"Locked-ring" C-glycoside derivatives may be prepared wherein the ring of the sugar molecule remains intact without the need for any protecting groups. These C-glycoside derivatives may be produced by first reacting an aldose reducing sugar, which may be a hexose or a pentose, with a .beta.-diketone to form a C-glycoside ketone. The C-glycoside ketone is then reacted with a ketone reactive compound, such as a hydrazine or hydroxylamine, optionally linked to a detectable label, to form a C-glycoside derivative wherein the ketone reactive compound is conjugated to the C-glycoside at the site of the ketone. The aldose reducing sugar used in the first reaction may a simple pentose or hexose monosaccharide, or it may be optionally substituted.