A MECHANISTIC STUDY OF THE PHOTOCHEMISTRY OF CARBOHYDRATE P-TOLUENESULFONATES

Photolysis of dene-6-O-(p-tolylsulfonyl)-alpha-D-galactopyranose (5) i n methanol under nitrogen in the presence of sodium hydroxide or diaza bicyclo[2.2.2]octane (3, DABCO) produces toluene and ,2:3,4-di-O-isopr opylidene-alpha-D-galactopyranose (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 g ive the p-tolylsulfonyl radical(2) and deprotonated 6, which accepts a proton from the solvent. The p-tolylsulfonyl radical (2) then abstrac ts a hydrogen atom from the solvent to give p-toluenesulfinic acid, a compound that is converted to the p-toluenesulfinate anion (8) under t he basic reaction conditions. Photolysis of 8 completes the reaction s equence by forming toluene.