Observed Stereoselectivities (observed + stereoselectivity)

Distribution by Scientific Domains


Selected Abstracts


Isolation and X-Ray Structures of Reactive Intermediates of Organocatalysis with Diphenylprolinol Ethers and with Imidazolidinones

HELVETICA CHIMICA ACTA, Issue 11 2008
5-Repulsion, A Survey, Comparison with Computed Structures, the Geminal-Diaryl Effect at Work, with 1-Acyl-imidazolidinones: The
Abstract Reaction of 2-phenylacetaldehyde with the Me3Si ether of diphenyl-prolinol, with removal of H2O, gives a crystalline enamine (1). The HBF4 salts of the MePh2Si ether of diphenyl-prolinol and of 2-(tert -butyl)-3-methyl- and 5-benzyl-2,2,3-trimethyl-1,3-imidazolidin-4-one react with cinnamaldehyde to give crystalline iminium salts 2, 3, and 4. Single crystals of the enamine and of two iminium salts, 2 and 3, were subjected to X-ray structure analysis (Figs.,1, 2, and 6), and a 2D-NMR spectrum of the third iminium salt was recorded (Fig.,7). The crystal and NMR structures confirm the commonly accepted, general structures of the two types of reactive intermediates in organocatalysis with the five-membered heterocycles, i.e., D, E (Scheme,2). Fine details of the crystal structures are discussed in view of the observed stereoselectivities of the corresponding reactions with electrophiles and nucleophiles. The structures 1 and 2 are compared with those of other diphenyl-prolinol derivatives (from the Cambridge File CSD; Table,1) and discussed in connection with other reagents and ligands, containing geminal diaryl groups and being used in enantioselective synthesis (Fig.,4). The iminium ions 3 and 4 are compared with N -acylated imidazolidinones F and G (Figs.,9, 12, and 13, and Table,3), and common structural aspects such as minimalization of 1,5-repulsion (the ,A1,3 -effect'), are discussed. The crystal structures of the simple diphenyl-prolinol,HBF4 salt (Fig.,3) and of Boc- and benzoyl-(tert -butyl)methyl-imidazolidinone (Boc-BMI and Bz-BMI, resp.; Figs.,10 and 11) are also reported. Finally, the crystal structures are compared with previously published theoretical structures, which were obtained from high-level-of-theory DFT calculations (Figs.,5 and 8, and Table,2). Delicate details including pyramidalization of trigonal N-atoms, distortions around iminium CN bonds, shielding of diastereotopic faces, and the , -interaction between a benzene ring and a Me group match so well with, and were actually predicting the experimental results that the question may seem appropriate, whether one will soon start considering to carry out such calculations before going to the laboratory for experimental optimizations. [source]


New Simple Hydrophobic Proline Derivatives as Highly Active and Stereoselective Catalysts for the Direct Asymmetric Aldol Reaction in Aqueous Medium

ADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 17 2008
Francesco Giacalone
Abstract New 4-substituted acyloxyproline derivatives with different hydrophobic properties of the acyl group were easily synthesized and used as catalysts in the direct asymmetric aldol reaction between cyclic ketones (cyclohexanone and cyclopentanone) and several substituted benzaldehydes. Reactions were carried out using water, this being the best reaction medium examined. Screening of these catalysts showed that compounds bearing the most hydrophobic acyl chains [4-phenylbutanoate and 4-(pyren-1-yl)butanoate] provided better results. The latter catalysts were successfully used in only 2 mol% at room temperature without additives to give aldol products in excellent stereoselectivities. These results demonstrate that derivatization of the proline moiety with the proper simple hydrophobic substituent in the 4-position can furnish highly active and stereoselective catalysts without the need of additional chiral backbones in the molecule. Finally, an explanation of the observed stereoselectivities in the presence of water is provided. [source]


Allylsilane-Modified Amino Acids from the Claisen Rearrangement

HELVETICA CHIMICA ACTA, Issue 12 2002
Mustafa Mohamed
The Claisen rearrangement of the N -protected, silylated allyl glycinates 11 and 12 led to the formation of allyl/silyl-functionalized amino acids 13 and 14 in yields up to 80%. The diastereoisomer ratio varied from 2,:,1 to 29,:,1 for 11mb, and from 2,:,1 to 46,:,1 (syn/anti) for 12mb, depending on reaction conditions, as shown by X-ray crystallographic analysis of 14mb. The relationship between the size of the alkyl groups on the chlorosilane reagent (Me2R,SiCl, R,=Cl, Me, t -Bu, Ph) used as an enolate trap and the observed stereoselectivity was investigated in the case of the Ireland,Claisen variant. Me3SiCl gave the best results. However, the size of the alkyl groups on the silylated ester (Me2R,Si, R=Me, t -Bu, Ph, i-Pr) did not exert a significant effect on the diastereoselectivity or yield of the rearrangement. [source]


Asymmetric biomimetic oxidations of phenols using oxazolidines as chiral auxiliaries: the enantioselective synthesis of (+)- and (,)-dehydrodiconiferyl alcohol,

JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 8-9 2006
Maurizio Bruschi
Abstract Stereoselective bimolecular radical coupling reactions of phenylpropenoid phenols are described. Evans's 2-oxazolidinone 11a,d derivatives of ferulic acid were prepared and oxidized to give dimeric benzofuran neolignan structures 12,13a,d in 40,50% overall yields. The chiral phenols were dimerized either enzymatically with hydrogen peroxide and horseradish peroxidase (HRP) or with silver oxide. The enantioselectivity after reductive cleavage of the chiral auxiliaries to give dehydrodiconiferyl alcohol ranged from 18% to 62% enantiomeric excess. The conformational analysis and the activation energy using semiempirical PM3 calculations on the intermediate quinomethides is used to explain the observed stereoselectivity. Copyright © 2006 John Wiley & Sons, Ltd. [source]