Sulfuric acid-catalyzed acetolysis of anomeric methyl 2,3,4,6-tetra-O-acetyl-D-mannopyranosides: kinetics and mechanism.
The kinetics of the acetolysis and accompanying anomerization of methyl 2,3,4,6-tetra-O-acetyl-alpha- and -beta-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 beta-mannopyranoside prefers endocyclic CO-bond rupture, while in the alpha anomer the endocyclic and exocyclic cleavages are comparatively rapid.
Kaczmarek
J
J
Department of Chemistry, University of Gdańsk, Poland.
Kaczyński
Z
Z
Trumpakaj
Z
Z
Szafranek
J
J
Bogalecka
M
M
Lönnberg
H
H
D000085:Acetates / Q000737:chemistry; D000465:Algorithms; D002236:Carbohydrate Conformation; D002849:Chromatography, Gas; D007700:Kinetics; D008757:Methylglucosides / Q000737:chemistry; D008766:Methylmannosides / Q000737:chemistry*; D013464:Sulfuric Acids / Q000737:chemistry
D000085:Acetates; D008757:Methylglucosides; D008766:Methylmannosides; D013464:Sulfuric Acids; C033158:sulfuric acid