Unusual Enzymatic C-C Bond Formation and Cleavage Reactions during Natural Product Biosynthesis.

Natural products from plants and microorganisms provide a valuable reservoir of pharmaceutical compounds. C-C bond formation and cleavage are crucial events during natural product biosynthesis, playing pivotal roles in generating diverse and intricate chemical structures that are essential for biological functions. This review summarizes our recent findings regarding biosynthetic enzymes that catalyze unconventional C-C bond formation and cleavage reactions during natural product biosynthesis.

Structure Elucidation of Two New Norlignans from Anemone vitifolia and Their Anti-Inflammatory Activities.

Two new norlignans together with two known phenylpropanoids were isolated from the whole herb of Anemone vitifolia. All compounds were reported from this plant for the first time. The structures of these compounds were identified by comprehensive HR-ESI-MS, 1D and 2D NMR spectroscopic data analysis and comparison with literature data. Additionally, bioactivity study results showed that two new compounds have potential anti-inflammatory activity. The plausible biosynthetic pathway for these compounds were also speculated in this article.

Noralashinol B, a norlignan with cytotoxicity from stem barks of Syringa pinnatifolia.

One new norlignan, noralashinol B (1), and one new natural product, proposed noralashinol C (2), were isolated in a continuous phytochemical investigation on the stem barks of Syringa pinnatifolia. Their structures were elucidated based on the analysis of spectroscopic data, including mass spectrometry and 1D and 2D NMR spectroscopies, and the absolute configuration was determined by experimental and calculated electronic circular dichroism. Compound 1 showed a weak cytotoxicity against HepG2 hepatic cancer cells with its IC50 value of 31.7 µM. Furthermore, 1 induced apoptosis of HepG2 cells in a dose-dependent manner at concentrations of 0-80.0 µM.

Enantioselective syntheses of both enantiomers of 9'-dehydroxyimperanene and 7,8-dihydro-9'-dehydroxyimperanene and the comparison of biological activity between 9-norlignans and dihydroguaiaretic acids.

To estimate the effect of methyl group of dihydroguaiaretic acid, which shows many kinds of biological activities, on biological activity, both enantiomers of 9'-dehydroxyimperanene (5, 6) and 7,8-dihydro-9'-dehydroxyimperanene (7, 8) lacking one of the methyl groups of dihydroguaiaretic acid were synthesized. (S)-7,8-Dihydro-9'-dehydroxyimperanene (7) showed 4-6-fold higher cytotoxic activity than all stereoisomers of dihydroguaiaretic acid (2-4). The IC50 values of (S)-7,8-dihydro-9'-dehydroxyimperanene (7) against HL-60 and HeLa cells were 6.1µM and 5.6µM, respectively. Though only one of three stereoisomers of dihydroguaiaretic acid showed antibacterial activity against a gram negative bacterium, both enantiomers of 5-8 showed antibacterial activity against a gram negative bacterium. This is a Letter on biological activity of 9-norlignan, in which one of methyl groups of lignan is absent.

Functions and mechanisms of enzymes assembling lignans and norlignans.

Lignans and norlignans are distributed throughout the plant kingdom and exhibit diverse chemical structures and biological properties that offer potential for therapeutic use. Originating from the phenylpropanoid biosynthesis pathway, their characteristic carbon architectures are formed through unique enzyme catalysis, featuring regio- and stereoselective C-C bond forming processes. Despite extensive research on these plant natural products, their biosynthetic pathways, and enzyme mechanisms remain enigmatic. This review highlights recent advancements in elucidating the functions and mechanisms of the biosynthetic enzymes responsible for constructing the distinct carbon frameworks of lignans and norlignans.

Chemical constituents from Pterocarpus santalinus and their inhibitory effects on nitric oxide production.

A tropolone (2) and an acorane sesquiterpene (3), along with twenty previously known compounds were isolated from the heartwood of Pterocarpus santalinus. The structure of the isolated compounds was elucidated via 1D and 2D NMR spectroscopy and HRESIMS analysis. The absolute configuration of 3 was determined by comparison of the experimental and calculated ECD data. All compounds were evaluated for their inhibitory effects against nitric oxide production in LPS-stimulated RAW 264.7 macrophages.

Two new nor-lignans, siamensinols A and B, from Selaginella siamensis Hieron. and their biological activities.

Two new nor-lignans siamensinols A-B (1-2) and seven known compounds agatharesinol (3), syringaresinol-glucoside (4), noreugenin (5), 8-methyleugenitol (6), melachromone (7), uncinoside A (8) and daucosterol (9) were isolated from Selaginella siamensis Hieron. The structures of the new compounds were elucidated on the basis of comprehensive spectroscopic methods, including 1 D, 2 D-NMR, HR-ESI-MS and CD spectrometry. Compounds 1-2 showed moderate inhibitory effect on MOLT-3 cells while 8-methyleugenitol (6) exhibited moderate inhibitory effect on three tumor cells (HepG2, A549 and HuCCA-1). Compounds 2-3 showed the potent cancer chemoprevention in DPPH, XXO, IXO and AIA assays.

A cyclic peptide and two pairs of norlignan lignanoside epimers from Selaginella pulvinata.

A new cyclopeptide, pulvpeptin A (1), two pairs of norlignan lignanosides, tamariscinosides G and H (2, 3), together with five known compounds (4-8) were isolated from Selaginella pulvinata. The structures were elucidated by spectroscopic data and chemical degradation. The activity of tamariscinoside G (2) was evaluated by stimulating glucose uptake in 3T3-L1 adipocytes. The results showed 1.49-fold increase in glucose uptake in 3T3-L1 cells relative to basal.

Norlignan glucosides from Hypoxis hemerocallidea and their potential in vitro anti-inflammatory activity via inhibition of iNOS and NF-κB.

Eleven diarylpentanoid/norlignan glucosides, along with five other specialized metabolites, were isolated and characterized from the hydro-alcoholic extract of Hypoxis hemerocallidea corms. Hypoxhemerolosides A-F were found to be undescribed compounds. Curcapicycloside was isolated and identified for the first time in its original form, previously it was reported as a methylated derivative. In addition, (1S,2R)-1-(3,4-dihydroxyphenyl)-5-(4-ß-D-glucopyranoxy-3-hydroxyphenyl)-1-methoxypent-4-yn-2-ol and (1S,2R)-1-(3,4-dihydroxyphenyl)-1-ethoxy-5-(4-ß-D-glucopyranoxy-3-hydroxyphenyl)pent-4-yn-2-ol were isolated and characterized as artifacts, generated during extraction/isolation procedures from possible 1-(3,4-dihydroxyphenyl)-5-(4-ß-D-glucopyranoxy-3-hydroxyphenyl)pent-4-yne-1,2-diol. Structure elucidation was mainly achieved by the interpretation of 1D and 2D NMR and HRESIMS data. The isolated compounds were screened for anti-inflammatory activity in terms of iNOS and NF-κB inhibition as well as for cytotoxicity. Hypoxhemerolosides C-E and obtuside A moderately inhibited nitric oxide production in LPS-stimulated mouse macrophages (RAW 264.7).

Spirosteroids and α-glucosidase inhibitory norlignans from Asparagus racemosus Willd. roots.

Three undescribed spirosteroids, asparacemosones A-C, an undescribed spiro-21-norsteroid, asparacemosone D, along with seven known compounds were isolated from Thai herbal plant Asparagus racemosus Willd. roots. Their structures were elucidated by spectroscopic analysis including NMR, UV, IR and mass spectrometry. The absolute configurations of asparacemosones A, B, and D were determined by single crystal X-ray diffraction using CuKα radiation. Among the isolated compounds, the norlignan nyasol and three acetylenic norlignans demonstrated potent α-glucosidase inhibition, with IC50 values ranging from 0.003 to 0.004 µM which is 5 × 104 fold more potent than the standard acarbose.

Design of 92 New 9-Norlignan Derivatives and Their Effect on Cell Viabilities of Cancer and Insect Cells.

Ninety-two new 9-norlignan derivatives containing more effective compounds against both cancer and insect cells than lead compounds were synthesized. Against HeLa cells, 7-(3,4-dimethoxyphenyl)-7'-(3'-hydroxy-4'-methoxyphenyl) derivative 63 (IC50 = 0.9 ± 0.2 µM) was to be around 6-fold more potent than lead compound 5. Moreover, against HL-60 cells, 7-(4-trifluoromethylphenyl)-7'-(3'/4'-hydroxyphenyl) derivatives 78 and 79 (IC50 = 2.2 ± 0.4 µM and 2.4 ± 0.6 µM) were 3-fold more potent than lead compound 5. Furthermore, against Sf9 cells from the common cutworm, the 7-(4-trifluoromethylphenyl) derivatives bearing electron-withdrawing groups 76-96 showed a wider range of activity (around 20-fold difference), giving valuable information on the structure-activity relationship. The 7-(4-trifluoromethylphenyl)-7'-(2'/3'-hydroxyphenyl) derivatives 77 and 78 (IC50 = 4.7 ± 0.6 µM and 4.9 ± 0.9 µM) had around 2-fold higher activity against Sf9 cells than lead compound 5. The 7-(4-trifluoromethylphenyl)-7'-(3'-hydroxyphenyl) derivative 78 was also effective against mosquito NIAS-AcAl-2 cells with an IC50 value of 5.4 ± 0.3.

Lobatamunsolides A-C, Norlignans from the Roots of Pueraria lobata and their Nitric Oxide Inhibitory Activities in Macrophages.

Phytochemical investigation of the methanol (MeOH) extract of Pueraria lobata roots, known as "kudzu", combined with liquid chromatography/mass spectrometry (LC/MS)-based analysis, resulted in the identification of four norlignans (1-4), including three new norlignans, lobatamunsolides A-C (1-3), and five known isoflavonoids (5-9). The structures of the new compounds were elucidated by a combination of spectroscopic methods, including 1D and 2D nuclear magnetic resonance (NMR) and high resolution (HR)-electrospray ionization mass spectrometry (ESIMS), and their absolute configurations were determined by chemical reaction and quantum chemical electronic circular dichroism (ECD) calculations. The isolated compounds (1-9) were evaluated for their inhibitory effects on nitric oxide (NO) production in lipopolysaccharide (LPS)-stimulated RAW 264.7 macrophages. Compound 9 displayed the strongest NO inhibitory effect and compound 2 showed a weak effect. The potential mechanism of the effect of compound 9 was investigated by analysis of its molecular docking on the active site of inducible nitric oxide synthase (iNOS), which showed the potential interactions of compound 9 with key amino acid residues and the heme cofactor of iNOS. The mechanism as the inhibition of transcriptional iNOS protein expression was confirmed by western blotting experiments.