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Effects of Trans Α-Hydroxyl Groups in Alkaline Degradation of Glycosidic Bonds

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Effects of Trans Α-Hydroxyl Groups in Alkaline Degradation of Glycosidic Bonds EFFECTS OF TRANS -HYDROXYL GROUPS IN ALKALINE DEGRADATION OF GLYCOSIDIC BONDS U.S.D.A., FOREST SERVICE RESEARCH PAPER FPL 188 1972 U.S. Department of Agriculture, Forest Service Forest Products Laboratory, Madison, Wis. ABSTRACT The influence of ß-dihydroxyl groups in the alkaline hydrolysis of glucosidic bonds is examined. In deriva­ tives of ß-D-glucopyranosides, all hydroxyl groups are trans to each other, and the alkaline elimination of sub­ stituted methyl ethers in positions C-2 and C-4 can be explained, in part, by assistance from neighboring trans a-hydroxyls. The rate of degradation of ethyl 2-2-methyl-ß-D-glucopyranoside is two times slower than the corresponding 2-hydroxyl derivative, whereas the rate for ethyl 2-deoxy-D-glucopyranoside is 8 times slower in 2.5N NaOH at 170° C. EFFECTS OF TRANS -HYDROXYL GROUPS IN ALKALINE DEGRADATION OF GLYCOSIDIC BONDS By R. M. ROWELL and J. GREEN, Chemist Forest Products Laboratory 1 Forest Service U.S. Department of Agriculture INTRODUCTION Of the chemical pulp produced in the United tion has been directed to the effects of reactive States, 80 percent is produced by an alkaline hydroxyl groups adjacent to the glucosidic link- kraft process; thus, determining the reactions ages. These ß-dihydroxyl groups contribute taking place when cellulose is subjected to aqueous significantly in degrading cellulose under alkaline alkali has been of great importance. Much of the conditions. work has dealt with the degradation caused by the It has long been recognized that linked hemi- endwise depolymerization (peeling) reaction that celluloses have much greater stability to dilute 2 gives rise to isosaccharinic acid (4, 13, 15, 22) . alkali than any of the polysaccharides (1, 21). Not only has the mechanism of this reaction been This results from the inability of the hydroxyl studied, but also a mechanism to modify the re- group on carbon atom 2 -hydroxyl) to form a ducing D-glucose end unit to stop or at least to carbonyl group necessary for the degradation to slow down this endwise degradation (16, 19). proceed at room temperature to saccharinic Of equal interest has been the study of the de- acids (22, p. 298). gradation of cellulose caused by the splitting of Several workers have studied the mechanism internal glycosidic linkages. Cleavage of these of alkaline glycosidic cleavage (2, 3, 6, 7, 11, 12, and bonds between internal D-glucose units gives rise 14), and the most widely accepted mechanism is to new end units that then undergo the endwise shown in figure 1. In the alkaline solution, the depolymerization reaction. Increases in cellulose hydroxyl oxygens will be ionized. As shown, the yield during alkaline pulping ultimately depend on C-2 oxygen anion is trans to the $-glycosidic retention of glycosidic bonds, linkage, and can easily displace the C-1 oxygen In the research on the alkaline degradation of bond to form an epoxide B between carbons 1 and cellulose, many environmental factors have been 2. This epoxide is then hydrolyzed to a glucosyl considered. The variables of temperature, type of anion, which degrades further. alkali, alkali concentration, addition of oxygen, and It can be seen from A in figure 1 that other various stabilizing chemicals have all been ex- trans arrangements are in the molecule. In fact, amined, and their individual and combined effects in the ß-D-glucopyranoside structure, all hydroxyl have been determined. Much has been written on groups are trans to each other. Thus, as easily these environmental effects; however, little atten- as C-2 hydroxyl can assist in the elimination of 1 Maintained at Madison, Wis., in cooperation with the University of Wisconsin. 2 Underlined numbers in parentheses refer to Literature Cited at the end of this report. Figure I.--Mechanism of alkaline glycosidic cleavage. (M 140 232) Figure 2.--Alkaline degradation of 2-2-substituted glycosides. (M 140 231) FPL 188 2 the C-1 hydroxyl, so C-2 can eliminate C-3; C-3, modification of the one originally given by Janson C-2; C-3, C-4; and even the C-6 hydroxyl is in and Lindberg (11). Pyridine (25 ml.) and acetic close proximity to eliminate C-1 or C-4 hydroxyls. anhydride (10 ml.) were added to 2-?-methyl- Lindberg and coworkers (6) studied the influence D-glucose (5 g.) (5, 9). The mixture was allowedto of cis-trans hydroxyl groups in model glycosides, stand overnight at room temperature, then con- and found that in all cases a trans hydroxyl in a centrated at 60° C. to a thick oil. The syrup was position alpha to the glycosidic bond hydrolyzed dissolved in acetic acid (20 ml,): then acetyl faster under alkaline conditions than did the cis bromide (25 ml.) was added. After cooling the configuration. They also found that by blocking solution to 10° C., water (0.5 ml.) was added the a-hydroxyl group, the rate of hydrolysis was dropwise over 15 minutes. The solution was slowed (11), but not stopped as might have been warmed to room temperature, and allowed to expected. These degradations were at 170° C. in stand for 2 hours. The mixture was then extracted 10 percent sodium hydroxide; under these condi- with chloroform (2 x 100 ml.), and the combined tions, blocking the C-2 hydroxyl did not offer very chloroform extracts were washed successively much protection. This not only results from the with sodium bicarbonate-ice solution and ice rather severe conditions but, as stated earlier, water. The chloroformwas then dried over sodium can result from the trans hydroxyl on C-3 aiding sulfate, and concentrated at 40°C. to a thick light- in eliminating the C-2 blocking group that in turn, yellow syrup. The syrup was dissolved in dry aided in eliminating the glycosidic linkage (fig. 2). benzene (7 ml.), and ethanol (30 ml.) was added. Since both the glycoside and the C-2 hydroxyl in To this calcium sulfate (5 g.) and silver carbonate these experiments were substituted with methyl (6 g.) were added, and the mixture stirred over- groups, it was impossible to determine the origin night at roomtemperature. After filtration through of the methyl alcohol detected after the degrada- a Celite pad, the filtrate was concentrated to a tion. For this reason, ethyl 2-0-methyl-ßD-- syrup that showed one major product of R Sol- f glucopyranoside triacetate was prepared here-to vent A, 0.60. The syrup was dissolved in a small differentiate between the points of elimination. De- volume of chloroform, and applied to the top of grading this model determines whether the block- a chromatographic column (2.5 x 50 cm.) packed ing group is eliminated simultaneously with glyco- with Mallinckrodt SilicAR CC-4 (100-200 mesh). side hydrolysis or whether a different mechanism Elution with solvent A followed by combination is in effect. Of equal interest was the complete and concentration of the pure fractions gave 1.7 removal of the a-hydroxyl group. For this purpose, grams of I. Crystallization from hot ethanol gave ethyl 2-deoxy-D-glucopyranoside was synthe- sized. m.p. 96-97° C., -22.7°C. (c22 chloroform). To closely simulate a cellulose molecule, a Anal, calc. for C H O : C, 51.72; H, 6.94. model compound, ethyl 4-0-methyl-ßD--gluco- 15 24 9 pyranoside, has been synthesized. Degrading this Found: C, 51.78; H, 6.72. derivative will determine the mode of degradation Et h y 1 2-deoxy-α-D-glucopyranoside (II).--2- in the internal linkages of cellulose. Deoxy-D-glucose (10 g.) and BioRad AG 50 W-X8 (H+) resin (30 g.) were added toethanol (200 ml.). The mixture was stirred and refluxed for 1 hour, EXPERIMENTAL and the resin removed by filtration. Air evapora- tion of the ethanol gave a small yield of crystal- Purity of crystalline products was determined line product, and on further evaporation gave a by thin-layer chromatography on silica g e 1 thick syrup, Redissolution of the syrup in hot H-coated glass plates irrigated with either ethyl ethanol followed by air evaporation gave more acetate-hexane (1:1, v/v) or chloroform-ethanol crystalline product. Repeating this procedure four (4: 1, v/v). Components were located by spraying times, a total crystalline yield of 3.2 grams was with sulfuric acid (10 pct.) in ethanol, then char- achieved. Recrystallization from hot ethanol gave ring until permanent spots became visible. m.p. 124° C. +120° C. (c, 3.09, water). Ethyl 3, 4, 6 - tri -0-acetyl-2-0-methyl-ß-D- R solvent B 0.43. glucopyranoside (I).--The following procedure is a f 3 Anal. calc. for C H O : C, 50.00; H, 8.33. Table I.--Rate of alkaline degradation of model glycoside 8 16 5 Found: C, 49.94; H, 8.16. Ethyl 4-0-methyl-ß-D-glucopyranoside (III).-- Compound K (h-1) This compound was prepared as described (18). Alkaline Degradation of Samples Solutions of the above compounds (10 ml., 4 mg./ ml.) were degraded in 2.5N NaOH in stainless steel (316, 1/2 in. x 10 in.) reaction vessels. Oxygen was purged from the alkali solutionprior to sugar addition with a stream of oxygen-free nitrogen, The vessels were sealed with threaded caps, and 1See Literature Cited (11). placed in an oil bath at 170° ±0.2° C. At various intervals, samples were collected by cooling and this reason, the stainless steel reactors were opening the containers. Aliquots of these solutions used. were treated with sulfuric acid, and analyzed spectrophotometrically as described by Scott Because of the possible leakage of gas, no quan- et al.
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