Search results for " Bases"

X-ray Single Crystal Structure, Tautomerism Aspect, DFT, NBO, and Hirshfeld Surface Analysis of a New Schiff Bases Based on 4-Amino-5-Indol-2-yl-1,2,…

2021

Four different new Schiff basses tethered indolyl-triazole-3-thione hybrid were designed and synthesized. X-ray single crystal structure, tautomerism, DFT, NBO and Hirshfeld analysis were explored. X-ray crystallographic investigations with the aid of Hirshfeld calculations were used to analyze the molecular packing of the studied systems. The H···H, H···C, S···H, Br···C, O···H, C···C and N···H interactions are the most important in the molecular packing of 3. In case of 4, the S···H, N···H, S···C and C···C contacts are the most significant. The results obtained from the DFT calculations indicated that the thione tautomer is energetically lower than the thiol one by 13.9545 and 13.7464 kcal…

Orientation of the acidity probes benzene, acetonitrile and pyridine in SAPO-5 and GaAPO-5 molecular sieves: an FTIR microscopy study

1997

The adsorption and desorption behaviour of benzene, acetonitrile (CD3CN) and pyridine for large crystals of SAPO-5 and GaAPO-5 is studied using FTIR microscopy with polarised IR radiation. From the analysis of the polarised adsorption bands it is concluded that the adsorbates are ordered in the molecular sieves at 373 K. The orientation of molecules interacting with Bronsted or Lewis acid sites and of physisorbed molecules are discussed. Benzene adsorbed at Bronsted sites is oriented with the aromatic ring parallel to the crystallographic c-axis of the molecular sieve, whereas physisorbed benzene is oriented with the aromatic ring parallel to the crystallographic a,bplane. Acetonitrile can …

Reversal of asymmetric induction in stereoselective strecker synthesis on galactosyl amine as the chiral matrix

1988

Abstract The reversal of the direction of asymmetric induction in Lewis acid catalyzed Strecker synthesis using the 2,3,4,6-tetra-O-pivaloyl-β-D-galactopyranosyl amine 1 is described. In isopropanol or tetrahydrofuran 1 had given ( R )-diastereomers of the corresponding α-amino nitriles preferably. However, in chloroform in the presence of heterogeneous zinc chloride the same auxiliary alternatively leads to an excess of the ( S )-diastereomers.

3.13 Carbohydrate Derived Auxiliaries: Amino Sugar and Glycosylamine Auxiliaries

2012

The stereodifferentiating potential of carbohydrate auxiliaries is illustrated by examples of oxazolidinones derived from amino sugars and aldimines of glycosylamines. Taking advantage of the bicyclic structure of amino sugar-derived oxazolidinones, reactions of their unsaturated or deprotonated N -acyl derivatives show an efficient exo - versus endo -differentiation. High diastereoselectivity is achieved in the syntheses of β -branched or α -substituted carboxylic acid derivatives. Imines of glycosylamines, in particular of 2,3,4,6-tetra- O -pivaloyl- β - d -galactopyranosylamine, are characterized by a conformation stabilized through the exo -anomeric effect and by their ability to coordi…

New heterogeneous catalysts for greener routes in the synthesis of fine chemicals

2007

Abstract New strong Lewis acid SnTf-MCM-41 and SnTf-UVM-7 catalysts with unimodal and bimodal pore systems were prepared in a two-step synthesis in which the triflic acid (Tf) was incorporated to previously synthesized mesoporous tin-containing silicas. The Sn incorporation inside the pore walls was carried out through the Atrane method. The SnTf-UVM-7 catalysts were prepared by aggregating nanometric mesoporous particles defining a hierarchic textural-type additional pore system. Following these procedures, catalysts with different Si/Sn ratios—21.8 to 50.8 for SnTf-MCM-41 and 18.4 for SnTf-UVM-7—were prepared. These new materials were tested in the acylation of aromatic sulfonamides using…

Crystal structure and theoretical study of (2E)-1-[4-hydroxy-3-(morpholin-4-ylmethyl)phenyl]-3-(thiophen-2-yl)prop-2-en-1-one

2018

WOS: 000437492100018

Oral leukoplakia; a proposal for simplification and consistency of the clinical classification and terminology

2019

There is a distinct lack of uniformity in the definitions and clinical terminologies related to oral leukoplakia and leukoplakialike lesions and disorders. Proposals have been put forward to subclassify leukoplakia into a homogeneous and a non-homogeneous type based on color only, being either predominantly white or mixed white-and-red, respectively, irrespective of the texture of the lesion. In this proposal there is no need anymore to regard the poorly defined proliferative verrucous leukoplakia as a separate entity. Since keratosis is primarily a histopathological term, its clinical use is discouraged. Alternative terminology for these so-called keratotic lesions and disorders has been p…

2018

Tetrel bonding is the noncovalent interaction of group IV elements with electron donors. It is a weak, directional interaction that resembles hydrogen and halogen bonding yet remains barely explored. Herein, we present an experimental investigation of the carbon-centered, three-center, four-electron tetrel bond, [N–C–N]+, formed by capturing a carbenium ion with a bidentate Lewis base. NMR-spectroscopic, titration-calorimetric, and reaction-kinetic evidence for the existence and structure of this species is reported. The studied interaction is by far the strongest tetrel bond reported so far and is discussed in comparison with the analogous halogen bond. The necessity of the involvement of …

Reconsidering TOF calculation in the transformation of epoxides and CO2 into cyclic carbonates

2020

Abstract The combination of Lewis acids and Lewis bases, currently defined as catalysts and co-catalysts (or promoter) respectively, in the reaction between epoxides and CO2 to give cyclic carbonates, is discussed, starting from examples in which the Lewis base was used in larger amount with respect to the Lewis acid. In these cases, turnover frequency (TOF) values have been usually calculated taking into account solely the amount of the Lewis acid employed. The occurrence of two distinct reaction pathways, one catalysed by the sole Lewis base and the other one catalysed by the Lewis acid/Lewis base couple, in which the Lewis acid alone does not play a catalytic role, should bring researche…

Highly enantioselective synthesis of fluorinated gamma-amino alcohols through proline-catalyzed cross-Mannich reaction.

2005

A new, simple route for the synthesis of fluorinated beta-alkyl gamma-amino alcohols in optically pure form in only two steps and featuring proline catalysis from inexpensive and readily available starting materials is described. The applied strategy allows for the introduction of diversity into both the beta-fluoroalkyl and alpha-alkyl groups of these compounds. [reaction: see text]