approximately 100 mg of triphenylcarbinol is dissolved with careful trituration in 2 ml of sulfuric acid (sp gr 1.84). This solution is poured quickly into 50 ml of distilled water and allowed to stand for 30 minutes. The triphenylcarbinol is collected on a weighed Gooch crucible, washed with distilled water, and dried at 110° C to constant weight. The results can be expressed as triphenylcarbinol, or as percentage of triphenylmethyl groups according to the formula:

Wt of triphenylcarbinol 0.9347×100

Wt of sample

=

(d) REFERENCES

percentage of C(CH).

[1] E. L. Jackson, R. C. Hockett, and C. S. Hudson, J. Am. Chem. Soc. 56, 947 (1934).

[2] D. D. Reynolds and W. L. Evans, J. Am. Chem. Soc. 60, 2559 (1938). [3] M. L. Wolfrom, J. L. Quinn, and C. Christman, J. Am. Chem. Soc. 57, 713 (1935).

[4] B. Helferich and J. Becker, Liebigs Ann. Chem. 440, 1 (1924).

9. GLYCOSIDIC DERIVATIVES

(a) PREPARATION OF GLYCOSIDES BY FISCHER METHOD (1)

General characteristics of the method. By treating dry methyl alcoholic solutions of the sugars with hydrogen chloride, the hydroxyl of the reducing carbon is replaced by a methoxyl group and an equilibrium is established between the alpha and beta methyl pyranosides and furanosides [2]. When other alcohols such as ethyl, propyl, amyl, isopropyl, allyl, and benzyl are used as solvents, the corresponding glycosides are formed. The pyranose modifications usually predominate in the equilibrium mixture; the furanose modifications are at their maximum during the early stages of the reaction. In the preparation of furanose and pyranose glycosides advantage is taken of these properties [3]. The furanosides are prepared by allowing the reaction to proceed, usually at room temperature, until the furanoside content reaches a maximum; the pyranosides are prepared by heating the alcoholic solution until equilibrium is reached. The time required for reaching the desired state varies for the different sugars.

Since the equilibrium mixture contains the alpha and beta pyranose and furanose modifications, it is often difficult to obtain crystalline products. The following methods are useful for the purification of glycosides:

(a) Acetylation of the crude glycoside mixture gives products which frequently can be separated in the crystalline state.

(b) Calcium chloride and other molecular compounds are occasionally useful for separating the isomeric glycosides [4, 5, 6, 7].

(c) Mixtures of the acetylated alpha and beta glycosides in chloroform solution are converted by titanium tetrachloride to mixtures in which the alpha isomer predominates [8].

(d) For some glycosides the selective action of enzymes may be used to remove one of the isomers from the crude mixture. By using the proper enzyme [9] (a-glucosidase or B-glucosidase, or the corresponding enzymes for other sugars) the alpha or beta methyl glycoside may be removed from a mixture of the two. This method has been successfully employed for the separation of the methyl fructosides [10].

(e) Since the methyl furanosides and their acetates can be distilled at a low pressure, distillation may be used to separate them from the less volatile pyranosides [2].

The Fischer method using methyl alcoholic hydrogen chloride is particularly suitable for preparing the methyl pyranosides. It is usually not applicable to disaccharides as hydrolysis occurs simultaneously with glycoside formation. When the sugar is insoluble in the alcohol, the method may be applied to an alcoholic solution of the halogeno-acetyl or the acetyl sugar. In this case, deacetylation and glycoside formation proceed simultaneously.

(1) METHYL L-ARABINOPYRANOSIDES.

Method.-[11].-One hundred grams of l-arabinose is refluxed for 3 hours with 1 liter of anhydrous methyl alcohol containing 1.5 percent of hydrogen chloride. The acid is neutralized with silver carbonate1 and the solution is filtered, treated with activated charcoal, and refiltered. The filtrate is evaporated in vacuo to a thin sirup, which is allowed to crystallize. About 30 g of crude methyl B-l-arabinoside is obtained which is purified by an extraction with hot ethyl acetate. The residue is recrystallized from absolute ethyl alcohol. A second extraction and recrystallization give pure methyl B-l-arabinopyranoside, which melts at 169° C and gives [a]2+245.5° (water, c=7).

The mother liquor from the beta arabinoside preparation is evaporated under reduced pressure to a thin sirup which, after standing in a vacum desiccator, crystallizes slowly. The crystalline material, which is a mixture of the alpha and beta isomers, is separated and fractionally recrystallized from hot ethyl acetate." The less soluble beta isomer crystallizes first and the alpha isomer accumulates in the mother liquors. Pure methyl a-l-arabinopyranoside melts at 131° C and gives [a]=+17.3° (water, c=3).

NOTES

1 Silver carbonate is prepared by mixing a 10-percent aqueous solution of silver nitrate with a molecularly equivalent solution of sodium bicarbonate. The resulting precipitate is collected on a filter and washed with water until free from sodium salts, and then with methyl alcohol. The product is air-dried and protected from light.

2 For other methods useful for separating glycoside mixtures, see page 513. (2) METHYL a-d-ARABINOFURANOSIDE.

3

Method [12]. One hundred grams of powdered and sieved darabinose is shaken at 20° C with 4 liters of anhydrous methyl alcohol containing 29.2 g of hydrogen chloride (0.7 percent). Complete solution occurs after about 30 minutes, and the solution is then allowed to stand for 17 hours. The acid is removed by treating the mixture with silver oxide, and the excess silver is precipitated with hydrogen sulfide. The filtered solution is concentrated in vacuo to a thick sirup, which is dried overnight in a vacuum desiccator. The sirup is then extracted six times with 400-ml portions of anhydrous ether. The ether extract is evaporated in vacuo to a sirup which weighs about 23 g. The sirup is taken up with ethyl acetate and allowed several days to crystallize. The crystalline material, methyl a-d-arabinofuranoside, when separated and dried, weighs about 11.5 g. It is recrystallized by slowly cooling a warm ethyl acetate solution of the material to 5° C. The crystals melt at 65° to 67° C and give [a]2+123° (water, c=1).

NOTES

1 Montgomery and Hudson [12] report that the solution, upon standing at 20° C, becomes nonreducing after 4.5 hours and has a specific rotation of [a]=+24°. On further standing the rotation increases to a maximum value of [a]B=+45° in 17 hours and thereafter decreases to [a] = -17° in 42 hours. 2 The acid must be completely removed, since the furanosides are very unstable in the presence of acids and water.

3 Unless a flask is used which is separable from the distilling head, it is well to transfer the solution while is it still dilute to another container in which the extraction may be readily carried out after the sirupy stage has been reached.

Each extraction may be readily carried out by shaking the ether and sirup in a shaking machine for 45 minutes.

5 This furanoside is very hygroscopic and should be kept out of contact with moist air during this and succeeding operations. Montgomery and Hudson used a cabinet (55 by 39 by 43 cm) constructed of copper and with a glass top and removable end with strip-felt closure. The other end was provided with 2 openings 13 cm in diameter into which long rubber gloves were fitted with adhesive tape.

The first crystals were obtained by allowing the solution to stand at 5° C for several months.

(b) USE OF CALCIUM CHLORIDE COMPOUNDS TO SEPARATE GLYCOSIDES

(1) METHYL d-GULOPYRANOSIDES.

Method' [4].-Forty grams of a-d-gulose CaCl2.H2O2 is refluxed for 6 hours with 300 ml of absolute methyl alcohol containing 4.5 g of anhydrous hydrogen chloride. The warm solution is slowly neutralized with a slight excess of finely powdered calcium carbonate. After the addition of about 1 g of decolorizing carbon, the solution is filtered. The filtrate is concentrated in vacuo to a thick sirup, which is diluted with 50 ml of absolute ethyl alcohol. In the course of several hours, a crystalline product separates. The crystals are collected on a filter and washed with a mixture of equal parts of absolute ethyl alcohol and ethyl acetate. The mother liquor is evaporated in vacuo to a sirup which is saturated with ethyl acetate, and additional crystals are obtained.3

The dextrorotatory crystals are combined and recrystallized from ethyl alcohol to give methyl a-d-guloside.CaCl2.2H2O and [a]2+66.8° (water, c=7). The levorotatory crystals are combined and recrystallized to give pure methyl ẞ-d-guloside.CaCl2.2H2O' and [a]=-45.7° (water, c=2).

The free glycosides are obtained by shaking the calcium chloride compound in water solution with silver oxalate, silver sulfate, or silver carbonate. The resulting insoluble calcium and silver salts are separated by filtration and the free glycoside is crystallized from the aqueous solution. Methyl a-d-gulopyranoside crystallizes as a monohydrate which melts at 77° C and gives [a]2+109.4° (water, c=2). Methyl B-d-gulopyranoside melts at 176° C and gives [a]20=-83.3° (water, c=3).

NOTES

1 Calcium chloride compounds have also proved useful for the preparation of methyl a-d-a-glucoheptopyranoside [5], of methyl 8-d-mannofuranoside [6], and for the separation of the methyl 2,3,6-trimethylglucofuranosides [7]. If calcium chloride is not present in the original sugar it must be added in the appropriate quantity.

2 The preparation of a-d-gulose. CaC12. H2O is described on page 465.

In a typical experiment the first crop of crystals weighed 15 g and gave [a]=+60.8°; the second crop, 3.72 g, [a]=+37.5°; the third crop, 6.0 g, [a]=+17.3°; the fourth crop, 3.5 g, [a] = — 36.0°.

These compounds crystallize from solutions containing 1 mole or more of calcium chloride; compounds containing less calcium chloride are obtained under different conditions.

(c) PREPARATION OF GLYCOSIDES FROM HALOGENO-ACETYL DERIVATIVES

The Koenigs-Knorr reaction [13]. By treating certain halogenoacetyl sugars in methyl alcoholic solution with silver carbonate, the halogen is replaced by a methoxyl to give the acetylated glycosides, of the same ring structure as that of the halogeno-acetyl sugar used. The replacement of the halogen is usually accompanied by Walden inversion, and as pointed out by Isbell [14] normal glycosides are formed in good yield from halogeno-acetyl sugars having the halogen and acetyl groups on the same side of the sugar ring, whereas orthoacetic esters are formed from halogeno-acetyl sugars having the halogen and acetyl groups on opposite sides of the sugar ring.

Glycosides having complex aglucone groups may be prepared from the halogeno-acetates by use of various alcohols and phenols. If a solid alcohol or phenol is to be used, the substance in solution in an inert solvent such as benzene, or quinoline, is condensed, usually in the presence of silver carbonate or silver oxide, with the halogenoacetyl sugar dissolved in a similar solvent [15,]. The addition of a dehydrating agent, such as calcium chloride or Drierite [16], frequently improves the yields of the desired product. This method has also been applied to the preparation of furanoside derivatives [17].

Pyridine, quinoline, and sodium hydroxide have been used in place of silver carbonate. By treating the halogeno-acetyl derivatives in alcoholic solution with mercury salts instead of silver salts, Zemplén and Gerecs have worked out optimum conditions for the selective preparation of either the alpha or beta ethyl cellobiosides [18]. In general, by this method the formation of alpha isomers seems to be favored by approximately equivalent concentrations of halogenoacetate, alcohol, and mercury salt [18, 19]. Another method consists in condensing alcoholic solutions of the fully acetylated sugars in the presence of sublimed ferric chloride [20]. By the use of 1 mole of ferric chloride for each mole of the acetate, 20-percent yields of ethyl heptaacetyl-a-cellobioside are obtained.

(1) METHYL

TETRAACETYL-B-d-GLUCOPYRANOSIDE. Method [13].-Ten grams of bromo-tetraacetylglucose is dissolved in 150 ml of absolute methyl alcohol at room temperature and shaken with 10 g of dry powdered silver carbonate. The mixture is shaken until a halogen test of the solution is negative (about 6 hours). The mixture is filtered and the silver salts are washed with ether. The filtrate is treated with water and a little barium carbonate, and after a second filtration it is concentrated in vacuo to a heavy sirup which is extracted with ether. The combined ether extract is washed with sodium carbonate solution, then with water, and finally dried with sodium sulfate.

Methyl tetraacetyl-6-d-glucopyranoside crystallizes upon concentration of the solution. It is recrystallized from methyl alcohol or ligroin. The crystals melt at 105° C and give [a]=-18.2° (chloroform, c=4).

(d) PREPARATION OF PHENOLIC GLYCOSIDES FROM ACETYL DERIVATIVES

A very good method for preparing glycosides of phenols is that of Helferich and Smitz-Hillebrecht [21]. The phenol is melted with the

acetyl sugar in the presence of zinc chloride or p-toluene sulfonic acid. The zinc chloride customarily yields alpha glycosides, and the p-toluene sulfonic acid, beta glycosides. Glycosides of polyhydric alcohols and phenols have been prepared with several sugar residues in the molecule. The method is carried out by condensing the polyhydric phenol with the acetylated sugar and then reacting the product with a bromo-acetyl sugar in an aqueous acetone solution of sodium hydroxide.

(1) PHENYL TETRACCETYL-a-d-GLUCOPYRANOSIDE.

Method [21].-A mixture of 50 g of 6-pentaacetylglucose, 46 g of phenol, and 12.5 g of anhydrous zinc chloride is heated in a bath at 125° to 130° C. for 45 minutes while mechanically stirred. The dark solution is allowed to cool and is then dissolved in 300 ml of benzene and several hundred milliliters of water. The benzene solution is washed with water, then several times with 2 N sodium hydroxide solution, and finally several times with water. The solution is dried with calcium chloride and evaporated to a thick sirup, which is dissolved in hot alcohol and allowed to crystallize. After several recrystallizations from absolute alcohol, about 13 g of pure phenyl tetraacetyla-d-glucopyranoside is obtained. The compound melts at 114° to 115° C and gives [a]2+168° (chloroform).

The phenyl a-d-glucopyranoside is readily prepared from this material by deacetylation, using the method of Zemplén (see p. 494). (2) PHENYL TETRAACETYL-B-d-GLUCOPYRANOSIDE.

Method [21].-A mixture of 292 g of phenol, 3.9 g of p-toluenesulfonic acid, and 300 g of B-pentaacetylglucose is heated for 90 minutes, while mechanically stirred, in a boiling-water bath. The solution when cool is taken up in 400 ml of benzene and worked up as described above for the alpha isomer. The crystals obtained after recrystallization from alcohol weigh about 140 g. Phenyl tetraacetyl-6-d-glucopyranoside melts at 124° to 125° C and gives [a]=-22.0° (chloroform).

(e) PREPARATION OF GLYCOSIDES FROM SUGAR MERCAPTALS

This method is particularly useful for preparing furanose glycosides [23], but it may also be used for obtaining the pyranose glycosides [24], the thiofuranosides, and the thiopyranosides. The method consists in treating the sugar mercaptal dissolved in the alcohol with mercuric chloride, and when furanosides are desired, mercuric oxide. The product formed depends upon the conditions employed. Thus the galactofuranosides are formed at room temperature and in neutral ethyl alcoholic solution in the presence of mercuric oxide. In a boiling solution containing hydrogen chloride (which is formed during the reaction) the ethyl a-d-galactopyranoside is produced in 90-percent yield. A water suspension of the galactose ethyl mercaptal, mercuric oxide, and mercuric chloride gives at 0° C the crystalline ethyl galactothiofuranoside.

(1) ETHYL d-GALACTOFURANOSIDES.

Method [25]. To a solution of 28 g of galactose ethyl mercaptal in 250 ml of absolute alcohol at 70° C, 35 g of yellow mercuric oxide and 10 g of powdered Drierite' are added. The mixture is stirred rapidly while a solution of 35 g of mercuric chloride in 150 ml of absolute ethyl alcohol is added over a period of 30 to 40 minutes. The reaction mixture is allowed to cool to 30° C over about an hour's