Reaction Kinetics and Mechanism of Sulfuric Acid-Catalyzed Acetolysis of Acylated Methyl L-Ribofuranosides

The mechanism of the sulfuric acid-catalyzed acetolysis of methyl 2,3,5-tri-O-acetyl- and methyl 2,3,5-tri-O-benzoyl-L-ribofuranosides and the accompanying anomerization of both the starting material and the 1,2,3,5-tetra-O-acetyl- and 1-O-acetyl-2,3,5-tri-O-benzoyl-L-ribofuranoses formed was investigated. The progress of the reactions was followed by H-1 NMR spectroscopy and the rate constants for the reactions were determined for a proposed kinetic model. The role of H+ and Ac+ as the catalytically active species was clarified, proving that the anomerization of the acylated methyl furanosides is activated by protonation, while, on the contrary, the anomerization of the 1-O-acetyl ribofuranoses is activated by the acetyl cation. The anomerization of the acylated methyl furanosides was verified to be activated on the ring oxygen leading to endocyclic CO-bond rupture while the 1-O-acetyl ribofuranoses are activated on the acetyloxy group on C(1) leading to exocyclic cleavage. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)