[CONTRIBUTION FROM
THE
RESEARCH LABORATORIES OF THE SCHOOL OF PHARMACY, PURDUE UNIVERSITY ]
SYNTHESIS OF IODOHIPPURIC ACIDS. I. 2,5-, 3,5- AND 3,4-DIIODOHIPPURIC ACIDS CARL J. KLEMME
AND
JAMES H. HUNTER
Received November 16, I939
Coincident with the recent developments in the field of clinical radiography, a demand has arisen for suitable contrast agents. Various organic iodine derivatives have attained a prominent position in this capacity. The success attending the use of sodium o-iodohippurate (Hippuran) suggested a study of other iodinated hippuric acids. The first phase of this study, Le., the synthesis of the desired iodohippuric acids, is the subject of this and other reports to be published later. The pharmacological and radiological investigations of these derivatives are problems for subsequent studies. With o-aminobenzoic acid as the starting point, 2,5- and 3,5- diiodohippuric acids were synthesized as indicated below:
10;~:10 0 OC1
/
L!J ' ( I
I
CONHCHzCOOH
-+
I
:0 y ) ~ ~ I ~ ~ I ~ L I
I
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while 3,4-diiodohippuric acid was obtained from p-aminobenzoic acid through the following steps:
I
I
1 From a portion of a thesis submitted by James H. Hunter in partial fulfilment for the degree of Doctor of Philosophy, August, 1938.
m
228
CARL J. KLEMME AND JAMES H. HUNTER
I
I
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The preparation of 2-amino-5-iodobenzoic acid has been effected by a number of different methods. Grothe (1) prepared it as early as 1878 by reduction of the corresponding nitro-iodo acid; Wheeler and Liddle (2) by acid hydrolysis of 2-acetamino-5-iodobenzoic acid ; Wheeler and Johns (3) by the action of finely powdered iodine on a solution of anthranilic acid in potassium hydroxide; Schoeller and Hueter (4) by treating the anhydride of 5-hydroxymercuri-anthranilic acid with iodine in potassium hydroxide; and Borsche, Weussmann, and Fritzsche (5) by the action of iodine monochloride on an acetic acid solution of anthranilic acid2. The method of Wheeler and Johns (3) appeared to afford a convenient means of preparing 2-amino-5iodobenzoic acid for our study. However, in our experience, this method proved unsatisfactory owing to an extremely contaminated crude product and low yields. By chance, our attention was attracted to a reaction between anthranilic acid and nascent iodine3. Based on this reaction, we have developed a satisfactory method for the preparation of this amino-iodobenzoic acid. It is not without interest to mention that 2-amino-5iodobenzoic acid can also be prepared by application of the iodination method of Datta and Prosad (6). This procedure was not critically studied; however, in our preliminary experiments, the yields were found to be low. 2-Amino-5-iodobenzoic acid was converted into the 2 ,5-diiodo acid by diazotization and treatment of the diazonium compound with aqueous potassium iodide according to the directions of Wheeler and Johns (3). This acid, upon treatment with thionyl chloride, readily yielded the corresponding acid chloride, and the latter, when shaken with glycine in the presence of dilute sodium hydroxide, gave the sodium salt of 2 ,bdiiodohippuric acid. The free acid was liberated from the sodium salt by treatment with dilute mineral acid. 2-hino-3 ,5diiodobenzoic acid, the essential intermediate for the synthesis of 3 ,5-diiodohippuric acid, has been prepared by iodinating 2-amino-5-iodobenzoic acid with iodine monochloride (3), and by the 2 Since the completion of this work, a detailed procedure for the preparation of 2-amino-5-iodobeneoic acid has appeared in Organic Syntheses, John Wiley and Sons, New York City, 1989, Vol. XIX, p. 52. s A brief description of this method of iodination is given by Kempf in Houben’s “Die Methoden der organischen Chemie,” Georg Thieme, Leipzig, 1933, Zweite Auflage, Vol. 111, p. 877.
SYNTHESIS OF IODOHIPPURIC ACIDS
229
action of two moles of this reagent on o-aminobenzoic acid in dilute hydrochloric acid (3). Preparation of this acid for our purpose was based on the latter method. Wheeler and Liddle (2) originally obtained 3 ,5-diiodobenzoic acid by diazotizing 4-amino-3,5-diiodobenzoicacid and decomposing the resulting diazonium compound with boiling alcohol. Later (7) they reported the preparation of this acid from 3-iodo-5-aminobenzoic acid. We have used a procedure of Wheeler and Johns (3), based on thedeaminationof 2-amino-3 ,5-diiodobenzoic acid. From 3 ,5-diiodobenzoic acid, 3 ,5diiodohippuric acid was obtained by the aforementioned procedure for the isomeric 2,5diiodohippuric acid. 3-Iodo-4-aminobenzoic acid was first prepared by hydrolysis of 3-iOdO4-acetaminobenzoic acid (2). In 1909, Wheeler and Liddle (2) reported that 3-iodo-4-aminobenzoic acid could be prepared by passing the vapors of iodine monochloride into a solution of p-aminobenzoic acid in cold, concentrated hydrochloric acid. We sought to employ the latter method for producing this acid. In spite of repeated attempts, we were unable to repeat their preparatory directions, and it was necessary to modify their procedure by using glacial acetic acid rather than concentrated hydrochloric acid. With this modification, 3-iodo-4-aminobenzoic acid was prepared, although in poor yields. An attempt to apply the method developed for the preparation of 2-amino-5-iodobenzoic acid, Le., iodination with nascent iodinea, to the preparation of the isomeric 3-iodo-4-aminobenzoicacid naturally suggested itself. While a small amount of the latter compound was obtained by adapting this method to p-aminobenzoic acid, the principal product formed was 2 ,4-diiodoaniline. Preliminary experiments on the iodination of p-aminobenzoic acid with elementary iodine in the presence of ammonia ( 6 ) gave no promise of being satisfactory. Efforts to iodinate p-aminobenzoic acid by treatment with the calculated quantities of potassium iodide and potassium iodate in dilute sulfuric acid were likewise unsuccessful. For use in the present investigation, 3-iodo-4-aminobenzoic acid was prepared both by the action of gaseous iodine monochloride on an acetic acid solution of p-aminobenzoic acid, and by the action of nascent iodine on aqueous potassium p-aminobenzoate. Neither of these methods may be regarded as practical. 3-Iodo-4-aminobenzoic acid gave, upon diazotization and treatment with aqueous potassium iodide, the corresponding diiodo acid (2). This acid has also been prepared by Wheeler and Johns (8) from 2-amino-4 ,5diiodobenzoic acid. Conversion of 3,4-diiodobenzoic acid into its chloride and the latter into 3,4-diiodohippuric acid was effected in a manner similar to that mentioned above for the 2,Bdiiodo derivative.
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CARL J. KLEMME AND JAMES H. HUNTER
Iodine was estimated, in most cases, by alight modifications of the official method demibed for tetraiodophenolphthalein (9). A potentiometric method waa used for analysis of the iodo acid chlorides. In brief: A sample of the compound waa decomposed by fusing with anhydrous sodium carbonate snd the fusion filtrate, in excess dilute sulfuric acid, titrated with N/10 silver nitrate using a silver-silver chloride indicator electrode and a reference electrode of mercurous sulfate. Molecular weight determinations were made by titration of an alcoholic solution of the appropriate compound in accordance with a recognized procedure for neutral equivalent determinations (10). EXPERIMENTAL
8-Amino-6-iodobenzoic ocid. With mechanical stirring, 25 g. (0.184 mole) of recrystalbed anthranilic acid (map.145")'waa dissolved in a solution of 16.9 g. of stick potaesium hydroxide in 600 cc. of water contained in a 2-liter beaker. A solution of 46.8g. (0.184 mole) of iodine in 250 cc. of water containing 24.75 g. of stick potassium hydroxide waa slowly run into the well-stirred potassium anthranilate solution. After one minute, 100 cc. of glacial acetic acid was quickly added and the reactionmixture immediately diluted with 600 cc. of water. A dark precipitate began to appear almost a t once; stirring was continued for one hour, during which time this precipitate aaaumed a light brown color. After standing undisturbed for two hours, excess iodine waa removed by adding 25 cc. of 15% sodium bisulfite and thoroughly agitating. The mixture was allowed t o stand a short while. The precipitate waa collected, repeatedly washed with water and air-dried. Yield of crude product, 41.69 g. (86.8%). One crystallization from dilute alcohol gave, after working up the mother liquor, 34.84 g. (72.2%) of a pure product melting a t 210-211.5". A portion of this product, when mixed with an authentic specimen of 2-amino-5-iodobenzoic acid, caused no depression of the melting point of the latter. Identity of our product was further confirmed by converting it into 2,5-diiodobenzoic acid, and identifying the latter by a mixed melting point with a known sample of this diiodobenzoic acid. Several repetitions of thin preparatory procedure with different amounts of starting materials gave equally good yields. S,b-Diiodobenzoic acid. This compound was prepared in good yields according to the directions reported by Wheeler and Johns (3). The alkali-acid purification step described by these investigators was omitted. The crude product was directly crystallized from dilute alcohol. Thus, from 15.0 g. (0.0593 mole) of 2-amino-5iodobenzoic acid, 17.8 g. (83.6%) of crude 2,S-diiodobenzoic acid was obtained. Crystallization from dilute alcohol gave 14.6 g. (68%) of the pure compound, m.p. 180.5-181.6'6. S,b-DiiodobenzoyZ chloride. Fifty grams (0.134 mole) of pure 2,5-diiodobenzoic acid waa treated with 50 cc. of thionyl chloride (Eastman's "Practical") and the mixture gently refluxed on a steam-bath for two hours. Excess thionyl chloride was distilled from a steam-bath and the residue crystdlized from carbon tetrachloride. All melting points are uncorrected. Wheeler and Johns (3) reported a quantitative yield for the crude diiodo acid; the yield of purified acid, m.p. 183O, ia not reported. Following their procedure, we have obtained, after purification, an acid melting a t 181" instead of 183". 4
6
SYNTHESIS OF IODOHIPPURIC ACIDS
231
After washing once with petroleum ether, the pale yellow crystals melted at 93-95". Yield: 45 g. (85.8%). For analysis, the compound was recrystallized from carbon tetrachloride. This gave pale, yellowish-green needles, melting at 93-94.5'. Anal. Calc'd for C7HsClIIO: I, 64.70; C1, 9.03. Found: I, 64.56; C1, 8.79. Alkaline hydrolysis of the above compound gave, after acidifying and crystallizing from dilute alcohol, an acid melting at 182.5-183". When this acid was mixed with pure 2,5-diiodobenzoic acid no depression of the melting point was observed. 9,6-Diiodohippuric acid. Sixteen grams of glycocoll (Eastman) was dissolved in 100 cc. of 5% sodium hydroxide and 20 g. (0.051 mole) of 2,5-diiodobenzoyl chloride was added. The mixture was vigorously shaken, whereupon most of the acid chloride went into solution. The reaction-mixture was diluted with 50 cc. of water and filtered. The clear, yellow filtrate was poured, with efficient stirring, into a mixture of 25 cc. of concentrated hydrochloric acid and 150 g. of crushed ice. The white, gelatinous precipitate was collected, washed, and air-dried. The dry product was pulverized and extracted with hot ether to remove any 2,5-diiodobenzoic acid present. Yield: 20.3 g. (92.6%). Crystallization from dilute alcohol gave white burrs melting a t 210.5-211". Anal. Calc'd for CoH7LNOs: N, 3.25; I, 58.67; M.W. 430.9. Found: N, 3.36; I, 57.53; M.W. 435.9. 3-Iodo-4-aminobenzoic acid. (A) A solution of 3 g. (0.022 mole) of p-aminobenzoic acid in 30 cc. of glacial acetic acid was placed in a small suction-flask fitted with a n inlet tube, the lower end of which dipped below the surface of the solution. The flask was connected t o a second small suction-flask containing 3.5 g. (0.0197 mole) of iodine monochloride (11) and also fitted with an inlet tube reaching almost to the bottom. While a current of air was drawn through the system, the iodine monochloride was vaporized by warming on a steam-bath. After all the iodine monochloride had passed over, the reaction-mixture was poured into 150 cc. of water and allowed t o stand for two hours. The brown, flocculent precipitate was collected, washed with water and air-dried. Yield: 2.1 g. (36.5 per cent) of asubstance melting at 202". Crystallization from water gave a small amount of a compound melting at 204". A mixture of this compound and 3-iodo-4-aminobenzoic acid melted a t 203204". Repetition of this procedure, using 10 g. of p-aminobenzoic acid and proportional quantities of other reagents, gave a much lower yield (15.6%). (B) Ten grams (0.073 mole) of p-aminobenzoic acid was dissolved in 200 cc. of water containing 8 g. of potassium hydroxide. To this was added a solution of 18.6 g. (0.073 mole) of iodine dissolved in 100 cc. of water containing 9 g. of potassium hydroxide. With stirring, 40 cc. of glacial acetic acid was run into the combined solutions and the whole immediately diluted with 190 cc. of water. The mixture was stirred vigorously and allowed to stand overnight. After treating with a small amount of 15% sodium bisulfite, the brown precipitate was filtered off, washed with water and air-dried. Yield: 8.35 g. (43.4%). The crude product was extracted with 40 cc. of 10% ammonia water and the residue washed with water. The dry, ammonia-insoluble residue weighed 5.04 g. (26.3%). Acidification of the ammoniacal extract with acetic acid gave, after long standing, 2.15 g. (11.2%) of dark crystals melting at 200-201". Crystallization from dilute alcohol yielded 1.7 g. (8.85%) of faintly colored crystals, m.p. 203-204". Identity of this product was supported by diazotizing and treating with aqueous potassium iodide; this yielded a product which, after purification, melted at 257-258'.
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CARL J. KLEMME AND JAMES H. HUNTER
Wheeler and Liddle (2) have reported the melting point of 3,4-diiodobenzoic acid as 257". The ammonia-insoluble product, after crystallizing from dilute alcohol, melted a t 94-96'. I t was soluble in dilute sulfuric acid; insoluble in water and alkali. From these properties, 2,4-diiodoaniline was suspected. Two more crystallizations from dilute alcohol gave a product which melted a t 95.5-96'. Anal. Calc'd for CsHJ2N: N, 4.06; I, 73.6. Found: N, 4.05; I, 70.22. Method (B) was carried out under various conditions, particularly of temperature, but no improvement was observed. Indeed, a t temperatures of 0" to lo", elementary iodine was deposited. 3,d-Diiodobenzoic acid was prepared from 3-iodo-4-aminobenzoic acid according to the procedure of Wheeler and Liddle (8). 3,d-Diiodobenzoyl c h l o d e . A mixture of 4 g. (0.0106 mole) of 3,4-diiodobenzoic acid and 10 cc. of thionyl chloride (Eastman's "Practical") was gently refluxed for three-fourths of an hour on a steam-bath. Excess thionyl chloride was distilled from a steam-bath and the residue extracted with four small portions of carbon tetrachloride. The combined extracts were evaporated to about 3 cc. and chilled. Separation of the yellow solid was completed by the addition of a small volume of petroleum ether. The product, after decanting the petroleum ether and drying, weighed 2.6 g. (62.4%) and melted a t 74-76". Anal. Calc'd for C7HaC1120:I, 64.70; C1, 9.03. Found: I, 64.35; C1, 9.16. Hydrolysis of this product gave an acid melting a t 258-259". No depression of the melting point was observed when this acid was mixed with 3,4-diiodobenzoic acid. 3,d-Diiodohippuric acid. One and nine-tenths grams (0.0048 mole) of 3,4-diiodobenzoyl chloride was added to a hot solution of 2 g. of glycocoll (Eastman) in about 40 cc. of approximately 1%sodium hydroxide. The temperature was held between 85" and 100' for thirty-five minutes. The reaction-mixture was then filtered and washed. After acidifying the filtrate with concentrated hydrochloric acid, the white precipitate was collected, washed with water and air-dried. The crude, dry product was repeatedly extracted with hot ether to remove 3,4-diiodobenzoic acid. The dry, ether-insoluble residue weighed 1.32 g. (64.0%). Crystallization from dilute alcohol gave pure white crystals melting a t 150-154" after beginning to soften a t 148". Anal. Calc'd for C O H ~ I ~ NN, O ~3.25; : I, 58.67; M.W. 430.9. Found: N, 3.02; I, 57.61; M.W. 436.8. 9-Amino-3, 6-diiodobenzoic acid. The preparation of this compound was based on the procedure of Wheeler and Johns (3), who treated anthranilic acid in 5% hydrochloric acid with aqueous iodine monochloride. In our experience, attempts to prepare an aqueous solution of iodine monochloride resulted in its prompt decomposition. The following modification was found to be satisfactory. Two hundred and fifty cubic centimeters of 25% hydrochloric acid contained in a 2-liter beaker was heated to 80" and stirred mechanically. Twelve and one-half grams (0.091 mole) of recrystallized anthranilic acid was dissolved in this warm solution and a solution of 29.5 g. (0.183 mole) of iodine monochloride (11) in 30 cc. of 25% hydrochloric acid was added. After stirring for one minute, the reaction-mixture was diluted with 11. of water, whereupon a yellow precipitate began to appear almost a t once. With continued stirring, the mixture was heated a t 80-90" for fifteen minutes and then cooled t o 28". The flesh-colored precipitate was filtered off, washed thoroughly with water, and dried in air. The crude product was dissolved in 300 cc. of
SYNTHESIS OF IODOHIPPURIC ACIDS
233
warm (45") sodium hydroxide and filtered into dilute hydrochloric acid. The flocculent precipitate was collected, washed with water, and air-dried. Yield: 17.5 g. (49.3%), m.p. 226-229". One crystallization from dilute alcohol, using a little decolorizing charcoal, gave 14.0 g. (39.4%) of the crystalline acid melting at 231". Wheeler and Johns (3) report yields of 85-90% of crude product. They gave no yield for the pure compound, m.p. 230-232". 3,5-Diiodobenzoic acid. This compound was prepared from the foregoing 2amino-3,5-diiodobenzoic acid according to the procedure of Wheeler and Johns (3). S,5-DiiodobenzoylchZoride6. A mixture of 2.5 g. (0.0067 mole) of 3,5-diiodobenzoic acid and 5 cc. of thionyl chloride (Eastman's "Practical") was gently refluxed on a steam-bath for three-fourths of an hour, and the excess thionyl chloride distilled. The residue was crystallized from a small volume of carbon tetrachloride using a little decolorizing charcoal. Yield: 2.07 g. (79%) of yellow crystals, m.p. 66-67". Recrystallization from carbon tetrachloride gave 1.6 g. (60.8%) of crystals melting a t 67-68'. Anal. Calc'd for CrHaClI20: I, 64.70; C1, 9.03. Found: I, 64.25; C1, 8.66. Hydrolysis of the above compound gave an acid melting at 237.5-238". When mixed with a known specimen of 3,5-diiodobenzoic acid (m.p. 233-LWo), the mixture melted at 236.5-237.5'. 3,5-Diiodohippuricacide. Five grams of glycocoll (Eastman) was dissolved in about 100 cc. of approximately 1% sodium hydroxide and the solution warmed t o 90". Four and eight-tenths grams (0.0122 mole) of 3,5-diiodobenzoyl chloride was added and the mixture shaken vigorously for ten minutes. During this time, the temperature was kept between 80" and 90". After cooling slightly, the supernatant liquid was filtered off. The filtrate was cooled t o room temperature, acidified with concentrated hydrochloric acid, and cooled to 16". The white precipitate was collected, washed with water and air-dried. Yield: 1.7 g.7. The crude product was extracted with hot ether and the ether-insoluble residue crystallized from dilute alcohol. Yield: 0.95 g. (39.9%) of white crystals melting a t 208-209". Anal. Calc'd for CsH?I2NOa:N, 3.25; I, 58.67, M.W. 430.9. Found: N, 3.11; I, 57.88; M.W. 430.9. SUMMARY
1. A new and practical method has been developed for the preparation of 2-amino-5-iodobenzoic acid. It was further found that this acid could be prepared by adapting the iodination method of Datta and Prosad (6) to anthranilic acid. 2. A new method for the preparation of 3-iodo-4-aminobenzoic acid by the iodination of p-aminobenzoic acid with nascent iodinea is reported. The method can not be recommended as practical; the principal product formed in this reaction was found to be 2,4-diiodoaniline.
* This compound is mentioned in English patent 465,994; Chem. Abstr., 51, 7446 (1937). 7 Two and one-half grams of impure 3,5-diiodobenzoyl chloride was recovered after filtering off the supernatant liquid, hence only 2.3 g. (0.00686 mole) of the acid chloride actually was used in this reaction.
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CARL J. KLEMME AND JAMES H. HUNTER
3. The preparation of 2,s)3,4- and 3,s- diiodobenzoyl chlorides, and conversion of these into the corresponding diiodohippuric acids is described in detail. LAFAYETTE, IND. REFERENCES
(1) GROTHE,J. prakt. Chem., 18,327 (1878). (2) WHEELER AND LIDDLE,A m . Chem. J., 43, 441 (1909). (3) WHEELER AND JOHNS, Am. Chem. J . , 43, 398 (1910). AND HUETER,Ber., 47, 1938 (1914). (4) SCHOELLER (5) BORSCHE,WEUSSMANN,AND FRITZSCHE, Ber., 67, 1774 (1924). (6) DATTAAND PROSAD, J. Am. Chem. SOC.,39, 441 (1917). (7) WHEELERAND LIDDLE,Am. Chem. J . , 43,505 (1909). (8) WHEELERAND JOHNS, Am. Chem. J., 44, 451 (1910). (9) “Pharmacopoeia of the United States, ” 11th Decennial Revision, Mack Printing Co., Easton, Pa., 1936, p. 196. (10) MULLIKEN,“A Method for the Identification of Pure Organic Compounds,’, John Wiley and Sons, New York, 1922, Vol. IV, p. 156. (11) Organic Syntheses, John Wiley and Sons, New York, 1934, Vol. XIV, p. 53.
