Albert Lévai

Enantioselective epoxidation of 2,2-dimethyl-2H-chromenes by dimethyldioxirane and jacobsen's Mn(III)salen catalysts

Enantioselective epoxidation of 2,2-dimethyl-2H-chromenes 1a-d has been performed by the Mn(III)salen complexes (R,R)-3 and (S,S)-3 as catalysts and dimethyldioxirane (DMD) as oxygen donor. The epoxychromans 2a-d were obtained in good yields and high enantioselectivities (up to 93% e.e.), which constitute the first examples of enantioselective epoxidation by DMD with the Jacobsen catalyst.

Synthesis and conformational analysis of some spiropyrazoline isomers

Abstract A series of spiropyrazolines has been synthesized by 1,3-dipolar cycloaddition of E- and Z-3-arylidene-chromanones, -1-thiochromanones, -flavanones, -1-thioflavanones as well as 2-benzylidene-1-indanone, -1-benzosuberone with diazomethane. It has been found that this cycloaddition is regio- and stereoselective affording trans- and cis- spiro-1-pyrazolines. Spiro-1-pyrazolines were converted into spiro-2-pyrazolines on acid-catalysed isomerization. Conformation and relative configuration of compounds prepared has been elucidated by various one- and two-dimensional n.m.r. methods.

Highly diastereoselective Michael reaction under solvent-free conditions using microwaves: conjugate addition of flavanone to its chalcone precursor

Microwave-assisted reaction of 2′-hydroxychalcones in the presence of DBU resulted in the formation of hitherto unknown dimers by conjugate addition of the intermediate cyclic ketone to the starting enone.

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.

Synthesis of pyrazolines by the reactions ofα,β-enones with diazomethane and hydrazines (review)

The first representatives of pyrazolines were synthesized in the last century. These nitrogen-containing heterocyclic compounds became important in the development of different bioactive substances. For this reason, various procedures have been worked out for their synthesis. In the present article we summarize those synthetic methods providing 1- or 2-pyrazolines by the reactions of α,β-unsaturated ketones with diazomethane or hydrazine derivatives.

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.

Enantioselective hydrogenation of exocyclic α, β-unsaturated ketones. Part II. Hydrogenation in the presence of (S)-proline

In the asymmetric synthesis of chiral compounds, the reduction of prochiral unsaturated reactants has a great importance. (S)-proline as a chiral auxiliary is used in the hydrogenations of exocyclic α,β-unsaturated ketones with palladium on carbon catalysts, producing the corresponding saturated ketones with an optical purity up to 20%. The influence of the parameters (solvents, additives) on the optical yield is also investigated. The highest enantioselectivity was obtained in ethyl acetate and acetonitrile (ee 20%), at atmospheric pressure and room temperature.

DIOXIRANE OXIDATION OF (Z)-1-THIOAURONES, (E)-3-ARYLIDENE-1-THIOCHROMAN-4-ONES AND (E)-3-ARYLIDENE-1-THIOFLAVAN-4-ONES

Abstract The oxidation of the title compounds 1, 4 and 7 with dimethyldioxirane (DMD) afforded the corresponding sulfoxides 2, 5 and 8 and/or sulfones 3, 6 and 9 in good yields (Scheme 1 and 2). Excess dimethyldioxirane gave the sulfones chemoselectively without formation of the epoxides. The epoxidation of the sulfones 6a,b,d to the respective spiroepoxides 10a,b,d required the more reactive methyl(trifluoromethyl)dioxirane (TFD) as oxidant.

Thermal, decomposition of some new spiro-1-pyrazolines

Abstract The thermal decomposition of spiro-1-pyrazolines 2 obtained by the cycloaddition of exocyclic α,β-unsaturated ketones with diazomethane gives spirocyclopropanes 4 with high selectivity and the new β-methyl-3-benzylidene derivatives 3. The configuration and conformation of the thermolysis products 3 and 4 were elucidated by different n.m.r. methods.

Structural effects on the enantioselective acetylation of 4-hydroxychromans catalyzed by microbial lipases

Abstract Kinetic resolutions of a series of racemic 4-hydroxychromans by the Candida cylindracea lipase catalysed acetylation are described. Correlation between structure (conformation) and enantioselectivity is discussed.