Synthesis, crystal structure and biological activity of novel diester cyclophanes

Abstracts

A series of novel diester cyclophanes was synthesized by esterification of 1,2-benzenedicarbonyl chloride with eight different diols under high dilution conditions. The structures of the compounds were verified by elemental analysis, ¹H nuclear magnetic resonance (NMR), IR spectroscopy and high resolution mass spectrometry (HRMS). The crystal structures of two compounds were characterized by single crystal X-ray diffractometry (XRD). All the new cyclophanes were evaluated for biological activities and the results showed that some of these compounds have low antibacterial or antifungal activities.

synthesis; ester cyclophanes; crystal; antibacterial activity; antifungal activity

Uma série de novos ciclofanos diésteres foi sintetizada por esterificação de cloreto de 1,2 benzenodicarbonilo com oito diferentes dióis sob condições de elevada diluição. As estruturas dos compostos foram verificadas por análise elementar, ressonância magnética nuclear (NMR) de ¹H, espectroscopia IR e espectrometria de massas de alta resolução (HRMS). As estruturas cristalinas de dois dos compostos foram caracterizadas por difratometria de raios X (XRD) de monocristais. Todos os novos ciclofanos foram avaliados com relação a suas atividades biológicas e os resultados mostraram que alguns destes compostos têm baixas atividades antibacterianas e antifúngicas.

Synthesis, crystal structure and biological activity of novel diester cyclophanes

Pengfei Zhang; Bingqin Yang^*; Xianwen Fang; Zhao Cheng; Meipan Yang

Department of Chemistry, Key Laboratory of Synthetic and Natural Functional Molecule Chemistry, Northwest University, Xi'an 710069, Shaanxi, P. R. China

ABSTRACT

A series of novel diester cyclophanes was synthesized by esterification of 1,2-benzenedicarbonyl chloride with eight different diols under high dilution conditions. The structures of the compounds were verified by elemental analysis, ¹H nuclear magnetic resonance (NMR), IR spectroscopy and high resolution mass spectrometry (HRMS). The crystal structures of two compounds were characterized by single crystal X-ray diffractometry (XRD). All the new cyclophanes were evaluated for biological activities and the results showed that some of these compounds have low antibacterial or antifungal activities.

Keywords: synthesis, ester cyclophanes, crystal, antibacterial activity, antifungal activity

RESUMO

Uma série de novos ciclofanos diésteres foi sintetizada por esterificação de cloreto de 1,2 benzenodicarbonilo com oito diferentes dióis sob condições de elevada diluição. As estruturas dos compostos foram verificadas por análise elementar, ressonância magnética nuclear (NMR) de ¹H, espectroscopia IR e espectrometria de massas de alta resolução (HRMS). As estruturas cristalinas de dois dos compostos foram caracterizadas por difratometria de raios X (XRD) de monocristais. Todos os novos ciclofanos foram avaliados com relação a suas atividades biológicas e os resultados mostraram que alguns destes compostos têm baixas atividades antibacterianas e antifúngicas.

Introduction

Cram and Steinberg¹ opened the field of cyclophane chemistry by their work in 1951. Since then, cyclophanes have attracted a great deal of attention from organic scientists, various kinds of compounds have been synthesized and their structures and properties have been studied.^2-6 Cyclophanes play an important role in host-guest chemistry because they can recognize specific molecules or ions by forming various inclusion complexes.^7-9 In some reaction systems, the host-guest complex can simulate the high efficiency of enzyme catalysis, which means the reaction yield can be greatly improved.^10,11 Besides, some ester cyclophanes were also used in the perfume industry because of their aromatic properties. Cyclophanes have so many important applications that it can be predicted a promising future in this field. However, the biological activities of cyclophanes were scarcely reported.^12,13 Therefore, there is a continuous need to synthetize novel cyclophanes through efficient and simple routes and to study their biological activities.

So far, scientists have synthesized a wide variety of cyclophanes with numerous modes of ring attachments. Medium-sized ring cyclophanes generally have a ring size of 8 to 11 atoms.¹⁴ Herein, it is reported the synthesis and biological activity of several diester cyclophanes. In continuation with our interest in cyclophane synthesis,¹⁵ an effective method for the synthesis of cyclophane compounds was designed, and a series of medium-sized cyclophanes 4a-4h was obtained by esterification of 1,2-benzenedicarbonyl chloride with some diols under high dilution conditions. Their antibacterial and antifungal activities are also described.

Results and Discussion

O-Phthalic acid undergoes a reaction with SOCl₂ and catalytic DMF (dimethylformamide) to form the diacid chlorides 1 (Scheme 1).¹⁶ Diols 3a-3h were also prepared and used for the synthesis of cyclophanes. 4-Methylaniline was treated with oxirane and catalytic propanoic acid by stirring at room temperature for 24 h. Diol 3a was obtained as white solid.¹⁷ Diols 3b, 3c, 3d, 3e and 3f were also prepared by similar procedures. Compounds 3g and 3h were obtained using CaCO₃, KI and 2-chloroethanol according to Lin et al.¹⁸

Different catalysts showed different catalytic effect in the cyclization procedure. The inorganic base K₂CO₃ showed no catalytic activity. The reaction could be improved by adding pyridine, but the product yield was still low. It was found that a high cyclization yield was achieved in the presence of pyridine and 4-dimethylaminopyridine (DMAP). The reaction 1 equiv. of diol 3a with 1.5 equiv. of diacid chloride 1 in CH₂Cl₂ and in the presence of pyridine and DMAP afforded the cyclophane 4a in 56% yield.^19-24 Using the new dual catalyst system not only increased the yield of the product, but also improved the reaction rate. The ¹H NMR spectrum of cyclophane 4a displayed the aromatic methyl protons as a singlet at δ 2.27, OCH₂CH₂ protons as two triplet at δ 3.80 and 4.55 with J 4.0 Hz, in addition to aromatic protons. Cyclophanes 4b-4h were also accessed by a familiar method from the reaction of diols 3b-3h with diacid chloride 1. In order to avoid polymerization, the cyclization procedure was run under high dilution conditions and the reaction solution had to be vigorously stirred.

The carbonyl absorption of cyclophanes in the IR was in 1700-1720 cm^-1 region, at lower frequency than that of normal esters due to the formation of a large p bonding. This peak was the strongest peak compared with all other peaks in the IR spectrum.

Crystal and molecular structures

Crystals of compounds 4g and 4h were obtained from a mixture of petroleum ether and ethyl acetate (5:1). Crystal data collection and refinement parameters for the two compounds are shown in Table 1. The structures of compounds are presented in Figures 1a and 2a. The crystal structure X-ray analysis of 4g showed that there are six intermolecular CH...OH-bonding interactions (Figure 1c): [H(2)...O(1) = 2.697 Å, C(2)-H(2)...O(1) = 126.18º];

[H(18)^...O(4) = 2.607 Å, C(18)-H(18)^...O(4) = 153.84º]; [H(10B)^...O(4) = 2.577 Å, C(10)-H(10B)^...O(4) = 135.39º]; [H(11A)^...O(6) = 2.614 Å, C(11)-H(11A)^...O(6) = 143.53º]; [H(10A)^...O(5) = 2.630 Å, C(10)-H(10A)^...O(5) = 163.21º] and [H(9B)^...O(5) = 2.552Å, C(9)-H(9B)^...O(5) = 159.46º]. The crystal structure X-ray analysis of 4h revealed cyclophanes connected by four intermolecular CH^...O hydrogen bonding (Figure 2c): [H(3)^...O(1) = 2.410 Å, C(3)-H(3)^...O(1) = 153.69º]; [H(7B)^...O(2) = 2.699 Å, C(7)-H(7B)^...O(2) = 158.01º]; [H(17B)^...O(4) = 2.587 Å, C(17)-H(17B)^...O(4) = 133.99º] and [H(18A)^...O(2) = 2.642 Å, C(18)-H(18A)^...O(2) = 164.49º]. The compounds were further stabilized by H-bond stacking interaction to construct supramolecular architectures (Figures 1b and 2b).

Biological activity

The synthesized novel compounds 4a-4h were screened for their in vitro antibacterial activity against Staphylococcus aureus (ATCC 25923), Bacillus subtilis (ATCC 6633), Escherichia coli (ATCC 25922), Pseudomonas aeruginosa (ATCC 27853) and Salmonella typhi (ATCC 14028). The new compounds 4a-4h were also evaluated for their antifungal activity against Candida albicans (ATCC 10231). The results of preliminary antibacterial activity of the compounds are shown in Table 2. A control test was performed with sterile DMSO. This revealed no inhibiting activity. The compounds 4a, 4b, 4c and 4h were found to exhibit higher antifungal activity against C. albicans among the new compounds.

The minimum inhibitory concentration (MIC) for the active compounds is presented in Table 3. Chloramphenicol and fluconazole were chosen as standard drugs whose MIC values are also provided in Table 3. The antibacterial activity (MIC of 128 µg mL^-1) of compound 4g against E. coli showed relatively better results than the other new compounds. This is attributed to the nitro unit substituent at the 4-position of the 128 µg mL^-1) when compared with compounds 4c and 4h. aryl ring. But the MIC value is relatively low for cyclophane However, cyclophanes 4a and 4b were less effective than 4g as compared with chloramphenicol. Compounds 4a and fluconazole. It is clear that all the synthesized cyclophanes 4b had relatively higher activity against C. albicans (MIC of have low antibacterial or antifungal activity.

Experimental

Materials and equipment

All reagents and solvents were purchased from Sinopharm Chemical Reagent Co., Ltd. The solvents were purified under conventional conditions or distilled immediately before use. The melting points were determined using an XT-4B micro-melting-point apparatus and are informed uncorrected. The elemental analyses for the compounds were carried out on a Vario EL III CHNOS instrument. The IR spectra (KBr) were recorded on an Equinox-55 spectrophotometer. ¹H NMR spectra were recorded on an INOVA-400 NMR spectrometer, using TMS as internal standard and CDCl₃ as the solvent. High resolution mass spectrometry (HRMS) were obtained with a MALDI-TOF mass spectrometer (Kratos Analytical Inc.). Column chromatography was performed on silica gel (200-300 mesh).

Antibacterial and antifungal assays

The bacteria and fungi were obtained from Shaanxi Province Institute of Microbiology (Xi'an, China) and maintained on Luria-Bertani (LB) medium consisting of the following: yeast extract 5.0 g, peptone 10.0 g, NaCl 10.0 g, distilled H₂O 1 L, pH 7.5 in slants or Petri plates at room temperature.

Preliminary antimicrobial activities of compounds 4a-4h were tested as follows: the new compound was dissolved in DMSO at the concentration of 200 µg mL^-1and placed on the LB plates that had been previously inoculated separately with the microorganisms. The plates were incubated at 35 ºC, and the growth-inhibition zones were measured after 12 h in the case of bacteria/fungi.

The MIC values of the compound assays were carried out using the broth microdilution method. The tested compounds were dissolved in DMSO to afford 128 µg mL^-1. Two fold dilutions of the solution were prepared (64, 32, 16, 8, 4, 2, 1, 0.5, 0.25 and 0.125 µg mL^-1). The bacterial cultures (ca. 10⁸ CFU mL^-1) in Müller-Hinton medium were added to the corresponding tubes. The plates were incubated at 35 ºC for 12 h. Chloramphenicol and fluconazole were used as reference agents. Suitable solvent control (DMSO), positive growth control and standard drug control were also run simultaneously. The MIC values of the compounds were recorded as the lowest concentration of each chemical compound in the tubes with no turbidity of inoculated bacteria/fungus.

Crystal structure X-ray data of 4g and 4h

All measurements of compounds 4g and 4h were made on a Bruker Smart APEX II CCD diffractometer with graphite monochromated Mo-K_α radiation. The data was collected at a temperature of 296 K using the ω-2952 scan technique. Crystallographic data for the structure analysis of compounds 4g and 4h was deposited at the Cambridge Crystallographic Data Centre, CCDC No. 846714 and 846711, respectively. ²⁵

Conclusions

Eight novel compounds (4a-4h) were synthesized. XRD studies were carried out for compounds 4g and 4h, which were further stabilized by H-bond stacking interaction. These new compounds 4a-4h represent a novel class of compounds that incorporate an 11-membered ring lactone type structure. Some of the synthesized cyclophanes have low antibacterial or antifungal activities, providing stimulus for further studies.

Supplementary Information

Supplementary data is available free of charge at http://jbcs.sbq.org.br as PDF file.

Acknowledgment

This work was financially supported by the National Natural Science Foundation of China (Grant No. 21172178 P. R. China).

Submitted: July 11, 2012

Published online: September 25, 2012

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Publication Dates

• Publication in this collection
  09 Oct 2012
• Date of issue
  Oct 2012

History

• Received
  11 July 2012
• Accepted
  25 Sept 2012