Xylomolones A–D from the Thai Mangrove Xylocarpus moluccensis: Assignment of Absolute Stereostructures and Unveiling a Convergent Strategy for Limonoid Biosynthesis

Ten limonoids and three protolimonoids from the Thai mangrove Xylocarpus moluccensis

Ten new limonoids, named xylomolones E-N (1-10), and two new protolimonoids, named xylomolones O (11) and P (12), were isolated from seeds of the Thai mangrove Xylocarpus moluccensis, together with the known compound, hispidone acetonide (13). The structures of these compounds were established by extensive NMR spectroscopic data, single-crystal X-ray diffraction analysis, and comparison of experimental ECD spectra. The absolute configurations of xylomolones E (1) and L (8) were unambiguously determined by single-crystal X-ray diffraction analyses, conducted with Cu Kα radiation. Xylomolones E-L (1-8) are mexicanolide-type limonoids, among which xylomolones J (6) and K (7) contain a C7/C28δ-lactone ring, whereas xylomolones M (9) and N (10) are phragmalin-type limonoids. Xylomolones O (11) and P (12) are two new protolimonoids. In addition, the 1H and 13C NMR spectroscopic data for hispidone acetonide (13) was first assigned completely. In bioassay, xylomolones F (2), M (9), and P (12) exhibited moderate inhibitory activity against the production of NO in LPS-induced RAW 264.7 cells with IC50 values of 31.54 ± 7.27, 62.84 ± 17.62, and 22.7 ± 6.56 μM, respectively.

Isolation and NMR Scaling Factors for the Structure Determination of Lobatolide H, a Flexible Sesquiterpene from Neurolaena lobata

A new flexible germacranolide (1, lobatolide H) was isolated from the aerial parts of Neurolaena lobata. The structure elucidation was performed by classical NMR experiments and DFT NMR calculations. Altogether, 80 theoretical level combinations with existing ¹³C NMR scaling factors were tested, and the best performing ones were applied on 1. ¹H and ¹³C NMR scaling factors were also developed for two combinations utilizing known exomethylene containing derivatives, and the results were complemented by homonuclear coupling constant (J_HH) and TDDFT-ECD calculations to elucidate the stereochemistry of 1. Lobatolide H possessed remarkable antiproliferative activity against human cervical tumor cell lines with different HPV status (SiHa and C33A), induced cell cycle disturbance and exhibited a substantial antimigratory effect in SiHa cells.

UV light-driven late-stage skeletal reorganization to diverse limonoid frameworks: A proof of concept for photobiosynthesis

Late-stage skeletal reorganization (LSSR) is a type of fascinating organic transformation processes in natural product total synthesis. However, few facile and effective LSSR methodologies have hitherto been developed. Here, LSSR of limonoid natural products via photochemical cascades is first reported. Starting from xyloelves A and B, nine distinct limonoid products with five unprecedented scaffolds are generated. The photocascade pathways of these natural products and mechanistic rationale via intramolecular triplet energy transfer are revealed by quantum mechanical calculations. Most notably, ultraviolet light-driven transannular and stereoselective C → C 1,4-acyl migration is first found as a photochemical approach, particularly for LSSR of natural products. This approach holds promise for designing LSSR strategies to access bioactive cage-like molecules. Besides that, our findings provide a clear proof of concept for natural product photobiosynthesis. Xyloelf A, substantially ameliorating concanavalin A-induced liver injury in mice, could be used as a unique molecular template for hepatoprotective drug discovery.

Secotrijugins A-D, four highly oxidized and rearranged limonoids from Trichilia sinensis and their anti-inflammatory activity

Four undescribed highly oxidized and rearranged limonoids, secotrijugins A-D, were purified from the leaves and twigs of Trichilia sinensis. Within them, secotrijugin A was characterized as a rare 30-nortrijugin-type limonoid with an unusual cleavage of 1,14-ether bond, secotrijugins B and C represented new examples with the cleavage of δ-lactone ring D, and secotrijugin D was a rare trijugin-type limonoid with an unusual 2,6-oxygen bridge. The structures of limonoids were characterized by means of spectroscopic analysis and ECD calculations. The cellular screening revealed that secotrijugin B was the most active against LPS-stimulated NO production in BV-2 cells, which played an anti-inflammatory role by downregulating COX-2 and iNOS protein expression. The further in vivo experiments confirmed that secotrijugin B had strong in vivo anti-inflammatory effect via suppressing NO and ROS generation.

Unusual Secondary Metabolites from the Mangrove Ecosystems: Structures, Bioactivities, Chemical, and Bio-Syntheses

Mangrove ecosystems are widely distributed in the intertidal zone of tropical and subtropical estuaries or coasts, containing abundant biological communities, for example, mangrove plants and diverse groups of microorganisms, featuring various bioactive secondary metabolites. We surveyed the literature from 2010 to 2022, resulting in a collection of 134 secondary metabolites, and classified them into two major families in terms of the biological sources and 15 subfamilies according to the chemical structures. To highlight the structural diversity and bioactivities of the mangrove ecosystem-associated secondary metabolites, we presented the chemical structures, bioactivities, biosynthesis, and chemical syntheses.

Research progress of meliaceous limonoids from 2011 to 2021

Covering: July 2010 to December 2021Limonoids, a kind of natural tetranortriterpenoids with diverse skeletons and valuable insecticidal and medicinal bioactivities, are the characteristic metabolites of most plants of the Meliaceae family. The chemistry and bioactivities of meliaceous limonoids are a continuing hot area of natural products research; to date, about 2700 meliaceous limonoids have been identified. In particular, more than 1600, including thirty kinds of novel rearranged skeletons, have been isolated and identified in the past decade due to their wide distribution and abundant content in Meliaceae plants and active biosynthetic pathways. In addition to the discovery of new structures, many positive medicinal bioactivities of meliaceous limonoids have been investigated, and extensive achievements regarding the chemical and biological synthesis have been made. This review summarizes the recent research progress in the discovery of new structures, medicinal and agricultural bioactivities, and chem/biosynthesis of limonoids from the plants of the Meliaceae family during the past decade, with an emphasis on the discovery of limonoids with novel skeletons, the medicinal bioactivities and mechanisms, and chemical synthesis. The structures, origins, and bioactivities of other new limonoids were provided as ESI. Studies published from July 2010 to December 2021 are reviewed, and 482 references are cited.

Synthesis of (-)-melazolide B, a degraded limonoid, from a natural terpene precursor

Degraded limonoids are a subclass of limonoid natural products that derive from ring-intact or ring-rearranged limonoids. Establishment of robust synthetic routes to access them could provide valuable materials to identify the simplest active pharmacophore responsible for the observed biological activities of the parent molecules. This communication delineates the development of a divergent strategy to furnish melazolide B and several other related congeners from a common keto-lactone intermediate, which was rapidly assembled from α-ionone. A chemoselective carbonyl α,β-dehydrogenation and a Wharton reduction were key strategic steps in this synthetic pathway.

Isolation, Structure Determination of Sesquiterpenes from Neurolaena lobata and Their Antiproliferative, Cell Cycle Arrest-Inducing and Anti-Invasive Properties against Human Cervical Tumor Cells

Seven new germacranolides (1–3, 5–8), among them a heterodimer (7), and known germacranolide (4), eudesmane (9) and isodaucane (10) sesquiterpenes were isolated from the aerial parts of Neurolaena lobata. Their structures were determined by using a combination of different spectroscopic methods, including HR-ESIMS and 1D and 2D NMR techniques supported by DFT-NMR calculations. The enantiomeric purity of the new compounds was investigated by chiral HPLC analysis, while their absolute configurations were determined by TDDFT-ECD and OR calculations. Due to the conformationally flexible macrocycles and difficulties in assigning the relative configuration, ¹³C and ¹H NMR chemical shift and ECD and OR calculations were performed on several stereoisomers of two derivatives. The isolated compounds (1–10) were shown to have noteworthy antiproliferative activities against three human cervical tumor cell line with different HPV status (HeLa, SiHa and C33A). Additionally, lobatolide C (6) exhibited substantial antiproliferative properties, antimigratory effect, and it induced cell cycle disturbance in SiHa cells.

Discovery of benthol A and its challenging stereochemical assignment: opening up a new window for skeletal diversity of super-carbon-chain compounds

Super-carbon-chain compounds (SCCCs) are marine organic molecules featuring long polyol carbon chains with numerous stereocenters. Polyol-polyene compounds (PPCs) and ladder-frame polyethers (LFPs) are two major families. It is highly challenging to establish the absolute configurations of SCCCs. In this century, few new SCCC families have been reported. Benthol A, an aberrant SCCC, was obtained from a South China Sea benthic dinoflagellate that should belong to a new taxon. Its planar structure and absolute configuration, containing thirty-five carbon stereocenters, were unambiguously established by a combination of extensive NMR spectroscopic investigations, periodate degradation of the 1,2-diol groups, ozonolysis of the carbon-carbon double bonds, J-based configurational analysis, NOE interactions, modified Mosher's MTPA ester method, and DFT-NMR ¹³C chemical-shift calculations aided by DP4+ statistical analysis. Benthol A displayed potent antimalarial activity against Plasmodium falciparum 3D7 parasites. This new molecule combines extraordinary structural features, particularly eight scattered ether rings on a C₇₂ backbone chain, which places it within a new SCCC family between PPCs and LFPs, herein termed polyol-polyether compounds. This suggestion was strongly supported by principal component analysis. The discovery of benthol A does not only provide new insights into the untapped biosynthetic potential of marine dinoflagellates, but also opens up a new window for skeletal diversity of SCCCs.

Chemical profile and therapeutic potentials of Xylocarpus moluccensis (Lam.) M. Roem.: A literature-based review

ETHNOPHARMACOLOGICAL RELEVANCE

Historically, mangrove plants are among the potential sources of foods and remedies for humans living in the forests and nearby communities. Xylocarpus moluccensis (Lam.) M. Roem., an important mangrove medicinal plant, has been traditionally used for many purposes such as treatment of fever, dysentery, diarrhea, swelling, and abdominal disorders. The aim of the present work was to summarize the chemical reports and biological activities of the mangrove medicinal plant X. moluccensis based on information collected from different databases.

MATERIALS AND METHODS

An up-to-date search (till Aug 2019) was carried out in databases such as PubMed, Science Direct, Google Scholar, and various patient offices (e.g., WIPO, CIPO, USPTO) using the keywords: 'Xylocarpus moluccensis', and/or paired with 'ethnobotanical use', and 'phytochemical'. In vitro, ex vivo, or in vivo studies were included.

RESULTS

Findings suggest that X. moluccensis contains various important minerals and phytochemicals, where flavonoids, terpenes and terpenoids are the most prominent isolated phyto-constituents of X. moluccensis. Extracts/fractions or isolated compounds from this plant possess diverse biological activities, including anti-inflammatory, anti-microbial, antineoplastic, anti-diarrheal, insecticidal, anti-feedant, neuropharmacological (e.g., central nervous system depressant), anti-atherosclerotic, and lipid-lowering activity. Only one report suggests that the methanol and aqueous extracts of this plant did not exert cytotoxic effects on normal mouse fibroblast cells. However, no clinical studies were reported.

CONCLUSIONS

Taken all together, X. moluccensis may be one of the best sources of pharmacologically active lead compounds. More research, however, is necessary to establish the safety and efficacy, and its toxicogenetic effects in animal models.

Computational prediction of chiroptical properties in structure elucidation of natural products

Covering: up to 2019This review covers the current status of the quantum mechanical prediction of chiroptical properties, such as electronic CD and optical rotation, as needed for stereochemical assignments in new natural products. The reliability of the prediction of chiroptical properties is steadily increasing, with a parallel decrease in the required computational resources. Now, quantum mechanical calculations for a medium-sized natural product can be reliably performed by natural product chemists on a mainstream PC. This review is aimed to guide natural product chemists through the numerous steps involved in such calculations. Through a concise, but comprehensive, discussion of the current computational practice, enriched by a few illustrative examples, this review provides readers with the theoretical background and practical knowledge needed to select the most appropriate parameters for performing the calculations, to anticipate possible problems, and to critically evaluate the reliability of their computational results. Common reasons for mistakes are also discussed; in particular, the importance of the correct evaluation of conformational ensembles of flexible molecules (an aspect often overlooked in current research) is stressed.

Total Synthesis of (+)-Granatumine A and Related Bislactone Limonoid Alkaloids via a Pyran to Pyridine Interconversion

We report the first total synthesis of (+)-granatumine A, a limonoid alkaloid with PTP1B inhibitory activity, in ten steps. Over the course of this study, two key methodological advances were made: a cost-effective procedure for ketone α,β-dehydrogenation using allyl-Pd catalysis, and a Pd-catalyzed protocol to convert epoxyketones to 1,3-diketones. The central tetrasubstituted pyridine is formed by a convergent Knoevenagel condensation and carbonyl-selective electrocyclization cascade, which was followed by a direct transformation of a 2 H-pyran to a pyridine. These studies have led to the structural revision of two members of this family.

Computationally-assisted discovery and structure elucidation of natural products

Computer hardware development coupled with the development of quantum chemistry, new computational models and algorithms, and user-friendly interfaces have lowered the barriers to the use of computation in the discovery and structure elucidation of natural products. Consequently, the use of computational chemistry software as a tool to discover and determine the structure of natural products has become more common in recent years. In this review, we provide several examples of recent studies that used computer technology to facilitate the discovery and structure determination of various natural products.