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Abstract.A method for the seleclive oxidation of the primary hydroxyl functions of polysaccharides is described. The oxidation is mediated by 2,2,6,6-tetramethyl-1 -piperidinyloxy (TEMPO) and hypobromite is used as the regenerating oxidant. At pH 10.5-1 1 a selectivity of 98% for cold water soluble potato starch and >90% for dahlia inulin was found.2,2,6,6-Tetramethyl-1 -piperidinyloxy (TEMPO, 1) is known to be a stable radical, which can be oxidised by several reagents to give a nitrosonium ion (2)'. The latter is a strong oxidant and shows selectivity towards primary hydroxyl functions over secondary ones. Only very recently this reagent has been uscd in sugar chemistry'. Partially protected mono-and disncchnrides were selectively oxidised in a TEMPO mediated reaction with hypohalite as the regenerating oxidant. However, selectivity was not reported*' or was not veryHere we present a modified method for the oxidation of polysac~hatides~. The reaction is mediated by TEMPO and hypobromite is used as the regenerating oxidalt. which in turn is regenerated by hypochlorite (Scheme 1). Obviously there will be a competing reaction: the undesired oxidation of secondary alcohols by hypobromite. This reaction is relatively slow (takes about 10 hours at osr-"X CI -OaX Br-0 (1) rooin temperature) and has its optiinuin around pH 9'. We investigated the influence of the pH on polysacchxide oxidation in the range 9-1 1.5. As a substrate cold water soluble potato starch was used. The forinntion of acid was followed with pH-stat, by which the pH was kept at the desired value by adding 0.5N NaOH. It appeared that the reaction was much faster at pH values higher than 9. This was profitable because non selective oxidation due to hypohalite is slower at high pH.Selectivity of the reaction under chosen conditions' was detennined by comparison of initial and final reaction rates6 (the latter being the reaction rate of non selective oxidation), I3C NMR and a uronic acid assay'. Forination of glucuronic acid units was proportional to the consumption of NaOH. When 0.95 mmol NaOH per ininol anhydroglucose unit (AGU) was consumed, more tli'an 90% glucuronic acid was formed. From comparison of the initial and final reaction rates and from '% NMR (Figure la) a selectivity in the oxidation of primary hydroxyl functions of about 98% was determined ( Table 1).Another substrate oxidised under these conditions was dahlia inulin (DP 30). Initial reaction rate was comparable to that of starch. However, after consumption of about 1 mmol NaOH per mmol anhydrofructose unit (MU), forination of acid proceeded at a steady rate higher than the reaction rate of a hypobromite oxidation under the same conditions (for starch these two rates were the same). Reaction of the sterically hindered nitrosonium ion with secondary alcohols of a furanoside unit might be faster than reaction with secondary alcohols of a pyranoside unit. Also, when C4 of inulin is oxidised in addition to C6, it can be expected that the molecule degrades under the applied conditions due...

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