Problems encountered in global deprotections of GXM oligosaccharides

Session: 
S7.3 Catalytic glycan transformations
Code: 
OL7.3.4
Location (hall): 
Galactose
Start/end time: 
Wednesday, July 3, 2019 - 12:30 to 12:45
Conor Joseph
Crawford

Conor Joseph Crawford1, Stefan Oscarson1

1University College Dublin, Dublin, Ireland

In a project aimed with synthesising a library of capsular polysaccharide fragments based on glucuronoxylomannan (GXM) polymer of Cryptococcus neoformans, we encountered a series of issues in the hydrogenolysis (global deprotection) of the benzyl ethers. Namely, the preservation of our desired 6-O-acetylation pattern in the target structures and the saturation of benzyl and napthyl rings to their corresponding ether (Figure 1).

This led to our efforts in finding a high quality source palladium on carbon catalyst in the hope of rectifying these issues. In this presentation details regarding the synthesis of GXM oligosaccharides will be presented with applications including FRET probes.[1] Also presented will be a procedure where the palladium catalysts reactivity is tuned through controlled poisoning. This gave rise to an efficient catalyst in terms of increased yield, reaction rate and chemoselectivity under hydrogenolysis conditions.  

Figure 1. Example of structure obtained upon global hydrogenolysis. In green desired structure and in orange the desired acetylation pattern. In red the undesired saturation of the napthyl and benzyl rings.

References: 
  1. Guazzelli, L., Ulc, R., Rydner, L. & Oscarson, S. A synthetic strategy to xylose-containing thioglycoside tri- and tetrasaccharide building blocks corresponding to Cryptococcus neoformans capsular polysaccharide structures. Org. Biomol. Chem. 13, 6598–6610 (2015).

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