Novel spirobicyclic artemisinin analogues (artemalogues): Synthesis and antitumor activities

Synthesis and antihepatoma activity of guaianolide dimers derived from lavandiolide I

Guaianolide dimers represent a unique class of natural products with anticancer activities, but their low content in plants has limited in-depth pharmacological studies. Lavandiolide I is a guaianolide dimer isolated from Artemisia species, and had been synthesized on a ten-gram scale in four steps with 60 % overall yield, which showed potent antihepatoma activity on the HepG2, Huh7, and SK-Hep-1 cell lines with IC50 values of 12.1, 18.4, and 17.6 µM, respectively. To explore more active dimers, 33 lavandiolide I derivatives were designed, synthesized, and evaluated for their inhibitory activity on human hepatoma cell lines. Among them, 10 derivatives were more active than lavandiolide I and sorafenib on the three cell lines. The primary structure-activity relationship concluded that the introduction of aldehyde, ester, azide, amide, carbamate and urea functional groups at C-14' of the guaianolide dimer significantly enhanced the antihepatoma activity. Among these compounds, derivatives 25, 27, and 33 enhanced antihepatoma activity more than 1.2-5.8 folds than that of lavandiolide I, and demonstrated low toxicity to the human liver cell lines (THLE-2) and good safety profiles with selective index ranging from 1.3 to 3.4, while lavandiolide I was more toxic to THLE-2 cells. This work provides new insights into enhancing the antihepatoma efficacy and reducing the toxicity of sesquiterpenoid dimers.

Regio- and stereoselective (3 + 2)-cycloaddition reactions of nitrones with cyclic allenes

An effective approach to access functionalized 2H-cyclonona(deca)[d]isoxazoles and 15-oxo-3,10-methanobenzo[b][1]azacyclododecines has been developed by the reaction of N-aryl-C,C-bis(methoxycarbonyl)nitrones with cyclonona(deca)-1,2-dienes in a one-pot fashion. The reaction of N-aryl-C-(phenylcarbamoyl)nitrones with these allenes proceeds strictly regioselectively giving the mixtures of diastereomeric isoxazolidines containing a double bond at the C⁴-position of the isoxazolidine ring. The quantum chemical calculations show that the regioselectivity of these reactions is in good agreement with the reactivity indices of the considered compounds.

Combination of Pseudo-Natural Product Design and Formal Natural Product Ring Distortion Yields Stereochemically and Biologically Diverse Pseudo-Sesquiterpenoid Alkaloids

We describe the synthesis and biological evaluation of a new natural product-inspired compound class obtained by combining the conceptually complementary pseudo-natural product (pseudo-NP) design strategy and a formal adaptation of the complexity-to-diversity ring distortion approach. Fragment-sized α-methylene-sesquiterpene lactones, whose scaffolds can formally be viewed as related to each other or are obtained by ring distortion, were combined with alkaloid-derived pyrrolidine fragments by means of highly selective stereocomplementary 1,3-dipolar cycloaddition reactions. The resulting pseudo-sesquiterpenoid alkaloids were found to be both chemically and biologically diverse, and their biological performance distinctly depends on both the structure of the sesquiterpene lactone-derived scaffolds and the stereochemistry of the pyrrolidine fragment. Biological investigation of the compound collection led to the discovery of a novel chemotype inhibiting Hedgehog-dependent osteoblast differentiation.

Anticancer potential of spirocompounds in medicinal chemistry: A pentennial expedition

Spirocompounds constitute an important class of organic frameworks enveloping numerous pharmacological activities, among them, the promising anticancer potential of spirocompounds have enthused medicinal chemists to explore new spiro derivatives with significantly improved pharmacodynamic and pharmacokinetic profile along with their mechanism of action. The current review intends to provide a sketch of the anticancer activity of various spirocompounds like spirooxindole, spiroisoxazole, spiroindole etc, from the past five years unfolding various aspects of pharmacological activities and their structure-activity relationships (SARs). This literature analysis may provide future direction for the efficient design of novel spiromolecules with enhanced safety and efficacy.

Synthesis of Spiroisoxazolines via TEMPO/NaNO2-Catalyzed Aerobic Oxidative Dearomatization

A catalytic, aerobic oxidative dearomatization protocol has been developed for the preparation of spiroisoxazline scaffolds from oximes using TEMPO and NaNO₂ as the catalyst and O₂ as the sole oxidant. This dearomatization methodology features its mild reaction conditions, good functional group tolerance, and an unprecedented broad substrate scope, encompassing phenols, aryl ethers, thiophenols, aryl sulfides, etc.

Twist-chair conformation of the tetraoxepane ring remains unchanged in tetraoxaspirododecane diamines

A detailed structural analysis has been performed for N,N'-bis(4-chlorophenyl)-7,8,11,12-tetraoxaspiro[5.6]dodecane-9,10-diamine, C₂₀H₂₂Cl₂N₂O₄, (I), N,N'-bis(2-fluorophenyl)-7,8,11,12-tetraoxaspiro[5.6]dodecane-9,10-diamine, C₂₀H₂₂F₂N₂O₄, (II), and N,N'-bis(4-fluorophenyl)-7,8,11,12-tetraoxaspiro[5.6]dodecane-9,10-diamine, C₂₀H₂₂F₂N₂O₄, (III). The seven-membered ring with two peroxide groups adopts a twist-chair conformation in all three compounds. The lengths of the C-N and O-O bonds are slightly shorter than the average statistical values found in the literature for azepanes and 1,2,4,5-tetraoxepanes. The geometry analysis of compounds (I)-(III), the topological analysis of the electron density at the (3, -1) bond critical points within Bader's quantum theory of `Atoms in molecules' (QTAIM) and NBO (natural bond orbital) analysis at the B3LYP/6-31G(d,2p) level of theory showed that there are n_O→σ*(C-O), n_N→σ*(C-O) and n_O→σ*(C-N) stereoelectronic effects. The molecules of compounds (I) and (III) are packed in the crystals as zigzag chains due to strong N-H...O and C-H...O hydrogen-bond interactions, whereas the molecules of compound (II) form chains in the crystals bound by N-H...O, C-H...π and C-H...O contacts. All these data show that halogen atoms and their positions have a minimal effect on the geometric parameters, stereoelectronic effects and crystal packing of compounds (I)-(III), so that the twist-chair conformation of the tetraoxepane ring remains unchanged.

Suppression of asparagine synthetase enhances the antitumor potency of ART and artemalogue SOMCL-14-221 in non-small cell lung cancer

Non-small cell lung cancer (NSCLC) is one of the leading causes of cancer-related mortality. Artemisinin (ART) and SOMCL-14-221 (221), a spirobicyclic analogue of ART, have been reported to inhibit the proliferation of A549 cells with unclear underlying mechanism. In the present study, we validated that both ART and 221 inhibited the proliferation and migration of NSCLC cells and the growth of A549 xenograft tumors without appreciable toxicity. The proteomic data revealed proteins upregulated in ART and 221 groups were involved in "response to endoplasmic reticulum stress" and "amino acid metabolism". Asparagine synthetase (ASNS) was identified as a key node protein in these processes. Interestingly, knockdown of ASNS improved the antitumor potency of ART and 221 in vitro and in vivo, and treatments with ART and 221 disordered the amino acid metabolism of A549 cells. Moreover, ART and 221 activated ER stress, and inhibition of ER stress abolished the anti-proliferative effects of ART and 221. In conclusion, this study demonstrates that ART and 221 suppress tumor growth by triggering ER stress, and the inhibition of ASNS enhances the antitumor activity of ART and 221, which provides new strategy for drug combination therapy.

Artemisinin-derived hybrids and their anticancer activity

The emergence of drug-resistance and the low specificity of anticancer agents are the major challenges in the treatment of cancer and can result in many side effects, creating an urgent demand to develop novel anticancer agents. Artemisinin-derived compounds, bearing a peroxide-containing sesquiterpene lactone moiety, could form free radicals with high reactivity and possess diverse pharmaceutical properties including in vitro and in vivo anticancer activity besides their typical antimalarial activity. Hybrid molecules have the potential to improve the specificity and overcome the drug resistance, therefore hybridization of artemisinin skeleton with other anticancer pharmacophores may provide novel anticancer candidates with high specificity and great potency against drug-resistant cancers. The review outlines the recent advances of artemisinin-derived hybrids as potential anticancer agents, and the structure-activity relationships are also discussed to provide an insight for rational designs of novel hybrids with high activity.

Phthalideisoquinoline Hemiacetal Alkaloids from Corydalis decumbens That Inhibit Spontaneous Calcium Oscillations, Including Alkyl Derivatives of (+)-Egenine That Are Strikingly Levorotatory

The new phthalideisoquinoline hemiacetal alkaloids (2-7) and the known analogues (1 and 8) were isolated from the bulbs of Corydalis decumbens. The new compounds were characterized by analysis of their NMR spectroscopic data, chemical degradation syntheses, X-ray crystallography, and comparison of experimental and calculated ECD data. All the isolates were screened in vitro for inhibitory activity of spontaneous calcium oscillations in primary cultured neocortical neurons. Compounds 1-3 and 5-7 were found to be active in the suppression of spontaneous calcium oscillations with IC₅₀ values of 6.8, 5.6, 11.6, 10.2, 8.3, and 3.1 μM, respectively. It was also observed that the presence of hydroxy, methoxy, and ethoxy groups at the remote stereogenic center C-7' of some isolated phthalideisoquinoline hemiacetal alkaloids could alter the preferred conformation and invert the sign of optical rotation, rather than this resulting from configurational isomerism at C-1 or C-9, and that the ³J_1,9 coupling constants of these analogues varied accordingly. For example, compounds 1 and 6 are levorotatory, despite these molecules having the same carbon skeleton and absolute configuration as (+)-egenine. This emphasizes the potential risk of incorrectly assigning absolute configuration based only on observed coupling constants or optical rotation when comparing the data of new compounds with literature values for known analogues, especially within this class of molecules.

Synthesis of the Tricyclic Picrotoxane Motif by an Oxidative Cascade Cyclization

An oxidative cascade cyclization of β-keto esters has been developed for the construction of the tricyclic picrotoxane motif in a single step, and DFT calculations suggested a possible cationic cyclization mechanism. This cascade cyclization can be operated on a 20 g scale to obtain a 77% total yield of the tricyclic products, which in turn can be converted to versatile intermediates for further elaboration to picrotoxanes and their structurally related compounds.

A Molecular Electron Density Theory Study of the Chemoselectivity, Regioselectivity, and Diastereofacial Selectivity in the Synthesis of an Anticancer Spiroisoxazoline derived from α-Santonin

The [3 + 2] cycloaddition (32CA) reaction of an α-santonin derivative, which has an exocyclic C–C double bond, with p-bromophenyl nitrile oxide yielding only one spiroisoxazoline, has been studied within the molecular electron density theory (MEDT) at the MPWB1K/6-311G(d,p) computational level. Analysis of the conceptual density functional theory (CDFT) reactivity indices and the global electron density transfer (GEDT) account for the non-polar character of this zwitterionic-type 32CA reaction, which presents an activation enthalpy of 13.3 kcal·mol⁻¹. This 32CA reaction takes place with total ortho regioselectivity and syn diastereofacial selectivity involving the exocyclic C–C double bond, which is in complete agreement with the experimental outcomes. While the C–C bond formation involving the β-conjugated carbon of α-santonin derivative is more favorable than the C–O one, which is responsible for the ortho regioselectivity, the favorable electronic interactions taking place between the oxygen of the nitrile oxide and two axial hydrogen atoms of the α-santonin derivative are responsible for the syn diastereofacial selectivity.

Artemisinin-indole and artemisinin-imidazole hybrids: Synthesis, cytotoxic evaluation and reversal effects on multidrug resistance in MCF-7/ADR cells

A series of artemisinin derivatives with MDR reversal activity were designed and synthesized. All hybrids were screened to anticancer activities against four human cancer cell lines (A549, MCF-7, HepG-2, MDA-MB-231) and normal human hepatic cell (L02) in vitro. Most of the new compounds showed higher anticancer activities than artemisinin, among which compounds 11a and 11c displayed superior potency with IC₅₀ 6.78 μM and 5.25 μM against MCF-7, respectively. The further research indicated that the most potent 11c induced cell cycle arrest at G2 phase in MCF-7. Additionally, compound 11c showed remarkable MDR reversal activity which reversed adriamycin against MCF-7/ADR cells with IC₅₀ 0.76 μM.

Anti-proliferative and anti-malarial activities of spiroisoxazoline analogues of artemisinin

A series of spiroisoxazoline analogues of artemisinin was synthesized by employing 1,3-dipolar cycloaddition between various in situ generated nitrile oxides and artemisitene. All the synthesized compounds were tested for their anti-proliferative and anti-malarial activities. Among the compounds tested, compound 11a was found to be potent against the HCT-15 cancer cell line with IC50  = 4.04 μM when compared to 5-fluorouracil (IC50  = 35.53 μM). DNA cell cycle analysis shows that 11a was inhibiting cell proliferation at the G2/M phase. Compound 11b was found to be most active against Plasmodium falciparum with IC50  = 0.1 μM and also blocked host hemoglobin hydrolysis by the falcipain-3 receptor. It was demonstrated to have better dynamics of parasite killing efficiency than artemisinin. Molecular docking studies revealed that these compounds interacted with falcipain-3 receptor sites.

Diversity Modification and Structure-Activity Relationships of Two Natural Products 1β-hydroxy Alantolactone and Ivangustin as Potent Cytotoxic Agents

Sesquiterpene lactones (STLs) are a class of plant secondary metabolites widely found in nature with potent antitumor activities. In this work, two isolated STLs 1β-hydroxy alantolactone (1) and ivangustin (2) were derivatized through diversity-oriented strategy, and in vitro cytotoxic activity assessments were conducted against six cell lines including HeLa, PC-3, HEp-2, HepG2, CHO and HUVEC. The cytotoxic structure-activity relationship showed that the double bond between C5 and C6 was beneficial to improve activity; C1-OH oxidized derivatives showed a slight stronger activity, comparable to the positive drug etoposide (VP-16). Yet, C1-OH esterified derivatives decreased the potency which were different from those of 1-O-acetylbritannilactone (ABL) reported previously by us, and C13-methylene reductive and spiro derivatives resulted in almost complete ablation of cytotoxic activity. Mechanistic basis of cytotoxicity of the representative compound 1i was assayed to relate with apoptosis and cell cycle arrest. Furthermore, 1i inhibited TNF-α-induced canonical NF-κB signaling in PC-3 cells. Molecular modeling studies exhibited additional hydrogen bond interaction between 1i and the residue Lys37 of p65, indicating that 1i could form covalent protein adducts with Cys38 on p65.

Semisynthesis, an Anti-Inflammatory Effect of Derivatives of 1β-Hydroxy Alantolactone from Inula britannica

1β-hydroxy alantolactone, a sesquiterpene lactone mainly isolated from Inula genus plants, exhibits potent anti-inflammatory and anticancer activities. In this work, 1β-hydroxy alantolactone was isolated and five derivatives were prepared through different reactions at the C1-OH and C13-methylene motifs. The structure-activity relationships (SAR) of anti-inflammatory effects against NO production in RAW264.7 cells showed that the α-methylene-γ-butyrolactone motif was essential for NO production suppression and that retaining the C1-OH group can remarkably improve this effect. The NF-κB signaling pathway plays a pivotal role in the regulation of NO expression. Moreover, the levels of p65 and p50 phosphorylation were investigated and active compound 1 inhibited phosphorylation of p65 and p50 in TNF-α-induced NF-κB signaling. Further molecular docking suggested that 1 may target the p65 of NF-κB.

Synthesis of novel spiro-isoxazoline and spiro-isoxazolidine derivatives of tomentosin

A series of novel enantiomerically pure spiro-(isoxazolidines/isoxazolines) were synthesized regioselectively by 1,3-dipolar cycloaddition using nitrones and nitrile oxides, on the exocyclic double bond of tomentosin extracted from Dittrichia viscosa.

Novel dihydroisoxazoline-alkyl carbon chain hybrid artemisinin analogues (artemalogs): synthesis and antitumor activities

Two new series of dihydroisoxazoline-alkyl carbon chain hybrid artemisinin analogues (artemalogs) were designed and synthesized though a 1,3-dipolar cycloaddition, leading to novel analogues with dramatically improved antiproliferative effects against tumor cells.