Benzoylethynyl derivatives of arabinogalactan via acylation of its propargyl ethers

To introduce a highly reactive acetylenic ketone function into the polysaccharide structure, arabinogalactan propargyl ethers (AGPg) with degree of substitution 1 were modified via the benzoylation of residual hydroxyl groups followed by cross-coupling of propargyl ether groups with benzoyl chloride. In the first stage, mixed propargyl ethers/benzoic esters of AG (degrees of substitution 1 and 2, respectively) were obtained in a yield of up to 95 % at room temperature in pyridine. In the second stage, the synthesized AG derivatives were involved in a palladium/copper-catalyzed cross-coupling of propargyl groups with benzoyl chloride (60-65 ^оC, dioxane, Et₃N) to afford arabinogalactans bearing benzoate and benzoylpropargyl ether groups in 87-96 % yield (degree of substitution up to 1.0). It was found that in dilute solutions containing a 5-10-fold excess of reagents relative to propargyl groups, the reaction of AGPg with benzoyl chloride prevails over the intermolecular cross-linking, giving organosoluble products with complete conversion of the terminal acetylenes. The addition of piperidine to the activated triple bond showed that the obtained AG derivatives are promising intermediates for design of new pharmacologically oriented polysaccharides. AGPg and the original arabinogalactan were benzoylated for the first time in a two-phase aqueous-organic system (37-60 % KOH aqueous solution/dioxane) to deliver the corresponding organosoluble benzoates in 54-81 % yields and a degree of substitution 1.2-2.8.