ChemInform Abstract: Photochemistry of α-Diketones in Carbohydrates: Anomalous Norrish Type II Photoelimination and Norrish-Yang Photocyclization Promoted by the Internal Carbonyl Group

A series of four α-diketones placed as 1α-pyruvoyl tethers on D-glucopyranose and D-glucopyranosiduronic acid skeletons was prepared in order to determine the influence of captodative and stereoelectronic effects on the regioselectivity of the hydrogen atom transfer (HAT) in Norrish typeII photochemical processes. We observed that the 1,5-HAT regioselectivity can be switched between the two potentially abstractable syn-1,3-diaxial hydrogens at H6 and H8. Highly unusual photoproducts from Norrish typeII photoelimination and Norrish-Yang photocyclization initiated by the excited internal carbonyl group were obtained, in some cases in excellent synthetic yield. The 1,5-HAT transition state in the Norrish typeII photoelimination was investigated by photochemical experiments in the crystalline state.