A Mechanistic Study of the Photochemistry of Carbohydrate p-Toluenesulfonates

Photolysis of 1,2:3,4-di-O-isopropylidene-6-O-(p-tolylsulfonyl)-α-D-galactopyranose (5) in methanol under nitrogen in the presence of sodium hydroxide or diazabicyclo[2.2.2]octane (3, DABCO) produces toluene and 1,2:3,4-di-O-isopropylidene-α-D-gatactopyranose (6). Electron transfer from DABCO or hydroxide ion to singlet excited 5 to generate a radical anion is the first step in this reaction. The radical anion rapidly fragments to give the p-tolylsulfonyl radical (2) and deprotonated 6, which accepts a proton from the solvent. The p-tolylsulfonyl radical (2) then abstracts a hydrogen atom from the solvent to give p-toluenesulfinic acid, a compound that is converted to the p-toluenesulfinate anion (8) under the basic reaction conditions. Photolysis of 8 completes the reaction sequence by forming toluene.