Partial synthesis of C-ring derivatives from oleanolic and maslinic acids. Formation of several triene systems by chemical and photochemical isomerization processes

Small Structural Differences Govern the Carbonic Anhydrase II Inhibition Activity of Cytotoxic Triterpene Acetazolamide Conjugates

Acetylated triterpenoids betulin, oleanolic acid, ursolic acid, and glycyrrhetinic acid were converted into their succinyl-spacered acetazolamide conjugates. These conjugates were screened for their inhibitory activity onto carbonic anhydrase II and their cytotoxicity employing several human tumor cell lines and non-malignant fibroblasts. As a result, the best inhibitors were derived from betulin and glycyrrhetinic acid while those derived from ursolic or oleanolic acid were significantly weaker inhibitors but also of diminished cytotoxicity. A betulin-derived conjugate held a K_i = 0.129 μM and an EC₅₀ = 8.5 μM for human A375 melanoma cells.

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

Synthesis of erythrodiol C-ring derivatives and their activity against Chlamydia trachomatis

To develop new potential agents against Chlamydia trachomatis among oleanane type triterpenoids the synthesis, spectral and X-ray analysis as well as antimicrobial screening of C-12 oxygen and nitrogen derivatives of erythrodiol is presented. The reduction of methyl 3β-acetoxy-12-oxo-oleanoate with LiAlH4 led to isomeric erythrodiol 12β- and 12α-hydroxy-derivatives, their stereochemistry with respect to the position of hydroxyl-group at C-12 was determined based on the multiplets splitting patterns, the magnitude of the spin-spin interaction, and NOESY interactions. Methyl 3β-acetoxy-12-oxo-oleanoate was transformed to 12E-hydroxyimino- and 12E-methoxyimino-derivatives by the interaction with NH2OH∙HCl or CH3ONH2∙HCl, respectively. By Beckmann rearrangement with SOCl2 in dioxane 12E-oxime was converted to C-lactame and its following reduction with LiAlH4 in THF or dioxane led to erythrodiol C-azepanone or C-azepane derivatives. The structure 3-O,12-N-bis-acetyl-derivative of C-azepane-erythrodiol was confirmed by the single crystal X-ray analysis. Erythrodiol 12β-hydroxy- and C-azepane derivatives were found to be lead compounds with significant activity against C. trachomatis with MIC 1.56 and 3.125 μg/mL. Molecular docking was employed to suggest potential binding interaction, the tested compounds are likely to act as Cdu1 protein inhibitors while 12β-hydroxy-erythrodiol exhibited the highest affinity towards this respective target protein. These results indicated that C-ring oxygen and nitrogen erythrodiol derivatives might be considered for further research in the design of antibacterial agents against Chlamydia trachomatis.

A simple but unusual rearrangement of an oleanane to a taraxerane-28,14 β -olide

Reaction of 3-O-acetyl-oleanolic acid (3) with formic acid/hydrogen peroxide at 100 °C for several hours provides an extraordinary but simple pathway to a taraxeran-28,14 β -olide type triterpenoid while the same reaction at 0 °C occurred without re-arrangement of the carbon skeleton, and an oleanane-28,13 β -olide was obtained instead. The products from these reactions were subjected to a cytotoxicity screening employing several human tumor cell lines showing the latter compound not cytotoxic while the former was cytotoxic especially for MCF-7 (breast adenocarcinoma), and FaDu (hypopharyngeal carcinoma) cells. The highest cytotoxicity, however, was observed for 3 β, 12α, 13 β -trihydroxy-oleanan-28-oic acid (6) holding with EC₅₀ = 4.2 μM for MCF-7 tumor cells.

Ursolic Acid and Its Derivatives as Bioactive Agents

Non-communicable diseases (NCDs) such as cancer, diabetes, and chronic respiratory and cardiovascular diseases continue to be threatening and deadly to human kind. Resistance to and side effects of known drugs for treatment further increase the threat, while at the same time leaving scientists to search for alternative sources from nature, especially from plants. Pentacyclic triterpenoids (PT) from medicinal plants have been identified as one class of secondary metabolites that could play a critical role in the treatment and management of several NCDs. One of such PT is ursolic acid (UA, 3 β-hydroxy-urs-12-en-28-oic acid), which possesses important biological effects, including anti-inflammatory, anticancer, antidiabetic, antioxidant and antibacterial effects, but its bioavailability and solubility limits its clinical application. Mimusops caffra, Ilex paraguarieni, and Glechoma hederacea, have been reported as major sources of UA. The chemistry of UA has been studied extensively based on the literature, with modifications mostly having been made at positions C-3 (hydroxyl), C12-C13 (double bonds) and C-28 (carboxylic acid), leading to several UA derivatives (esters, amides, oxadiazole quinolone, etc.) with enhanced potency, bioavailability and water solubility. This article comprehensively reviews the information that has become available over the last decade with respect to the sources, chemistry, biological potency and clinical trials of UA and its derivatives as potential therapeutic agents, with a focus on addressing NCDs.

Cytotoxic and NF-κB and mitochondrial transmembrane potential inhibitory pentacyclic triterpenoids from Syzygium corticosum and their semi-synthetic derivatives

Syzygium is a large genus of flowering plants, with several species, including the clove tree, used as important resources in the food and pharmaceutical industries. In our continuing search for anticancer agents from higher plants, a chloroform extract of the leaves and twigs of Syzygium corticosum collected in Vietnam was found to be active toward the HT-29 human colon cancer cell line. Separation of this extract guided by HT-29 cells and nuclear factor-kappa B (NF-κB) inhibition yielded 19 known natural products, including seven triterpenoids, three ellagic acid derivatives, two methylated flavonoids, a cyclohexanone, four megastigmanes, a small lactone, and an aromatic aldehyde. The full stereochemistry of (+)-fouquierol (2) was defined for the first time. Biological investigations showed that (+)-ursolic acid (1) is the major cytotoxic component of S. corticosum, which exhibited also potent activities in the NF-κB and mitochondrial transmembrane potential (MTP) inhibition assays conducted, with IC50 values of 31 nM and 3.5 µM, respectively. Several analogues of (+)-ursolic acid (1) were synthesized, and a preliminary structure-activity relationship (SAR) study indicated that the C-3 hydroxy and C-28 carboxylic acid groups and 19,20-dimethyl substitution are all essential in the mediation of the bioactivities observed for this triterpenoid.

Synthesis of ring-C modified oleanolic acid derivatives and their cytotoxic evaluation

Ring-C of oleanolic acid was chemically modified by treating with NBS under a variety of experimental conditions. The structures of the synthesized compounds were established by spectral analysis ((1)H &(13)C NMR and Mass). All the compounds were evaluated against a panel of five human cancer cell lines by using MTT assay. Among the tested compounds, 2 and 7 showed significant activity against breast cancer cell line, MCF-7. Most significantly, compound 7 showed several folds enhanced activity against MCF-7 cancer cell lines (IC50: 2.96μM) than that of the parent (1) and the intermediate compound (6). Flow cytometric analysis revealed that these compounds arrested the cell cycle in G0/G1 phase and induced mitochondrial mediated apoptosis.

Biotransformation of oleanolic and maslinic methyl esters by Rhizomucor miehei CECT 2749

The pentacyclic triterpenoids methyl oleanolate, methyl maslinate, methyl 3β-hydroxyolean-9(11),12-dien-28-oate, and methyl 2α,3β-dihydroxy-12β,13β-epoxyolean-28-oate were biotransformed by Rhizomucor miehei CECT 2749. Microbial transformation of methyl oleanolate produced only a 7β,30-dihydroxylated metabolite with a conjugated 9(11),12-diene system in the C ring. Biotransformation of the substrate with this 9(11),12-diene system gave the same 7β,30-dihydroxylated compound together with a 7β,15α,30-trihydroxyl derivative. The action of this fungus (R. miehei) on methyl maslinate was more varied, isolating metabolites with a 30-hydroxyl group, a 9(11),12-diene system, an 11-oxo group, or an 12-oxo group. Microbial transformation of the substrate with a 12β,13β-epoxy function resulted in the isolation of two metabolites with 12-oxo and 28,13β-olide groups, hydroxylated or not at C-7β, together with a 30-hydroxy-12-oxo derivative. The structures of these derivatives were deduced by extensive and rigorous spectroscopic studies.

An efficient synthesis of methyl 2-cyano-3,12-dioxoursol-1,9-dien-28-oate (CDDU-methyl ester): analogues, biological activities, and comparison with oleanolic acid derivatives

An efficient synthesis of methyl 2-cyano-3,12-dioxoursol-1,9-dien-28-oate (CDDU-methyl ester) from commercially available ursolic acid, which features an oxidative ozonolysis-mediated C-ring enone formation, and provides the first access to ursolic acid-derived cyano enone analogues with C-ring activation. These new ursolic acid analogues show potent biological activities, with potency of approximately five-fold less than the corresponding oleanolic acid derivatives.

Genus caulophyllum: an overview of chemistry and bioactivity

Recently, some promising advances have been achieved in understanding the chemistry, pharmacology, and action mechanisms of constituents from genus Caulophyllum. Despite this, there is to date no systematic review of those of genus Caulophyllum. This review covers naturally occurring alkaloids and saponins and those resulting from synthetic novel taspine derivatives. The paper further discussed several aspects of this genus, including pharmacological properties, mechanisms of action, pharmacokinetics, and cell membrane chromatography for activity screening. The aim of this paper is to provide a point of reference for pharmaceutical researchers to develop new drugs from constituents of Caulophyllum plants.

The chemical and biological potential of C ring modified triterpenoids

A convenient and elegant route has been developed to separate the natural regioisomers triterpenoids ursolic acid (UA) and oleanolic acid (OA) as well as derivatives thereof. Eleven unknown derivatives of OA were designed, synthesized, and their cytotoxicity was investigated. Further sixteen compounds were prepared to correlate all compounds in a SAR study. It could be shown that C-ring modifications of OA and UA have only a moderate influence onto the cytotoxic activity of the compounds but a significant impact onto the ability to trigger apoptosis in ovarian cancer cells (cell line A2780).

C-Lactam Derivatives of Oleanolic Acid. The Synthesis of C-lactam by Beckmann Rearrangement of C-oxime

Oleanolic acid, one of the most known triterpenes, was subjected to different chemical transformations within C-3 β-hydroxyl group, a double bond between C-12 and C-13, and a carboxyl function at C-17 in order to obtain new derivatives. The key compound consists of four six-membered rings (A, B, D, E) and one enlarged ring (C ring) containing a nitrogen atom and a carbonyl function – lactam. This type of derivative can be obtained by Beckmann rearrangement of the appropriate oxime. The lactam can be transformed into thiolactam with the use of Lavesson's reagent. The method is also presented for new derivatives synthesis, as well as their structure elucidation by spectroscopic means.

Gastroprotective and ulcer-healing activity of oleanolic acid derivatives: In vitro–in vivo relationships

The triterpene oleanolic acid 1 and its semisynthetic derivatives 2-7 were assessed for gastroprotective and ulcer-healing effect using human epithelial gastric cells (AGS) and human lung fibroblasts (MRC-5). The ability of the compounds to protect the AGS cells against the damage induced by sodium taurocholate (NaT), to stimulate the cellular reduced glutathione (GSH) and prostaglandin E(2) content, to enhance AGS and MRC-5 cell proliferation and to scavenge superoxide anion in vitro was studied. The cytotoxicity of the compounds was assessed towards MRC-5 and AGS cells. In addition, the gastroprotective activity of the compounds was assessed in vivo using the HCl/EtOH-induced ulcer model in mice. All the assayed compounds displayed a significant reduction of AGS cells damage after incubation with NaT. None of the studied compounds was active as a superoxide anion scavenger nor stimulated the GSH content in AGS cell cultures. Compounds 1, 2, 4 and 6 were able to increase the prostaglandin content in AGS cell cultures. Concerning the proliferation assays, a significant stimulating effect was observed for compounds 3 and 7 on AGS cells and for 1 and 7 on MRC-5 fibroblasts. Regarding cytotoxicity, derivatives 2, 4, 6 and 7 were less toxic than the parent compound oleanolic acid. Our results strongly support the predictive capacity of the in vitro assessment of gastroprotective activity allowing the reduction of experimental animals.