A Diazido Mannose Analogue as a Chemoenzymatic Synthon for Synthesizing Di‐N‐acetyllegionaminic Acid‐Containing Glycosides

Abstract

A chemoenzymatic synthon was designed to expand the scope of the chemoenzymatic synthesis of carbohydrates. The synthon was enzymatically converted into carbohydrate analogues, which were readily derivatized chemically to produce the desired targets. The strategy is demonstrated for the synthesis of glycosides containing 7,9-di-N-acetyllegionaminic acid (Leg5,7Ac ), a bacterial nonulosonic acid (NulO) analogue of sialic acid. A versatile library of α2-3/6-linked Leg5,7Ac -glycosides was built by using chemically synthesized 2,4-diazido-2,4,6-trideoxymannose as a chemoenzymatic synthon for highly efficient one-pot multienzyme (OPME) sialylation followed by downstream chemical conversion of the azido groups into acetamido groups. The syntheses required 10 steps from commercially available d-fucose and had an overall yield of 34-52 %, thus representing a significant improvement over previous methods. Free Leg5,7Ac monosaccharide was also synthesized by a sialic acid aldolase-catalyzed reaction.

Synthesis of Nonulosonic Acids

“…In 2018, the first chemo‐enzymatic approach for legionaminic acid synthesis and its application towards enzymatic α ‐sialoside glycosylation was reported by Chen and co‐workers. [ 25 ] The strategy relied on the chemical synthesis of 6‐deoxy‐ d ‐mannosamine derivative 77 via S N 2 displacement of 2,4‐bis‐triflates of d ‐fucose, following Kulkarni's protocol established earlier on l ‐fucose [ 26 ] (Scheme 7). Accordingly, starting from d ‐fucose 75 , peracetylation followed by Lewis acid‐catalyzed anomeric displacement of acetate group by 4‐methoxy phenol (PMPOH) and then subsequent Zemplèn deacetylation gave α ‐coupled 6‐deoxy‐ d ‐fucose 76 .…”

Access to Galectin-3 Inhibitors from Chemoenzymatic Synthons

“…Two small hydrophobic interactions are detected for 23 with R183 and R186. In total, the number of interactions between compound 22 (7), 23 (9) and 24 (8) does not seem to be enough to explain the relative differences in binding efficiency on galectin. The galactose binding position is, however, displaced from its location in lactosamine type I structure with a RMSD of 1.58 Å ( 22), 3.23 Å ( 23) and 1.55 Å ( 24), indicative of a counterbalance between galectin affinity provided by the galactose and by additional interaction energy provided by substituents.…”

“…This was achieved starting from chemoenzymatic synthons, first enzymatically used either in cellulo or in vitro as substrates to form oligosaccharides prior to their chemical modifications. 6,7,8,9 Interestingly a reverse strategy consisting in using temporary protecting groups on acceptor substrates to divert and control the reactivity of glycosyltransferases has been applied to synthesize complex glycoforms. 10,11 Trans-glycosidases whether natural 12 or mutant glycoside hydrolases obtained by directed evolution 13 or rational design 14 as well as glycosynthases 15 are powerful tools for the synthesis of oligosaccharides or glycoconjugates.…”

A new approach to the synthesis of legionaminic acid analogues

“…21 In 2017, a complete synthesis of disaccharides of legionaminic acid was published by the Crich group, and involved converting commercially available N-acetylneuraminic acid (Neu5Ac, 3) into a legionaminic acid glycosyl donor via a eenstep synthetic route with an overall yield of 17%. 22 A chemoenzymatic approach to legionaminic acid synthesis has recently been reported, by Santra et al, 23 wherein D-fucose was converted through 9 synthetic steps (44% overall yield) into a trideoxy mannose derivative that was subsequently condensed with sodium pyruvate in the presence of the sialic acid aldolase from Pasteurella multocida to give 1. Despite the important contribution of all these nonulosonic acid syntheses, to date there does not exist a synthetic pathway towards C-7 analogues of Leg.…”

“… 22 A chemoenzymatic approach to legionaminic acid synthesis has recently been reported, by Santra et al , 23 wherein d -fucose was converted through 9 synthetic steps (44% overall yield) into a trideoxy mannose derivative that was subsequently condensed with sodium pyruvate in the presence of the sialic acid aldolase from Pasteurella multocida to give 1. 23 …”