Publications by authors named "Margus Lopp"

The Chikungunya virus (CHIKV) is an arbovirus belonging to the genus of the family. CHIKV is transmitted by the mosquitoes and causes Chikungunya fever. CHIKV outbreaks have occurred in Africa, Asia, Europe, and the countries of Indian and Pacific Oceans.

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Despite the remarkable advances in the area of asymmetric catalytic oxidations over the past decades, the development of sustainable and environmentally benign enantioselective oxidation techniques, especially with the efficiency level similar to natural enzymes, still represents a challenge. The growing demand for enantiopure compounds and high interest to industry-relevant green technological advances continue to encourage the research pursuits in this field. Among various oxidants, molecular oxygen is ubiquitous, being available at low cost, environmentally benign and easy-to-handle material.

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A method for the synthesis of 9,11-secosteroids starting from the natural corticosteroid cortisol is described. There are two key steps in this approach, combining chemistry and synthetic biology. Stereo- and regioselective hydroxylation at C9 (steroid numbering) is carried out using whole-cell biocatalysis, followed by the chemical cleavage of the C-C bond of the vicinal diol.

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Oxidative fragmentation of tertiary cyclopropanols with phenyliodine(III) dicarboxylates in aprotic solvents (dichloromethane, chloroform, toluene) produces mixed anhydrides. The fragmentation reaction is especially facile with phenyliodine(III) reagents bearing electron-withdrawing carboxylate ligands (trifluoroacetyl, 2,4,6-trichlorobenzoyl, 3-nitrobenzoyl), and affords 95-98% yields of the corresponding mixed anhydride products. The latter can be straightforwardly applied for the acylation of various nitrogen, oxygen and sulfur-centered nucleophiles (primary and secondary amines, hydroxylamines, primary alcohols, phenols, thiols).

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Until now, the oil shale kukersite has been used mainly for energy and oil production. To broaden the possible applications of oil shales, the wet air oxidation of kukersite (an organic-rich sedimentary rock from Estonia) was studied. Kukersite was oxidized with an oxygen-rich gas in water at temperatures up to 200 °C and pressures up to 60 bar.

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Article Synopsis
  • A new strategy is introduced for creating histone deacetylase inhibitory peptides using a method that efficiently adds zinc-binding features late in the synthesis process.
  • The approach focuses on breaking down a specific cyclopropane compound that serves as a versatile starting material.
  • The method's effectiveness is demonstrated through the successful synthesis of the natural product chlamydocin and its various analogues, highlighting the ability to incorporate complex structures into the final products.
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Tertiary cyclopropanols undergo ring-opening oxidative sulfonylation to afford γ-keto sulfones when reacting with sulfinate salts in the presence of a copper(ii) acetate catalyst and an oxidant (tert-butyl hydroperoxide or atmospheric oxygen). Various fluoroalkyl, aryl and alkyl sulfinate salts are successfully employed as sulfonylation reagents, affording the corresponding sulfones in up to 94% yields. The experimental protocol is mild and tolerates a number of functionalities in the cyclopropanol substrate.

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An efficient, mild, and environmentally benign method was developed for the asymmetric synthesis of 2-oxyranyl ketones from easily available tertiary cyclopropanols. The one-pot protocol includes the aerobic oxidation of cyclopropanol derivatives catalyzed by Mn(III) complexes followed by the poly-l-leucine-assisted stereoselective elimination of water from the intermediate peroxides with DBU to afford the corresponding epoxy ketones in high yields and good-to-excellent enantioselectivities (up to 97%).

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Tertiary cyclopropanols easily available from carboxylic esters have been used in the synthesis of distally fluorinated ketones. Cyclopropane ring cleavage reactions in methanol with aqueous tert-butyl hydroperoxide in the presence of a copper(ii) acetate catalyst and sodium triflinate (Langlois reagent) afford β-trifluoromethyl ketones in 16-74% isolated yields. Sodium triflinate serves as a precursor of reactive trifluoromethyl copper species, enabling ring-opening trifluoromethylation, as evidenced by mechanistic studies.

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Chikungunya virus (CHIKV; genus Alphavirus) is the causative agent of chikungunya fever. CHIKV replication can be inhibited by some broad-spectrum antiviral compounds; in contrast, there is very little information about compounds specifically inhibiting the enzymatic activities of CHIKV replication proteins. These proteins are translated in the form of a nonstructural (ns) P1234 polyprotein precursor from the CHIKV positive-strand RNA genome.

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The virtual screening for new scaffolds for TrkA receptor antagonists resulted in potential low molecular weight drug candidates for the treatment of neuropathic pain and cancer. In particular, the compound (Z)-3-((5-methoxy-1H-indol-3-yl)methylene)-2-oxindole and its derivatives were assessed for their inhibitory activity against Trk receptors. The IC50 values were computationally predicted in combination of molecular and fragment-based QSAR.

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An indirect capillary electrophoresis method for a quantitative determination of mono-, di- and oligosaccharides was developed to investigate biomass degradation, the isomerization of glucose into fructose and conversion of fructose to 5-hydroxymethylfurfural (5-HMF) in ionic liquids (ILs). Three chromophores, namely 2,6-pyridinedicarboxylic acid (PDC), maleic acid and phthalic acid, were used to perform indirect detection. The electroosmotic flow (EOF) was reversed to reduce analysis time, using 1-tetradecyl-3-methylimidazolium chloride (C14MImCl).

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Tertiary cyclopropanols react rapidly with Togni reagent in methanol at room temperature in the presence of catalytic amounts (3 mol%) of CuCl affording β-trifluoromethyl ketones in 65-73% isolated yields. Ring opening in 1,2-dialkylsubstituted cyclopropanols gives a mixture of isomeric β-trifluoromethyl ketones in about 50% combined yield.

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A general and efficient approach was developed for the introduction of S-functionality at the C-5 position of cytosine and uracil nucleosides and their analogues. The key step is a palladium-catalyzed C-S coupling of the corresponding 5-bromo nucleoside derivative and alkyl thiol. The butyl 3-mercaptopropionate coupling products were further converted to the corresponding disulphides, the stable precursors of 5-mercaptopyrimidine nucleosides.

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The concept of the remote activation of reactivity was first applied in asymmetric organocatalysis. An isatin 3-phenylimine derivative acts as a donor in the thiourea catalyzed asymmetric addition to unsaturated 1,4-ketoesters, affording aza-Michael adducts in high enantiomeric purity and yield.

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Starting from simple alkylidene oxindoles and nitroketones, a highly stereoselective methodology was developed for the synthesis of spiro-cyclopentaneoxindoles with four consecutive stereogenic centers. Using an organocatalytic cascade of Michael and aldol reactions in the presence of a chiral thiourea catalyst products were obtained in moderate to high yields and excellent enantioselectivities. Nitro, ester, and hydroxyl groups were introduced to the spiro ring, which could be used to facilitate further functionalization of the products.

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Article Synopsis
  • A three-component triple cascade reaction creates [3.2.0]heterobicycles with high diastereoselectivity, starting from a hetero-Michael addition to α,β-unsaturated aldehydes.
  • The reaction's rate and selectivity are influenced by the (E)-4-heterocrotonate and the size of the secondary amine used.
  • Major diastereoisomer enantiomers of oxa- and azabicyclo[3.2.0]heptane derivatives are separated via enzymatic kinetic resolution using CALB, achieving high E values, and the absolute configuration of one enantiomer is confirmed through single crystal X-ray analysis.
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The organocatalytic Michael addition of malonates to symmetric unsaturated 1,4-diketones catalyzed by thiourea and squaramide derivatives with Cinchona alkaloids afforded the formation of a new C-C bond in high yields (up to 98%) and enantiomeric purities (up to 93%). The absolute configuration of the product was suggested from comparison of the experimental and calculated VCD spectra of the reaction product 3a.

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Article Synopsis
  • * The synthesis results in high yields (up to 99%), significant diastereoselectivity (up to 11:1), and good enantiomeric purity (up to 92%).
  • * This research marks a pioneering use of 3-chloroxindoles as nucleophiles in a highly stereoselective organocatalytic process.
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A new enantiomerically pure carbacyclic nucleoside analogue with bimorpholine as a nonaromatic nucleobase was synthesized. The nucleoside analogue and bimorpholine were tested for cytotoxicity using an MTT assay and the xCELLigence System. Both assays revealed that compound 3 was highly cytotoxic at a 50 μM concentration while the cytotoxic effect of compound 1 was much less prominent.

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A new general methodology was developed to access highly enantiomerically enriched 1,4-dihydropyridines (DHPs) 3 via an organocatalytic asymmetric aza-ene-type cascade reaction, cocatalyzed by (S)-diarylprolinol-TMS ether V and benzoic acid (BA). Both aliphatic and aryl enals 1 reacted smoothly with enaminones and β-enamino esters 2, affording highly functionalized 1,4-DHPs 3 in high enantioselectivities and good yields.

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Spin-spin coupling constants between nuclei in NMR spectroscopy reflect their spatial arrangement. A number of calculation methods, applying different levels of theory, have been developed to support the stereochemical assignment of novel compounds. Nevertheless, revisions of the assignment of structures in the literature are not rare.

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A novel multicomponent cascade reaction led to the formation of a strained 3-azabicyclo[3.2.0]heptane derivative 4.

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A complex derived from the enantiomeric bipiperidine and copper(II) acetate hydrate is an efficient catalyst for the enantioselective Henry reaction. The easy availability of both catalyst components, mild reaction conditions, high yield, and good to excellent enantioselectivity make the catalyst useful for everyday practice.

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Article Synopsis
  • - The compound C(15)H(22)O(5) is a chiral intermediate used in the synthesis of biologically active 9,11-secosterols.
  • - In its crystal structure, the cyclohexane rings are arranged in a trans-fused configuration and both take on chair conformations.
  • - The molecules in the crystal are held together loosely in layers parallel to the (100) plane due to weak intermolecular hydrogen bonds involving carbonyl oxygen atoms from acetyl groups.
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