C-Glycosyl radicals obtained from iodide 1 react with benzo- and naphtho-quinone to afford, after deprotection, C-glycosyl quinones 4 and 6 which inhibit yeast α-glucosidase; employing a naphthoquinone linked to the non-reducing end of a sugar 7, a spacer-connected C-disaccharide 8 is obtained.