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177
MONOESTERS OF CHLORAMPHENICOL [CONTRIBUTION FROM THE
CHEMICAL RESEARCH LABORATORIES OF CABLO EEBA S.p.A.1
Monoesters of Chloramphenicol, Esterified on the Secondary Hydroxyl Group in the Presence of Trifiuoroacetic Anhydride and Methanesulfonic Anhydride LUIGI ALMIRANTE
AND
GIAMPAOLO TOSOLINI
Received May 9,1960 Chloramphenicol esters, esterified on the secondary hydroxyl group, have been prepared by condensing chloramphenicol with weak organic acids in the presence of tduoracetic and methanesulfonic anhydrides. The mechanism of the reaction and the infrared spectra of the compounds obtained are d i a c d .
During the last ten years, chloramphenicol esters have been greatly used in medicine for the parenteral administration1 of the antibiotic and for obtaining tasteless derivatives2 mainly for pediatric use. Chloramphenicol monoesters, synthesized for the above purposes, are esters on the primary hydroxyl group; their general formula is as follows: o z N + - ~ ~ - ~I ~ 2 ~ ~ ~ - ~ NHCOCHCla
In order to study the difference in behavior between chloramphenicol esters on the primary hydroxyl group and the corresponding esters on the secondary hydroxyl group of general formula OCO-R
I
NI-ICCCHQ,
with regard to the enxymntic hydrolysis, we considered the possibilities of synthesis of the latter class of compounds. In a previous article, one of us3 had already reported obtaining the stearic ester of chloramphenicol on the secondary hydroxyl group according to the following scheme of reaction: NO*
NOz I
I
CHOH I CHNHz
/
I
CHOH CHN, t ‘C-CHC12 CH20’
I
CHzOH
6 - Q CHN+
I
, C-CHClz
CUNHCOCHC~~
I
However, the total yield was low. Later, Edgerton, et al.‘ prepaied D( -) threo-2dichloroacetamid~l( p - nitrophenyl) - 1 - pnlmitoyl 1,3 propanediol through N + 0’ migration of the palmitoyl group of the corresponding 2-palmitoylamidopropanediol and successive dichloroacetylation a t the nitrogen. However, the total yield was only 2-3%. Continuing the studies in this field we followed the possibilities of direct esterification; in particular, our attention was drawn to the use of trifiuoroacetic anhydride as a condensing agent. A s is known,6 an alcohol reacts with an acid, in the presence of trifluoroacetic anhydride, according to the following scheme:
-
R 4 H
-
+ R ‘ 4 O O H + (CF&O)zO + R ’ 4 O O R + 2CF+2OOH
This is therefore a method of esterification under very mild conditions which is of great interest, particularly in carbohydrate chemistry. Specifically on studying the applications of this reaction to polyvalent alcohols, our attention was drawn to a work by Schmidt and Staab6 regarding the synthesis of 3,5diphenoylglucose in which is described the intermolecular closure of the diphenic 3-monoester of 1,2-acetoneglucofuranose to 1,2acetone-3,5-diphenoylglucofuranose, occurring in benzol, in the presence of trifluoroacetic anhydride. Closure involved a secondary and not a primary hydroxyl group. The investigators attribute this particular reaction to trifluoroacetylation in position 6, subsequent esterification in position 5 , and splitting of the trifluoroacetic radical by treating the reaction mixture with alkali. In the case of chloramphenicol, where the primary hydroxyl group is much more active than the (1) G. Ceriotti, A. Defranceschi, I. de Carneri, and V. Zamboni, F a m w , Ed. sei., 9,21 (1954). (2) G. Pauletta, Fannaco, Ed. sci., 7, 3 (1952);A. J. Glaako, W. H. Edgerton, W. A. Dill, and W. R. Lenz, Antibiotics and Chemotherapy, 2, 234 (1952). (3) L. Almirante and L. Caprio, 1st. “Carlo Erbrr,” Ricerche terap., Raccolta pubbl. chim. biol. e med., 2, 1-9 (1956);Chem. Abstr., 53, 12278d (1959). (4) W. H. Edgerton, V. H. Maddox, and J. Controulis, J . Am. Chem. Soc., 77, 27 (19%). (5) J. M.Tedder, Chem.Rev., 55, 787 (1955). (6) 0.T. Schmidt and W. Staab, Chem. Ber., 87, 388 (1954).
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MONOESTERS O F CHLORAMPHENICOL
The infrared spectra of 1-acyl derivatives showed secondary one, there was a good probability of obtaining esters a t the secondary hydroxyl group using the presence of bands due to G-0 stretching trifluoroacetic anhydride aa a condensing agent. vibration of the free primary hydroxyl group. In In fact, the reaction between one mole of stearic compound (111) the band was a t 1066 cm.-' in acid, one mole of chloramphenicol and an excess of (V) at 1072 cm.', in (VII) a t 1070 cm.-l, in (X) a t trifluoroacetic anhydride gave an excellent yield 1071 cm.-' and in (XII) at 1063 cm.-' In the case of the benzoic esters (V) and (XII) of a product identical with that already obtained by one of us3 by the above-mentioned way. The iden- the band was split for an absorption due to C-H tity was determined by mixed-melting point and in plane deformation vibration of the monombby infrared spectrum. stituted benzene ring'). The ditrifluoroacetyl Using infrared spectra we were able to demon- ester (IX) does not show bands in the 1020-1100 strate the mechanism of reaction. After allowing cm.-' zone; but it does a very intense band at 1792 one mole of stearic acid to react with one mole of cm.-', thus confirming its structure. The attempt to chloramphenicol (I) in an excess of trifluoroacetic esterify chloramphenicol with strong acids such as anhydride, we evaporated the mixture to dryness, formic, monochloroacetic, or dichloroacetic acids, removing traces of stearic acid with petroleum using trifluoroacetic anhydride as condensing agent, ether. The crude solid obtained was examined by did not succeed as was to be expected.b In these elementary fluorine analysis and infrared spectrum. cases, apart from chloramphenicol ditrifluoroaceThe product is 1-p-nitrophenyl-1-stearoyl-3-tri- tate (IX), we also isolated a monotrifluoroacetic fluoroacetyl-l,3-propanediol (11). On suspending ester (X) to which, by the aid of the infrared specthis ester in water made acid with hydrochloric acid trum, we attribute a structure similar to the aboveover night, the trifluoroacetic acid radical is split mentioned esters of the secondary hydroxyl group. l-Phenyl-l,3-propanediol (XIV) with stearic and the stearic ester of chloramphenicol on the secacid and trifluoroacetic anhydride formed almost ondary hydroxyl group is obtained (111): equal quantities of l-phenyl-l,3-distearoylpropaneR diol (XV) and 1-phenyl-3-stearoyl-1,3-propanediol I (XVI) according to the reaction:
+ HOOC-R'
CHOH I
CHNHCOCHClz
I
CHzOH I.R=NOz R
trifluoroacetic anhydride
0 I
CHzOH
Q
Q
1
I
'
I
CHz I CH2OCOR
CHzOCOCF, 11. R = NO2 R ' = (CH*)i&H3
anhydride
CH*
CHOCOR
CHNHCOCHClz
trifluoroacetic
CHOH
I
CHOCOR'
+ HOOC-R
I
4-
CHOH
I I
CHz CHzOCOR
Benzoic acid gave only l-phenyl-3-benzoyl-l,3propanediol (XVII)and 3,5dinitrobenzoic acid only 1-phenyl-1,3-dinitrobenzoyl-propanediol (XVIII), besides otber products containing fluorine which The same method was used to synthesize the ben- we did not purify. In the reaction for obtainzoic ester (V) and the acetic ester (VII) of the ing product I11 with trifluoroacetic anhydride as secondary hydroxyl group. 1-Phenyl-2dichloroacet- the condensing agent and as the main product using amido-l13-propanediol (XI) reacting with benzoic the methanesulphonic anhydride, we obtained a acid in the presence of trifluoroacetic anhydride has second compound of identical elementary formula also given 1-phenyl-1-benzoyl-2-dichloroacetamido- and spectrum (in carbon tetrachloride solution) which melted at 40' but did not crystallize from 1,3-propanediol (XII). The melting point and the infrared spectra of trichloroethylene. The product, purified from dilute these compounds differ from the corresponding alcohol, is probably amorphous. Thus, also in this esters at the primary hydroxyl group. The stearic case, the stearic ester of chloramphenicol (111) is ester (IV), benzoic esters (VI and XIII), and the present in two forms, one crystalline and the other :wetic ester (VI111 were prepared from one mole of amorphous, as we recently showed with a roentthe corresponding dichloroacetamido-propanediols genographic study for the stearic ester of chloramand one mole of the acid chloride in anhydrous (7) A. R. Katritzky and J. M. Lagowski, J. Chem. Soc., pyridine.2 4155 (1958).
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phenyl>3-stearoyl-2-dicMoroacetamido-l,3-propanedi~l,the two f o m have not been transformed into each other by crystallization. Using metlroncsu2fonic anhgdride aa the condensing agent, the reaction proceeded in the same way, always giving the ester (111); but the total yield of the product waa lower and the form melting at 40" was almost exclueively obtained. In the same way, using tduoroacetic anhydride aa condensing agent, D( - >threa-l-(pnitrophenyl>l-benzoyl-2dichloroacetamido-l,%propanediol (V) and D( - >thre+l(p-nitrophenyl>l-acetyl-2dichloroacetamido- 1,3-propanediol (VIII) and Dbthreo-l-phenyl-l-benzoyl-2diohloroacetamido-1,3-propanediol (XII) were obtained. The later waa produced starting from Dtthrea-l-phenyl-Zdichloroacee amido-1,bpropanediol (XI), already mentioned in literature,u with a melting point of 95-97' after crystallization from water. l-PhenyGl,~~tearoyLl,bpropanediol (XV) and l-phenyl3-steuroytl,3-propanedwl(XVI). Stearic acid (9.4 g., 0.033 mole) waa added to 10 ml. (0.071 mole) of tduoroacetic EXPERIMENTAL anhydride and the whole shaken for an hour, heating D( - ~ r e o - l - ( p N i t r o p h e n y l t l - s t e a r o y G ~ d ~ h l o r o a c e t -gently until complete solution waa obtained. The mass am&l,S-propanedwl (111). Stearic acid (8.3 g., 0.0294 solidified immediately afterwards. l-Phenyl-1,3-propanediol mole) waa suspended in 12 ml. (0.086 mole) of trifluoro- (XIV) (5 g., 0.033 mole) waa added, cooling with ice. On acetic anhydride. After 30 min. shaking at 25", 10 g. (0.031 bringing to room temperature, an exothermic reaction mde) of D( - >threo-l-(pnitrophenyl>fldichloracetamido- occurred and the whole maa went into solution. After 45 1,3-propanediol (CAF; I) waa added a little at a time. Com- min. the liquid was poured on ice and neutralked with plete solution occurred at the end of the addition. The sodium bicarbonate. It was filtered after being allowed t o solution waa allowed to stand at 25" for 45 min., and then stand for 2 days, and the solid crystallized from 95% at 50" for 5 min. It waa then poured into ice and neutralized ethanol. The product (XV) melting a t 60-61" was obwith sodium bicarbonate aftera few minutes. The mixture tained. Evaporation of the ethanol gave a waxy product was filtered and the crystals so obtained were purified by which waa crystallized from a mixture of ethanol and water dissolving in methanol and precipitating with water. Crude and filtered after cooling. This product (XVI) was dried in prodclct (14.2 g.) waa obtained, melting a t 80-86'; yield, a dessicator and melted at 41-42'. It did not depress the 82.5%. The product contained 2% of stearic acid and was melting point of a sample of bstearoyl ester obtained from recrystaked twice from trichloroethylene. Pure product, 1-phenyl-1,bpropanediol with stoichiometric quantities of B g., waa obtained, melting a t 103-104". On evaporation of stearoyl chloride in pyridine. The infrared spectrum was the trichlorethylene and trituration with petroleum ether also identical. 1-PhenyM-benzoyl-1,S-propanedwl (XVII) and l-phenyl(b.p. 60-70") 7 g. of a product melting at 40-42" waa 1 ,S-(S,6dinitro)bazoyGl,3-propanediol (XVIII) were preobtained. A d . Calcd. for C2&ft&1~"t06: c, 59.08; H, 7.86; c1, pared in the same way. 12.03. Found: C, 60.12; H, 8.01; C1, 11.57. Achmledgment. The authors acknowledge the The infrared spectrum confirmed the chloramphenicol technical assistance of S. Colombo and G. Salvamonostearic ester structure a t the secondary hydroxyl group. The product (111) waa therefore in two forms, one dori and wish to thank Dr. E. Pella for microcrystalline and the other probably amorphous. However, analyses. unlike what occurred in the cam of D( -)-threo-l-(pnitroMILAN,ITALY (8) L. Almirante, I. de Cameri, and G. Coppi, F a r " , (10) C. 0.Alberti, L. Bernardi, B. Camerino, D. C a b Ed. pact., 15, 671 (1960). (9) A. J. Glazko, W. A. Dill, A. Kazenko, L. Wolf, and tapan, G. Larini, and A. Vercellone, Gozz. Chim. Ztal., 84, H. E. Carnes, Antibiotics and Chemotherapy, 8 , 516 (1958). 519 (1954).
phenicol a t the primary hydroxyl group (IV).* I n the latter case, the biological activity is closely linked to the amorphous form of the ester and to the size of the particlese: the crystalline product does not undergo enzyme hydrolysis and therefore does not produce detectable blood levels of chloramphenicol, but is eliminated unaltered in the feces. In contrast, the amorphous product is completely absorbed from the gastrointestinal tract and is therefore therapeutically efficacious. In the case of esters of the secondary hydroxyl group, enzyme hydrolysis occurs only to a minimum extent and there is no appreciable daerence between the crystalline and amorphous forms of the stearic ester (111).
