Available online on 15.03.2024 at http://jddtonline.info

Journal of Drug Delivery and Therapeutics

Open Access to Pharmaceutical and Medical Research

Copyright  © 2024 The   Author(s): This is an open-access article distributed under the terms of the CC BY-NC 4.0 which permits unrestricted use, distribution, and reproduction in any medium for non-commercial use provided the original author and source are credited

Open Access  Full Text Article                                                                                                                                                      Research Article

Design, Synthesis, Characterization and Biological Evaluation of Novel Depsides as Potential Antibacterials

Manoj Kumari More*

Shri G. S. Institute of Technology and Science, 23 Sir M. Visvesvaraya Marg, Indore, MP, 452003

INTRODUCTION 

Depsides, in the strict sense, are compounds comprised of two or more aromatic rings bound by a phenolic oxygen ester linkage. They are most often found in lichens, but have also been isolated from higher plants, including species of the Ericaceae, Lamiaceae and Papaveraceae ^1,2. Some depsides isolated from lichens have been reported to have activity against mycobacteria, gram-positive bacteria, insects and nematodes. In addition, several well characterized depsides exhibit antiproliferative, analgesic, antipyretic, anticancer, anti-HIV-1 integrase and antiviral properties^3-11. As inhibitors of prostaglandin biosynthesis and leukotriene B4 biosynthesis, depsides are potent nonsteroidal anti-inflammatories¹². Furthermore, it is reported that members of the macrocyclic salicylate family (Fig.1)¹³ are potent inhibitors of the mammalian vacuolar ATPase with a potential novel mode-of action¹⁴, whereas resorcinylic macrolides (b, Fig. 1) such as radicicol¹⁵ inhibit the molecular chaperone Hsp90¹⁶. Recently, a new depside, named jaboticabin (Fig.1), was demonstrated to inhibit chemokine interleukin (IL)-8 production before and after cigarette smoke treatment of cells ¹⁷. Depsides, in the strict sense, are compounds comprised of two or more aromatic rings bound by a phenolic oxygen-ester linkage. They are most often found in lichens, but have also been isolated from higher plants. Depsides isolated from lichens have been reported to have activity against mycobacteria, gram-positive bacteria, insects, and nematodes. In addition, several well-characterized depsides exhibit antiproliferative, analgesic, antipyretic, anticancer, anti-HIV-1 integrase and antiviral properties. Depsides are potent nonsteroidal anti-inflammatory agent. Lichens are a symbiotic association of mostly ascomycetous fungi (mycobiont) and cyanobacterial (photobiont) partners. They occur in a wide variety of habitats and natural environmental conditions such as low temperatures, prolonged darkness, drought, continuous light, etc. It has been suggested that in response to these stresses, natural selection has favoured species producing high concentrations of characteristics phenolic compounds (mainly depsides, depsidones).However, few attentions have focused on the antibacterial activity of depsides up to now, especially gram-negative bacteria. In view of that, we have designed series of novel depsides derivative which are structurally similar to jaboticabin (Fig. 1), and evaluated their antibacterial activities against two gram-positive bacterial strains (Bacillus subtilis ATCC 6633),(Staphylococcus aureus ATCC29737) and gram-negative bacterial strains (Escherichia coli ATCC 8739),(Pseudomonas aeruginosa ATCC25619) by cup-plate method and serial dilution method and two fungal strains (Aspergillus niger ATCC 90292) and (Candida albicans ATCC 24433) by the micro-broth dilution method.

MATERIAL AND METHODS

Chemistry

Chemicals (starting material) used were purchased from Aldrich (U.S.A). Separation of the compounds by and column chromatography was carried out with silica gel 60 (200-300mesh ASTM, E. Merck). The quantity of silica gel used was50-100 times the weight charged on the column. Then, the eluates were monitored using TLC. Melting points (uncorrected) were determined on an open capillary method. ESI mass spectra were obtained on an EI-MS technique on Shimadzu QP 2010 Plus GC-MS mass spectrometer and ¹H NMR spectra were recorded on a Bruker advance II 400 NMR spectrometer at25⁰C with TMS and solvent signals allotted as internal standards. Chemical shifts were reported in parts per million (d).

Experiments

Antibacterial and antifungal activities

The antibacterial activities of the synthesized compounds were tested against (Bacillus subtilis ATCC 6633), (Staphylococcus aureus ATCC 29747), (Escherichia coli ATCC 8739) and (Pseudomonas aeruginosa ATCC 25619) the MIC and zone of inhibition calculated by cup-plate method and serial dilution method. The minimum inhibitory concentration of the test compounds were determined by two folds serial dilution technique. The stock solutions of 400µg/ml of each test compounds were prepared. Then 7 sterilized test tubes were taken and numbered. 2ml of stock solution was taken into first test tube and add 1ml of sterile broth to all other test tubes. From the first tube, 1ml of test compound solution was taken and added to the second tube and mixed properly. Again pipette out 1ml from second tube and added to third tube and mixed properly. Same procedure was repeated for all remaining tubes and 1ml was expelled out from last tube to obtain the dilution from 100µg/ml to 3.12µg/ml. Then all tubes were inoculated with a 1ml of corresponding microorganism inoculums suspension and incubated at 37±1°C for 24 hrs. Control tubes (without drug or test compounds) were also incubated similarly. Similar procedure was repeated for all test compounds and standard drugs. The antifungal activities of the synthesized compounds were tested against (Aspergillus niger ATCC 90292) and (Candida albicans ATCC 24433) by the micro-broth dilution method. 12.80mg of sample was dissolved in 2% DMSO and made up to 100 ml in a volumetric flask to produce a concentration of 128μg/ml. All the sample solution had been prepared in the same manner in Aseptic condition. Amphotericin B was used as a standard drug. The lowest concentration of the test compounds showing no visible microbial growth were considered as minimum inhibitory concentration. The observed MIC values are presented in Table 2.

General procedure for esterification of 2-hydroxy phenyl acetic acid

A solution of 2-hydroxyphenylacetic acid (7.6 g, 50mmol) in methanol or ethanol (50 ml) containing conc. H₂SO₄ (5ml) was refluxed overnight. Water (100 ml) was added, the organic phase was washed with saturated NaCl (100 ml) and was dried over Na₂S0₄, and the solvent was evaporated.

Methyl 2-(2-hydroxyphenyl) acetate (B1): White powder, yield 90%, mp: 61-62⁰ C, ¹H NMR (300 MHz, d6-DMSO): 3.54 (s, 2H); 3.58 (s, 3H); 6.78 (m, 2H); 7.06 (m, 2H); 9.47 (s,n1H). MS (ESI): 167.1 (C₉H₁₁O₃, [M+H]⁺).Anal. Calcd forC₉H₁₀O₃: C, 65.05; H, 6.07%; Found: C, 65.03; H, 6.10%.

Propyl 2-(2-hydroxyphenyl) acetate (B2): yellow oil, yield 86%, mp: 67-68⁰C; 1H NMR (300 MHz, d6-DMSO): 1.17 (t, J ¼ 7.1 Hz, 3H); 3.53 (s, 2H); 4.01 (m,2H); 6.75 (m, 2H); 7.07 (m, 2H); 9.46 (s, 1H). MS (ESI): 181.1 (C₁₁H₁₄O₃,[M + H]⁺).Anal. Calcd for C₁₁H₁₄O₃: C, 65.65; H, 6.71%; Found: C, 65.69; H, 6.74%.

Butyl 2-(2-hydroxyphenyl) acetate (B3): yellow oil, yield 85%, mp: 65-70⁰C; 1H NMR (300 MHz, d6-DMSO): 1.17 (t, J ¼ 7.1 Hz, 3H); 3.53 (s, 2H); 4.01 (m,2H); 6.75 (m, 2H); 7.07 (m, 2H); 9.46 (s, 1H). MS (ESI): 181.1 (C₁₂H₁₆O₃,[M +H]⁺).Anal. Calcd for C₁₂H₁₆O₃: C, 65.65; H, 6.71%; Found: C, 65.69; H, 6.84%.

General procedure for the synthesis of depsides: 

To a stirred solution of B (1.04 g, 3.1mmol) in dichloromethane (50 ml) was added 2 (864 mg, 1.96mmol), N, N-dimethyl amino pyridine (248mg, 2.03mmol) and N, N-0-dicyclohexylcarbodiimide (740 mg, 3.6mmol). The mixture was refluxed overnight. Then ethanol (1 ml) and acetic acid (1 ml) was added; the mixture was refluxed for 2 h and concentrated in vacuum.

2-(2-methoxy-2-oxoethyl) phenyl 4-aminobenzoate (P1): Creamy powder, Yield 71.52%, IR spectra (in cm^-1) 1573.81 (C=C), 3035.75 (C-H), 1072.34 (C-O), 1724.24 (C=O), 3325.05 (N-H), 1345.9 (C-N),NMR: ¹H NMR (400 MHz, DMSO): δ (ppm): 3.41 (m, 3H); 5.54 (s, 2H); 6.56(t, 4H); 7.67 (m, 1H); 7.89 (s, 3H); 8.01 (s, 2H).MS (ESI): 299.116 (C₁₆H₁₅NO₄₎, [M+H]⁺).

2-(2-methoxy-2-oxoethyl)phenyl4-methoxybenzoate(P2):  White powder, Yield 64.2%, IR spectra (in cm^-1) 1539.09 (C=C), 3031.89 (C-H), 1087.78 (C-O), 1737.74 (C=O)NMR: ¹H NMR (400 MHz, DMSO): δ (ppm): 3.73 (s, 2H); 3.52 (s, 3H); 7.31 (m, 1H); 7.43 (m, 3H); 7.94 (m, 1H); 8.58 (m, 1H); 8.76 (m, 1H). MS (ESI): 314.115 (C₁₇H₁₆O₅₎, [M+ H] ⁺).

2-(2-methoxy-2-oxoethyl)phenyl4-methylbenzoate(P3): Red powder, Yield71.0%, IR spectra (in cm^-1) 1520.34 (C=C), 3014.53 (C-H), 1072.36 (C-O), 1712.67 (C=O), 1444.53 (CH₃)NMR: ¹HNMR (400MHz, DMSO): δ (ppm): 2.46 (s, 3H); 3.47 (s, 3H); 3.68 (s, 2H); 7.27 (m, 2H); 7.39 (m, 4H); 7.98 (d, 2H). MS (ESI): 298.121 (C₁₇H₁₆O₄₎, [M+ H] ⁺).

2-(2-methoxy-2-oxoethyl) phenyl 3-methoxybenzoate(P4): White powder, Yield 65.6%, IR spectra (in cm^-1) 1573.81 (C=C), 3035.75 (C-H), 1054.99 (C-O), 1774.39 (C=O) NMR: ¹HNMR (400 MHz, DMSO):  δ (ppm): 3.61(s, 3H); 6.81-7.29 (m, 9H); 8.53 (s, 1H); 7.71(s, 1H). MS (ESI): 314.115 (C₁₇H₁₆O₅₎, [M+ H] ⁺).

2-(2-propoxy-2-oxoethyl) phenyl 4-aminobenzoate (P5):   Creamy powder, Yield 71%, IR spectra (in cm^-1) 1577.66 (C=C), 3039.60 (C-H), 1047.27 (C-O), 1737.75 (C=O) NMR: ¹H NMR (400 MHz, DMSO): δ (ppm): 3.41 (m, 1H);    5.54 (s, 1H); 6.56 (t, 1H,); 7.67 (m, 1H, CH); 7.89 (s, 1H); 8.01 (s, 1H).MS (ESI): 327.147 (C₁₈H₁₉NO₄), [M+ H] ⁺).

2-(2- propoxy-2-oxoethyl) phenyl4-methoxybenzoate(P6): Red powder, Yield 68.2%, IR spectra (in cm^-1) 1575.20 (C=C), 3030.89 (C-H), 1058.85 (C-O), 1778.25 (C=O), 3386.77 (N-H), 1323.08 (C-N) NMR: ¹HNMR (400 MHz, DMSO): δ (ppm): 3.54 (s, 2H); 3.55 (s, 3H); 3.76 (s, 3H); 3.90 (s, 2H); 6.87 (m, 1H); 6.94 (m, 2H); 7.09 (m, 1H); 7.28 (m, 4H). MS (ESI): 305.0 (C₁₉H₂₀O₅₎, [M+ H] ⁺).

2-(2- propoxy-2-oxoethyl) phenyl 4-methylbenzoate (P7): Yellowish powder. Yield64.5%, IR spectra (in cm^-1) 1490.75 (C=C), 3124.47 (C-H), 1089.71 (C-O), 1772.46 (C=O), 1440.73 (CH₃) NMR: ¹HNMR (400MHz, DMSO): δ (ppm2.46 (s, 3H); 3.47 (s, 3H); 3.68 (s, 2H); 7.27 (m, 2H); 7.39 (m, 4H); ): 8.98 (d, 2H). MS (ESI): 297.1 (C₁₉H₂₀O₄₎, [M+ H] ⁺).

2-(2-propoxy-2-oxoethyl) phenyl 3-methoxybenzoate (P8): Reddish brown powder. Yield62.3%, IR spectra (in cm^-1) 1589.95 (C=C), 3026.93 (C-H), 1085.85 (C-O), 1728.39 (C=O)NMR:¹HNMR (400MHz, DMSO): δ(ppm): 3.49 (s, 2H); 3.51 (s, 3H); 3.72 (s, 3H); 3.73 (s, 3H); 3.82 (s, 2H); 6.84 (m, 1H); 6.90(m, 1H); 6.94 (m, 1H); 7.08 (m, 1H); 7.18 (m, 1H); 7.30 (m, 2H) MS (ESI): 322.147 (C₁₉H₂₀O₅), [M+ H] ⁺).

2-(2-butoxy-2-oxoethylphenyl) 4-aminobenzoate (P9): creamy powder. Yield62.7%, IR spectra (in cm^-1) 1545.32 (C=C), 3085.51 (C-H), (C-O), 1730.89 (C=O), 3481.41 (NH), 1090.17 (C-N) NMR: ¹HNMR (400MHz, DMSO): δ (ppm): 3.41 (m, 4H); 5.54 (s, 2H, NH2); 6.56 (t, 3H); 7.67 (m, 4H); 7.89 (s, 3H); 8.0(s, 2H). MS (ESI): 3241.163 (C₁₉H₂₁NO₄), [M+ H] ⁺).

2-(2-butoxy-2-oxoethyl) phenyl 4-methoxybenzoate (P10): White powder, Yield72.6%IR spectra (in cm^-1) 1575.75 (C=C), 3035.75 (C-H), 1049.20 (C-O), 1781.81 (C=O) NMR: ¹H NMR (400 MHz, DMSO): δ (ppm): 2.46 (s, 3H); 3.47 (s, 3H); 3.68,(s, 2H); 7.27 (m, 2H);  7.39 (m, 4H); 7.98 (d, 2H). MS (ESI): 336.1 (C₂₀H₂₂O₅), [M+ H] ⁺).

2-(2-butoxy-2-oxoethyl)phenyl3-methoxyacetate(P11): White powder, Yield 53.3%,IR spectra (in cm^-1) 1539.09 (C=C), 3033.82 (C-H), 1047.27 (C-O), 1772.46 (C=O) NMR: ¹H NMR (400 MHz, DMSO): δ (ppm): 3.49326.1 (s, 3H); 3.69 (s, 2H); 3.84 (s, 6H); 6.88 (t, 1H); 7.20 (d, 2H); 7.28 (m, 2H); 7.39 (m, 2H). MS (ESI): 319.0(C₂₀H₂₂O₅), [M+ H] ⁺).

2-(2-butoxy-2-oxoethyl) phenyl 3-methylacetate (P12): Reddish powder, Yield74.8%, IR spectra (in cm^-1) 1575.73 (C=C), 3122.54 (C-H), 1014.49 (C-O), 1735.81 (C=O), 1438.80 (CH₃) NMR: ¹H NMR (400MHz, DMSO): δ (ppm): 2.46(s, 3H); 3.47   (s, 3H); 3.68 (s, 2H); 7.27 (m, 2H); 7.39 (m, 4H); 8.98 (d, 2H). MS (ESI): 373.1 (C₂₀H₂₂O₄), [M+ H] ⁺).

2-(2-methoxy-2-oxoethyl)phenyl 4-iodobenzoate(P13): Creamy powder, Yield73%, IR spectra (in cm^-1) 1438.80 (C=C), 2931.60 (C-H), 1026.06 (C-O), 1749.02 (C=O), 489.89 (C-I).NMR: ¹H NMR (400 MHz, DMSO): δ (ppm): 2.46 (s, 3H); 3.47 (s, 3H); 3.68 (s, 2H); 7.27 (m, 2H); 7.39 (m, 4H); 8.80 (d, 2H). MS (ESI): 310.0 (C₁₆H₁₃IO₄), [M+ H] ⁺).

2-(2-butoxy-2-oxoethyl) phenyl 3,5-dinitrobenzoate(P14): White powder, Yield 55.5%, IR spectra (in cm^-1) 1527.52 (C=C), 3041.53 (C-H), 1062.34 (C-O), 1645.17 (C=O),   3481.41 (NH), 1081.99 (C-N), 1346.22 (R-NO₂) NMR: ¹H NMR (400 MHz, DMSO): δ (ppm): 3.52 (s, 3H); 3.73 (s, 2H); 7.94 (t, 2H); 7.31 (m, 3H); 7.43 (m, 3H); 8.51 (m, 1H);   8.58 (m, 2H); 8.76 (m, 2H). MS (ESI): 330.105 (C₁₉H₁₈N₂O₈), [M+ H] ⁺).

2-(2-propoxy-2-oxoethyl) phenyl 3, 5-dinitrobenzoate (P15): White powder, Yield 61.6%, IR spectra (in cm^-1) 1533.30 (C=C), 3105.18 (C-H), 1018.34 (C-O), 1627.81 (C=O), 3326.98 (NH), 1090.17 (C-N), 1450.37 (R-NO₂) NMR: ¹H NMR (400 MHz, DMSO): δ (ppm): 3.52 (s, 3H); 3.73 (s, 2H); 7.94 (t, 3H); 8.51 (m, 4H); 8.58 (m, 2H); 8.76 (m, 1H); 7.31 (m, 1H); 7.43 (m, 3H). MS (ESI): 325.00 (C₁₈H₁₆N₂O₈), [M+ H] ⁺).

2-(2-methoxy-2-oxoethyl) phenyl 3,5-dinitrobenzoate(P16): White powder, Yield 51.7%, IR spectra (in cm^-1) 1551.43 (C=C), 3073.62 (C-H), 1062.34 (C-O), 1642.51 (C=O), 3472.3 (NH), 1213.12 (C-N), 1473.91 (R-NO₂) NMR: ¹H NMR (400 MHz, DMSO): δ (ppm): 0.99 (t, 3H); 3.71 (s,2H); 3.94 (q, 3H); 7.31 (m, 1H); 7.42 (m, 3H); 7.93 (t, 1H); 8.52 (d, 1H); 8.59 (d, 1H); 8.76 (m, 1H) MS (ESI): 330.10(C₁₆H₁₂N₂O₈), [M+ H] ⁺).

RESULTS AND DISCUSSION

Sixteen depsides were synthesized to screen for the antibacterial activity. All of them were reported for the first time. The synthesis of compounds P₁-P₁₆ followed the general pathway outlined in Scheme 1. They are prepared in two steps. Firstly, a solution of 2-hydroxyphenylacetic acid (compound A) in methanol or ethanol containing concentrated H₂SO₄ was refluxed overnight ¹⁸. This step can yield the corresponding ester. Secondly, the coupling reaction between the obtained esters and the different substituted phenyl acetic acid or benzoic acid was performed through ‘step ii’ by using N,N-dicyclohexyl- carbodiimide (DCC) and 4-dimethyl amino pyridine (DMAP) in anhydrous CH₂C_l2¹⁹. Then, compounds P₁-P₁₆ were obtained by subsequent purification. All of the synthetic compounds gave satisfactory analytical and spectroscopic data, which were in full accordance with their depicted structures.

Antibacterial and Antifungal Activity

All the compounds prepared were evaluated for their antibacterial and antifungal activities against two gram-positive bacterial strains (B. subtilis ATCC 6633 and S. aureus ATCC 29737) and two gram-negative bacterial strains (E. coli ATCC 8739 and P. aeruginosa ATCC 25619) activities by serial dilution method and two fungal strains (Aspergillus niger ATCC 90292) and (Candida albicans ATCC 24433) by the micro-broth dilution method. The results are shown in Table1 properties, low toxicity, inertness and chemical stability²². Moreover, depsides have been reported to have antibacterial activity^3,4. Enlightened by the above two factors,2-Hydroxy phenyl acetic acid which has phenolic hydroxyl group in the ortho position was used to synthesize novel depsides with a view to find new antibacterial agents. Then, studies were performed by modification of the parent compounds to determine how the substituents of the subunits affected the antibacterial activities.

Scheme 1 (i) CH₃OH, C₃H₇OH and C₄H₉OH, H₂SO₄, reflux overnight, (ii) DCC, DMAP, CH₂Cl₂, Reflux overnight.

QSAR Analysis

To obtain a significant correlation between structural features and biological activities, QSAR studies were performed using the linear free energy relationship (LFER) model of Hansch and Fujita. Biological activity data were reported as _log MIC on molar basis and used as dependent variable to get the linear relationship in the QSAR model. The physicochemical parameters taken from the list of Skagerberg et al ²⁵ were then correlated with varied molecular descriptors like partition coefficient (log p), lowest unoccupied molecular orbital (LUMO), molar refractivity (MR) and melting point (MP). All the calculations to figure out molecular descriptors were done at SCF level using AM1²⁶ Hamiltonian incorporated in MOPAC 6.0 package²⁷. Geometries were optimized at minimum gradient level using CHEM 3D-6.0 software ²⁸. In order to perform correlation studies for various descriptors, inter correlated parameters were discarded depending upon their individual correlation with biological activities. Using the stepwise selection and elimination procedures the resultant parameters were subjected to MLR analysis with the help of Valstat software²⁹. The best fit between _log MIC and these explaining parameters were obtained through multiple regression analysis (MRA) using least square. Out of a number of parametric evaluations, the best correlation (r2 > 0.93) was found to exist between molar refractivity and biological activity. The resulting mono-parametric models are displayed in Eqs. (1)-(2) with statistical parameters of regression. No outliers have been detected and these Eqs. (1)-(2) were derived using entire training data set. The overall quality of the models is indicated by coefficient of determination (r2), standard error of estimate (s), and Fisher statistics (F).

QSAR model for activity against B. subtilis:

Eqn.1 _log MIC = [3.8516 (± 4.4673)] + mrl [3.3645 (± 1.22073)] + se [26.4534 (± 14.9436)] 

 n=19, r=0.809503, r^2=0.655296, Variance=0.0960711, std=0.309953, F=17.1093, FIT=136.875

QSAR model for activity against P. aeruginosa

Eqn.2 _log MIC= [4.68168 (±0.158409)] +se [-0.0258718 (±0.0152161)] + Lumo [0.0437538 (± 0.0272712)]

 n=21, r=0.859191, r^2=0.738209, variance=0.0379778, std =0.194879, F=15.5092, FIT=172.324

CONCLUSIONS

In this paper, we designed a series of novel depsides derivatives and evaluated their antibacterial and antifungal activities against two gram-positive bacterial strains (B. subtilis ATCC 6633 and S. aureus ATCC 29737) and two gram-negative bacterial strains (E. coli ATCC 8739 and P. aeruginosa ATCC 25619) by MIC and zone of inhibition method. Compounds (P₇) and (P₈) showed powerful antibacterial activities against B. subtilis ATCC 6633 with MIC of 2.0 40mg/ml while compounds p7 and p8 exhibited significant antibacterial activities against E. coli ATCC 8739 with MIC of 3.12 40mg/ml, which were superior to the positive controls amoxycillin trihydrate and ciprofloxacin HCl, respectively. Fungal strains (Aspergillus niger ATCC 90292) and (Candida albicans ATCC 24433) by the micro-broth dilution method. Compound 2-(2- propoxy -2-oxoethyl) phenyl 4-methylbenzoate (P₈) was found to have highest antifungal activity against Candida albicans ATCC 24433 In addition, a comparison of the substitution on E-ring demonstrated that 3-position-substituted derivatives have more potent activity against B. subtilis ATCC 6633 than the 4-position-substituted ones. Most significantly, the stronger electron withdrawing substituent the compound.

ACKNOWLEDGEMENTS

We are grateful to Central Drug Research Institute, Lucknow, India for providing spectro analytical facilities and one of the authors (V.S.) is thankful to Council of Scientific and Industrial Research, New Delhi for providing financial assistance. The work was co-financed by a grant from Shri S.G.I.T.S. Indore (M.P.).

Table1: Structure of proposed compounds

Table 2: MICs (minimum inhibitory concentrations) (µg/ml) Zone of inhibition of the synthetic compounds

REFERENCES

1. Ono M, Masuoka C, Koto M, Tateishi M, Komatsu H, Kobayashi H, Igoshi K, Ito Y, Okawa M, Nohara T. Antioxidant ortho-benzoyloxyphenyl acetic acid ester, vaccihein A, from the fruit of rabbiteye blueberry (Vaccinium ashei). Chemical and Pharmaceutical Bulletin 2002; 50(10):1416-7. https://doi.org/10.1248/cpb.50.1416 PMid:12372879

2. Hillenbrand M, Zapp J, Becker H. Depsides from the petals of Papaver rhoeas. Planta Medica 2004;70(04):380-2. https://doi.org/10.1055/s-2004-818956 PMid:15095160

3. Vartia KO. Antibiotics in lichens. In The lichens 1973 Jan 1 (pp. 547-561). Academic Press. https://doi.org/10.1016/B978-0-12-044950-7.50022-2

4. Armstrong RA, Bradwell T. Growth of foliose lichens: a review. Symbiosis 2011;53:1-6. https://doi.org/10.1007/s13199-011-0108-4

5. Ingolfsdottir K, Bloomfield SF, Hylands PJ. In vitro evaluation of the antimicrobial activity of lichen metabolites as potential preservatives. Antimicrobial Agents and Chemotherapy 1985;28(2):289-92. https://doi.org/10.1128/AAC.28.2.289 PMid:3834834 PMCid:PMC180233

6. Sankawa U, Shibuya M, Ebizuka Y, Noguchi H, Iitaka Y, Endo A, Kitahara N. Depside as potent inhibitor of prostaglandin biosynthesis: a new active site model for fatty acid cyclooxygenase. Prostaglandins 1982;24(1):21-34. https://doi.org/10.1016/0090-6980(82)90174-5 PMid:6812170

7. Proksa B, Adamcova J, Sturdikova M, Fuska J. Metabolites of Pseudevernia furfuracea (L.) Zopf. and their inhibition potential of proteolytic enzymes. Die Pharmazie 1994;49(4):282-3.

8. Yanmamoto Y, Miura Y, Kinoshita Y, Higuchi M, Yamada Y, Murakami A, Ohigashi H, Koshimuzi K. Screening of tissue cultures and thalli and some of their active constituents for inhibition of tumor promoter-induced Epstein-Barr virus activation. Chem Pharm Bull 1995;43:1388-90. https://doi.org/10.1248/cpb.43.1388 PMid:7553984

9. Kumar KC S, Müller K. Lichen metabolites. 2. Antiproliferative and cytotoxic activity of gyrophoric, usnic, and diffractaic acid on human keratinocyte growth. Journal of Natural Products 1999;62(6):821-3. https://doi.org/10.1021/np980378z PMid:10395495

10. Nielsen J, Nielsen PH, Frisvad JC. Fungal depside, guisinol, from a marine derived strain of Emericella unguis. Phytochemistry 1999;50(2):263-5. https://doi.org/10.1016/S0031-9422(98)00517-2

11. Neamati N, Hong H, Mazumder A, Wang S, Sunder S, Nicklaus MC, Milne GW, Proksa B, Pommier Y. Depsides and depsidones as inhibitors of HIV-1 integrase: discovery of novel inhibitors through 3D database searching. Journal of Medicinal Chemistry 1997;40(6):942-51. https://doi.org/10.1021/jm960759e PMid:9083483

12. KC SK, Müller K. Depsides as non-redox inhibitors of leukotriene B4 biosynthesis and HaCaT cell growth, 2. Novel analogues of obtusatic acid. European Journal of Medicinal Chemistry 2000;35(4):405-11. https://doi.org/10.1016/S0223-5234(00)00132-X PMid:10858601

13. Beutler JA, McKee TC. Novel marine and microbial natural product inhibitors of vacuolar ATPase. Current Medicinal Chemistry. 2003;10(9):787-96. https://doi.org/10.2174/0929867033457827 PMid:12678782

14. Boyd MR, Farina C, Belfiore P, Gagliardi S, Kim JW, Hayakawa Y, Beutler JA, McKee TC, Bowman BJ, Bowman EJ. Discovery of a novel antitumor benzolactone enamide class that selectively inhibits mammalian vacuolar-type (H+)-atpases. Journal of Pharmacology and Experimental Therapeutics 2001;297(1):114-20.

15. Mirrington RN, Ritchie E, Shoppee CW, Taylor WC, Sternhell S. The constitution of radicicol. Tetrahedron Letters 1964;5(7):365-70. https://doi.org/10.1016/0040-4039(64)80029-0

16. Roe SM, Prodromou C, O'Brien R, Ladbury JE, Piper PW, Pearl LH. Structural basis for inhibition of the Hsp90 molecular chaperone by the antitumor antibiotics radicicol and geldanamycin. Journal of Medicinal Chemistry 1999;42(2):260-6. https://doi.org/10.1021/jm980403y PMid:9925731

17. Reynertson KA, Wallace AM, Adachi S, Gil RR, Yang H, Basile MJ, D'Armiento J, Weinstein IB, Kennelly EJ. Bioactive Depsides and Anthocyanins from Jaboticaba (Myrciaria c auliflora). Journal of Natural Products 2006;69(8):1228-30. https://doi.org/10.1021/np0600999 PMid:16933884

18. Solladie G, Ziani-Cherif C. Total synthesis of natural gingerols, the three active principles of ginger. The Journal of Organic Chemistry 1993;58(8):2181-5. https://doi.org/10.1021/jo00060a038

19. Tranchimand S, Tron T, Gaudin C, Iacazio G. First chemical synthesis of three natural depsides involved in flavonol catabolism and related to quercetinase catalysis. Synthetic Communications 2006;36(5):587-97. https://doi.org/10.1080/00397910500406534

20. Xiao ZP, Fang RQ, Li HQ, Xue JY, Zheng Y, Zhu HL. Enamines as novel antibacterials and their structure-activity relationships. European Journal of Medicinal Chemistry 2008;43(9):1828-36. https://doi.org/10.1016/j.ejmech.2007.11.026 PMid:18192085

21. Xiao ZP, Fang RQ, Shi L, Ding H, Xu C, Zhu HL. Synthesis, crystal structure, and growth inhibition of human hepatoma cell (HepG2) of polyphenolic compounds based on gallates. Canadian Journal of Chemistry 2007;85(11):951-7.https://doi.org/10.1139/v07-107

22. Fujita F, Moriyama T, Higashi T, Shima A, Tominaga M. Methyl p‐hydroxybenzoate causes pain sensation through activation of TRPA1 channels. British Journal of Pharmacology 2007;151(1):134-41. https://doi.org/10.1038/sj.bjp.0707219 PMid:17351650 PMCid:PMC2012982

23. Sheldrick 33. SHELXL-97. Program for crystal structure refinement. 1997.

24. Meletiadis J, Meis JF, Mouton JW, Donnelly JP, Verweij PE. Comparison of NCCLS and 3-(4, 5-dimethyl-2-thiazyl)-2, 5-diphenyl-2H-tetrazolium bromide (MTT) methods of in vitro susceptibility testing of filamentous fungi and development of a new simplified method. Journal of Clinical Microbiology 2000;38(8):2949-54. https://doi.org/10.1128/JCM.38.8.2949-2954.2000 PMid:10921957 PMCid:PMC87156

25. Skagerberg B, Cruciani G, Ebert C. Sweden zyxwvutsrqponmlkjih. Quant Struct-Act Relat 1989;8:32-8 https://doi.org/10.1002/qsar.19890080105

26. Scano P, Thomson C. Comparison of semiempirical MO methods applied to large molecules. Journal of Computational Chemistry 1991;12(2):172-4. https://doi.org/10.1002/jcc.540120205

27. Stewart JJ. Plain text manual pages for MOPAC 6. J Am Chem Soc.1985; 107:3902. https://doi.org/10.1021/ja00299a024

28. Software CHEM 3D-6.0, Cambridge Soft Corporation, 100 Cambridge Park, MA 02140-2317, USA.

29. Valstat software developed at the Department of Pharmacy, SGSITS, 23, Park Road, Indore, India