RESUMEN
The development of a general organocatalyst for the alpha-functionalization of aldehydes, via an enamine intermediate, is presented. Based on optically active alpha,alpha-diarylprolinol silyl ethers, the scope and applications of this catalyst for the stereogenic formation of C-C, C-N, C-F, C-Br, and C-S bonds are outlined. The reactions all proceed in good to high yields and with excellent enantioselectivities. Furthermore, we will present mechanistic insight into the reaction course applying nonlinear effect studies, kinetic resolution, and computational investigations leading to an understanding of the properties of the alpha,alpha-diarylprolinol silyl ether catalysts.
RESUMEN
The application of cinchona alkaloid derivatives as catalysts for enantioselective alpha-sulfenylation of activated C-H bonds in lactones, lactams, and beta-dicarbonyl compounds by different electrophilic sulfur reagents is presented. Optically active products are obtained in good to excellent yields and up to 91 % ee. Furthermore, the diastereoselective reduction of alpha-sulfenylated beta-keto esters to give optically active alpha-sulfenylated beta-hydroxy esters has been studied. A model for the intermediate is presented, in which the protonated cinchona alkaloid interacts with the substrate leading to face-shielding in accordance with the enantioselective alpha-sulfenylation step.
Asunto(s)
Carbono/química , Hidrógeno/química , Cetonas/química , Lactamas/química , Lactonas/química , Sulfamerazina/química , Catálisis , Espectroscopía de Resonancia Magnética , Espectrometría de Masas , Modelos MolecularesRESUMEN
The first catalytic inverse-electron demand hetero-Diels-Alder reaction of nitroso alkenes has been developed. Nitroso alkenes were generated in situ from alpha-halooximes and underwent [4 + 2]-cycloadditions with enamines as dienophiles formed from aldehydes and pyrrolidine (10 mol%) as an organocatalyst. The presence of a suitable heterogeneous buffer system was found to be essential and best results were obtained with sodium acetate trihydrate. The resulting 5,6-dihydro-4H-oxazines were obtained in moderate to good yields under mild reaction conditions. A catalytic cycle has been proposed and evidence for the cycloaddition mechanism has been obtained. Moderate asymmetric induction (42% ee) was observed when a chiral secondary amine was used.
RESUMEN
The mechanism of Lewis acid catalysed hetero-Michael addition reactions of weakly basic nucleophiles to alpha,beta-unsaturated ketones was investigated. Protons, rather than metal ions, were identified as the active catalysts. Other mechanisms have been ruled out by analyses of side products and of stoichiometric enone-catalyst mixtures and by the use of radical inhibitors. No evidence for the involvement of pi-olefin-metal complexes or for carbonyl-metal-ion interactions was obtained. The reactions did not proceed in the presence of the non-coordinating base 2,6-di-tert-butylpyridine. An excellent correlation of catalytic activities with cation hydrolysis constants was obtained. Different reactivities of mono- and dicarbonyl substrates have been rationalised. A (1)H NMR probe for the assessment of proton generation was established and Lewis acids have been classified according to their propensity to hydrolyse in organic solvents. Brønsted acid-catalysed conjugate addition reactions of nitrogen, oxygen, sulfur and carbon nucleophiles are developed and implications for asymmetric Lewis acid catalysis are discussed.
RESUMEN
[reaction: see text] Strong Brønsted acids such as bis(trifluoromethanesulfon)imide catalyze the hetero-Michael addition of carbamates, alcohols, and thiols to alpha,beta-unsaturated ketones, alkylidene malonates, and acrylimides. Scope, reaction rates, and yields are superior to comparable Lewis acid-catalyzed processes.
RESUMEN
On the basis of observations from Vitis vinifera cv. Pinotage wines and experiments performed in model wine medium, a new chemical pathway responsible for the formation of anthocyanin-vinylphenol adducts in red wines is described. Until now, these pigments have been considered to be reaction products of anthocyanins and vinylphenols, the latter being generated during fermentation by enzymatic decarboxylation of the respective cinnamic acids. The mechanism of the novel pathway, involving intact hydroxycinnamic acid and anthocyanin, is explained. Only cinnamic acids with electron-donating substituents on the aromatic ring, such as coumaric acid, ferulic acid, caffeic acid, and sinapic acid, undergo this conversion, as they stabilize an intermediately formed carbenium ion. Decarboxylation and oxidation of the pyran moieties are the final steps in the generation of the corresponding 4-vinylphenol, 4-vinylguaiacol, 4-vinylcatechol, and 4-vinylsyringol adducts of anthocyanins in red wine.