Approaches to Construction of the Medium-Sized Ring Structure in Zetekitoxin AB by Ring-Closing Metathesis

Zetekitoxin AB (ZTX), a member of the saxitoxin (STX) family isolated from the Panamanian golden frog Atelopus zeteki, shows extremely potent Na_V-inhibitory activity. Here, we investigate the synthesis of 12-membered ring structure with the C11 tertiary hydroxyl group in ZTX by means of the Mislow-Evans rearrangement reaction and subsequent ring-closing metathesis reaction. Although this approach did not provide access to the 12-membered macrocycle, we obtained a new STX analog with an 18-membered macrolactam structure as a synthetic mimic of ZTX.

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.

Silylpyrrole Oxidation En Route to Saxitoxin Congeners Including 11-Saxitoxinethanoic Acid

Saxitoxin (STX) is the archetype of a large family (>50) of architecturally distinct, bisguanidinium natural products. Among this collection of isolates, two members, 11-saxitoxinethanoic acid (11-SEA) and zetekitoxin AB (ZTX), are unique, bearing carbon substitution at C11. A desire to efficiently access these compounds has motivated the development of new tactical approaches to a late-stage C11-ketone intermediate 26, designed to enable C-C bond formation using any one of a number of possible reaction technologies. Highlights of the synthesis of 26 include a metal-free, silylpyrrole oxidative dearomatization reaction and a vinylsilane epoxidation-rearrangement cascade to generate the requisite ketone. Nucleophilic addition to 26 makes possible the preparation of unnatural C11-substituted STXs. Olefination of this ketone is also demonstrated and, when followed by a redox-neutral isomerization reaction, affords 11-SEA.

Synthesis of Saxitoxin and Its Derivatives

The chiral synthesis of (+)-saxitoxin and its derivatives is described. Two consecutive carbon-nitrogen bonds at C-5 and C-6 in saxitoxin were effectively installed by the sequential Overman rearrangement of an allylic vicinal diol derived from d-malic acid. The bicyclic guanidine unit was constructed by the intramolecular aminal formation of an acyclic bis-guanidine derivative possessing a ketone carbonyl at C-4. From the bicyclic aminal intermediate, (+)-saxitoxin, (+)-decarbamoyl-β-saxitoxinol [(+)-dc-β-saxitoxinol], and the unnatural skeletal isomer, (-)-iso-dc-saxitoxinol, were synthesized.

The chemistry and biology of guanidine secondary metabolites

Covering: 2017-2019Guanidine natural products isolated from microorganisms, marine invertebrates and terrestrial plants, amphibians and spiders, represented by non-ribosomal peptides, guanidine-bearing polyketides, alkaloids, terpenoids and shikimic acid derived, are the subject of this review. The topics include the discovery of new metabolites, total synthesis of natural guanidine compounds, biological activity and mechanism-of-action, biosynthesis and ecological functions.

Synthetic Approaches to Zetekitoxin AB, a Potent Voltage-Gated Sodium Channel Inhibitor

Voltage-gated sodium channels (Na_Vs) are membrane proteins that are involved in the generation and propagation of action potentials in neurons. Recently, the structure of a complex made of a tetrodotoxin-sensitive (TTX-s) Na_V subtype with saxitoxin (STX), a shellfish toxin, was determined. STX potently inhibits TTX-s Na_V, and is used as a biological tool to investigate the function of Na_Vs. More than 50 analogs of STX have been isolated from nature. Among them, zetekitoxin AB (ZTX) has a distinctive chemical structure, and is the most potent inhibitor of Na_Vs, including tetrodotoxin-resistant (TTX-r) Na_V. Despite intensive synthetic studies, total synthesis of ZTX has not yet been achieved. Here, we review recent efforts directed toward the total synthesis of ZTX, including syntheses of 11-saxitoxinethanoic acid (SEA), which is considered a useful synthetic model for ZTX, since it contains a key carbon-carbon bond at the C11 position.

Divergent Synthesis of Natural Derivatives of (+)-Saxitoxin Including 11-Saxitoxinethanoic Acid

The bis-guanidinium toxins are a collection of natural products that display nanomolar potency against select isoforms of eukaryotic voltage-gated Na+ ion channels. We describe a synthetic strategy that enables access to four of these poisons, namely 11-saxitoxinethanoic acid, C13-acetoxy saxitoxin, decarbamoyl saxitoxin, and saxitoxin. Highlights of this work include an unusual Mislow-Evans rearrangement and a late-stage Stille ketene acetal coupling. The IC50 value of 11-saxitoxinethanoic acid was measured against rat NaV 1.4, and found to be 17.0 nm, similar to those of the sulfated toxins gonyautoxin II and III.

The chemistry and biology of guanidine natural products

The chemistry and biology of natural guanidines isolated from microbial culture media, from marine invertebrates, as well as from terrestrial plants and animals, are reviewed.

A Synthetic Strategy for Saxitoxin Skeleton by a Cascade Bromocyclization: Total Synthesis of (+)-Decarbamoyl-α-saxitoxinol

A new synthetic strategy for the formation of the ABC tricyclic framework of saxitoxin was developed. The BC ring moiety, including a spiro-aminal structure, was first constructed stereoselectively by a newly designed cascade bromocyclization of a readily available internal alkyne bearing guanidine and urea. The A ring was then synthesized by a guanylation of a cyclic urea, easily prepared via the oxidative cleavage of the diol of the cascade product, followed by addition of cyanide. This strategy enables the concise stereocontrolled total synthesis of (+)-decarbamoyl-α-saxitoxinol, which is a naturally occurring saxitoxin analogue.