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 type II photochemical processes. We observed that the 1,5‐HAT regioselectivity can be switched between the two potentially able syn‐1,3‐diaxial hydrogens at H6 and H8. Highly unusual photoproducts from Norrish type II 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 type II photoelimination was investigated by photochemical experiments in the crystalline state. Elimination versus cyclization: Photolysis of 1α‐pyruvoyl diketones of D‐glucopyranosyl and D‐glucopyranosiduronic acid carbohydrate skeletons led to Norrish type II photoelimination or Norrish–Yang photocyclization products via a completely regioselective 1,5‐hydrogen atom transfer promoted by the excited internal carbonyl group, depending on the stability of the 1,4‐biradical intermediate (see scheme, DPS=tert‐butyldiphenylsilyl).