Rainer T. Fell

Synthesis of optically active α-hydroxy ketones by enantioselective oxidation of silyl enol ethers with a fructose-derived dioxirane

Abstract Optically active α-hydroxy ketones 3 have been prepared in moderate to high enantioselectivities by the asymmetric oxidation of the silyl enol ethers 2 with in situ generated dioxirane from the fructose-derived ketone 1 . Best results (ee values up to 82%) for this novel non-transition metal mediated asymmetric α-hydroxylation may be obtained, when an excess of the fructose-derived ketone 1 is employed at pH ca. 8 and short reaction times. Valuable mechanistic information on the spiro versus planar transition states for the oxygen-transfer process has been aquired through the absolute configuration of the resulting α-hydroxy ketone 3 products.

Kinetic resolution of racemic α-hydroxy ketones by lipase-catalyzed irreversible transesterification

Abstract Asymmetric acetylation of racemic α-hydroxy ketones with isopropenyl acetate catalyzed by lipases afforded the optically active keto alcohols and acetates in high enantiomeric excess (up to 99%); an enzymatic kinetic resolution which may be performed on preparative scale.

Enantioselective synthesis and chiroptical properties of optically active isoflavone epoxides

Abstract Enantioselective epoxidation of isoflavones 1a-f has been performed by using Mn(III)salen complexes (R,R)- 3 and (S,S)- 3 as catalysts and dimethyldioxirane (DMD) or NaOCl together with 4-phenylpyridine N-oxide (PPNO) axial ligand as oxygen donors to obtain nonracemic isoflavone epoxides 2a-f. With the help of circular dichroism (CD) spectra of three enantiomeric pairs, and Snatzkes inverse octant rule, the absolute configurations of these optically active isoflavone epoxides have been determined.

Kinetic resolution of chiral α-hydroperoxy esters by horseradish peroxidase-catalyzed enantioselective reduction to α-hydroxy esters

Abstract The kinetic resolution of the methyl α-hydroperoxy esters 4 has been investigated by horseradish peroxidase (HRP)-catalyzed reduction to the corresponding optically active α-hydroxy ester 5 in the presence of guaiacol. The method allows for the first time the preparation of enantiomerically pure ( ee > 97%) methyl ( R )-2-hydroperoxybutyrate ( 4a ). The HRP enzyme is sensitive to the steric demand of the ester alkyl side chain, as manifested by ethyl ( ee > 97%) and isopropyl ( ee 79%), while the tert -butyl derivative is not accepted by the enzyme (no reduction).

Synthesis of optically active α-hydroxycarbonyl compounds by (salen)Mn(III)-catalyzed oxidation of silyl enol ethers and silyl ketene acetals

Optically active α-hydroxy ketones and esters have been prepared by (salen)Mn(III)-catalyzed asymmetric oxidation of silyl enol ethers and silyl ketene acetals in high enantioselectivity, with ee values up to 81 % for the α-hydroxy ketones and up to 57% for α-hydroxy esters. The extent of conversion and the enantioselectivity depends strongly on the type of oxygen donor, the pH of the aqueous bleach medium, the additive, and the substitution pattern at the enol functionality.

Enantioselective epoxidation of isoflavones by Jacobsen's Mn(III)salen catalysts and dimethyldioxirane oxygen-atom source

Abstract The catalytic enantioselective epoxidation of the isoflavones 1a–f has been performed by the Mn(III)salen complexes (R,R)-3 and (S,S)-3 as catalysts and dimethyldioxirane as the oxygen-atom source to afford optically active isoflavone epoxides 2a–f. The absolute configuration of the nonracemic epoxides 2 have been determined by X-ray diffraction analysis. Our present results constitute the first examples of the preparation of optically active isoflavone epoxides.

Synthesis and fluoride ion - Triggered chemiluminescence (CIEEL mechanism) of siloxy-substituted benzofuran dioxetanes

Abstract The dioxetanes 2 with tert -butyldimethylsilyoxy-substituted aryl groups have been prepared from the corresponding benzofurans by photooxygenation and their fluoride ion-triggered CIEEL chemiluminescence was investigated; facile saponification by fluoride and even phenoxide ions of the aryl esters 4 is held responsible for the low chemiluminescence efficiencies.

Horseradish peroxidase - a biocatalyst for the one-pot synthesis of enantiomerically pure hydroperoxides and alcohols

The kinetic resolution of racemic hydroperoxides by horseradish peroxidase (HRP)-catalyzed reduction was investigated, with major emphasis on catalytic efficiency and enantioselectivity. The kinetic parameters of the enzymatic reaction were determined, enantiomeric excess (ee) and absolute configurations of hydroperoxides and alcohols were measured, and a broad spectrum of structurally different hydroperoxides were investigated to assess the scope and limitation of this method. Both the catalytic efficiency and the stereoselectivity of HRP highly depend on the structure of the hydroperoxides. The enzyme selectively recognizes sterically unencumbered hydroperoxides, which allows kinetic resolution by means of enantioselective reduction to yield optically active hydroperoxides and alcohols in excellent ee values (up to 99%). Functional groups in the hydroperoxide molecule do not affect the stereoselectivity of the enzyme, which permits a large number of functionalized hydroperoxides to be resolved by HRP.