Total Synthesis and Structural Elucidation of Azaspiracid-1. Final Assignment and Total Synthesis of the Correct Structure of Azaspiracid-1

New azaspiracid analogues detected as bi-charged ions in Azadinium poporum (Amphidomataceae, Dinophyceae) isolated from Japanese coastal waters

Lipophilic marine biotoxin azaspiracids (AZAs) are produced by dinoflagellates Azadinium and Amphidoma. Recently, several strains of Azadinium poporum were isolated from Japanese coastal waters, and detailed toxin profiles of two strains (mdd421 and HM536) among them were clarified by several detection techniques on liquid chromatography-tandem mass spectrometry (LC-MS/MS) and liquid chromatography-quadrupole time of flight mass spectrometry (LC-QTOFMS). In our present study, AZA analogues in seven strains of A. poporum from Japanese coastal waters (including two previously reported strains) were determined by these detection techniques. The dominant AZA in the seven strains was AZA2 accompanied by small amounts of several known AZAs and twelve new AZA analogues. Eight of the twelve new AZA analogues discovered in our present study were detected as bi-charged ions on the positive mode LC/MS/MS. This is the first report describing AZA analogues detected as bi-charged ions with hexose and sulfate groups in their structures.

Complex profiles of azaspiracid analogues in two culture strains of Azadinium poporum (Amphidomataceae, Dinophyceae) isolated from Japanese coastal waters determined by LC-MS/MS

Lipophilic marine biotoxins azaspiracids (AZAs) are produced by dinoflagellates Azadinium and Amphidoma. Recently, several strains of Azadinium poporum were isolated from Japanese coastal waters. In our present study, AZA analogues in two strains (mdd421 and HM536) of A. poporum were analyzed by several detection techniques on the liquid chromatography-tandem mass spectrometry (LC-MS/MS) and liquid chromatography-quadrupole time of flight mass spectrometry (LC-QTOFMS). The dominant AZA analogue in the Japanese A. poporum strains was AZA2. Other known AZA analogues were AZA11, AZA35, AZA2 methyl ester and AZA2 phosphate ester. Besides these AZAs, thirteen new AZA analogues were discovered in the two strains. A putative AZA analogue (Compound 1) with the smallest molecular weight ever found in nature was also discovered in the two strains. This is the first report describing detailed AZA profiles in Japanese isolates of A. poporum.

The enantioselective total synthesis of laurendecumallene B

For decades, the Laurencia family of halogenated C15-acetogenins has served as a valuable testing ground for the prowess of chemical synthesis, particularly as it relates to generating functionalized 8-membered bromoethers. Herein, we show that a readily modified and predictable approach that generates such rings and an array of attendant stereocenters via a bromenium-induced cyclization/ring-expansion process can be used to synthesize laurendecumallene B and determine the configuration of two of its previously unassigned stereocenters. In particular, this work highlights how the use of the bromenium source BDSB (Et2SBr·SbCl5Br) in non-conventional solvents is essential in generating much of the target's complexity in optimal yields and stereoselectivity. Moreover, the final structural assignment of laurendecumallene B reveals that it has one element of bromine-based chirality that, to the best of our knowledge, is not shared with any other member of the class.

Variability and profiles of lipophilic toxins in bivalves from Great Britain during five and a half years of monitoring: azaspiracids and yessotoxins

Cefas has been responsible for the delivery of official control biotoxin testing of bivalve molluscs from Great Britain for just over a decade. Liquid chromatography tandem mass spectrometric (LC-MS/MS) methodology has been used for the quantitation of lipophilic toxins (LTs) since 2011. The temporal and spatial distribution of okadaic acid group toxins and profiles in bivalves between 2011 and 2016 have been recently reported. Here we present data on the two other groups of regulated lipophilic toxins, azaspiracids (AZAs) and yessotoxins (YTXs), over the same period. The latter group has also been investigated for a potential link with Protoceratium reticulatum and Lingulodinium polyedra, both previously recognised as YTXs producing phytoplankton. On average, AZAs were quantified in 3.2% of all tested samples but notable inter-annual variation in abundance was observed. The majority of all AZA contaminated samples were found between July 2011 and August 2013 in Scotland, while only two, three-month long, AZA events were observed in 2015 and 2016 in the south-west of England. Maximum concentrations were generally reached in late summer or early autumn. Reasons for AZAs persistence during the 2011/2012 and 2012/2013 winters are discussed. Only one toxin profile was identified, represented by both AZA1 and AZA2 toxins at an approximate ratio of 2 : 1, suggesting a single microalgal species was the source of AZAs in British bivalves. Although AZA1 was always the most dominant toxin, its proportion varied between mussels, Pacific oysters and surf clams. The YTXs were the least represented group among regulated LTs. YTXs were found almost exclusively on the south-west coast of Scotland, with the exception of 2013, when the majority of contaminated samples originated from the Shetland Islands. The highest levels were recorded in the summer months and followed a spike in Protoceratium reticulatum cell densities. YTX was the most dominant toxin in shellfish, further strengthening the link to P. reticulatum as the YTX source. Neither homo-YTX, nor 45-OH homo-YTX were detected throughout the monitored period. 45-OH YTX, thought to be a shellfish metabolite associated with YTX elimination, contributed on average 26% in mussels. Although the correlation between 45-OH YTX abundance and the speed of YTX depuration could not be confirmed, we noted the half-life of YTX was more than two-times longer in queen scallops, which contained 100% YTX, than in mussels. No other bivalve species were affected by YTXs. This is the first detailed evaluation of AZAs and YTXs occurrences and their profiles in shellfish from Great Britain over a period of multiple years.

Iterative Assembly of Polycyclic Saturated Heterocycles from Monomeric Building Blocks

Polycyclic saturated heterocycles with predictable shapes and structures are assembled by iterative couplings of bifunctional stannyl amine protocol (SnAP) reagents and a single morpholine-forming assembly reaction. Combinations of just a few monomers enable the programmable construction of rotationally restricted, nonplanar heterocyclic arrays with discrete sizes and molecular shapes. The three-dimensional structures of these constrained scaffolds can be quickly and reliably predicted by DFT calculations and the target structures immediately decompiled into the constituent building blocks and assembly sequences. As a demonstration, in silico combinations of the building blocks predict saturated heptacyclic structures with elementary shapes including helices, S-turns and U-turns, which are synthesized in 5-6 steps from the monomers using just three chemical reactions.

Synthesis of the C1-C19 Domain of Azaspiracid-34

Azaspiracid-34 (AZA34) is a recently described structurally unique member of the azaspiracid class of marine neurotoxins. Its novel structure, tentatively assigned on the basis of MS and ¹H NMR spectroscopy, is accompanied by a 5.5-fold higher level of toxicity against Jurkat T lymphocytes than AZA1. To completely assign the structure of AZA34 and provide material for in-depth biological evaluation and detection, synthetic access to AZA34 was targeted. This began with the convergent and stereoselective assembly of the C1-C19 domain of AZA34 designed to dovetail with the recent total synthesis approach to AZA3.

Phycotoxins in Marine Shellfish: Origin, Occurrence and Effects on Humans

Massive phytoplankton proliferation, and the consequent release of toxic metabolites, can be responsible for seafood poisoning outbreaks: filter-feeding mollusks, such as shellfish, mussels, oysters or clams, can accumulate these toxins throughout the food chain and present a threat for consumers' health. Particular environmental and climatic conditions favor this natural phenomenon, called harmful algal blooms (HABs); the phytoplankton species mostly involved in these toxic events are dinoflagellates or diatoms belonging to the genera Alexandrium, Gymnodinium, Dinophysis, and Pseudo-nitzschia. Substantial economic losses ensue after HABs occurrence: the sectors mainly affected include commercial fisheries, tourism, recreational activities, and public health monitoring and management. A wide range of symptoms, from digestive to nervous, are associated to human intoxication by biotoxins, characterizing different and specific syndromes, called paralytic shellfish poisoning, amnesic shellfish poisoning, diarrhetic shellfish poisoning, and neurotoxic shellfish poisoning. This review provides a complete and updated survey of phycotoxins usually found in marine invertebrate organisms and their relevant properties, gathering information about the origin, the species where they were found, as well as their mechanism of action and main effects on humans.

Chemo- and regioselective reductive deoxygenation of 1-en-4-yn-ols into 1,4-enynes through FeF3 and TfOH co-catalysis

We report chemo- and regioselective direct reductive deoxygenation of 1-en-4-yn-3-ols into 1,4-enynes through FeF₃ and TfOH cooperative catalysis under NBSH-mediated conditions.

Microsphere-based immunoassay for the detection of azaspiracids

Azaspiracids (AZAs) are a group of lipophilic toxins discovered in mussels from Ireland in 1995 following a human poisoning incident. Nowadays the regulatory limit for AZAs in many countries is set at 160 μg of azaspiracid equivalents per kilogram of shellfish meat. In this work a microsphere-based immunoassay has been developed for the detection of AZAs using a Luminex system. This method is based on the competition between AZA-2 immobilized onto the surface of microspheres and free AZAs for the interaction with a monoclonal anti-azaspiracid antibody (mAb 8F4). In this inhibition immunoassay the amount of mAb 8F4 bound to AZA-2 microspheres was quantified using a phycoerythrin-labeled anti-mouse antibody, and the fluorescence was measured with a Luminex analyzer. Simple acetate/methanol or methanol extractions yielded final extracts with no matrix interferences and adequate recovery rates of 86.5 and 75.8%, respectively. In summary, this work presents a sensitive and easily performed screening method capable of detecting AZAs at concentrations below the range of the European regulatory limit using a microsphere/flow cytometry system.

Constructing molecular complexity and diversity: total synthesis of natural products of biological and medicinal importance

The advent of organic synthesis and the understanding of the molecule as they occurred in the nineteenth century and were refined in the twentieth century constitute two of the most profound scientific developments of all time. These discoveries set in motion a revolution that shaped the landscape of the molecular sciences and changed the world. Organic synthesis played a major role in this revolution through its ability to construct the molecules of the living world and others like them whose primary element is carbon. Although the early beginnings of organic synthesis came about serendipitously, organic chemists quickly recognized its potential and moved decisively to advance and exploit it in myriad ways for the benefit of mankind. Indeed, from the early days of the synthesis of urea and the construction of the first carbon-carbon bond, the art of organic synthesis improved to impressively high levels of sophistication. Through its practice, today chemists can synthesize organic molecules--natural and designed--of all types of structural motifs and for all intents and purposes. The endeavor of constructing natural products--the organic molecules of nature--is justly called both a creative art and an exact science. Often called simply total synthesis, the replication of nature's molecules in the laboratory reflects and symbolizes the state of the art of synthesis in general. In the last few decades a surge in total synthesis endeavors around the world led to a remarkable collection of achievements that covers a wide ranging landscape of molecular complexity and diversity. In this article, we present highlights of some of our contributions in the field of total synthesis of natural products of biological and medicinal importance. For perspective, we also provide a listing of selected examples of additional natural products synthesized in other laboratories around the world over the last few years.

Production and isolation of azaspiracid-1 and -2 from Azadinium spinosum culture in pilot scale photobioreactors

Azaspiracid (AZA) poisoning has been reported following consumption of contaminated shellfish, and is of human health concern. Hence, it is important to have sustainable amounts of the causative toxins available for toxicological studies and for instrument calibration in monitoring programs, without having to rely on natural toxin events. Continuous pilot scale culturing was carried out to evaluate the feasibility of AZA production using Azadinium spinosum cultures. Algae were harvested using tangential flow filtration or continuous centrifugation. AZAs were extracted using solid phase extraction (SPE) procedures, and subsequently purified. When coupling two stirred photobioreactors in series, cell concentrations reached 190,000 and 210,000 cell·mL⁻¹ at steady state in bioreactors 1 and 2, respectively. The AZA cell quota decreased as the dilution rate increased from 0.15 to 0.3 day⁻¹, with optimum toxin production at 0.25 day⁻¹. After optimization, SPE procedures allowed for the recovery of 79 ± 9% of AZAs. The preparative isolation procedure previously developed for shellfish was optimized for algal extracts, such that only four steps were necessary to obtain purified AZA1 and -2. A purification efficiency of more than 70% was achieved, and isolation from 1200 L of culture yielded 9.3 mg of AZA1 and 2.2 mg of AZA2 of >95% purity. This work demonstrated the feasibility of sustainably producing AZA1 and -2 from A. spinosum cultures.

Survey of marine natural product structure revisions: a synergy of spectroscopy and chemical synthesis

The structural assignment of new natural product molecules supports research in a multitude of disciplines that may lead to new therapeutic agents and or new understanding of disease biology. However, reports of numerous structural revisions, even of recently elucidated natural products, inspired the present survey of techniques used in structural misassignments and subsequent revisions in the context of constitutional or configurational errors. Given the comparatively recent development of marine natural products chemistry, coincident with modern spectroscopy, it is of interest to consider the relative roles of spectroscopy and chemical synthesis in the structure elucidation and revision of those marine natural products that were initially misassigned. Thus, a tabulated review of all marine natural product structural revisions from 2005 to 2010 is organized according to structural motif revised. Misassignments of constitution are more frequent than perhaps anticipated by reliance on HMBC and other advanced NMR experiments, especially when considering the full complement of all natural products. However, these techniques also feature prominently in structural revisions, specifically of marine natural products. Nevertheless, as is the case for revision of relative and absolute configuration, total synthesis is a proven partner for marine, as well as terrestrial, natural products structure elucidation. It also becomes apparent that considerable 'detective work' remains in structure elucidation, in spite of the spectacular advances in spectroscopic techniques.

Synthesis-guided structure revision of the sarcodonin, sarcoviolin, and hydnellin natural product family

A sweeping structural revision of the sarcodonin natural product family (published structures 1a-13a) is proposed after extensive studies aimed at their chemical synthesis. Key features of revised structure 1b include replacement of the N,N-dioxide moiety with an oxime, ring-opening of the central diketopiperazine, and transposition of the terphenyl wing from the 1β-2β position of 1a to the 2β-3β position of 1b. This structure revision arose from the serendipitous synthesis of a benzodioxane aminal (44) whose structure was unambiguously determined by X-ray crystallography and whose spectral properties bore considerable resemblance to the published data for the sarcodonins. A versatile new method for O-arylation of hydroxamic acids is also reported herein, as well as a manganese(III)-mediated α-oxidation of hydroxamic acids to aminals.

Involvement of caspase activation in azaspiracid-induced neurotoxicity in neocortical neurons

Azaspiracids (AZAs) are a novel group of marine phycotoxins that have been associated with severe human intoxication. We found that AZA-1 exposure increased lactate dehydrogense (LDH) efflux in murine neocortical neurons. AZA-1 also produced nuclear condensation and stimulated caspase-3 activity with an half maximal effective concentration (EC(50)) value of 25.8 nM. These data indicate that AZA-1 triggers neuronal death in neocortical neurons by both necrotic and apoptotic mechanisms. An evaluation of the structure-activity relationships of AZA analogs on LDH efflux and caspase-3 activation demonstrated that the full structure of AZAs was required to produce necrotic or apoptotic cell death. The similar potencies of AZA-1 to stimulate LDH efflux and caspase-3 activation and the parallel structure-activity relationships of azaspiracid analogs in the two assays are consistent with a common molecular target for both responses. To explore the molecular mechanism for AZA-1-induced neurotoxicity, we assessed the influence of AZA-1 on Ca(2+) homeostasis. AZA-1 suppressed spontaneous Ca(2+) oscillations (EC(50) = 445 nM) in neocortical neurons. A distinct structure-activity profile was found for inhibition of Ca(2+) oscillations where both the full structure as well as analogs containing only the FGHI domain attached to a phenyl glycine methyl ester moiety were potent inhibitors. The molecular targets for inhibition of spontaneous Ca(2+) oscillations and neurotoxicity may therefore differ. The caspase protease inhibitor Z-VAD-FMK produced a complete elimination of AZA-1-induced LDH efflux and nuclear condensation in neocortical neurons. Although the molecular target for AZA-induced neurotoxicity remains to be established, these results demonstrate that the observed neurotoxicity is dependent on a caspase signaling pathway.

Cell volume decrease as a link between azaspiracid-induced cytotoxicity and c-Jun-N-terminal kinase activation in cultured neurons

Azaspiracids (AZAs) are a group of marine toxins recently described that currently includes 20 members. Not much is known about their mechanism of action, although the predominant analog in nature, AZA-1 targets several organs in vivo, including the central nervous system, and exhibits high neurotoxicity in vitro. AZA distribution is increasing globally with mussels being most widely implicated in AZA-related food poisoning events, with human poisoning by AZAs emerging as an increasing worldwide problem in recent years. We used pharmacological tools to inhibit the cytotoxic effect of the toxin in primary cultured neurons. Several targets for AZA-induced neurotoxicity were evaluated. AZA-1 elicited a concentration-dependent hyperpolarization in cerebellar granule cells of 2-3 days in vitro; however, it did not modify membrane potential in mature neurons. Furthermore, in immature cells, AZA-1 decreased the membrane depolarization evoked by exposure of the neurons to 50mM K(+). Preincubation of the neurons with 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS), 4-acetamido-4'-isothiocyanato-2,2'-stilbenedisulfonic acid (SITS), 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB), amiloride, or ouabain before addition of AZA-1 decreased the AZA-1-induced neurotoxicity and the increase in phosphorylated c-Jun-N-terminal kinase (JNK) caused by the toxin, indicating that disruption in ion fluxes was involved in the neurotoxic effect of AZA-1. Furthermore, short exposures of cultured neurons to AZA-1 caused a significant decrease in neuronal volume that was reverted by preincubation of the neurons with DIDS or amiloride before addition of the toxin. The results presented here indicate that the JNK activation induced by AZA-1 is secondary to the decrease in cellular volume elicited by the toxin.

Monoclonal antibodies with orthogonal azaspiracid epitopes

Azaspiracid antibodies: Immunization of azaspiracid immunoconjugates has elicited monoclonal antibodies with distinct epitopes on the marine toxin; this will open the way toward azaspiracid diagnostics and the detection of contaminated shellfish before they can enter the food supply.

Inspirations, discoveries, and future perspectives in total synthesis

The last one hundred years have witnessed a dramatic increase in the power and reach of total synthesis. The pantheon of accomplishments in the field includes the total synthesis of molecules of unimaginable beauty and diversity such as the four discussed in this article: endiandric acids (1982), calicheamicin gamma(1)(I) (1992), Taxol (1994), and brevetoxin B (1995). Chosen from the collection of the molecules synthesized in the author's laboratories, these structures are but a small fraction of the myriad constructed in laboratories around the world over the last century. Their stories, and the background on which they were based, should serve to trace the evolution of the art of chemical synthesis to its present sharp condition, an emergence that occurred as a result of new theories and mechanistic insights, new reactions, new reagents and catalysts, and new synthetic technologies and strategies. Indeed, the advent of chemical synthesis as a whole must be considered as one of the most influential developments of the twentieth century in terms of its impact on society.

Total synthesis of (+)-azaspiracid-1. An exhibition of the intricacies of complex molecule synthesis

The synthesis of the marine neurotoxin azaspiracid-1 has been accomplished. The individual fragments were synthesized by catalytic enantioselective processes: A hetero-Diels-Alder reaction to afford the E- and HI-ring fragments, a carbonyl-ene reaction to furnish the CD-ring fragment, and a Mukaiyama aldol reaction to deliver the FG-ring fragment. The subsequent fragment couplings were accomplished by aldol and sulfone anion methodologies. All ketalization events to form the nonacyclic target were accomplished under equilibrating conditions utilizing the imbedded configurations of the molecule to adopt one favored conformation. A final fragment coupling of the anomeric EFGHI-sulfone anion to the ABCD-aldehyde completed the convergent synthesis of (+)-azaspiracid-1.

From nature to the laboratory and into the clinic

Natural products possess a broad diversity of structure and function, and they provide inspiration for chemistry, biology, and medicine. In this review article, we highlight and place in context our laboratory's total syntheses of, and related studies on, complex secondary metabolites that were clinically important drugs, or have since been developed into useful medicines, namely amphotericin B (1), calicheamicin gamma(1)(I) (2), rapamycin (3), Taxol (4), the epothilones [e.g., epothilones A (5) and B (6)], and vancomycin (7). We also briefly highlight our research with other selected inspirational natural products possessing interesting biological activities [i.e., dynemicin A (8), uncialamycin (9), eleutherobin (10), sarcodictyin A (11), azaspiracid-1 (12), thiostrepton (13), abyssomicin C (14), platensimycin (15), platencin (16), and palmerolide A (17)].

Synthetic and Computational Studies on the ABC Trioxadispiroketal Subunit of the Marine Biotoxin Azaspiracid-1

The trioxadispiroketal residue in the marine biotoxin azaspiracid-1, which exists in a configuration capable of exhibiting a double anomeric effect, is believed to be the thermodynamically most stable bis-spiroketal diastereomer. In order to get insight into how structural factors affect this equilibrium, a simplified ABC trioxadispiroketal analog of azaspiracid-1 was synthesized and subjected to equilbration and computational studies. Compound 7, which represents a double anomeric effect was obtained as the major isomer, together with diastereomers 14 and 15, in a respective ratio of 62:22:16. DFT calculations for 7, 14 and 15 qualitatively matched this observation. These results suggest that while a double anomeric effect may play a major role in the stability of the trioxadispiroketal configuration in the more complex natural product, the substitution pattern of the C ring is also a contributing factor.

Azaspiracid substituent at C1 is relevant to in vitro toxicity

The azaspiracids are a group of marine toxins recently described that currently includes 20 analogues. Not much is known about their mechanism of action, although effects on some cellular functions have been found in vitro. We used the reported effects on cell viability, actin cytoskeleton, and caspase activation to study the structure-activity relationship of AZA-1 and AZA-2 and the role of the carboxylic acid moiety in toxicity. AZA-1, AZA-2, and the synthetic AZA-2-methyl ester (AZA-2-ME), where the C1 carboxylic acid moiety of AZA-2 was esterified to the corresponding methyl ester moiety, induced a reduction of cell viability in neuroblastoma and hepatocyte cell lines with similar potency and kinetics. Interestingly, the mast cell line HMC-1 was resistant to AZA-induced cytotoxicity. Actin cytoskeleton alterations and caspase activation appeared after treatment with AZA-1, AZA-2, AZA-2-ME, and biotin-AZA-2 (AZA-2 labeled with biotin at C1) in neuroblastoma cells with similar qualitative, quantitative, and kinetics characteristics. Irreversibility of AZA effects on the actin cytoskeleton and cell morphology after short incubations with the toxin were common to AZA-1, AZA-2, and AZA-2-ME; however, 10-fold higher concentrations of biotin-AZA-2 were needed for irreversible effects. AZA-2-ME was rapidly metabolized in the cell to AZA-2, while transformation of biotin-AZA-2 into AZA-2 was less efficient, which explains the different potency in short exposure times. The moiety present at C1 is related to AZA toxicity in vitro. However, the presence of a methyl moiety at C8 is irrelevant to AZA toxicity since AZA-1 and AZA-2 were equipotent regardless of the readout effect.

Studies Toward the Synthesis of (-)-Zampanolide: Preparation of the Macrocyclic Core

Studies towards the synthesis of the macrocyclic core of (-)-zampanolide are reported. The synthetic approach features a one-pot reduction/vinylogous aldol reaction for construction of the C15-C20 fragment, an intramolecular silyl-modified Sakurai (ISMS) reaction for construction of the 2,6-cis-disubstituted exo-methylene pyran subunit, and use of an sp2-sp3 Stille reaction for macrocyclization.

Azaspiracid shellfish poisoning: a review on the chemistry, ecology, and toxicology with an emphasis on human health impacts

Azaspiracids (AZA) are polyether marine toxins that accumulate in various shellfish species and have been associated with severe gastrointestinal human intoxications since 1995. This toxin class has since been reported from several countries, including Morocco and much of western Europe. A regulatory limit of 160 μg AZA/kg whole shellfish flesh was established by the EU in order to protect human health; however, in some cases, AZA concentrations far exceed the action level. Herein we discuss recent advances on the chemistry of various AZA analogs, review the ecology of AZAs, including the putative progenitor algal species, collectively interpret the in vitro and in vivo data on the toxicology of AZAs relating to human health issues, and outline the European legislature associated with AZAs.

Marine natural products

This review covers the literature published in 2006 for marine natural products, with 758 citations (534 for the period January to December 2006) referring to compounds isolated from marine microorganisms and phytoplankton, green algae, brown algae, red algae, sponges, cnidaria, bryozoans, molluscs, tunicates and echinoderms. The emphasis is on new compounds (779 for 2006), together with their relevant biological activities, source organisms and country of origin. Biosynthetic studies, first syntheses, and syntheses that lead to the revision of structures or stereochemistries, have been included.