The Nitrogenous Hamigerans: Unusual Amino Acid-Derivatized Aromatic Diterpenoid Metabolites from the New Zealand Marine Sponge Hamigera tarangaensis

Enantioconvergent 6π Electrocyclization Enabled by Photoredox Racemization

This study presents a novel photoredox-enabled enantioconvergent catalytic strategy used to construct chiral 2H-1,3-benzoxazines via an unprecedented oxa-6π electrocyclization utilizing racemic α-substituted glycinates as substrates. The approach leverages a cobalt-based chiral Lewis acid catalyst, which promotes the transformation under thermal or photoredox conditions. While the thermal reaction selectively converts only the (S)-configured glycinates into enantioenriched 2H-1,3-benzoxazines (up to 96:4 e.r.), the addition of 0.5 mol % of a commercially available iridium photocatalyst under visible light irradiation transforms the reaction into an enantioconvergent process. Detailed mechanistic and time course studies of optically pure α-deuterated substrates revealed the presence of an enantiospecific kinetic isotope effect, which helped to clarify the role of both the photo- and chiral Lewis acid catalyst in the reaction sequence. In this dual catalytic system, the photocatalyst promotes a dynamic interconversion between the substrate enantiomers─a process not accessible via ground-state chemistry─while the chiral Lewis acid selectively transforms only the (S)-configured substrates. Further mechanistic evidence for the proposed mechanism is provided by linear free energy relationship analysis, which suggests that the stereodetermining step involves a 6π electrocyclization under both thermal and photoredox conditions.

Concise Total Syntheses of the 6-7-5 Hamigeran Natural Products

Herein, we report the total syntheses of four hamigeran natural products featuring a 6-7-5 tricyclic carbon skeleton. We utilized a palladium-catalyzed intramolecular cyclopropanol ring opening cross-coupling to build the central seven-membered ring and a series of oxidations including a challenging aromatic C-H oxidation to introduce the peripheral functionalities. This approach enabled us to achieve the first total syntheses of hamigeran C (14 steps), debromohamigeran I (12 steps), and hamigeran I (13 steps). Our synthesis also resulted in hamigeran G in 13 steps, which is significantly shorter than the previously reported one (24 steps, longest linear sequence).

Naturally Occurring Organohalogen Compounds-A Comprehensive Review

The present volume is the third in a trilogy that documents naturally occurring organohalogen compounds, bringing the total number-from fewer than 25 in 1968-to approximately 8000 compounds to date. Nearly all of these natural products contain chlorine or bromine, with a few containing iodine and, fewer still, fluorine. Produced by ubiquitous marine (algae, sponges, corals, bryozoa, nudibranchs, fungi, bacteria) and terrestrial organisms (plants, fungi, bacteria, insects, higher animals) and universal abiotic processes (volcanos, forest fires, geothermal events), organohalogens pervade the global ecosystem. Newly identified extraterrestrial sources are also documented. In addition to chemical structures, biological activity, biohalogenation, biodegradation, natural function, and future outlook are presented.

Enantioselective Total Synthesis of (-)-Hamigeran F and Its Rearrangement Product

Herein, we report the first enantioselective total synthesis of the highly complex hamigeran diterpenoid (-)-hamigeran F and its rearrangement product. The synthetic strategy features key steps of asymmetric hydrogenation, Horner-Wadsworth-Emmons olefination, and intramolecular Friedel-Crafts acylation to construct the [6,6,5]-tricyclic skeleton bearing three consecutive stereocenters, a sequence of steps involving Rosenmund reduction, Wittig reaction, dihydroxylation to assemble the α-acetoxy ketone group, and an intramolecular aldol reaction to build the tetracyclic core structure.

11-Step and Scalable Total Synthesis of Hamigeran M Enabled by Five C-H Functionalizations

We report the convergent total synthesis of (±)-hamigeran M, enabled by five C-H functionalization reactions and proceeding in 11 steps in 3.9% overall yield. The C-H functionalizations include a hydroxy-directed C-H borylation, one C-H metalation-1,2-addition, one C-H metalation-Negishi coupling, a late-stage oxazole-directed C-H borylation-oxidation, and one electrophilic bromination. Two of these five C-H functionalizations forged strategic C-C bonds in the seven-membered ring of hamigeran M. The oxazole-directed C-H borylation-oxidation was unprecedented and ensured a late-stage hydroxylation. Other key steps include a tandem Suzuki reaction-lactonization to join the cyclopentane building block with the aromatic moiety and a hydrogen-atom transfer reaction to reduce a challenging tetrasubstituted double bond.

The Phosphinamide-Based Catalysts: Discovery, Methodology Development, and Applications in Natural Product Synthesis

In the total synthesis of natural products, synthetic efficiency has been an important driver for designing and developing new synthetic strategies and methodologies. To this end, the step, atom, and time economy and the overall yield are major factors to be considered. On the other hand, developing unified routes that can be used for synthesizing multiple molecules, specifically skeletally different classes of molecules, are also important aspects with which to be concerned. In the efforts toward efficient and flexible synthesis of structurally unique terpenoid and indole alkaloid natural products, we have designed and developed several phosphinamide-based new catalysts and reaction methodologies that have been compellingly demonstrated to be widely useful as strategic protocols for the diverse synthesis of various complex terpenoids and indole alkaloids. The important progress of these results will be summarized in this Account.In the first part, we present the stories of successful design and establishment of a novel method for the synthesis of P-stereogenic phosphinamides (P-SPhos) via a Pd-catalyzed C-H desymmetric enantioselective arylation, as well as the flexible derivatization of the P-stereogenic phosphinamides into various types of skeletally unique tricyclic and N,P-bidentate P-stereogenic compounds. Subsequently, the discovery of P-stereogenic phosphinamides as chiral organocatalysts for the desymmetric enantioselective reduction of cyclic 1,3-diketones and of phosphinamide-based cyclopalladium complex (C-Pd) as precatalysts for highly efficient Suzuki-Miyaura cross-coupling reaction of sterically congested nonactivated enolates is introduced. The notable features of the P-stereogenic phosphinamide-catalyzed desymmetric enantioselective reduction are highlighted by the broad substrate compatibility and excellent stereoselectivity, as well as most significantly, the good recoverability and reusability of catalysts. With regard to the sterically congested nonactivated enolates, such substrates are challenging for Suzuki cross-coupling reactions. We demonstrate that the phosphinamide-based cyclopalladium is a type of highly active precatalyst that allows the reaction to proceed under mild conditions and to be easily scaled up. Following the methodology development, the practical applications of these methods serving as strategic transformations are highlighted by the unified synthesis of four cyathane-type and two hamigeran-type terpenoids.In the second part, we describe the development of a robust method for oxidative Heck cross-coupling of indolyl amides by using the phosphinamide-based cyclopalladium as catalyst or phosphinamide as coligand. The method provides a general and straightforward method for diverse synthesis of indolyl δ-lactam derivatives, which present as a common core in a variety of Aspidosperma-derived indole alkaloids. The successful demonstration of this protocol for a concise and divergent synthesis of leuconodine-type indole alkaloids is also presented. We believe the results presented in this Account would have significant implications beyond our results and would find further applications in the field of synthetic methodology and natural product synthesis.

An Overview of Bioactive 1,3-Oxazole-Containing Alkaloids from Marine Organisms

1,3-Oxazole chemicals are a unique class of five-membered monocyclic heteroarenes, containing a nitrogen atom and an oxygen. These alkaloids have attracted extensive attention from medicinal chemists and pharmacologists owing to their diverse arrays of chemical structures and biological activities, and a series of 1,3-oxazole derivatives has been developed into therapeutic agents (e.g., almoxatone, befloxatone, cabotegravir, delpazolid, fenpipalone, haloxazolam, inavolisib). A growing amount of evidence indicates that marine organisms are one of important sources of 1,3-oxazole-containing alkaloids. To improve our knowledge regarding these marine-derived substances, as many as 285 compounds are summarized in this review, which, for the first time, highlights their sources, structural features and biological properties, as well as their biosynthesis and chemical synthesis. Perspective for the future discovery of new 1,3-oxazole compounds from marine organisms is also provided.

Asymmetric Hydrogenation of Racemic α-Aryl-β-ethoxycarbonyl Cyclopentanones via Dynamic Kinetic Resolution and Its Application to the Synthesis of (+)-Burmaniol A

An efficient asymmetric hydrogenation of racemic α-aryl-β-ethoxycarbonyl cyclopentanones via dynamic kinetic resolution is reported. Via catalysis by a chiral iridium Ir-SpiroPAP catalyst, a range of racemic α-aryl-β-ethoxycarbonyl cyclopentanones were hydrogenated to the corresponding ester-functionalized chiral 2-arylcyclopentanols with three contiguous stereocenters in high yields with excellent enantio- and diastereoselectivities. This method was successfully applied in the enantioselective synthesis of cyclopentane-based γ-amino ester/alcohol derivatives and phenylpropanoid (+)-burmaniol A.

Bioactive natural products from marine sponges belonging to family Hymedesmiidae

Natural products of marine origin exhibit extensive biological activities, and display a vital role in the exploration of new compounds for drug development. Marine sponges have been reported at the top with respect to the discovery of biologically active metabolites that have potential pharmaceutical applications. The family Hymedesmiidae belonging to the Demospongiae class includes ten accepted genera, of which four genera were explored for their bioactive metabolites, namely Phorbas, Hamigera, Hemimycale, and Kirkpatrickia. Genus Phorbas has received more attention due to the isolation of various classes of compounds with unique structures mainly diterpenes, alkaloids, sesterterpenes, and steroids that exhibited diverse biological activities including: antiviral, antimicrobial, and anti-inflammatory, whereas anticancer compounds predominated. This review focuses on the isolated secondary metabolites from family Hymedesmiidae with their biological potential and covers the literature from 1989 to 2020.

Natural products of marine origin exhibit extensive biological activities, and display a vital role in the exploration of new compounds for drug development.

Recent advances in total syntheses of natural products containing the benzocycloheptane motif

Covering: 2010 to 2020Benzocycloheptane is a fundamental and unique structural motif found in pharmaceuticals and natural products. The total syntheses of natural products bearing the benzocycloheptane subunit are challenging and there are only a few efficient approaches to access benzocycloheptane. Thus, new methods and innovative strategies for preparing such natural products need to be developed. In this review, recent progress in the total syntheses of natural products bearing the benzocycloheptane motif is presented, and key transformations for the construction of benzocycloheptane are highlighted. This review provides a useful guide for those engaged in the syntheses of natural products containing the benzocycloheptane motif.

Synthetic Studies toward the Hamigerans with a 6-7-5 Tricyclic Core

An approach toward the 6-7-5 tricyclic carbon skeleton of the hamigeran natural products was developed. The key steps include a benzyne-β-ketoester annulative ring expansion to form the 7-membered ring, a Nazarov reaction to form the 5-membered ring, a Ni-catalyzed conjugate methyl addition or a Corey-Chaykovsky reaction to install the all-carbon quaternary center, and a Suzuki cross coupling followed by reduction to introduce the isopropyl group.

The 6π-azaelectrocyclization of azatrienes. Synthetic applications in natural products, bioactive heterocycles, and related fields

Covering: 2006 to 2018 The application of the 6π-azaelectrocyclization of azatrienes as a key strategy for the synthesis of natural products, their analogs and related bioactive or biomedically-relevant compounds (from 2006 to date) is comprehensively reviewed. Details about reaction optimization studies, relevant reaction mechanisms and conditions are also discussed.

Natural Products from Sponges

The sponge is one of the oldest multicellular invertebrates in the world. Marine sponges represent one of the extant metazoans of 700–800 million years. They are classified in four major classes: Calcarea, Demospongiae, Hexactinellida, and Homoscleromorpha. Among them, three genera, namely, Haliclona, Petrosia, and Discodemia have been identified to be the richest source of biologically active compounds. So far, 15,000 species have been described, and among them, more than 6000 species are found in marine and freshwater systems throughout tropical, temperate, and polar regions. More than 5000 different compounds have been isolated and structurally characterized to date, contributing to about 30% of all marine natural products. The chemical diversity of sponge products is high with compounds classified as alkaloids, terpenoids, peptides, polyketides, steroids, and macrolides, which integrate a wide range of biological activities, including antibacterial, anticancer, antifungal, anti-HIV, anti-inflammatory, and antimalarial. There is an open debate whether all natural products isolated from sponges are produced by sponges or are in fact derived from microorganisms that are inhaled though filter-feeding or that live within the sponges. Apart from their origin and chemoecological functions, sponge-derived metabolites are also of considerable interest in drug development. Therefore, development of recombinant microorganisms engineered for efficient production of sponge-derived products is a promising strategy that deserves further attention in future investigations in order to address the limitations regarding sustainable supply of marine drugs.

Two new 4-methylidene containing steroids, craterol A and B, from the New Zealand two sponge association between Stelletta crater and Desmacella dendyi

NMR-directed investigation of the two sponge association between Stelletta crater and Desmacella dendyi has resulted in the isolation of two new members of the rare 4-methylidene class of sterols. Craterol A (1) and B (2) represent the first examples of natural products reported from the species S. crater. The isolation of these compounds challenges the role of 4-methylidene sterols as chemotaxonomic markers for the sponge genus Theonella.

A Total Synthesis of (-)-Hamigeran B and (-)-4-Bromohamigeran B

A concise synthesis of (-)-hamigeran B and (-)-4-bromohamigeran B is presented. The key reactions include a Suzuki coupling of enol triflate 15 with arylboronic ester for efficient synthesis of the densely 1,2,3-trisubstituted cyclopentene 23, a coordination-controlled intramolecular Friedel-Crafts cyclization of free phenol 13 for highly regioselective construction of tricyclic core 12, and a LiOH/O₂-promoted hydrolysis and concomitant aerobic oxidation of 31 for atom- and step-economic accessing of diketone 32. The application of these key transformations allowed for a rapid and efficient synthesis of (-)-hamigeran B and (-)-4-bromohamigeran B in 13 steps from the readily available chiral material 18.

Hamigerans R and S: Nitrogenous Diterpenoids from the New Zealand Marine Sponge Hamigera tarangaensis

Seven new members of the hamigeran family of diterpenoids have been isolated from the New Zealand marine sponge Hamigera tarangaensis. Among the new additions are hamigeran R (1), considered to be the first benzonitrile-based marine natural product, and hamigeran S (2), the first dimeric structure in the series. The formation of 1 and 2 is thought to occur via the reaction of hamigeran G with a nitrogen source, where the nitrile carbon of 1 is derived from the terpenoid skeleton.

Marine natural products

Covering: 2016. Previous review: Nat. Prod. Rep., 2017, 34, 235-294This review covers the literature published in 2016 for marine natural products (MNPs), with 757 citations (643 for the period January to December 2016) referring to compounds isolated from marine microorganisms and phytoplankton, green, brown and red algae, sponges, cnidarians, bryozoans, molluscs, tunicates, echinoderms, mangroves and other intertidal plants and microorganisms. The emphasis is on new compounds (1277 in 432 papers for 2016), together with the relevant biological activities, source organisms and country of origin. Reviews, biosynthetic studies, first syntheses, and syntheses that led to the revision of structures or stereochemistries, have been included.

Marine natural products

Covering: 2015. Previous review: Nat. Prod. Rep., 2016, 33, 382-431This review covers the literature published in 2015 for marine natural products (MNPs), with 1220 citations (792 for the period January to December 2015) referring to compounds isolated from marine microorganisms and phytoplankton, green, brown and red algae, sponges, cnidarians, bryozoans, molluscs, tunicates, echinoderms, mangroves and other intertidal plants and microorganisms. The emphasis is on new compounds (1340 in 429 papers for 2015), together with the relevant biological activities, source organisms and country of origin. Reviews, biosynthetic studies, first syntheses, and syntheses that lead to the revision of structures or stereochemistries, have been included.

Rearranged Terpenoids from the Marine Sponge Darwinella cf. oxeata and Its Predator, the Nudibranch Felimida grahami

Marine sponges are a rich source of terpenoids with rearranged spongian carbon skeletons. Investigation of extracts from the sponge Darwinella cf. oxeata yielded four new rearranged diterpenoids, oxeatine (2) and oxeatamides H-J (3-5), as well as the known metabolites oxeatamide A (6), oxeatamide A methyl ester (7), and membranolide (1). Oxeatine (2) has a new heterocyclic skeleton, while oxeatamide J (5) has an N-methyl urea group included in a γ-lactam moiety. UPLC-QTOF analysis of the extract obtained from the mantle of the nudibranch Felimida grahami indicated the presence of 1 and 4.

Hot off the press

A personal selection of 33 recent papers is presented covering various aspects of current developments in bioorganic chemistry and novel natural products such as artesin A from Artemisia sieversiana.