Golgi-modifying properties of macfarlandin E and the synthesis and evaluation of its 2,7-dioxabicyclo[3.2.1]octan-3-one core

Bioactive Compounds from Marine Heterobranchs

The natural products of heterobranch molluscs display a huge variability both in structure and in their bioactivity. Despite the considerable lack of information, it can be observed from the recent literature that this group of animals possesses an astonishing arsenal of molecules from different origins that provide the molluscs with potent chemicals that are ecologically and pharmacologically relevant. In this review, we analyze the bioactivity of more than 450 compounds from ca. 400 species of heterobranch molluscs that are useful for the snails to protect themselves in different ways and/or that may be useful to us because of their pharmacological activities. Their ecological activities include predator avoidance, toxicity, antimicrobials, antifouling, trail-following and alarm pheromones, sunscreens and UV protection, tissue regeneration, and others. The most studied ecological activity is predation avoidance, followed by toxicity. Their pharmacological activities consist of cytotoxicity and antitumoral activity; antibiotic, antiparasitic, antiviral, and anti-inflammatory activity; and activity against neurodegenerative diseases and others. The most studied pharmacological activities are cytotoxicity and anticancer activities, followed by antibiotic activity. Overall, it can be observed that heterobranch molluscs are extremely interesting in regard to the study of marine natural products in terms of both chemical ecology and biotechnology studies, providing many leads for further detailed research in these fields in the near future.

General Access to Concave-Substituted cis-Dioxabicyclo[3.3.0]octanones: Enantioselective Total Syntheses of Macfarlandin C and Dendrillolide A

The evolution of a strategy to access the family of rearranged spongian diterpenoids harboring a concave-substituted cis-2,8-dioxabicyclo[3.3.0]octan-3-one fragment is described. The approach involves late-stage fragment coupling of a tertiary-carbon radical and an electron-deficient double bond to form vicinal quaternary and tertiary stereocenters with high fidelity. A stereoselective Mukaiyama hydration is the key step in the subsequent elaboration of the cis-2,8-dioxabicyclo[3.3.0]octan-3-one moiety. This strategy was utilized in enantioselective total syntheses of (-)-macfarlandin C and (+)-dendrillolide A. An efficient construction of enantiopure tetramethyloctahydronaphthalenes was developed during the construction of (-)-macfarlandin C.

Bridging the gap between natural product synthesis and drug discovery

Covering: 1986 to 2020Natural products are an enduring source of chemical information useful for probing biologically relevant chemical space. Toward gathering further structure-activity relationship (SAR) information for a particular natural product, synthetic chemists traditionally proceeded first by a total synthesis effort followed by the synthesis of simplified derivatives. While this approach has proven fruitful, it often does not incorporate hypotheses regarding structural features necessary for bioactivity at the synthetic planning stage, but rather focuses on the rapid assembly of the targeted natural product; a goal that often supersedes the opportunity to gather SAR information en route to the natural product. Furthermore, access to simplified variants of a natural product possessing only the proposed essential structural features necessary for bioactivity, typically at lower oxidation states overall, is sometimes non-trivial from the original established synthetic route. In recent years, several synthetic design strategies were described to streamline the process of finding bioactive molecules in concert with fathering further SAR studies for targeted natural products. This review article will briefly discuss traditional retrosynthetic strategies and contrast them to selected examples of recent synthetic strategies for the investigation of biologically relevant chemical space revealed by natural products. These strategies include: diversity-oriented synthesis (DOS), biology-oriented synthesis (BIOS), diverted-total synthesis (DTS), analogue-oriented synthesis (AOS), two-phase synthesis, function-oriented synthesis (FOS), and computed affinity/dynamically ordered retrosynthesis (CANDOR). Finally, a description of pharmacophore-directed retrosynthesis (PDR) developed in our laboratory and initial applications will be presented that was initially inspired by a retrospective analysis of our synthetic route to pateamine A completed in 1998.

Transition Metal-Mediated C-C Single Bond Cleavage: Making the Cut in Total Synthesis

Transition-metal-mediated cleavage of C-C single bonds can enable entirely new retrosynthetic disconnections in the total synthesis of natural products. Given that C-C bond cleavage inherently alters the carbon framework of a compound, and that, under transition-metal catalysis, the generated organometallic or radical intermediate is primed for further complexity-building reactivity, C-C bond-cleavage events have the potential to drastically and rapidly remodel skeletal frameworks. The recent acceleration of the use of transition-metal-mediated cleavage of C-C single bonds in total synthesis can be ascribed to a communal recognition of this fact. In this Review, we highlight ten selected total syntheses from 2014 to 2019 that illustrate how transition-metal-mediated cleavage of C-C single bonds at either the core or the periphery of synthetic intermediates can streamline synthetic efforts.

Total Syntheses of Norrisolide-Type Spongian Diterpenes Cheloviolene C, Seconorrisolide B, and Seconorrisolide C

The first total syntheses of three unusual norrisolide-type rearranged spongian diterpenes, cheloviolene C, seconorrisolide B, and seconorrisolide C, have been accomplished via a common intermediate through late-stage ring-scissoring. The synthesis features a Wolff ring contraction for the synthesis of the trans-hydrindane system, and a crucial retro Diels-Alder reaction/intramolecular ene cyclization for the rapid stereoselective construction of the furo[2,3-b]furan system, which is commonly seen in rearranged spongian diterpenes.

Simplified immunosuppressive and neuroprotective agents based on gracilin A

The architecture and bioactivity of natural products frequently serve as embarkation points for the exploration of biologically relevant chemical space. Total synthesis followed by derivative synthesis has historically enabled a deeper understanding of structure-activity relationships. However, synthetic strategies towards a natural product are not always guided by hypotheses regarding the structural features required for bioactivity. Here, we report an approach to natural product total synthesis that we term 'pharmacophore-directed retrosynthesis'. A hypothesized, pharmacophore of a natural product is selected as an early synthetic target and this dictates the retrosynthetic analysis. In an ideal application, sequential increases in the structural complexity of this minimal structure enable development of a structure-activity relationship profile throughout the course of the total synthesis effort. This approach enables the identification of simpler congeners retaining bioactivity at a much earlier stage of a synthetic effort, as demonstrated here for the spongiane diterpenoid, gracilin A, leading to simplified derivatives with potent neuroprotective and immunosuppressive activity.

Structural Simplification of Natural Products

Natural products (NPs) are important sources of clinical drugs due to their structural diversity and biological prevalidation. However, the structural complexity of NPs leads to synthetic difficulties, unfavorable pharmacokinetic profiles, and poor drug-likeness. Structural simplification by truncating unnecessary substructures is a powerful strategy for overcoming these limitations and improving the efficiency and success rate of NP-based drug development. Herein, we will provide a comprehensive review of the structural simplification of NPs with a focus on design strategies, case studies, and new technologies. In particular, a number of successful examples leading to marketed drugs or drug candidates will be discussed in detail to illustrate how structural simplification is applied in lead optimization of NPs.

Two Competitive but Switchable Organocatalytic Cascade Reaction Pathways: The Diversified Synthesis of Chiral Acetal-Containing Bridged Cyclic Compounds

The organocatalytic enantioselective synthesis of methanobenzodioxepine derivatives bearing a 6,6,5-bridged ring system is presented. The m-CPBA-triggered in situ α-oxidation of β-oxoesters to provide the required but unstable α-hydroxy-β-dicarbonyl substrates is the key to this three-step sequence, providing the desired cyclic acetals with excellent stereoselectivities containing two bridgehead and one fully substituted stereocenters. It is noteworthy that the absence of m-CPBA furnished the acetal products bearing a 6,6,6-bridged ring system with similar good results from the same starting materials.

Total synthesis of complex terpenoids employing radical cascade processes

Covering: 2011-2017Radical cyclizations have a rich history in organic chemistry and have been particularly generous to the field of natural product synthesis. Owing to their ability to operate in highly congested molecular quarters, and with significant functional group compatibility, these transformations have enabled the synthesis of numerous polycyclic terpenoid natural products over the past several decades. Moreover, when programmed accordingly into a synthetic plan, radical cascade processes can be used to rapidly assemble molecular complexity, much in the same way nature rapidly constructs terpene frameworks through cationic cyclization pathways. This review highlights recent total syntheses of complex terpenoids (from 2011-2017) employing C-C bond-forming radical cascade sequences.

Total Synthesis of (-)-Chromodorolide B By a Computationally-Guided Radical Addition/Cyclization/Fragmentation Cascade

The first total synthesis of a chromodorolide marine diterpenoid is described. The core of the diterpenoid is constructed by a bimolecular radical addition/cyclization/fragmentation cascade that unites two complex fragments and forms two C-C bonds and four contiguous stereogenic centers of (-)-chromodorolide B in a single step. This coupling step is initiated by visible-light photocatalytic fragmentation of a redox-active ester, which can be accomplished in the presence of an iridium or a less-precious electron-rich dicyanobenzene photocatalyst, and employs equimolar amounts of the two addends. Computational studies guided the development of this central step of the synthesis and provide insight into the origin of the observed stereoselectivity.

Domino construction of a bullataketal core via double bond cleavage in activated dihydrofurans

A new rapid approach to structurally challenging and biologically relevant methanobenzodioxepines was developed via a Brønsted acid-triggered domino transformation of 3-carbonyl-4,5-dihydrofurans, substituted with MOM-protected o-hydroxyaryls at the C4 atom.

The Potential of Indonesian Heterobranchs Found around Bunaken Island for the Production of Bioactive Compounds

The species diversity of marine heterobranch sea slugs found on field trips around Bunaken Island (North Sulawesi, Indonesia) and adjacent islands of the Bunaken National Marine Park forms the basis of this review. In a survey performed in 2015, 80 species from 23 families were collected, including 17 new species. Only three of these have been investigated previously in studies from Indonesia. Combining species diversity with a former study from 2003 reveals in total 140 species from this locality. The diversity of bioactive compounds known and yet to be discovered from these organisms is summarized and related to the producer if known or suspected (might it be down the food chain, de novo synthesised from the slug or an associated bacterium). Additionally, the collection of microorganisms for the discovery of natural products of pharmacological interest from this hotspot of biodiversity that is presented here contains more than 50 species that have never been investigated before in regard to bioactive secondary metabolites. This highlights the great potential of the sea slugs and the associated microorganisms for the discovery of natural products of pharmacological interest from this hotspot of biodiversity.

Covalent modification of biological targets with natural products through Paal-Knorr pyrrole formation

Covering: up to June 2017Natural products and endogenous metabolites engage specific targets within tissues and cells through complex mechanisms. This review examines the extent to which natural systems have adopted the Paal-Knorr reaction to engage nucleophilic amine groups within biological targets. Current understanding of this mode of reactivity is limited by only a few examples of this reaction in a biological context. This highlight is intended to stimulate the scientific community to identify potential research directions and applications of the Paal-Knorr reaction in native and engineered biological systems.

Spongian-16-one Diterpenes and Their Anatomical Distribution in the Australian Nudibranch Goniobranchus collingwoodi

Six new (1-6) spongian-16-one analogues have been characterized from the Australian nudibranch species Goniobranchus collingwoodi, along with four known spongian-16-one derivatives. The structures and relative configuration were suggested by spectroscopic analyses informed by molecular modeling. Dissection of animal tissue revealed that the mantle and viscera differ in their terpene composition. Whole body extracts were not toxic to brine shrimp (Artemia sp.), but were unpalatable to palaemon shrimp (Palaemon serenus) at a concentration found within the nudibranch. Individual terpenes were not cytotoxic to human lung (NCIH-460), colorectal (SW620), and liver (HepG2) cancer cells.

Fragment Coupling with Tertiary Radicals Generated by Visible-Light Photocatalysis

Convergent synthesis strategies in which a target molecule is prepared by a branched approach wherein two or more complex fragments are combined at a late stage are almost always preferred over a linear approach in which the overall yield of the target molecule is eroded by the efficiency of each successive step in the sequence. As a result, bimolecular reactions that are able to combine complex fragments in good yield and, where important, with high stereocontrol are essential for implementing convergent synthetic strategies. Although intramolecular reactions of carbon radicals have long been exploited to assemble polycyclic ring systems, bimolecular coupling reactions of structurally complex carbon radicals have rarely been employed to combine elaborate fragments in the synthesis of structurally intricate molecules. We highlight in this Account recent discoveries from our laboratories that demonstrate that bimolecular reactions of structurally elaborate tertiary carbon radicals and electron-deficient alkenes can unite complex fragments in high yield using nearly equimolar amounts of the two coupling partners. Our discussion begins by considering several aspects of the bimolecular addition of tertiary carbon radicals to electron-deficient alkenes that commend these transformations for the union of structurally complex, sterically bulky fragments. We then discuss how in the context of synthesizing rearranged spongian diterpenoids we became aware of the exceptional utility of coupling reactions of alkenes and tertiary carbon radicals to unite structurally complex synthetic intermediates. Our initial investigations exploit the early report of Okada that N-(acyloxy)phthalimides reductively fragment at room temperature in the presence of visible light and catalytic amounts of the photocatalyst Ru(bpy)3Cl2 to form carbon radicals that react with alkenes. We show that this reaction of a tertiary radical precursor and an enone can combine two elaborate enantioenriched fragments in good yield with the formation of new quaternary and secondary stereocenters. As a result of the ready availability of tertiary alcohols, we describe two methods that were developed, one in collaboration with the MacMillan group, to generate tertiary radicals from tertiary alcohols. In the method that will be preferred in most instances, the tertiary alcohol is esterified in high yield to give a tert-alkyl hemioxalate salt, which-without purification-reacts with electron-deficient alkenes in the presence of visible light and an Ir(III) photocatalyst to give coupled products having a newly formed quaternary carbon in high yield. Hemioxalate salts containing Li, Na, K, and Cs countercations can be employed in this reaction, whose only other product is CO2. These reactions are carried out using nearly equimolar amounts of the addends, making them ideal for coupling of complex fragments at the late stage in a synthetic sequence. The attractive attributes of the fragment-coupling chemistry that we discuss in this Account are illustrated by an enantioselective total synthesis of a tricyclic trans-clerodane diterpenoid in eight steps and 34% overall yield from commercially available precursors. We anticipate that bimolecular reactions of carbon radicals will be increasingly used for fragment coupling in the future.

Rearranged Diterpenes and Norditerpenes from Three Australian Goniobranchus Mollusks

Three new norditerpenes (1, 6, and 7) and four diterpenes (2-5) with extensively rearranged carbon skeletons have been characterized from Australian nudibranchs. The relative configuration of the cyclopropyl-containing verrielactone (1) from Goniobranchus verrieri was suggested by spectroscopic analysis at 500 MHz informed by a combination of molecular modeling and DFT calculations. The nudibranchs G. splendidus and G. cf. splendidus provided 2-7, for which the structures and stereochemistry were deduced by 2D NMR studies at either 500 or 700 MHz. Each of the seven terpenoids exhibited a carbon skeleton modified from one of the tetrahydroaplysulphurin, spongionellin, or gracilane series of terpenes. A biosynthetic pathway to terpenes 1-7 from spongialactone is proposed.

Total Synthesis of (-)-Chromodorolide B

The first total synthesis of a chromodorolide diterpenoid is described. The synthesis features a bimolecular radical addition/cyclization/fragmentation cascade that unites butenolide and trans-hydrindane fragments while fashioning two C-C bonds and stereoselectively forming three of the ten contiguous stereocenters of chromodorolide B.

Capturing Biological Activity in Natural Product Fragments by Chemical Synthesis

Natural products have had an immense influence on science and have directly led to the introduction of many drugs. Organic chemistry, and its unique ability to tailor natural products through synthesis, provides an extraordinary approach to unlock the full potential of natural products. In this Review, an approach based on natural product derived fragments is presented that can successfully address some of the current challenges in drug discovery. These fragments often display significantly reduced molecular weights, reduced structural complexity, a reduced number of synthetic steps, while retaining or even improving key biological parameters such as potency or selectivity. Examples from various stages of the drug development process up to the clinic are presented. In addition, this process can be leveraged by recent developments such as genome mining, antibody–drug conjugates, and computational approaches. All these concepts have the potential to identify the next generation of drug candidates inspired by natural products.

New Cytotoxic Norditerpenes from the Australian Nudibranchs Goniobranchus Splendidus and Goniobranchus Daphne

This study reports the isolation and characterisation of six new metabolites with ‘gracilin’-type carbon skeletons and of aplytandiene-3 from the Australian nudibranch Goniobranchus splendidus. The structure of gracilin G is revised, and the C-6 configuration deduced by comparison of calculated 3JC/H values with values measured using the EXSIDE pulse sequence. A lactone isolated from Goniobranchus daphne contains a rearranged spongionellin-type skeleton. Screening of selected metabolites revealed significant cytotoxicity against a HeLa S3 cell line by five of the new terpenes.

“Pruning of biomolecules and natural products (PBNP)”: an innovative paradigm in drug discovery

Smart Schneider: ‘Nature’ is the most intelligent tailor with an ability to utilize the resources. Researchers are still at an infant stage learning this art. The present review highlights some of the man made pruning of bio-molecules and NPs (PBNP) in finding chemicals with a better therapeutic index.

Concise synthesis and biological assessment of (+)-neopeltolide and a 16-member stereoisomer library of 8,9-dehydroneopeltolide: identification of pharmacophoric elements

We describe herein a concise synthesis of (+)-neopeltolide, a marine macrolide natural product that elicits a highly potent antiproliferative activity against several human cancer cell lines. Our synthesis exploited the powerful bond-forming ability and high functional group compatibility of olefin metathesis and esterification reactions to minimize manipulations of oxygen functionalities and to maximize synthetic convergency. Our findings include a chemoselective olefin cross-metathesis reaction directed by H-bonding, and a ring-closing metathesis conducted under non-high dilution conditions. Moreover, we developed a 16-member stereoisomer library of 8,9-dehydroneopeltolide to systematically explore the stereostructure-activity relationships. Assessment of the antiproliferative activity of the stereoisomers against A549 human lung adenocarcinoma, MCF-7 human breast adenocarcinoma, HT-1080 human fibrosarcoma, and P388 murine leukemia cell lines has revealed marked differences in potency between the stereoisomers. This study provides comprehensive insights into the structure-activity relationship of this important antiproliferative agent, leading to the identification of the pharmacophoric structural elements and the development of truncated analogues with nanomolar potency.

Design and synthesis of skeletal analogues of gambierol: attenuation of amyloid-β and tau pathology with voltage-gated potassium channel and N-methyl-D-aspartate receptor implications

Gambierol is a potent neurotoxin that belongs to the family of marine polycyclic ether natural products and primarily targets voltage-gated potassium channels (K(v) channels) in excitable membranes. Previous work in the chemistry of marine polycyclic ethers has suggested the critical importance of the full length of polycyclic ether skeleton for potent biological activity. Although we have previously investigated structure-activity relationships (SARs) of the peripheral functionalities of gambierol, it remained unclear whether the whole polycyclic ether skeleton is needed for its cellular activity. In this work, we designed and synthesized two truncated skeletal analogues of gambierol comprising the EFGH- and BCDEFGH-rings of the parent compound, both of which surprisingly showed similar potency to gambierol on voltage-gated potassium channels (K(v)) inhibition. Moreover, we examined the effect of these compounds in an in vitro model of Alzheimer's disease (AD) obtained from triple transgenic (3xTg-AD) mice, which expresses amyloid beta (Aβ) accumulation and tau hyperphosphorylation. In vitro preincubation of the cells with the compounds resulted in significant inhibition of K(+) currents, a reduction in the extra- and intracellular levels of Aβ, and a decrease in the levels of hyperphosphorylated tau. In addition, pretreatment with these compounds reduced the steady-state level of the N-methyl-D-aspartate (NMDA) receptor subunit 2A without affecting the 2B subunit. The involvement of glutamate receptors was further suggested by the blockage of the effect of gambierol on tau hyperphosphorylation by glutamate receptor antagonists. The present study constitutes the first discovery of skeletally simplified, designed polycyclic ethers with potent cellular activity and demonstrates the utility of gambierol and its synthetic analogues as chemical probes for understanding the function of K(v) channels as well as the molecular mechanism of Aβ metabolism modulated by NMDA receptors.

Divergent synthesis and chemical reactivity of bicyclic lactone fragments of complex rearranged spongian diterpenes

The synthesis and direct comparison of the chemical reactivity of the two highly oxidized bicyclic lactone fragments found in rearranged spongian diterpenes (8-substituted 6-acetoxy-2,7-dioxabicyclo[3.2.1]octan-3-one and 6-substituted 7-acetoxy-2,8-dioxabicyclo[3.3.0]octan-3-one) are reported. Details of the first synthesis of the 6-acetoxy-2,7-dioxabicyclo[3.2.1]octan-3-one ring system, including an examination of several possibilities for the key bridging cyclization reaction, are described. In addition, the first synthesis of 7-acetoxy-2,8-dioxabicyclo[3.3.0]octanones containing quaternary carbon substituents at C6 is disclosed. Aspects of the chemical reactivity and Golgi-modifying properties of these bicyclic lactone analogs of rearranged spongian diterpenes are also reported. Under both acidic and basic conditions, 8-substituted 2,7-dioxabicyclo[3.2.1]octanones are converted to 6-substituted-2,8-dioxabicyclo[3.3.0]octanones. Moreover, these dioxabicyclic lactones react with primary amines and lysine side chains of lysozyme to form substituted pyrroles, a conjugation that could be responsible for the unique biological properties of these compounds. These studies demonstrate that acetoxylation adjacent to the lactone carbonyl group, in either the bridged or fused series, is required to produce fragmented Golgi membranes in the pericentriolar region that is characteristic of macfarlandin E.

Enantioselective total synthesis of aplyviolene

The enantioselective total synthesis of the rearranged spongian diterpene aplyviolene has been completed in 14 steps from the known hydroazulenone 8. The key junction of the hydrocarbon and oxygenated fragments to form the critical C8 quaternary carbon stereocenter and set the stage for elaborating the delicate bicyclic lactone functionality was accomplished in high yield and exquisite stereoselectivity by Michael addition of an enantioenriched hydroazulenone enolate to an enantiopure α-bromocyclopentenone.

Structures and Anatomical Distribution of Oxygenated Diterpenes in the Australian Nudibranch Chromodoris reticulata

The structures and stereochemistry of six new diterpenes (1–6), two of which contain cyclic imine functionality, have been deduced by 2D NMR spectroscopy. The anatomical distribution of these, and of 17 other diterpenes (7–23) that were also isolated, has been investigated. The known compound aplyroseol-2 (14) was the major compound in the mantle tissue along with some dialdehydes, while the linear furan ambliofuran (7) was the only diterpene found solely in the internal organs. The presence of lactone-acetal-hemiacetal functionality in many of the isolated compounds is a consequence of the reactive dialdehydes present in the mollusc.