Recent advances in synthesis and biological activity of triterpenic acylated oximes

Rongalite/iodine-mediated C(sp3)-H bond oximation and thiomethylation reaction of methyl ketones using copper nitrate as the [NO] reagent: synthesis of thiohydroximic acids

In this work, a highly efficient rongalite/iodine-mediated oxime formation reaction for the preparation of thiohydroximic acids from methyl ketones by employing copper nitrate as the [NO] reagent has been developed. Notably, copper nitrate participated as both a catalyst and the mild oximation reagent in the transformation. This reaction is highly efficient and facile, with a broad substrate scope, especially for fused ring skeleton substrates, heterocyclic skeleton substrates, and acetyl-substituted natural products. Mechanistic studies revealed that copper nitrate might be converted into a NO2 radical or the NO2 radical dimeric forms as an ion-pair equivalent to participate in the transformation.

Production of Oximes Directly from Sustainable Lignocellulose-Derived Aldehydes and Ammonia over HTS-1 Catalyst

Oxime chemicals are the building blocks of many anticancer drugs and widely used in industry and laboratory. A simple but robust hierarchically porous zeolite (HTS-1) catalyst was prepared by hydrothermal methods and used for the preparation of vanillin oxime from vanillin in NH3  ⋅ H2 O/DIO (v/v 1/10) system. The results of the catalyst characterization showed that the larger pore size and more framework Ti were conducive to promote the transformation of the substrates. The conversion of vanillin and the yield of vanillin oxime were both higher than 99 % under optimized reaction conditions. It was found that the reaction proceeded by oxidation of NH3 to hydroxylamine (NH2 OH), and oximation of hydroxylamine with vanillin to obtain vanillin oxime, where the rate-controlling step was the hydroxylamine formation, and the apparent activation energy was 26.22 kJ/mol. The corresponding oximation products could also be obtained by extending this method to other compounds derived from lignin. Furthermore, the catalytic system was used directly to the conversion of birch biomass to obtain oxime products such as vanillin oxime, syringaldehyde oxime, and furfural oxime etc. This work might give insights into the sustainable production of N-containing high-value products from lignocellulose.

Direct Synthesis of Aldoximes: Ruthenium-Catalyzed Coupling of Alcohols and Hydroxylamine Hydrochloride

A catalytic method for the direct synthesis of oximes from alcohols and hydroxyl amine hydrochloride salt is reported. The reaction is catalyzed by a ruthenium pincer catalyst, which oxidizes alcohols involving amine-amide metal-ligand cooperation, and the in situ formed aldehydes condense with hydroxyl amine to deliver the oximes. Notably, the reaction requires only a catalyst and base; water and liberated hydrogen are the only byproducts, making this protocol attractive and environmentally benign.

Development and Assessment of 1,5-Diarylpyrazole/Oxime Hybrids Targeting EGFR and JNK-2 as Antiproliferative Agents: A Comprehensive Study through Synthesis, Molecular Docking, and Evaluation

New 1,5-diarylpyrazole oxime hybrid derivatives (scaffolds A and B) were designed, synthesized, and then their purity was verified using a variety of spectroscopic methods. A panel of five cancer cell lines known to express EGFR and JNK-2, including human colorectal adenocarcinoma cell line DLD-1, human cervical cancer cell line Hela, human leukemia cell line K562, human pancreatic cell line SUIT-2, and human hepatocellular carcinoma cell line HepG2, were used to biologically evaluate for their in vitro cytotoxicity for all the synthesized compounds 7a-j, 8a-j, 9a-c, and 10a-c. The oxime containing compounds 8a-j and 10a-c were more active as antiproliferative agents than their non-oxime congeners 7a-j and 9a-c. Compounds 8d, 8g, 8i, and 10c inhibited EGFR with IC50 values ranging from 8 to 21 µM when compared with sorafenib. Compound 8i inhibited JNK-2 as effectively as sorafenib, with an IC50 of 1.0 µM. Furthermore, compound 8g showed cell cycle arrest at the G2/M phase in the cell cycle analysis of the Hela cell line, whereas compound 8i showed combined S phase and G2 phase arrest. According to docking studies, oxime hybrid compounds 8d, 8g, 8i, and 10c exhibited binding free energies ranging from -12.98 to 32.30 kcal/mol at the EGFR binding site whereas compounds 8d and 8i had binding free energies ranging from -9.16 to -12.00 kcal/mol at the JNK-2 binding site.

Synthesis and bioactivities evaluation of oleanolic acid oxime ester derivatives as α-glucosidase and α-amylase inhibitors

Different oleanolic acid (OA) oxime ester derivatives (3a-3t) were designed and synthesised to develop inhibitors against α-glucosidase and α-amylase. All the synthesised OA derivatives were evaluated against α-glucosidase and α-amylase in vitro. Among them, compound 3a showed the highest α-glucosidase inhibition with an IC₅₀ of 0.35 µM, which was ∼1900 times stronger than that of acarbose, meanwhile compound 3f exhibited the highest α-amylase inhibitory with an IC₅₀ of 3.80 µM that was ∼26 times higher than that of acarbose. The inhibition kinetic studies showed that the inhibitory mechanism of compounds 3a and 3f were reversible and mixed types towards α-glucosidase and α-amylase, respectively. Molecular docking studies analysed the interaction between compound and two enzymes, respectively. Furthermore, cytotoxicity evaluation assay demonstrated a high level of safety profile of compounds 3a and 3f against 3T3-L1 and HepG2 cells.

Highlights

Oleanolic acid oxime ester derivatives (3a–3t) were synthesised and screened against α-glucosidase and α-amylase.

Compound 3a showed the highest α-glucosidase inhibitory with IC50 of 0.35 µM.

Compound 3f presented the highest α-amylase inhibitory with IC50 of 3.80 µM.

Kinetic studies and in silico studies analysed the binding between compounds and α-glucosidase or α-amylase.

Newly synthesised oxime and lactone derivatives from Dipterocarpus alatus dipterocarpol as anti-diabetic inhibitors: experimental bioassay-based evidence and theoretical computation-based prediction

Dipterocarpus alatus-derived products are expected to exhibit anti-diabetes properties. Natural dipterocarpol (1) was isolated from Dipterocarpus alatus collected in Quang Nam province, Vietnam; afterwards, 20 derivatives including 13 oxime esters (2 and 3a–3m) and 7 lactones (4, 5, 6a–6e) were semi-synthesised. Their inhibitory effects towards diabetes-related proteins were investigated experimentally (α-glucosidase) and computationally (3W37, 3AJ7, and PTP1B). Except for compound 2, the other 19 compounds (3a–3m, 4, 5, and 6a–6d) are reported for the first time, which were modified at positions C-3, C-24 and C-25 of the dipterocarpol via imidation, esterification, oxidative cleavage and lactonisation reactions. A framework based on docking-QSARIS combination was proposed to predict the inhibitory behaviour of the ligand-protein complexes. Enzyme assays revealed the most effective α-glucosidase inhibitors, which follow the order 5 (IC₅₀ of 2.73 ± 0.05 μM) > 6c (IC₅₀ of 4.62 ± 0.12 μM) > 6e (IC₅₀ of 7.31 ± 0.11 μM), and the computation-based analysis confirmed this, i.e., 5 (mass: 416.2 amu; polarisability: 52.4 ų; DS: −14.9 kcal mol⁻¹) > 6c (mass: 490.1 amu; polarisability: 48.8 ų; DS: −13.7 kcal mol⁻¹) > 6e (mass: 549.2 amu; polarisability: 51.6 ų; DS: −15.2 kcal mol⁻¹). Further theoretical justifications predicted 5 and 6c as versatile anti-diabetic inhibitors. The experimental results encourage next stages for the development of anti-diabetic drugs and the computational strategy invites more relevant work for validation.

Dipterocarpus alatus-derived products are expected to exhibit anti-diabetes properties.

Visible-light mediated stereospecific C(sp2)-H difluoroalkylation of (Z)-aldoximes

A visible light mediated stereospecific C(sp²)-H difluoroalkylation of (Z)-aldoximes to (E)-difluoroalkylated ketoximes has been described. In this reaction, (hetero)-aromatic and aliphatic difluoroalkylated ketoximes could be obtained with the retention of the configuration of the starting aldoximes. A preliminary mechanism study showed that a difluoromethyl radical via an SET pathway was involved.

Oximes: Novel Therapeutics with Anticancer and Anti-Inflammatory Potential

Oximes have been studied for decades because of their significant roles as acetylcholinesterase reactivators. Over the last twenty years, a large number of oximes have been reported with useful pharmaceutical properties, including compounds with antibacterial, anticancer, anti-arthritis, and anti-stroke activities. Many oximes are kinase inhibitors and have been shown to inhibit over 40 different kinases, including AMP-activated protein kinase (AMPK), phosphatidylinositol 3-kinase (PI3K), cyclin-dependent kinase (CDK), serine/threonine kinases glycogen synthase kinase 3 α/β (GSK-3α/β), Aurora A, B-Raf, Chk1, death-associated protein-kinase-related 2 (DRAK2), phosphorylase kinase (PhK), serum and glucocorticoid-regulated kinase (SGK), Janus tyrosine kinase (JAK), and multiple receptor and non-receptor tyrosine kinases. Some oximes are inhibitors of lipoxygenase 5, human neutrophil elastase, and proteinase 3. The oxime group contains two H-bond acceptors (nitrogen and oxygen atoms) and one H-bond donor (OH group), versus only one H-bond acceptor present in carbonyl groups. This feature, together with the high polarity of oxime groups, may lead to a significantly different mode of interaction with receptor binding sites compared to corresponding carbonyl compounds, despite small changes in the total size and shape of the compound. In addition, oximes can generate nitric oxide. This review is focused on oximes as kinase inhibitors with anticancer and anti-inflammatory activities. Oximes with non-kinase targets or mechanisms of anti-inflammatory activity are also discussed.

Controllable Z/E-selective synthesis of α-amino-ketoximes from N-nitrososulfonamides and aryl alkenes under neutral conditions

An amidoximation of alkenes with N-nitrososulfonamides enabled by triplet energy transfer under neutral conditions is presented. Both (Z)- and (E)-α-amino-ketoximes are selectively accessible depending on the triplet energy of the photosensitizer.

Visible-light-mediated synthesis of quinolines

Visible light promoted synthetic routes of quinolines using different strategies are hereby documented.

OxymaPure Coupling Reagents: Beyond Solid-Phase Peptide Synthesis

OxymaPure [ethyl 2-cyano-2-(hydroxyimino)acetate] is an exceptional reagent with which to suppress racemization and enhance coupling efficiency during amide bond formation. The tremendous popularity of OxymaPure has led to the development of several Oxyma-based reagents. OxymaPure and its derived reagents are widely used in solid- and solution-phase peptide chemistry. This review summarizes the recent developments and applications of OxymaPure and Oxyma-based reagents in peptide chemistry, in particular in solution-phase chemistry. Moreover, the side reaction associated with OxymaPure is also discussed.

1 Introduction

2 Oxyma-Based Coupling Reagents

2.1 Aminium/Uronium Salts of OxymaPure

2.2 Phosphonium Salts of OxymaPure

2.3 Oxyma-Based Phosphates

2.4 Sulfonate Esters of OxymaPure

2.5 Benzoate Esters of OxymaPure

2.6 Carbonates of OxymaPure Derivatives

3 OxymaPure Derivatives

4 Other Oxime-Based Additives and Coupling Reagents

5 Side Reactions Using OxymaPure Derivatives

6 Conclusion

7 List of Abbreviations

Recent advances in natural anti-HIV triterpenoids and analogs

The human immunodeficiency virus/acquired immunodeficiency syndrome (HIV/AIDS) epidemic is one of the world's most serious health challenges. Although combination antiretroviral therapy provides effective viral suppression, current medicines used against HIV cannot completely eradicate the infectious disease and often have associated toxicities and severe side effects in addition to causing drug resistance. Therefore, the continued development of new antiviral agents with diverse structures and novel mechanisms of action remains a vital need for the management of HIV/AIDS. Natural products are an important source of drug discovery, and certain triterpenes and their analogs have demonstrated potential as pharmaceutical precursors for the treatment of HIV. Over the past decade, natural triterpenoids and analogs have been extensively studied to find new anti-HIV drugs. This review discusses the anti-HIV triterpenoids and analogs reported during the period of 2009-2019. The article includes not only a comprehensive review of the recent anti-HIV agent development from the perspective of medicinal chemistry, but also discusses structure-activity relationship analyses of the described triterpenoids.

Recent Developments in the Functionalization of Betulinic Acid and Its Natural Analogues: A Route to New Bioactive Compounds

Betulinic acid (BA) and its natural analogues betulin (BN), betulonic (BoA), and 23-hydroxybetulinic (HBA) acids are lupane-type pentacyclic triterpenoids. They are present in many plants and display important biological activities. This review focuses on the chemical transformations used to functionalize BA/BN/BoA/HBA in order to obtain new derivatives with improved biological activity, covering the period since 2013 to 2018. It is divided by the main chemical transformations reported in the literature, including amination, esterification, alkylation, sulfonation, copper(I)-catalyzed alkyne-azide cycloaddition, palladium-catalyzed cross-coupling, hydroxylation, and aldol condensation reactions. In addition, the synthesis of heterocycle-fused BA/HBA derivatives and polymer‒BA conjugates are also addressed. The new derivatives are mainly used as antitumor agents, but there are other biological applications such as antimalarial activity, drug delivery, bioimaging, among others.

Therapeutic potential of glycyrrhetinic acids: a patent review (2010-2017)

INTRODUCTION

Glycyrrhetinic acids (GAs) viz., 18β-glycyrrhetinic acid and 18α-glycyrrhetinic acid, are oleanane-type triterpenes having a carboxylic acid group at C-30, and are extracted from the Chines herbal medicine licorice (Glycyrrhiza uralensis). Although the pharmacological properties of GAs have long been known, attention to them has greatly increased in recent times due to their cytotoxic activity.

AREAS COVERED

This review represents the patents granted about natural and synthetic glycyrrhetinic acid analogs from January 2010 to December 2017, the advances made by research groups in conjunction with pharmaceutical companies in the discovery of new natural or synthetic glycyrrhetinic acid analogs.

EXPERT OPINION

GAs demonstrate excellent cytotoxic, antimicrobial, enzyme inhibitory, antiinflammatory, antioxidant, analgesic, and antiviral effects. It is interesting to note that the C-₃(OH) and C_30-CO₂H functional groups make GAs very attractive lead structures for medicinal scientists since these functionalities allow the generation of further chemical diversity for improved pharmacological effects. Moreover, various GA analogues have been prepared via modification of the C_30-CO₂H. It is noteworthy that the C-30 amide of GA demonstrated better cytotoxic effects compared to the parent compounds. In addition, GAs have the capability to conjugate with other anticancer drugs or be converted into their halo or amino analogs which is expected to stimulate medicinal chemist to synthesize new lead compounds in cancer drug discovery.

Enhancing anticancer activity through the combination of bioreducing agents and triterpenes

AIM

Triterpenes are natural compounds, whose wide biological activity predestines them for application as promising new chemotherapeutics. In this paper, we report the results of our investigations into the substitution of oleanolic acid with aromatic and nitroaromatic moieties acting as bioreducing agents.

RESULTS

The process of reduction of nitro groups was investigated through cyclic voltammetry, UV-Vis and electron paramagnetic resonance spectroelectrochemistry. The cytotoxic activity against selected cancer cell lines was determined, showing a significant increase in cytotoxicity when the triterpene is equipped with a nitroaromatic moiety.

CONCLUSION

We believe this approach to the functionalization is promising in terms of enhancing anticancer activity. We also indicate electrochemical techniques as advantageous preclinical screening methods for the identification of cytotoxic agents.

Metal-Involving Synthesis and Reactions of Oximes

This review classifies and summarizes the past 10-15 years of advancements in the field of metal-involving (i.e., metal-mediated and metal-catalyzed) reactions of oximes. These reactions are diverse in nature and have been employed for syntheses of oxime-based metal complexes and cage-compounds, oxime functionalizations, and the preparation of new classes of organic species, in particular, a wide variety of heterocyclic systems spanning small 3-membered ring systems to macroheterocycles. This consideration gives a general outlook of reaction routes, mechanisms, and driving forces and underlines the potential of metal-involving conversions of oxime species for application in various fields of chemistry and draws attention to the emerging putative targets.

Synthesis of novel lupane triterpenoid-indazolone hybrids with oxime ester linkage

An efficient protocol for the synthesis of novel lupane triterpenoid-indazolone hybrids with oxime ester linkage has been developed from naturally accessible precursor betulin. For the first time a series of betulonic acid-indazolone hybrids have been synthesized via an acylation of corresponding 6,7-dihydro-1H-indazol-4(5H)-one oximes with betulonic acid chloride. Diastereoselective reduction of the obtained betulonic acid conjugates with NaBH₄ resulted in a formation of betulinic acid-indazolone hybrids in excellent yields. The configuration of the key compounds has been fully established by X-ray and 2D NMR analysis.

Hybrid Compounds Strategy in the Synthesis of Oleanolic Acid Skeleton-NSAID Derivatives

The current study focuses on the synthesis of several hybrid individuals combining a natural oleanolic acid skeleton and synthetic nonsteroidal anti-inflammatory drug moieties (NSAIDs). It studied structural modifications of the oleanolic acid structure by use of the direct reactivity of hydroxyl or hydroxyimino groups at position C-3 of the triterpenoid skeleton with the carboxylic function of anti-inflammatory drugs leading to new perspective compounds with high potential pharmacological activities. Novel ester- and iminoester-type derivatives of oleanolic unit with the different NSAIDs, such as ibuprofen, aspirin, naproxen, and ketoprofen, were obtained and characterized. Moreover, preliminary research of compounds obtaining structure stability under acidic conditions was examined and the PASS method of prediction of activity spectra for substances was used to estimate the potential biological activity of these compounds.