Isolation, Structure Elucidation, and Absolute Configuration of Syncarpic Acid-Conjugated Terpenoids from Rhodomyrtus tomentosa

Direct Organocatalytic Reductive Alkylation of Syncarpic Acid: Scope and Applications

Biologically important 4-alkylsyncarpic acids, which resemble the core structure of many natural products, were synthesized in one-pot through the organocatalytic three-component reductive alkylation with excellent yields and C-selectivity. Synthetic applications of 4-alkylsyncarpic acids were demonstrated by converting into the functionally rich molecules through different reactions like Michael, retro-Michael, reduction, and oxidation reactions. In a continuation, formal total synthesis of (±)-triumphalone, (±)-isotriumphalone, and monomeric phloroglucinol derivatives was reported in a few steps starting from 4-alkylsyncarpic acids in overall very good yields. Further showcasing the importance of C-alkylated products, 4-benzylsyncarpic acid and its Michael adduct with methyl vinyl ketone were synthesized in a gram scale without compromising rate/yields.

Discovery and Biomimetic Synthesis of a Polycyclic Polymethylated Phloroglucinol Collection from Rhodomyrtus tomentosa

Inspired by a previously reported biomimetic synthesis study, four new naturally occurring phloroglucinol trimers 1-4 with unusual 6/5/5/6/6/6-fused hexacyclic ring systems, along with two known analogues (5 and 6) and two known biogenetically related dimers (10 and 11), were isolated from Rhodomyrtus tomentosa. Their structures and absolute configurations were unambiguously elucidated by spectroscopic analysis, X-ray diffraction, and electronic circular dichroism calculation. By mimicking two potentially alternative biosynthetic pathways, the first asymmetric syntheses of 1-4 and the racemic syntheses of 5 and 6 were achieved in only five to six steps without the need for protecting groups. Furthermore, phloroglucinol dimers 10 and 11 exhibited significant in vitro antiviral activity against the respiratory syncytial virus.

Meroterpenoids: A Comprehensive Update Insight on Structural Diversity and Biology

Meroterpenoids are secondary metabolites formed due to mixed biosynthetic pathways which are produced in part from a terpenoid co-substrate. These mixed biosynthetically hybrid compounds are widely produced by bacteria, algae, plants, and animals. Notably amazing chemical diversity is generated among meroterpenoids via a combination of terpenoid scaffolds with polyketides, alkaloids, phenols, and amino acids. This review deals with the isolation, chemical diversity, and biological effects of 452 new meroterpenoids reported from natural sources from January 2016 to December 2020. Most of the meroterpenoids possess antimicrobial, cytotoxic, antioxidant, anti-inflammatory, antiviral, enzyme inhibitory, and immunosupressive effects.

A biomimetic synthesis-enabled stereochemical assignment of rhodotomentones A and B, two unusual caryophyllene-derived meroterpenoids from Rhodomyrtus tomentosa

Rhodotomentones A and B (1 and 2), two unusual caryophyllene-derived meroterpenoids (CDMTs) featuring a rare 6/6/9/4/6/6 hexacyclic ring system, along with their biogenetically-related CDMTs 7 and 12–15, were isolated from Rhodomyrtus tomentosa.

Cajuputones A-C, β-Triketone Flavanone Hybrids from the Branches and Leaves of Melaleuca cajuputi

Three new β-triketone flavanone hybrids, cajuputones A-C were obtained from Melaleuca cajuputi (the Australian 'tea tree'). The structures of cajuputones A-C were elucidated by 1D/2D NMR spectroscopy and HR-ESI-MS analyses; and their absolute configurations were established by electric circular dichroism (ECD) calculations using TDDFT method. Structurally, cajuputones A-C feature a rare 6/6/6/6 oxatetracyclic ring system fused between an acylphloroglucinol-derived β-triketone and a pinocembrin or strobopinin moiety via an angle-type pyran-like motif. DFT-based conformational optimization in chloroform explained the similarity of the 1D NMR data of cajuputones B and C (C-2 epimers).

Unraveling Plant Natural Chemical Diversity for Drug Discovery Purposes

The screening and testing of extracts against a variety of pharmacological targets in order to benefit from the immense natural chemical diversity is a concern in many laboratories worldwide. And several successes have been recorded in finding new actives in natural products, some of which have become new drugs or new sources of inspiration for drugs. But in view of the vast amount of research on the subject, it is surprising that not more drug candidates were found. In our view, it is fundamental to reflect upon the approaches of such drug discovery programs and the technical processes that are used, along with their inherent difficulties and biases. Based on an extensive survey of recent publications, we discuss the origin and the variety of natural chemical diversity as well as the strategies to having the potential to embrace this diversity. It seemed to us that some of the difficulties of the area could be related with the technical approaches that are used, so the present review begins with synthetizing some of the more used discovery strategies, exemplifying some key points, in order to address some of their limitations. It appears that one of the challenges of natural product-based drug discovery programs should be an easier access to renewable sources of plant-derived products. Maximizing the use of the data together with the exploration of chemical diversity while working on reasonable supply of natural product-based entities could be a way to answer this challenge. We suggested alternative ways to access and explore part of this chemical diversity with in vitro cultures. We also reinforced how important it was organizing and making available this worldwide knowledge in an "inventory" of natural products and their sources. And finally, we focused on strategies based on synthetic biology and syntheses that allow reaching industrial scale supply. Approaches based on the opportunities lying in untapped natural plant chemical diversity are also considered.

Triketone-terpene meroterpenoids from the leaves of Rhodomyrtus tomentosa

Eight new meroterpenoids (1-8) featuring β-triketone-conjugated terpenoids, rtomentones A-H, were isolated from the leaves of Rhodomyrtus tomentosa. Structures of the isolates were unambiguously established by a combination of NMR and ECD spectroscopy and X-ray diffraction analysis. Rtomentone C (3) was the first example of aromadendrane-based meroterpenoid containing an oxa-spiro[5.6] ring. Rtomentone D (4) was obtained as a racemic mixture confirmed by chiral HPLC analysis. The cytotoxicity against MDA-MB-231, A549, and DLD-1 cells of all isolates was evaluated.

Meroterpene-Like α-Glucosidase Inhibitors Based on Biomimetic Reactions Starting from β-Caryophyllene

BACKGROUND

Natural meroterpenes derived from phloroglucinols and β-caryophyllene have shown high inhibitory activity against α-glucosidase or cancer cells, however, the chemical diversity of this type of skeletons in Nature is limited.

METHODS

To expand the chemical space and explore their inhibitory activities against α-glucosidase (EC 3.2.1.20), we employed β-caryophyllene and some natural moieties (4-hydroxycoumarins, lawsone or syncarpic acid) to synthesize new types of meroterpene-like skeletons. All the products (including side products) were isolated and characterized by NMR, HR-MS, and ECD.

RESULTS

In total, 17 products (representing seven scaffolds) were generated through a one-pot procedure. Most products (12 compounds) showed more potential activity (IC₅₀ < 25 μM) than the positive controls (acarbose and genistein, IC₅₀ 58.19, and 54.74 μM, respectively). Compound 7 exhibited the most potent inhibition of α-glucosidase (IC₅₀ 3.56 μM) in a mixed-type manner. The CD analysis indicated that compound 7 could bind to α-glucosidase and influence the enzyme's secondary structure.

CONCLUSIONS

Compound 7 could serve as a new type of template compound to develop α-glucosidase inhibitors. Full investigation of a biomimic reaction can be used as a concise strategy to explore diverse natural-like skeletons and search for novel lead compounds.

The Health Beneficial Properties of Rhodomyrtus tomentosa as Potential Functional Food

Rhodomyrtus tomentosa (Aiton) Hassk. is a flowering plant belonging to the family Myrtaceae, native to southern and southeastern Asia. It has been used in traditional Vietnamese, Chinese, and Malaysian medicine for a long time for the treatment of diarrhea, dysentery, gynecopathy, stomachache, and wound healing. Moreover, R. tomentosa is used to make various food products such as wine, tea, and jam. Notably, R. tomentosa has been known to contain structurally diverse and biologically active metabolites, thus serving as a potential resource for exploring novel functional agents. Up to now, numerous phenolic and terpenoid compounds from the leaves, root, or fruits of R. tomentosa have been identified, and their biological activities such as antioxidant, antibacterial, anti-inflammatory, and anticancer have been evidenced. In this contribution, an overview of R. tomentosa and its health beneficial properties was focused on and emphasized.

Rare dimeric guaianes from Xylopia vielana and their multidrug resistance reversal activity

Thirteen undescribed dimeric guaianes were isolated from the leaves of Xylopia vielana Pierre. Their structures were elucidated by NMR spectroscopy and mass spectrometry, and the absolute configurations of vielanins G-Q were determined by a combination of the circular dichroism (CD) exciton chirality method, chemical conversion, and electronic CD (ECD) spectroscopy analysis. Vielaninors A and B are the first examples of trinor-guaiane-dimers. Multidrug resistance reversal activity assay of the isolates was evaluated in doxorubicin-resistant human breast cancer cells. Vielanins H, K-M, P, and Q were noncytotoxic and enhanced the cytotoxicity of doxorubicin by 2.1-41.6-fold at 10 μM.

A new polyhydroxylated oleanane triterpenoid from the roots of Rhodomyrtus tomentosa

A new oleanane triterpenoid, 2α,3β,6β,23,29-pentahydroxyolean-12-en-28- oic acid (1), was isolated from the roots of Rhodomyrtus tomentosa, together with four known oleanane triterpenoids (2-5) and two known ursane triterpenoids (6-7). The structure of compound 1 was determined by extensive NMR and HR-ESI-MS data analysis. Compounds 4-5 showed cytotoxicity against PC12 cell lines at a concentration of 50 μM, and compound 1 exhibited moderate neuroprotective activity against corticosterone induced PC12 cell death at the same concentration.

Cytotoxic and anti-inflammatory active phloroglucinol derivatives from Rhodomyrtus tomentosa

Seven undescribed phloroglucinol derivatives, tomentodiones N-T, and eleven known ones were isolated from Rhodomyrtus tomentosa. Tomentodione N is the first example of a β-triketone unit coupled with an isoamyl alcohol through a furan fused-ring, and tomentodiones N, S, T were three racemates. The undescribed structures were elucidated by means of spectroscopic, X-ray diffraction, electronic circular dichroism calculation, and chemical methods. In addition, the isolated compounds were determined for the cytotoxic activities on HeLa cells and anti-inflammatory activities in LPS-stimulated RAW 264.7 cells.

Tomentodione M sensitizes multidrug resistant cancer cells by decreasing P-glycoprotein via inhibition of p38 MAPK signaling

In this study, we investigated the mechanism by which tomentodione M (TTM), a novel natural syncarpic acid-conjugated monoterpene, reversed multi-drug resistance (MDR) in cancer cells. TTM increased the cytotoxicity of chemotherapeutic drugs such as docetaxel and doxorubicin in MCF-7/MDR and K562/MDR cells in a dose- and time-dependent manner. TTM reduced colony formation and enhanced apoptosis in docetaxel-treated MCF-7/MDR and K562/MDR cells, and it enhanced intracellular accumulation of doxorubicin and rhodamine 123 in MDR cancer cells by reducing drug efflux mediated by P-gp. TTM decreased expression of both P-gp mRNA and protein by inhibiting p38 MAPK signaling. Similarly, the p38 MAPK inhibitor SB203580 reversed MDR in cancer cells by decreasing P-gp expression. Conversely, p38 MAPK-overexpressing MCF-7 and K562 cells showed higher P-gp expression than controls. These observations indicate that TTM reverses MDR in cancer cells by decreasing P-gp expression via p38 MAPK inhibition.