Antispasmodic effects and structure-activity relationships of labdane diterpenoids from Marrubium globosum ssp. libanoticum

Application of Pauson-Khand reaction in the total synthesis of terpenes

The Pauson-Khand reaction (PKR) is a formal [2 + 2 + 1] cycloaddition involving an alkyne, an alkene and carbon monoxide mediated by a hexacarbonyldicobaltalkyne complex to yield cyclopentenones in a single step. This versatile reaction has become a method of choice for the synthesis of cyclopentenone and its derivatives since its discovery in the early seventies. The aim of this review is to point out the applications of PKR in the total synthesis of terpenes.

[Studies on the Syntheses of Bioactive Natural Products Having a Fused Ring System Based on Novel Skeletal Construction Methods]

Here, I describe a part of our efforts to develop synthetic strategies to construct bioactive natural products having a fused ring system. We have designed four chiral building blocks bearing contiguous quaternary stereocenters for the syntheses of bioactive C17-oxygenated steroids/triterpenoids and C9-oxygenated labdane diterpenoids. The compounds were stereoselectively synthesized from α-substituted glycolic acid (R)-3-methylcyclohex-2-enyl esters through Ireland-Claisen rearrangement to construct the stereocenters simultaneously. Synthetic utility of a β-type building block is highlighted by total syntheses of marrubiin (11 steps, 22%) and related seven labdane diterpene lactones, cyllenine C (12 steps, 29%), marrulactone (13 steps, 11%), marrulanic acid (14 steps, 10%), marrubasch F (12 steps, 14%), marrulibacetal (14 steps, 4%), marrulibacetal A (14 steps, 2%), and desertine (15 steps, 0.5%). These syntheses feature the construction of the [6.6.5]-tricyclic ring portion via a Pauson-Khand reaction, cleavage of the resultant cyclopentenone ring and an elongation of the C9 side chain by an epoxide opening reaction. The relative stereochemistry of desertine was determined to be 13R, 14S, 15S, 16R by some chemical conversions and NMR analysis. Further efforts toward total syntheses of oxygenated terpenoids using three other chiral building blocks and structure-activity relationship study of synthesized labdane diterpene lactones are currently underway in our laboratory and will be reported in due course.

Stereoselective Synthesis of trans-Decalin-Based Spirocarbocycles via Photocyclization of 1,2-Diketones

Diastereoselective synthesis of the trans-decalin-based α-hydroxyl butanone spirocarbocycles bearing all-carbon quaternary stereogenic centers has been achieved via Norrish-Yang photocyclization of trans-decalin-substituted-2,3-butanediones using daylight. Density functional theory (DFT) calculations suggest that this diastereoselective reaction is affected by both substrate conformation and intramolecular hydrogen bonds. The developed chemistry could be applied to synthesizing the derivatives of the trans-decalin-based biologically important natural products.

In vitro and in silico studies of 8(17),12E,14-labdatrien-18-oic acid in airways smooth muscle relaxation: new molecular insights about its mechanism of action

In the field of experimental pharmacology, researchers continuously investigate new relaxant agents of the airway smooth muscle cells (ASMCs), since the pathophysiology of respiratory illnesses, such as asthma, involves hyperresponsiveness and changes in ASMC homeostasis. In this scenario, labdane-type diterpenes, like forskolin (FSK), are a class of compounds known for their relaxing action on smooth muscle cells (SMCs), being this phenomenon related to the direct activation of AC-cAMP-PKA pathway. Considering the continuous effort of our group to study the mechanism of action and prospecting for compounds isolated from natural sources, in this paper, we presented how the diterpene 8(17),12E,14-labdatrien-18-oic acid (LBD) promotes relaxant effect on ASMC, performing in vitro experiments using isolated guinea pig trachea and in silico molecular docking/dynamics simulations. In vitro experiments showed that in the presence of aminophylline, FSK and LBD had their relaxant effect potentiated (EC₅₀ from 1.4 ± 0.2 × 10^-5 M to 1.5 ± 0.3 × 10^-6 M for LBD and from 2.0 ± 0.2 × 10^-7 M to 6.4 ± 0.4 × 10^-8 M for FSK) while in the presence of Rp-cAMPS this effect was attenuated (EC₅₀ from 1.4 ± 0.2 × 10^-5 M to 3 × 10^-4 M for LBD and from 2.0 ± 0.2 × 10^-7 to 3.1 ± 1.0 × 10^-6 M for FSK). Additionally, in silico simulations evidenced that the lipophilic character of LBD is probably responsible for its stability on AC binding site. LBD presented two preferential orientations, where the double bonds of the isoprene moiety as well as the unique polar group (carboxylic acid) in this compound form important anchoring points. In this sense, we consider that the LBD can interact stabilizing the catalytic dimmer of AC as the FSK, although less efficiently.

Total Syntheses of Marrubiin and Related Labdane Diterpene Lactones

Total syntheses of the labdane diterpene lactones marrubiin, marrulibacetal, desertine, marrulibacetal A, marrubasch F, cyllenine C, marrulanic acid, and marrulactone are described. The trans-decalin moiety of these molecules was constructed in a stereoselective manner by a Pauson-Khand reaction, and the resultant cyclopentenone was oxidatively cleaved for formation of the lactone ring. Elongation of the side chain at C9 was achieved by an epoxide-opening reaction with a variety of nucleophiles, and the functional group manipulations completed the syntheses of these natural products. Stereochemistries of desertine could be established by the transformations.

Natural Antispasmodics: Source, Stereochemical Configuration, and Biological Activity

Natural products with antispasmodic activity have been used in traditional medicine to alleviate different illnesses since the remote past. We searched the literature and compiled the antispasmodic activity of 248 natural compounds isolated from terrestrial plants. In this review, we summarized all the natural products reported with antispasmodic activity until the end of 2017. We also provided chemical information about their extraction as well as the model used to test their activities. Results showed that members of the Lamiaceae and Asteraceae families had the highest number of isolated compounds with antispasmodic activity. Moreover, monoterpenoids, flavonoids, triterpenes, and alkaloids were the chemical groups with the highest number of antispasmodic compounds. Lastly, a structural comparison of natural versus synthetic compounds was discussed.

Total Syntheses of (+)-Marrubiin and (-)-Marrulibacetal

A stereoselective total synthesis of (+)-marrubiin has been accomplished starting from a chiral building block via the CyNH2-promoted Pauson-Khand reaction (PKR) followed by oxidative cleavage of the resultant cyclopentenone ring. Two successive oxidations and internal transacetalization culminated in the total synthesis of the antispasmodic labdane diterpenoid (-)-marrulibacetal.

The vasorelaxant effect of 8(17),12E,14-labdatrien-18-oic acid involves stimulation of adenylyl cyclase and cAMP/PKA pathway: Evidences by pharmacological and molecular docking studies

The relaxant effect of 8(17),12E,14-labdatrien-18-oic acid (LBD) was investigated on isolated aortic rings and compared with forskolin (FSK), a standard and potent activator of adenylyl cyclase (AC) with relaxing effect. The presence of potassium channel blockers, such as glibenclamide (ATP-blocker), apamin (SKCa-blocker), charybdotoxin (BKCa-blocker) did not significantly affect either the LBD or FSK concentration-response curves. However, in the presence of 4-aminopyridine (KV-blocker), the relaxant effect for both diterpenes was significantly attenuated, with reduction of its relative potencies. Moreover, the relaxation induced by 8-Br-cAMP, an analog of cAMP, was also significantly attenuated in the same conditions, i.e., in the presence of 4-aminopyridine. The presence of aminophylline, a nonselective phosphodiesterase inhibitor, caused a significant increasing in the potency for both LBD and FSK. On the other hand, the presence of Rp-cAMPS, a selective PKA-inhibitor, significantly attenuated the relaxant effect of LBD. In this work, in the same experimental conditions, both labdane-type diterpenes presented remarkably similar results; FSK, however, presented a higher potency (100-fold) than LBD. Thus, the hypothesis that LBD could be a novel AC-activator emerged. To assess that hypothesis, computational molecular docking studies were performed. Crystallographic structure of adenylyl cyclase/forskolin complex (1AB8) was obtained from RSCB Protein Data Bank and used to compare the modes of interaction of the native ligand and LBD. The computational data shows many similarities between LBD and FSK concerning the interaction with the regulatory site of AC. Taken together, the results presented here pointed to LBD as a novel AC-activator.

Evaluating Antiproliferative and Antioxidant Activity of Marrubium crassidens

PURPOSE

Naturally occurring substances as novel drugs in cancer therapy, at all times, represent a challenge to science since medicinal plants are proving to be brilliant sources of new chemopreventive agents.

METHODS

In the present study, methanol extract from aerial parts of Marrubium crassidens was assessed for its antiproliferative activity in the breast cancer cell line MCF-7 through MTT bioassay using cell viability and cytotoxicity indices. The antioxidant property of M. crassidens extract together with its phenolic and flavonoids content were evaluated, as well.

RESULTS

According to data obtained in the study, M. crassidens exhibited antiproliferative activity with a gradual rise in cytotoxicty effect setting out on 240µg/mL concentration of the extract. Moreover, the RC50 value for antioxidant activity of the extract was determined as 40µg/mL and values for the total phenolic and flavonoids were calculated as 512.64mg gallic acid equivalent and 212.73mg quercetin equivalent per 100g of dry plant material.

CONCLUSION

Generally, the observed antiproliferative and antioxidant properties of M. crassidens could be certified to the high amounts of phenolic and flavonoid content detected in the extract.

Characterization of Non-Terpenoids in Marrubium crassidens Boiss. Essential Oil

PURPOSE

Marrubium crassidens, a plant belonging to the family Lamiaceae, was studied for its volatile components present in the aerial parts of the plant during the flowering stage.

METHODS

The essential oil of the plant obtained through hydrodistillation of the dried plant material was assessed for its chemical composition by GC/MS and GC-FID analyses.

RESULTS

Twenty-five compounds were identified, which constituted 94.3% of the total oil composition. The major components were identified as, m-tolualdehyde (23.3%), acetophenone (15.8%), nonacosane (13.1%), docosane (7.2%), o-tolualdehyde (4.1%), β-caryophyllene (3.8%) and caryophyllene oxide (3.4%). Non-terpenoids with 75.7% were the most abundant components of the essential oil.

CONCLUSION

Overall, M. crassidens essential oil revealed to include rather higher proportions of non-terpenoid compounds compared with other species of genus Marrubium.

Antiproliferative activity of Marrubium persicum extract in the MCF-7 human breast cancer cell line

AIM

Developing antitumor drugs from natural products is receiving increasing interest worldwide due to limitations and side effects of therapy strategies for the second leading cause of disease related mortality, cancer.

METHODS

The antiproliferative activity of a methanolic extract from the aerial parts of Marrubium persicum extract was assessed with the MCF-7 breast cancer cell line using the MTT test for cell viability and cytotoxicity indices. In addition, antioxidant properties of the extract were evaluated by measuring its ability to scavenge free DPPH radicals. Moreover, the total phenolic and flavonoid content of the extract was determined based on Folin-Ciocalteu and colorimetric aluminum chloride methods.

RESULTS

The findings of the study for the antiproliferative activity of the methanolic extract of M. persicum showed that growth of MCF-7 cells was inhibited by the extract in a dose and time dependent manner, where a gradual increase of cytotoxicity effect has been achieved setting out on 200 μg/mL concentration of the plant extract. The antioxidant assay revealed that the extract was a strong scavenger of DPPH radicals with an RC50 value of 52 μg/mL. The total phenolic and flavonoids content of the plant extract was 409.3 mg gallic acid equivalent and 168.9 mg quercetin equivalent per 100g of dry plant material.

CONCLUSION

Overall, M. persicum possesses potential antiproliferative and antioxidant activities on the malignant MCF-7 cell line that could be attributed to the high content of phenolics and flavonoids, and therefore warrants further exploration.

Convergent approach to complex spirocyclic pyrans: practical synthesis of the oxa-pinnaic acid core

The enantioselective synthesis of the oxa-pinnaic acid framework has been achieved through internal asymmetric induction. The synthetic strategy pursued illustrates the adaptability of the Achmatowicz oxidative rearrangement for the synthesis of complex spirocyclic pyrans starting from tertiary alcohols.

Cytotoxic Properties of Marrubium globosum ssp. libanoticum and its Bioactive Components

Marrubium globosum Montbr. et Auch. ex Benth. ssp. libanoticum Boiss. (Lamiaceae) is a medicinal plant used in Lebanon for the treatment of inflammatory diseases, asthma, coughs and other pulmonary and urinary problems. The goal of our study was to assess the biological activity of M. globosum by testing different extracts of the aerial parts for their antiproliferative activity against human melanoma cells using the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay. The chloroform fraction showed the greatest activity. The compounds isolated from the extracts were also tested: the mixture of (13 S)-9α,13α-epoxylabda-6β(19),16(15)-diol dilactone and (13 R)-9α,13α-epoxylabda-6β(19),16(15)-diol dilactone was the most active fraction, with an IC50 value of 29.2 ± 0.06 μg/mL.

Anti-inflammatory mechanism of 15,16-epoxy-3α-hydroxylabda-8,13(16),14-trien-7-one via inhibition of LPS-induced multicellular signaling pathways

Phytochemical investigation of Leonurus japonicus has led to the isolation of a labdane diterpene derivative, 15,16-epoxy-3α-hydroxylabda-8,13(16),14-trien-7-one (1), which was tested for its in vitro anti-inflammatory effects. The results demonstrated that 1 exhibits an inhibitory effect on LPS-stimulated RAW 264.7 macrophages. The anti-inflammatory action shown by 1 suppressed LPS-induced NF-κB activation, resulting in the down-regulation of iNOS and COX-2 protein expression, attributable to the inhibitory action of LPS-induced NO and PGE(2) production. Compound 1 inhibited LPS-induced phosphorylation and the degradation of inhibitory kappa B (IκBα) and decreased the nuclear translocation of p50 and p65. In addition, 1 exhibited an inhibitory effect on LPS-induced NF-κB-DNA and AP-1-DNA binding activity, using an electrophoretic mobility shift assay with NF-κB- and AP-1-specific (32)P-labeled probes. The LPS-induced mitogen-activated protein kinases (p-JNK, p-p38, and p-ERK) and p-Akt were inhibited after 30 and 10 min of LPS stimulation, respectively. In addition, TNF-α production was suppressed by 1.

Antioxidant and antigenotoxic properties of compounds isolated from Marrubium deserti de Noé

In our continual course toward the valorization of traditionally used endemic flora through the analysis of its chemobiodiversity, the phytochemical analysis of aerial parts of Marrubium deserti de Noé was undertaken. Dichloromethane and methanol extracts led to the isolation of terpenoid derivatives among which two were new labdane diterpenes named marrulibacetal A and desertine, respectively. Six of them were known compounds (a mixture of the isomers cyllenin A and 15-epi-cyllenin A, marrubiin, marrulactone, marrulibacetal and β-stigmasterol) and seven known phenolic compounds were also isolated: apigenin and several 7-O-substituted derivatives (apigenin-7-O-β-neohesperidoside, apigenin-7-O-glucoside, terniflorin and apigenin-7-O-glucuronide) together with two phenylethanoid glucosides (acteoside and forsythoside B). The structures and relative configurations of the new compounds were elucidated by MS and a series of 1D and 2D NMR analyses. Some pure compounds have been evaluated for their antioxidant activities through different methods: DPPH and ABTS assays as well as CUPRAC assay. Genotoxic and antigenotoxic activities of extracts and pure compounds were also evaluated in vitro on Escherichia coli PQ37 cells by the SOS Chromotest. Some of the isolated compounds like phenylethanoid derivatives showed stronger antioxidant capacity than trolox and were also able to significantly inhibit β-galactosidase induction caused by the mutagen agent nitrofurantoin.