An expeditious and efficient synthesis of β-d-galactopyranosyl-(1→3)-d-N-acetylglucosamine (lacto-N-biose) using a glycosynthase from Thermus thermophilus as a catalyst

Mutant glycosynthases or transglycosidases obtained from a Thermus thermophilus β-d-glycosidase (TtbGly) efficiently catalyzed the synthesis of β-(1→3)-disaccharides. Unfortunately, this regioselectivity was changed to the β-(1→4) one when N-acetylglucosamine derivatives were used as acceptors, thus precluding the possibility of synthesizing d-Galp-β-(1→3)-d-GlcpNAc (lacto-N-biose) or d-Glcp-β-(1→3)-d-GlcpNAc, which are useful synthons for the synthesis of antigen determinants. In contrast, we show in this work that, in the presence of phenyl 2-amino-1-thio-β-d-glucopyranoside, the 'normal' β-(1→3) regioselectivity of E338G TtbGly glycosynthase takes place. Thus, transglycosylations using α-galactosyl or α-glucosyl fluorides gave the corresponding phenyl β-d-glycopyranosyl-(1→3)-2-amino-2-deoxy-1-thio-β-d-glucopyranosides in high yields (88-97%). Subsequent selective N-acylation followed by NBS/water deprotection of the thiophenyl group afforded lacto-N-biose in high overall yields.