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.

Selective blocking effects of 4,9-anhydrotetrodotoxin, purified from a crude mixture of tetrodotoxin analogues, on NaV1.6 channels and its chemical aspects

Tetrodotoxin (TTX) is a potent neurotoxin found in a number of marine creatures including the pufferfish, where it is synthesized by bacteria and accumulated through the food chain. It is a potent and selective blocker of some types of voltage-gated Na⁺ channel (Na_V channel). 4,9-Anhydrotetrodotoxin (4,9-anhydroTTX) was purified from a crude mixture of TTX analogues (such as TTX, 4-epiTTX, 6-epiTTX, 11-oxoTTX and 11-deoxyTTX) by the use of liquid chromatography-fluorescence detection (LC-FLD) techniques. Recently, it has been reported that 4,9-anhydroTTX selectively blocks the activity of Na_V1.6 channels with a blocking efficacy 40–160 times higher than that for other TTX-sensitive Na_V1.x channel isoforms. However, little attention has been paid to the molecular properties of the α-subunit in Na_V1.6 channels and the characteristics of binding of 4,9-anhydroTTX. From a functional point of view, it is important to determine the relative expression of Na_V1.6 channels in a wide variety of tissues. The aim of this review is to discuss briefly current knowledge about the pharmacology of 4,9-anhydroTTX, and provide an analysis of the molecular structure of native Na_V1.6 channels. In addition, chemical aspects of 4,9-anhydroTTX are briefly covered.

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.

The chemical synthesis of tetrodoxin: an ongoing quest

This contribution reviews all the synthetic work on tetrodotoxin that has appeared in the literature through June 2011.

An efficient total synthesis of optically active tetrodotoxin from levoglucosenone

Tetrodotoxin, a toxic principal of puffer-fish poisoning, is one of the most famous marine natural products, and has been known as a formidable synthetic target in synthesis owing to its multifunctional structure and unusual chemical properties. From the perspective of supplying tetrodotoxin derivatives such as labeled molecules for biochemical research, we have completed our second total synthesis of tetrodotoxin from a synthetic intermediate for 11-deoxytetrodotoxin, which was previously prepared from levoglucosenone as a chiral starting material in this laboratory. This paper discloses the details of the total synthesis with special reference to significant influences on the neighboring functional groups found in the installation of guanidine. The established route should allow us to prepare the tetrodotoxin-related compounds required for biochemical studies.

Total Synthesis of (−)-5,6,11-Trideoxytetrodotoxin and Its 4-Epimer

[reaction: see text] The first total synthesis of 5,6,11-trideoxytetrodotoxin (1) and its 4-epimer were achieved. The synthesis is characterized by the stereoselective construction of the quaternary amino carbon center at C8a by an asymmetric transferring Strecker synthesis and the highly efficient conversion of cyanohydrin 4 to 1 via intramolecular cyclization reactions.

Marine natural products: synthetic aspects

An overview of marine natural products synthesis during 2004 is provided. As with the previous installment of this series, the emphasis is on total syntheses of molecules of contemporary interest, new total syntheses, and syntheses that have resulted in structure confirmation or stereochemical assignments.

Novel and Stereocontrolled Synthesis of (±)-Tetrodotoxin from myo-Inositol

[reactions: see text] The novel and stereocontrolled synthesis of (+/-)-tetrodotoxin from myo-inositol is described. The key steps involve the stepwise oxidation of hydroxyl groups to the carbonyl function, followed by the addition of specific nucleophiles, including the successive spiro alpha-chloroepoxide formation and its ring-opening with the azide anion, to give the desired branched chain structures (5-->6, 17-->18-->19-->20 and 23-->24-->25) with the desired regio- and stereoselectivities in high yields. The stepwise conversion of the alpha-azido aldehyde 25 to the delta-lactone 29, followed by reduction of the azide, introduction of a guanidine moiety, aldehyde formation, and deprotection, produced the (+/-)-tetrodotoxin.

Marine natural products: synthetic aspects

An overview of marine natural products synthesis during 2003 is provided. The emphasis on total syntheses of molecules of contemporary interest, new total syntheses, and syntheses that have resulted in structure conformation or stereochemical assignments.

Synthesis of (-)-tetrodotoxin: preparation of an advanced cyclohexenone intermediate

The preparation of an advanced intermediate toward the enantioselective synthesis of tetrodotoxin is outlined. The enantiomerically pure cyclopentene 15 was generated from ketone 14 by alkylidene carbene insertion with retention of absolute configuration. An ozonolysis/aldol sequence first produced the trans cyclohexenone, which upon epimerization gave the more stable cis enone 18.

Stereocontrolled Synthesis of 8,11-Dideoxytetrodotoxin, An Unnatural Analogue of Puffer Fish Toxin

8,11-Dideoxytetrodotoxin, an unnatural tetrodotoxin analogue, was synthesized in a highly stereoselective manner from a common intermediate from our synthetic studies on tetrodotoxin. The key features in the synthesis were as follows: neighboring group participation of a trichloroacetamide to allow regioselective and stereoselective hydroxylation, protection of a delta-hydroxylactone as an ortho ester, and guanidine installation through the use of Boc-protected isothiourea. Global deprotection of the fully protected intermediate under acidic conditions gave 8,11-dideoxytetrodotoxin, which exhibited very weak biological activities.

A stereoselective synthesis of (-)-tetrodotoxin

An asymmetric synthesis of the fugu fish poison, (-)-tetrodotoxin, is described. The route to this extraordinary target employs a number of unique transformations, foremost of which are two stereospecific C-H bond functionalization reactions. Accordingly, Rh-catalyzed carbene and nitrene C-H insertions facilitate rapid entry to the cyclohexane core of the natural product and make possible the late-stage installation of the tetrasubstituted carbinolamine center.

First asymmetric total synthesis of tetrodotoxin

Tetrodotoxin, a toxic principle of puffer fish poisoning, is one of the most famous marine natural products because of the complex structure having many functional groups and its potent biological activity leading to death. Since the structure elucidation in 1964, this toxin has been recognized as a formidable target molecule for total synthesis. We have recently achieved the first asymmetric total synthesis from 2-acetoxy-tri-O-acetyl-d-glucal as a chiral starting material. The highly hydroxylated cyclohexane ring was constructed by Claisen rearrangement and regioselective hydroxylations of an acetone moiety and an intramolecular directed aldol condensation of the precursor having methyl ketone with dihydroxyacetone, which was synthesized through Sonogashira coupling. Installation of nitrogen functionality was unsuccessful through an attempted Overman rearrangement. We, therefore, employed a new intramolecular conjugate addition strategy between the carbamate and unsaturated ester groups. The alpha-hydroxyl lactone moiety was synthesized through an intramolecular epoxide opening by the Z-enolate of aldehyde, which was followed by oxidation-reduction of the resulting cyclic vinyl ether. The lactone was then converted to a protected ortho ester, and then gunanidinylation was followed by cleavage of the 1,2-glycol to give the fully protected tetrodotoxin. Selection of the protective groups has finally led us to accomplish the total synthesis of tetrodotoxin in an enantiomerically pure form. All the stereogenic centers were controlled with high selectivity, and the hydroxyl groups were differently protected to discriminate for the future analogue synthesis of a bioorganic program. The synthetic tetrodotoxin was purified by ion exchange chromatography and characterized to be identical with the natural compound.

Stereocontrolled synthesis of 8,11-dideoxytetrodotoxin, unnatural analogue of puffer fish toxin

[structure: see text] 8,11-Dideoxytetrodotoxin, an unnatural tetrodotoxin analogue, was synthesized in a highly stereoselective manner from a common intermediate in our synthetic studies on tetrodotoxin. The synthesis features neighboring group participation of trichloroacetamide for stereoselective hydroxylation, protection of ortho ester, and guanidine installation with Boc-protected isothiourea.

Asymmetric total synthesis of 11-deoxytetrodotoxin, a naturally occurring congener

Tetrodotoxin, a toxic principle of puffer fish poisoning, is a specific blocker of sodium channel. Despite many synthetic efforts since the structure elucidation in 1964, the only total synthesis of the racemic tetrodotoxin has been reported by Kishi and co-workers. In the course of our studies directed toward the total synthesis to analyze biologically interesting issues associated with tetrodotoxin, we accomplished a highly stereocontrolled synthesis of (-)-5,11-dideoxytetrodotoxin in 1999. Based on the synthesis, we describe herein the first total synthesis of 11-deoxytetrodotoxin, a naturally occurring analogue. The synthesis started from an allylic alcohol, the same intermediate for the synthesis of 5,11-dideoxytetrodotoxin. Epoxidation of the allylic alcohol was followed by isomerization with Ti(i-PrO)(4) to give an alpha-hydroxy allylic alcohol, in which the configurations of the two hydroxyl groups were inverted by oxidation and then a 2-step reduction. Further epoxidation of the allylic alcohol and ozonolysis of the remaining vinyl group gave an aldehyde, which reacted with magnesium acetylide to give a propargyl alcohol in a stereoselective manner. Oxidative cleavage of the acetylenic moiety with RuO(4) afforded a fully functionalized lactone for 11-deoxytetrodotoxin. Crucial guanidinylation was achieved from trichloroacetamide according to our own method to give acetyldibenzylguanidine. Finally, deprotection of benzyl groups, acetates, and acetal furnished 11-deoxytetrodotoxin.

Effects of specific modifications of several hydroxyls of tetrodotoxin on its affinity to rat brain membrane

The widely used sodium channel blocker tetrodotoxin (TTX) is a compound that has six hydroxyl residues at the C-4, C-6, C-8, C-9, C-10, and C-11 positions in addition to a guanidinium group, which is positively charged in biological pH range. Thirteen analogs of this toxin with structural modifications involving one or more of these hydroxyls were examined on their affinity to a rat brain membrane preparation, which is known to contain sodium channels abundantly. The equilibrium dissociation constants associated with the binding of TTX and its analogs to the sodium channels were estimated, from their ability to inhibit the binding of [3H]saxitoxin, as follows (in nM): TTX, 1.8; chiriquitoxin, 1.0; 11-oxoTTX, 1.5; 11-norTTX-6,6-diol, 1.6; 11-norTTX-6(S)-ol, 23; 11-norTTX-6(R)-ol, 31; 11-deoxyTTX, 37; 6-epiTTX, 39; 4-epiTTX, 68; 4,9-anhydroTTX, 180; TTX-8-O-hemisuccinate, >380; TTX-11-carboxylic acid, >2300; tetrodonic acid, >3600; 5,6,11-trideoxyTTX, >5000. The reduction of the affinity observed with the analogs involving reduction or translocation of the hydroxyls at C-6 and C-11 is indicative of the contribution of these residues to the binding to sodium channels as hydrogen bond donors. The especially large value of the dissociation constant for TTX-11-carboxylic acid is consistent with the idea that the C-11-hydroxyl forms a hydrogen bond with a carboxylic acid residue of the channel protein. The markedly low affinity of TTX-8-O-hemisuccinate may possibly be ascribable to intramolecular salt-bridge formation, which neutralizes the positive charge of the guanidinium group.

Photoaffinity labeling of the electroplax sodium channel with tetrodotoxin derivatives. II. Comparison of the photoreactivity of different photoactivable groups in the tetrodotoxin binding site

Four photoactivable tetrodotoxin (TTX) derivatives and the corresponding tritiated compounds were synthesized and purified. The photoactivable groups introduced were a nitro-azidophenyl (NAP) group and a trifluoromethyl diazirinobenzoyl (TDB) group, as nitrene and carbene precursors, respectively, as well as two isomers of the fluoro-nitrophenoxy group, a new photoreactive group selective for nucleophiles. The binding affinities of the four derivatives to the sodium channel were similar to that of TTX, but the values of specific photoincorporation were unexpectedly low (up to 2.5%), suggesting that the photoactivable groups are likely to be oriented unfavorably for labeling reactions in the TTX binding site. Two of the labeled sodium channel proteins were purified and digested. Peptides labeled with TTXenNAP lost most of the label during high performance liquid chromatography in 0.1% trifluoroacetic acid-acetonitrile, while peptides labeled with TTXenTDB retained the labels during the procedures. The TTXenTDB-labeled peptides were eluted in four major fractions with 80% recovery of the amount applied on a reverse-phased C4 column. However, further purification is necessary to identify the labeled sites of the peptides.

High-yield synthesis of a [3H]ethylenediamine ditetrodotoxin derivative

[3H]Tetrodotoxin [( 3H]TTX) and a [3H]ethylenediamine derivative of TTX are the most widely used ligands for the study of the Na channel. The former ligand presents a low specific radioactivity (1 Ci/mmol) while the latter is highly labeled (30 Ci/mmol). However, its two-step synthesis, i.e., mild oxidation followed by coupling of [3H]ethylenediamine, has been described with a low overall yield of 1.7%. In this work, more favorable experimental conditions are defined for the limiting reaction, i.e., the oxidation step, using [14C]testosterone as a model molecule. Applied to the oxidation of tetrodotoxin, this procedure produces yield values of 30-50%, as determined by high-performance liquid chromatography. Moreover, two oxidized TTX molecules appear to be covalently linked to [3H]ethylenediamine, yielding a new labeled tetrodotoxin derivative with a specific radioactivity of 45 Ci/mmol and a dissociation constant of 0.6 nM for electroplax membranes.