The Experts below are selected from a list of 114 Experts worldwide ranked by ideXlab platform
Michelangelo Parrilli - One of the best experts on this subject based on the ideXlab platform.
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The role of sugar configuration in the Acetolysis of 6-deoxyhexose methyl glycosides.
Carbohydrate research, 2009Co-Authors: Luigi Cirillo, Emiliano Bedini, Annalida Di Nola, Michelangelo ParrilliAbstract:Abstract The Acetolysis of several perbenzylated 6-deoxyhexose methyl glycosides under two mild conditions (10 equiv ZnCl 2 in 2:1 v/v Ac 2 O–AcOH at 5 °C; 10:10:1 v/v/v Ac 2 O–AcOH–TFA at 70 °C) was studied. We focused on the effect of sugar configuration on the competition between mechanisms with activation at exocyclic or endocyclic oxygen site. No effect was detected in Acetolysis using the TFA protocol promoting an exo -activation mechanism, which affords 1- O -Ac-pyranosides regardless of sugar configuration. On the contrary, it has a primary role in determining the endo - versus exo -product distribution on ZnCl 2 -promoted Acetolysis.
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Acetolysis of 6 deoxysugar disaccharide building blocks exo versus endo activation
European Journal of Organic Chemistry, 2008Co-Authors: Luigi Cirillo, Emiliano Bedini, Michelangelo ParrilliAbstract:Two different protocols for the mild and selective Acetolysis of 6-deoxysugar methyl disaccharides under thermodynamic or kinetic control have been developed. The structures of the disaccharides obtained depend on the protocol used and, in the kinetically controlled cases, on the 6-deoxysugar configuration and protecting group pattern too. The behavior of 6-deoxyhexose oligosaccharides of different series (rhamno, quinovo, and fuco) under these two reaction conditions has been studied and rationalized based on the competition between exo versus endo oxygen activation in the Acetolysis mechanism.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)
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Selective Acetolysis of 6-deoxy-sugar oligosaccharide building blocks governed by the armed-disarmed effect
Tetrahedron Letters, 2008Co-Authors: Emiliano Bedini, Daniela Comegna, Annalida Di Nola, Michelangelo ParrilliAbstract:The effect of the arming–disarming protection in the Acetolysis of 6-deoxy-sugar oligosaccharides has been for the first time systematically investigated. Starting from the newly synthesized methyl glycosides, the Acetolysis conditions employed here afforded 1-O-Ac oligosaccharides selectively without cleavage of the interglycosidic bonds, if a suitable protecting group pattern was used. Actually, the behavior of armed–disarmed, armed-armed, and disarmed–disarmed 6-deoxy-sugar disaccharides in Acetolysis reactions was investigated: the results fit well with the prediction made on the basis of the armed–disarmed effect.
Harri Lönnberg - One of the best experts on this subject based on the ideXlab platform.
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Sulfuric acid-catalyzed Acetolysis of anomeric methyl 2,3,4,6-tetra-O-acetyl-d-mannopyranosides: kinetics and mechanism
Carbohydrate research, 2000Co-Authors: Józef Kaczmarek, Zbigniew Kaczyński, Zygmunt Trumpakaj, Janusz Szafranek, Magda Bogalecka, Harri LönnbergAbstract:Abstract The kinetics of the Acetolysis and accompanying anomerization of methyl 2,3,4,6-tetra- O -acetyl-α- and -β- d -mannopyranosides at different concentrations of sulfuric acid in acetic anhydride–acetic acid mixtures were studied. The progress of the reactions was followed by gas chromatography, and the rate constants of the partial reactions were calculated on the basis of the time-dependent product distribution obtained. The mechanisms of the reactions involved are discussed. The involvement of unstable ionic intermediates is taken into account in the evaluation of the kinetic results, and simplified and extended models are used in the mathematical treatment of the results. A fourth-order Runge–Kutta algorithm is used to calculate rate constants. Acetolysis was found to be faster for mannosides than for glucosides relative to their anomerization. The β-mannopyranoside prefers endocyclic CO-bond rupture, while in the α anomer the endocyclic and exocyclic cleavages are comparatively rapid.
Emiliano Bedini - One of the best experts on this subject based on the ideXlab platform.
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The role of sugar configuration in the Acetolysis of 6-deoxyhexose methyl glycosides.
Carbohydrate research, 2009Co-Authors: Luigi Cirillo, Emiliano Bedini, Annalida Di Nola, Michelangelo ParrilliAbstract:Abstract The Acetolysis of several perbenzylated 6-deoxyhexose methyl glycosides under two mild conditions (10 equiv ZnCl 2 in 2:1 v/v Ac 2 O–AcOH at 5 °C; 10:10:1 v/v/v Ac 2 O–AcOH–TFA at 70 °C) was studied. We focused on the effect of sugar configuration on the competition between mechanisms with activation at exocyclic or endocyclic oxygen site. No effect was detected in Acetolysis using the TFA protocol promoting an exo -activation mechanism, which affords 1- O -Ac-pyranosides regardless of sugar configuration. On the contrary, it has a primary role in determining the endo - versus exo -product distribution on ZnCl 2 -promoted Acetolysis.
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Acetolysis of 6 deoxysugar disaccharide building blocks exo versus endo activation
European Journal of Organic Chemistry, 2008Co-Authors: Luigi Cirillo, Emiliano Bedini, Michelangelo ParrilliAbstract:Two different protocols for the mild and selective Acetolysis of 6-deoxysugar methyl disaccharides under thermodynamic or kinetic control have been developed. The structures of the disaccharides obtained depend on the protocol used and, in the kinetically controlled cases, on the 6-deoxysugar configuration and protecting group pattern too. The behavior of 6-deoxyhexose oligosaccharides of different series (rhamno, quinovo, and fuco) under these two reaction conditions has been studied and rationalized based on the competition between exo versus endo oxygen activation in the Acetolysis mechanism.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)
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Selective Acetolysis of 6-deoxy-sugar oligosaccharide building blocks governed by the armed-disarmed effect
Tetrahedron Letters, 2008Co-Authors: Emiliano Bedini, Daniela Comegna, Annalida Di Nola, Michelangelo ParrilliAbstract:The effect of the arming–disarming protection in the Acetolysis of 6-deoxy-sugar oligosaccharides has been for the first time systematically investigated. Starting from the newly synthesized methyl glycosides, the Acetolysis conditions employed here afforded 1-O-Ac oligosaccharides selectively without cleavage of the interglycosidic bonds, if a suitable protecting group pattern was used. Actually, the behavior of armed–disarmed, armed-armed, and disarmed–disarmed 6-deoxy-sugar disaccharides in Acetolysis reactions was investigated: the results fit well with the prediction made on the basis of the armed–disarmed effect.
Józef Kaczmarek - One of the best experts on this subject based on the ideXlab platform.
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Sulfuric acid-catalyzed Acetolysis of anomeric methyl 2,3,4,6-tetra-O-acetyl-d-mannopyranosides: kinetics and mechanism
Carbohydrate research, 2000Co-Authors: Józef Kaczmarek, Zbigniew Kaczyński, Zygmunt Trumpakaj, Janusz Szafranek, Magda Bogalecka, Harri LönnbergAbstract:Abstract The kinetics of the Acetolysis and accompanying anomerization of methyl 2,3,4,6-tetra- O -acetyl-α- and -β- d -mannopyranosides at different concentrations of sulfuric acid in acetic anhydride–acetic acid mixtures were studied. The progress of the reactions was followed by gas chromatography, and the rate constants of the partial reactions were calculated on the basis of the time-dependent product distribution obtained. The mechanisms of the reactions involved are discussed. The involvement of unstable ionic intermediates is taken into account in the evaluation of the kinetic results, and simplified and extended models are used in the mathematical treatment of the results. A fourth-order Runge–Kutta algorithm is used to calculate rate constants. Acetolysis was found to be faster for mannosides than for glucosides relative to their anomerization. The β-mannopyranoside prefers endocyclic CO-bond rupture, while in the α anomer the endocyclic and exocyclic cleavages are comparatively rapid.
Yves L. Dory - One of the best experts on this subject based on the ideXlab platform.
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Experimental and Theoretical Evidence of Through-Space Electrostatic Stabilization of the Incipient Oxocarbenium Ion by an Axially Oriented Electronegative Substituent During Glycopyranoside Acetolysis
The Journal of Organic Chemistry, 1997Co-Authors: Momcilo Miljkovic, David Yeagley, And Pierre Deslongchamps, Yves L. DoryAbstract:The rate of Acetolysis of methyl 2,3,6-tri-O-methyl-α-d-galacto- and -glucopyranosides depends strongly on the electronegativity of the C4 substituent. Thus, of the three derivatives studied (4-methoxy, 4-acetoxy, and 4-acetamido-4-deoxy derivatives of d-galacto- and d-glucopyranosides), the glycopyranosides bearing the most electronegative C4 substituent (methoxy group) acetolyze at the fastest rate, whereas those having the least electronegative C4 substituent (N-acetamido group) acetolyze at the slowest rate. Furthermore, whereas the influence of the electronegativity of the C4 substituent upon the Acetolysis rate in the d-gluco series is relatively moderate (kmax/kmin = 2.9−3.5), this influence is very large in the d-galacto series (kmax/kmin = 44.4−58.6). We propose that this observation can only be explained by the existence of an electron donation process from the axially oriented electronegative substituent at the C4 carbon atom of the galactopyranoside ring to the forming oxocarbenium ion. Such a...