Search results for "SELE"

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: Low-Loading Asymmetric Organocatalysis

2012

Asymmetric organocatalysis is now recognized as the third pillar of asymmetric synthesis. Recent years have witnessed increasing interest towards the use of highly active and stereoselective organocatalysts. This critical review documents the advances in the development of chiral organocatalysts which are systematically used in ≤3 mol% loading in all the sub-areas of the field, namely aminocatalysis, Bronsted acids and bases, Lewis acids and bases, hydrogen bond-mediated catalysis, phase transfer and N-heterocyclic carbene catalyses (194 references).

ChemInform Abstract: Syntheses of Sialyl Lewis A Glycopeptides - A Partial Sequence of the P-Selectin Ligand PSGL-1.

2000

ChemInform Abstract: First Synthesis of the Chiral Mixed O/S Ligands, 1,2-Sulfinyl Thiols: Application as Chiral Proton Sources in Enantioselective P…

2001

Abstract A suitable method for the preparation of the chiral mixed O/S ligands 1,2-sulfinyl thiols is described. These compounds have then been used as a chiral proton source in the enantioselective protonation of 2-methyl tetralone enolate and the results are compared with those obtained from the analogous alcohols. A theoretical model is proposed to explain the different behaviors exhibited in the protonation reaction for each of these proton sources. Configurational assignments for the new chiral thiols have been carried out by means of X-ray analysis.

Highly Enantio- and Diastereoselective Inverse Electron Demand Hetero-Diels-Alder Reaction using 2-Alkenoylpyridine N-Oxides as Oxo-Heterodienes

2008

A general catalytic inverse electron demand hetero-Diels Alder reaction for 2-alkenoylpyridine N-oxides is presented. 2-Alkenoylpyridine N-oxides react very efficiently with alkenes in the presence of bisoxazolidine-copper(II) [BOX-Cu(II)] complexes to give chiral dihydropyrans bearing a pyridine ring at the 6-position with very high yields and excellent diastereo- and enantioselectivity. These heterodienes exhibited higher reactivity and enantioselectivity than the corresponding non-oxidized 2-alkenoylpyridines.

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.

Zwitterionic Aza-Claisen Rearrangements Controlled by Pyrrolidine Auxiliaries - Useful Key Steps in Convergent Enantioselective Syntheses

2012

Chiral pyrrolidine substituents served as efficient auxiliaries in diastereoselective zwitterionic ketene aza-Claisen rearrangements. Palladium-catalysed N-allylation starting from optically active proline and prolinol derivatives, as well as from (2S,5S)-2,5-dimethoxymethylpyrrolidine, gave various allylamines bearing trisubstituted olefin moieties. Treatment with complex carboxylic acid fluorides in the presence of trimethylaluminium induced activated ketene addition to the nitrogen and subsequent [3,3] sigmatropic rearrangement to give γ,δ-unsaturated amides with excellent simple diastereoselectivities and up to 11:1 auxiliary-induced diastereomeric ratios. Cleavage of the pyrrolidine am…

ChemInform Abstract: Two-Carbon Bridge Substituted Cocaines: Enantioselective Synthesis, Attribution of the Absolute Configuration, and Biological Ac…

2010

In an effort to learn more about the general structure-activity relationships of cocaine with the aim to elucidate those structural features that might confer antagonistic properties to such analogues, we describe herein our synthetic efforts to prepare two-carbon bridge functionalized (methoxylated and hydroxylated) analogues. Our approach makes use of a modification of the classical Willstatter synthesis of cocaine: Mannich type cyclization of acetonedicarboxylic acid monomethyl ester with methylamine hydrochloride and 2-methoxysuccindialdehyde in a citrate buffer solution afforded the 6- and 7-substituted 2-carbomethoxy-3-tropinones 3a,b and 4a,b in approximate yields of 64%. Reduction o…

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.