Efficient synthesis of O-(2-acetamido-2-deoxy-beta-D-glucopyranosyl)-serine and -threonine building blocks for glycopeptide formation

Glucosamine donors 1-3 having N-TCP, N,N-diacetyl and N-Teoc protection, respectively, give with N-alpha-Boc-protected serine and threonine benzyl esters 4a,b as acceptors exclusively the beta-glycosides; they can be transformed into O-GlcNAc serine and threonine derivatives 8a,b. The high yielding O-glycosylation of compounds 4a,b with trichloroacetimidate 3 and the ease of replacement of the N-Teoc group by the N-acetyl group prompted the use of N-alpha-Fmoc-protected serine and threonine allyl (9a,b) and Pfp active esters (12a,b) as accepters, thus very efficiently yielding the corresponding O-(N, O-acetylglucosamino) serine and threonine derivatives 11a,b and 14a,b as active esters.