over the past three decades. Tsitsikammamines, wakayin, and several of their analogs show inhibition of topoisomerases. One additional possible mechanism of anticancer activity of tsitsikammamines analogs that has been discovered recently is through the inhibition of indoleamine 2, 3-dioxygenase, an enzyme involved in tumoral immune resistance. This review discusses the isolation, synthesis, and evaluation of bioactivities of bispyrroloquinone alkaloids and their analogs.

Marine pyrroloiminoquinone alkaloids

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

Efficient total synthesis of ammosamide B

A total synthesis of ammosamide B, a metabolite of the marine-derived Streptomyces strain CNR-698, has been executed in nine steps and 6.9% overall yield. The key step involves the condensation of a 4,6-diBoc-protected 1,3,4,6-tetraaminobenzene derivative with dimethyl 2-ketoglutaconate, which effectively constructs the pyrrolidinone ring and the quinoline ring in a single step. This contributes a unique approach to the synthesis of pyrroloquinoline alkaloids that offers the advantages of brevity and relatively high overall yield.

Total synthesis of the ammosamides

The ammosamides A-C are chlorinated pyrrolo[4,3,2-de]quinoline metabolites isolated from the marine-derived Streptomyces strain CNR-698. The natural products, which possess a dense array of heteroatoms, were synthesized in 17-19 steps from 4-chloroisatin. That the five nitrogen atoms were introduced at the appropriate time and in a suitable oxidation state was key to the success of the total synthesis. Compared to synthetic deschloro ammosamide B, natural ammosamide B is much less susceptible to oxidative degradation.

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.

[Development of novel synthetic organic reactions: synthesis of antitumor natural products and leading compounds for new pharmaceuticals]

Biologically active natural products with unique, highly complex molecular skeletons have been used as leading compounds for raw materials of new drugs. Due to the limitations of natural supply, highly efficient, large-scale syntheses and molecular design have been sought in drug discovery. For this purpose, we have focused on a synthetic strategy effective in developing novel reactions and reagents and found several useful regio- and stereospecific reactions, contributing to the synthesis of otherwise unattainable target molecules. The application of these reactions for the total synthesis of three types of potent cytotoxic natural products for the first time is described in this paper. The basic concept is first described. Then the total synthesis of anthracyclines, fredericamycin A, and discorhabdins is reported. Novel reactions using hypervalent iodine reagents under environmentally benign conditions are also described. The future prospects for this method are discussed.

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.