Total Synthesis of (-)-Bicubebin A, B, (+)-Bicubebin C and Structural Reassignment of (-)-cis-Cubebin

Intermedianin, a new furofuran lignan from the leaves of Knema intermedia Warb

Phytochemical investigation of the leaves of Knema intermedia has led to the isolation of a new furofuran lignan, intermedianin 1 together with five known lignans, α-cubebin 2, β-cubebin 3, bicubebin A 4, bicubebin B 5, and bicubebin C 6. The characterisation and structural elucidation of the isolated compounds were established by extensive spectroscopic data analysis and comparison with literature data. The antifungal activity was tested using the broth microdilution assay, whereas the microbial biofilms were determined using a semi-quantitative static biofilm. Compound 1 exhibited activity against C. albicans, C. lusitanae, and C. auris, (each with MIC/MFC value 250 µg/mL) and increased the biofilm of C. auris (64.07 ± 3.83%) and Candida lusitanae (62.90 ± 3.41%) when treated with 500 µg/mL.

Organocatalytic aldol approach for the protecting group-free asymmetric synthesis of (7R')-parabenzlactone, (-)-hinokinin, (-)-yatein, (-)-bursehernin, (-)-pluviatolide, (+)-isostegane and allied lignans

A short and efficient catalytic asymmetric protection-free synthesis of dibenzylbutyrolactone lignans, such as (-)-hinokinin, (-)-yatein, (-)-bursehernin, (-)-pluviatolide, and their 7'-hydroxylignans - (7'R)-parabenzlactone, (7'R)-hydroxyyatein, (7'R)-hydroxybursehernin, and (7'R)-hydroxy pluviatolide, respectively, is described. The syntheses of (+)-isostegane and the formal synthesis of (-)-podophyllotoxin and bicubebins are also described. Organocatalytic aldol-reduction-lactonization and Pd/C-catalyzed hydrogenative debromination are two-pot sequential reactions for the enantioselective synthesis of hydroxybutyrolactone 13b with excellent diastereo- and enantioselectivity (dr 33 : 1 and >99% ee). The protecting group-free chemoselective α-alkylation of 13b directly led to 7'-hydroxydibenzylbutyrolactone lignans, followed by hydrogenative dehydroxylation, which led to their (deoxy) dibenzylbutyrolactone lignans, and the syntheses were completed in three to five steps from 6-bromopiperonal.

Anti-hyperglycemic potential and chemical constituents of Aristolochia triangularis Cham. leaves - A medicinal species native to Brazilian forests

ETHNOPHARMACOLOGICAL RELEVANCE

Aristolochia triangularis Cham. has been used in Brazilian traditional medicine for various therapeutic purposes, including as a leaf-based infusion for diabetes management.

AIM OF THE STUDY

This study was designed to chemically characterize an infusion of in natura A. triangularis leaves and evaluate the in vivo anti-hyperglycemic properties of this infusion.

MATERIALS AND METHODS

Chemical composition was examined using liquid-liquid extraction procedure, chromatographic methods, NMR, and LC-MS/MS. The in vivo anti-hyperglycemic activity of the freeze-dried infusion of A. triangularis leaves (Inf-L-At) was assessed using oral glucose tolerance test (OGTT). Initially, normoglycemic male rats were pre-treated with orally administered Inf-L-At at doses of 62.5, 125, and 250 mg/kg for two consecutive days. On the day of the OGTT, fasting animals received a glucose load (4 g/kg) 30 min after treatment with Inf-L-At, and the blood glucose levels were verified at 15, 30, 60, and 180 min. Intestinal maltase, lactase, and sucrase activities and muscle and liver glycogen contents were also assessed after the OGTT.

RESULTS

Inf-L-At extract led to glycemic reduction with no dose-response at 15, 30, and 60 min comparable to that of the antidiabetic drug glibenclamide and was accompanied by an increase in hepatic and muscle glycogen contents. Additionally, there was a significant statistically decrease in the in vitro activity of disaccharidases. Maltase and sucrase activities were inhibited at all doses, whereas lactase activity was inhibited only at 62.5 and 250 mg/kg. In total, 75 compounds were found in the infusion, including seven new ones, (7S*,8S*,7ꞌS*,8ꞌR*)-4,4ꞌ-dihydroxy-3,3ꞌ-dimethoxy-7,9ꞌ-epoxylignan-7ꞌ-ol; 4ꞌ-hydroxy-3ꞌ-methoxy-3,4-methylenedioxy-7,9ꞌ-epoxylignan-9,7ꞌ-diol; triangularisines A, B, and C; N-ethyl-N-methyl-affineine; and N-methyl pachyconfine, and one previously not described as a natural product, epi-secoisolariciresinol monomethyl ether.

CONCLUSION

The results demonstrated the anti-hyperglycemic activity of the infusion from A. triangularis leaves and showed that it is a rich source of lignoids, alkaloids, and glycosylated flavonoids, which are known to exhibit antidiabetic effects and other biological properties that can be beneficial for patients with chronic hyperglycemia, thus certifying the popular use of this herbal drink.

Total Synthesis of Polysubstituted γ-Butyrolactone Lignans (-)-Hinokinin, (-)-Bicubebin B, and (-)-Isodeoxypodophyllotoxin via Oxime Carbonate Formation

The diverse structures and profound biological activities of lignan natural products have enticed significant effort in the exploration of new methodologies for their total synthesis. We have prepared γ-butyrolactone oximes from readily available δ-nitro alcohols via Boc2O mediated cyclization. The mild conditions are compatible with a wide range of functional groups, and this methodology has been applied to the total synthesis of five lignan natural products.

The enantioselective total syntheses of (+)-7-oxohinokinin, (+)-7-oxoarcitin, (+)-conicaol B and (-)-isopolygamain

A flexible approach to C7 keto dibenzyl butyrolactone lignans was developed and the synthesis of several natural products and their related derivatives is described herein. The developed pathway proceeds through enantioenriched β-substituted butyrolactones, from which facile aldol addition and subsequent oxidation affords the desired benzylic ketone moiety. This methodology was used to complete the first enantioselective total syntheses of three natural products, (+)-7-oxohinokinin, (+)-7-oxoarcitin and (+)-conicaol B, and a further five analogues. The utility of this method was further demonstrated through a 1-2 step modification to access another class of natural product, aryltetralin lignans, allowing the asymmetric total synthesis of (-)-isopolygamain and a polygamain derivative. Anti-proliferative testing determined (-)-isopolygamain was the most active of the compounds prepared, with IC₅₀ values of 2.95 ± 0.61 μM and 4.65 ± 0.68 μM against MDA-MB-231 (triple negative breast cancer) and HCT-116 (colon cancer) cell lines, respectively.

NMR Characterization of Lignans

Lignans are particularly interesting secondary metabolites belonging to the phenyl-propanoid biosynthetic pathway. From the structural point of view, these molecules could belong to the aryltetralin, arylnaphtalene, or dibenzylbutyrolactone molecular skeleton. Lignans are present in different tissues of plants but are mainly accumulated in seeds. Extracts from plant tissues could be characterized by using the NMR-based approach, which provides a profile of aromatic molecules and detailed structural information for their elucidation. In order to improve the production of these secondary metabolites, elicitors could effectively stimulate lignan production. Several plant species are considered in this review with a particular focus on Linum species, well recognized as the main producer of lignans.

Stereoselective Total Synthesis of (+)-Aristolactam GI

Aristolactams are an important subgroup of aporphinoids, which all share a common phenanthrene chromophore motif that is thought to be responsible for the range of interesting physicochemical and biological properties exhibited by these compounds. Among all of the aristolactams discovered, (+)-aristolactam GI displays a unique structural feature of having the aristolactam scaffold linked via a benzodioxane ring to a phenyl propanoid unit, resulting in the compound being an aporphinoid-lignan hybrid. The synthesis of (+)-aristolactam GI was achieved first by synthesis of an orthogonally protected aristolactam, which was prepared using a Suzuki/aldol cascade to convert a differentially protected isoindolin-1-one to the required phenanthrene. The required enantiopure phenyl propanoid unit was prepared from readily available ( R)-methyl lactate. A selective Mitsunobu reaction was used to combine these two key fragments, prior to the formation of the linking benzodioxane in the final step. The absolute stereochemistry of the natural product was confirmed to be 7' S, 8' S.

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.

Reaction Mechanism of a Nonheme Iron Enzyme Catalyzed Oxidative Cyclization via C-C Bond Formation

A complementary study including design of mechanistic probes, biochemical assays, model analysis, and liquid chromatography coupled mass spectrometry was conducted to establish the reaction mechanism for a nonheme iron enzyme catalyzed (-)-podophyllotoxin formation. Our results indicate that the originally proposed hydroxylated intermediate is unlikely to be involved in this reaction. Instead, the formation of benzylic radical/carbocation intermediate can be utilized to trigger the C-C bond formation to construct the C-ring of (-)-podophyllotoxin.

Modular Synthesis and Biological Investigation of 5-Hydroxymethyl Dibenzyl Butyrolactones and Related Lignans

Dibenzyl butyrolactone lignans are well known for their excellent biological properties, particularly for their notable anti-proliferative activities. Herein we report a novel, efficient, convergent synthesis of dibenzyl butyrolactone lignans utilizing the acyl-Claisen rearrangement to stereoselectively prepare a key intermediate. The reported synthetic route enables the modification of these lignans to give rise to 5-hydroxymethyl derivatives of these lignans. The biological activities of these analogues were assessed, with derivatives showing an excellent cytotoxic profile which resulted in programmed cell death of Jurkat T-leukemia cells with less than 2% of the incubated cells entering a necrotic cell death pathway.

Lignans: A Chemometric Analysis

The physicochemical properties of classical lignans, neolignans, flavonolignans and carbohydrate-lignan conjugates (CLCs) were analysed to assess their ADMET profiles and establish if these compounds are lead-like/drug-like and thus have potential to be or act as leads in the development of future therapeutics. It was found that while no studied compounds were lead-like, a very large proportion (>75%) fulfilled all the requirements to be deemed as present in drug-like space and almost all compounds studied were in the known drug space. Principal component analysis was an effective technique that enabled the investigation of the relationship between the studied molecular descriptors and was able to separate the lignans from their sugar derivatives and flavonolignans, primarily according to the parameters that are considered when defining chemical space (i.e., number of hydrogen bond donors, acceptors, rotatable bonds, polar surface area and molecular weight). These results indicate that while CLCs and flavonolignans are less drug-like, lignans show a particularly high level of drug-likeness, an observation that coupled with their potent biological activities, demands future pursuit into their potential for use as therapeutics.

Medicinal Attributes of Lignans Extracted from Piper Cubeba: Current Developments

Lignans are a large class of natural products that have been isolated from many plants. They reveal diverse biological activities, especially antiviral and antitumor properties. From Piper cubeba, lignans of several classes can be isolated from the roots, rhizomes, stems, leaves, seeds, and fruits. Among its various chemical constituents, (-)-cubebin and (-)-hinokinin are found in significant quantities. Although they have been known for some time, during the last few decades their biological properties have been studied by several research groups. The cubebins have been identified as a lactol monomer and dimers as a mixture of diastereoisomers. Recently, their structural characterization and the synthesis of the possible structures have led to the correction of some earlier structural proposals. This review describes the more recent developments in the study of the medicinal attributes of cubebin and hinokinin extracted from Piper cubeba and the synthesis and biological testing of some analogues.