A physical organic chemistry approach to dissolution of cellulose: effects of cellulose mercerization on its properties and on the kinetics of its decrystallization

The effects of alkali treatment on the structural characteristics of cotton linters and sisal cellulose samples have been studied. Mercerization results in a decrease in the indices of crystallinity and the degrees of polymerization, and an increase in the alpha-cellulose contents of the samples. The relevance of the structural properties of cellulose to its dissolution is probed by studying the kinetics of cellulose decrystallization, prior to its solubilization in LiCl/N,N-dimethylacetamide (DMAc). Our data show that the decrystallization rate constants and activation parameters are only slightly dependent on the physico-chemical properties of the starting celluloses. This multi-step reaction is accompanied by a small enthalpy and large, negative, entropy of activation. These results are analyzed in terms of the interactions within the biopolymer chains during decrystallization, as well as those between the two ions of the electrolyte and both DMAc and cellulose.

Metal-free synthesis of indanes by iodine(III)-mediated ring contraction of 1, 2-dihydronaphthalenes

A metal-free protocol was developed to synthesize indanes by ring contraction of 1, 2-dihydronaphthalenes promoted by PhI(OH)OTs (HTIB or Koser's reagent). This oxidative rearrangement can be performed in several solvents (MeOH, CH3CN, 2 , 2, 2-trifluoroethanol (TFE), 1 , 1, 1, 3, 3, 3-hexafluoroisopropanol (HFIP), and a 1:4 mixture of TFE:CH2Cl2) under mild conditions. The ring contraction diastereoselectively gives functionalized trans-1, 3-disubstituted indanes, which are difficult to obtain in synthetic organic chemistry

Rhodium-catalyzed carbonylation of allylaminoalcohols: Catalytic synthesis of N-(2-hydroxy-alkyl)-gamma-lactams and bicyclic oxazolidines

Gamma-lactams and bicyclic oxazolidines are important structural frameworks in both synthetic organic chemistry and related pharmacological fields. These heterocycles can be prepared by the rhodium-catalyzed carbonylation of unsaturated amines. In this work, allylaminoalcohols, derived from the aminolysis of cyclohexene oxide, styrene oxide, (R)-(+)-limonene oxide, and ethyl-3-phenyl-glicidate, were employed as substrates. These allylaminoalcohols were carbonylated by employing RhClCO(PPh3)(2) as a precatalyst under varying CO/H-2 mixtures, and moderate to excellent yields were obtained, depending on the substrate used. The results indicated that an increase in the chelating ability of the substrate (-OH and -NHR moieties) decreased the conversion and selectivity of the ensuing reaction. Additionally, the selectivity could be optimized to favor either the gamma-lactams or the oxazolidines by controlling the CO/H-2 ratio. A large excess of CO provided a lactam selectivity of up to 90%, while a H-2-rich gas mixture improved the selectivity for oxazolidines, resulting from hydroformylation/cyclization. Studies of the reaction temperature indicated that an undesirable substrate deallylation reaction occurs at higher temperature (>100 degrees C). Further, kinetic studies have indicated that the oxazolidines and gamma-lactams were formed through parallel routes. Unfortunately, the mechanism for oxazolidines formation is not yet well understood. However, our results have led us to propose a catalytic cycle based on hydroformylation/acetalyzation pathways. The gamma-lactams formation follows a carbonylation route, mediated by a rhodium-carbamoylic intermediate, as previously reported. To this end, we have been able to prepare and isolate the corresponding iridium complex, which could be confirmed by X-ray crystallographic analysis. (C) 2008 Elsevier B.V. All rights reserved.

Facile Synthesis of Koser`s Reagent and Derivatives from Iodine or Aryl Iodides

The first one-pot synthesis of neutral and electron-rich [hydroxy(tosyloxy)iodo]arenes (HTIBs) from iodine and arenes is presented, thereby avoiding the need for expensive iodine(III) precursors. A large set of including a polyfluorinated analogue, can be obtained from the corresponding aryl iodide under the same conditions. The reaction proceeds under mild conditions, without excess reagents, and is fast and high-yielding. Together, the two presented routes give access to a wide range of HTIBs, which are useful reagents in a variety of synthetic transformations.

Tellurium in organic synthesis: the enantioselective synthesis of the pheromone blend components of Mayetiola destructor, Drosophila mulleri and Contarinia pisi

The components of the pheromone blend of Mayetiola destructor, Drosophila mulleri, and Contarinia pisi were synthesized in high enantiomeric excess (99% ee) from a common enantiopure dianion prepared from an enantiopure hydroxytelluride. (C) 2009 Elsevier Ltd. All rights reserved.

Tellurium tetrachloride: an improved method of preparation

An efficient and practical synthesis of tellurium tetrachloride from elemental tellurium and sulfuryl chloride is described. (C) 2008 Elsevier Ltd. All rights reserved.

Synthesis of 4-iodopyrazoles: A Brief Review

The selective incorporation of halogen into organic molecules provides a challenge to academic and industrial research. This microreview presents an overview of the available methodologies for the synthesis of 4-iodopyrazoles, valuable precursors for the selective construction of highly functionalized organic molecules of synthetic and biological importance.

Catalytic Enantioselective alpha-Arylation of Carbonyl Compounds

Enantioselective creation of benzylic quaternary centers still is a continuous challenge to many synthetic organic chemists. Among the existing methods for installation of this type of center, the direct asymmetric alpha-arylation of carbonyl compounds is very attractive due to the ready availability of the coupling substrates. Herein, we present some new tools to the catalytic asymmetric alpha-arylation of carbonyl compounds that overcame many of the drawbacks posted in previous methods for this type of reaction.

Extração de β-caroteno de cenouras: uma proposta para disciplinas experimentais de química

This paper reports on the development of a simple and fast procedure for β-carotene extraction from carrots and its quantification by UV/Vis spectroscopy. Carotenoids extracted from carrots may also be used as alternative reagents for TLC (thin layer chromatography) detection of natural compounds with antioxidant properties, replacing the commercial p.a. grade β-carotene. Although this reagent had around 10% b-carotene, it proved to be as efficient for TLC analysis as the commercial p.a. grade β-carotene. This practice is a useful alternative for teaching undergraduate organic chemistry laboratory classes.

Aspectos mecanísticos da bioatividade e toxicidade de nitrocompostos

Nitrocompounds are bioactive molecules used as antibacterial, antiparasitic and antitumoral agents. In the past of years, these molecules have been broadly studied in several fields, such as medicinal chemistry, organic chemistry, biochemical, toxicology and electrochemistry. The nitrocompounds mode of action involves the biotransformation of the nitro group, releasing intermediates in the redox process. Some of those intermediates attack enzymes, membranes and DNA, providing the basis for their biological activity and adverse effects. In this report, some aspects regarding the biological activity, mechanism of action and toxicity of nitrocompounds are explored, purposing the research of new bioactive derivatives having low toxicity.

Mechanistic aspects of the isomerization of Z-vinylic tellurides double bonds in the synthesis of potassium Z-vinyltrifluoroborate salts

Through direct transmetalation reaction of Z-vinylic tellurides with nBuLi was observed the unexpected isomerization of double bonds leading to potassium E-vinyltrifluoroborates salts in low to moderate yields. Using EPR spin trapping experiments the radical species that promoted the stereoinversion of Z-vinylic organometallic species during the preparation of potassium vinyltrifluoroborate salts was identified. The experiments support the proposed mechanism, which is based on the homolytic cleavage of the TenBu bond.

Regiospecific synthesis of 5-trichloromethyl-1H-pyrazole and 1H-pyrazole-5-carboxylic ester derivatives

Reaction of 1,1,1-trichloro-4-methoxy-3-penten-2-one (1) with hydrazines (2a-h) (NH2NHR, R = H, Me, t-Bu, Ph, 4-NO2-C6H4, C6F5, CO2Me, CONH2) under differing conditions regiospecifically affords different pyrazole derivatives, 3-methyl-5-trichloromethyl-5-hydroxy-4,5-dihydropyrazoles (3a, d-h), 3-methyl-5-trichloromethyl-1H-pyrazoles (4a,b,d-g) and 5-carboxyethyl-3-methyl-1H-pyrazoles (5a-e). The structural assignments were based on the analysis of their H-1/C-13 NMR and ESI-MS data.

Antioxidant Capacity of 2-(3,5-diaryl-4,5-dihydro-1H-pyrazol-1-yl)-4-phenylthiazoles

The antioxidant capacity of 2-(3,5-diaryl-4,5-dihydro-1H-pyrazol-1-yl)-4-phenylthiazoles was evaluated. The values of antioxidant capacities of compounds 2d and 2e were found to be, respectively, 2,700 +/- 150 and 3,135 +/- 230 TE by the ORAC method, corresponding to a significant antioxidant capacity.

Chiral Triphenylprolinol Ligands for the Efficient Catalytic Asymmetric Arylation of Aldehydes

The synthesis of new chiral amino alcohols by Heck arylation of an enecarbamate is described. These compounds were used as chiral ligands for the catalytic asymmetric arylation of aldehydes and can be easily recovered. Chiral, nonracemic diarylmethanols were obtained in high yields and enantioselectivities.

Enantioselective Arylations Catalyzed by Carbohydrate-Based Chiral Amino Alcohols

The application of carbohydrate-derived amino alcohols in the asymmetric arylation of aldehydes by using arylboronic acids as the source of transferable aryl groups is described. The best ligand is derived from the readily available sugar D-xylose and it mediates the addition of a range of arylboronic acids to various aromatic aldehydes in excellent yields and high enantiomeric excesses.