Aglycone

An aglycone is the nonsugar partner recovered when the carbohydrate residue of a glycoside is removed. For the classical oxygen-linked description, IUPAC identifies that partner as the compound $\mathrm{ROH}$. The sugar-derived residue is sometimes called the glycone.^[ref]

Methyl α-D-glucopyranoside supplies a concrete example. Its glycosidic attachment contains $\mathrm{OC}{\mathrm{H}}_3$, so $R$ is a methyl group, $\mathrm{C}{\mathrm{H}}_3$. The corresponding aglycone is methanol, $\mathrm{C}{\mathrm{H}}_3\mathrm{OH}$. The fragment $R$ and the compound $\mathrm{ROH}$ are different chemical objects; a label on a structural drawing should not silently replace the recovered partner molecule.

As original formula bookkeeping, combining free glucose with methanol and subtracting water gives the molecular formula ${\mathrm{C}}_7{\mathrm{H}}_{14}{\mathrm{O}}_6$ of the methyl glycoside. Reversing that balance accounts for the two partners of formal hydrolysis. It says nothing about reaction speed or the conditions needed for cleavage.

Calling a compound an aglycone describes its relationship to a particular glycoside; it does not by itself identify a biological activity. Oxygen-, nitrogen-, sulfur- and carbon-linked sugar derivatives require their corresponding attachment descriptions, so the $\mathrm{ROH}$ example should not be applied indiscriminately to every sugar-containing compound.^[ref]