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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 abstractable 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., This work was supported by the Investigation Programs of the Ministerio de Economía y Competitividad (CTQ2010‐18244) and of the Gobierno de Canarias (SolSub20081000192). D.A.‐D. thanks the Ministerio de Economía y Competitividad for a fellowship. C.R.‐F. thanks the Programa I3P‐CSIC for a postdoctoral contract.

A tandem 1,5-, 1,6- or 1,8-intramolecular hydrogen atom transfer (HAT)/intermolecular radical allylation using mono- or (1→4)-O-disaccharide models is described. It is a simple, sequential, stereocontrolled methodology for C-C bond formation by remote free radical functionalization, giving access to interesting carbohydrate structures carrying differently functionalized tethers. A tandem intramolecular hydrogen atom transfer (HAT)/intermolecular radical allylation is described. This process occurs by a completely regio- and stereoselective route that provides a simple methodology for C-C bond formation on remote positions without modifying the remainder of the carbohydrate., This work was supported by the Research Programs of the Gobierno de Canarias (SolSubC200801000192 and CTQ2010‐18244) of the Ministerio de Ciencia e Innovación, cofinanced by the Fondo Europeo de Desarrollo Regional (FEDER). I. P.‐M. thanks the Spanish Ministerio de Ciencia e Innovación for the Research Program PIE 200930I138. L. M. Q. thanks the Consejo Superior de Investigaciones Científicas (CSIC) (Program I3P) for fellowships.

The glycopyran-6-O-yl radical promoted hydrogen atom transfer reaction (HAT) between the two pyranose units of ¿-D-Manp-(1-4)-¿-D-Glcp and ¿-D-Manp-(1-4a)-4a-carba-¿-D-Glcp disaccharides provides supporting chemical evidence for the conformational differences between O- and C-glycosyl compounds. In the O-disaccharide the 6-alkoxyl radical, generated under oxidative or reductive conditions, abstracts exclusively the hydrogen at C-5' via a completely regioselective 1,8-HAT reaction. This may be attributable to the conformational restriction of the glycosidic and aglyconic bonds due principally to steric and stereoelectronic effects. On the contrary, very little regioselectivity is observed in the homologous C-disaccharide and a mixture of compounds generated by 1,5-, 1,6-, and 1,8-HAT processes where the abstraction occurs at hydrogen atoms positioned at C-4a, C-1', and C-5', respectively, has been obtained. This study has been extended to simpler O- and C-glycosides, where the aglycon was a straight n-alkyl alcohol tether of five atoms; in general, all of the results obtained are shown to be consistent with a major conformational flexibility of the C-glycosidic bond., This work was supported by the Spanish Ministerio de Educación y Ciencia (MEC); Research Programs CTQ2004‐06381/BQU, CTQ2004‐02367/BQU and CTQ2007‐67492/BQU, cofinanced by the Fondo Europeo de Desarrollo Regional (FEDER). I. P.‐M. and L. M. Q. thank the CSIC (Program I3P) for fellowships.