Selective synthesis of curdlan ω-carboxyamides by Staudinger ylide nucleophilic ring-opening

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Abstract

Chemoselective modification of polysaccharides is a significant challenge, and regioselective modification is even more difficult, due to the low and similar reactivity of the various polysaccharide hydroxyl groups. Bromination of glycans that possess free 6-OH groups is exceptional in that regard, giving regiospecific, high-yield access to 6-bromo-6-deoxyglycans. Herein we report a simple and efficient pathway for synthesizing 6-ω-carboxyalkanamido-6-deoxy-containing polysaccharide derivatives in a sequence starting from 6-bromo-6-deoxycurdlan, via azide displacement, then conversion of the azide to the iminophosphorane ylide by triphenylphosphine (Ph3P). We take advantage of the nucleophilicity of the iminophosphorane nitrogen by subsequent regioselective ring-opening reactions of cyclic anhydrides. These reactions of the useful polysaccharide curdlan were essentially completely regio- and chemo-selective, proceeding under mild conditions in the presence of ester groups, yet preserving those groups. These interesting polysaccharide-based materials have pendant carboxyls attached through a hydrocarbon tether and hydrolytically stable amide linkage; as such they are promising for diverse application areas, including aqueous dispersions for coatings, adhesives, and other consumer products, and for amorphous solid dispersions in oral drug delivery.

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Liu, S., Gao, C., Mosquera-Giraldo, L. I., Taylor, L. S., & Edgar, K. J. (2018). Selective synthesis of curdlan ω-carboxyamides by Staudinger ylide nucleophilic ring-opening. Carbohydrate Polymers, 190, 222–231. https://doi.org/10.1016/j.carbpol.2018.02.074

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