Tõnis Kanger

Asymmetric Baeyer-Villiger oxidation of cyclobutanones

Abstract The asymmetric Baeyer-Villiger oxidation of racemic cyclobutanones 1, 2, and 3 and a prochiral cyclobutanone 4 under Sharpless oxidation conditions resulted in the enantiomeric lactones 5 ee 34%, 6 ee 53%, 7 ee 75% and 8 40% ee respectively.

Highly Enantio‐ and Diastereoselective Generation of Two Quaternary Centers in Spirocyclopropanation of Oxindole Derivatives.

Spirocyclopropanes: Only one out of eight possible stereoisomers was obtained in the asymmetric cascade cyclopropanation of alkylidene oxindoles with ethyl 2-chloroacetoacetate. Improved catalyst design ensured that spirocyclopropyl oxindoles featuring two quaternary centers were synthesized in high yield and high enantio- and diastereoselectivity (see scheme).

Asymmetric synthesis of congested spiro-cyclopentaneoxindoles via an organocatalytic cascade reaction.

Starting from simple alkylidene oxindoles and nitroketones, a highly stereoselective methodology was developed for the synthesis of spiro-cyclopentaneoxindoles with four consecutive stereogenic centers. Using an organocatalytic cascade of Michael and aldol reactions in the presence of a chiral thiourea catalyst products were obtained in moderate to high yields and excellent enantioselectivities. Nitro, ester, and hydroxyl groups were introduced to the spiro ring, which could be used to facilitate further functionalization of the products.

Enantioselective ortho-Lithiation of Aminals of benzaldehyde chromiumtricarbonyl complex

The asymmetric ortho lithiation of aminals of benzaldehyde chromiumtricarbonyl complex is described. The best regioisomeric composition is attained with aminal 2d. Quenching with an electrophile and hydrolysis of the aminal back to the aldehyde afford a single enantiomer of the o-substituted benzaldehyde chromiumtricarbonyl complex.

Asymmetric oxidation of cyclobutanones: modification of the Sharpless catalyst

Abstract Oxidation of prochiral and racemic cyclobutanones with t-BuOOH and Ti–TADDOL-based complexes afforded lactones in up to 44% ee. The enantioselectivity of the reaction clearly depends on the amount of the reagent and the highest enantioselectivities were obtained with stoichiometric amounts of the complex. Modification of the TADDOL structure and use of the mixed complex derived from TADDOL and a tartaric ester led to more reactive but less selective oxidation systems.

Enantioselective ortho-lithiation of benzaldehyde chromiumtricarbonyl complex

The asymmetric ortho lithiation of benzaldehyde chromiumtricarbonyl complex is performed by in situ temporary protection of the aldehyde functionality with a chiral amide and BuLi as metallating agent. Reaction with an electrophile followed by acidic hydrolysis leads to the enantiomerically enriched ortho substituted benzaldehyde chromiumtricarbonyl complex.

Asymmetric oxidation of 3-alkyl-1,2-cyclopentanediones. Part 2: Oxidative ring cleavage of 3-alkyl-1,2-cyclopentanediones: synthesis of 2-alkyl-γ-lactone acids

Abstract Ti(O i Pr) 4 /diethyl tartrate/ t BuOOH system oxidizes 3-alkyl-1,2-cyclopentanediones resulting in hydroxylated ring cleavage products 2-alkyl-γ-lactone acids, in high enantioselectivity (∼95% ee) and satisfactory isolated yields (up to 55%).

Asymmetric oxidation of 1,2-cyclopentanediones

Abstract Cyclic 3-alkyl-1,2-cyclopentanediones undergo a direct asymmetric oxidation with the DET/Ti(OiPr)4/tBuOOH oxidative system, resulting in enantiomeric α-hydroxy compounds and ring-cleaved hydroxylated acids (lactones) up to 95% ee.

Remote Activation of the Nucleophilicity of Isatin

The concept of the remote activation of reactivity was first applied in asymmetric organocatalysis. An isatin 3-phenylimine derivative acts as a donor in the thiourea catalyzed asymmetric addition to unsaturated 1,4-ketoesters, affording aza-Michael adducts in high enantiomeric purity and yield.

Direct asymmetric α-hydroxylation of β-hydroxyketones

Abstract Direct oxidation of racemic β-hydroxyketones 1a-c under Sharpless oxidation conditions resulted in the enantiomeric α,β-dihydroxyketones 2a in 97% ee, 2b in 86% ee and 2c in 95% ee respectively, in 37–58% of isolated yield. The oxidation is assumed to proceed via an allylic enolate intermediate.