Representative RP-HPLC analysis of coupling reactions. Traces of three potential products in the coupling reaction with Fmoc-Ser(Ac3GalNAcα)-OH: Fmoc-Pro-Gly-Hex-OH (6) (starting material, black); Fmoc-d-Ser(Ac3GalNAcα)-Pro-Gly-Hex-OH (2b, non-natural epimer, red); Fmoc-l-Ser(Ac3GalNAcα)-Pro-Gly-Hex-OH (2a, desired product, blue); mixture of three components at a ratio of 1:2:4 (purple).