Regioselective Acetylation
of Diols and Polyols by
Acetate Catalysis: Mechanism and Application
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Abstract
We propose a principle for H-bonding
activation in acylation of
hydroxyl groups, where the acylation is activated by the formation
of hydrogen bonds between hydroxyl groups and anions. With the guidance
of this principle, we demonstrate a method for the selective acylation
of carbohydrates. By this method, diols and polyols are regioselectively
acetylated in high yields under mild conditions using catalytic amounts
of acetate. In comparison to other methods involving reagents such
as organotin, organoboron, organosilicon, organobase, and metal salts,
this method is more environmentally friendly, convenient, and efficient
and is also associated with higher regioselectivity. We have performed
a thorough quantum chemical study to decipher the mechanism, which
suggests that acetate first forms a dual H-bond complex with a diol,
which enables subsequent monoacylation by acetic anhydride under mild
conditions. The regioselectivity appears to originate from the inherent
structure of the diols and polyols and their specific interactions
with the coordinating acetate catalyst