Search results for "Selectivity"
showing 10 items of 1148 documents
ChemInform Abstract: P-Chirogenic Secondary Phosphine Oxides: New Stereoselective Synthesis and Applications
2015
ChemInform Abstract: Water in Stereoselective Organocatalytic Reactions
2009
In this review, recent advances in asymmetric organocatalytic reactions carried out with variable amounts of water, from substoichiometric to a large excess (reaction medium), are discussed. We also summarize several proposed mechanisms for the different possibilities of the action of water both in the increased activity of the catalyst and in the asymmetric induction. Finally, the application of this catalytic methodology to the enantioselective synthesis of valuable compounds through enamine or iminium catalysis is presented.
ChemInform Abstract: Triazolopyridines. Part 26. The Preparation of Novel [1,2,3]Triazolo[1,5-a]pyridine Sulfoxides.
2008
Abstract The regioselective synthesis of new triazolopyridine halides and sulfoxides with the substituent in all different ring positions of [1,2,3]triazolo[1,5- a ]pyridines is presented. The triazolo ring opening reaction of some representative sulfoxides to obtain disubstituted pyridines is also studied.
ChemInform Abstract: A New Method for the Regioselective Synthesis of β-Enamino Acid Derivatives.
2010
Abstract A mild, simple and efficient route to β-enamino imidazole carbonilic derivatives 3 by reaction of ketimines with N,N′ -carbonyl diimidazole in the presence of boron triflouride as catalyst has been developed. The conversion of 3 to esters has also been explored.
ChemInform Abstract: Reactivity and Selectivity of ortho-Metalated Rhodium(II) Complexes in C-H Insertion Reactions of α-Diazo Compounds.
2010
Regioselective intramolecular C−H insertion reactions of α-diazo β-keto esters and α-diazo ketones are mediated by Rh2[OOCR]4-x[PC]x (x = 1, 2; PC = ortho-metalated phosphine; R = CH3, C3F7). In particular, the intramolecular transformation of 1-diazo-5-methyl-3-propyl-2-hexanone catalyzed by Rh2[OOCCH3]2[PC]2 (PC = (C6H4)P(C6H5)2, head-to-tail (H−T) configuration) afforded only the tertiary C−H insertion product. By comparison, no C−H insertion reaction was promoted by doubly metalated complexes with a head-to-head (H−H) configuration. Rh2[OOCR]3[PC] compounds were found to be less suited for these type of reactions.
Triazolopyridines. Part 26: The preparation of novel [1,2,3]triazolo[1,5-a]pyridine sulfoxides
2008
Abstract The regioselective synthesis of new triazolopyridine halides and sulfoxides with the substituent in all different ring positions of [1,2,3]triazolo[1,5- a ]pyridines is presented. The triazolo ring opening reaction of some representative sulfoxides to obtain disubstituted pyridines is also studied.
A combined experimental and theoretical study of the alkylation of 3,5-dithioxo-[1,2,4]triazepines
2008
The chemo- and regioselective alkylation reactions of 3,5-dithioxo[1,2,4]triazepine 1 in a basic medium with α,ω-dibromoalkanes 2a–c, Br(CH2)nBr (n = 1–3), are studied experimentally and theoretically. These alkylations, which occur at the thioxo sulfur atom in position 5, afford mainly 5-bromomethylthio-2,7-dimethyl- 2,3-dihydro- 4H[1,2,4]triazepin-3-one 3 for n = 1, 6,8-dimethyl-5-thioxo-2,3,4,5-tetrahydro-6H[1,3]thiazolo[4,5-d][1,2,4]triazepine 4 for n = 2 and 7,9-dimethyl-6-thioxo-2,3,4,5,6,7-hexahydro[1,3]thiazino [4,5-d][1,2,4]triazepine 5 for n = 3. Theoretical calculations have been carried out at the B3LYP/6-31G* and B3LYP(benzene)/6-311+G*//B3LYP/6-31G* levels, in order to rationa…
Hyperconjugative Control by Remote Substituents of Diastereoselectivity in the Oxygenation of Hydrocarbons.
2000
The oxidation of 2-substituted adamantanes (2) with TFDO (1) is reported. The data show a stereodifferentiation of the chemical environments induced by remote electron-withdrawing substituents which produces remarkable Z/E diastereoselectivity in the oxidation of the tertiary C(5)-H and C(7)-H bonds. The results show a bell-shaped correlation between the Z/E stereoselectivity and the substituent constant sigma(I), which is interpreted in terms of hyperconjugative stabilization of the diastereomeric transition states.
The reaction of amino-imidazoles, -pyrazoles and -triazoles with α,β-unsaturated nitriles
2005
The reactions of α,β-unsaturated nitriles (1, 9, 12) as bielectrophiles with aminoazoles (2, 4, 6) as binu-cleophiles were investigated. Acrylonitrile (1) reacts almost exclusively in a chemoselective Michael-type addition yielding the substituted azoles 3, 5 and 7, respectively. Cinnamonitriles 9a,b behave in a similar way, but the free CN group adds a second molecule 4 yielding 10a,b and its cyclocondensation product 11a,b as minor component. The attempted formation of azolopyrimidines is best achieved by the reaction of the benzylidenemalononitriles 12a - f with 2 or 4. The process is chemo- and regioselective. The structure determinations were based on NMR measurements including DEFT, C…