Papers by Gilbert Tumambac
Steroids, 2009
s t e r o i d s 7 4 ( 2 0 0 9 ) 30-41 a v a i l a b l e a t w w w . s c i e n c e d i r e c t . c... more s t e r o i d s 7 4 ( 2 0 0 9 ) 30-41 a v a i l a b l e a t w w w . s c i e n c e d i r e c t . c o m j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / s t e r o i d s A comparative study of enol aldehyde formation from betamethasone, dexamethasone, beclomethasone and related compounds under acidic and alkaline conditions Betamethasone Dexamethasone Beclomethasone Enol aldehyde a b s t r a c t
Organic Letters, 2005
An axially chiral 1,8-dipyridylnaphthalene N,N&am... more An axially chiral 1,8-dipyridylnaphthalene N,N'-dioxide has been employed in enantioselective fluorescence analysis of the enzymatic kinetic resolution of trans-1,2-diaminocyclohexane. The procedure eliminates cumbersome purification and derivatization steps required by traditional methods. The results demonstrate the potential of fluorescence spectroscopy using suitable chiral chemosensors for real-time analysis of the enantiomeric composition of chiral compounds and for high-throughput screening of asymmetric reactions. [structure: see text]
Journal of Organic Chemistry, 2005
Axially chiral 1,8-bis(2,2′-diphenyl-4,4′-diquinolyl)naphthalene, 8, and 1, 8-bis(2,2′-diisopropy... more Axially chiral 1,8-bis(2,2′-diphenyl-4,4′-diquinolyl)naphthalene, 8, and 1, 8-bis(2,2′-diisopropyl-4,4′diquinolyl)naphthalene N, N′-dioxide, 9, have been prepared to study the stereodynamics of these and other 1,8-diheteroarylnaphthalenes based on reversible first-order isomerization kinetics and crystallographic data. The ratio of the two enantiomeric anti-conformers to the meso syn-isomer of 8 and 9 was determined as 1.2:1 and 9.6:1. Investigation of the conformational stability of the atropisomers at enhanced temperatures using HPLC and NMR spectroscopy revealed a Gibbs activation energy of 122.4 (121.8) kJ/mol and 115.2 (109.0) kJ/mol for the anti/syn-(syn/anti)isomerization of 8 and 9, respectively. Comparison of the conformational stability of a series of 1,8-dipyridylnaphthalenes and 1,8-diquinolylnaphthalenes shows that the latter exhibit a significantly higher rotational energy barrier. While the syn-and anti-isomers of 1,8-dipyridylnaphthalenes interconvert rapidly at room temperature the stereoisomers of 1,8-diquinolylnaphthalenes can be isolated by chromatography or crystallization and stored at 25°C for several months without any sign of racemization. The conformational stability of 1,8-diquinolylnaphthalenes is a consequence of significantly increased steric hindrance to isomerization in a highly congested T-shaped transition state. Conversion of 1,8-diheteroarylnaphthalenes to their corresponding N,N′-dioxides was found to result in an increased anti/syn-ratio and decreased rotational energy barrier, which was attributed to synergistic repulsive dipole/dipole interactions destabilizing the diastereomeric ground states and facilitated out-of-plane bending reducing the steric hindrance in the T-shaped transition state.
Chirality, 2005
The kinetics of the racemization of 2-benzoylcyclohexanone 1 in hexanes, ethanol, and mixtures th... more The kinetics of the racemization of 2-benzoylcyclohexanone 1 in hexanes, ethanol, and mixtures thereof have been investigated by time dependence of enantiomeric purity using enantioselective HPLC. In pure hexanes and ethanol, the racemization half-lives were determined as 552 and 23.8 min, respectively, at 66°C. Surprisingly, racemization of 1 in mixtures of hexanes and ethanol was found to involve an induction period followed by a sigmoidal decrease of the enantiomeric excess with half-lives varying between 11.5 and 24.0 min. This unexpected solvent influence on the rate of racemization of 1 was attributed to complex isomerization mechanisms involving three possibly interconverting enol tautomers of 1.Chirality 17:171–176, 2005. © 2005 Wiley-Liss, Inc.
Cheminform, 2004
For Abstract see ChemInform Abstract in Full Text.
Tetrahedron, 2004
The use of anti-1,8-bis(2,2′-diisopropyl-4,4′-diquinolyl)naphthalene, 1, for metal ion-selective ... more The use of anti-1,8-bis(2,2′-diisopropyl-4,4′-diquinolyl)naphthalene, 1, for metal ion-selective fluorescence recognition has been investigated. Employing CuCl2, ZnCl2, FeCl2, and FeCl3 in fluorescence titration experiments of 1 revealed formation of a bluegreen light emitting bimetallic complex. A dramatic red-shift of the fluorescence maximum of 1 and metal ion-selective quenching was observed in the presence of Cu(II), Fe(II), and Fe(III)chlorides in acetonitrile. By
Journal of Physical Chemistry A, 2003
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Papers by Gilbert Tumambac