Penta-O-acetyl-β-D-glycopyranoses and 1,4-dimethoxybenzene led selectively by electrophilic substitution to C-β-Dglycopyranosyl-1,4-dimethoxybenzenes which were converted by simple and efficient reactions (oxidation, reduction and deacetylation) to the corresponding C-glycosylhydroand C-glycosylbenzoquinones, with either an acetylated or deprotected sugar moiety. C-β-D-Glucosylbenzoquinone 19 and C-β-D-Glucosylhydroquinone 23 were found to be competitive inhibitors of rabbit muscle glycogen phosphorylase b (GPb), with respect to the substrate α-D-glucose-1-phosphate, with Ki values of 1.3 and 0.9 mM, respectively, whereas C-β-D-glucosylhydroquinone 17 was not effective up to a concentration of 8 mM. In order to elucidate the structural basis of inhibition, we determined the crystal structures of 19 and 23 in complex with GPb at a 2.03-2.05 ? resolution. The complex structures reveal that the inhibitors can be accommodated at the catalytic site at approximately the same position as α-D-glucose and stabilise the transition state conformation of the 280s loop by making several favourable contacts to Asp283 and Asn284 of this loop.