Chemoenzymatic Synthesis of Asymmetrically Branched Human Milk Oligosaccharide Lacto--Hexaose.

We herein reported the first chemoenzymatic synthesis of lacto--hexaose (LNH) by combining chemical carbohydrate synthesis with a selectively enzymatic glycosylation strategy. A tetrasaccharide core structure GlcNHβ1→3 (GlcNAcβ1→6) Galβ1→4Glc, a key precursor for subsequent enzymatic glycan extension toward asymmetrically branched human milk oligosaccharides, was synthesized in this work. When the order of galactosyltransferase-catalyzed reactions was appropriately arranged, the β1,4-galactosyl and β1,3-galactosyl moieties could be sequentially assembled on the C6-arm and C3-arm of the tetrasaccharide, respectively, to achieve an efficient LNH synthesis. Lacto--neotetraose (LNH), another common human milk oligosaccharide, was also synthesized en route to the target LNH.