time. It is then filtered, 10 ml of pyridine is added to the filtrate, and the solution is allowed to stand at 0° C overnight. The pyridinemercuric-chloride compound is separated by filtration; the filtrate is evaporated in vacuo to a sirup, which is dissolved in 100 ml of water, neutralized with dilute alkali (phenolphthalein indicator), and the solution again evaporated to a sirup. This sirup is dehydrated by several distillations with absolute alcohol, and finally a thick sirup is left. This is taken up with hot ethyl acetate, and the solution cooled and decanted from the sirupy phase which forms during the cooling. Seed of ethyl B-d-galactofuranoside is added and crystallization is allowed to take place in a refrigerator. About 10 g of ethyl-p-dgalactofuranoside is obtained. The material, after recrystallization from 30 ml of hot ethyl acetate, melts at 85° to 86° C and gives [a]=-102° (water, c=1).

2

The mother liquor is concentrated in vacuo to a volume of 200 ml and crystallization is allowed to take place. The crystalline mixture of alpha and beta galactofuranosides may be mechanically separated since ethyl 8-d-galactofuranoside crystallizes as white fragile needles, while ethyl a-d-galactofuranoside 2 separates as round translucent buttons which cling to the wall of the flask. The crude alpha isomer is purified by recrystallization from 100 times its weight of hot ethyl acetate from which it crystallizes in short needles. Ethyl a-d-galactofuranoside melts at 140° C and gives [a]2+92° (water, c=Ï).

NOTES

1 Drierite is a trade name for an anhydrous calcium sulfate.

2 Nomenclature as given in reference [25].

(2) METHYL d-GLUCOPYRANOSIDES.

Method [24].-Glucose dibenzyl mercaptal (8.2 g) is dissolved in 100 ml of boiling methyl alcohol and a solution of 16.3 g of mercuric chloride in 30 ml of warm methyl alcohol is added. After the solution has been heated for 15 minutes on the water bath, the precipitate of CH-CH2-S-HgCl is removed by a filtration. Dry hydrogen sulfide gas is passed into the solution. and it is again filtered. The filtrate is neutralized with silver carbonate, treated with a decolorizing carbon, and filtered. The filtrate is evaporated in vacuo to a sirup, which is taken up in a little cold water and filtered to remove the insoluble portion. The aqueous solution is concentrated under reduced pressure to a heavy sirup, which is brought to crystallization by the addition of absolute alcohol. The product is separated and recrystallized from 18 parts of hot absolute alcohol. About 2.8 g of methyl a-dglucopyranoside is obtained. It melts at 166° C and gives [a] 20= +158.9° (water, c=10).

The mother liquors from the preparation of the alpha isomer are allowed to stand in a refrigerator. After several days the resulting crystals of methyl B-d-glucopyranoside are separated and recrystallized from 8 parts of absolute alcohol. About 0.7 g of methyl B-d-glucopyranoside is obtained. The compound melts at 105° C and gives [a] 20=-34.2° (water, c=10).

(f) PREPARATION OF GLYCOSIDES FROM SUGARS WITH DIMETHYL SULFATE

By careful methylation with one equivalent of dimethyl sulfate, methyl glycosides may be obtained directly from the sugars [5, 26].

(1) METHYL 8-d-MANNOPYRANOSIDE ISOPROPYL ALCOHOLATE. Method [5].-A solution of 18 g of mannose dissolved in 100 ml of water is kept in an ice bath and stirred while dimethyl sulfate and 30-percent sodium hydroxide are added dropwise over a period of 2 to 3 hours at such a rate as to maintain the reaction of the solution alkaline to acyl blue (pH 12). Fifteen milliliters of dimethyl sulfate is added over a period of 2 to 3 hours, and 20 to 22 ml of alkali in the course of 8 hours. After the solution has stood overnight at room temperature, it is neutralized with sulfuric acid and filtered. A small quantity of barium carbonate is added to the filtered solution, which is then concentrated in vacuo to a volume of approximately 30 ml. After the addition of 60 ml of dioxane and a second evaporation, the solution is concentrated to a thick sirup, which is dissolved in 60 ml of pyridine. The solution is filtered, mixed with an equal volume of acetic anhydride, and allowed to stand overnight at room temperature. It is then poured with stirring into a mixture of cracked ice and water, and the acetylated glycoside is extracted with chloroform. The chloroform solution is washed successively with solutions of sodium bicarbonate and copper sulfate (until free from pyridine), and finally with water. The chloroform solution is dried, filtered, and evaporated to a sirup which is brought to crystallization by the addition of ethyl alcohol and further concentration. The material is removed from the flask with ether (100 ml) and petroleum ether added to saturation. After the mixture stands for a day at 0° C, the crystals are collected on a filter and washed with a mixture of ether and petroleum ether. The yield of the mixed alpha and beta methyl tetraacetylmannosides is about 18 g. This material is stirred with 100 ml of ether and filtered. The insoluble portion, about 6 g, is nearly pure methyl tetraacetyl-6-d-mannopyranoside. The compound melts at 161° C and gives [a]=-50.4° (chloroform,c=1). The acetylated glycoside is deacetylated by the barium methylate method described on page 493. After the barium is removed, the resulting solution is treated with a decolorizing carbon, filtered, and evaporated in vacuo to a heavy sirup, which is taken up in 10 ml of isopropyl alcohol. After the mixture has stood for a short time, methyl 6-d-mannoside, containing isopropyl alcohol of crystallization (CH1406. C2H,O) separates in large thin plates. The yield is nearly quantitative. Methyl 8-d-mannopyranoside isopropyl alcoholate melts at 74° to 75° C and gives [a]=-53.3° (water, c=4).

NOTE

The methylation described here is like that used by Schlubach and Maurer [26] for the preparation of 8-methyl glucoside. The dimethyl sulfate method is particularly useful for the preparation of those glycosides in which the methoxyl and the hydroxyl on the adjacent carbon are cis, because such glycosides are not produced in satisfactory yield by the Koenigs-Knorr reaction. Methyl B-dmannopyranoside may be prepared also by the hydrogen chloride method [27].

(g) PREPARATION OF GLYCOSIDES BY THE USE OF ALKYL IODIDES AND SILVER OXIDE

(1) METHYL TETRAACETYL-B-d-FRUCTOPYRANOSIDE.

Method [28].-Sixty grams of powdered tetraacetylfructose, 375 g of freshly prepared silver oxide, and 340 ml of methyl iodide are boiled on the water bath for 6 hours in a 2-liter flask fitted with a

reflux condenser which is supplied with ice water in order to prevent loss of the solvent. The methyl iodide is then distilled off and recovered. The residue from the distillation is mixed with ether, and the solution filtered and evaporated in the air. On seeding the resulting colorless sirup, crystalline methyl tetraacetyl-p-d-fructopyranoside is obtained in almost the theoretical yield. The product, purified by recrystallization from petroleum ether, melts at 75° to 76° C and gives [a]20=-124.6° (chloroform, c=8).

NOTES

1 A general review of the methylation method is given in reference [29].

2 The reaction can also be carried out at room temperature if the mixture is shaken and a longer reaction time is allowed.

3 A large flask should be used for this evaporation in order to prevent loss by bumping.

(2) METHYL a-d-MANNOFURANOSIDE.

Method [30].-Four-tenths gram of 2,3-5,6-mannose dicarbonate is dissolved in methyl iodide containing a little acetone, and small amounts of silver oxide are added at intervals during a half hour, while the solution is heated below the boiling point. Prolonged contact with large excesses of silver oxide is harmful. The solution is filtered and the residue extracted with boiling acetone. The original filtrate and the acetone extracts are evaporated and the residue is again treated with the methylating agents as before. The filtrate from the second treatment yields on evaporation 0.15 g of methyl 2,3-5,6-a-d-mannofuranoside dicarbonate. This product separates in colorless crystals which are sparingly soluble in ethyl acetate and melt at 172° to 173° C with decomposition. The carbonate groups are removed by saponification with barium hydroxide. The relatively pure compound thus obtained crystallizes readily. Methyl a-dmannofuranoside melts at 118° to 119° C and gives [a]=+113° (water, c=1).

Other methods for preparing glycosides.-A number of other methods of limited applicability have been used for the preparation of glycosides. Thus methyl a-d-glucoside has been obtained by enzymatic synthesis from aqueous methyl alcoholic solutions of glucose and a-glucosidase [31].

Certain glycosides can be prepared conveniently by the oxidation of glycals with perbenzoic acid in the presence of an alcohol. Methyl a-d-mannoside is prepared in good yield by this method from glucal [32].

(h) REFERENCES

[1] E. Fischer, Ber. deut. chem. Ges. 26, 2400 (1893).

[2] E. Fischer, Ber. deut. chem. Ges. 47, 1980 (1914).

[3] P. A. Levene, A. L. Raymond, and R. T. Dillon, J. Biol. Chem. 95, 699 (1932).

[4] H. S. Isbell, BS J. Research 8, 5 (1932) RP396.

[5] H. S. Isbell and H. L. Frush, J. Research NBS 24, 125 (1940) RP1274.

[6] E. Pacsu and A. Scattergood, J. Am. Chem. Soc. 61, 534 (1939).

[7] K. Hess and K. E. Heumann, Ber. deut. chem. Ges. 72, 149 (1939).

[8] E. Pacsu, J. Am. Chem. Soc. 52, 2571 (1930).

[9] E. F. Armstrong and K. F. Armstrong, The Carbohydrates, p. 21 (Longmans, Green, and Co., London, 1934).

[10] C. B. Purves and C. S. Hudson, J. Am, Chem. Soc. 56, 702, 708, 1969, 1973 (1934).

[11] C. S. Hudson, J. Am. Chem. Soc. 47, 267 (1925).

[12] E. Montgomery and C. S. Hudson, J. Am. Chem. Soc. 59, 992 (1937).

[13] W. Koenigs and E. Knorr, Ber. deut. chem. Ges. 34, 957 (1901).

[14] H. S. Isbell, Ann. Rev. Biochem. 9, 70 (1940); H. L. Frush and H. S. Isbell,

J. Research NBS 27, 413 (1941) RP1429.

[15] A. Robertson and R. B. Waters, J. Chem. Soc. 1930, 2729; 1931 (1881). [16] D. D. Reynolds and W. L. Evans, J. Am. Chem. Soc. 60, 2559 (1938). [17] H. Schlubach and K. Meisenheimer, Ber. deut. chem. Ges. 67, 429 (1934). [18] G. Zemplén and A. Gerecs, Ber. deut. chem. Ges. 63, 2720 (1930). [19] G. Zemplén and Z. S. Nagy, Ber. deut. chem. Ges. 63, 368 (1930). [20] G. Zemplén and Z. Csürös, Ber. deut. chem. Ges. 64, 993 (1931).

[21] B. Helferich and E. Smitz-Hillebrecht, Ber. deut. chem. Ges. 66, 378 (1933). [22] B. Helferich, R. Hiltmann, and W. Reischel, Liebigs Ann. Chem. 534, 276 (1938).

[23] J. W. Green and E. Pacsu, J. Am. Chem. Soc. 59, 1205 (1937); 60, 2056 (1938). [24] E. Pacsu, Ber. deut. chem. Ges. 58, 509 (1925).

[25] J. W. Green and E. Pacsu, J. Am. Chem. Soc. 59, 2569 (1937).

[26] H. Schlubach and K. Maurer, Ber. deut. chem. Ges. 57, 1686 (1924).

[27] H. G. Bott, W. N. Haworth, and E. L. Hirst, J. Chem. Soc. 1930, 2653. [28] C. S. Hudson and D. H. Brauns, J. Am. Chem. Soc. 38, 1216 (1916). [29] C. R. Young, A General Review of Purdie's Reaction, Memorial Volume of Scientific Papers of St. Andrews University 500th Anniversary.

[30] W. N. Haworth and C. R. Porter, J. Chem. Soc. 1930, 649.

[31] E. Bourquelot, H. Hérissey, and M. Bridel, Compt. rend. 156, 491 (1913). [32] M. Bergmann and H. Schotte, Ber. deut. chem. Ges. 54, 1564 (1921).

10. MERCAPTALS

(a) PREPARATION OF MERCAPTALS

These substances are thioacetal derivatives of the aldehydo forms of the sugars and are used for the preparation of acetylated aldehydo derivatives (see p. 490), for the preparation of pyranose and furanose glycosides (see p. 517), and for the isolation of sugars from impure mixtures. The mercaptals of a large number of sugars have been prepared by shaking the sugar with the mercaptan in the presence of concentrated hydrochloric acid at room temperature. The process is not applicable to disaccharides, as partial hydrolysis occurs. The detailed procedure for the preparation of galactose ethyl mercaptal given in this section is typical of the method in general. (1) GALACTOSE ETHYL MERCAPTAL.

Method. [1,2].-Fifty grams of galactose is placed in a 500-ml glassstoppered wide-mouth bottle and is dissolved at room temperature in 75 ml of concentrated hydrochloric acid (sp. gr 1.19). Fifty ml of technical ethyl mercaptan is then added and the mixture is shaken, the pressure being released at intervals. After 5 minutes ice and water is added to the reaction mixture. The crystalline product which forms is separated by filtration and recrystallized, first from absolute alcohol and then from hot water. About 37 g of pure

material is obtained which melts at 140° to 142° C.

(b) REFERENCES

[1] P. A. Levene and G. M. Meyer, J. Biol. Chem. 74, 695 (1927). [2] M. L. Wolfrom, J. Am. Chem. Soc. 52, 2466 (1930).

11. OXIDATION PRODUCTS

(a) PREPARATION OF ALDONIC ACIDS

General characteristics.—In aqueous solution the aldonic acids establish an equilibrium between the free acid (I), the gamma lactone (II), the delta lactone (III), and frequently condensation products.

The relative proportions of the free acid and delta and gamma lactones vary greatly according to the configuration of the sugar acid and according to the temperature and concentration of the solution. To prepare the free acids, it is generally necessary to avoid lactone formation. Usually the crystalline acid can be obtained by concentrating a freshly prepared aqueous alcoholic solution of the acid at a low temperature as rapidly as possible. The use of isoamyl or butyl alcohol is advantageous because these alcohols retard the rate of lactone formation, facilitate removal of the water, and after partial evaporation, leave a solvent from which the acid crystallizes readily. The lactones, when pure, also crystallize readily, but frequently this condition cannot be realized experimentally because of the simultaneous formation of two lactones. If one dissolves mannonic acid in water, the delta lactone is formed more rapidly than the gamma lactone, but the two are formed simultaneously. After several hours the concentration of the delta lactone reaches a maximum and at this point it may be crystallized from the solution. As the solution stands, the concentration of the more slowly formed gamma lactone increases at the expense of the free acid and the delta lactone until equilibrium is reached, when nearly all the sugar acid may be present as the gamma lactone. The position of equilibrium depends on the experimental conditions and on the configuration of the acid. The equilibrium state for gluconic acid permits the crystallization of either the free acid or the delta lactone from the equilibrium solution. At temperatures below 25° C free gluconic acid is obtained, whereas at temperatures above 25° C the delta lactone crystallizes. Heating gluconic acid in a high-boiling solvent, such as butyl alcohol, results in the formation of gluconic y-lactone, which crystallizes when the solution is cooled. The sugar acids combine with alcohols readily to form esters which sometimes interfere with the crystallization of the acids or lactones. Heating the esters in alcoholic solution, especially in the presence of a mineral acid, causes the formation of lactones. The ethyl ester of gluconic acid crystallizes well and decomposes on heating to give gluconic y-lactone. Lactone formation is accelerated by acid catalysts; consequently, if one wishes to prepare the free acid, the presence of mineral acid is avoided, but if one wishes to establish the equilibrium state quickly, as in the preparation of mannonic y-lactone, a mineral acid is added.

The aldonic acids may be prepared from the aldoses with the same number of carbon atoms by oxidation, from the ketoses with more