Aldose to ketose sugar isomerization, an ubiquitous reaction in carbohydrate chemistry, can occur via intermolecular proton transfer or intramolecular hydride shift. In biological systems the first mechanism is preferred in presence of enzymes that do not have a metal center, while the latter has been reported to be favored by metalloenzymes, even when the metal is not involved in the stabilization of the transition state. In the hydride transfer mechanism, at the transition state the hydride may be stabilized by hydridic-to-protonic hydrogen bonding if proton donors such as hydroxyl or amino are present nearby. Gas phase computational studies of lithium salts of 7-substituted endo-3-hydroxybicyclo[2.2.1]hept-5-en-2-on activation barrier is lowered by up to 3.3 kcal/mol by hydridic-to-protonic hydrogen bonding when the proton donor is an ammonium group. Efforts to experimentally verify the computational results have focused on synthesis and characterization of a series of compounds with similar rigid framework, 3-hydroxybicyclo[2.2.1]heptan-2-one, functionalized at C(7) with different proton donor groups. The compounds were then subjected to kinetic measurements and the preliminary results are presented and discussed from the perspective of hydridic-to-protonic hydrogen bonding.

Sugar isomerization via hydride transfer mechanism assisted by hydridic-to-protonic hydrogen bonding

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