slow uniform stream of carbon dioxide is passed through the apparatus and the reaction flask is gently heated until the liquid refluxes halfway up the condenser. Ordinarily 30 to 45 minutes are required to complete the reaction and sweep out the apparatus. After the reaction is complete, the contents of the 2 receivers are washed into a 250-ml Erlenmeyer flask containing 5 ml of a 25-percent aqueous sodium acetate solution. The volume of the solution is adjusted to about 125 ml and 6 drops of 90-percent formic acid is added. Then the flask is rotated until the color due to bromine disappears, whereupon 12 more drops of formic acid is added. The mixture is allowed to stand 2 minutes, 1 g of potassium iodide and a few milliliters of 10-percent sulfuric acid are added, and after 3 minutes the liberated iodine is titrated with 0.1 N sodium thiosulfate.

A blank should be run on the reagents, as all phenol appears to contain some substance which gives a small blank. This value is to be subtracted only from the first determination made with a given charge, as the material responsible for the blank is destroyed with the first determination. If the carbon dioxide is introduced

A

B

above the surface of the hydrogen FIGURE 113-Apparatus for the volumetric iodide-phenol charge, bumping

may be prevented by introducing a capillary boiling tube.

determination of methoxyl groups.

Each methoxyl group liberates six atoms

of iodine, and hence requires 6 moles of thiosulfate. ml 0.1 N thiosulfate X0.5172X100

mg sample

percentage of OCH

(g) REFERENCES

[1] T. Purdie and J. C. Irvine, J. Chem. Soc. 83, 1021 (1903).

[2] W. N. Haworth, J. Chem. Soc. 107, 8 (1915).

[3] K. Freudenburg and R. M. Hixon, Ber. deut. chem. Ges. 56, 2125 (1923). [4] C. M. Fear and R. C. Menzies, J. Chem. Soc. 1926, 937.

[5] C. C. Barker, E. L. Hirst, J. K. N. Jones, J. Chem. Soc. 1938, 1695.

[6] I. E. Muskat, J. Am. Chem. Soc. 56, 693, 2449 (1934).

[7] J. C. Irvine and A. Cameron, J. Chem. Soc. 85, 1071 (1904).

[8] C. B. Young, A General Review of Purdie's Reaction, Memorial Volume of Scientific Papers of St. Andrews University 500th Anniversary.

[9] W. N. Haworth and H. R. L. Streight, Helv. Chim. Acta 15, 614 (1932). [10] Hans Meyer, Lehrbuch der Organisch-Chemischen Methodik, I, 891 (Julius

Springer, Berlin, 1922).

[11] Franz Vieböck and Adolf Schwappach, Ber. deut. chem. Ges. 63, 2818 (1930). [12] E. P. Clark, J. Assn. Official Agr. Chem. 15, 136 (1932). [13] E. P. Clark, J. Am. Chem. Soc. 51, 4180 (1929).

8. TRIPHENYLMETHYL ETHERS

General characteristics.-Triphenylmethyl chloride reacts with sugars and glycosides in the presence of pyridine to give triphenylmethyl ethers, which are called "trityl" derivatives. It has been

shown that triphenylmethyl chloride condenses, preferably with the primary alcohol group. However, under more severe conditions secondary alcoholic groups will react [1]. Since the trityl derivatives are stable under the conditions required for acetylation and benzoylation, and since the trityl group may be removed easily by mild acid treatment, these compounds have proved useful for the production of sugar derivatives containing free primary hydroxyl groups [2]. The trityl group may also be replaced directly by bromine or iodine atoms [3].

(a) PREPARATION OF TRITYL DERIVATIVES

(1) METHYL 6-TRITYL-a-d-GLYCOPYRA NOSIDE.

Method [4].-One part of dry methyl a-d-glucoside and 1.4 parts of triphenylchloromethane (equivalent proportions) are dissolved in 8 parts of pure dry pyridine. The solution is heated on a boiling-water bath for 1 hour and then is allowed to cool. Water is added dropwise until the point of turbidity is reached, and the solution is allowed to stand for an hour. The solution is then poured into ice water. The sirupy layer is separated and rubbed up repeatedly with fresh portions of water. Crystallization takes place slowly. About 1.5 parts of the trityl derivative are obtained. The crude product is recrystallized from 5 parts of alcohol. When air-dried the compound contains 1.5 moles of alcohol of crystallization and melts at 80° C if heated rapidly. Methyl 6-trityl-a-d-glucopyranoside, when alcoholfree, melts at 151° to 152° C and gives [a]=+86.3° (pyridine).

2

NOTES

1 Traces of water will inhibit this reaction markedly.

? Or allowed to stand 24 hours at room temperature. For those glycosides without a primary alcohol group a considerably longer time is required (14 days at room temperature).

(b) REMOVAL OF TRITYL GROUPS

(1) CONVERSION OF METHYL 2,3,4-TRIBENZOYL-6-TRITY L-a-d-GLUCOSIDE TO METHYL 2,3,4-TRIBENZOYL-a-d-GLUCOSIDE.

Method [4]. Two parts of methyl 2,3,4-tribenzoyl-6-trityl-a-dglucoside are dissolved in 2.5 parts (by volume) of chloroform. The solution is cooled in an ice bath and is rapidly saturated with dry hydrogen chloride. After 30 minutes an excess of a saturated aqueous solution of potassium bicarbonate is mixed with the ice-cold solution. The chloroform layer is washed with water, dried with calcium chloride and evaporated in vacuo to a sirup which is taken up with a small amount of methyl alcohol. Upon standing at 0° C the solution deposits crystals of triphenylcarbinol and triphenylmethyl methyl ether. The crystals are separated and the mother liquor is evaporated again in vacuo to a thick sirup, which is dissolved in 1.5 parts (by volume) of hot absolute alcohol. Methyl tribenzoyl-a-d-glucoside crystallizes from the alcoholic solution in a yield of about 65 per cent and gives [a]=+131.7° (pyridine).

(c) QUANTITATIVE DETERMINATION OF TRITYL GROUPS

The trityl groups in a compound can be determined by their conversion to triphenylcarbinol. A weighed sample which will yield

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)3

[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 I-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]=+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.

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]+123° (water, c=1).

5

NOTES

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]=+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] =+66.8° (water, c=7). The levorotatory crystals are combined and recrystallized to give pure methyl ẞ-d-guloside.CaCl2.2H2O* and [a]2o=—45.7° (water, e=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]=+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 3-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.

3 In a typical experiment the first crop of crystals weighed 15 g and gave [a]2+60.8°; the second crop, 3.72 g, [a] = +37.5°; the third crop, 6.0 g, [a]20 17.3°; the fourth crop, 3.5 g, [a]20=-36.0°.