Synthesis of 3-substituted isoxazolidin-4-ols using hydroboration-oxidation reactions of 4,5-unsubstituted 2,3-dihydroisoxazoles

Isoxazolidines represent a very important class of N/O-containing heterocycles used as the key intermediates in the synthesis of more complex cyclic and acyclic compounds, including various biologically active molecules. Here, we present a fast and highly stereoselective approach towards both C-3/4-cis and C-3/4-trans isomers of 3-substituted isoxazolidin-4-ols. The strategy relies on a highly regio- and trans-stereoselective hydroboration-oxidation reaction of the 4,5-unsubstituted 2,3-dihydroisoxazoles with basic hydrogen peroxide. The consecutive oxidation/reduction route, sequentially employing Dess-Martin periodinane and ʟ-selectride, is used for the inversion of the C-3/4-trans relative configuration of the isoxazolidine ring. The significance of the method lies in its variability and applicability to a concise synthesis of various 4-hydroxyisoxazolidines, starting from the readily available C-alkyl/aryl-nitrones. The resemblance to 3-hydroxypyrrolidines certainly makes the 4-hydroxyisoxazolidines important and valuable structural fragments in drug discovery.

N-Oxides rescue Ru(v) in catalytic Griffith–Ley (TPAP) alcohol oxidations

Electrochemistry of tetrapropylammonium perruthenate (TPAP) reveals that an unstable Ru(v) complex is reoxidised by amine N-oxides in catalytic Griffith–Ley alcohol oxidations.

Asymmetric synthesis of enantiopure isoxazolidinone monomers for the synthesis of β-oligopeptides by chemoselective amide ligation

The design and general synthesis of enantiopure isoxazolidinone monomers as precursors for the preparation of enantiopure N-terminal hydroxylamine-β(3)-oligopeptides, which may be used as reaction partners with α-ketoacids in the decarboxylative amide ligation reaction, is described.

Regio- and stereoselectivity of captodative olefins in 1,3-dipolar cycloadditions. A DFT/HSAB theory rationale for the observed regiochemistry of nitrones

Captodative olefins 1-acetylvinyl carboxylates proved to be highly regioselective dipolarophiles in 1,3-dipolar cycloadditon to propionitrile oxide, arylphenylnitrile imines, diazoalkanes, and nitrones to yield the corresponding 5-substituted heterocycles. The addition of the latter was also stereoselective, being slightly susceptible to steric demand of the carboxylate substituent in the olefin. All atempts to cleave the isoxazolidine N-O bond under reductive conditions failed, providing diverse products with side-group reduction. FMO theory was unsuccessful to explain the regioselectivity observed with nitrones, since the opposite orientation was predicted. The recently formulated DFT/HSAB theoretical model was able to rationalize this regioselectivity, identifying the nucleophilic and electrophilic atoms involved in the process via calculation of interaction energies, suggesting the specific direction of the electronic process at each of the reaction sites.

Improvement of TADDOLate-TiCl(2)-Catalyzed 1,3-Dipolar Nitrone Cycloaddition Reactions by Substitution of the Oxazolidinone Auxiliary of the Alkene with Succinimide

A significant improvement of metal-catalyzed asymmetric 1,3-dipolar cycloaddition reactions of acyclic nitrones with alpha,beta-unsaturated carbonyl compounds is described using succinimide as a new auxiliary for the alpha,beta-unsaturated carbonyl moiety. In the absence of a catalyst, N-crotonoylsuccinimide reacts with C,N-diphenylnitrone to give the endo-isoxazolidine, whereas in the presence of 10 mol % TiCl(2)(i-PrO)(2) the exo-product is obtained. Four different TiCl(2)-TADDOLate complexes have been tested as catalysts for the 1,3-dipolar cycloaddition reactions, and the most successful catalyst was applied (5 mol %) in a series of reactions between two different alkenoylsuccinimides and three different nitrones. The crude products containing an N-acylsuccinimide moiety were converted directly into the corresponding carboxamides upon treatment with hydrazine. The 1,3-dipolar cycloaddition reactions proceed with a high degree of exo-selectivity, often >90% de, which is an improvement compared with previous experiments performed using the oxazolidinone auxiliary for the alkenoyl moiety. Furthermore, the enantioselectivities are also improved compared with previous work, and ee up to 73% is obtained, which is the highest ee found for these metal-catalyzed exo-selective 1,3-dipolar cycloaddition reactions. The ee can be improved to >90% by recrystallization. The absolute structure of the 1,3-dipolar cycloaddition adduct is determined on the basis of the X-ray crystal structure of a compound with a known configuration. On the basis of this knowledge of the absolute structure of the product the mechanism of the reaction is briefly discussed.