Biomimetic Total Synthesis of Rhodonoids C and D, and Murrayakonine D

Structurally diverse chromane meroterpenoids from Rhododendron capitatum with multifunctional neuroprotective effects

Eleven new chromane meroterpenoids (1-11), along with 24 known ones (12-35) were isolated from Rhododendron capitatum, a Tibetan medicine. Their structures were determined via extensive spectroscopic methods. The absolute configurations of 1 and 2 were determined by comparison of the experimental and theoretically calculated ECD data. For compounds 3-9, the absolute configurations at the C-2 were assigned according to the empirical chromane helicity rule. The stereochemistry of the chiral alcohols at C-13 in 3 and C-15 in 4 were determined using the Rh2(OCOCF3)4-induced ECD spectra based on the bulkiness rule. Additionally, the absolute configurations of secondary alcohols at C-13 in 8 and 9 were unambiguously established by Mosher's method. Neuroprotection evaluations in vitro and in vivo revealed that compounds 1, 18, and 21 can significantly inhibit the inducible nitric oxide synthase (iNOS) and cyclooxygenase 2 (COX-2) protein expressions. Compound 21 also down-regulated MAPK signal pathway in BV-2 cells. The PC-12 cell damage induced by H2O2 and 6-hydroxydopamine (6-OHDA) was attenuated by compounds 1, 21, and 22, especially for 22. Moreover, compounds 3, 6, 22, 23, and 28 significantly enhanced NGF-induced neurite growth in PC-12 cells. Notably, compound 6 demonstrated the most potent neurite growth promotion with a rate of 22.93 ± 2.24 % at 10 μM, which was approximately 3-fold higher than that induced by nerve growth factor (NGF). In AD Caenorhabditis elegans CL4176 model, compounds 1 and 21 delayed Aβ-induced paralysis and reduced ROS expression levels. These studies provide new potential neuroprotective agents for the prevention and treatment of neurodegenerative diseases.

Exploring the synthetic potential of epoxide ring opening reactions toward the synthesis of alkaloids and terpenoids: a review

Epoxides are oxygen containing heterocycles which are significantly employed as crucial intermediates in various organic transformations. They are considered highly reactive three-membered heterocycles due to ring strain and they undergo epoxide ring opening reactions with diverse range of nucleophiles. Epoxide ring-opening reactions have gained prominence as flexible and effective means to obtain various functionalized molecules. These reactions have garnered substantial attention in organic synthesis, driven by the need to comprehend the synthesis of biologically and structurally important organic compounds. They have also found applications in the synthesis of complex natural products. In this review article, we have summarized the implementation of epoxide ring opening reactions in the synthesis of alkaloids and terpenoids based natural products reported within the last decade (2014-2023).

An updated review of the genus Rhododendron since 2010: Traditional uses, phytochemistry, and pharmacology

Rhododendron, the largest genus of Ericaceae, consists of approximately 1000 species that are widely distributed in Europe, Asia, and North America but mainly exist in Asia. Rhododendron plants have not only good ornamental and economic value but also significant medicinal potential. In China, many Rhododendron plants are used as traditional Chinese medicine or ethnic medicine for the treatment of respiratory diseases, pain, bleeding and inflammation. Rhododendron is known for its abundant metabolites, especially diterpenoids. In the past 13 years, a total of 610 chemical constituents were reported from Rhododendron plants, including 222 diterpenoids, 122 triterpenoids, 103 meroterpenoids, 71 flavonoids and 92 other constituents (lignans, phenylpropanoids, phenolic acids, monoterpenoids, sesquiterpenoids, coumarins, steroids, fatty acids). Moreover, the bioactivities of various extracts and isolates, both in vitro and in vivo, were also investigated. Our review summarized the research progress of Rhododendron regarding traditional uses, phytochemistry and pharmacology in the past 13 years (2010 to December 2022), which will provide new insight for prompting further research on Rhododendron application and drug development.

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.

Gambogic acid: Multi-gram scale isolation, stereochemical erosion toward epi-gambogic acid and biological profile

Introduction

Extracted from gamboge resin, gambogic acid (GBA) is a natural product that displays a complex caged xanthone structure and exhibits promising antitumor properties. However, efforts to advance this compound to clinical applications have been thwarted by its limited availability that in turn, restricts its pharmacological optimization.

Methods

We report here an efficient method that allows multigram scale isolation of GBA in greater than 97% diastereomeric purity from various sources of commercially available gamboge. The overall process includes: (a) isolation of organic components from the resin; (b) separation of GBA from the organic components via crystallization as its pyridinium salt; and (c) acidification of the salt to isolate the free GBA.

Results and Discussion

We found that GBA is susceptible to epimerization at the C2 center that produces epi-gambogic acid ( epi-GBA), a common contaminant of all commercial sources of this compound. Mechanistic studies indicate that this epimerization proceeds via an ortho-quinone methide intermediate. Although the observed stereochemical erosion accounts for the chemical fragility of GBA, it does not significantly affect its biological activity especially as it relates to cancer cell cytotoxicity. Specifically, we measured similar levels of cytotoxicity for either pure GBA or an equilibrated mixture of GBA/ epi-GBA in MBA-MB-231 cells with IC50 values at submicromolar concentration and induction of apoptosis after 12 hours of incubation. The results validate the pharmacological promise of gambogic acid and, combined with the multigram-scale isolation, should enable drug design and development studies.

Biomimetic Total Synthesis of the Rubiginosin Meroterpenoids

Total synthesis of the Rhododendron meroterpenoids rubiginosins A and G, which both contain unusual 6-6-6-4 ring systems, has been achieved using a bioinspired cascade approach. Stepwise synthesis of these natural products, and the related 6-6-5-4 meroterpenoids fastinoid B and rhodonoid B, from naturally occurring chromene precursors is also reported.

Meroterpenoids with diverse structures and anti-inflammatory activities from Rhododendron anthopogonoides

Eight pairs of meroterpenoid enantiomers and four achiral meroterpenoids were isolated from Rhododendron anthopogonoides Maxim. Seventeen of them, named (+)-/(-)-anthoponoids A-G, (+)-daurichromene D, and anthoponoids H and I, are undescribed compounds with structural diversity. Their structures were characterized herein by a combined application of spectroscopic techniques, X-ray crystallographic analysis, ECD calculation, and the modified Mosher's method. (+)-/(-)-Anthoponoid A and anthoponoid I are the first Rhododendron meroterpenoids found to possess a hexahydroxanthene motif and a diterpene unit, respectively. Some isolates were identified as NF-κB pathway inhibitors, and (+)-anthoponoid E, (-)-anthoponoid G, and anthoponoid H showed suppressive effects on LPS-induced inflammatory responses in RAW 264.7 macrophages.

Enantiomeric pairs of meroterpenoids from Rhododendron fastigiatum

Five pairs of optically pure meroterpenoid enantiomers (1a/1b-5a/5b) and two known compounds (6 and 7) were isolated from Rhododendron fastigiatum. Compounds 1a/1b-5a/5b were resolved from naturally scalemic mixtures by chiral HPLC. Their structures were elucidated by spectroscopic methods, X-ray crystallographic experiments, and ECD analyses. Compounds 1a/1b, 2a/2b, 3b, 4a/4b, and 5a/5b were new meroterpenoids with different polycyclic systems. Two enantiomeric pairs (2a/2b and 3a/3b), 6, and 7 exhibited inhibitory effects on protein tyrosine phosphatase 1B (PTP1B) in vitro.

ortho-Quinone Methide Cyclizations Inspired by the Busseihydroquinone Family of Natural Products

A series of cascade reactions of o-quinone methides have been developed based on the proposed biosynthesis of busseihydroquinone and parvinaphthol meroterpenoid natural products. The polycyclic framework of the most complex family members, busseihydroquinone E and parvinaphthol C, was assembled by an intramolecular [4 + 2] cycloaddition of an electron-rich chromene substrate. The resultant cyclic enol ether underwent rearrangements under acidic or oxidative conditions, which led to a new total synthesis of rhodonoid D.

Synthesis of 6,12-Disubstituted Methanodibenzo[b,f][1,5]dioxocins: Pyrrolidine Catalyzed Self-Condensation of 2′-Hydroxyacetophenones

The preparation of unprecedented 6,12-disubstituted methanodibenzo[b,f][1,5]dioxocins from pyrrolidine catalyzed self-condensation of 2′-hydroxyacetophenones is herein described. This method provides easy access to this highly bridged complex core, resulting in construction of two C–O and two C–C bonds, a methylene bridge and two quaternary centers in a single step. The intricate methanodibenzo[b,f][1,5]dioxocin compounds were obtained in up to moderate yields after optimization of the reaction conditions concerning solvent, reaction times and the use of additives. Several halide substituted methanodibenzo[b,f][1,5]dioxocins could be prepared from correspondent 2′-hydroxyacetophenones.

Synthesis of Diversely Functionalized 2 H-Chromenes through Pd-Catalyzed Cascade Reactions of 1,1-Dibromoolefin Derivatives with Arylboronic Acids

In this work, an unprecedented synthesis of (2 H-chromen-2-ylidene)acetates and (2 H-chromen-2-ylidene)ethanones with controlled stereoselectivity via Pd-catalyzed cascade reactions of 3-(2-(2,2-dibromovinyl)phenoxy)acrylates or 3-(2-(2,2-dibromovinyl)phenoxy)enones with aryl boronic acids has been established. This protocol combines two C-C bond forming reactions (an intermolecular Suzuki coupling followed by an intramolecular Heck coupling) in one pot under the catalysis of the same catalyst. Compared with literature methods for the preparation of 2 H-chromene derivatives, it has advantages such as easily obtainable or commercially available substrates, diverse substitution pattern of products, simple procedure, and excellent stereoselectivity. Interestingly, this cascade reaction could distinguish the subtle difference of the electron-withdrawing capacity and the size of various functional groups and thus resulted in a different chemoselectivity. In addition, the utility of the (2 H-chromen-2-ylidene)acetate thus obtained was showcased by its facile transformation into the synthetically and photophysically significant 3 H-xanthen-3-one derivative.

Enantiomeric Pairs of Meroterpenoids with Diverse Heterocyclic Systems from Rhododendron nyingchiense

Eight enantiomeric pairs of new meromonoterpenoids (1a/1b-8a/8b) and four known compounds (9-12) were isolated from Rhododendron nyingchiense. Their structures were established by spectroscopic methods, quantum chemical calculations, and X-ray crystallography. The enantiomeric pairs were acquired from scalemic mixtures by chiral-phase HPLC and showed diverse heterocyclic frameworks. Compounds 1a/1b possess a rare 6/7/5/5 heterocyclic system, and 2a/2b incorporate a new 6/6/3/5 heterocyclic system featuring a quinone motif. Compounds 3a/3b represent the first meroterpenoids with a 6/6/5 ring system from the Rhododendron genus. Putative biosynthetic pathways of these compounds are proposed. Compounds 1b, 2a-4a, 8a, 8b, and 11 exhibited weak inhibitory effects on PTP1B, with IC₅₀ values ranging from 5.7 ± 0.5 to 61.0 ± 4.8 μM.

Total Syntheses of (±)-Rhodonoids C, D, E, F, and G and Ranhuadujuanine B

Here we describe the divergent, biosynthetically inspired syntheses of (±)-rhodonoids C-G and (±)-ranhuadujuanine B. The key steps of the syntheses include the construction of the chromene unit through a formal oxa-[3 + 3] annulation and a biomimetic acid-catalyzed ring cyclization. Cationic [2 + 2] cycloaddition is accomplished to form the cyclobutane core of (±)-rhodonoids E and F.