Biomimetic Synthesis toward the Transtaganolides/Basiliolides

Emergence of 2-Pyrone and Its Derivatives, from Synthesis to Biological Perspective: An Overview and Current Status

Pyrone moieties are present in natural products and can be synthesized by a diverse range of synthetic methods, resulting in the formation of various derivatives through chemical modifications. Many pyrone-based derivatives are commercially available and are biocompatible. They are building blocks of various intermediates in organic synthesis. They possess remarkable biological properties including antimicrobial, antiviral, cytotoxic, and antitumor activity. These characteristics have made them valuable for the development of drugs. We have summarized recent developments in the synthesis of 2-pyrone and its derivatives and their potential applications. With regard to synthetic approaches, the focus has been on metal-free and transition metal-catalyzed reactions.

Alkynoates as Versatile and Powerful Chemical Tools for the Rapid Assembly of Diverse Heterocycles under Transition-Metal Catalysis: Recent Developments and Challenges

Heterocycles, heteroaromatics and spirocyclic entities are ubiquitous components of a wide plethora of synthetic drugs, biologically active natural products, marketed pharmaceuticals and agrochemical targets. Recognizing their high proportion in drugs and rich pharmacological potential, these invaluable structural motifs have garnered significant interest, thus enabling the development of efficient catalytic methodologies providing access to architecturally complex and diverse molecules with high atom-economy and low cost. These chemical processes not only allow the formation of diverse heterocycles but also utilize a range of flexible and easily accessible building units in a single operation to discover diversity-oriented synthetic approaches. Alkynoates are significantly important, diverse and powerful building blocks in organic chemistry due to their unique and inherent properties such as the electronic bias on carbon-carbon triple bonds posed by electron-withdrawing groups or the metallic coordination site provided by carbonyl groups. The present review highlights the comprehensive picture of the utility of alkynoates (2007-2019) for the synthesis of various heterocycles (> 50 types) using transition-metal catalysts (Ru, Rh, Pd, Ir, Ag, Au, Pt, Cu, Mn, Fe) in various forms. The valuable function of versatile alkynoates (bearing multifunctional groups) as simple and useful starting materials is explored, thus cyclizing with an array of coupling partners to deliver a broad range of oxygen-, nitrogen-, sulfur-containing heterocycles alongside fused-, and spiro-heterocyclic compounds. In addition, these examples will also focus the scope and reaction limitations, as well as mechanistic investigations into the synthesis of these heterocycles. The biological significance will also be discussed, citing relevant examples of drug molecules highlighting each class of heterocycles. This review summarizes the recent developments in the synthetic methods for the synthesis of various heterocycles using alkynoates as readily available starting materials under transition-metal catalysis.

From Plant to Patient: Thapsigargin, a Tool for Understanding Natural Product Chemistry, Total Syntheses, Biosynthesis, Taxonomy, ATPases, Cell Death, and Drug Development

Thapsigargin, the first representative of the hexaoxygenated guaianolides, was isolated 40 years ago in order to understand the skin-irritant principles of the resin of the umbelliferous plant Thapsia garganica. The pronounced cytotoxicity of thapsigargin is caused by highly selective inhibition of the intracellular sarco-endoplasmic Ca²⁺-ATPase (SERCA) situated on the membrane of the endo- or sarcoplasmic reticulum. Thapsigargin is selective to the SERCA pump and to a minor extent the secretory pathway Ca²⁺/Mn²⁺ ATPase (SPCA) pump. Thapsigargin has become a tool for investigation of the importance of SERCA in intracellular calcium homeostasis. In addition, complex formation of thapsigargin with SERCA has enabled crystallization and structure determination of calcium-free states by X-ray crystallography. These results led to descriptions of the mechanism of action and kinetic properties of SERCA and other ATPases. Inhibition of SERCA depletes Ca²⁺ from the sarco- and endoplasmic reticulum provoking the unfolded protein response, and thereby has enabled new studies on the mechanism of cell death. Development of protocols for selective transformation of thapsigargin disclosed the chemistry and facilitated total synthesis of the molecule. Conversion of trilobolide into thapsigargin offered an economically feasible sustainable source of thapsigargin, which enables a future drug production. Principles for prodrug development were used by conjugating a payload derived from thapsigargin with a hydrophilic peptide selectively cleaved by proteases in the tumor. Mipsagargin was developed in order to obtain a drug for treatment of cancer diseases characterized by the presence of prostate specific membrane antigen (PSMA) in the neovascular tissue of the tumors. Even though mipsagargin showed interesting clinical effects the results did not encourage funding and consequently the attempt to register the drug has been abandoned. In spite of this disappointing fact, the research performed to develop the drug has resulted in important scientific discoveries concerning the chemistry, biosynthesis and biochemistry of sesquiterpene lactones, the mechanism of action of ATPases including SERCA, mechanisms for cell death caused by the unfolded protein response, and the use of prodrugs for cancer-targeting cytotoxins. The presence of toxins in only some species belonging to Thapsia also led to a major revision of the taxonomy of the genus.

Sarco/Endoplasmic Reticulum Calcium ATPase Inhibitors: Beyond Anticancer Perspective

The sarco/endoplasmic reticulum calcium ATPase (SERCA), which plays a key role in the maintenance of Ca²⁺ ion homeostasis, is an extensively studied enzyme, the inhibition of which has a considerable impact on cell life and death decision. To date, several SERCA inhibitors have been thoroughly studied and the most notable one, a derivative of the sesquiterpene lactone thapsigargin, is gradually approaching a clinical application. Meanwhile, new compounds with SERCA-inhibiting properties of natural, synthetic, or semisynthetic origin are being discovered and/or developed; some of these might also be suitable for the development of new drugs with improved performance. This review brings an up-to-date comprehensive overview of recently discovered compounds with the potential of SERCA inhibition, discusses their mechanism of action, and highlights their potential clinical applications, such as cancer treatment.

Base Mediated Tandem Vinylogous Addition and Cyclization of γ-Phosphonyl/Sulfonyl Crotonates and Ynones: Synthesis of Functionalized 2-Pyrones

A general method for highly functionalized 2-pyrones via a base-mediated sequential vinylogous addition and cyclization of γ-phosphonyl/sulfonyl crotonates and ynones are described. An exclusive E-geometry with respect to the newly generated olefin substituent at C3 of pyrone was observed. Imino glyoxalates and glycine imines similarly reacted with ynones to deliver 3-imino pyrones.

Palladium-Catalyzed Cascade Intramolecular Cyclization and Allylation of Enynoates with Allylic Alcohols

A Pd(II)-catalyzed mild and highly regioselective 6- endo cyclization/allylation reaction of enynoates with simple allylic alcohols has been developed. Under mild reaction conditions, the vinyl palladium species generated in situ after cyclization could insert C-C double bond of allylic alcohol through cross-coupling reaction and lead to the formation of allyl pyrone via β-OH elimination. This cascade cross-coupling reaction represents a direct and atom economic methodology for the construction of novel allyl pyrones in moderate to good yields.

Structural basis for the selective addition of an oxygen atom to cyclic ketones by Baeyer-Villiger monooxygenase from Parvibaculum lavamentivorans

Baeyer-Villiger monooxygenase (BVMO) catalyzes insertion of an oxygen atom into aliphatic or cyclic ketones with high regioselectivity. The BVMOs from Parvibaculum lavamentivorans (BVMO_Parvi) and Oceanicola batsensis (BVMO_Ocean) are interesting because of their homologies, with >40% sequence identity, and reaction with the same cyclic ketones with a methyl moiety to give different products. The revealed BVMO_Parvi structure shows that BVMO_Parvi forms a two-domain structure like other BVMOs. It has two inserted residues, compared with BVMO_Ocean, that form a bulge near the bound flavin adenine dinucleotide in the active site. Furthermore, this bulge is linked to a nearby α-helix via a disulfide bond, probably restricting access of the bulky methyl group of the substrate to this bulge. Another sequence motif at the entrance of the active site (Ala-Ser in BVMO_Parvi and Ser-Thr in BVMO_Ocean) allows a large volume in BVMO_Parvi. These minute differences may discriminate a substrate orientation in both BVMOs from the initial substrate binding pocket to the final oxygenation site, resulting in the inserted oxygen atom being in different positions of the same substrate.

Expeditious assembly of fused dihydropyranones via N-heterocyclic carbene-catalyzed tandem Michael addition/lactonization

A convergent and efficient NHC-catalyzed enantioselective tandem Michael addition/lactonization sequence of ynals with 1,2-dione is disclosed. This strategy expeditiously assembles the valuable optically active fused dihydropyranones in good to high yields and with excellent enantioselectivities.

Translating endoplasmic reticulum biology into the clinic: a role for ER-targeted natural products?

ER stress has been identified as a hallmark, and sometimes trigger, of several pathologies, notably cancer, inflammation and neurodegenerative diseases like Alzheimer's and Parkinson's. Among the molecules described in literature known to affect ER function, the majority are natural products, suggesting that natural molecules may constitute a significant arsenal of chemical entities for modulating this cellular target. In this review, we will start by presenting the current knowledge of ER biology and the hallmarks of ER stress, thus paving the way for presenting the natural products that have been described as being ER modulators, either stress inducers or ER protectors. The chemistry, distribution and mechanism of action of these compounds will be presented and discussed.

Short, enantioselective total synthesis of chatancin

An enantioselective total synthesis of the polycyclic diterpene (+)-chatancin, a potent PAF antagonist, is reported. Proceeding in seven steps from dihydrofarnesal, this synthetic route was designed to circumvent macrocyclization-based strategies to complex, cyclized cembranoids. The described synthesis requires only six chromatographic purifications, is high yielding, and avoids protecting-group manipulations. An X-ray crystal structure of this fragile marine natural product was obtained.

The total syntheses of basiliolide C, epi-basiliolide C, and protecting-group-free total syntheses of transtaganolides C and D

The total syntheses of basiliolide C and previously unreported epi-basiliolide C are achieved by an Ireland-Claisen/Diels-Alder cascade. The development of a palladium catalyzed cross-coupling of methoxy alkynyl zinc reagents allows for the protecting-group-free syntheses of transtaganolides C and D. Syntheses of transtaganolides C and D are accomplished in a single operation to generate three rings, two all-carbon quaternary centers, and four tertiary stereocenters from a monocyclic, achiral precursor.