Search results for "Regioselectivity"
showing 10 items of 298 documents
Diastereo- and enantioselective synthesis of orthogonally protected 2,4-diaminocyclopentanecarboxylates: a flip from beta-amino- to beta,gamma-diamin…
2007
Conformationally restricted, orthogonally protected 2,4-diaminocarboxylates with a cyclopentane skeleton were efficiently synthesized from beta-lactam 6, the syntheses involving strategies of diastereoselective epoxidation of the beta-lactam and the corresponding monoprotected amino esters with opposite selectivities followed by regioselective opening of the oxirane ring with sodium azide. The enantiomers were also prepared. This new class of compounds can be regarded not only as conformationally constrained beta,gamma-diamino acid derivatives but also as potential functionalized carbocyclic nucleoside precursors.
Reactions at Interfaces: Oxygenation of n-Butyl Ligands Anchored on Silica Surfaces with Methyl(trifluoromethyl)dioxirane
2011
The oxygenation of n-butyl and n-butoxy chains bonded to silica with methyl(trifluoromethyl)dioxirane (1) revealed the ability of the silica matrix to release electron density toward the reacting C(2)-H σ-bond through the Si-C(1) and Si-O(1) σ-bonds connecting the alkyl chain to the surface (silicon β-effect). The silica surface impedes neither the alkyl chain adopting the conformation required for the silicon β-effect nor dioxirane 1 approaching the reactive C(2) methylene group. Reaction regioselectivity is insensitive to changes in the solvation of the reacting system, the location of organic ligands on the silica surface, and the H-bonding character of the silica surface. Reaction rates…
A dft analysis of the participation of zwitterionic tacs in polar [3+2] cycloaddition reactions
2014
A set of seven non-substituted tri-atom-components (TACs) participating in [3+2] cycloaddition (32CA) reactions has been studied using the reactivity indices defined within the conceptual DFT at the B3LYP/6-31G(d) level of theory. This series of TACs shows a zwitterionic structure and low reactivity towards ethylene. The general characteristic of these TACs is their high nucleophilic and a low electrophilic behaviour. Activation energies computed at the MPWB1K/6-311G(d) level in dichloromethane point to that non-substituted TACs react quickly toward dicyanoethylene showing their ability to react towards electron-deficient ethylenes. However, when the TACs are electrophilically activated by …
Synthesis, molecular structure and stability of fused bicyclic Δ4-1,2,4-oxadiazoline Pt(II) complexes
2015
Abstract The reaction of trans -[PtCl 2 (NCR) 2 ] (R 1 = CH 2 CO 2 Me ( 1a ), R 2 = CH 2 Cl ( 1b )) with pyrroline N -oxide − O + N CHCH 2 CH 2 C Me 2 ( 4 ), at room temperature for 15 min, furnishes via [3+2] cycloaddition the fused bicyclic Δ 4 -1,2,4-oxadiazoline Pt(II) complexes trans- [PtCl 2 { N C(R)O N C(H)CH 2 CH 2 C Me 2 } 2 ] (R 1 = CH 2 CO 2 Me ( 5a ), R 2 = CH 2 Cl ( 5b )). Compounds 5a and 5b were refluxed in CH 2 Cl 2 for 1 week to afford the derived ketoimine Pt(II) complexes trans- [PtCl 2 {N(C( O)(R)) C CH 2 CH 2 C(Me 2 ) N H} 2 ] (R 1 = CH 2 CO 2 Me ( 6a ), R 2 = CH 2 Cl ( 6b )), respectively, as a result of the N–O bond cleavage of the oxadiazoline ring in 5 . The c…
Understanding the domino reaction between 3-chloroindoles and methyl coumalate yielding carbazoles. A DFT study.
2014
The molecular mechanism of the reaction between N-methyl-3-chloroindole and methyl coumalate yielding carbazole has been studied using DFT methods at the MPWB1K/6-311G(d,p) level in toluene. This reaction is a domino process that comprises three consecutive reactions: (i) a polar Diels-Alder (P-DA) reaction between indole and methyl coumalate yielding two stereoisomeric [2 + 4] cycloadducts (CAs); (ii) the elimination of HCl from these CAs affording two stereoisomeric intermediates; and (iii) the extrusion of CO2 in these intermediates, finally yielding the carbazole. This P-DA reaction proceeds in a completely regioselective and slightly exo selective fashion. In spite of the highly polar …
18F-labeling of peptides by means of an organosilicon-based fluoride acceptor.
2006
Fluorine-18 is among the most commonly used radionuclides for positron emission tomography (PET). This non-invasive imaging technique is capable of providing in vivo information about the distribution of radiolabeled biomolecules by 1808 coincidence detection of two simultaneously emitted photons from positron–electron annihilation. Although a number of different radiotracers have been successfully employed in PET, only a few, such as 2-[F]fluoro-2-deoxy-d-glucose (FDG) and [F]fluorodopa, have gained widespread application in nuclear medicine. The reason for this is that the regioselective introduction of F into tracer molecules is often non-specific and radiochemical yields (RCY) of the Fl…
Selective derivatisation of resorcarenes: Part 7. The reason for the diastereoselectivity of Mannich reactions with chiral amines
2001
The formation of tetrakis(benzo-3,4-dihydro-1,3-oxazines) 2 by condensation of resorcarenes 1 with various primary amines and an excess of formaldehyde is not only regioselective but, in the case of (R)- or (S)-1-phenylethylamine and their para-substituted (Br, Me) derivatives, also completely diastereoselective. The reaction with the structurally similar (R)-1-aminoindane and (S)-1-(1-naphthyl)ethylamine, on the other hand, produced only a slight excess of one of the two possible epimers 2 and with (R)-1-cyclohexylethylamine no diastereoselectivity was found at all. The presence and the ratio of both diastereomeric structures can be clearly determined by NMR spectroscopy. Inspection of par…
Lipase-catalysed preparation of acyl derivatives of the germacranolide cnicin
2009
Several acyl derivatives of cnicin were obtained through lipase-catalysed acylation and alcoholysis reactions. In most reactions lipases showed a regioselective behaviour affording only one product. Longer chain acyl derivatives were prepared at lower temperature than the used in lipase-catalysed reactions, to preclude side products formation. The enzymatic approach let to prepare a family of novel acetyl and fatty acid derivatives of cnicin which are not obtainable following traditional organic synthetic procedures. Fil: Monsalve, Leandro Nicolas. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Unidad de Microanálisis…
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…
Highly regioselective synthesis of a 3-O-sulfonated arabino Lewisa asparagine building block suitable for glycopeptide synthesis
2006
Using the stannylene method, the trisaccharide 2-acetamido-3-O-[6-O-benzyl-beta-D-galactopyranosyl]-4-O-[2,3,4-tri-O-benzyl-beta-D-arabinopyranosyl]-6-O-benzyl-2-deoxy-beta-D-glucopyranosyl azide was regioselectively sulfonated and, after reduction of the anomeric azide, coupled to Fmoc alpha-allyl aspartate. After Pd(0)-catalyzed deallylation, the sulfatyl Lewis(a) asparagine building block was obtained, suitable for solid-phase glycopeptide synthesis applying the fluoride labile PTMSEL linker system.