The interaction of alkyl α-(S)-Malates with N,N′-dicyclohexylcarbodiimide under mild conditions

Eur. Polym. J'. Vol. 27, No. 6, pp. 479-482, 1991 Printed in Great Britain. All rights reserved

0014-3057/91 $3.00 + 0.00 Copyright © 1991 Pergamon Press plc

THE INTERACTION OF ALKYL ~-(S)-MALATES WITH N,N'-DICYCLOHEXYLCARBODIIMIDE UNDER MILD CONDITIONS NADYA BELCHEVA,* IVAN M. PANAYOTOVand CHRISTO TSVETANOV Institute of Polymers, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria (Received 9 August 1990)

Abstract--The autopolycondensation of alkyl ct-(S)-malates in the presence of N,N'-dicyclohexylcarbodiimide under mild conditions was studied. It was established that the final product consists of low molecular N-acyl derivatives and optically active polyester fraction with 3~n up to 4000 and low polydispersity Jl~w/AT. = 1.1 - 1.5. When malic acid was used as a monomer surprisingly low molecular weight products of ester and anhydride nature were obtained instead of the expected polymalic network. All the polymer fractions separated were optically active oligoesters which could be used as biodegradable carriers of pharmaceutical agents.

chloroform/methanol at 2:1 vol. ratio) sample with 0.02 M alcoholic KOH according to Korshak et al. [11]. Specific rotation [~] was determined on POLAMAT A---Carl Zeiss Jena. t-Butyl ~t-(S)-malate (t-BuM) and benzyl ~-(S)-malate (BM) were obtained as described by Miller et al. [12] t-BuM--yield: 30%. i.r. (neat): 1735cm i. [~t].~= 11.97 + 0.1 (c = 1.4 g/dl; methanol). BM--yield: 60%. i.r. (neat) = 1735cm-~. [~t]ss9 20° - - 10.16 +_0.1 (c = 2.5 g/dl; methanol). [ct]~6 = - 12.33 _+0.1 (c = 1.5 g/dl; methanol). The two ~-monoesters are viscous liquids. Octyl ~t-(S)-malate (OM) and ~t-(S)-octylmalamide (OMA) were synthesized by the method of Liesen et al. [13]. OM--yield: 60%. i.r. (neat): 1740 cm -l. [ct]~6 = -7°_+ 0.1 (c =0.7g/dl; methanol). OMA--yield: 70%. i.r. (neat): 1720, 1640, 1570cm -t. [~t]~=-36°_+0.1 ( c = l . 3 g / d l ; methanol). The compounds are viscous liquids. Kinetic data on the interaction of alkyl ct-(S)-malates with DCC in solvents were obtained via titration of aliquots of 1 ml with 0.02 M alcoholic KOH (content of monomer in each sample from 3 to 6 mmol) [6]. The autopolycondensation process was monitored by i.r. spectrometry, measuring the band at 2115cm -~ for DCC. The reaction mixture was filtered after disappearance of this band and the filtrate was treated as described for MM [6].

INTRODUCTION Biodegradable polyesters synthesized on the basis of natural acids have potential advantages as biodegradable drug carriers [1-3]. C o m p a r e d with synthetic polypeptides, they are more stable against tissue enzymes and in many cases they are preferable for their better biocompatibility [4]. Some reports have appeared on various applications of biodegradable oligoesters with h4"n up to 104 as suitable carriers of anticancer agents [5]. In our previous paper [6] the interaction of methyl ~t-(S)-malate ( M M ) with N,N'-dicyclohexylcarbodiimide (DCC) was investigated as a model reaction for preparing biodegradable polymalates via polycondensation. The aim of this work is to complete the investigation on the autopolycondensation of alkyl ct-(S)-malates, including t-butyl-, benzyl-, octyl ct-(S)malates and octylmalamide in the presence of D C C , p-toluenesulphonic acid (p-TSA) and pyridine (Py) under mild conditions, particular attention being paid to the influence of the ct-substituent on the macromolecular characteristics o f the polymer fractions. Linear oligoesters were directly prepared from (R, S)malic acid using the carbodiimide method. EXPERIMENTAL

RESULTS AND DISCUSSION

PROCEDURES

Materials and methods

All starting compounds were recrystallized or distilled under vacuum. All solvents were dried and distilled prior to use. The i.r. spectra were registered on a Perkin-Elmer 983 spectrophotometer at a resolution of 3.0 cm- ~. The ~/w, -~/, and h/w/-£/, values were determined using a Waters gelpermeation chromatograph equipped with a differential refractometer R-401, u.v.-detector M440 and Ultrastyragel columns with porosities 500, 500 and 100 A,. THF was used as eluent at a flow rate 0.8 ml/min at 45°C. Calibration was made with polystyrene standards. Carboxyl end-groups were determined by titration of the dissolved (water or *To whom all correspondence should be addressed.

In our previous work [6] a convenient kinetic procedure was developed for following the interaction between ~t-monoesters of (S)-malic acid and D C C in various solvents. The observed correlation between the solvent donor number (DN) and the conversion (XcooH) gives indication both for the relative reactivity of the carboxyi groups and the kind of final product. The method can be used as a qualitative test for the reliability of the solvent as a factor directing the reaction to polymer formation. The influence of the solvent on the reaction between BM and O M with D C C is shown in Fig. 1, where kinetic data in various solvents are given. The c o m m o n course of the curves is identical with that for M M [6]. Unfortunately, the

479

t

NADYA BELCHEVAet al.

480

(a)

(b)

100 o

100

oN

8O "

o

- ~

,".--

20

70 DN

60

-=

50

>~ 4 0 (..) 14.1

14 • 1

20

I

2

20

ff

~7

I

I

n I

I

I

I

4

6

2

4

6

r(hr)

r(hr)

Fig. 1. Conversion of BM (a) and OM (b) in various solvents in the presence of DCC. p-TSA (4 mol%) and Py to monomer mole ratio of 0.25 at 0°C. Disappearance of acid groups was followed by titration with K O H (0.02 M in absol, ethanol). (O, O) THF; (I-q, I I ) CH3CN; (A, &) benzene; (~7, V ) CHC13.

interaction of t-butyl ~-(S)-malate (t-BuM) and DCC cannot be followed by the same method because of hydrolysis of the ester bond since the procedure represents titration with alcoholic potassium hydroxide. In the i.r.-spectra of the final products obtained in various solvents, besides the ester band, peaks at 3330, 1650 and 1520cm -1 appeared, most probably because of the presence of stable low molecular N-acyl derivatives. The latter block the carboxy group and hinder further propagation of the polyester chain. This process could not be eliminated either by variation of the ~-substituent or by the type of the solvent. Another proof of this assumption is that the yields of polymer fractions do not exceed 25-30% in all cases of polycondensation under different reaction conditions. The molecular weight char'acteristics of the polymer fractions isolated by precipitation of the reaction mixture in ether after filtering of N,N'-dicyclohexylurea (DCU) are shown in Table 1. The data from experiments giving polyesters with highest molecular weights are presented in Table 1. It can be concluded that polymer formation is favoured when reaction of the corresponding malate is carried out in a solvent similar in nature to the ~-substituent. The polycondensation process is strongly hindered,

also, when malates with bulky moieties are used. Most probably the steric hindrance additionally complicates the multistage condensation process in the presence of carbodiimides. Thus our efforts to obtain poly-t-BuM via polycondensation of t-BuM were unsuccessful. As seen from Table 1, the length of the aliphatic chain of the ~-substituent does not dramatically influence the course of the reaction. In the i.r. spectra of all polymer fractions, a band at 1750-1735 cm -1 shows their ester nature. The advantage of the developed method is that the polymers obtained by polycondensation of optically active monomers are also optically active. Specific rotations [~] of the monomers and their polymalates are given in Table 2. Unfortunately, it is difficult to find any direct explanation of the results observed or any correlation between characteristics given. Preparation of optically active high molecular weight polymalates by anionic and cationic polymerization is known [3] but no data of any correlation exist. The authors suppose that chirality in the biodegradable carriers indirectly affects the process of the controlled release of pharmaceutical agent in the body. Reactions with carbodiimides are widely used in organic and biochemical synthesis but surprisingly

Table 1. Molecular weight characteristics of polyalkyl =-(S)-malates obtained as precipitates in ether and determined by gel permeation chromatography (GPC) No. Monomer 1.

MM

2. 3. 4. 5. 6. 7.

t-BuM BM BM OM OM OMA

Solvent Benzene/THF 5:2 THF THF Benzene THF n-Octane THF

Time* (hr)

Yield of polymer (%)

h3.

~/M.

P.

- 5

3

22

4000

1.3

3I

0 0 10 0 0 0

4 9 7 8 5 4

18 15 20 10 22 26

650 1400 4200 1600 3800 700

2.2 1.2 I. I 1.2 1.4 1.1

3 6 20 7 17 3

0 (°C)

All reactions were carried out in the presence of p-TSA (4% tool) and Py to monomer mole ratio 0.25. *The reaction was carried out until DCC was consumed (i.r. band at 2115 cm-t).

Interaction o f malates with carbodiimides

481

Table 2. Specific rotation [~] of alkyl a-(S)-malates and polyalkyl a-(S)-malates and their molecular weight characteristics determined by G P C No.

Product

I.

9. 10.

MM Poly-MM Poly-MM BM BM Poly-BM Poly-BM OM Poly-OM t-BuM

l 1.

Poly-t-BuM

700

12. 13.

OMA Poly-OMA

700

2. 3. 4. 5. 6. 7.

8.

t,,]]~;

~n
- 12.33 -25.38

700 4200

- 18.38 -7 - 1.7 - 11.97 -42.7 -36.0 -28.93

2100

only a few reports on their use in oligomer or polymer reactions have appeared [7, 8]. This can be explained by the multistage complex mechanism of the reactions with carbodiimides resulting mainly in indefinable mixtures of products. Our study on the autopolycondensation of unprotected (R, S)-malic acid broadens the outlook on the esterification process in the presence of DCC. The availability of two carboxylic groups and one hydroxy group in the molecule of malic acid is a prerequisite for the formation of a crosslinked product with mixed ester and anhydride structure. Our results however, did not confirm this assumption. The interaction between (R, S)-malic acid and DCC leads only to a mixture of low molecular products soluble in water, THF, DMF, C H C I 3 and insoluble in acetone (Table 3).

I.

HO-CH-COOR I I + RN=C=NR CH2COOH

~

t,,]~

-5.29 - 18.19 - 19.69 - 10.16

Cone. 2.5, 2.1, 1.8, 2.5, 1.5, 1.3, 1.6, 0.7, 0.8, 1.4, 1.4, 1.3, 1.2,

(%) (solvent) methanol chloroform chloroform methanol methanol chloroform chloroform methanol methanol methanol acetone methanol chloroform

To clarify the character of this reaction, model experiments were performed with succinic acid and glycolic acid. According to the tentative reaction scheme of Holmberg [7], the interaction between succinic acid and DCC leads only to cyclic anhydride whereas ester formation is preferred for glycolic acid (Scheme 1). i.r. Spectra listed in Table 3 confirm the proposed structure of the final product. It is logical to assume that the composition of the final product in the case of autopolycondensation of (R, S)-malic acid in the presence of DCC is affected by the quantity of DCC. When the stoichiometric ratio of DCC to carboxy groups is 1:2, mostly linear oligoesters are obtained, while at ratio 1:1 the final product most probably is a mixture of linear oligoesters and cyclic anhydride. The investigation of Mironova et al. [9] on the interaction of dicarboxylic

HOCHC O0R I I CH2~-~NHR

HOCHC00R I I

CH~C-NR-C-~YHR

HOCHC00R I I

c 2.

HOOCCHCH2C00H

+ RN=C=NR ~

HOCHC00H

OH 0

NR

c-o-cHc ocooH

OH 0 I II CH-C ~ 0 I /

CH2-~ 0

HOOCCHCH~C-O-CHCH~COOH OH 3.

HOOCCH2CH2COOH

+

PaN=C=I~R ~

~H2COOH CH2~I-O-~I-NHR 0

4.

HOCH2COOH

+

RN=C=~

NR

HOCH2~-0-1~-NHR 0

Scheme 1

0

C00H

0 II H 2 - C~ 0

~

CH 2 - C / 11 0 HOCH2~-0-CH2C00H 0

482

NADYABELCI-mVAet al.

Table 3. Autopolycondensationof dicarboxylicacids in THF at 0°C in the presence of DCC, p-TSA (4 tool%) and lay to monomer mole ratio of 0.25 for 3 hr DCC to monomer No. Monomer (mole ratio of) /17n(oec) M~/M.~oPc) /~n(COOH) i.r. bands (cm-1) 1. (R, S)-malicacid 1:1 600 1.7 142 1760 2. (R, S)-malicacid 2:1 900 1.5 180 1880, 1800, 1750 3. Succinicacid 1:1 100 102 1875, 1790 4. Succinicacid 2:1 200 122 1875, 1790 5. Glycolicacid 1: 1 450 1.1 350 1750

acids and carbodiimides confirms cyclic anhydride formation when malic acid is used. In spite of the preliminary character of our results, the fact that no crosslinking occurs when malic acid interacts with DCC is rather interesting but difficult to explain. It is known [6] that the last stage of the esterification process in the presence of D C C is a nucleophilic attack of the hydroxy group on the carboxylic group activated as the acyl pyridinium complex. This is probably the rate-limiting step depending on the nucleophilicity of the hydroxy group. The impossibility to prepare crosslinked polymalic acid as well as the low yields of polymer fractions obtained from different aliphatic ~-(S)-malates by the method developed can be explained by low reactivity of the hydroxy group in malic acid. On the contrary, when another hydroxy component such as triethylene glycol was used in the condensation process with malic acid, only an insoluble product was obtained [10]. The results from our investigations on the polycondensation of alkyl ~-(S)-malates in the presence of DCC, p - T S A and Py under mild conditions show that, regardless of the kind of ~-substituent, all monoesters preferentially form low molecular N-acyl derivatives in high yields. The proposed method permits the preparation of optically active oligomalates with ~Q'n from 1000 to 4000, polydispersities of 1.1 to 1.5 and yields up to

30%, suitable for preparing controlled release systems in the form of biodegradable microspheres.

REFERENCES

1. J. Heller. Biomaterials 1, 51 (1980). 2. D. Gilding and A. Reed. Polymer 20, 149 (1979). 3. S. Arnold and R. Lenz. Makromolek. Chem., Macromolec. Syrup. 6, 285 (1986). 4. P. Hutchinson and B. Furr. In Drug Delivery Systems. (edited by Johnson and Lloyd-Jones), p. 106. (1987). 5. R. Wada, S. Hyon, Y. Ikada, Y. Nakao, H.Yoshikawa and S. Miranishi. J. Bioact. Compatible Polym. 3, 126 (1988). 6. N. Belcheva, Ch. Tsvetanov, I. M. Panayotov and S. Lazarova. Makromolek. Chem. 191, 213 (1990). 7. K. Holmberg and B. Hansen. Acta Chem. Scand., Set. B 33, 410 (1978). 8. F. Jones and D. Lu. Polym. Prepr. (Am. Chem. Soc., Die. Polym. Chem.) 26(1), 264 (1985). 9. D. Mironova and G. Dvorko. Ukr. Khim. Zh. 33, 602 (1967). 10. I. Panayotov, N. Belcheva and Ch. Tsvetanov. Makromolek. Chem. 188, 2821 (1987). 11. V. Korshak and S. Vinogradova. Linear Polyesters. Nauka, Moscow (1958). 12. M. Miller, J. Bajwa, Ph. Mattingly and K. Peterson. J. org. Chem. 47, 4928 (1092). 13. G. Liesen and Ch. Sukenik. J. org. Chem. 52, 455 (1987).