Chemoselective nucleophilic fluorination induced by selective solvation of the SN2 transition state

Reaction of the fluoride ion with secondary alkyl halides leads to 90% of elimination reaction and only 10% of nucleophilic substitution in dipolar aprotic solvents. Adding water to the organic phase, the SN2 yield increases in the cost of decreased reactivity. Using ab initio calculations, we have shown that it is possible to increase the reaction rate and the selectivity toward the SN2 process through supramolecular organocatalysis. The catalytic concept is based on selective solvation of the transition state through two hydrogen bonds provided by the 1,4-benzenedimethanol. The two hydrogen bonds between the catalyst and the SN2 transition state favor this pathway while just one strong hydrogen bond between the catalyst and the fluoride ion leads to a lower stabilization of the nucleophile, resulting in a higher reaction rate. Our calculations predict that the substitution product increases to 40% yield because of the selective catalysis provided by the 1,4-benzenedimethanol.     

Asymmetric intermolecular heck-type reaction of acyclic alkenes via oxidative palladium(II) catalysis

Herein, we report an asymmetric intermolecular Heck-type reaction of acyclic alkenes by using a palladium-pyridinyl oxazoline diacetate complex under oxidative palladium(II) catalysis conditions. A premade palladium-ligand complex afforded higher enantioselectivities than a corresponding premixed palladium-ligand system, while offering enhanced asymmetric induction when compared to known intermolecular Heck-type protocols.     

Base sequence- and <Emphasis Type="Italic">T</Emphasis><Subscript>m</Subscript>-dependent DNA oligomer separation by open tubular capillary columns carrying complementary DNA oligomers as probes

DNA chips prepared on a flat glass surface have unavoidable drawbacks when used for quantitative analysis. In an attempt to overcome this problem, we constructed an HPLC-type system suitable for quantitative analysis that enables base sequence- and T _m-dependent DNA oligomer separation in a flow system. A small open tubular capillary column (300-mm × 100-μm I.D.) was used. The DNA oligomers used as probes had an amino group at the 5′-end and were immobilized on the inner silica surface of the capillary column which had been sequentially treated with 3-aminopropyltriethoxysilane, butyltrimethoxysilane, and disuccinimidylglutarate. Using the combination of probe-immobilized column placed in a column oven equipped with temperature gradient function, a nano-flow-controllable pump, a small sample-loading injector, and a capillary-fitted UV detector, we succeeded in separating complementary and non-complementary DNA oligomers in specific and quantitative modes. We also designed a temperature gradient strategy for efficient separation of target DNA oligomers in DNA mixture samples. Using a column carrying two different probes with similar T _m values, their complementary target DNA oligomers were also separated and detected. The developed DNA open tubular capillary column system investigated in the present study could be further improved as an alternative tool to DNA chips to be used for the quantitative analysis of DNA or mRNA samples. Kamakshaiah Charyulu Devarayapalli and Seung Pil Pack contributed equally to this paper.     

Amidophenacylating reagents in synthesis of new derivatives of 1,3-oxazole- and 1,3-thiazole-4-sulfonyl chlorides and corresponding sulfonamides

Derivatives of 4-benzylsulfanyl-1,3-oxazole and 4-benzylsulfanyl-1,3-thiazole were synthesized using available amidophenacylating reagents. By the oxidative chlorination compounds obtained were converted to 1,3-oxazole-4-sulfonyl and 1,3-thiazol-4-sulfonyl chlorides. The latter were used to prepare the corresponding sulfonamides.     

Hydrophobic Variations of N‐Oxide Amphiphiles for Membrane Protein Manipulation: Importance of Non‐hydrocarbon Groups in the Hydrophobic Portion

Amphipathic agents called detergents serve as membrane‐mimetic systems to maintain the native structures of membrane proteins during their manipulation. However, membrane proteins solubilized in conventional detergents tend to undergo denaturation and aggregation, necessitating the development of novel amphipathic agents with enhanced properties. Here we describe several new amphiphiles that contain an N‐oxide group as the hydrophilic portion. The new amphiphiles have been evaluated for the ability to solubilize and stabilize a fragile multi‐subunit assembly from biological membranes. We found that cholate‐based agents were promising in supporting retention of the native protein quaternary structure, while deoxycholate‐based amphiphiles were highly efficient in extracting/solubilizing the intact superassembly from the native membrane. Monitoring superassembly solubilization and stabilization as a function of variation in amphiphile structure led us to propose that a non‐hydrocarbon moiety such as an amide, ether, or a hydroxy group present in the lipophilic regions can manifest distinctive effects in the context of membrane protein manipulation.     

Transformations of Acylation Products of Functionally 4-Substituted 2-Alkyl(aryl)-5-hydrazino-1,3-oxazoles into 1,3,4-Oxadiazole Derivatives

Acylation products of 2-aryl-5-hydrazino-4-X-1,3-oxazoles [X = C(O)OAlk, P(O)(OAlk)²], when heated in acetic acid or ethanol, undergo recyclization and transform into the derivatives of 1,3,4-oxadiazol-2-ylglycine or its phosphonyl analog. A similar rearrangement also occurs in the reactions of 2-alkyl(aryl)-5-hydrazino-1,3-oxazole-4-carbonitriles with carboxylic acid chlorides in pyridine, but it is accompanied by additional cyclization involving the amide residue and nitrile group, yielding 2-(5-amino-1,3-oxazol-4-yl)-1,3,4-oxadiazole derivatives.__________Translated from Zhurnal Obshchei Khimii, Vol. 75, No. 3, 2005, pp. 461–467.Original Russian Text Copyright © 2005 by Golovchenko, Pil’o, Brovarets, Chernega, Drach.     

Catalytic Performance and Kinetic Models on Zirconium Phosphate Modified Ru/Co/SiO2 Fischer–Tropsch Catalyst

The present paper represents the promising ways to improve catalytic performance by introducing zirconium phosphate (ZP) on Ru/Co/SiO₂ catalysts and the related kinetic models using the optimized Fischer?CTropsch synthesis (FTS) catalyst. A lot of works has been reported using cobalt-based catalyst for FTS reaction, and many authors have continuously tried to find out highly efficient FTS catalyst by modifying support as well as by introducing promoters. Silica is one of the excellent candidates as catalytic supports, and the present works intensively represents how to modify SiO₂ support for a high catalytic performance by using ZP species. The effect of ZP-modification of SiO₂ support with respect to cobalt aggregation and catalytic deactivation was mainly investigated for FTS reaction. The surface modification at P/(Zr?+?P) molar ratio between 0.029 and 0.134, enhanced the spatial confinement effect of cobalt clusters, and resulted in high catalytic stability with the help of well-dispersed ZP particle formation. The enhanced catalytic performance, in terms of CO conversion, C₅+ selectivity and catalytic stability, is mainly attributed to the suppressed aggregation, a homogeneous distribution of cobalt clusters with a proper size and a low mobility of cobalt clusters at an optimum molar ratio of P/(Zr?+?P) because of the formation of thermally stable ZP particles. The kinetic parameters and rate equations on the optimized catalyst are also derived in terms of CO conversion and product distribution.     

Arbitrarily vertex decomposable suns with few rays

A graph G of order n is called arbitrarily vertex decomposable if for each sequence (n₁,…,n_k) of positive integers with n₁+?+n_k=n, there exists a partition (V₁,…,V_k) of the vertex set of G such that V_i induces a connected subgraph of order n_i, for all i=1,…,k. A sun with r rays is a unicyclic graph obtained by adding r hanging edges to r distinct vertices of a cycle. We characterize all arbitrarily vertex decomposable suns with at most three rays. We also provide a list of all on-line arbitrarily vertex decomposable suns with any number of rays.