Reaction kinetics and mechanism of acid-catalyzed anomerization of 1-O-acetyl-2,3,5-tri-O-benzoyl-l-ribofuranose

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

“…[3,18] Our previous study evidenced that the ring-opening process takes place via protonation of the ring oxygen and that this acyclic intermediate (C) with a free hydroxyl group at C(4) can either ring-close or be acetylated to form the acyclic compound 5. [17] This proposal is in accordance with the results of Painter [19] and Miljković [5a] indicating that the acetylated oxygen at C(4) is not a strong enough nucleophile for ringclosing.…”

“…Recently, we reported on the acid-catalyzed anomerization of 1-O-acetyl-2,3,5-tri-O-benzoyl--ribofuranose [17] and hence decided to use the same reaction conditions in the present study in order to enable the utilization of our earlier results in the current experiments. Both the reactions from the acetylated (1 and 2) as well as the benzoylated derivatives (7 and 8) gave a reaction mixture containing seven compounds: the anomeric methyl furanosides, the anomeric 1-O-acetyl furanoses, the isomeric methyl hemiacetals and the fully acylated ribose hydrate.…”

“…Based on our earlier results, [17] we propose a mechanism for the anomerization of 1-O-acetylated ribofuranoses (3, 4 and 9,10), where the acetyl cation formed in the acetylation media activates the anomeric acetyl group for exo-cyclic cleavage. The main pathway for the formation of the fully acylated acyclic -ribose hydrates (6 and 12) proceeds via acetolysis of the acetyl methyl hemiacetals (5 and 11), while being also to some extent formed via ring-opening of the 1-O-acetylated ribofuranoses (3, 4 and 9,10).…”

“…Further, some formation of the per-acetylated ribose hydrate (6) was observed which is in good agreement with the results on the anomerization of the benzoylated ribofuranose acetates 9 and 10 under these conditions. [17] When the acetolysis reactions were carried out starting from the methyl 2,3,5-tri-O-benzoyl-α-(7) and β--ribofuranoside (8), the reaction followed a similar pattern as for the acetylated counterparts 1 and 2. The time-dependent product distribution for the acetolysis reaction is plotted in Figure 4.…”

“…Nevertheless, we do not question the presence of the acetyl cation in this type of acetylating mixtures, neither do we question its role as a catalytically active source. In our previous study, [17] we reported that the acetyl cation is the source of catalytic activity in the anomerization of the benzoylated 1-O-acetyl ribofuranoses 9 and 10. We showed that under various reaction conditions there is no change in the overall reaction path of the anomerization as long as acetic anhydride, the source of the acetyl cation, is present.…”