Selective Oxidation of the Primary Hydroxyl Group

The direct oxidation of hydroxyls on inulin allows the potential introduction of carbonyl and carboxyl groups, altering the properties of the polysaccharide and opening additional commercial applications (Bragd et al., 2004). The primary hydroxyl in the C-6 position on the fructofuranoside subunits can be selectively oxidized using 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO). This forms a stable radical that can be oxidized by hypobromite, or similar reagent, to give a nitrosonium ion (Bragd et al., 2004; de Nooy et al., 1994, 1995). The preferred reaction pH is 9.5 (de Nooy, 1997); however, at pH 10.5, 6-carboxy-inulin is formed with a good yield (82%) (de Nooy et al., 1995). A variation in this approach utilizes 4-acetamido-2,2,6,6-tetramethyl-piperidine-1-oxyl (4-acNH-TEMPO) and peracetic acid or monoperoxysulfate as the oxidant, the former giving the better conversion efficiency (Bragd et al., 2002).

Platinum (catalyst) and molecular oxygen (oxidant) have been used to selectively oxidize carbohydrates (Abbadi and van Bekkum, 1996; Gallezot and Besson, 1995) such as sucrose (Fritsche-Lang et al., 1985; Kunz and Recker, 1995). Inulin is readily oxidized using O2 and platinum in the presence of sodium hydroxide (Verraest et al., 1998). Oxidation occurs selectively at the C-6 position with a relatively high yield (79%). Sucrose is also selectively oxidized at the C-6 and C-6' positions under similar conditions (Edye et al., 1991, 1994). The molecular weight of the inulin influences the reaction rate and degree of oxidation. Nystose (GF3) had a degree of oxidation of 40%. As the chain length increases, the degree of oxidation declines. Inulin with an average degree of polymerization of 10 had a degree of oxidation of 28%, while at an average degree of polymerization of 30, it decreased to only 20%. In addition, the amount of by-products increases with increasing chain length and degree of oxidation.

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