Syntheses of (-)-Tripterifordin and (-)-Neotripterifordin from Stevioside

Recent Developments in the Syntheses of C-20-Oxygenated ent-Kaurane Diterpenoids

Ent-kaurane diterpenes are a large group of natural products, with more than 1,000 compounds since their discovery. Due to their excellent biological activities and complex polycyclic structures, these compounds have attracted organic synthesis chemists around the world to be devoted to achieve their total synthesis. At present, the isolated C-20-oxygenated ent-kaurane diterpenes are the most abundant of these natural products, reaching more than 350 in number. However, only total syntheses of 3,20-epoxy, 7,20-epoxy and 19,20-lactone ent-kaurane diterpenes have been reported. In this review, we elaborate the synthesis of these three types of C-20 oxygenated ent-kaurane natural products, discuss these synthetic strategies in detail, and provide good guidance and reference for the synthesis of other C-20 oxygenated compounds.

Catalytic Asymmetric Total Synthesis of (-)-Garryine via an Enantioselective Heck Reaction

The first asymmetric total synthesis of the hexacyclic veatchine-type C20-diterpenoid alkaloid (-)-garryine is presented. Key steps include a Pd-catalyzed enantioselective Heck reaction, a radical cyclization, and a photoinduced C-H activation/oxazolidine formation sequence. Of note, a highly enantioselective Heck reaction developed in this work provides efficient access to 6/6/6 tricyclic compounds, in particular, containing a C19-functionalitiy, which is useful for diverse transformations.

Direct and indirect targets of carboxyatractyloside, including overlooked toxicity toward nucleoside diphosphate kinase (NDPK) and mitochondrial H+ leak

CONTEXT

The toxicity of atractyloside/carboxyatractyloside is generally well recognized and commonly ascribed to the inhibition of mitochondrial ADP/ATP carriers, which are pivotal for oxidative phosphorylation. However, these glycosides may 'paralyze' additional target proteins.

OBJECTIVE

This review presents many facts about atractyloside/carboxyatractyloside and their plant producers, such as Xanthium spp. (Asteraceae), named cockleburs.

METHODS

Published studies and other information were obtained from databases, such as 'CABI - Invasive Species Compendium', 'PubMed', and 'The World Checklist of Vascular Plants', from 1957 to December 2022. The following major keywords were used: 'carboxyatractyloside', 'cockleburs', 'hepatotoxicity', 'mitochondria', 'nephrotoxicity', and 'Xanthium'.

RESULTS

In the third decade of the twenty first century, public awareness of the severe toxicity of cockleburs is still limited. Such toxicity is often only perceived by specialists in Europe and other continents. Interestingly, cocklebur is among the most widely distributed invasive plants worldwide, and the recognition of new European stands of Xanthium spp. is provided here. The findings arising from field and laboratory research conducted by the author revealed that (i) some livestock populations may instinctively avoid eating cocklebur while grazing, (ii) carboxyatractyloside inhibits ADP/GDP metabolism, and (iii) the direct/indirect target proteins of carboxyatractyloside are ambiguous.

CONCLUSIONS

Many aspects of the Xanthium genus still require substantial investigation/revision in the future, such as the unification of the Latin nomenclature of currently distinguished species, bur morphology status, true fruit (achene) description and biogeography of cockleburs, and a detailed description of the physiological roles of atractyloside/carboxyatractyloside and the toxicity of these glycosides, mainly toward mammals. Therefore, a more careful interpretation of atractyloside/carboxyatractyloside data, including laboratory tests using Xanthium-derived extracts and purified toxins, is needed.

Benzamidine Conjugation Converts Expelled Potential Active Agents into Antifungals against Drug-Resistant Fungi

Fungal infections present a growing global public health concern, necessitating the development of novel antifungal drugs. However, many potential antifungals, particularly the expelled potential active agents (EPAAs), are often underestimated owing to their limitations in cellular entry or expulsion by efflux pumps. Herein, we identified 68 EPAAs out of 2322 candidates with activity against a Candida albicans efflux pump-deficient strain and no inhibitory activity against the wild-type strain. Using a novel conjugation strategy involving benzamidine (BM) as a mitochondrion-targeting warhead, we successfully converted EPAAs into potent antifungals against various urgent-threat azole-resistantCandida strains. Among the obtained EPAA-BM conjugates, IS-2-BM (11) exhibited excellent antifungal activities and induced negligible drug resistance. Furthermore, IS-2-BM prevented biofilm formation, eradicated mature biofilms, and exhibited excellent therapeutic effects in a murine model of systemic candidiasis. These findings provide a promising strategy for increasing the possibilities of discovering more antifungals.

Charting the Evolution of Chemoenzymatic Strategies in the Syntheses of Complex Natural Products

Bolstered by recent advances in bioinformatics, genetics, and enzyme engineering, the field of chemoenzymatic synthesis has enjoyed a rapid increase in popularity and utility. This Perspective explores the integration of enzymes into multistep chemical syntheses, highlighting the unique potential of biocatalytic transformations to streamline the synthesis of complex natural products. In particular, we identify four primary conceptual approaches to chemoenzymatic synthesis and illustrate each with a number of landmark case studies. Future opportunities and challenges are also discussed.

Synthetic utility of oxygenases in site-selective terpenoid functionalization

Terpenoids are one of the largest classes of natural products whose members possess a wide variety of biological activities. With several exceptions, scalable production of complex terpenoids with either purely biological or chemical methods still remains a major challenge. However, recent efforts to combine the two approaches in chemoenzymatic synthesis hold tremendous promise to address this challenge. Central to this paradigm is the development of useful biocatalytic methods, such as regioselective C–H oxidation, for terpene modifications. This review highlights recent applications of biocatalytic hydroxylation for site-selective modification of terpenoids.

Reinvigorating the Chiral Pool: Chemoenzymatic Approaches to Complex Peptides and Terpenoids

Biocatalytic transformations that leverage the selectivity and efficiency of enzymes represent powerful tools for the construction of complex natural products. Enabled by innovations in genome mining, bioinformatics, and enzyme engineering, synthetic chemists are now more than ever able to develop and employ enzymes to solve outstanding chemical problems, one of which is the reliable and facile generation of stereochemistry within natural product scaffolds. In recognition of this unmet need, our group has sought to advance novel chemoenzymatic strategies to both expand and reinvigorate the chiral pool. Broadly defined, the chiral pool comprises cheap, enantiopure feedstock chemicals that serve as popular foundations for asymmetric total synthesis. Among these building blocks, amino acids and enantiopure terpenes, whose core structures can be mapped onto several classes of structurally and pharmaceutically intriguing natural products, are of particular interest to the synthetic community.In this Account, we summarize recent efforts from our group in leveraging biocatalytic transformations to expand the chiral pool, as well as efforts toward the efficient application of these transformations in natural products total synthesis, the ultimate testing ground for any novel methodology. First, we describe several examples of enzymatic generation of noncanonical amino acids as means to simplify the synthesis of peptide natural products. By extracting amino acid hydroxylases from native biosynthetic pathways, we obtain efficient access to hydroxylated variants of proline, lysine, arginine, and their derivatives. The newly installed hydroxyl moiety then becomes a chemical handle that can facilitate additional complexity generation, thereby expanding the pool of amino acid-derived building blocks available for peptide synthesis. Next, we present our efforts in enzymatic C-H oxidations of diverse terpene scaffolds, in which traditional chemistry can be combined with strategic applications of biocatalysis to selectively and efficiently derivatize several commercial terpenoid skeletons. The synergistic logic of this approach enables a small handful of synthetic intermediates to provide access to a plethora of terpenoid natural product families. Taken together, these findings demonstrate the advantages of applying enzymes in total synthesis in conjunction with established methodologies, as well as toward the expansion of the chiral pool to enable facile incorporation of stereochemistry during synthetic campaigns.

Antimicrobial and cytotoxic effects of ammonium derivatives of diterpenoids steviol and isosteviol

Antimicrobial and cytotoxic activities of several ammonium derivatives of diterpenoids steviol and isosteviol have been investigated in vitro. The results have showed that these compounds possess high antibacterial activity against MRSA strains and cytotoxic effect against cancer cell lines MCF-7, M-HeLa, A-549, PC3, HepG2, T98G. Lead compounds 4 and 5 were detected, which, in the case of the MCF-7 cell line (human breast adenocarcinoma), showed IC₅₀ at the doxorubicin level with a selectivity index of 5.0-5.2. Flow cytometry and laser confocal microscopy analysis demonstrated that the mechanism of cytotoxic effects of the tested compounds on MCF-7 cells could be associated with the induction of apoptosis along the mitochondrial pathway. At the same time, they did not cause hemolysis and showed only slight cytotoxicity with respect to normal human cells of embryonic lung (Wi-38). The obtained results allow us to consider the studied compounds as promising scaffolds for the design of new effective antibacterial drugs and anticancer agents targeting mitochondria.

Divergent synthesis of complex diterpenes through a hybrid oxidative approach

Polycyclic diterpenes exhibit many important biological activities, but de novo synthetic access to these molecules is highly challenging because of their structural complexity. Semisynthetic access has also been limited by the lack of chemical tools for scaffold modifications. We report a chemoenzymatic platform to access highly oxidized diterpenes by a hybrid oxidative approach that strategically combines chemical and enzymatic oxidation methods. This approach allows for selective oxidations of previously inaccessible sites on the parent carbocycles and enables abiotic skeletal rearrangements to additional underlying architectures. We synthesized a total of nine complex natural products with rich oxygenation patterns and skeletal diversity in 10 steps or less from ent-steviol.

Secondary metabolites, their structural diversity, bioactivity, and ecological functions: An overview

Natural products are also called secondary metabolites to distinguish them from the primary metabolites, i.e. those natural compounds like glucose, amino acids, etc. that are present in every living cell and are used and required in the essential life processes of cells. Natural products are classified according to their metabolic building blocks into alkaloids, fatty acids, polyketides, phenyl propanoids and aromatic polyketides, and terpenoids. The structural diversity of natural products is explored using the scaffold approach focusing on the characteristic carbon frameworks.  Aside from discussing specific alkaloids that are either pharmacologically (e.g. boldine, berberine, galantamine, etc.) or historically (caffeine, atropine, lobeline, etc.) important alkaloids, a single chart is presented which shows the typical scaffolds of the most important subclasses of alkaloids.  How certain classes of natural products are formed in nature from simple biochemical ‘building blocks’ are shown using graphical schemes. This has been done for a typical tetra-ketide (6-methylsalicylic acid) from acetyl coenzyme A, or in general to all the major subclasses of terpenes.   An important aspect of understanding the structural diversity of natural products is to recognize how some compounds can be visualized as key intermediates for enzyme mediated transformation to several other related structures.  This is seen in the case of how arachidonic acid can transform into prostaglandins, or geranyl diphosphate to various monoterpenes, or squalene epoxide to various pentacyclic triterpenes, or cholesterol transforming to sex hormones, bile acids and the cardioactive cardenolides and bufadienolides. These are presented in carefully designed schemes and charts that are appropriately placed in the relevant sections of the narrative texts.  The ecological functions and pharmacological properties of natural products are also presented showing wherever possible how the chemical scaffolds have led to developing drugs as well as commercial products like sweeteners.

Total Synthesis of (+)-Strongylophorines 2 and 9

The first enantioselective total syntheses of highly complex hexacyclic meroterpenoids STR-2 and -9 (strongylophorine (STR)) are reported. Key elements of the synthetic route include the use of Robinson-type annulation reaction to construct the tricyclic terpenoid building block and a highly efficient PIDA-mediated 1,3-diaxial sp³ C-H activation to incorporate the requisite δ-lactone moiety. This route also enables the determination of absolute configuration of the synthesized natural products.

Bioactivity Profile of the Diterpene Isosteviol and its Derivatives

Steviosides, rebaudiosides and their analogues constitute a major class of naturally occurring biologically active diterpene compounds. The wide spectrum of pharmacological activity of this group of compounds has developed an interest among medicinal chemists to synthesize, purify, and analyze more selective and potent isosteviol derivatives. It has potential biological applications and improves the field of medicinal chemistry by designing novel drugs with the ability to cope against resistance developing diseases. The outstanding advancement in the design and synthesis of isosteviol and its derivative has proved its effectiveness and importance in the field of medicinal chemical research. The present review is an effort to integrate recently developed novel drugs syntheses from isosteviol and potentially active pharmacological importance of the isosteviol derivatives covering the recent advances.