Search results for "Regioselectivity"
showing 10 items of 298 documents
Copper-catalyzed regioselective synthesis of (E)-β-fluorovinyl sulfones
2019
Organofluorine compounds are finding increasing application in a variety of fields such as pharmaceutical, agrochemical, and material sciences. However, given the scarcity of fluorine-containing natural products, advancement in this area depends almost entirely on the development of new synthetic methodologies. In this article, we present the synthesis of a series of previously undescribed (E)-&beta
Proton Acid-Catalysed Transformations of Estrogen Derivatives: New Results and Some Mechanistic Aspects of theKober Colour Reaction
1994
The 3-O-methylated estrogen derivatives 1a-d and α-estradiol (1e) underwent sulfuric acid-catalysed transformations to furnish the steroids 2–6. The processes involved in the reaction sequence are regioselective sulfonation and, above all, the Wagner-Meerwein rearrangement of the methyl group at C-13. With the objective of obtaining further information on the course of the Kober colour reaction of estrogens, some UV/VIS and ESR spectroscopic investigations were also carried out. Protonenkatalysierte Transformationen von Estrogen-Derivaten: Neue Ergebnisse und einige mechanistische Aspekte der Kober-Farbreaktion Die O-methylierten Estrogenderivate 1a-d und α-Estradiol (1e) reagieren in Schwe…
Tuning the Basicity of Synergic Bimetallic Reagents: Switching the Regioselectivity of the Direct Dimetalation of Toluene from 2,5‐ to 3,5‐Positions
2008
Meta-meta metalation: Remarkably, toluene can be directly dimanganated or dimagnesiated at the 3,5-positions using bimetallic bases with active Me3SiCH2 ligands (see scheme, blue). In contrast, n-butyl ligands lead to 2,5-metalation (red). tmp=2,2,6,6-tetramethylpiperidide.
Efficient synthesis of new C-functionalized macrocyclic polyamines
2010
A powerful synthetic route for the preparation of new poly-azamacrocycles, valuable precursors of bifunctional chelating agents with applications in nuclear medicine, is reported. The desired functional group was introduced onto the macro-cycle backbone during the cyclization step, thus avoiding the tedious preparation of a C-functionalized synthon. The regioselective reaction of macrocycles bearing an aminomethyl pendant arm with aldehydes is also described.
Unveiling the Different Reactivity of Bent and Linear Three-Atom-Components Participating in [3 + 2] Cycloaddition Reactions
2021
The reactivity of a series of pairs of bent and linear three-atom-component (B-TACs and L-TACs) participating in [3 + 2] cycloaddition (32CA) reactions towards ethylene and electrophilic dicyanoethylene (DCE) have been studied within the Molecular Electron Density Theory. While the pseudodiradical structure of B-TACs changes to that of pseudoradical or carbenoid L-TACs upon dehydrogenation, zwitterionic B-TACs remain unchanged. Conceptual Density Functional Theory (CDFT) indices characterize five of the nine TACs as strong nucleophiles participating in polar reactions towards electrophilic ethylenes. The activation energies of the 32CA reactions with electrophilic DCE range from 0.5 to 22.0…
Selective derivatisation of resorcarenes: 1. The regioselective formation of tetra-benzoxazine derivatives
1997
Abstract Four 5,6-benzo-1,3-oxazine rings are formed by the condensation of resorcarenes with various aliphatic or aromatic primary amines and formaldehyde. From four possible regioisomers only the C4 symmetrical compound is isolated in yields of up to 90%. Semiempirical calculations confirm its relative stability, which is due to the possible formation of four intramolecular OH…O hydrogen bonds. The regioselectivity of the reaction is further established for two examples by single crystal X-ray analysis. A solvent molecule is included in the extended cavity.
BENZOTHIETES—VERSATILE SYNTHONS FOR THE PREPARATION OF HETEROCYCLES
1994
Abstract Thermal or photochemical ring opening transforms benzo[b]thiete to o-thiobenzoquinonemethide, a highly reactive 8π electron system. Cycloaddition reactions or linear additions with subsequent cyclizations lead to larger heterocyclic rings. A variety of compounds containing sulfur and possibly further heteroatoms like nitrogen, oxygen or phosphorus in normal, medium and large ring systems can be synthesized on this route.
Relevance of the Electronegativity of Boron inη5-Coordinating Ligands: Regioselective Monoalkylation and Monoarylation in Cobaltabisdicarbollide[3,3′…
2003
Regioselective monoalkylation and monoarylation in cobaltabisdicarbollide clusters has been achieved starting from Cs[8-I-3,3'-Co(1,2-C 2 B 9 H 1 0 )(1',2'-C 2 B 9 H 1 1 )] by cross-coupling reactions between a B-I fragment andan appropriate Grignard reagent in the presence of a Pd catalyst and CuT. A considerable number of monoalkylated and monoarylated derivatives have been synthesized, which allowed study of the influence of boron in metallocene-type ligands and the effect of alkyl and aryl substituents on boron in boron anionic clusters. Experimental data from UV/ Vis spectroscopy, E 1 / 2 measurements, and X-ray diffraction analysis, and supported by EHMO and ab initio analyses, indica…
Influence of the Ligand of Palladium(0) Complexes on the Rate of the Oxidative Addition of Aryl and Activated Alkyl Bromides: Csp2 −Br versus Csp3 −B…
2017
Kinetic data by means of electrochemical techniques are used to characterize the reactivity of aryl bromides and activated alkyl bromides in oxidative addition to palladium(0) complexes generated from three precursors: Pd0(PPh3)4, {Pd0(dba)2 + 2 PPh3} and {Pd0(dba)2 + 2 P(o Tol)3} in DMF at 25 °C. It is established that, for the investigated substrates 1-6 and 7-11, the oxidative addition at the Csp3 Br bond is much faster than that at the Csp2 Br bond when the palladium(0) is ligated by two PPh3. This explains why the regioselectivity in Suzuki-Miyaura reactions performed from substrates bearing both Csp2 Br and Csp3 Br groups is in favour of the substitution at the Csp3 Br bond. It is in …
Understanding the different reactivity of (Z)- and (E)-β-nitrostyrenes in [3+2] cycloaddition reactions. An MEDT study
2021
The experimental reactivity of isomeric (Z)- and (E)-b-nitrostyrenes participating in [3+2] cycloaddition (32CA) reactions has been analysed on the basis of molecular electron density theory (MEDT) at the HF/6-311G(d,p), B3LYP/6-311G(d,p) and uB97X-D/6-311G(d,p) computational levels. It was found that the polar zw-type 32CA reactions with 5,5-dimethylpyrroline-N-oxide proceed via a one-step mechanism, characterised by the attack of the nucleophilic oxygen centre of the nitrone on the electrophilically activated b-position of these nitrostyrenes. This behaviour is completely understood by means of the analysis of the conceptual DFT reactivity indices. These 32CA reactions present low activat…