O-Glycosyl Amino Acids by 2-Nitrogalactal Concatenation − Synthesis of a Mucin-Type O-Glycan

Base‐promoted Michael‐type addition of N‐Boc‐ and N‐Fmoc‐protected serine and threonine esters to 2‐nitrogalactal derivatives 2 and 26 led highly selectively to α‐glycosides 4a−d and 27a,c, respectively. Ensuing transformation of threonine derivative 4d and serine derivatives 4a,b resulted in compounds useful as lysine and dipeptide mimetics. 6‐O‐Desilylation of 27a,c, then 6‐O‐sialylation, and transformation of the nitro group of the galactose moiety into a 2‐acetamido functionality, afforded N‐Boc‐protected serine and threonine tert‐butyl esters 31a,c carrying the O‐protected STN‐antigen at the hydroxy group. The threonine derivative 31c was then transformed into the N‐Fmoc‐protected amino acid building block 33, which was employed for the synthesis of mucin repeating unit partial structure Ac‐GS(STN)‐TAPPAHG‐NH2 (1). (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)