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Biomedical subjects

Jon A Tunge

Publications and source records attributed to Jon A Tunge.

16 recordsLinked to original sources
PubMed2006

Stereochemical control in the reduction of 2-chromanols.

[reaction: see text] Reduction of C5-substituted 2-hydroxychromans selectively provides 2,4-cis-chromans using large silane reductants and 2,4-trans-chromans using the smaller silane PhSiH(3). The stereochemical outcome has been rationalized on the basis of a Curtin-Hammett kinetic situation arising from hydride delivery to two different conformations of an intermediate oxocarbenium ion. This method provides a powerful way to control the relative stereochemistry of these substructures which are prevalent in bioactive natural products.

Biological Products↗
PubMed2006

Synthesis of homoallylic amines via the palladium-catalyzed decarboxylative coupling of amino acid derivatives.

Protected homoallylic amines are synthesized by the decarboxylative coupling of alpha-amino acid derivatives. The catalytic C-C bond-forming reaction relies on the bioinspired decarboxylative metalation of alpha-amino acids to produce alpha-amino anion equivalents. The alpha-amino anion equivalents are intercepted by pi-allyl palladium electrophiles to produce substituted homoallylic amines.

Amines↗
PubMed2006

Decarboxylative ring contractions and olefin insertions of vinyl oxazinanones.

[Structure: see text] 6-Vinyl oxazinanones undergo catalytic, diastereoselective, decarboxylative ring contraction to form vinyl azetidines in good yield. Performing the decarboxylation in the presence of Michael acceptors results in decarboxylative olefin insertion to provide diastereoenriched substituted vinyl piperidines.

Journal Article↗
PubMed2006

Identification of sulfation sites of metabolites and prediction of the compounds' biological effects.

Characterizing the biological effects of metabolic transformations (or biotransformation) is one of the key steps in developing safe and effective pharmaceuticals. Sulfate conjugation, one of the major phase II biotransformations, is the focus of this study. While this biotransformation typically facilitates excretion of metabolites by making the compounds more water soluble, sulfation may also lead to bioactivation, producing carcinogenic products. The end result, excretion or bioactivation, depends on the structural features of the sulfation sites, so obtaining the structure of the sulfated metabolites is critically important. We describe herein a very simple, high-throughput procedure for using mass spectrometry to identify the structure-and thus the biological fate-of sulfated metabolites. We have chemically synthesized and analyzed libraries of compounds representing all the biologically relevant types of sulfation products, and using the mass spectral data, the structural features present in these analytes can be reliably determined, with a 97% success rate. This work represents the first example of a high-throughput analysis that can identify the structure of sulfated metabolites and predict their biological effects.

Alcohols↗
PubMed2005

Catalytic decarboxylative sp-sp3 coupling.

A palladium-catalyzed 1,4-enyne synthesis was developed based the decarboxylative coupling of acetylides with allyl electrophiles. Stereochemical studies have implicated palladium-allyl-acetylides as intermediates. Thus, decarboxylative metalation was established as an environmentally benign alternative to transmetalation from alkynyl tin reagents.

Journal Article↗
PubMed2005

Sequential Pd(II)-Pd(0) catalysis for the rapid synthesis of coumarins.

Electrophilic palladium-catalyzed cycloisomerization of brominated aryl propiolates produces brominated coumarins. The brominated coumarins can be diversified by reduction of the Pd(II) catalyst to Pd(0) followed by Suzuki, Sonogashira, Heck, or Hartwig-Buchwald coupling. Thus, a single loading of precatalyst can be used to conduct sequential reactions, allowing the synthesis of functionalized coumarins. Extension of this methodology toward the synthesis of coumarin libraries is discussed.

Bromine↗
PubMed2005

Ruthenium-catalyzed decarboxylative insertion of electrophiles.

[reaction: see text]. A ruthenium complex, Cp*Ru(bipyridyl)Cl, has been developed as a catalyst for the first regioselective tandem Michael addition-allylic alkylation of activated Michael acceptors. The net transformation is the decarboxylative insertion of Michael acceptors into allyl beta-ketoesters.

Journal Article↗
PubMed2004

Asymmetric allylic alkylation of ketone enolates: an asymmetric Claisen surrogate.

[reaction: see text] The combination of catalytic palladium(0) and Trost ligand provides an effective catalyst for the rearrangement of allyl beta-ketoesters. The mechanism of the transformation involves formation of pi-allyl palladium intermediates which undergo enantioselective attack by ketone enolates. Decarboxylation of beta-ketocarboxylates allows regiospecific generation of enolates under extremely mild conditions.

Journal Article↗
PubMed2004

Ruthenium-catalyzed decarboxylative allylation of nonstabilized ketone enolates.

[reaction: see text] Bipyridyl(pentamethylcyclopentadienyl)ruthenium chloride is an efficient catalyst for the formal [3,3] rearrangement of allyl beta-ketoesters. The mechanism of the transformation involves formation of pi-allyl ruthenium intermediates, which are selectively attacked at the more substituted allyl terminus by freely diffusing enolates. Decarboxylation of beta-ketocarboxylates allows generation of enolates under extremely mild conditions.

Journal Article↗
PubMed2004

Selective selenocatalytic allylic chlorination.

Ene-chlorination of olefins by N-chlorosuccinimide is catalyzed by phenylselenenyl chloride. This reaction demonstrates the catalytic conversion of C-H bonds into more reactive C-Cl bonds. [reaction: see text]

Journal Article↗
PubMed2003

Structural dependence of thermodynamics of alkene binding to yttrium alkyl complexes and of kinetics of alkyl migration to coordinated alkenes.

Agostic interactions in yttrium alkyls are structure dependent. Primary alkyl yttrium complexes have beta-CH(2) agostic interactions at low temperature, but a shift toward alpha-agostic interactions occurs on warming. For the more crowded beta-disubstituted yttrium alkyls, an alpha-CH(2) agostic interaction is seen. The thermodynamics of alkene binding to the primary alkyl yttrium complex Cp(2)YCH(2)CH(2)CH(CH(3))(2) (2) depend strongly on the structure of the alkene. A single allylic substituent on the alkene has a small effect on alkene binding, but a second allylic substituent has a large destabilizing effect. Propene binding to yttrium alkyls is largely independent of the nature of the alkyl ligand. Equilibrium constants for propene binding to n-, gamma-substituted, beta-substituted, and secondary alkyl yttrium complexes are similar. The rate of migration of an alkyl group to a coordinated alkene depends strongly on the structure of the alkyl group: n-alkyl approximately gamma-substituted >> beta-substituted >> alpha-substituted. The approximately 200-fold slower insertion of propene into Cp(2)YCH(2)CH(CH(3))(2) (6) than that into Cp(2)YCH(2)CH(2)CH(CH(3))(2) (2) is therefore due to kinetically slow migration of the beta-disubstituted alkyl group of 6 and not to differences in the equilibrium binding of propene. Processes related to chain transfer and site epimerization at the metal center are also reported.

Journal Article↗
PubMed2002

Asymmetric synthesis of silylated alpha-amino acid esters through dynamic kinetic resolution.

Esters of three types of silylated alpha-amino acids have been prepared from appropriate zirconaaziridines. Slow addition (syringe pump) of the (R,R) carbonate of trans-stilbene gave metallacycles with the maximum possible diastereomeric excess (as determined by the diastereomeric excess produced by the Hoffmann test-the same reaction but with racemic carbonate). Methanolysis gave esters (RO(2)C)CH(R')(NHPh) (R' = Me(3)Si, Me(3)SiCH(2), and p-Me(3)SiC(6)H(4)) with the same optical purity at the alpha carbon.

Algorithms↗