Uronic acids are important constituents of polysaccharides found on the cell membranes of different organisms. To prepare uronic‐acid‐containing oligosaccharides, uronic acid 6,3‐lactones can be employed as they display a fixed conformation and a unique reactivity and stereoselectivity. Herein, we report a highly β‐selective and efficient mannosyl donor based on C‐4 acetyl mannuronic acid 6,3‐lactone donors. The mechanism of glycosylation is established using a combination of techniques, including infrared ion spectroscopy combined with quantum‐chemical calculations and variable‐temperature nuclear magnetic resonance (VT NMR) spectroscopy. The role of these intermediates in glycosylation is assayed by varying the activation protocol and acceptor nucleophilicity. The observed trends are analogous to the well‐studied 4,6‐benzylidene glycosides and may be used to guide the development of next‐generation stereoselective glycosyl donors.

Uronic acids are important constituents of polysaccharides found on the cell membranes of different organisms. To prepare uronic-acid-containing oligosaccharides,u ronic acid 6,3-lactones can be employed as they displayaf ixed conformation and au nique reactivity and stereoselectivity. Herein, we report ah ighly b-selective and efficient mannosyl donor based on C-4 acetyl mannuronic acid 6,3-lactone donors.T he mechanism of glycosylation is established using ac ombination of techniques,i ncluding infrared ion spectroscopyc ombined with quantum-chemical calculations and variable-temperature nuclear magnetic resonance (VT NMR) spectroscopy. The role of these intermediates in glycosylation is assayed by varying the activation protocol and acceptor nucleophilicity.T he observed trends are analogous to the well-studied 4,6-benzylidene glycosides and mayb eu sed to guide the development of next-generation stereoselective glycosyl donors.