A simple strategy for the synthesis of the glycosylated collagen biomarkers alfa-D-glucopyranosyl-(1->2)-beta-D-galactopyranosyl-O-pyridinoline (Glc-Gal-PYD), alfa-D-glucopyranosyl-(1->2)-beta-D-galactopyranosyl-5-O-hydroxylysine (Glc-Gal-Hyl) and of their unnatural epimers Glc-Gal-epiPYD and Glc-Gal-epiHyl