Synthesis, In Silico Study, and In Vitro Antifungal Activity of New 5-(1,3-Diphenyl-1H-Pyrazol-4-yl)-4-Tosyl-4,5-Dihydrooxazoles.

The increase in multi-drug resistant Candida strains has caused a sharp rise in life-threatening fungal infections in immunosuppressed patients, including those with SARS-CoV-2. Novel antifungal drugs are needed to combat multi-drug-resistant yeasts. This study aimed to synthesize a new series of 2-oxazolines and evaluate the ligands in vitro for the inhibition of six Candida species and in silico for affinity to the CYP51 enzymes (obtained with molecular modeling and protein homology) of the same species. The 5-(1,3-diphenyl-1H-pyrazol-4-yl)-4-tosyl-4,5-dihydrooxazoles 6a-j were synthesized using the Van Leusen reaction between 1,3-diphenyl-4-formylpyrazoles 4a-j and TosMIC 5 in the presence of K2CO3 or KOH without heating, resulting in short reaction times, high compound purity, and high yields. The docking studies revealed good affinity for the active site of the CYP51 enzymes of the Candida species in the following order: 6a-j > 4a-j > fluconazole (the reference drug). The in vitro testing of the compounds against the Candida species showed lower MIC values for 6a-j than 4a-j, and for 4a-j than fluconazole, thus correlating well with the in silico findings. According to growth rescue assays, 6a-j and 4a-j (like fluconazole) inhibit ergosterol synthesis. The in silico toxicity assessment evidenced the safety of compounds 6a-j, which merit further research as possible antifungal drugs.

Inhibitors of 3-Hydroxy-3-methylglutaryl Coenzyme A Reductase Decrease the Growth, Ergosterol Synthesis and Generation of petite Mutants in Candida glabrata and Candida albicans.

Candida glabrata and Candida albicans, the most frequently isolated candidiasis species in the world, have developed mechanisms of resistance to treatment with azoles. Among the clinically used antifungal drugs are statins and other compounds that inhibit 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMGR), resulting in decreased growth and ergosterol levels in yeasts. Ergosterol is a key element for the formation of the yeast cell membrane. However, statins often cause DNA damage to yeast cells, facilitating mutation and drug resistance. The aim of the current contribution was to synthesize seven series of compounds as inhibitors of the HMGR enzyme of Candida ssp., and to evaluate their effect on cellular growth, ergosterol synthesis and generation of petite mutants of C. glabrata and C. albicans. Compared to the reference drugs (fluconazole and simvastatin), some HMGR inhibitors caused lower growth and ergosterol synthesis in the yeast species and generated fewer petite mutants. Moreover, heterologous expression was achieved in Pichia pastoris, and compounds 1a, 1b, 6g and 7a inhibited the activity of recombinant CgHMGR and showed better binding energy values than for α-asarone and simvastatin. Thus, we believe these are good candidates for future antifungal drug development.

Pyrrole-Based Enaminones as Building Blocks for the Synthesis of Indolizines and Pyrrolo[1,2-a]pyrazines Showing Potent Antifungal Activity.

As a new approach, pyrrolo[1,2-a]pyrazines were synthesized through the cyclization of 2-formylpyrrole-based enaminones in the presence of ammonium acetate. The enaminones were prepared with a straightforward method, reacting the corresponding alkyl 2-(2-formyl-1H-pyrrol-1-yl)acetates, 2-(2-formyl-1H-pyrrol-1-yl)acetonitrile, and 2-(2-formyl-1H-pyrrol-1-yl)acetophenones with DMFDMA. Analogous enaminones elaborated from alkyl (E)-3-(1H-pyrrol-2-yl)acrylates were treated with a Lewis acid to afford indolizines. The antifungal activity of the series of substituted pyrroles, pyrrole-based enaminones, pyrrolo[1,2-a]pyrazines, and indolizines was evaluated on six Candida spp., including two multidrug-resistant ones. Compared to the reference drugs, most test compounds produced a more robust antifungal effect. Docking analysis suggests that the inhibition of yeast growth was probably mediated by the interaction of the compounds with the catalytic site of HMGR of the Candida species.

Synthesis and Molecular Docking Studies of Alkoxy- and Imidazole-Substituted Xanthones as α-Amylase and α-Glucosidase Inhibitors.

Current antidiabetic drugs have severe side effects, which may be minimized by new selective molecules that strongly inhibit α-glucosidase and weakly inhibit α-amylase. We have synthesized novel alkoxy-substituted xanthones and imidazole-substituted xanthones and have evaluated them for their in silico and in vitro α-glucosidase and α-amylase inhibition activity. Compounds 6c, 6e, and 9b promoted higher α-glucosidase inhibition (IC50 = 16.0, 12.8, and 4.0 µM, respectively) and lower α-amylase inhibition (IC50 = 76.7, 68.1, and >200 µM, respectively) compared to acarbose (IC50 = 306.7 µM for α-glucosidase and 20.0 µM for α-amylase). Contrarily, derivatives 10c and 10f showed higher α-amylase inhibition (IC50 = 5.4 and 8.7 µM, respectively) and lower α-glucosidase inhibition (IC50 = 232.7 and 145.2 µM, respectively). According to the structure-activity relationship, attaching 4-bromobutoxy or 4'-chlorophenylacetophenone moieties to the 2-hydroxy group of xanthone provides higher α-glucosidase inhibition and lower α-amylase inhibition. In silico studies suggest that these scaffolds are key in the activity and interaction of xanthone derivatives. Enzymatic kinetics studies showed that 6c, 9b, and 10c are mainly mixed inhibitors on α-glucosidase and α-amylase. In addition, drug prediction and ADMET studies support that compounds 6c, 9b, and 10c are candidates with antidiabetic potential.

Divergent Pd-catalyzed Functionalization of 4-Oxazolin-2-ones and 4-Methylene-2-oxazolidinones and Synthesis of Heterocyclic-Fused Indoles.

Palladium-catalyzed functionalization was presently performed on two building blocks: 4-oxazolin-2-ones and 4-methylene-2-oxazolidinones. Direct Heck arylation of 4-oxazolin-2-ones led to a series of 5-aryl-4-oxazolin-2-ones, including analogues with N-chiral auxiliary, in an almost quantitative yield. The Pd(II)-catalyzed homocoupling reaction of 4-oxazolin-2-ones provided novel heterocyclic across-ring dienes. Meanwhile, the intramolecular cross-coupling of N-aryl-4-methylene-2-oxazolidinones furnished a series of oxazolo[3,4-a]indol-3-ones. Further functionalization of 4-methylene-2-oxazolidinones afforded substituted indoles and heterocyclic-fused indoles with aryl, bromo, carbinol, formyl, and vinyl groups. A computational study was carried out to account for the behavior of the formylated derivatives. The currently developed methodology was applied to a new formal total synthesis of ellipticine.

Cascade synthesis of indolizines and pyrrolo[1,2-a]pyrazines from 2-formyl-1-propargylpyrroles.

A straightforward synthesis of indolizines and pyrrolo[1,2-a]pyrazines was performed through a cascade condensation/cyclization/aromatization reaction of substituted 2-formyl-N-propargylpyrroles with active methylene compounds such as nitromethane, alkyl malonates, methyl cyanoacetate and malononitrile. Under basic conditions, the reaction proceeded satisfactorily to provide the corresponding 6,7-disubstituted indolizines. The condensation of the pyrrolic analogues with ammonium acetate gave rise to pyrrolo[1,2-a]pyrazines in high yields. N-Allenyl-2-formylpyrroles behaved as more reactive substrates than 2-formyl-N-propargylpyrroles, furnishing the expected indolizines in higher yields. Hence, an allenyl-containing intermediate was probably generated as the reactive species in the reaction mechanism of some N-propargyl pyrroles prior to the cyclization reaction.

Regioselective Mercury(I)/Palladium(II)-Catalyzed Single-Step Approach for the Synthesis of Imines and 2-Substituted Indoles.

An efficient synthesis of ketimines was achieved through a regioselective Hg(I)-catalyzed hydroamination of terminal acetylenes in the presence of anilines. The Pd(II)-catalyzed cyclization of these imines into the 2-substituted indoles was satisfactorily carried out by a C-H activation. In a single-step approach, a variety of 2-substituted indoles were also generated via a Hg(I)/Pd(II)-catalyzed, one-pot, two-step process, starting from anilines and terminal acetylenes. The arylacetylenes proved to be more effective than the alkyl derivatives.

Selective synthesis of trisubstituted pyrroles through the reactions of alkynyl Fischer carbene complexes with oxazolones.

An efficient and simple synthesis of novel trisubstituted 1H-pyrroles 4a-qvia 1,3-dipolar cycloaddition of Δ3-trifluoromethyloxazolones 2a-d with both chromium and tungsten alkynyl Fischer carbene complexes (1a-h) is described. An unexpected and unreported -CF3 group elimination process was observed in the pyrrole structure. Our experimental and theoretical data suggested that the metal fragment may be responsible for this phenomenon. The dipolar cycloaddition proceeded efficiently to produce a single regioisomer, which was unambiguously established through NMR and single-crystal X-ray diffraction studies. Nevertheless, the reaction of alkynyl carbenes bearing an α,ß,γ,δ-unsaturated moiety with excess oxazolone 2a produced a polycyclic compound 6 speculatively formed through a cascade reaction involving 1,6-, 1,4- and 1,2-nucleophilic addition steps.

Highly selective Diels-Alder and Heck arylation reactions in a divergent synthesis of isoindolo- and pyrrolo-fused polycyclic indoles from 2-formylpyrrole.

A highly regio-, chemo- and stereoselective divergent synthesis of isoindolo- and pyrrolo-fused polycyclic indoles is herein described, starting from 2-formylpyrrole and employing Diels-Alder and Heck arylation reactions. 3-(N-Benzyl-2-pyrrolyl)acrylates and 4-(pyrrol-2-yl)butenones underwent a highly endo-Diels-Alder cycloaddition with maleimides to furnish octahydropyrrolo[3,4-e]indoles, which served as precursors in the regioselective synthesis of aza-polycyclic skeletons via an intramolecular Heck arylation reaction. Through the latter reaction, the 3-(N-benzyl-2-pyrrolyl)acrylates give rise to 3-(pyrrolo[2,1-a]isoindol-3-yl)acrylates. A further oxidative aromatization of the polycyclic intermediates provides the corresponding polycyclic pyrrolo-isoindoles and isoindolo-pyrrolo-indoles. A theoretical study on the stereoselective Diels-Alder reactions, carried out by calculating the endo/exo transition states, revealed the assistance of non-covalent interactions in governing the endo stereocontrol.

Synthesis and highly potent anti-inflammatory activity of licofelone- and ketorolac-based 1-arylpyrrolizin-3-ones.

Since NSAIDs are commonly used anti-inflammatory agents that produce adverse effects, there have been ongoing efforts to develop more effective and less toxic compounds. Based on the structure of the anti-inflammatory pyrrolizines licofelone and ketorolac, a series of 1-arylpyrrolizin-3-ones was synthesized. Also prepared was a series of substituted pyrroles, mimicking similar known anti-inflammatory agents. The anti-inflammatory activity of the test compounds was determined with a phorbol ester (TPA)-induced murine ear edema protocol. For the most active derivatives, 19b-c/20b-c, the anti-inflammatory effect was the same as that of the reference compound (indomethacin) and was dose-dependent. These compounds have an aryl ring at the C-1 position and a methoxycarbonyl group at the C-2 position of the pyrrolizine framework, which represent plausible pharmacophore groups with anti-inflammatory activity. The anti-inflammatory activity of 1-substituted analogs containing a five- or six-membered heterocycles was lower but still good, while that of the pyrroles was only moderate. Although the docking studies suggests that the effect of analogs 19a-c/20a-c is associated with the inhibition of cyclooxygenase-2, experimental assays did not corroborate this idea. Indeed, a significant inhibition of NO was found experimentally as a plausible mechanism of action.

Enantiopure 4-oxazolin-2-ones and 4-methylene-2-oxazolidinones as chiral building blocks in a divergent asymmetric synthesis of heterocycles.

Enantiopure 3-((R)- and 3-((S)-1-phenylethyl)-4-oxazoline-2-ones were evaluated as chiral building blocks for the divergent construction of heterocycles with stereogenic quaternary centers. The N-(R)- or N-(S)-1-phenylethyl group of these compounds proved to be an efficient chiral auxiliary for the asymmetric induction of the 4- and 5-positions of the 4-oxazolin-2-one ring through thermal and MW-promoted nucleophilic conjugated addition to Michael acceptors and alkyl halides. The resulting adducts were transformed via a cascade process into fused six-membered carbo- and heterocycles. The structure of the reaction products depended on the electrophiles and reaction conditions used. Alternative isomeric 4-methylene-2-oxazolidinones served as chiral precursors for a versatile and divergent approach to highly substituted cyclic carbamates. DFT quantum calculations showed that the formation of bicyclic pyranyl compounds was generated by a diastereoselective concerted hetero-Diels-Alder cycloaddition.

Synthesis of exo-Imidazolidin-2-one Dienes, Their Isomerization, and Selectivity in Diels-Alder Cycloadditions.

An efficient and alternative synthesis of exo-imidazolidin-2-one dienes is described. A condensation reaction was carried out with bis-imino derivatives, diacetyl, and triphosgene, affording symmetrically N, N-disubstituted dienes. The use of alkyl methyl α-diketones led to the formation of nonsymmetrical dienes, which underwent isomerization to provide more stable inner-outer-ring dienes under Lewis acid conditions. Evaluation was made of the reactivity as well as regio- and stereoselectivity of these dienes in Diels-Alder reactions. They proved to be highly reactive and selective. DFT calculations of the transition states accounted for their behavior.

Synthesis of Polyheterocyclic Pyrrolo[3,4-b]pyridin-5-ones via a One-Pot (Ugi-3CR/aza Diels-Alder/N-acylation/aromatization/SN2) Process. A Suitable Alternative towards Novel Aza-Analogues of Falipamil.

We describe the one-pot synthesis of twenty polyheterocyclic pyrrolo[3,4-b]pyridin-5-ones via a cascade process (Ugi-3CR/aza Diels-Alder/N-acylation/aromatization) in 20 to 95% overall yields, as well as four pharmacologically promising analogues via an improved cascade process (Ugi-3CR/aza Diels-Alder/N-acylation/aromatization/SN2): two piperazine-linked pyrrolo[3,4-b]pyridin-5-ones in 33 and 34%, and a couple of Falipamil aza-analogues in 30 and 35% overall yields. It is worth highlighting the good substrate scope found, because final products are furnished with alkyl, aryl, and heterocyclic substituents. The use of chain-ring tautomerizable isocyanides (as key reagents for the Ugi-type three component reaction) allowed for a rapid and efficient assembly of the polysubstituted oxindoles, which were used in situ toward the complex products, conferring features like robustness, sustainability, and the one-pot approach to this synthetic methodology.

Synthesis of novel polysubstituted N-benzyl-1H-pyrroles via a cascade reaction of alkynyl Fischer carbenes with α-imino glycine methyl esters.

An efficient and simple synthesis of novel and densely substituted N-benzyl-1H-pyrroles 6a-r is described by a 1,4-addition/isomerization/ring closure/demetalation cascade process of alkynyl Fischer carbene complexes 1a-f and 2a and α-imino glycine methyl esters 3a, b, d, g, h, and k promoted with LDA.

Thermochemical study of 1-acetyl vinyl p-nitrobenzoate: vinyl bond enthalpy in captodative olefins.

Captodative olefins are highly reactive and selective substrates in Diels-Alder and 1,3-dipolar cycloadditions. Seeking an explanation of this fact based on molecular energetics, the thermochemical analysis of 1-acetyl vinyl p-nitrobenzoate, a captodative olefin, has been performed using semi-micro-combustion calorimetry, effusion measurements through a quartz crystal microbalance, and differential scanning calorimetry. The molar standard combustion energy and enthalpy as well as the molar standard formation enthalpy are reported along with sublimation and melting enthalpies. From these data, experimental formation enthalpy of the gas-phase is derived and compared with the theoretical value calculated through the density functional theory procedure. The olefinic bond enthalpy is also computed from experimental data, and the relevance of the results is discussed.

Condensation of Diacetyl with Alkyl Amines: Synthesis and Reactivity of p-Iminobenzoquinones and p-Diiminobenzoquinones.

Condensation reactions between diacetyl and α-branched primary alkylamines under mild and neutral conditions provided a mixture of 2,5-dimethylbenzoquinone(alkylimines), 2,5-dimethylbenzoquinone(bis-alkyldiimines), and N,N'-dialkyl-2,5-dimethylbenzene-1,4-diamines, which were efficiently separated as pure products by column chromatography. Both 2,5-dimethylbenzoquinone(alkylimines) and 2,5-dimethylbenzoquinone(bis-alkyldiimines) underwent an interchange of the alkylimino group when treated with anilines, followed by reductive aromatization, to provide diarylamines and 1,4-dianilinobenzenes, respectively. Evaluation was also made of the reactivity and selectivity of these compounds in the presence of anilines, thiophenols and alkylhalides.

Synthesis and highly potent hypolipidemic activity of alpha-asarone- and fibrate-based 2-acyl and 2-alkyl phenols as HMG-CoA reductase inhibitors.

In the search for new potential hypolipidemic agents, the present study focused on the synthesis of 2-acyl phenols (6a-c and 7a-c) and their saturated side-chain alkyl phenols (4a-c and 5a-c), and on the evaluation of their hypolipidemic activity using a murine Tyloxapol-induced hyperlipidemic protocol. The whole series of compounds 4-7 greatly and significantly reduced elevated serum levels of total cholesterol, LDL-cholesterol, and triglycerides, with series 6 and 7 showing the greatest potency ever found in our laboratory. At the minimum dose (25mg/kg/day), the latter compounds lowered cholesterol by 68-81%, LDL by 72-86%, and triglycerides by 59-80%. This represents a comparable performance than that shown by simvastatin. Experimental evidence and docking studies suggest that the activity of these derivatives is associated with the inhibition of HMG-CoA reductase.

Regioselective synthesis of 1,2-dihydroquinolines by a solvent-free MgBr2-catalyzed multicomponent reaction.

A highly efficient and regioselective synthesis of 1,2-dihydroquinolines via a multicomponent reaction between an aniline and two ketones is described. This reaction was catalyzed by magnesium bromide and carried out under solvent-free conditions. When the reaction was performed by using 3-substituted anilines and nonsymmetrically substituted ketones, principally a single product was found among the four expected regioisomers. A variety of anilines and ketones, including cyclic ketones, were evaluated providing a series of 1,2-dihydroquinolines with diverse substitution patterns. A study of the mechanism is discussed. There is evidence of the in situ formation of the imine as a result of the reaction between the aniline and one of the ketones, before annulation to the heterocyclic ring.

Palladium-catalyzed synthesis of natural and unnatural 2-, 5-, and 7-oxygenated carbazole alkaloids from N-arylcyclohexane enaminones.

A palladium-catalyzed synthesis of the carbazole framework is described, including the preparation of 2-, 5-, and 7-oxygenated natural and unnatural carbazole alkaloids. A series of N-arylcyclohexane enaminones, generated by condensation of cyclohexane-1,3-dione with diverse anilines, were aromatized by a Pd(0)-catalyzed thermal treatment to afford the corresponding diarylamines. The latter were submitted to a Pd(II)-catalyzed cyclization and methylation processes to provide the desired carbazoles, including clausine V. Following an inverse strategy, a new and short total synthesis of glycoborine is also reported.

Highly regio- and stereoselective Diels-Alder cycloadditions via two-step and multicomponent reactions promoted by infrared irradiation under solvent-free conditions.

Infrared irradiation promoted the Diels-Alder cycloadditions of exo-2-oxazolidinone dienes 1-3 with the Knoevenagel adducts 4-6, as dienophiles, leading to the synthesis of new 3,5-diphenyltetrahydrobenzo[d]oxazol-2-one derivatives (7, 9, 11 and 13-17), under solvent-free conditions. These cycloadditions were performed with good regio- and stereoselectivity, favoring the para-endo cycloadducts. We also evaluated the one-pot three-component reaction of active methylene compounds 20, benzaldehydes 21 and exo-2-oxazolidinone diene 2 under the same reaction conditions. A cascade Knoevenagel condensation/Diels-Alder cycloaddition reaction was observed, resulting in the final adducts 13-16 in similar yields. These procedures are environmentally benign, because no solvent and no catalyst were employed in these processes. The regioselectivity of these reactions was rationalized by Frontier Molecular Orbital (FMO) calculations.