 Glycosyl group

A glycosyl group is a substituent obtained formally by removing the hydroxy group at the anomeric carbon of a cyclic monosaccharide. The resulting attachment site connects that sugar-derived unit to another part of a molecule. Removing the whole hydroxy group differs from removing only its hydrogen: the latter leaves an oxygen at the attachment site.^[ref]

For an atom-count example, cyclic glucose has molecular formula ${\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6$. Subtracting an anomeric $\mathrm{OH}$ gives a glycosyl group fragment with composition ${\mathrm{C}}_6{\mathrm{H}}_{11}{\mathrm{O}}_5$. Attaching that fragment to $\mathrm{OC}{\mathrm{H}}_3$ gives ${\mathrm{C}}_7{\mathrm{H}}_{14}{\mathrm{O}}_6$. The oxygen joining the sugar-derived group to the methyl group belongs to the attached $\mathrm{OC}{\mathrm{H}}_3$ fragment in this bookkeeping. The formal subtraction describes connectivity, rather than prescribing a preparative reaction.

A name also needs the ring form and, when specified, the anomer configuration. Thus the glycosyl naming rules retain information about the cyclic sugar and its attachment; a molecular formula alone does not distinguish all such structures. Glycosyl groups occur in names of glycosides and oligosaccharides, connecting functional group nomenclature with glycosidic bonds.^[ref]