Chemo‐Enzymatic Synthesis of Isomeric I‐branched Polylactosamines Using Traceless Blocking Groups

Abstract

Poly‐N‐acetyl lactosamines (polyLacNAc) are common structural motifs of N‐ and O‐linked glycan, glycosphingolipids and human milk oligosaccharides. They can be branched by the addition of β1,6‐linked N‐acetyl‐glucosamine (GlcNAc) moieties to internal galactoside (Gal) residues by the I‐branching enzyme beta‐1,6‐N‐acetylglucosaminyltransferase 2 (GCNT2). I‐branching has been implicated in many biological processes and is also associated with various diseases such as cancer progression. Currently, there is a lack of methods that can install, in a regioselective manner, I‐branches and allows the preparation of isomeric poly‐LacNAc derivatives. Here, we described a chemo‐enzymatic strategy that addresses this deficiency and is based on the enzymatic assembly of an oligo‐LacNAc chain that at specific positions is modified by a GlcNTFA moiety. Replacement of the trifluoroacetyl (TFA) moiety by tert‐butyloxycarbonyl (Boc) gives compounds in which the galactoside at the proximal site is blocked from modification by GCNT2. After elaboration of the antennae, the Boc group can be removed, and the resulting amine acetylated to give natural I‐branched structures. It is also shown that fucosides can function as a traceless blocking group that can provide complementary I‐branched structures from a single precursor. The methodology made it possible to synthesize a library of polyLacNAc chains having various topologies.

A Biomimetic Synthetic Strategy Can Provide Keratan Sulfate I and II Oligosaccharides with Diverse Fucosylation and Sulfation Patterns

“…These enzymes can install glycosidic linkages in a regio- and stereospecific manner and reactions can be driven to completion by using alkaline phosphatases to hydrolyze nucleotide products that can act as product inhibitor. − The efficiency of glycosyltransferase mediated glycan assembly has made it possible to prepare complex oligosaccharides in an automated fashion. − Complex glycans can have several acceptor sites for a given glycosyltransferase, making it difficult to prepare discrete compounds. Site selective glycosylations can, however, be accomplished by careful synthetic planning, the use of unnatural sugar nucleotide donors, , chemical modifications, − or temporary monosaccharide blocking groups …”

“…54−56 Complex glycans can have several acceptor sites for a given glycosyltransferase, making it difficult to prepare discrete compounds. Site selective glycosylations can, however, be accomplished by careful synthetic planning, 45 the use of unnatural sugar nucleotide donors, 57,58 chemical modifications, 59−61 or temporary monosaccharide blocking groups. 42 Complex carbohydrates can be modified by entities such as sulfates; 62−64 biomimetic approach that can provide a wide range of differently sulfated and fucosylated KS oligosaccharides.…”