Studies in Competitive [3,3] Sigmatropic Rearrangement: Synthesis of Oxygen Heterocycles

[3,3] Sigmatropic rearrangement of unsymmetrically substituted 1,4-but-2-ynes exhibit preference for rearrangement on the aryloxy prop-2-ynyl moiety to N-methyl-N-pyrimidinyl amino prop-2-ynyl moiety in case of 5-substituted uracils (la-f).     

One-Step Protection-Free Synthesis of 3-Aryl-5-hydroxyalkyl-1,2,4-oxadiazoles as Building Blocks

A simple and straightforward synthesis of 3-aryl-5-hydroxyalkyl-1,2,4-oxadiazoles is described. The reaction among arylamidoximes, ethyl glycolate or ethyl lactate, and potassium carbonate in refluxing toluene afforded the desired 1,2,4-oxadiazoles in moderate to good yields. The synthesis has been accomplished in a single step, avoiding protection–deprotection protocols.

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The unambiguous synthesis and NMR assignment of 4-alkoxy and 3-alkylquinazolines

Contrary to a number of reports, alkylations of the privileged 3,4-dihydroquinazoline scaffold provide N3-alkylated products, and not 4-alkoxyquinazolines. To correctly assign the structure, ¹³C NMR shifts of the –Z–CH_n– (Z=O, N) fragment are necessary; resonances in the 45–55 ppm range are indicative of N3-alkylation. Treatment of 3,4-dihydroquinazoline-4-one with p-TsCl afforded the N3-tosylated compound, whose reaction with an amine yielded the corresponding N3-alkyl derivative. A mechanism corroborated by ¹⁵N-labeling involving pyrimidine ring opening and recyclisation is proposed. Finally, the unambiguous preparation of 4-alkoxyquinazolines is described via treatment of 3,4-dihydroquinazoline-4-ones with PCl₅ followed by an alkoxide.     

The preparation of the 239Np tracer from 243Am and the purification of the stock solution

The present work describes preparation of ²³⁹Np tracer from ²⁴³Am stock solution and the purification of this solution from ferric cation. The method of the preparation of tracer involves stabilization of Np(IV) by ascorbic acid and ferric nitrate, separation of ²³⁹Np from ²⁴³Am by extraction chromatography and determination of recoveries of ²³⁹Np by means of gamma spectroscopy. We used the commercially available sorbents TEVA^®Resin for the ²³⁹Np preparation and DGA Resin for ²⁴³Am purification. All sorbents were purchased from Eichrom Industries, Inc. The first eluate from the column can be stored for a future preparation of the tracer and fraction with ²³⁹Np will be used to monitor radiochemical yield of ²³⁷Np.     

Magnetic hollow poly(N-isopropylacrylamide-co-N,N′-methylenebisacrylamide-co-glycidyl acrylate) particles prepared by inverse emulsion polymerization

To prepare functionalized magnetic polymer particles that are thermally responsive, inverse emulsion copolymerization of N-isopropylacrylamide, N,N′-methylenebisacrylamide and glycidyl acrylate (GA) was investigated in paraffin oil in the presence of γ-Fe₂O₃ nanoparticles dispersed in a water/glycerol mixture. The resulting polymer particles were characterized regarding the morphology, size, polydispersity, iron content, and the temperature-dependent phase transition using optical microscopy, transmission electron microscopy, scanning electron microscopy, atomic absorption spectroscopy, and differential scanning calorimetry. Magnetic properties were examined using hysteresis loop measurements and by analyzing the magnetic susceptibility with respect to temperature. We have also investigated the influence of the concentration of γ-Fe₂O₃ and GA in monomers on properties of the particles (morphology, size, and presence of oxirane groups). The particles possessed a hollow structure as a result of phase separation between water/glycerol hydrophilic solvents in the polymerization feed and the forming polymer. Depending on the concentration of γ-Fe₂O₃ in the monomer phase, the magnetic hollow particles contained 5–24 wt% iron. In water, the particles gradually collapsed when the temperature was raised to 40 °C because the elevated temperature weakened hydration and the PNIPAAm chains gradually became more hydrophobic.     

Stereoselective total synthesis of protected sulfamisterin and its analogues

The stereoselective synthesis of sulfamisterin I and its unnatural analogues II and V in their protected form was achieved through a common strategy. The Wittig reaction of aldehydes VIII and IX with the C₁₄ hydrophobic side-chain X served as the key C-C connecting transformation. Subsequent functional group inter-conversions in the coupling products XI and XX completed the total synthesis.     

Synthesis of Some Unusual (1,2,4‐Oxadiazole)‐Linked Hexenopyranosides and Mannopyranosides

A copper‐catalyzed reaction of propargyl 4,6‐di‐O‐acetyl‐2,3‐dideoxy‐α‐D‐erythro‐hex‐2‐enopyranoside with 3‐(4‐azidophenyl)‐1,2,4‐oxadiazoles gave the corresponding hexenopyranosides bearing an 1,2,4‐oxadiazole subunit in the aglyconic part of the molecule. The same reaction between ethyl 4‐azido‐2,3,4‐trideoxy‐α‐D‐erythro‐hex‐2‐enopyranoside and acetylenic 1,2,4‐oxadiazoles afforded the corresponding hexenopyranosides carrying a triazole and a 1,2,4‐oxadiazole ring at C‐4 of the carbohydrate. Combination of the two sequences gave hexenopyranosides displaying two 1,2,4‐oxadiazole subunits, each one being embedded in the C‐1 and C‐4 frameworks, of the carbohydrate moiety. A simple dihydroxylation reaction of these unsaturated carbohydrates yielded a series of mannopyranosides bearing one or two 1,2,4‐oxadiazole subunits at C‐1 or C‐4. These new compounds were evaluated for their cytotoxic activities against two cell strains: NCI‐H₂₉₂ (lung carcinoma) and Hep‐2 (larynx carcinoma), some of them presenting impressive cell growth inhibitions.     

Coordination of extraframework Li+ cation in the MCM-22 and MCM-36 zeolite: FTIR study of CO adsorbed

Nature and population of Li⁺ cationic sites in MCM-22 zeolite and its pillared form (MCM-36) were investigated by means of adsorption of CO as a probe molecule. CO stretching frequency and adsorption heat were measured by FTIR spectroscopy and adsorption microcalorimetry. Intrazeolitic carbonyl complexes on Li⁺ cations in MCM-22 and MCM-36 are characterized by two main vibrational bands at 2,195 and 2,188 cm^?1. Band at higher wavenumbers is ascribed to carbonyls on Li⁺ ions coordinated only to two oxygen atoms at the intersection of 10-ring channels and interacting with CO molecule by energy around 45 kJ mol^?1. Band at 2,188 cm^?1 was assigned to the carbonyls on Li⁺ cations located on top of 5 or 6-rings on the channel walls and coordinated to three or four oxygen atoms, interacting with CO molecule by energy 33–36 kJ mol^?1. Effect of pillaring and layered form of zeolite on nature and population of Li⁺ cationic sites is also discussed, as well as the formation of dicarbonyl complexes.     

Aggregation Effects in Visible‐Light Flavin Photocatalysts: Synthesis,Structure, and Catalytic Activity of 10‐Arylflavins

A series of 10‐arylflavins (10‐phenyl‐, 10‐(2′,6′‐dimethylphenyl)‐, 10‐(2′,6′‐diethylphenyl)‐, 10‐(2′,6′‐diisopropylphenyl)‐, 10‐(2′‐tert‐butylphenyl)‐, and 10‐(2′,6′‐dimethylphenyl)‐3‐methylisoalloxazine ( 2 a – f )) was prepared as potentially nonaggregating flavin photocatalysts. The investigation of their structures in the crystalline phase combined with ¹H‐DOSY NMR spectroscopic experiments in CD₃CN, CD₃CN/D₂O (1:1), and D₂O confirm the decreased ability of 10‐arylflavins 2 to form aggregates relative to tetra‐O‐acetyl riboflavin ( 1 ). 10‐Arylflavins 2 a – d do not interact by π–π interactions, which are restricted by the 10‐phenyl ring oriented perpendicularly to the isoalloxazine skeleton. On the other hand, N3? H???O hydrogen bonds were detected in their crystal structures. In the structure of 10‐aryl‐3‐methylflavin ( 2 f ) with a substituted N3 position, weak C? H???O bonds and weak π–π interactions were found. 10‐Arylflavins 2 were tested as photoredox catalysts for the aerial oxidation of 4‐methoxybenzyl alcohol to the corresponding aldehyde (model reaction), thus showing higher efficiency relative to 1 . The quantum yields of 4‐methoxybenzyl alcohol oxidation reactions mediated by arylflavins 2 were higher by almost one order of magnitude relative to values in the presence of 1 .     

Benzofurazane as a New Redox Label for Electrochemical Detection of DNA: Towards Multipotential Redox Coding of DNA Bases

Benzofurazane has been attached to nucleosides and dNTPs, either directly or through an acetylene linker, as a new redox label for electrochemical analysis of nucleotide sequences. Primer extension incorporation of the benzofurazane‐modified dNTPs by polymerases has been developed for the construction of labeled oligonucleotide probes. In combination with nitrophenyl and aminophenyl labels, we have successfully developed a three‐potential coding of DNA bases and have explored the relevant electrochemical potentials. The combination of benzofurazane and nitrophenyl reducible labels has proved to be excellent for ratiometric analysis of nucleotide sequences and is suitable for bioanalytical applications.