Intramolecular Hydrogen Atom Abstraction: The β-Oxygen Effect in the Norrish Type II Photoreaction

Article abstract of DOI:10.1016/0040-4020(95)00755-W

The preparation of phenacyl glycosides of α- and β-tetrabenzylglucopyranose, of α-tetrabenzylmannopyranose, and of 2-hydroxytetrahydropyran is described.Photolysis of each compound through Pyrex results cleanly in lactone formation by the Norrish type II photocleavage.Competition experiments are described which demonstrate that both glycosyl anomers are consumed at the same rate in the course of the photolysis.Similar experiments establish that the phenacyl α-glucoside is marginally more reactive than the corresponding α-mannoside.The tetrahydropyran derivative is cleaved much more rapidly the carbohydrates.This effect is interpreted in terms destabilization of the polarized transition state for hydrogen atom abstraction by the β-oxygen bonds in the carbohydrate series. - Key words: Hydrogen atom abstraction; Norrish II reaction; β-oxygen effect; 1-alkoxy-1-glycosyl radicals; phenacyl ethers