Biomimetic Total Synthesis of the Rubiginosin Meroterpenoids

Chromene meroterpenoids from Rhododendron dauricum L. and their anti-inflammatory effects

Rhododendron dauricum L. is a perennial herb belonging to the genus Rhododendron, commonly utilized in formulations for treating coughs and bronchitis, as well as in herbal teas for enhancing immunity and preventing tracheitis. In this study, fifteen previously undescribed chromene meroterpenoids (1a/1b-4a/4b, 5-8, 9b, 10a, 11b), along with twenty-one known compounds were isolated from the dried twigs and leaves of Rhododendron dauricum L. Of these, (-)-rhodonoid E (9b), (+)-confluentin (10a), and (-)-rubiginosin D (11b) were separated for the first time by chiral HPLC separation. The elucidation of their structures, including absolute configurations, was achieved through a combination of techniques such as NMR, HRESIMS, modified Mosher's method and quantum-chemical calculation of electronic circular dichroism (ECD) spectra. Seven pairs of enantiomers, compounds 1a/1b-4a/4b and 9a/9b-11a/11b, were initially obtained in a racemic manner and were further separated by chiral HPLC preparation. The biological assessment of these compounds against NO production was conducted in the LPS-induced RAW264.7 macrophage cells model. Compounds 9a, 9b, and 11a displayed inhibitory rates exceeding 80%, with IC50 values ranging from 8.69 ± 0.94 to 13.01 ± 1.11 μM. A preliminary examination of the structure-activity relationship (SAR) for these isolates indicated that chromene meroterpenoids with α, β-unsaturated ketone carbonyl and Δ12(13) double bond functionalities exhibited enhanced anti-inflammatory properties.

Recent advances in the application of [2 + 2] cycloaddition in the chemical synthesis of cyclobutane-containing natural products

Cyclobutanes are distributed widely in a large class of natural products featuring diverse pharmaceutical activities and intricate structural frameworks. The [2 + 2] cycloaddition is unequivocally the primary and most commonly used method for synthesizing cyclobutanes. In this review, we have summarized the application of the [2 + 2] cycloaddition with different reaction mechanisms in the chemical synthesis of selected cyclobutane-containing natural products over the past decade.

Discovery, biosynthesis, organic synthesis, and bioactivities of meroterpenoids from Rhododendron species

Meroterpenoids discovered in Rhododendrons species possess unique chemical structures and biological activities and are expected to become new drug targets for Alzheimer's disease, metabolic disorders, and chronic kidney disease, and these compounds have attracted increasing attention in recent years. In this study, Rhododendron meroterpenoids and their structures, classifications, racemate distribution, biosynthetic pathways, chemical synthesis, and bioactivities are reviewed prior to 2023.

[2 + 2]-Cycloaddition-derived cyclobutane natural products: structural diversity, sources, bioactivities, and biomimetic syntheses

This review summarizes the structural diversity, bioactivities, and biomimetic synthesis of [2 + 2]-type cyclobutane natural products, along with discussion of their biosynthesis, stereochemical analysis, racemic occurrence, and biomimetic synthesis.

Synthesis and Anti-Leishmanial Properties of Quinolones Derived from Zanthosimuline

Quinoline derivatives and especially quinolones are considered as privileged structures in medicinal chemistry and are often associated with various biological properties. We recently isolated a series of original monoterpenyl quinolones from the bark of Codiaeum peltatum. As this extract was found to have a significant inhibitory activity against a Leishmania species, we decided to study the anti-leishmanial potential of this type of compound. Leishmaniasis is a serious health problem affecting more than 12 million people in the world. Available drugs cause harmful side effects and resistance for some of them. With the aim of finding anti-leishmanial compounds, we developed a synthetic strategy to access natural quinolones and analogues derived from zanthosimuline. We showed the versatility of this natural compound toward cyclization conditions, leading to various polycyclic quinolone-derived structures. The natural and synthetic compounds were evaluated against amastigote forms of Leishmania infantum. The results obtained confirmed the interest of this family of natural compounds but also revealed promising activities for some intermediates deriving from zanthosimuline. Following the same synthetic strategy, we then prepared 14 new analogues. In this work, we identified two promising molecules with good activities against intramacrophage L. infantum amastigotes without any cytotoxicity. We also showed that slight changes in amide functional groups affect drastically their anti-parasitic activity.

Anti-infammatory scalemic chromanoids and chromenoids from Rhododendron dauricum

Four pairs of undescribed chromane and chromene meroterpenoid scalemic mixtures (1a/1b-4a/4b), together with three pairs of known chromane meroterpenoid ones (5a/5b-7a/7b) were isolated from the twigs and leaves of Rhododendron dauricum L. Among them, 1a/1b-3a/3b and 5a/5b-7a/7b were the chromane ones derived from an intramolecular [2 + 2] cyclic addition of their respective chromene precursors, forming a 6/6/6/4 and 6/6/5/4 ring fused scaffold. The absolute configurations of the chiral center at C-15 of 2a/2b were determined by Snatzke's method, and comparing the experimental and calculated electronic circular dichroism (ECD) data. The inhibitory effects of the isolated compounds were tested against lipopolysaccharide (LPS)-induced nitric oxide production in RAW264.7 macrophage cells to evaluate their anti-inflammatory activity. Compounds 4a, 4b and 6a displayed inhibitory effects on nitric oxide (NO) production, and compound 4b exhibited the obvious anti-inflammatory activity, with an IC₅₀ value of 6.91 ± 0.97 μM, by downregulating nuclear factor kappa B (NF-κB) and reducing the expression of inducible nitric oxide synthase (iNOS) in LPS-induced RAW264.7 cells. These results intimated that 4b could be used as a leading compound to develop anti-inflammatory drugs and is worthy of further investigated.

Diastereoselective construction of tetracyclic chromanes via a triply annulative strategy

A triple Michael/aldol cascade reaction has been established to construct tetracyclic chromanes in a diastereoselective fashion (≥5 : 1 dr). The polycyclic products were generated in 50-78% isolated yields under mild and metal-free conditions. Five reactive sites of enolate-tethered divinyl ketones were sequentially utilized to form four C-C bonds in a one-pot operation, leading to a construction of three new rings. Up to six consecutive stereocenters, including two quarternary stereogenic centers, were created in this domino process.

On the Erosion of Enantiopurity of Rhodonoids via Their Asymmetric Total Synthesis

Rhodonoid natural products are found in nature as a scalemic mixture. This interesting phytochemical feature is presumed to originate from a reversible electrocyclic ring opening of the chromene core present in the biogenetic precursors of rhodonoids. Herein, we systematically investigated factors that are responsible for this racemization event. This eventually led us to complete the asymmetric total synthesis of rhodonoids A, C, D, and G.

A Fungal Promiscuous UbiA Prenyltransferase Expands the Structural Diversity of Chrodrimanin-Type Meroterpenoids

Prenyltransferases play important roles in the diversification of natural products and the improvement of biological activities. A UbiA-type prenyltransferase CdnC with substrate promiscuity was identified as the pivotal builder of the noncanonical chrodrimanin skeletons, which carry a benzo-cyclohexanone structure as the nonterpene part. In vitro and heterologous expression studies with CdnC led to the production of a series of novel chrodrimanin-like structures. The discovery of CdnC offers a referable strategy for the biosynthesis and structural diversification of farnesyl-derived meroterpenoids.

A series of meroterpenoids with rearranged skeletons from an endophytic fungus Penicillium sp. GDGJ-285

Five new meroterpenoids (1–5), including three novel skeleton meroterpenoids, peniclactones A–C (1–3), and two new isoaustinone analogues (4 and 5), 6-hydroxyisoaustinone (4) and 6-ketoisoaustinone (5), were isolated from the fungus Penicillium sp. GDGJ-285.