Stereoselectivity ketones, hydrogen abstraction

Stereoselective radical addition on the exo-face of levoglucosenone 250 leads to the C-linked dimer 251 in a modest 26% yield. The expected competing reaction is the direct hydrogen abstraction by the initial radical species. Stereoselective hemiketal and ketone reductions afford the 1,6-anhydro derivative 252, which is deprotected and opened to give the final C-ana-log of a /3-(1 4) disaccharide 253. 

Like (5-alkoxy ketones, (5-amido ketones also undergo photoinduced 8-hydrogen abstraction to give proline derivatives (in <50% chemical yield) (Scheme 6.126).967 The reaction stereoselectivity depends on the biradical cyclization rate, which competes with conformational changes. Whereas singlet biradicals couple without reaching conformational equilibrium, the triplet biradicals allow bond rotation before the ring forms. 

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