A Chemoenzymatic Approach for the Synthesis of Unnatural Disaccharides Containing D-Galacto- or D-Fucofuranosides

Unusual diglycosides composed of D-hexofuranosyl entities were prepared by a chemoenzymatic route using the -L-arabinofuranosidase, AbfD3. The required, unprotected monosaccharidic donors were first prepared according to multi-step syntheses. Since one goal of this study was the investigation of donor -1 subsite in the active site of the enzyme, we focused on D-fucofuranosyl and 6-deoxy-6-fluoro-D-galactofuranosyl derivatives which present stereochemical similarities with L-arabinose series, but also structural variations on the side arm. These substrates were then used in AbfD3-catalysed hydrolyses to determine the parameters Km and kcat and in AbfD3-catalysed transglycosylation to evaluate their ability to serve as donor/acceptor. Four disaccharides were thus isolated and characterised, two resulting from -(1,2) connection along with two -(1,3)-regioisomers.