Total synthesis of brevetoxin A

Palladium-Catalyzed Ligand-Controlled Switchable Hetero-(5 + 3)/Enantioselective [2σ+2σ] Cycloadditions of Bicyclobutanes with Vinyl Oxiranes

For nearly 60 years, significant research efforts have been focused on developing strategies for the cycloaddition of bicyclobutanes (BCBs). However, higher-order cycloaddition and catalytic asymmetric cycloaddition of BCBs have been long-standing formidable challenges. Here, we report Pd-catalyzed ligand-controlled, tunable cycloadditions for the divergent synthesis of bridged bicyclic frameworks. The dppb ligand facilitates the formal (5+3) cycloaddition of BCBs and vinyl oxiranes, yielding valuable eight-membered ethers with bridged bicyclic scaffolds in 100% regioselectivity. The Cy-DPEphos ligand promotes selective hetero-[2σ+2σ] cycloadditions to access pharmacologically important 2-oxabicyclo[3.1.1]heptane (O-BCHeps). Furthermore, the corresponding catalytic asymmetric synthesis of O-BCHeps with 94-99% ee has been achieved using chiral (S)-DTBM-Segphos, representing the first catalytic asymmetric cross-dimerization of two strained rings. The obtained O-BCHeps are promising bioisosteres for ortho-substituted benzenes.

Synthesis of enol phosphates directly from ketones via a modified one-pot Perkow reaction

A modified Perkow reaction, named Perkow-Shi reaction, was developed based on the one-pot α-tosyloxylation of ketones following by addition of P(iii)-reagents and 4 Å molecular sieves. Diversity of enol phosphates, as well as enol phosphonates, enol phosphinates, and enol phosphoramidates were synthesized in high yields directly from the ubiquitously available ketones instead of the unfavourable α-chloroketones under a mild and environmental friendly condition.

A modified Perkow reaction was developed based on the one-pot α-tosyloxylation of ketones following by addition of P(iii)-reagents and 4 Å molecular sieves.

The current state of knowledge on taxonomy, modulating factors, ecological roles, and mode of action of phytoplankton allelochemicals

Allelopathy is widespread in marine, brackish, and freshwater habitats. Literature data indicate that allelopathy could offer a competitive advantage for some phytoplankton species by reducing the growth of competitors. It is also believed that allelopathy may affect species succession. Thus, allelopathy may play a role in the development of blooms. Over the past few decades, the world's coastal waters have experienced increases in the numbers of cyanobacterial and microalgal blooming events. Understanding how allelopathy is implicated with other biological and environmental factors as a bloom-development mechanism is an important topic for future research. This review focuses on a taxonomic overview of allelopathic cyanobacteria and microalgae, the biological and environmental factors that affect allelochemical production, their role in ecological dynamics, and their physiological modes of action, as well as potential industrial applications of allelopathic compounds.

Perspectives from nearly five decades of total synthesis of natural products and their analogues for biology and medicine

Covering: 1970 to 2020By definition total synthesis is the art and science of making the molecules of living Nature in the laboratory, and by extension, their analogues. Although obvious, its application to the synthesis of molecules for biology and medicine was not always the purpose of total synthesis. In recent years, however, the field has acquired momentum as its power to reach higher molecular complexity and diversity is increasing, and as the demand for rare bioactive natural products and their analogues is expanding due to their recognised potential to facilitate biology and drug discovery and development. Today this component of total synthesis endeavors is considered highly desirable, and could be part of interdisciplinary academic and/or industrial partnerships, providing further inspiration and momentum to the field. In this review we provide a brief historical background of the emergence of the field of total synthesis as it relates to making molecules for biology and medicine. We then discuss specific examples of this practice from our laboratories as they developed over the years. The review ends with a conclusion and future perspectives for natural products chemistry and its applications to biology and medicine and other added-value contributions to science and society.

Total Synthesis Endeavors and Their Contributions to Science and Society:A Personal Account

The advent of organic synthesis in the 19th century, serendipitous as it was, set in motion a revolution in science that continues to evolve into increasing levels of sophistication and to expand into new domains of science and technology for the benefits of science and society. Its evolution was always driven by the challenges posed by natural products, whose structures were becoming increasingly complex and diverse. In response to these challenges, synthetic organic chemists were prompted to sharpen their art to reach their target molecules, whose structures were often confirmed only after their synthesis in the laboratory through the art and science of total synthesis. The latter became the “locomotive” and the “flagship” of organic synthesis, for through this practice novel synthetic methods were discovered and invented, and also tested for their generality, applicability, and scope with regard to molecular complexity and diversity. The purpose of total synthesis has also evolved over the years to include aspects beyond the synthesis of the molecule and confirmation of its structure. In this article, we briefly review the evolution of total synthesis in terms of its power and reach and demonstrate its current state of the art that combines fundamentals with translational aspects through examples from our laboratories. The highlighted examples reflect the newly emerged paradigm of the discipline that includes—in addition to the total synthesis of the target molecule—structural elucidations, method discovery and development, design, synthesis, and biological evaluation of analogues for biology and medicine, and training of young students, preparing them for academic and industrial careers in the various disciplines that require knowledge and skills to practice the central science of chemical synthesis. Such disciplines include chemical biology, drug discovery and development, materials science and nanotechnology, and other endeavors whose fundamentals depend and rely on the structure of the molecule and its synthesis.

Metal-Free Chemoselective Oxidative Dehomologation or Direct Oxidation of Alcohols: Implication for Biomass Conversion

A transition metal-free, chemoselective reaction was performed using the sodium tert-butoxide-oxygen (NaO(t) Bu-O2 ) system, resulting in either oxidative dehomologation or direct oxidation of alcohols. In particular, the newly developed protocol may be used to predict the major product formed, which depends on alkyl chain length of the alcohols and reaction conditions. The rational mechanism of this transformation was also demonstrated by performing an (18) O isotopic labelling experiment. This protocol presents a straightforward method for biomass conversion of a lignin model compound to phenol and benzoic acid.

Synthesis and biological evaluation of QRSTUVWXYZA' domains of maitotoxin

The synthesis of QRSTUVWXYZA' domains 7, 8, and 9 of the highly potent marine neurotoxin maitotoxin (1), the largest secondary metabolite isolated to date, is described. The devised synthetic strategy entailed a cascade Takai-Utimoto ester olefination/ring closing metathesis to construct ring Y, a hydroxydithioketal cyclization/methylation sequence to cast ring X, a Horner-Wadsworth-Emmons coupling of WXYZA' ketophosphonate 11 with QRSTU aldehyde 12 to form enone 10, and a reductive hydroxyketone ring closure to forge ring V. 2D NMR spectroscopic analysis and comparison of (13)C chemical shifts with those of the corresponding carbons of maitotoxin revealed close similarities supporting the originally assigned structure of this region of the natural product. Biological evaluations of various synthesized domains of maitotoxin in this and previous studies from these laboratories led to fragment structure-activity relationships regarding their ability to inhibit maitotoxin-elicited Ca(2+) influx in rat C6 glioma cells.

Total synthesis and complete structural assignment of gambieric acid A, a large polycyclic ether marine natural product

More than thirty years after the discovery of polycyclic ether marine natural products, they continue to receive intense attention from the chemical, biological, and pharmacological communities because of their potent biological activities and highly complex molecular architectures. Gambieric acids are intriguing polycyclic ethers that exhibit potent antifungal activity with minimal toxicity against mammals. Despite the recent advances in the synthesis of this class of natural products, gambieric acids remain unconquered due to their daunting structural complexity, which poses a formidable synthetic challenge to organic chemists. This paper reviews our long-term studies on the total synthesis, complete configurational reassignment, and structure-activity relationships of gambieric acid A over the last decade.

The endeavor of total synthesis and its impact on chemistry, biology and medicine

The synthesis of urea in 1828 set in motion the discipline of organic synthesis in general and of total synthesis in particular, the art and science of synthesizing natural products, the molecules of living nature. Early endeavors in total synthesis had as their main objective the proof of structure of the target molecule. Later on, the primary goal became the demonstration of the power of synthesis to construct complex molecules through appropriately devised strategies, making the endeavor an achievement whose value was measured by its elegance and efficiency. While these objectives continue to be important, contemporary endeavors in total synthesis are increasingly focused on practical aspects, including method development, efficiency, and biological and medical relevance. In this article, the emergence and evolution of total synthesis to its present state is traced, selected total syntheses from the author's laboratories are highlighted, and projections for the future of the field are discussed.

Organic synthesis: the art and science of replicating the molecules of living nature and creating others like them in the laboratory

Synthetic organic chemists have the power to replicate some of the most intriguing molecules of living nature in the laboratory and apply their developed synthetic strategies and technologies to construct variations of them. Such molecules facilitate biology and medicine, as they often find uses as biological tools and drug candidates for clinical development. In addition, by employing sophisticated catalytic reactions and appropriately designed synthetic processes, they can synthesize not only the molecules of nature and their analogues, but also myriad other organic molecules for potential applications in many areas of science, technology and everyday life. After a short historical introduction, this article focuses on recent advances in the field of organic synthesis with demonstrative examples of total synthesis of complex bioactive molecules, natural or designed, from the author's laboratories, and their impact on chemistry, biology and medicine.

Catalysis of diazoalkane-carbonyl homologation. How new developments in hydrazone oxidation enable the carbon insertion strategy for synthesis

Diazo compounds continue both to challenge and to fascinate practitioners of chemical synthesis. The most strategically powerful and unique type of reactivity observed with these reagents is a formal insertion of the donor-acceptor carbon into C-C or C-H bonds alpha to carbonyl groups. Although the reaction does not involve discrete carbon-metal bonds, it can be catalyzed by metal-based Lewis acids. This chapter investigates both classical and modern developments in diazoalkyl carbon insertion with a special emphasis on nonstabilized nucleophiles.

A New Variant of Fused Cyclic Ether Synthesis Based on Ireland-Claisen Rearrangement and RCM

A new variant of fused cyclic ether synthesis based on Ireland-Claisen rearrangement and ring-closing olefin metathesis (RCM) was developed. The Ireland-Claisen rearrangement and ring-closing olefin metathesis (RCM) was developed. The Ireland-Claisen rearrangement of a ( Z)-3-alkoxyprop-2-en-1-yl glycolate ester having a cyclic ether on the oxygen at C3 of the ( Z)-prop-2-en-1-yl group stereoselectively produced an anti-α,β-dialkoxyester which was successfully transformed to a fused bicyclic ether via a reaction sequence including RCM.

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.

Transition metal-catalysed cross-coupling reactions of P-activated enols

Transition metal catalysed cross-coupling reactions are ubiquitous in organic chemistry providing an impressive technique for C-C bond formation. Whilst many electrophilic partners have been described for these reactions, aryl and vinyl phosphates, phosphonates and phosphonites can offer advantages in terms of preparation, stability and reactivity profile. This critical review summarises the advances made to date utilising P-activated enols in metal-catalysed cross-coupling reactions (97 references).

Maitotoxin: An Inspiration for Synthesis

Maitotoxin holds a special place in the annals of natural products chemistry as the largest and most toxic secondary metabolite known to date. Its fascinating, ladder-like, polyether molecular structure and diverse spectrum of biological activities elicited keen interest from chemists and biologists who recognized its uniqueness and potential as a probe and inspiration for research in chemistry and biology. Synthetic studies in the area benefited from methodologies and strategies that were developed as part of chemical synthesis programs directed toward the total synthesis of some of the less complex members of the polyether marine biotoxin class, of which maitotoxin is the flagship. This account focuses on progress made in the authors' laboratories in the synthesis of large maitotoxin domains with emphasis on methodology development, strategy design, and structural comparisons of the synthesized molecules with the corresponding regions of the natural product. The article concludes with an overview of maitotoxin's biological profile and future perspectives.

A multiply convergent platform for the synthesis of trioxacarcins

Many first-line cancer drugs are natural products or are derived from them by chemical modification. The trioxacarcins are an emerging class of molecules of microbial origin with potent antiproliferative effects, which may derive from their ability to covalently modify duplex DNA. All trioxacarcins appear to be derivatives of a nonglycosylated natural product known as DC-45-A2. To explore the potential of the trioxacarcins for the development of small-molecule drugs and probes, we have designed a synthetic strategy toward the trioxacarcin scaffold that enables access to both the natural trioxacarcins and nonnatural structural variants. Here, we report a synthetic route to DC-45-A2 from a differentially protected precursor, which in turn is assembled in just six steps from three components of similar structural complexity. The brevity of the sequence arises from strict adherence to a plan in which strategic bond-pair constructions are staged at or near the end of the synthetic route.

Synthesis of the WXYZA' domain of maitotoxin

A synthesis of the WXYZA' domain (7) of the marine neurotoxin maitotoxin (1) is reported. The convergent synthetic strategy involves construction of key building blocks 11 and 12, their coupling, and the elaboration of the resulting ester (10) to the target molecule through a ring-closing metathesis and a hydroxy dithioketal cyclization as the key steps. For the construction of fragment 11, the Noyori reduction/Achmatowicz rearrangement and hydroxy epoxide opening technologies were applied (starting from furfuryl alcohol (13)), whereas for the synthesis of fragment 12, a carbohydrate-based approach was adopted (starting from 2-deoxy-D-ribose (14)). The synthesized WXYZA' domain (7) of maitotoxin (1) exhibited the expected (13)C NMR chemical shifts, supporting the originally assigned structure of the corresponding region of the natural product.

Synthesis of the QRSTU domain of maitotoxin and its 85-epi- and 86-epi-diastereoisomers

A devised synthetic strategy toward the QRSTU ring system 4 of the marine-derived biotoxin maitotoxin (1) delivered, in addition to 4, its diastereoisomers 85-epi-QRSTU and 86-epi-QRSTU ring systems 5 and 6. The convergent route to these maitotoxin fragments involved coupling of UT and Q building blocks 9 (obtained from 2-deoxy-D-ribose) and 10 (obtained from D-ribose) followed by ring-closing metathesis to afford enol ether 8, whose elaboration to the targeted QRSTU ring system 4 required its conversion to hydroxy ketone 7. The latter compound (7) was transformed to the final product through a hydroxy dithioketal cyclization, followed by oxidation/methylation of the resulting O,S-mixed ketal to install the last of the five methyl groups contained within the target molecule (4). (13)C NMR spectroscopic analysis of synthesized fragments 4, 5, and 6 and comparisons with maitotoxin provided strong support for the originally assigned structure of the QRSTU domain of the natural product.

SmI2-induced cyclizations and their applications in natural product synthesis

Since the isolation of brevetoxin-B, a red tide toxin, many bioactive marine natural products featuring synthetically challenging trans-fused polycyclic ether ring systems have been reported. We have developed SmI(2)-induced cyclization of beta-alkoxyacrylate with aldehyde, affording 2,6-syn-2,3-trans-tetrahydropyran (THP) or 2,7-syn-2,3-trans-oxepane with complete stereoselection, as a key reaction of efficient iterative and bi-directional strategies for the construction of these polycyclic ethers. This reaction is also applicable to the synthesis of 3-, 5-, and 6-methyl-THPs and 3,5-dimethyl-THP. The synthesis of 2-methyl- and 2,6-dimethyl-THPs was accomplished by means of a unique methyl insertion. Recently, the SmI(2)-induced cyclization was extended to similar reactions using beta-alkoxyvinyl sulfone and sulfoxide. Reaction of (E)- and (Z)-beta-alkoxyvinyl sulfone-aldehyde afforded 2,6-syn-2,3-trans- and 2,6-syn-2,3-cis- THPs, respectively. Reaction of (E)-beta-alkoxyvinyl (R)- and (S)-sulfoxides gave 2,6-anti-2,3-cis- and 2,6-syn-2,3-trans-THPs, respectively. Reaction of (Z)-beta-alkoxyvinyl (R)-sulfoxides gave 2,6-syn-2,3-cis-THP and an olefinic product, while that of (Z)-beta-alkoxyvinyl (S)-sulfoxide afforded a mixture of many products. These SmI(2)-induced cyclizations have been applied to the total syntheses of various natural products, including brevetoxin-B, mucocin, pyranicin, and pyragonicin. Synthetic studies on gambierol and maitotoxin are also introduced.

Synthesis of marine polycyclic polyethers via endo-selective epoxide-opening cascades

The proposed biosynthetic pathways to ladder polyethers of polyketide origin and oxasqualenoids of terpenoid origin share a dramatic epoxide-opening cascade as a key step. Polycyclic structures generated in these biosynthetic pathways display biological effects ranging from potentially therapeutic properties to extreme lethality. Much of the structural complexity of ladder polyether and oxasqualenoid natural products can be traced to these hypothesized cascades. In this review we summarize how such epoxide-opening cascade reactions have been used in the synthesis of ladder polyethers and oxasqualenoid natural products.

The development of endo-selective epoxide-opening cascades in water

This tutorial review traces the development of endo-regioselective epoxide-opening reactions in water. Templated, water-promoted epoxide-opening cyclization reactions can offer rapid access to subunits of the ladder polyethers, a fascinating and complex family of natural products. This review may be of interest to those curious about the ladder polyethers and their hypothesized biogenesis, about organic reactions in water, and about the development and application of cascade reactions in organic synthesis.

Enantioselective total synthesis of brevetoxin A: unified strategy for the B, E, G, and J subunits

Brevetoxin A is a decacyclic ladder toxin that possesses 5-, 6-, 7-, 8-, and 9-membered oxacycles, as well as 22 tetrahedral stereocenters. Herein, we describe a unified approach to the B, E, G, and J rings based upon a ring-closing metathesis strategy from the corresponding dienes. The enolate technologies developed in our laboratory allowed access to the precursor acyclic dienes for the B, E, and G medium-ring ethers. The strategies developed for the syntheses of these four monocycles ultimately provided multigram quantities of each of the rings, supporting our efforts toward the completion of a convergent synthesis of brevetoxin A.

Enantioselective total synthesis of brevetoxin A: convergent coupling strategy and completion

A highly convergent, enantioselective total synthesis of brevetoxin A is reported. The development of a [X+2+X] Horner-Wadsworth-Emmons/cyclodehydration/reductive etherification convergent coupling strategy allowed a unified approach to the synthesis of two advanced tetracyclic fragments from four cyclic ether subunits. The Horner-Wittig coupling of the two tetracyclic fragments provided substrates that were explored for reductive etherification, the success of which delivered a late-stage tetraol intermediate. The tetraol was converted to the natural product through an expeditious selective oxidative process followed by methylenation.

Total synthesis of ciguatoxin

Something fishy: Ciguatoxin (see structure) is one of the principal toxins involved in ciguatera poisoning and the target of a total synthesis involving the coupling of three segments. The key transformations in this synthesis feature acetylene-dicobalthexacarbonyl complexation.

Total synthesis of brevetoxin A

A total synthesis of brevetoxin A is reported. Two tetracyclic coupling partners, prepared from previously reported advanced fragments, were effectively united via a Horner-Wittig olefination. The resulting octacycle was progressed to substrates that were explored for reductive etherification, the success of which led to a penultimate tetraol intermediate. The tetraol was converted to the natural product through an expeditious selective oxidative process followed by methylenation.

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.

Sodium channel inhibiting marine toxins

Saxitoxin (STX), tetrodotoxin (TTX) and their many chemical relatives are part of our daily lives. From killing people who eat seafood containing these toxins, to being valuable research tools unveiling the invisible structures of their pharmacological receptor, their global impact is beyond measure. The pharmacological receptor for these toxins is the voltage-gated sodium channel which transports Na ions between the exterior to the interior of cells. The two structurally divergent families of STX and TTX analogues bind at the same location on these Na channels to stop the flow of ions. This can affect nerves, muscles and biological senses of most animals. It is through these and other toxins that we have developed much of our fundamental understanding of the Na channel and its part in generating action potentials in excitable cells.

Epoxide-opening cascades in the synthesis of polycyclic polyether natural products

The structural features of polycyclic polyether natural products can, in some cases, be traced to their biosynthetic origin. However in case that are less well understood, only biosynthetic pathways that feature dramatic, yet speculative, epoxide-opening cascades are proposed. We summarize how such epoxide-opening cascade reactions have been used in the synthesis of polycyclic polyethers (see scheme) and related natural products.The group of polycyclic polyether natural products is of special interest owing to the fascinating structure and biological effects displayed by its members. The latter includes potentially therapeutic antibiotic, antifungal, and anticancer properties, and extreme lethality. The polycyclic structural features of this class of compounds can, in some cases, be traced to their biosynthetic origin, but in others that are less well understood, only to proposed biosynthetic pathways that feature dramatic, yet speculative, epoxide-opening cascades. In this review we summarize how such epoxide-opening cascade reactions have been used in the synthesis of polycyclic polyethers and related natural products.

The continuing saga of the marine polyether biotoxins

The unprecedented structure of the marine natural product brevetoxin B was elucidated by the research group of Nakanishi and Clardy in 1981. The ladderlike molecular architecture of this fused polyether molecule, its potent toxicity, and fascinating voltage-sensitive sodium channel based mechanism of action immediately captured the imagination of synthetic chemists. Synthetic endeavors resulted in numerous new methods and strategies for the construction of cyclic ethers, and culminated in several impressive total syntheses of this molecule and some of its equally challenging siblings. Of the marine polyethers, maitotoxin is not only the most complex and most toxic of the class, but is also the largest nonpolymeric natural product known to date. This Review begins with a brief history of the isolation of these biotoxins and highlights their biological properties and mechanism of action. Chemical syntheses are then described, with particular emphasis on new methods developed and applied to the total syntheses. The Review ends with a discussion of the, as yet unfinished, story of maitotoxin, and projects into the future of this area of research.