The chemical synthesis of tetrodoxin: an ongoing quest

A concise synthesis of tetrodotoxin

Tetrodotoxin (TTX) is a neurotoxic natural product that is an indispensable probe in neuroscience, a biosynthetic and ecological enigma, and a celebrated target of synthetic chemistry. Here, we present a stereoselective synthesis of TTX that proceeds in 22 steps from a glucose derivative. The central cyclohexane ring of TTX and its α-tertiary amine moiety were established by the intramolecular 1,3-dipolar cycloaddition of a nitrile oxide, followed by alkynyl addition to the resultant isoxazoline. A ruthenium-catalyzed hydroxylactonization set the stage for the formation of the dioxa-adamantane core. Installation of the guanidine, oxidation of a primary alcohol, and a late-stage epimerization gave a mixture of TTX and anhydro-TTX. This synthetic approach could give ready access to biologically active derivatives.

Synthesis of pyrimidine-containing alkaloids

This review deals with the synthesis of naturally occurring alkaloids containing partially or completely saturated pyrimidine nuclei. The interest in these compounds is associated with their structural diversity, high biological activity and toxicity. The review is divided into four parts, each of which describes a number of synthetic methodologies toward structurally different naturally occurring alkaloids containing saturated cyclic six-membered amidine, guanidine, aminal and urea (thiourea) moieties, respectively. The development of various synthetic strategies for the preparation of these compounds has remarkably increased during the past few decades. This is primarily due to the fact that some of these compounds are isolated only in limited quantities, which makes it practically impossible to study their full structural characteristics and biological activity.

The skin microbiome facilitates adaptive tetrodotoxin production in poisonous newts

Rough-skinned newts (Taricha granulosa) use tetrodotoxin (TTX) to block voltage-gated sodium (Nav) channels as a chemical defense against predation. Interestingly, newts exhibit extreme population-level variation in toxicity attributed to a coevolutionary arms race with TTX-resistant predatory snakes, but the source of TTX in newts is unknown. Here, we investigated whether symbiotic bacteria isolated from toxic newts could produce TTX. We characterized the skin-associated microbiota from a toxic and non-toxic population of newts and established pure cultures of isolated bacterial symbionts from toxic newts. We then screened bacterial culture media for TTX using LC-MS/MS and identified TTX-producing bacterial strains from four genera, including Aeromonas, Pseudomonas, Shewanella, and Sphingopyxis. Additionally, we sequenced the Nav channel gene family in toxic newts and found that newts expressed Nav channels with modified TTX binding sites, conferring extreme physiological resistance to TTX. This study highlights the complex interactions among adaptive physiology, animal-bacterial symbiosis, and ecological context.

Guanidine cyclic diimides and their polymers

We report the formation and degradation of a unique guanidine cyclic diimide (GCDI) structure under mild conditions. Furthermore, the GCDI-based polymers can be readily synthesized from guanidine and dianhydride monomers.

Production and Synthetic Modifications of Shikimic Acid

Shikimic acid is a natural product of industrial importance utilized as a precursor of the antiviral Tamiflu. It is nowadays produced in multihundred ton amounts from the extraction of star anise ( Illicium verum) or by fermentation processes. Apart from the production of Tamiflu, shikimic acid has gathered particular notoriety as its useful carbon backbone and inherent chirality provide extensive use as a versatile chiral precursor in organic synthesis. This review provides an overview of the main synthetic and microbial methods for production of shikimic acid and highlights selected methods for isolation from available plant sources. Furthermore, we have attempted to demonstrate the synthetic utility of shikimic acid by covering the most important synthetic modifications and related applications, namely, synthesis of Tamiflu and derivatives, synthetic manipulations of the main functional groups, and its use as biorenewable material and in total synthesis. Given its rich chemistry and availability, shikimic acid is undoubtedly a promising platform molecule for further exploration. Therefore, in the end, we outline some challenges and promising future directions.

Highly Functionalized Tricyclic Oxazinanones via Pairwise Oxidative Dearomatization and N-Hydroxycarbamate Dehydrogenation: Molecular Diversity Inspired by Tetrodotoxin

Benzenoids in principle represent attractive and abundant starting materials for the preparation of substituted cyclohexanes; however, the synthetic tools available for overcoming the considerable aromatic energies inherent to these building blocks limit the available product types. In this paper, we demonstrate access to heretofore unknown heterotricyclic structures by leveraging oxidative dearomatization of 2-hydroxymethyl phenols with concurrent N-hydroxycarbamate dehydrogenation using a common oxidant. The pairwise-generated, mutually reactive species then participate in a second stage acylnitroso Diels-Alder cycloaddition. The reaction chemistry of the derived [2.2.2]-oxazabicycles, bearing four orthogonal functional groups and three stereogenic centers, is shown to yield considerable diversity in downstream products. The methodology allows for the expeditious synthesis of a functionalized intermediate bearing structural and stereochemical features in common with the complex alkaloid tetrodotoxin.

Formation of the Main Cores Present in Natural Products by Tandem Additions

A rapid route to 5,5- and 5,6- bicyclic systems is provided by an 1,3-alkyl-shift process mediated by a hypervalent iodine reagent on aromatics. The structures obtained contain several unsaturations with different behaviors and reactivities. Such diversity allows further elaborations for the rapid formation of compact systems present in a variety of natural products. The potential for further transformations has been demonstrated by performing a double Michael addition. This cyclization process is regio- and stereoselective due to the presence of a former benzylic substituent. Furthermore, an extension of this approach has been accomplished on indole derivatives.

Synthetic approaches towards alkaloids bearing α-tertiary amines

Alkaloids account for some of the most beautiful and biologically active natural products. Although they are usually classified along biosynthetic criteria, they can also be categorized according to certain structural motifs. Amongst these, the α-tertiary amine (ATA), i.e. a tetrasubstituted carbon atom surrounded by three carbons and one nitrogen, is particularly interesting. A limited number of methods have been described to access this functional group and fewer still are commonly used in synthesis. Herein, we review some approaches to asymmetrically access ATAs and provide an overview of alkaloid total syntheses where those have been employed.

Tetrodotoxin, an Extremely Potent Marine Neurotoxin: Distribution, Toxicity, Origin and Therapeutical Uses

Tetrodotoxin (TTX) is a potent neurotoxin responsible for many human intoxications and fatalities each year. The origin of TTX is unknown, but in the pufferfish, it seems to be produced by endosymbiotic bacteria that often seem to be passed down the food chain. The ingestion of contaminated pufferfish, considered the most delicious fish in Japan, is the usual route of toxicity. This neurotoxin, reported as a threat to human health in Asian countries, has spread to the Pacific and Mediterranean, due to the increase of temperature waters worldwide. TTX, for which there is no known antidote, inhibits sodium channel producing heart failure in many cases and consequently death. In Japan, a regulatory limit of 2 mg eq TTX/kg was established, although the restaurant preparation of “fugu” is strictly controlled by law and only chefs qualified are allowed to prepare the fish. Due to its paralysis effect, this neurotoxin could be used in the medical field as an analgesic to treat some cancer pains.

Stereocontrolled Total Synthesis of Tetrodotoxin from myo-Inositol and D-Glucose by Three Routes: Aspects for Constructing Complex Multi-Functionalized Cyclitols with Branched-Chain Structures

This report describes the stereocontrolled total synthesis of the multi-functionalized cyclitol derivative, tetrodotoxin, containing eight asymmetric carbons and different types of branched-chains, from myo-inositol and D-glucose using three different methods. The tetrodotoxin derivatives possess a relatively small molecular weight but unique structural and chemical properties. Selection of the appropriate synthetic method may be useful not only for compounds related to TTX (including related derivatives), but also for other highly complex multi-functionalized cyclitols containing branched-chains.

Syntheses of Cyclic Guanidine-Containing Natural Products

Naturally occurring guanidine derivatives frequently display medicinally useful properties. Among them, the higher order pyrrole-imidazole alkaloids, the dragmacidins, the crambescidins/batzelladines, and the saxitoxins/tetradotoxins have stimulated the development of many new synthetic methods over the past decades. We provide here an overview of the syntheses of these cyclic guanidine-containing natural products.

Synthetic studies on heterocyclic natural products

This paper highlights ongoing efforts toward Erythrina alkaloids, himandrine, tetrodotoxin, and thiopeptide antibiotics such as nosiheptide and describes representative spinoffs in biomedicine that emanated from the author’s research in synthetic organic chemistry.

Recent progress in neuroactive marine natural products

This review covers the isolation, chemical structure, biological activity, structure activity relationships including synthesis of chemical probes, and pharmacological characterization of neuroactive marine natural products; 302 references are cited.

Stereocontrolled Total Synthesis of Tetrodotoxin from myo-Inositol and D-Glucose by Three Routes: Aspects for Constructing Complex Multi-Functionalized Cyclitols with Branched-Chain Structures

This report describes the stereocontrolled total synthesis of the multi-functionalized cyclitol derivative tetrodotoxin containing eight asymmetric carbons and different types of branched-chains from myo-inositol and D-glucose using three different methods. The tetrodotoxin derivatives possess a relatively small molecular weight but unique structural and chemical properties. Selection of the appropriate synthetic method may be useful not only for compounds related to TTX (including related derivatives) but also for other highly complex multi-functionalized cyclitols containing branched-chains.

Synthesis of tetrodotoxin, a classic but still fascinating natural product

Tetrodotoxin, a toxic principle of puffer fish intoxication, is one of the most famous marine natural products due to its densely functionalized structure and potent toxicity. Despite its small molecular size (MW 319 g mol⁻¹), tetrodotoxin has long been well known as a formidable molecule in natural product synthesis. We have devoted more than twenty years to developing synthetic strategies for this molecule, resulting in the preparation of a variety of analogues of tetrodotoxin for biological experiments. This account describes a brief history of tetrodotoxin research and an overview of our synthetic efforts toward tetrodotoxin with the underlying logic and strategy.

An improved synthesis of (-)-5,11-dideoxytetrodotoxin

We describe an improved synthesis of (-)-5,11-dideoxytetrodotoxin from an enone, which was used for synthesis of tetrodotoxin and its analogues in this laboratory. One of the major modifications was to establish a two-step guanidinylation of trichloroacetamide of a highly functionalized intermediate, which allowed us to prepare (15)N(2)-labeled 5,11-dideoxytetrodotoxin for biosynthetic investigations.

The molecular mystique of tetrodotoxin

In many respects tetrodotoxin (TTX) is the quintessential natural toxin. It is unequivocally toxic to mammals with LD(50) values for mice in the range of 10 μg/kg (intraperitoneal), 16 μg/kg (subcutaneous), and 332 μg/kg (oral) (Kao, 1966). Its biothreat status is recognized by its listing as a "Select Agent" by the US Department of Health and Human Services which includes regulated agents "determined to have the potential to pose a severe threat to both human and animal health" (http://www.selectagents.gov/). It has a well-defined cellular target (i.e., NaV channels) and pharmacological mode of action (i.e., block of nerve and muscle action potentials), and it is an indispensable chemical tool in neuroscience. It is widely distributed in marine and terrestrial ecosystems where it plays a role in the chemical ecology of predator-prey relationships and drives evolutionary selection of TTX-resistance (Hanifin, 2010; Williams, 2010; Zimmer and Ferrer, 2007). Lastly, TTX has acquired a certain mystique in scientific lore attributable to many fascinating aspects of its natural history and molecular interactions as presented in selected summary below. Additional information may be found in other excellent reviews (Fozzard and Lipkind, 2010; Kao, 1966; Lee and Ruben, 2008; Narahashi, 2001, 2008).

Enantioselective synthesis of protected nitrocyclohexitols with five stereocenters. Total synthesis of (+)-pancratistatin

2-Methoxymethylpyrrolidine best performed, among several other proline derivatives, to control the enantioselective [3+3] annulation of β-(hetero)aryl-α-nitro-α,β-enals with commercial 2,2-dimethyl-1,3-dioxan-5-one, a procedure that renders highly oxygenated nitrocyclohexanes endowed with five new stereocenters. Use of this reaction allowed the development of a total synthesis of the antitumoral natural product (+)-pancratistatin; it also converted our previous racemic route to tetrodotoxin into an enantioselective one.

The chemistry and biology of organic guanidine derivatives

The chemistry and biology of organic natural guanidines are reviewed, including the isolation, structure determination, synthesis, biosynthesis and biological activities of alkaloids, non-ribosomal peptides, guanidine-bearing terpenes, polyketides and shikimic acid derivatives from natural sources.