General Strategy for Stereoselective Synthesis of 1-Substituted Exo,Endo-2,6-Diaryl-3,7-dioxabicyclo[3.3.0]octanes: Total Synthesis of (±)-Gmelinol

Phytochemical investigation, molecular docking studies and DFT calculations on the antidiabetic and cytotoxic activities of Gmelina philippensis CHAM

ETHNOPHARMACOLOGICAL RELEVANCE

Gmelina philippensis CHAM is an ornamental plant that is distributed in South Asia and warm regions of the Mediterranean area. The plant is traditionally applied in folk medicine for the treatment of diabetes.

AIM OF THE STUDY

To evaluate the cytotoxic and the antidiabetic activities of the ethanolic extract of G. philippensis aerial parts. To isolate the metabolite(s) responsible for these activities and to elucidate the mechanism of action by molecular docking study.

MATERIALS AND METHODS

Compounds (1-11) were isolated using various chromatographic techniques and their structures were determined by NMR spectroscopic and mass spectrometric analysis. The cytotoxic effect was tested using viability test and MTT assay. Antidiabetic activity was evaluated by measuring the inhibitory activity of the ethanolic extracts and compounds against α-glucosidase and α-amylase activities. Modeling and docking simulations were performed using Molecular Operating Environment software and the crystal structure of protein kinases CDK2, (1PYE) and AKT1 (4GV1), in addition to α-glucosidase (3TOP) and α-amylase (2QV4).

RESULTS

Compounds 2, 3 and 8 were isolated for the first time from the plant and identified as: gmelinol (2), apigenin (3) and tyrosol (8). While β-sitosterol-3-O- β-D-glucopyranoside (4) vicenin-II (7), rhoifolin (9), isorhoifolin (11) were isolated for the first time from the genus, along with and the new iridoid 6-O-α-L-(2″-O-benzoyl-4″-O-trans-p-methoxycinnamoyl)rhamnopyranosyl-1α- β-D-glucopyranoside catalpolgenin (6). In addition, to the previously reported compounds: mixture of β -sitosterol and stigmasterol (1), and 6- O- α-L-(2″,3″,4″-tri-O -benzoyl)rhamnopyranosylcatalpol (5) and 6-O-α-L-(2″-O-trans-p-methoxycinnamoyl)rhamnopyranosylcatalpol (10). The cytotoxic activity against hepatocellular carcinoma (HepG-2) cell lines for compounds 2, 5, 7, 9 and 11 was conducted using cisplatin as a standard. Gmelinol (2) exhibited strong cytotoxic activity against HepG-2 cell lines with IC ₅₀ value of 3.6 ± 0.1 μg/ml which is more potent than the standard cisplatin IC ₅₀ = 8.7 ± 0.9 μg/ml. Molecular modeling of 2 against diverse targets of protein kinases suggested that CDK-2 and AKT-1 could be the dual probable kinase targets for its cytotoxic action. Compound 2 showed α-amylase inhibition activity with IC ₅₀ value of 60.9 (μg/ml) while, compounds 5 showed strong α-glucosidase inhibition activity with IC ₅₀ values of 41.7 (μg/ml) compared to acarbose with IC ₅₀ value of 34.7, 30.6 (μg/ml). Molecular docking of compounds 2 and 5 on α-glucosidase (3TOP) and α-amylase (2QV4) enzymes revealed high binding affinity and active site interactions comparable to native ligand acarbose.

CONCLUSION

The ethanolic extract of G. philippensis CHAM aerial parts is effective against HepG-2 cell lines, α-amylase and α-glucocidase activities. Biologically guided isolation indicated that compounds 2 and 5 are responsible for these activities. These results were supported by DMF calculations that detected the molecular areas responsible for protein interactions shown via docking studies.

Thioether and Ether Furofuran Lignans: Semisynthesis, Reaction Mechanism, and Inhibitory Effect against α-Glucosidase and Free Radicals

The transformation of sesame lignans is interesting because the derived products possess enhanced bioactivity and a wide range of potential applications. In this study, the semisynthesis of 28 furofuran lignans using samin (5) as the starting material is described. Our methodology involved the protonation of samin (5) to generate an oxocarbenium ion followed by the attack from two different nucleophiles, namely, thiols (RSH) and alcohols (ROH). The highly diastereoselective thioether and ether furofuran lignans were obtained, and their configurations were confirmed by 2D NMR and X-ray crystallography. The mechanism underlying the reaction was studied by monitoring ¹H NMR and computational calculations, that is, the diastereomeric α- and β-products were equally formed through the S_N1-like mechanism, while the β-product was gradually transformed via an S_N2-like mechanism to the α-congener in the late step. Upon evaluation of the inhibitory effect of the synthesized lignans against α-glucosidases and free radicals, the lignans 7f and 7o of the phenolic hydroxyl group were the most potent inhibitors. Additionally, the mechanisms underlying the α-glucosidase inhibition of 7f and 7o were verified to be of a mixed manner and noncompetitive inhibition, respectively. The results indicated that both 7f and 7o possessed promising antidiabetic activity, while simultaneously inhibiting α-glucosidases and free radicals.

Oxidative Transformations of Lignans

Numerous oxidative transformations of lignan structures have been reported in the literature. In this paper we present an overview on the current findings in the field. The focus is put on transformations targeting a specific structure, a specific reaction, or an interconversion of the lignan skeleton. Oxidative transformations related to biosynthesis, antioxidant measurements, and total syntheses are mostly excluded. Non-metal mediated as well as metal mediated oxidations are reported, and mechanisms based on hydrogen abstractions, epoxidations, hydroxylations, and radical reactions are discussed for the transformation and interconversion of lignan structures. Enzymatic oxidations, photooxidation, and electrochemical oxidations are also briefly reported.

Direct Lactone Formation by Using Hypervalent Iodine(III) Reagents with KBr via Selective C−H Abstraction Protocol

We have developed a new and reliable method for the direct construction of biologically important aryl lactones and phthalides from carboxylic and benzoic acids, using a combination of hypervalent iodine(III) reagents with KBr.

Electrospray ionization ion-trap time-of-flight tandem mass spectrometry of two furofurans: sesamin and gmelinol

High-resolution electrospray ionization multistage tandem mass spectrometry (MS(1-7)) in positive ion mode was used to determine the accurate masses and the fragmentation pathways of two furofurans, sesamin and gmelinol. The protonated molecules [M+H]+ were absent in the conventional mass spectra of both compounds, but two characteristic ions, [M+H-H(2)O]+ and [M+H-H(2)]+, were always observed. Successive losses of CH(2)O and CO are the common characteristic fragmentations. Based on the exact masses acquired from 21 different tandem mass spectra, two or three fragmentation pathways for each compound are proposed. The consecutive losses of two H(2)O molecules and one H(2) molecule readily take place from the furan rings for both sesamin and gmelinol, resulting in the absence of the protonated molecules in the single-stage experiments. HCHO loss is observed at least three times in the tandem mass spectra, mainly from methylenedioxy groups (OCH(2)O) for sesamin but only from tetrahydrofuran rings for gmelinol. Moreover, CO loss is found at least three times in the tandem mass spectra of both sesamin and gmelinol from the 3,4-methylenedioxyphenyl (ArOCH(2)O) moieties for sesamin and from both the dimethoxyphenyl and the tetrahydrofuran ring moieties for gmelinol. In addition, the disubstituted benzyl cation ArCH(2)+ at m/z 135 for sesamin and at m/z 151 for gmelinol was found in the MS(3) spectra of both sesamin and gmelinol, which is very helpful in the identification of the compositions of 3,4-disubstituted groups on the benzene rings of the furofurans.