Search results for "SELECTIVITY"
showing 10 items of 1148 documents
Reaktion von elektronenreichen Heterocyclen mit Orthocarbonsäure-Derivaten, 12. Formylierung von Vinylindolen mit Diethoxycarbenium-tetrafluoroborat
1988
Reactions of Electron-Rich Heterocycles with Orthocarboxylic Acid Derivatives, 12. – Formylation of Vinylindoles with Diethoxycarbenium Tetrafluoroborate 2- and 3-vinylindoles 1, 3 are formylated with diethoxycarbenium tetrafluoroborate regioselectively. The significant selectivity and reactivity of formylation of diethoxycarbenium ion in comparison with other thermodynamically more stable dialkoxycarbenium ions are compatible with the known results of hetaryl functionalisation with orthocarboxylic acid derivatives.
A novel and selective fluoride opening of aziridines by XtalFluor-E. synthesis of fluorinated diamino acid derivatives.
2015
The selective introduction of fluorine onto the skeleton of an aminocyclopentane or cyclohexane carboxylate has been developed through a novel and efficient fluoride opening of an activated aziridine ring with XtalFluor-E. The reaction proceeded through a stereoselective aziridination of the olefinic bond of a bicyclic lactam and regioselective aziridine ring opening with difluorosulfiliminium tetrafluoroborate with the neighboring group assistance of the sulfonamide moiety to yield fluorinated diamino acid derivatives. The method based on the selective aziridine opening by fluoride has been generalized to afford access to mono- or bicyclic fluorinated substances.
New Studies and a Reinvestigation on [4+2] Cycloadditions of (−)-Thebaine: Asymmetrical Diels-Alder Reactions with a Conformationally Fixed Chiral Di…
1993
Some novel Diels-Alder reactions of the opium alkaloid (−)-thebaine (1) as an electron-rich diene and a reinvestigation of its reactions with cyclic and acyclic dienophiles are described. The π-facial selectivity has been studied on the basis of structural analyses of the cycloadducts. Some related results on [4 + 2] cycloaddition reactions of 1-methoxy-1,3-cyclohexadiene (11) are also reported.
The role of the trifluoromethyl group in reactivity and selectivity in polar cycloaddition reactions. A DFT study
2012
Abstract The role of the trifluoromethyl group in reactivity, regio-, and stereoselectivity in cycloaddition reactions has been analyzed by studying the 1,3-dipolar cycloaddition (13DC) reactions of nitrone 4 with 3,3,3-trifluoro-1-nitropropene 7b and 3,3,3-trifluoro-2-nitropropene 14 using density functional theory (DFT) methods at the B3LYP/6-311G(d) level. The recent M06-2X functional has also been evaluated but, although the thermodynamic performance is improved, the kinetic data are not necessarily better than those obtained by the popular B3LYP functional for cycloaddition reactions. Four reactive channels associated with the meta and ortho regio- and endo and exo stereoselective appr…
Selective Formal Transesterification of Fluorinated 2-(Trimethylsilyl)ethyl α-Imino Esters Mediated by TBAF
2008
The scope of the transesterification reaction between beta-fluorinated alpha-imino esters and various electrophiles in the presence of TBAF as fluorine source is described. The reaction is highly selective for alkyl iodides, bromides, and mesylates, while alkyl chlorides react at a significantly slower rate and tosylates do not react under the reaction conditions. This methodology represents a simple and useful alternative for the preparation of a wide variety of fluorinated alpha-imino esters.
Stereoselective synthesis of C-glycosyl compounds via Michael addition of trimethylsilyl enol ethers and enamines to hex-1-enopyran-3-uloses
1987
Abstract The titanium(IV)-catalyzed addition of trimethylsilyl enol ethers to 2,4,6-tri- O -acylhex - 1 - enopyran - 3 - uloses ( 3 and 4 ) gave stereoselectively 3,6 - di - O - acyl - 4 - deoxy - β - d - glycero -hex-3-enopyranosyl-2-ulose derivatives. The formation of the C -glycosyl bond was accompanied by a 2→3 acyl shift, followed by the elimination of the 4-acyloxy substituent. Similarly, the reaction of 1-pyrrolidinocyclohexene with 3 and 4 also led stereoselectively to 2-(2,4,6-tri- O -acyl-β- d - ribo -hexopyranosyl)cyclohexanones. In this Michael addition, among the eight possible diastereomers, only one was formed. The high stereocontrol of both C -glycosyl compound syntheses is …
Polar [3 + 2] cycloaddition of ketones with electrophilically activated carbonyl ylides. Synthesis of spirocyclic dioxolane indolinones
2008
International audience; The [3 + 2] cycloaddition reaction between carbonyl ylides generated from epoxides and ketones (ethyl pyruvate, ethyl phenylglyoxylate, isatin, N-methylisatin and 5-chloroisatin) to give substituted dioxolanes and spirocyclic dioxolane indolinones was investigated. The effect of microwave irradiation on the outcome of the reaction was studied. The thermal reaction between 2,2-dicyano-3-phenyloxirane and N-methylisatin was theoretically studied using DFT methods. This reaction is a domino process that comprises two steps. The first is the thermal ring opening of the epoxide to yield a carbonyl ylide intermediate, whereas the second step is a polar [3 + 2] cycloadditio…
Target Analogue Imprinted Polymers with Affinity for Folic Acid and Related Compounds
2001
Two approaches to synthesize molecularly imprinted polymers with affinity for folic acid and other substituted pteridines have been compared. In the first approach, the folic acid analogue methotrexate was used as template and functional monomers capable of generating selective binding sites were searched in a miniaturized screening system based on binding assessment in the batch mode. Highest selectivity was seen using 2-vinylpyridine as functional monomer, which was confirmed in the chromatographic mode for a batch synthesized on a gram scale. However, the retentivity and selectivity of this phase were insufficient for anticipated applications. In a second approach, using methacrylic acid…
Dysidotronic acid, a new and selective human phospholipase A2 inhibitor from the sponge Dysidea sp.
2000
Abstract A new bioactive sesquiterpenoid, named dysidotronic acid 1, with a rearranged drimane skeleton has been isolated from the sponge Dysidea sp. from Vanuatu islands, along with bolinaquinone 2. The chemical structure of 1 was determined on the basis of spectroscopic data. Dysidotronic acid significantly inhibited human synovial phospholipase A2 (PLA2) at 10 μM, with an IC50 value of 2.6 μM and a higher selectivity and potency towards this enzyme than the reference inhibitor manoalide.
ChemInform Abstract: Diastereodivergent Synthesis of Fluorinated Cyclic β3-Amino Acid Derivatives.
2016
The ability of 2-p-tolylbenzyl carbanions to behave as a source of chiral benzylic nucleophiles has been shown in its reaction with fluorinated imines. The process takes place with high levels of stereocontrol, rendering the corresponding amines as single diastereoisomers. Subsequent cross-metathesis followed by intramolecular aza-Michael reaction makes the synthesis of fluorinated homoproline derivatives bearing three stereogenic centers possible. Furthermore, the selectivity of the cyclization process can easily be tuned up in a diastereodivergent manner simply by changing the reaction conditions.