Search results for "Diastereomer"

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…

Aminosäureester als chirale Hilfsgruppen in Lewis-Säure-katalysierten Umsetzungen elektronenreicher Siloxydiene mit Iminen

1990

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.

Generation of Stereocenters Around a C3-Symmetric Cyclotriveratrylene Crown

2012

The oxidation of racemic methylthio-substituted C3-symmetric cyclotriveratrylene (CTV) 2 produces the corresponding trisulfoxides 3 as follows. Oxidation with m-chloroperoxybenzoic acid (m-CPBA) in dichloromethane leads to the formation of a distribution of the four possible diastereomers that is relatively close to the statistical outcome, in which the C3-symmetric stereoisomers are the minor species. In contrast, the optically active Davis reagent [oxaziridine (+)-4] in carbon tetrachloride at room temperature leads to a biased (73 %) distribution in favor of the C3-symmetric MRRR/PSSS diastereomer of 3 with 56 % ee (PSSS enantiomer according to the model of Davis). Interestingly, when th…

ChemInform Abstract: Reversal of Asymmetric Induction in Stereoselective Strecker Synthesis on Galactosyl Amine as the Chiral Matrix.

1989

Abstract The reversal of the direction of asymmetric induction in Lewis acid catalyzed Strecker synthesis using the 2,3,4,6-tetra-O-pivaloyl-β-D-galactopyranosyl amine 1 is described. In isopropanol or tetrahydrofuran 1 had given ( R )-diastereomers of the corresponding α-amino nitriles preferably. However, in chloroform in the presence of heterogeneous zinc chloride the same auxiliary alternatively leads to an excess of the ( S )-diastereomers.

Unusual Regioselectivity in the Opening of Epoxides by Carboxylic Acid Enediolates

2008

Addition of carboxylic acid dianions appears to be a potential alternative to the use of aluminium enolates for nucleophilic ring opening of epoxides. These conditions require the use of a sub-stoichiometric amount of amine (10% mol) for dianion generation and the previous activation of the epoxide with LiCl. Yields are good, with high regioselectivity, but the use of styrene oxide led, unexpectedly, to a mixture resulting from the attack on both the primary and secondary carbon atoms. Generally, a low diastereoselectivity is seen on attack at the primary center, however only one diastereoisomer was obtained from attack to the secondary carbon of the styrene oxide.

Efficient Conversion of Light to Chemical Energy : Directional, Chiral Photoswitches with Very High Quantum Yields

2020

Abstract Photochromic systems have been used to achieve a number of engineering functions such as light energy conversion, molecular motors, pumps, actuators, and sensors. Key to practical applications is a high efficiency in the conversion of light to chemical energy, a rigid structure for the transmission of force to the environment, and directed motion during isomerization. We present a novel type of photochromic system (diindane diazocines) that converts visible light with an efficiency of 18 % to chemical energy. Quantum yields are exceptionally high with >70 % for the cis–trans isomerization and 90 % for the back‐reaction and thus higher than the biochemical system rhodopsin (64 %). T…

Prolin-benzylester als chirale Hilfsgruppen in asymmetrischen 1,3-dipolaren Cycloadditionen mit Nitriloxiden

1990

Proline Benzyl Esters as Chiral Auxiliaries in Asymmetric 1,3-Dipolar Cycloadditions N-Acryloylproline benzyl ester (1b) reacts with nitrile oxides at temperatures from −78 to 0°C to give isoxazolines 4/5 with diastereomeric ratios of ca. 3:1 and with the (5S) isomer as the predominating. The corresponding N-methacryloylproline benzyl ester (1c) gives inferior results. The diastereoselectivity is not influenced by the temperature and the substituents of the nitrile oxides. To prove the absolute configuration of the predominating stereoisomer, a reaction sequence is developed which makes use of an enantioselective hydrolysis with pig liver esterase as the key step to generate an optically ac…

Diastereomeric control of enantioselectivity: evidence for metal cluster catalysis

2014

Enantioselective hydrogenation of tiglic acid effected by diastereomers of the general formula [(μ-H)2Ru3(μ3-S)(CO)7(μ-P–P*)] (P–P* = chiral Walphos diphosphine ligand) strongly supports catalysis by intact Ru3 clusters. A catalytic mechanism involving Ru3 clusters has been established by DFT calculations. peerReviewed