Novel pyrrolidine-thiohydantoins/thioxotetrahydropyrimidinones as highly effective catalysts for the asymmetric Michael addition

The synthesis of novel organocatalysts consisting of a pyrrolidine moiety and a thiohydantoin or a thioxotetrahydropyrimidinone ring is described. The compound combining the pyrrolidine with the thioxotetrahydropyrimidinone was found to be a highly effective catalyst for the Michael reaction. Low catalyst loadings (1-2.5%) can be employed leading to quantitative yields and excellent stereoselectivities in the reaction between cyclic ketones and nitroolefins.     

Modification of Guanine with Photolabile N‐Hydroxypyridine‐2(1H)‐thione: Monomer Synthesis,Oligonucleotide Elaboration,and Photochemical Studies

The photochemistry of N‐hydroxypyridine‐2(1H)‐thione (NHPT), inserted as a photolabile modifier at the 6‐position of 2′‐deoxyguanosine or guanosine, has been evaluated. In particular, 6‐[(1‐oxidopyridin‐2‐yl)sulfanyl]‐ ( 1a ) and 6‐[(pyridin‐2‐yl)sulfanyl]‐2′,6‐dideoxyguanosine ( 2a ), novel photolabile derivatives of the natural nucleosides, were synthesized and characterized. The observed photolysis products of 1a in organic solvents could only be rationalized by assuming a rapid equilibrium with the corresponding 6‐[(2‐thioxopyridin‐1(2H)‐yl)oxy] analogue 3a (Scheme 5). Transient spectroscopy of 1a indicated a strong triplet‐excited state suitable for triplet → triplet energy transfer or singlet‐oxygen generation. The NHPT function was stable enough for (slightly modified) automated solid‐phase oligonucleotide synthesis. The utility of the above compounds is discussed, as well as their potential use in photosensitization of reactive oxygen species in DNA.     

Furanyl nucleosides: synthesis and kinetics of their formation

The thermal reaction (at 140 °C) of various 1′,2′-didehydro-2′-deoxynucleosides afforded the corresponding furanyl nucleosides in good yields. The reaction kinetics were monitored by ¹H NMR and the mechanism in terms of `four-center complex fission' is discussed.     

Isolation of a Dodecanuclear Polyoxo Cluster {Nb12O21} Showing a Rare Case of Five-fold Coordinated Niobium(V) Centers with a Square Pyramidal Geometry

The reactivity of the complexing anthracene-9-carboxylate ligand has been investigated with a niobium(IV) tetrachloride precursor (NbCl₄ ⋅ 2THF) in isopropanol solvent. This resulted in the crystallization of a molecular assembly containing two distinct {Nb₁₂O₂₁} cores surrounded by multiple isopropanolate and anthracenoate ligands. The compound is formulated [Nb₁₂₍(μ₃-O)₃(μ-O)₁₈(C₁₅H₉O₂)₈(OⁱPr)₁₀] ⋅ [Nb₁₂(μ₃-O)₂(μ-O)₁₉(C₁₅H₉O₂)₈(OⁱPr)₁₀] illustrating the two different dodecameric oxo-clusters, for which the niobium(IV) precursor was oxidized in the niobium(V) state during the reactional process. The two distinct {Nb₁₂O₂₁} units mainly differs by the environment of the niobium centers, which exhibits unexpected five-fold coordination (square pyramid) for some of them, together with the classical six-fold coordination (octahedron) as usually found for niobium(V). In the crystallization process, the. IR spectroscopy was used to analyze the esterification reaction occurring between the anthracene acid an isopropanolate ligands responsible of the production of water used in the oxo-condensation of the niobium centers. ⁹³Nb Solid state NMR was tentatively used to assess the occurrence of the different niobium environments.     

Synthesis and self‐assembly of thermoresponsive poly(N‐isopropylacrylamide)‐b‐poly(oligo ethylene glycol methyl ether acrylate) double hydrophilic block copolymers

We report on the synthesis of novel poly(N‐isopropylacrylamide)‐b‐poly(oligo ethylene glycol methyl ether acrylate) (PNIPAM‐b‐POEGA) thermoresponsive block copolymers using reversible addition–fragmentation chain transfer polymerization methodologies. The synthesized block copolymers are characterized by gel permeation chromatography, nuclear magnetic resonance, Fourier transform infrared (FTIR) techniques in terms of molecular weight and composition. Their thermoresponsive self‐assembly in aqueous media is investigated using dynamic and static light scattering. The PNIPAM‐b‐POEGA thermoresponsive block copolymers formed aggregates in water by increasing the temperature above the lower critical solution temperature value of PNIPAM block. Solution pH seems to affect the self‐assembly behavior in some cases due to the presence of ? COOH end groups. Therefore, the copolymers were utilized as “smart” nanocarries for the hydrophobic drug indomethacin, implementing a novel encapsulation protocol taking advantage of the thermoresponsive character of the PNIPAM block. The empty and loaded self‐assembled nanocarriers systems were studied by light scattering techniques, ultraviolet–visible, and FTIR spectroscopy, which gave information on the size and structure of the nanocarriers, the drug loading content and the interactions between the drug and the components of the block copolymers. Drug loaded nanostructures show stability at room temperature, due to active drug/block copolymer interactions. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019, 57, 1467–1477     

Adsorption of Remazol Red 3BS from aqueous solutions using APTES- and cyclodextrin-modified HMS-type mesoporous silicas

The removal of Remazol Red 3BS (C.I. 239) dye by HMS ordered mesoporous silica, aminopropyl-modified HMS (HMS-NH₂) and β-cyclodextrin-modified HMS (HMS-CD) materials was studied in the present work. The modified materials were functionalized in situ by adding the organic modifiers (3-aminopropyltriethoxysilane and a silylated derivative of MCT-β-CD) in the synthesis mixture and using dodecylamine as the mesopore structure directing agent. The successful incorporation of aminopropyl groups in HMS-NH₂ and of cyclodextrin moieties in HMS-CD was verified by means of FT-IR spectroscopy, elemental analysis and N₂ porosimetry. The HMS-CD material exhibited significantly higher adsorption capacity compared to that of the HMS-NH₂ material, while the parent HMS mesoporous silica showed negligible adsorption capability. The maximum adsorption capacities obtained (at the optimum pH 2) on the basis of the Langmuir analysis were 0.28 mmol/g for HMS-CD and 0.14 mmol/g for HMS-NH₂. It was shown that the HMS-CD sorbent can be effectively regenerated by the surfactant-enhanced regeneration method using SDS and that can be reused without significant loss of its adsorption capabilities.     

Structural Variety of Niobium(V) Polyoxo Clusters Obtained from the Reaction with Aromatic Monocarboxylic Acids: Isolation of {Nb2O}, {Nb4O4} and {Nb8O12} Cores

The reactivity of aryl monocarboxylic acids (benzoic, 1- or 2-naphtoic, 4’-methylbiphenyl-4-carboxylic, and anthracene-9-carboxylic acids) as complexing agents for the ethoxide niobium(V) (Nb(OEt)₅ precursor has been investigated. A total of eight coordination complexes were isolated with distinct niobium(V) nuclearities as well as carboxylate complexation states. The use of benzoic acid gives a tetranuclear core Nb₄(μ₂-O)₄(L)₄(OEt)₈] (L=benzoate ( 1 )) with four Nb−(μ₂-O)−Nb linkages in a square plane configuration. A similar tetramer, 7 , was obtained with 2-naphtoic acid by using a 55 % humid atmosphere synthetic route. Two types of dinuclear brick were identified with one central Nb−(μ₂-O)−Nb linkage; they differ in their complexation state, with one bridging carboxylate ([Nb₂(μ₂-O)(μ₂-OEt)(L)(OEt)₆], with L=1-naphtoate ( 3 ) or anthracene-9-carboxylate ( 5 )) or two bridging carboxylate groups ([Nb₂(μ₂-O)(L)₂(OEt)₆], with L=4’-methylbiphenyl-4-carboxylic ( 4 ) or anthracene-9-carboxylate ( 6 )). An octanuclear moiety [Nb₈(μ₂-O)₁₂(L)₈(η₁-L)_4−x(OEt)_4+x] (with L=2-naphtoate, x=0 or 2; 8 ) was obtained by using a solvothermal route in acetonitrile; it has a cubic configuration with niobium centers at each node, linked by 12 μ₂-O groups. The formation of the niobium oxo clusters was characterized by infrared and liquid ¹H NMR spectroscopy in order to analyze the esterification reaction, which induces the release of water molecules that further react through oxolation with niobium atoms, in different {Nb₂O}, {Nb₄O₄} and {Nb₈O₁₂} nuclearities.