Remote Hydroxylation of Methyl Groups by Regioselective Cyclopalladation. Partial Synthesis of Hyptatic Acid-A

Chemical Synthesis of Saponins

Saponins are a large family of amphiphilic glycosides of steroids and triterpenes found in plants and some marine organisms. By expressing a large diversity of structures on both sugar chains and aglycones, saponins exhibit a wide range of biological and pharmacological properties and serve as major active principles in folk medicines, especially in traditional Chinese medicines. Isolation of saponins from natural sources is usually a formidable task due to the microheterogeneity of saponins in Nature. Chemical synthesis can provide access to large amounts of natural saponins as well as congeners for understanding their structure-activity relationships and mechanisms of action. This article presents a comprehensive account on chemical synthesis of saponins. First highlighted are general considerations on saponin synthesis, including preparation of aglycones and carbohydrate building blocks, assembly strategies, and protecting-group strategies. Next described is the state of the art in the synthesis of each type of saponins, with an emphasis on those representative saponins having sophisticated structures and potent biological activities.

Advances in the Semi-Synthesis of Triterpenoids

Recent achievements in triterpenoid semi-synthesis are discussed in this short review, which is divided into three parts according to the type of synthetic strategy being employed. These strategies include functionalization, modification of the carbon skeleton, and glycosylation. In the section on functionalization strategies, both functional group interconversions and new functional group installations on triterpenoid starting materials are described. The section on modification of the carbon skeleton is divided into three parts according to the tactic being applied, and incorporates rearrangement of the carbon skeleton, ring scission, and introduction of an additional heterocyclic ring. Meanwhile, in the section on glycosylation, notable achievements in the semi-synthesis of both natural and artificial triterpene saponins are discussed. Overall, the pivotal transformations that have brought about striking chemical structure variations of triterpenoid starting materials are highlighted herein, and it is hoped that this short review will provide inspiration to both established and new investigators engaged in this field of research.

1 Introduction

2 Semi-Synthesis of Triterpenoids via Functionalization Strategies

2.1 Functionalization of Rings with Functional Groups

2.2 Functionalization of a Side Chain

2.3 Functionalization of Rings without Existing Functional Groups

2.4 Functionalization of Angular Methyl Groups

2.5 Functionalization of Angular Methyl Groups and Functional-Group-Free Rings

2.6 Multisite Modifications

3 Semi-Synthesis of Triterpenoids via C-Skeleton Modification Strategies

3.1 Rearrangement Tactics

3.2 Ring-Opening Tactics

3.3 Additional Ring Introduction Tactics

4 emi-Synthesis of Triterpenoids via a Glycosylation Strategy

5 Conclusions and Outlook

Type and position of linkage govern the cytotoxicity of oleanolic acid rhodamine B hybrids

Oleanolic acid/rhodamine B hybrids exhibit different cytotoxicity depending on the way these two structural elements are linked. While a hybrid holding a piperazinyl spacer at C-28 proved to be cytotoxic in the nano-molar concentration range, hybrids with a direct linkage of the Rho B residue to C-3 of the triterpenoid skeleton are cytotoxic only in the low micro-molar concentration range without any selectivity. This once again underlines the importance of selecting the right spacer and the most appropriate position on the skeleton of the triterpene to achieve the most cytotoxic hybrids possible.

Facile Synthesis of Saikosaponins

Saikosaponin A (SSa) and D (SSd) are typical oleanane-type saponins featuring a unique 13,28-epoxy-ether moiety at D ring of the aglycones, which exhibit a wide range of biological and pharmacological activities. Herein, we report the first synthesis of saikosaponin A/D (1-2) and their natural congeners, including prosaikosaponin F (3), G (4), saikosaponin Y (5), prosaikogenin (6), and clinoposaponin I (7). The present synthesis features ready preparation of the aglycones of high oxidation state from oleanolic acid, regioselective glycosylation to construct the β-(1→3)-linked disaccharide fragment, and efficient gold(I)-catalyzed glycosylation to install the glycans on to the aglycones.

NF-κB inhibitory and cytotoxic activities of hexacyclic triterpene acid constituents from Glechoma longituba

BACKGROUND

Non-Small-Cell Lung Cancer (NSCLC) is the most-frequent cause of cancer death, and novel chemotherapeutic drugs for treating NSCLC are urgently needed. 2α, 3α, 23-trihydroxy-13α, 27-cyclours-11-en-28-oic acid (euscaphic acid G) is a new hexacyclic triterpene acid isolated by our group from Glechoma longituba (Nakai) Kupr. However, the underlying mechanisms responsible for the anticancer effects of hexacyclic triterpene acid have not been elucidated.

PURPOSE

In the present work, we evaluated growth inhibitory effect of the new isolated hexacyclic triterpene acid and explored the underlying molecular mechanisms.

METHODS/STUDY DESIGNS

Herbs were extracted and constituents were purified by chromatographic separation, including silica gel, ODS, MCI, Sephadex LH-20 and preparative HPLC. The compound structures were elucidated by the use of UV, NMR and MS spectral data. The anticancer activity of euscaphic acid G was evaluated by MTT assay. Cell cycle, apoptosis, reactive oxygen species and mitochondrial membrane potential were determined by flow cytometry. To display the possible mechanism of euscaphic acid G on NCI-H460 cells, RT-PCR, immunofluorescence and Western blot analysis were carried out.

RESULTS

A new hexacyclic triterpene acid, euscaphic acid G, together with fifteen known triterpenoids, was isolated from the aerial parts of G. longituba. Our results showed that euscaphic acid G exerted strong anti-proliferative activity against NCI-H460 cells in a concentration- and time-dependent manner. Flow cytometry demonstrated euscaphic acid G arrested the cell cycle at G1 phase, induced cellular apoptosis, accompanied by ROS generation and mitochondrial membrane potential reduction. Mechanistic studies revealed that euscaphic acid G treatment inhibited IKKα/β phosphorylation and IκBα phosphorylation, which subsequently caused the blockage of NF-κB p65 phosphorylation and nuclear translocation.

CONCLUSION

In conclusion, these results suggested that euscaphic acid G from G. longituba showed potential anticancer effects against lung cancer cells via inducing cell cycle arrest and apoptosis, at least partly, through NF-κB signaling pathways.

In silico evaluation of apoptogenic potential and toxicological profile of triterpenoids

AIM:

Caspases-3 and 8 are key mediators of intrinsic and extrinsic pathway of apoptosis, respectively. Triterpenoids of natural and synthetic origin reported as anticancer agents with apoptotic potential and hence may prove to be good candidates for in silico testing against caspases-3 and 8.

MATERIALS AND METHODS:

Various naturally-occurring and synthetic triterpenoids were subjected to activity prediction using PASS Online software, and among them, 67 compounds were selected for further processing. Protein structure of caspase-3 (3DEI) and caspase-8 (3KJQ) was obtained from the protein data bank and docked with selected triterpenoids using AutoDock Tools and AutoDock Vina. Toxicological profile was predicted based on clinical manifestations using PASS online software.

RESULTS:

The high docking score of -10.0, -9.9, -9.8, and -9.5 were shown by friedelin, tingenone, albiziasaponin A, and albiziasaponin C, respectively, for caspase-3, and -11.0, -9.6, -9.6, and -9.4 by β-boswellic acid, bryonolic acid, canophyllic acid, and CDDO, respectively, for caspase-8. Possible adverse events were predicted with varying degree of probability and major relevant effects were reported. Hydrostatic interactions along with formation of hydrogen bonds with specific amino acids in the binding pocket were identified with each triterpenoid.

CONCLUSION:

Lead molecules identified through this in silico study such as friedelin, tingenone, albiziasaponin, bryonolic acid, and canophyllic acid may be utilized for further in vitro/in vivo studies as apoptotic agents targeting caspases-3 and 8.

Iridium-Catalyzed Silylation of Unactivated C-H Bonds

The functionalization of primary C-H bonds has been a longstanding challenge in catalysis. Our group has developed a series of silylations of primary C-H bonds that occur with site selectivity and diastereoselectivity resulting from an approach to run the reactions as intramolecular processes. These reactions have become practical by using an alcohol or amine as a docking site for a hydrosilyl group, thereby leading to intramolecular silylations of C-H bonds at positions dictated by the presence common functional groups in the reactants. Oxidation of the C-Si bond leads to the introduction of alcohol functionality at the position of the primary C-H bond of the reactant. The development, scope, and applications of these functionalization reactions is described in this minireview.

Transition-Metal-Catalyzed Selective Functionalization of C(sp3 )-H Bonds in Natural Products

Direct functionalization of natural products is important for studying the structure-activity and structure-property relationships of these molecules. Recent advances in the transition-metal-catalyzed functionalization of C(sp3 )-H bonds, the most abundant yet inert bonds in natural products, have allowed natural product derivatives to be created selectively. Strategies to achieve such transformation are reviewed.

Applications of Nonenzymatic Catalysts to the Alteration of Natural Products

The application of small molecules as catalysts for the diversification of natural product scaffolds is reviewed. Specifically, principles that relate to the selectivity challenges intrinsic to complex molecular scaffolds are summarized. The synthesis of analogues of natural products by this approach is then described as a quintessential "late-stage functionalization" exercise wherein natural products serve as the lead scaffolds. Given the historical application of enzymatic catalysts to the site-selective alteration of complex molecules, the focus of this Review is on the recent studies of nonenzymatic catalysts. Reactions involving hydroxyl group derivatization with a variety of electrophilic reagents are discussed. C-H bond functionalizations that lead to oxidations, aminations, and halogenations are also presented. Several examples of site-selective olefin functionalizations and C-C bond formations are also included. Numerous classes of natural products have been subjected to these studies of site-selective alteration including polyketides, glycopeptides, terpenoids, macrolides, alkaloids, carbohydrates, and others. What emerges is a platform for chemical remodeling of naturally occurring scaffolds that targets virtually all known chemical functionalities and microenvironments. However, challenges for the design of very broad classes of catalysts, with even broader selectivity demands (e.g., stereoselectivity, functional group selectivity, and site-selectivity) persist. Yet, a significant spectrum of powerful, catalytic alterations of complex natural products now exists such that expansion of scope seems inevitable. Several instances of biological activity assays of remodeled natural product derivatives are also presented. These reports may foreshadow further interdisciplinary impacts for catalytic remodeling of natural products, including contributions to SAR development, mode of action studies, and eventually medicinal chemistry.

Evolution of C-H Bond Functionalization from Methane to Methodology

This Perspective presents the fundamental principles, the elementary reactions, the initial catalytic systems, and the contemporary catalysts that have converted C-H bond functionalization from a curiosity to a reality for synthetic chemists. Many classes of elementary reactions involving transition-metal complexes cleave C-H bonds at typically unreactive positions. These reactions, coupled with a separate or simultaneous functionalization process lead to products containing new C-C, C-N, and C-O bonds. Such reactions were initially studied for the conversion of light alkanes to liquid products, but they have been used (and commercialized in some cases) most often for the synthesis of the more complex structures of natural products, medicinally active compounds, and aromatic materials. Such a change in direction of research in C-H bond functionalization is remarkable because the reactions must occur at an unactivated C-H bond over functional groups that are more reactive than the C-H bond toward classical reagents. The scope of reactions that form C-C bonds or install functionality at an unactivated C-H bond will be presented, and the potential future utility of these reactions will be discussed.

Chemical synthesis of saponins

Saponins are a large family of amphiphilic glycosides of steroids and triterpenes found in plants and some marine organisms. By expressing a large diversity of structures on both sugar chains and aglycones, saponins exhibit a wide range of biological and pharmacological properties and serve as major active principles in folk medicines, especially in traditional Chinese medicines. Isolation of saponins from natural sources is usually a formidable task due to the microheterogeneity of saponins in Nature. Chemical synthesis can provide access to large amounts of natural saponins as well as congeners for understanding their structure-activity relationships and mechanisms of action. This article presents a comprehensive account on chemical synthesis of saponins. First highlighted are general considerations on saponin synthesis, including preparation of aglycones and carbohydrate building blocks, assembly strategies, and protecting-group strategies. Next described is the state of the art in the synthesis of each type of saponins, with an emphasis on those representative saponins having sophisticated structures and potent biological activities.

Triterpenoids from Gymnema sylvestre and their pharmacological activities

Because plants are estimated to produce over 200,000 metabolites, research into new natural substances that can be used in the pharmaceutical, agrochemical and agro-industrial production of drugs, biopesticides and food additives has grown in recent years. The global market for plant-derived drugs over the last decade has been estimated to be approximately 30.69 billion USD. A relevant specific example of a plant that is very interesting for its numerous pharmacological properties, which include antidiabetic, anticarcinogenic, and neuroprotective effects is Gymnema sylvestre, used as a medicinal plant in Asia for thousands of years. Its properties are attributed to triterpenoidic saponins. In light of the considerable interest generated in the chemistry and pharmacological properties of G. sylvestre triterpenes and their analogues, we have undertaken this review in an effort to summarise the available literature on these promising bioactive natural products. The review will detail studies on the isolation, chemistry and bioactivity of the triterpenoids, which are presented in the tables. In particular the triterpenoids oxidised at C-23; their isolation, distribution in different parts of the plant, and their NMR spectral data; their names and physico-chemical characterisation; and the biological properties associated with these compounds, with a focus on their potential chemotherapeutic applications.

Recent advances in synthesis and biological activity of triterpenic acylated oximes

During the last few decades more and more attention has been paid to triterpenes-a group of compounds with five- or four-ring skeleton and carboxyl, hydroxyl or oxo groups. Triterpenes with unsubstituted C-3 hydroxyl group can be easily transformed into appropriate ketones and then into oximes. The carbonyl group can be created not only from the hydroxyl group at C-3 position, but also at C-2, C-12 or C-28 positions. Several methods of creation of two = NOH groups within one molecule of triterpene are known. There are also known triterpenes with two carbonyl groups, e.g. at C-3 and C-11 positions, which differ in reactivity: among them only C-3 group can be transformed into oxime. A reactive hydroxyimine group can undergo the action of acylating agents, such as carboxylic acids or their derivatives, also the ones with significant pharmacological activity. Acyl derivatives of triterpenic oximes exhibit important pharmacological activity. The biological tests performed with the use of cell cultures inoculated with viruses showed inhibitory activity of some triterpenic acyloximes against type 1 HSV (H7N1), ECHO-6 and HIV-1 viruses. Another acylated oximes derived from triterpenes shown cytotoxic or antiproliferative activity against many lines of cancer cells. In many cases the pharmacological effects of the tested acyloxyiminotriterpenes were comparable to those of appropriate standard drugs. One of the newest application of acyl derivatives of triterpenic oximes is their ability to form organogels.

A Partial Synthesis of 23-Hydroxybetulonic Acid and 23-Hydroxybetulinic Acid Starting from Betulinic Acid

A partial syntheses of the biologically active 23-hydroxybetulonic acid and 23-hydroxybetulinic acid starting from the naturally abundant betulinic acid has been developed in which the key step was the Baldwin's cyclopalladation of the 23-methyl group from a 3-one oxime of betulinic acid, an important drug in Chinese medicine.

Catalytic functionalization of unactivated primary C-H bonds directed by an alcohol

New synthetic methods for the catalytic functionalization of C-H bonds have the potential to revolutionize the synthesis of complex molecules. However, the realization of this synthetic potential requires the ability to functionalize selectively one C-H bond in a compound containing many such bonds and an array of functional groups. The site-selective functionalization of aliphatic C-H bonds is one of the greatest challenges that must be met for C-H bond functionalization to be used widely in complex-molecule synthesis, and processes catalysed by transition-metals provide the opportunity to control selectivity. Current methods for catalytic, aliphatic C-H bond functionalization typically rely on the presence of one inherently reactive C-H bond, or on installation and subsequent removal of directing groups that are not components of the desired molecule. To overcome these limitations, we sought catalysts and reagents that would facilitate aliphatic C-H bond functionalization at a single site, with chemoselectivity derived from the properties of the catalyst and site-selectivity directed by common functional groups contained in both the reactant and the desired product. Here we show that the combination of an iridium-phenanthroline catalyst and a dihydridosilane reagent leads to the site-selective γ-functionalization of primary C-H bonds controlled by a hydroxyl group, the most common functional group in natural products. The scope of the reaction encompasses alcohols and ketones bearing many substitution patterns and auxiliary functional groups; this broad scope suggests that this methodology will be suitable for the site-selective and diastereoselective functionalization of complex natural products.

Bioactive pentacyclic triterpenes from the stems of Combretum laxum

Two new triterpene glucosides, β-d-glucopyranosyl 2α,3β,24-trihydroxyolean-12-en-28-oate and β-d-glucopyranosyl 2α,3β,23,24-tetrahydroxyurs-12-en-28-oate, in addition to nine known compounds belonging to three different triterpene classes (oleanane-, ursane- and lupane-type) have been isolated from the stems of a specimen of Combretum laxum growing in the “Pantanal” of the central-western region of Brazil. Among the known triterpenes, β-d-glucopyranosyl 2α,3β,6β-trihydroxyolean-12-en-28-oate is reported for the first time in the Combretaceae, while bellericoside and asiatic acid are described for the first time in the genus Combretum. The structures of the isolated compounds have been established on the basis of spectral techniques (1D-, 2D-NMR and MS). Their in vitro antifungal activities against standard strains of Candida albicans, C. krusei and Cryptococcus neoformans were also evaluated in this work.

A simple route for renewable nano-sized arjunolic and asiatic acids and self-assembly of arjuna-bromolactone

While separating two natural nano-sized triterpenic acids via bromolactonization, we serendipitously discovered that arjuna-bromolactone is an excellent gelator of various organic solvents. A simple and efficient method for the separation of two triterpenic acids and the gelation ability and solid state 1D-helical self-assembly of nano-sized arjuna-bromolactone are reported.

Total synthesis of lobatoside E, a potent antitumor cyclic triterpene saponin

Lobatoside E, a novel and complex cyclic triterpene saponin showing potent antitumor activities, has been synthesized for the first time, employing a highly modular approach. The synthesis, starting with oleanolic acid, D-glucose, D-galactose, L-arabinose, and L-rhamnose, requires a total of 73 steps, with the longest linear sequence of 31 steps and in 1.2% overall yield.

Antibacterial compounds from Planchonia careya leaf extracts

AIM OF THE STUDY

The leaves of Planchonia careya (F. Muell.) R. Knuth (Lecythidaceae) have been traditionally implemented in the treatment of wounds by the indigenous people of northern Australia, although the compounds responsible for the medicinal properties have not been identified. In this study, antibacterial compounds from the leaf extracts were isolated and characterized, and the biological activity of each compound was assessed.

MATERIALS AND METHODS

Compounds were isolated from the leaf extracts using HPLC-piloted activity-guided fractionation. The minimum inhibitory concentrations (MICs) were assessed with plate-hole diffusion assays, and the cytotoxicity was determined with MTT assays using monkey kidney epithelial (MA104) cells.

RESULTS

Six known compounds were isolated from the leaf extracts and were identified as 1, (+)-gallocatechin; 2, gallocatechin-(4alpha-->8)-gallocatechin; 3, 9(S)-hydroxy-10E,12Z-octadecadienoic acid (alpha-dimorphecolic acid); 4, 2alpha,3beta,24-trihydroxyolean-12-en-28-oic acid (hyptatic acid-A); 5, 3beta-O-cis-p-coumaroyltormentic acid; and 6, 3beta-O-trans-p-coumaroyltormentic acid. Compounds 5 and 6 were weakly selective for vancomycin-resistant Enterococcus (VRE) compared with eukaryotic cells, with an MIC of 59.4microg/mL and a 50% inhibitory concentration (IC(50)) of 72.0microg/mL for MA104 cells.

CONCLUSIONS

The isolation of six antibacterial compounds from the leaves of Planchonia careya validates the use of this species as a topical wound-healing remedy.

Palladium-catalyzed direct functionalization of imidazolinone: synthesis of dibromophakellstatin

The direct C-H functionalization of imidazolinone is achieved with Pd(OAc)2/NaOAc in DMSO. Dibromophakellstatin can be synthesized in 5 steps with 40% overall yield using this new C-H activation method.