1,721,032 research outputs found
Building Up Quaternary Stereocenters Through Biocatalyzed Direct Insertion of Carbon Nucleophiles on Ketones
No full textQuaternary stereocenters are privileged structural motifs, widely distributed in natural products and in pharmaceutically active compounds. Asymmetric methods for the efficient construction of these prominent frameworks are rapidly increasing. Biocatalysis represents an alternative to the existing methods of using hazardous metals and chemicals. Enzymes discussed in this mini-review involve thiamine-bisphosphate (ThDP)-dependent lyases, aldolases and hydroxynitrile lyases. The chiral products, (α-alkyl-α-hydroxy ketones, α-alkyl-α-hydroxy-β-diketones, β-hydroxy ketones, ketone cyanohydrins) obtained by these enzymes, are valuable intermediates suitable for further transformations affording a variety of multi-functionalized useful building blocks bearing a tetrasubstituted stereocenter, such as amino alcohols, diols, α-hydroxy carboxylic acids, β-hydroxy-α-amino acids to cite only a few
Albumin as promiscuous biocatalyst in organic chemistry
No full textAlbumin has emerged as biocatalyst since 1980 and the everlasting interest for this protein is proved by numerous papers. The use of albumin was initially confined in the field of asymmetric oxidations and reductions whereas more recently it has found a broader application to chemical reactions such as addition, condensations and eliminations. This review reports the main applications of albumin in organic synthesis appeared in the literature in the last decade
N-heterocyclic carbene catalysis as a tool for gaining access to the 3,4-dihydropyran-2-one skeleton
No full textAn overview of the progress made in the synthesis of 3,4-dihydropyran-2-ones since the development of NHC catalysis is reported. Two distinct classes of species - electrophilic alpha,beta-unsaturated acylazolium and nucleophilic azolium enol/enolate intermediates - can easily be produced from different substrates through the intermediacy of NHC catalysis. They enable the construction of the 3,4-dihydropyran-2-one skeleton, the core of natural and synthetic products with significant biological activities, through Michael additions, cycloadditions and rearrangements
BINOL derived C2-symmetric bis-sulfinates as efficient sulfinyl transfer agents in the synthesis of tert-butyl sulfoxides
No full textThe reaction of novel C2-symmetric bis-sulfinate esters derived from (R)-BINOL with Grignard reagents affords tert-butyl sulfoxides in ee up to 97%. The desired enantiomer can be generated at will by the proper selection of BINOL
High-performance liquid chromatographic separation of chiral metallocenic ketones and alcohols
No full textThe enantiomers of chiral (arene)tricarbonylchromium ketones and alcohols were separated by high-performance liquid chromatography with a Chiralcel OD column. The absolute configuration of the ketones was assigned on the basis of the sign of optical rotation determined with an on-line detector
An organocatalytic strategy for stereoselective additions of a glyoxylate anion equivalent
No full textBecause of the possibility of manipulating their functional groups, alfa-ketoesters are regarded as interesting products, being precursors of highly functionalized compounds that find applications in different fields; therefore, several strategies have been developed for their stereoselective synthesis. Herein, we report an organocatalytic method for the preparation of chiral beta-substituted-alfa-ketoesters relying on the unpolung strategy; in particular, we employed 1,3-dithiane-2-carboxy derivatives as acyl anion mimics in a metal-free catalytic enantioselective addition to nitroalkenes. Different bifunctional catalysts, endowed with two functionalities through which coordinating both reaction partners, have been tested in the model reaction between nitrostyrene and 2-S-trifluoroethyl carboxy-thioester-1,3-dithiane. Among these, the thiourea-based ones revealed the best; in particular, catalyst derived from 9-amino-9-deoxy epiquinidine afforded the product in 71% yield and 87% e.e. at room temperature. Different reaction conditions were screened and the reaction scope was investigated; the best results were obtained in toluene at 0 °C (60% yield and 92% e.e.). The Michael addition products were transformed into alfa-ketoesters through subsequent conversion of the dithiane moiety into a carbonyl one and transesterification without loss of the stereochemical integrity, and into beta-lactam through dithiane removal and nitro-group reduction. Known compounds were obtained, allowing the determination of the absolute stereochemistry
Enantioselective oxidation of sulphides by dioxiranes in the presence of bovine serum albumin
No full textOxidation of sulphides by in situ generated dioxiranes in buffered (pH 7.5) aqueous solutions, using bovine serum albumin (BSA) as a chiral auxiliary, affords the corresponding sulphoxides in up to 89% enantiomeric excess (e.e.)
Enantioselective Sulfoxidations Catalyzed by Chloroperoxidase and Horseradish Peroxidase
No full textAdvances in chemoselective intermolecular cross-benzoin-type condensation reactions
No full textThe intermolecular cross-benzoin and acyloin condensation reactions are powerful approaches to α-hydroxy carbonyls in a single step. However, their potentiality suffers from the occurrence of side reactions including self-condensation and the formation of the undesired cross-acyloin. The broad range of azolium salt precatalysts available confers high tunability to NHC mediated benzoin condensation, assuring a good level of selectivity to the direct coupling between two non-equivalent aldehydes. Many efforts have also been devoted to the design of strategies that expand the range of suitable reaction partners beyond the traditional aldehydes and to the discovery of novel umpolung catalytic systems. The synthesis of both racemic and enantiomerically enriched acyloins is reviewed
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