Phenylsulfinyl-lithium exchange on glycosyl phenylsulfoxides leads to configurationally stable anomeric carbanions which can react stereoselectively with electrophiles. Thus, the reaction of 3,4-O-isopropylidene-α-l-fucopyranosyl phenylsulfoxide with tBuLi followed by treatment with isobutyraldehyde led to the α-configured C-glycoside; the β-anomer furnished the corresponding β-C-glycoside.