Roscotanes and roscoranes: Oxygenated abietane and pimarane diterpenoids from Kaempferia roscoeana

Bi- and tricyclic diterpenoids: landmarks from a decade (2013-2023) in search of leads against infectious diseases

Covering: 2013 to 2023In an era where antimicrobial resistance severely threatens our ability to treat infections, the discovery of new drugs that belong to different chemical classes and/or bear original modes of action is urgently needed. In this case, diterpenoids comprise a productive field with a proven track record in providing new anti-infectives to tackle bacterial infections and malaria. This review highlights the potential of both naturally occurring and semi-synthetic bi- and tricyclic diterpenoids to become leads in search of new drugs to treat infections caused by bacteria, fungi, viruses and protozoan parasites. The literature from the last decade (2013-2023) is covered, focusing on naturally occurring and semi-synthetic bicyclic (labdanes and labdane-type) and tricyclic (all classes) diterpenoids, detailing their relevant biological activities in the context of infection, which are explained through structure-activity relationships.

Abietane Diterpenoids from the Rhizomes of Kaempferia roscoeana and Their Anti-Inflammatory Activities

Six new abietane diterpenes, roscoeananes A-F (1-6), along with two known compounds (7-8) were isolated from the rhizomes of Kaempferia roscoeana. The structures of all compounds were elucidated by analysis of spectroscopic data, and the absolute configurations were assigned by a comparison of the theoretical and experimental electronic circular dichroism (ECD) spectra and a comparison with literature values. The unreported compound 5 is an ether-linked dimer of roscoeanane B (2). Most of the isolated compounds were tested for their nitric oxide inhibitory effects in lipopolysaccharide-activated RAW264.7 cells. Among them, roscoeanane A (1) was found to reduce NO levels in murine macrophage cells with an IC50 value of 3.58 ± 0.95 μM and exhibited low cytotoxicity (IC50 > 50 μM).

Pimarane diterpenoids: sources, structures and biological activities

The pimarane diterpenoids, a widespread class of secondary metabolites, have been found in several dozens of plant species from various families and in organisms from other taxonomic groups. According to the different chiral centres, pimarane diterpenes can be divided into four types, including pimarane, isopimarane, ent-pimarane, and ent-isopimarane. Meanwhile, these compounds possessed many pharmacological activities, such as cytotoxic, anti-inflammatory, and antibacterial activities. Due to their notable structure and biological activities these substances have attracted interest in recent years. A comprehensive account of the structural diversity (368 structures, 117 references) and biological activities of pimarane diterpenes discovered from 2000 until 2023 is given in this review.

Albiflorenes A-L, polyoxygenated cyclohex(a/e)ne diterpene esters from Kaempferia albiflora

Twelve polyoxygenated cyclohex(a/e)ne diterpene esters, named albiflorenes A-L (1-12), were isolated from the whole plants of Kaempferia albiflora, known as "Prao Mang Mum." Their structures and relative stereochemistry were determined by extensive spectroscopic analysis. Furthermore, the comparison of experimental electronic circular dichroism (ECD) curves with the curves predicted by TDDFT was used to determine the absolute configurations. Albiflorenes contain polyoxygenated cyclohexane (or cyclohexene) derivatives, which are linked to either isopimarane or abietane diterpene acid units. The discovery marks the first occurrence of a conjugate between polyoxygenated cyclohexane (or cyclohexene) rings and diterpenoids. Among the isolates, albiflorene C specifically exhibited antibacterial activity against Bacillus cereus with MIC and MBC values of 3.13 and 6.25 μg/mL, respectively.

Kaemtakols A-D, highly oxidized pimarane diterpenoids with potent anti-inflammatory activity from Kaempferia takensis

Four highly oxidized pimarane diterpenoids were isolated from Kaempferia takensis rhizomes. Kaemtakols A-C possess a tetracyclic ring with either a fused tetrahydropyran or tetrahydrofuran motif. Kaemtakol D has an unusual rearranged A/B ring spiro-bridged pimarane framework with a C-10 spirocyclic junction and an adjacent 1-methyltricyclo[3.2.1.02,7]octene ring. Structural characterization was achieved using spectroscopic analysis, DP4 + and ECD calculations, as well as X-ray crystallography, and their putative biosynthetic pathways have been proposed. Kaemtakol B showed significant potency in inhibiting nitric oxide production with an IC50 value of 0.69 μM. Molecular docking provided some perspectives on the action of kaemtakol B on iNOS protein.

Inhibitory mechanisms of promising antimicrobials from plant byproducts: A review

Plant byproducts and waste present enormous environmental challenges and an opportunity for valorization and industrial application. Due to consumer demands for natural compounds, the evident paucity of novel antimicrobial agents against foodborne pathogens, and the urgent need to improve the arsenal against infectious diseases and antimicrobial resistance (AMR), plant byproduct compounds have attracted significant research interest. Emerging research highlighted their promising antimicrobial activity, yet the inhibitory mechanisms remain largely unexplored. Therefore, this review summarizes the overall research on the antimicrobial activity and inhibitory mechanisms of plant byproduct compounds. A total of 315 natural antimicrobials from plant byproducts, totaling 1338 minimum inhibitory concentrations (MIC) (in μg/mL) against a broad spectrum of bacteria, were identified, and a particular emphasis was given to compounds with high or good antimicrobial activity (typically <100 μg/mL MIC). Moreover, the antimicrobial mechanisms, particularly against bacterial pathogens, were discussed in-depth, summarizing the latest research on using natural compounds to combat pathogenic microorganisms and AMR. Furthermore, safety concerns, relevant legislation, consumer perspective, and current gaps in the valorization of plant byproducts-derived compounds were comprehensively discussed. This comprehensive review covering up-to-date information on antimicrobial activity and mechanisms represents a powerful tool for screening and selecting the most promising plant byproduct compounds and sources for developing novel antimicrobial agents.

Isopimarane-type diterpenoids from the rhizomes of Kaempferia galanga L. and their biological activities

Fourteen isopimarane diterpenoids (1-14) were isolated from the rhizomes of Kaempferia galanga, including four new compounds (1-4). The isolated secondary metabolites were identified through analysis of spectroscopic (1 D and 2 D NMR) and mass spectrometric data, together with X-ray diffraction studies. Compounds 4-5, 7-11, and 13 showed strong antimalarial activities, with IC₅₀ values in the range of 1.46-3.99 μg/mL. Moreover, compounds 4, 5, 8, and 12 showed cytotoxicity against KB cell line with IC₅₀ values in the range of 6.13-38.2 μg/mL, while compounds 4, 5, and 12 showed cytotoxicity against MCF-7 cell line with IC₅₀ values in the range of 11.75-47.4 μg/mL. Eventually, the isolated compounds were screened against six bacterial strains and Mycobacterium tuberculosis, demonstrating weak to moderate activities.

Sustainable biosynthesis of valuable diterpenes in microbes

Diterpenes, or diterpenoids, are the most abundant and diverse subgroup of terpenoids, the largest family of secondary metabolites. Most diterpenes possess broad biological activities including anti-inflammatory, antiviral, anti-tumoral, antimicrobial, anticancer, antifungal, antidiabetic, cardiovascular protective, and phytohormone activities. As such, diterpenes have wide applications in medicine (e.g., the anticancer drug Taxol and the antibiotic pleuromutilin), agriculture (especially as phytohormones such as gibberellins), personal care (e.g., the fragrance sclareol) and food (e.g., steviol glucosides as low-calorie sweeteners) industries. Diterpenes are biosynthesized in a common route with various diterpene synthases and decoration enzymes like cytochrome P450 oxidases, glycosidases, and acyltransferases. Recent advances in DNA sequencing and synthesis, omics analysis, synthetic biology, and metabolic engineering have enabled efficient production of diterpenes in several chassis hosts like Escherichia coli, Saccharomyces cerevisiae, Yarrowia lipolytica, Rhodosporidium toruloides, and Fusarium fujikuroi. This review summarizes the recently discovered diterpenes, their related enzymes and biosynthetic pathways, particularly highlighting the microbial synthesis of high-value diterpenes directly from inexpensive carbon sources (e.g., sugars). The high titers (>4 g/L) achieved mean that some of these endeavors are reaching or close to commercialization. As such, we envisage a bright future in translating microbial synthesis of diterpenes into commercialization.

Diterpenoids and p-methoxycinnamic acid diol esters from Kaempferia saraburiensis Picheans. (Zingiberaceae): Structural assignment of saraburol and their biological activities

Five undescribed compounds, including three diterpenoids namely, saraburol, saraburanes A and B, and two p-methoxycinnamic acid monoterpene diol esters, named E/Z-saraburinic esters, together with ten known oxygenated isopimarane diterpenoids, were isolated from the whole plant of Kaempferia saraburiensis Picheans. Among these compounds, saraburol possesses an unusual 6/9/6 tricyclic ring system bearing a 1,3-dioxonane-4-one scaffold, which is rarely found in natural products. The structure of isolated compounds was elucidated by spectroscopic methods, including HRESIMS, FTIR, 1D and 2D-NMR, and by comparison with published data, and their absolute configurations were determined by comparison of experimental with calculated ECD spectra and hydrolysis reaction. Using gauge-independent atomic orbital (GIAO) NMR shift calculations coupled with DP4+ probability analyses, biogenetic considerations, and optical rotation allowed for the complete characterization of saraburol. A plausible biosynthetic pathway for saraburol and saraburane A was proposed. The cytotoxicity result indicated that E-saraburinic ester exhibited the most potent activity with an IC₅₀ value of 12.0 μM against MOLT-3 cells with a selectivity index of 12.5. Saraburane B exhibited the most potent activity against Gram-positive bacteria strain Staphylococcus epidermidis with MIC (MBC) value of 25 (50) μg/mL.

Antimicrobial Diterpenes: Recent Development From Natural Sources

Antimicrobial resistance has been posing an alarming threat to the treatment of infectious diseases over the years. Ineffectiveness of the currently available synthetic and semisynthetic antibiotics has led the researchers to discover new molecules with potent antimicrobial activities. To overcome the emerging antimicrobial resistance, new antimicrobial compounds from natural sources might be appropriate. Secondary metabolites from natural sources could be prospective candidates in the development of new antimicrobial agents with high efficacy and less side effects. Among the natural secondary metabolites, diterpenoids are of crucial importance because of their broad spectrum of antimicrobial activity, which has put it in the center of research interest in recent years. The present work is aimed at reviewing recent literature regarding different classes of natural diterpenes and diterpenoids with significant antibacterial, antifungal, antiviral, and antiprotozoal activities along with their reported structure-activity relationships. This review has been carried out with a focus on relevant literature published in the last 5 years following the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines. A total of 229 diterpenoids from various sources like plants, marine species, and fungi are summarized in this systematic review, including their chemical structures, classification, and significant antimicrobial activities together with their reported mechanism of action and structure-activity relationships. The outcomes herein would provide researchers with new insights to find new credible leads and to work on their synthetic and semisynthetic derivatives to develop new antimicrobial agents.

Shanpanootols G and H, Diterpenoids from the Rhizomes of Kaempferia pulchra Collected in Myanmar and Their Vpr Inhibitory Activities

Two new trihydroxy derivative of Δ^8(14),15-isopimarane diterpenoids, shanpanootols G (1) and H (2), along with three known analogues were isolated from the ethyl acetate-soluble extract of Kaempferia pulchra rhizomes collected in Shan State of Myanmar. The structures of these compounds including their absolute configurations were elucidated by the combination of one dimensional (1D) and 2D-NMR spectroscopic methods, high resolution mass spectrometric technique, and the experimental and the calculated electronic circular dichroism (ECD) data. The isopimarane diterpenoids (1-5) were tested for their Viral protein R (Vpr) inhibitory activities against TREx-HeLa-Vpr cells. Shanpanootol H (2) and (1R,2S,5S,9R,10S,13R)-1,2-dihydroxypimara-8(14),15-dien-7-one (4) exhibited anti-Vpr activities at the 5 µM treated dose.

Diterpenoids with Aromatase Inhibitory Activity from the Rhizomes of Kaempferia elegans

Investigation of bioactive compounds from the rhizomes of Kaempferia elegans led to the isolation and characterization of ten new diterpenoids, namely, five 12,13-seco-diterpenoids named elegansins A-E (1-5) and five new abietanes, elegansols A-E (6-10), together with seven known diterpenoids (11-17). The structure elucidation of the new compounds was achieved by HRESIMS, NMR, and ECD spectroscopic analysis. Compounds (1-5) are the first examples of 12,13-seco-diterpenoid-type compounds representing a decalin fused dihydropyran skeleton. Plausible biosynthetic pathways for compounds 1-5 are proposed. Aromatase inhibitory activities of all compounds were evaluated, and abieta-8,11,13-trien-11-ol (16) was found to be the most potent aromatase inhibitor with an IC₅₀ value of 3.7 μM.

Anti-Vpr activities of sesqui- and diterpenoids from the roots and rhizomes of Kaempferia candida

New copaene-type and nerolidol-type sesquiterpenoids, 7-hydroxymustakone (1) and 15-hydroxynerolidol (2), and a 15-norlabdane diterpenoid, kaempcandiol (3), together with four known compounds (4-7) were isolated from the chloroform extract of Kaempferia candida roots and rhizomes. The structures of the new compounds 1-3 were elucidated based on 1D and 2D NMR and HRESIMS spectroscopic analyses. The extract of the K. candida roots and rhizomes and all isolated compounds 1-7 possessed HIV-1 viral protein R (Vpr) inhibitory activities on the TREx-HeLa-Vpr cell line at a 5 μM concentration, without detectable cytotoxicity.

New diterpenoids from the rhizomes of Hedychium forrestii

A novel hexanorditerpenoid, hedychin C (1), and a new diterpenoid, hedychin D (2), were isolated from the rhizomes of Hedychium forrestii. Their structures and absolute configurations were unambiguously established by means of extensive spectroscopic data (IR, UV, HRMS, and NMR) and electronic circular dichroism (ECD) calculations. This is the first report of naturally occurring labdane-type hexanorditerpenoid. Compound 1 was proved to have moderate cytotoxicity against XWLC-05 cell line with an IC₅₀ value of 53.6 μM.

Recent Advances in Kaempferia Phytochemistry and Biological Activity: A Comprehensive Review

Background: Plants belonging to the genus Kaempferia (family: Zingiberaceae) are distributed in Asia, especially in the southeast region, and Thailand. They have been widely used in traditional medicines to cure metabolic disorders, inflammation, urinary tract infections, fevers, coughs, hypertension, erectile dysfunction, abdominal and gastrointestinal ailments, asthma, wounds, rheumatism, epilepsy, and skin diseases. Objective: Herein, we reported a comprehensive review, including the traditional applications, biological and pharmacological advances, and phytochemical constituents of Kaempheria species from 1972 up to early 2019. Materials and methods: All the information and reported studies concerning Kaempheria plants were summarized from library and digital databases (e.g., Google Scholar, Sci-finder, PubMed, Springer, Elsevier, MDPI, Web of Science, etc.). The correlation between the Kaempheria species was evaluated via principal component analysis (PCA) and agglomerative hierarchical clustering (AHC), based on the main chemical classes of compounds. Results: Approximately 141 chemical constituents have been isolated and reported from Kaempferia species, such as isopimarane, abietane, labdane and clerodane diterpenoids, flavonoids, phenolic acids, phenyl-heptanoids, curcuminoids, tetrahydropyrano-phenolic, and steroids. A probable biosynthesis pathway for the isopimaradiene skeleton is illustrated. In addition, 15 main documented components of volatile oils of Kaempheria were summarized. Biological activities including anticancer, anti-inflammatory, antimicrobial, anticholinesterase, antioxidant, anti-obesity-induced dermatopathy, wound healing, neuroprotective, anti-allergenic, and anti-nociceptive were demonstrated. Conclusions: Up to date, significant advances in phytochemical and pharmacological studies of different Kaempheria species have been witnessed. So, the traditional uses of these plants have been clarified via modern in vitro and in vivo biological studies. In addition, these traditional uses and reported biological results could be correlated via the chemical characterization of these plants. All these data will support the biologists in the elucidation of the biological mechanisms of these plants.

Diterpenoids of terrestrial origin

This review covers the isolation and chemistry of diterpenoids from terrestrial sources from 2017.

Kaemgalangol A: Unusual seco-isopimarane diterpenoid from aromatic ginger Kaempferia galanga

A new unusual seco-isopimarane, kaemgalangol A (1) and 12 usual analogs (2-13) were isolated from the rhizomes of Kaempferia galanga (Family: Zingiberaceae). KaemgalangolA (1) represented a rarely isolated 9,10-seco-isopimarane skeleton. The chemical structures of the isolated compounds were mainlyinvestigated by spectroscopic techniques such as 1D, 2D NMR, and HRMS. The absolute configuration of 1-3 was studied by X-ray diffraction analysis as well as experimental and TDDFT-calculated electronic circular dichroism. Among the isolated diterpenoids, 5, 6 and 9 exhibited cytotoxic activity against HeLa (IC₅₀ 75.1, 74.2 and 76.5 μM, respectively) and HSC-2 (IC₅₀ 69.9, 53.3 and 58.2 μM, respectively) cancer cells.