Origin of the stereo- and regioselectivities in the Diels-Alder reactions of azaphospholes: a DFT investigation

Computations of the Diels-Alder (DA) reactions of azaphosphole representative namely, thiazolo[3,2-d][1,4,2]diazaphosphole with 1,3-butadiene and isoprene at the density functional theory level reveal concerted mechanisms via asynchronous transition states. The activation energies (B3LYP/6-311++G**// B3LYP/6-311G**), 16-19 kcal mol⁻¹, are much smaller than the value (32.57 kcal mol⁻¹) calculated for the DA reaction of the non-phosphorus analogue, imidazo[2,1-b]thiazole with 1,3-butadiene. An electron-withdrawing group at the 3-position of the dienophile enhances both stereo- and regioselectivities, which agree nicely with the experimental values. Inclusion of solvent effect (PCM model) reveals that the stereo- and regioselectivities are not affected appreciably. The relative stabilities of the transition structures corresponding to the endo/exo stereoisomers and meta (P/Me, 1:3)/para (P/Me, 1:4) regioisomers have been rationalized on the basis of the secondary molecular orbital interactions.     

Isotope‐Coded Affinity Tagging Technique Using Avidin Functional Affinity Electrophoresis: An Alternative to an Avidin Affinity Column

Avidin functional affinity electrophoresis (AFAEP) is substituted for an avidin affinity column (AAC) to capture biotinylated peptides in the Isotope‐Coded Affinity Tagging (ICAT) technique which is a valuable tool in quantitative proteomics. In this new technique, the AFAEP‐captured ICAT‐labeled biotinylated peptides are extracted with the biotin tag intact from the polyacrylamide gel piece with aqueous 95% formamide (pH 8.2) at 65 °C for 20 min, and then detected by a matrix‐assisted laser desorption/ionization time‐of‐flight mass spectrometer. Bovine serum albumin (BSA) and the ¹²C‐ and ¹³C‐ICAT reagents are used to test this AFAEP‐ICAT technique. The results show that both AFAEP and AAC methods provide quantitative information of the relative amounts of ¹²C‐ and ¹³C‐ICAT‐labeled biotinylated tryptic peptides of BSA in a sample. Compared with AAC, the AFAEP is cheaper to perform, more stringent in capturing the biotinylated peptides, and capable of simultaneously processing multiple samples.     

Study of EPR, optical properties and electrical conductivity of vanadyl doped Bi2O3.PbO.B2O3 glasses

Heavy metal based oxide glasses having composition xBi(2)O(3).(0.30 - x)PbO.0.70B(2)O(3) have been prepared (0.00 < or = x < or = 0.15, mol%) containing 2.0mol% of V(2)O(5) by normal melt-quenching technique. Electron paramagnetic resonance (EPR), optical spectra and dc conductivity of these glasses have been studied. Spin Hamiltonian parameters (SHP) of VO(2+) ions, dipolar hyperfine parameter, P and Fermi contact interaction parameter, K, molecular orbital coefficients (alpha(2) and gamma(2)) and optical band gap have been calculated. It is observed that in these glasses, the tetragonal nature of V(4+)O(6) complex increases with Bi(2)O(3) content. Increase in Bi(2)O(3):PbO ratio results in the contraction of 3d(xy) orbit of the unpaired electron in the vanadium ion, and the SHP are dependent on the theoretical optical basicity, Lambda(th). In present glasses, the conductivity (activation energy) first decreases (increases) with increase in mol% of Bi(2)O(3) content upto x = 0.08 and then shows a maxima (minima) at x = 0.10 and then starts decreasing (increasing) upto x < or = 0.15 with mol% of Bi(2)O(3) content.     

Resolution of (+/-)-ibuprofen using (-)-brucine as a chiral selector by thin layer chromatography

The enantiomeric resolution of (+/-)-ibuprofen into its enantiomers was achieved by TLC on silica gel plate using optically pure (-)-brucine as a chiral selector and acetonitrile-methanol (5:1, v/v) as the solvent system. Spots were located in an iodine chamber. The detection limit was 4.9 microg. The effect of concentration of the chiral selector, temperature and pH on resolution has been studied.     

Reactions of dimethyl acetylenedicarboxylate—X: Reaction with salicylaldehyde,ortho hydroxyacetophenone, 2-hydroxychalcones and 2,2′-dihydroxychalcones

Salicylaldehyde reacts with dimethyl acetylenedicarboxylate in benzene solution to give a mixture of dimethyl o-formylphenoxymaleate, dimethyl o-formylphenoxyfumarate, 2,3-dicarbomethoxychrom - 2 - en - 4 - ol, 2,3 - dicarbomethoxychrom - 3 - en - 2 - ol, dimethyl (2,3 - dicarbomethoxychrom - 2 - en - 4 - yl) - oxalacetate and dimethyl fumarate. 2,3 - Dicarbomethoxychrom - 3 - en - 2 - ol in this reaction is formed from 2,3 - dicarbomethoxychrom - 2 - en - 4 - ol through a benzopyrylium intermediate. The reaction of salicylaldehyde with excess of dimethyl acetylenedicarboxylate, however, gives a mixture of 2,3 - dicarbomethoxychrom - 3 - en - 2 - ol and dimethyl (2,3 - dicarbomethoxychrom - 3 - en) - 2 - oxyfumarate. 2,3 - Dicarbomethoxychrom - 3 - en - 2 - ol itself reacts further with dimethyl acetylenedicarboxylate to give 2,3,12 - tricarbomethoxychrom - 3,4 - eno[2,3 - b] pyrone. Similarly, the reaction of o-hydroxyacetophenone with dimethyl acetylendicarboxylate gives a mixture of dimethyl o-acetylphenoxymaleate, dimethyl o-acetylphenoxyfumarate, 2,3 - dicarbomethoxy - 4 - methylchrom - 2 - en - 4 - ol and 2,3 - dicarbomethoxy - 4 - methylchrom - 3 - en - 2 - ol. Both 2-hydroxychalcone and 2 - hydroxy - 4′ - methoxychalcone give mixtures of chalcone - 2 - oxymaleate and chalcone - 2 - oxyfumarate. The reaction of 2,2t?-dihydroxychalcone, however, gives 2′-hydroxyflavone, in addition to the expected maleate and fumarate. Similar reactions of 5 - chloro - 2,2′ - dihydroxychalcone and of 3,5 - dibromo - 2,2′ - dihydroxychalcone, on the other hand, give 2,3 - dicarbomethoxy - 4 - (o - hydroxyphenacyl) - 6 - chlorochrom - 2 - ene and 2,3 - dicarbomethoxy - 4 - (o - hydroxyphenacyl) 6,8 - dibromochrom - 2 - ene, respectively, together with the corresponding maleates and fumarates.     

Über triterpene—XXIX : Zur struktur des avenacins

From the roots of oat (Avena sativa) a second antibiotic active glycoside, avenacine B, was isolated in addition to the main glycoside avenacine A. In the aglycone of A the double bond could not be established contrary to the statement in the literature but an aldehyde group in addition to the keto group was identified and therefore the proposed structure has to be revised. In the aglycone of B the —CH₂OH-group of A has been substituted by CH₃. The structures of the sugar chains in both glycosides have been determined, in avenacine A as [β-d-gluco-pyranosyl(1 → 4)-[β-d-glucopyranosyl (1 → 2)]-α-l-arabinosyl (1)- and in B as [β-d-glucopyranosyl (1 → 4)]-β-d-glucopyranosyl (1)-.     

Photometric determination of trivalent gallium,indium and thallium with Xylenol orange

Summary Xylenol orange reacts very sensitively with gallium(III), indium (III) and thallium(III) to form reddish violet colored chelates having _max 560 nm in case of Ga and In and _max 590 nm in case of Ti at P_H 4.0. The molar ratio for all the chelates is 1 1 (metal reagent). Optimum conditions including the range for adherence to Beer's law, effect of P_H on the color intensity, effect of excess reagent, and sensitivity are reported for the photometric determination of these metal ions using Xylenol orange.

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Zusammenfassung Xylenolorange reagiert sehr empfindlich mit Gallium(III), Indium(III) und Thallium(III) unter Bildung rötlich-violetter Chelate mit einem Absorptionsmaximum bei 560 nm im Falle von Ga und In bzw. 590 nm für Tl bei p_H 4,0. Das Molverhältnis ist in jedem Fall 11. Die besten Arbeitsbedingungen, der Gültigkeitsbereich des Beerschen Gesetzes, der Einfluß des P_H auf die Farbintensität und des Reagensüberschusses sowie die Empfindlichkeit werden für die photometrische Bestimmung der genannten Ionen angegeben.

     

Hydrophobic interaction chromatography coupled with atomic fluorescence spectrometric detection Effect of the denaturation on the determination of thiolic proteins

Hydrophobic interaction chromatography coupled online with chemical vapour atomic fluorescence spectrometry (HIC-CVGAFS) has been optimized for the analysis of thiolic proteins in denaturing conditions. Proteins are pre-column simultaneously denatured and derivatized in phosphate buffer solution containing 8.0 mol dm⁻³ urea and p-hydroxymercurybenzoate (PHMB) and the derivatized denatured proteins are separated on a silica HIC Eichrom Propyl column in the presence of 8.0 M urea in the mobile phase. Post-column online reaction of derivatized denatured proteins with bromine, generated in situ by KBr/KBrO₃ in HCl medium, allowed the fast conversion of the uncomplexed PHMB and of the PHMB bound to proteins to inorganic mercury also in presence of urea. Hg²⁺, present in solution as Hg²⁺-urea complex, is selectively detected by AFS in a Ar/H₂ miniaturized flame after sodium borohydride reduction to Hg. Under optimized conditions, online bromine treatment gives a 100±2% recovery of both free and protein-complexed PHMB. Denatured glyceraldehyde-3-phosphate dehydrogenase, aldolase, lactate dehydrogenase, trioso phosphate isomerase and β-lactoglobulin have been examined. As the sensitivity and limit of detection of proteins in the HIC-CVGAFS apparatus depends on number of SH groups reacting with PHMB, the denaturation process, which increases the number of PHMB-reactive thiolic groups in proteins, improves the analytical performances of the described system in protein analysis. The detection limit for the denatured proteins examined was found in the range of 10⁻¹⁰-10⁻¹² mol dm⁻³, depending on the considered protein, with linear calibration curves spanning over four decades of concentration.     

Ternary complexes of copper(II), nickel(II) and zinc(II) with nitrilotriacetic acid as a primary ligand and some phenolic acids as secondary ligands

Summary The formation of ternary complexes of the MAL^3– type [where M = Cu^II, Ni^II and Zn^II ; A = nitrilotriacetic acid (NTA); L = 1-hydroxy-2-naphthoic acid (1,2 HNA) and 2-hydroxy-1-naphthoic acid (2,1 HNA)] have been studied potentiometrically in 50% v/v aqueous — ethanol (25° and µ = 0.1). Under identical conditions the binary complexes of the 1,2- and 2,1-HNA ligands have also been examined. The values of mixed ligand formation constants K_MAL have been found to be lower than K_ML (first step formation constant of binary complexes) and even less than (second step formation constant of binary complexes).     

Synthesis of (±) lupinifoun, di-o-methyl xanthohumol and isoxanthohumol and related compounds

Nuclear prenylation of naringenin (7) with 2-methylbut-3-en-2-ol in the presence of boron trifluoride etherate gives a mixture of 6-C-prenyl-(11), 8-C-prenyl-(15) and 6,8-di-C-prenyl-(8) derivatives. On formic acid cyclisation, 11 yielded two monodihydropyrans (12 and 13), but 15 afforded only one viz 16; similarly 8 formed the bisdihydropyran 10. Methylation of 8-C-prenyl naringenin (15) with Me₂SO₄ resulted in the formation of di-O-methyl derivatives of xanthohumol (22) and isoxanthohumol (23).

Cyclodehydrogenation of 6,8-di-C-prenyl-naringenin (8) with DDQ gave a mono-C-prenyl-2,2-dimethylpyran (1) corresponding to (±) lupinifolin. The angular isomer (2) was also formed. The structure of natural flemichin-B therefore needs further consideration. Similarly, cyclodehydrogenation of 6-C-(11)- and 8-C-prenyl-(15) naringenins afforded the corresponding linear (24) and angular (25) derivatives which have been characterized by conversion into known chalcones 26 and 27 by O-methylation.