- Email: [email protected]
Natural products as sources of new fungicides (III): Antifungal activity of 2,4-dihydroxy-5-methylacetophenone derivatives
Accepted Manuscript Natural Products as Sources of New Fungicides (III): Antifungal activity of 2,4dihydroxy-5-methylacetophenone derivatives Wei Shi, Wen-Jia Dan, Jiang-Jiang Tang, Yan Zhang, Tseden Nandinsuren, AnLing Zhang, Jin-Ming Gao PII: DOI: Reference:
S0960-894X(16)30305-5 http://dx.doi.org/10.1016/j.bmcl.2016.03.073 BMCL 23716
To appear in:
Bioorganic & Medicinal Chemistry Letters
Received Date: Revised Date: Accepted Date:
26 January 2016 29 February 2016 18 March 2016
Please cite this article as: Shi, W., Dan, W-J., Tang, J-J., Zhang, Y., Nandinsuren, T., Zhang, A-L., Gao, J-M., Natural Products as Sources of New Fungicides (III): Antifungal activity of 2,4-dihydroxy-5-methylacetophenone derivatives, Bioorganic & Medicinal Chemistry Letters (2016), doi: http://dx.doi.org/10.1016/j.bmcl.2016.03.073
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Natural Products as Sources of New Fungicides (III): Antifungal activity of 2,4-dihydroxy-5-methylacetophenone derivatives
Wei Shi, Wen-Jia Dan, Jiang-Jiang Tang, Yan Zhang, Tseden Nandinsuren, An-Ling Zhang*, and Jin-Ming Gao*
Shaanxi Key Labotory of Natural Products & Chemical Biology, College of Science, Northwest A&F University, Yangling 712100, Peoples Republic of China
* to whom correspondence should be addressed: Fax: 86-29-87092515; Email: [email protected] (A.L. Zhang, J.M. Gao)
Abstract: A series of new 2,4-dihydroxy-5-methylacetophenone 2 derivatives were synthesized, and characterized by 1H,
13
C NMR and ESI-MS. Their antifungal activities were
evaluated in vitro against five important plant fungal pathogens including Cytospora sp., Glomerella cingulate, Pyricularia oryzaecar, Botrytis cinerea and Alternaria solani by the mycelial growth inhibitory rate assay. Compounds 2b-d, 2g and 2h displayed a broad-spectrum activity. The log P value of these active compounds is ranging from 1.71 to 2.54. Especially, isopropyl ketone 2g (log P 2.27) was found to be the most active to the tested organisms with IC50 values of 17.28 to 32.32 μg/mL. The results suggest that compound 2g might be a promising candidate in the development of new agrochemical antifungal agents. Preliminary structure-activity relationship (SAR) studies of the acetophenone derivatives are also discussed.
Keywords:
Natural
products;
agrochemicals;
acetophenone
phytopathogenic fungi; antifungal agents; structure-activity relationship
derivatives;
Agricultural crops are facing tremendous losses due to pests, diseases, and weed damages which result in direct economic losses including reduction in grain yield and quality.1,2 Despite the availability of effective synthetic fungicides, new fungicides with novel chemical structures which have higher potency and broader spectrum of activity against resistant fungal strains are still needed. Recent studies have demonstrated that acetophenones and their derivatives can be seen as important intermediates for the synthesis of some agrochemicals and pharmaceuticals. For example, two antifungal agents (Figure 1) 2-hydroxy-4,6-dimethoxyacetophenone (xanthoxylin, 1) and 2,4-dihydroxy-5-methylacetophenone (2) were isolated from a medicinal plant Melicope borbonica3 and from the higher fungus Polyporus picipes (Polyporaceae),4 respectively. In our previous studies,4-6 compound 2 exhibited in vitro significant growth inhibitory activity, and initial structure-activity relationship (SAR) studies also showed the importance of 2,4-dihydroxy groups for antifungal activity. It has been reported that the mode of antimicrobial action of some new antifungal agent, such as alkyl parabens (p-hydroxybenzoic acid alkyl esters),7 which depends on their ability to disrupt the native membrane due to their hydrophobicity. 8 To explore the effects of the improved hydrophobicity on antifungal activity of acetophenone derivatives, and as a continuation of our research on the synthesis of structurally diverse antifungal compounds, in this Letter, we report the synthesis of lipophilic acetophenone-like compounds through aryl acetoxylation reaction with two methods, and evaluated antifungal activities against several fungal phytopathogens in vitro.
Figure 1. Structures of two antifungal natural products 1 (xanthoxylin) and 2 (2,4-dihydroxy-5-methylacetophenone).
The synthesis of 2,4-dihydroxy-5-methylacetophenone derivatives commenced from starting material 2,4-dihydroxybenzaldehyde. In a conventional procedure, 2,4-dihydroxybenzaldehyde was reduced to 2,4-dihydroxytoluene with NaBH3CN and HCl in THF in high yield (95%). With 2,4-dihydroxytoluene in hands, two methods were conducted to obtain these derivatives 2a–s (Scheme 1). Method A for compounds 2a–h employed Fries-type rearrangement of appropriate phenolic esters to hydroxyaryl ketones by catalysis of Lewis acids. A mixture of 2,4-dihydroxytoluene, corresponding acid anhydride and boron trifluoride-diethyl ether was refluxed. Then reaction mixture was poured into crushed ice and extracted with ethyl acetate. The organic layer was dried and concentrated under reduced pressure. The resulting oil was purified by silica gel column chromatography; Method B for compounds 2i–s involved the reaction of 2,4-dihydroxytoluene with appropriate acyl chlorides to yield ortho-acyl phenols via Friedel–Crafts acylation in a single step as reported in our earlier study.5 Commercially unavailable acyl chlorides were prepared according to the reported methods.9 In the case of compounds 2i–s, besides the desired ortho-acyl phenols, we observed the formation of the very small amount of the corresponding esters as byproducts, and the target phenols were difficult to obtain; as a result, a further purification by crystallization after flash chromatography was needed. The structures of the derivatives 2a–s were shown in Scheme 1.10 (see the Supporting Material). The tested five phytopathogenic fungi, namely, Cytospora sp., Glomerella cingulata Schr, Pyricularia oryzaecar, Botrytis cinerea Pers et Tris and Alternaria solani were provided by College of Science, Northwest A&F University.
R -CH3
log P 1.05
2k
2b
-CH2CH3
1.71
2l
2c 2d 2e 2f 2g 2h 2i 2j
-CH2CH2CH3 -CH2(CH2)2CH3 -CH2(CH2)3CH3 -CH2(CH2)4CH3 -CH(CH3)2 -C(CH3)3 -CH2Cl -CF3
2.13 2.54 2.96 3.38 2.27 2.98 1.58 2.20
2m 2n 2o 2p 2q 2r 2s
2 (2a)
R -(CH3)C=CH2
log P 2.09 1.78
-p-F-Ph -p-Cl-Ph -p-Br-Ph -p-I-Ph -p-NO2-Ph -p-OCH3-Ph -p-CH3-Ph
3.11 3.51 3.78 4.31 2.21 2.83 3.44
Scheme1. Synthetic route of the title compounds and log P values.
We estimated the role of alkyl and aryl ketone derivatives against a panel of plant pathogenic fungi.11 As shown in Table 1, aryl ketone derivatives were not effective against all the fungal strains tested in this study. As for alkyl ketone derivatives, methyl, ethyl, propyl and butyl ketones showed strong to moderate activities, an increase in the chain length of linear alkyl ketones led to enhanced antifungal activity. Among the linear alkyl ketones (2a–f), butyl ketone (2d) had the highest activity; the decrease of activities of compounds 2e (R = pentyl) and 2f (R = hexyl) possibly due to either to an unfavorable steric hindrance or to an increased lipophilicity. Consistent with the antifungal activity of alkyl ketones, branched alkyl ketones showed stronger activity than linear alkyl ketones. Compounds 2g and 2h showed the increased antifungal activities in comparison with the compounds 2c and 2d, respectively. This result suggested that the introduction of methyl group in the linear carbon chain is
important for these derivatives to show the antifungal effects. Notably, 2g, bearing an isopropyl substituent, had the best activity against all the five organisms and displayed significantly more potent potency for Cytospora sp., G. cingulata, P. oryzaecar and A. solani than a commercial fungicide, thiabendazole. Table 1. IC50 values (μg/mL) of derivatives against five plant pathogenic fungi. Compd. Cytospora sp.
Glomerella
Pyricularia
Botrytis
Alternaria
cingulata
oryzaecar
cinerea
solani
2 (2a)
>50
47.25 ± 1.42
>50
>50
>50
2b
>50
42.64 ± 0.34
>50
48.45 ± 1.13
>50
2c 2d
49.53 ± 1.52
23.18 ± 1.25
36.62 ± 0.86
45.23 ± 1.29
46.58 ± 1.32
23.63 ± 0.87
21.30 ± 1.07
24.17 ± 1.18
35.95 ± 0.52
42.52 ± 1.16
>100 >100 24.73 ± 1.56 33.52 ± 1.85 >50 >100 45.59± 1.55 44.41± 1.18 >100 >100 >100 >100 >200 >100 >100
>50 >100 19.84 ± 0.72 25.43 ± 0.96 46.12 ± 1.24 >100 47.20 ± 0.52 >50 >100 >100 >100 >100 >200 >100 >100
>50 >100 17.28 ± 0.75 33.16 ± 1.26 >50 >100 48.97± 1.28 >50 >100 >100 >100 >100 >200 >100 >100
>50 >100 32.32 ± 1.56 35.46 ± 1.73 >50 >100 >50 >50 >50 >100 >100 >100 >200 >100 >100
>50
>50
30.13 ± 1.12
24.78 ± 0.52
>50 >100 37.45 ± 0.87 >50 >50 >100 >50 >50 >100 >100 >100 >100 >200 >100 >100 >50
2e 2f 2g 2h 2i 2j 2k 2l 2m 2n 2o 2p 2q 2r 2s thiabendazole
Thiabendazole was used as the positive control. All data represent the mean ± S.D. of at least three independent experiments performed in triplicates.
The results listed in the Table 1 also revealed that the introduction of a chlorine atom (2i) or an unsaturated aliphatic chain (2k) at the position of the C=O group generally did not affect the activity compared to compound 2a. Thus, we can hypothesize that the Cl group or a conjugated (-MeC=CH-C=O-) unit in the alkyl chain is sensitive to the tested pathogens, especially G. cingulata. A similar result was observed as we introduced a cyclopropyl group (2l) on the side chain (2l vs. 2a). In addition, compounds 2b–2d, 2h and 2k were significantly more active than the
positive control, thiabendazole, for some fungi. These results suggested the possibility of the alkyl acetophenones as antifungal agents. To gain a further insight into the SAR, we calculated the lipophilicity values (log P) of the compounds 2a–2s (Scheme 1). According to Ghose and Crippen’s fragmentation method using ChemDrawUltra software, log P was expressed as logarithm of partition coefficient in octan-1-ol/H2O.12 As shown in Scheme 1, compounds 2c (log P 2.13), 2d (log P 2.54) and 2g (log P 2.27) exhibited obviously higher antifungal activities than lipophilic compounds 2e (log P 2.96) and 2f (log P 3.38), while 2b (log P 1.71) and 2k (log P 2.09) displayed high antifungal activities. The chloromethyl derivative 2i (log P 1.58) and the cyclopropyl derivative 2l (log P 1.78) exhibited better antifungal activities than the parent product 2a (log P 1.05) on a wide range of organisms. The introduction of benzene rings (i.e., 2m, log P 3.11; 2n, log P 3.51) led to a decrease in antifungal activity. The results revealed that the most active compounds have a log P value in the range of 1.71–2.54, the two exceptions being compounds 2j (log P 2.2) and 2q (log P 2.21) which decreased antifungal effects, despite their reasonable log P value. This was most likely due to the electron-attracting substituent in the side chain; the decrease in antifungal activity for compounds with log P> 2.5 could be due to an unfavorable steric hindrance. This result suggests that the addition of the aliphatic side chain increases the lipophilicity of the compounds, leading to an improved antifungal activity, which might be associated with better penetration through the plasmatic membrane of the fungi. In conclusion, we synthesized a series of new acetophenone analogs and assayed their antifungal activities in vitro. Among the synthetic compounds, alkyl ketone derivatives exerted broad-spectrum antifungal activities to several plant pathogens, and an increase in the chain length of linear alkyl ketones led to enhanced antifungal activities; branched alkyl ketones exhibited stronger activity than linear alkyl ketones. And these active compounds have a log P value ranging from 1.71 to 2.54. In particular, compound 2g (log P 2.27), with an isobutyryl group, produced a remarkable in vitro antifungal activity, which might be considered a promising candidate in the development of new fungicides.
Acknowledgements This work was supported by grants from the Program of Unified Planning Innovation
Engineering
of
Science
&
Technology
in
Shaanxi
Province
(No.2015KTCQ02-14) and the National Natural Science Foundation of China (No. 21542016, 30971882). Conflicts of Interest The authors declare no conflict of interest. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://.
References and notes 1.
Villegas, A. M.; Peralta, K. D.; Tapia, C. G.; Marin, K. C.; Catalan, L. E. Nat. Prod. Res.2015, 29,
586.
2.
Thind, T. S. Indian Phytopathology 2012,65, 109.
3.
Simonsen, H. T.; Adsersen, A.; Bremner, P.; Heinrich, M.; Smitt, U. W.; Jaroszewski, J. W.
Phytother. Res. 2004, 18, 542. 4.
Gao J.-M., L. Y., Xie S.-D., Zhang A.-L., Shi X.-W., Zhang W.-L. China Patent; 2009,
200910020963.7. 5.
Nandinsuren, T.; Shi, W.; Zhang, A.
L.; Bai, Y.
B.;
Gao,
J. M.
Nat. Prod.
Res.2015,DOI:10.1080/14786419.2015.1041140. 6.
Ma, Y. T.; Fan, H. F.; Gao, Y. Q.; Li, H.; Zhang, A. L.; Gao, J. M. Chem. Biol. Drug Des. 2013, 81,
545. 7.
Ito, S.; Yazawa, S.; Nakagawa, Y.; Sasaki, Y.; Yajima, S. Bioorg. Med. Chem. Lett. 2015, 25, 1774.
8.
Ma, Y.; Marquis, R. E. Lett. Appl. Microbiol. 1996, 23, 329.
9.
Liu, K.; Xu, L.; Berger, J. P.; MacNaul, K. L.; Zhou, G.; Doebber, T. W.; Forrest, M. J.; Moller, D.
E.; Jones, A. B. J. Med. Chem. 2005, 48, 2262. 10.
1
H and 13C NMR spectra were recorded with a Bruker Avance 500 MHz. MS spectra was taken on
a Bruker Daltonics esquire 6000 (ESI-ION TRAP). Spectral data of representative compound 2g. Light yellow solid, yield 70%, m.p. 119.2-120.7 °C. 1H NMR (500 MHz, CD3OD) δ 7.71(s, 1 arom. H), 6.51 (s, 1 arom. H), 3.63-3.52 (m, 1H, CH), 2.07 (s, 3H, CH3), 1.17 (6H, d, J = 12.8 Hz, 2CH3); 13C NMR (125 MHz，CD3OD) δ210.1, 164.7, 163.0, 130.0, 112.1, 112.1, 107.8, 35.0, 19.9, 7.8; HRESI-MS: m/z [M-H]- 193.1834 (calcd. for C11H13O3, 193.1825).
11. Antifungal bioassay: The in vitro antifungal activity was determined by measuring half maximal inhibitory concentration (IC 50) according to the reported method.5,13 Briefly, after incubation with these compounds for 72 h, the mycelial growth of fungi (mm) in both treated (T) and control (C) Petri dishes was measured diametrically in three different directions (radial fungal-growth assay) till the fungal growth in the control dishes was almost complete. The percentage of growth inhibition (I) was calculated using the following formula. I (%) = [(C-T)/C] × 100%. 12. Ghose, A. K.; Crippen, G. M. J. Chem. Inf. Comput. Sci. 1987,27, 21. 13. Bai, Y. B.; Zhang, A. L.; Tang, J. J.; Gao, J. M. J. Agr. Food Chem.2013, 61, 2789.
Graphical Abstract
S0960-894X(16)30305-5 http://dx.doi.org/10.1016/j.bmcl.2016.03.073 BMCL 23716
To appear in:
Bioorganic & Medicinal Chemistry Letters
Received Date: Revised Date: Accepted Date:
26 January 2016 29 February 2016 18 March 2016
Please cite this article as: Shi, W., Dan, W-J., Tang, J-J., Zhang, Y., Nandinsuren, T., Zhang, A-L., Gao, J-M., Natural Products as Sources of New Fungicides (III): Antifungal activity of 2,4-dihydroxy-5-methylacetophenone derivatives, Bioorganic & Medicinal Chemistry Letters (2016), doi: http://dx.doi.org/10.1016/j.bmcl.2016.03.073
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Natural Products as Sources of New Fungicides (III): Antifungal activity of 2,4-dihydroxy-5-methylacetophenone derivatives
Wei Shi, Wen-Jia Dan, Jiang-Jiang Tang, Yan Zhang, Tseden Nandinsuren, An-Ling Zhang*, and Jin-Ming Gao*
Shaanxi Key Labotory of Natural Products & Chemical Biology, College of Science, Northwest A&F University, Yangling 712100, Peoples Republic of China
* to whom correspondence should be addressed: Fax: 86-29-87092515; Email: [email protected] (A.L. Zhang, J.M. Gao)
Abstract: A series of new 2,4-dihydroxy-5-methylacetophenone 2 derivatives were synthesized, and characterized by 1H,
13
C NMR and ESI-MS. Their antifungal activities were
evaluated in vitro against five important plant fungal pathogens including Cytospora sp., Glomerella cingulate, Pyricularia oryzaecar, Botrytis cinerea and Alternaria solani by the mycelial growth inhibitory rate assay. Compounds 2b-d, 2g and 2h displayed a broad-spectrum activity. The log P value of these active compounds is ranging from 1.71 to 2.54. Especially, isopropyl ketone 2g (log P 2.27) was found to be the most active to the tested organisms with IC50 values of 17.28 to 32.32 μg/mL. The results suggest that compound 2g might be a promising candidate in the development of new agrochemical antifungal agents. Preliminary structure-activity relationship (SAR) studies of the acetophenone derivatives are also discussed.
Keywords:
Natural
products;
agrochemicals;
acetophenone
phytopathogenic fungi; antifungal agents; structure-activity relationship
derivatives;
Agricultural crops are facing tremendous losses due to pests, diseases, and weed damages which result in direct economic losses including reduction in grain yield and quality.1,2 Despite the availability of effective synthetic fungicides, new fungicides with novel chemical structures which have higher potency and broader spectrum of activity against resistant fungal strains are still needed. Recent studies have demonstrated that acetophenones and their derivatives can be seen as important intermediates for the synthesis of some agrochemicals and pharmaceuticals. For example, two antifungal agents (Figure 1) 2-hydroxy-4,6-dimethoxyacetophenone (xanthoxylin, 1) and 2,4-dihydroxy-5-methylacetophenone (2) were isolated from a medicinal plant Melicope borbonica3 and from the higher fungus Polyporus picipes (Polyporaceae),4 respectively. In our previous studies,4-6 compound 2 exhibited in vitro significant growth inhibitory activity, and initial structure-activity relationship (SAR) studies also showed the importance of 2,4-dihydroxy groups for antifungal activity. It has been reported that the mode of antimicrobial action of some new antifungal agent, such as alkyl parabens (p-hydroxybenzoic acid alkyl esters),7 which depends on their ability to disrupt the native membrane due to their hydrophobicity. 8 To explore the effects of the improved hydrophobicity on antifungal activity of acetophenone derivatives, and as a continuation of our research on the synthesis of structurally diverse antifungal compounds, in this Letter, we report the synthesis of lipophilic acetophenone-like compounds through aryl acetoxylation reaction with two methods, and evaluated antifungal activities against several fungal phytopathogens in vitro.
Figure 1. Structures of two antifungal natural products 1 (xanthoxylin) and 2 (2,4-dihydroxy-5-methylacetophenone).
The synthesis of 2,4-dihydroxy-5-methylacetophenone derivatives commenced from starting material 2,4-dihydroxybenzaldehyde. In a conventional procedure, 2,4-dihydroxybenzaldehyde was reduced to 2,4-dihydroxytoluene with NaBH3CN and HCl in THF in high yield (95%). With 2,4-dihydroxytoluene in hands, two methods were conducted to obtain these derivatives 2a–s (Scheme 1). Method A for compounds 2a–h employed Fries-type rearrangement of appropriate phenolic esters to hydroxyaryl ketones by catalysis of Lewis acids. A mixture of 2,4-dihydroxytoluene, corresponding acid anhydride and boron trifluoride-diethyl ether was refluxed. Then reaction mixture was poured into crushed ice and extracted with ethyl acetate. The organic layer was dried and concentrated under reduced pressure. The resulting oil was purified by silica gel column chromatography; Method B for compounds 2i–s involved the reaction of 2,4-dihydroxytoluene with appropriate acyl chlorides to yield ortho-acyl phenols via Friedel–Crafts acylation in a single step as reported in our earlier study.5 Commercially unavailable acyl chlorides were prepared according to the reported methods.9 In the case of compounds 2i–s, besides the desired ortho-acyl phenols, we observed the formation of the very small amount of the corresponding esters as byproducts, and the target phenols were difficult to obtain; as a result, a further purification by crystallization after flash chromatography was needed. The structures of the derivatives 2a–s were shown in Scheme 1.10 (see the Supporting Material). The tested five phytopathogenic fungi, namely, Cytospora sp., Glomerella cingulata Schr, Pyricularia oryzaecar, Botrytis cinerea Pers et Tris and Alternaria solani were provided by College of Science, Northwest A&F University.
R -CH3
log P 1.05
2k
2b
-CH2CH3
1.71
2l
2c 2d 2e 2f 2g 2h 2i 2j
-CH2CH2CH3 -CH2(CH2)2CH3 -CH2(CH2)3CH3 -CH2(CH2)4CH3 -CH(CH3)2 -C(CH3)3 -CH2Cl -CF3
2.13 2.54 2.96 3.38 2.27 2.98 1.58 2.20
2m 2n 2o 2p 2q 2r 2s
2 (2a)
R -(CH3)C=CH2
log P 2.09 1.78
-p-F-Ph -p-Cl-Ph -p-Br-Ph -p-I-Ph -p-NO2-Ph -p-OCH3-Ph -p-CH3-Ph
3.11 3.51 3.78 4.31 2.21 2.83 3.44
Scheme1. Synthetic route of the title compounds and log P values.
We estimated the role of alkyl and aryl ketone derivatives against a panel of plant pathogenic fungi.11 As shown in Table 1, aryl ketone derivatives were not effective against all the fungal strains tested in this study. As for alkyl ketone derivatives, methyl, ethyl, propyl and butyl ketones showed strong to moderate activities, an increase in the chain length of linear alkyl ketones led to enhanced antifungal activity. Among the linear alkyl ketones (2a–f), butyl ketone (2d) had the highest activity; the decrease of activities of compounds 2e (R = pentyl) and 2f (R = hexyl) possibly due to either to an unfavorable steric hindrance or to an increased lipophilicity. Consistent with the antifungal activity of alkyl ketones, branched alkyl ketones showed stronger activity than linear alkyl ketones. Compounds 2g and 2h showed the increased antifungal activities in comparison with the compounds 2c and 2d, respectively. This result suggested that the introduction of methyl group in the linear carbon chain is
important for these derivatives to show the antifungal effects. Notably, 2g, bearing an isopropyl substituent, had the best activity against all the five organisms and displayed significantly more potent potency for Cytospora sp., G. cingulata, P. oryzaecar and A. solani than a commercial fungicide, thiabendazole. Table 1. IC50 values (μg/mL) of derivatives against five plant pathogenic fungi. Compd. Cytospora sp.
Glomerella
Pyricularia
Botrytis
Alternaria
cingulata
oryzaecar
cinerea
solani
2 (2a)
>50
47.25 ± 1.42
>50
>50
>50
2b
>50
42.64 ± 0.34
>50
48.45 ± 1.13
>50
2c 2d
49.53 ± 1.52
23.18 ± 1.25
36.62 ± 0.86
45.23 ± 1.29
46.58 ± 1.32
23.63 ± 0.87
21.30 ± 1.07
24.17 ± 1.18
35.95 ± 0.52
42.52 ± 1.16
>100 >100 24.73 ± 1.56 33.52 ± 1.85 >50 >100 45.59± 1.55 44.41± 1.18 >100 >100 >100 >100 >200 >100 >100
>50 >100 19.84 ± 0.72 25.43 ± 0.96 46.12 ± 1.24 >100 47.20 ± 0.52 >50 >100 >100 >100 >100 >200 >100 >100
>50 >100 17.28 ± 0.75 33.16 ± 1.26 >50 >100 48.97± 1.28 >50 >100 >100 >100 >100 >200 >100 >100
>50 >100 32.32 ± 1.56 35.46 ± 1.73 >50 >100 >50 >50 >50 >100 >100 >100 >200 >100 >100
>50
>50
30.13 ± 1.12
24.78 ± 0.52
>50 >100 37.45 ± 0.87 >50 >50 >100 >50 >50 >100 >100 >100 >100 >200 >100 >100 >50
2e 2f 2g 2h 2i 2j 2k 2l 2m 2n 2o 2p 2q 2r 2s thiabendazole
Thiabendazole was used as the positive control. All data represent the mean ± S.D. of at least three independent experiments performed in triplicates.
The results listed in the Table 1 also revealed that the introduction of a chlorine atom (2i) or an unsaturated aliphatic chain (2k) at the position of the C=O group generally did not affect the activity compared to compound 2a. Thus, we can hypothesize that the Cl group or a conjugated (-MeC=CH-C=O-) unit in the alkyl chain is sensitive to the tested pathogens, especially G. cingulata. A similar result was observed as we introduced a cyclopropyl group (2l) on the side chain (2l vs. 2a). In addition, compounds 2b–2d, 2h and 2k were significantly more active than the
positive control, thiabendazole, for some fungi. These results suggested the possibility of the alkyl acetophenones as antifungal agents. To gain a further insight into the SAR, we calculated the lipophilicity values (log P) of the compounds 2a–2s (Scheme 1). According to Ghose and Crippen’s fragmentation method using ChemDrawUltra software, log P was expressed as logarithm of partition coefficient in octan-1-ol/H2O.12 As shown in Scheme 1, compounds 2c (log P 2.13), 2d (log P 2.54) and 2g (log P 2.27) exhibited obviously higher antifungal activities than lipophilic compounds 2e (log P 2.96) and 2f (log P 3.38), while 2b (log P 1.71) and 2k (log P 2.09) displayed high antifungal activities. The chloromethyl derivative 2i (log P 1.58) and the cyclopropyl derivative 2l (log P 1.78) exhibited better antifungal activities than the parent product 2a (log P 1.05) on a wide range of organisms. The introduction of benzene rings (i.e., 2m, log P 3.11; 2n, log P 3.51) led to a decrease in antifungal activity. The results revealed that the most active compounds have a log P value in the range of 1.71–2.54, the two exceptions being compounds 2j (log P 2.2) and 2q (log P 2.21) which decreased antifungal effects, despite their reasonable log P value. This was most likely due to the electron-attracting substituent in the side chain; the decrease in antifungal activity for compounds with log P> 2.5 could be due to an unfavorable steric hindrance. This result suggests that the addition of the aliphatic side chain increases the lipophilicity of the compounds, leading to an improved antifungal activity, which might be associated with better penetration through the plasmatic membrane of the fungi. In conclusion, we synthesized a series of new acetophenone analogs and assayed their antifungal activities in vitro. Among the synthetic compounds, alkyl ketone derivatives exerted broad-spectrum antifungal activities to several plant pathogens, and an increase in the chain length of linear alkyl ketones led to enhanced antifungal activities; branched alkyl ketones exhibited stronger activity than linear alkyl ketones. And these active compounds have a log P value ranging from 1.71 to 2.54. In particular, compound 2g (log P 2.27), with an isobutyryl group, produced a remarkable in vitro antifungal activity, which might be considered a promising candidate in the development of new fungicides.
Acknowledgements This work was supported by grants from the Program of Unified Planning Innovation
Engineering
of
Science
&
Technology
in
Shaanxi
Province
(No.2015KTCQ02-14) and the National Natural Science Foundation of China (No. 21542016, 30971882). Conflicts of Interest The authors declare no conflict of interest. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://.
References and notes 1.
Villegas, A. M.; Peralta, K. D.; Tapia, C. G.; Marin, K. C.; Catalan, L. E. Nat. Prod. Res.2015, 29,
586.
2.
Thind, T. S. Indian Phytopathology 2012,65, 109.
3.
Simonsen, H. T.; Adsersen, A.; Bremner, P.; Heinrich, M.; Smitt, U. W.; Jaroszewski, J. W.
Phytother. Res. 2004, 18, 542. 4.
Gao J.-M., L. Y., Xie S.-D., Zhang A.-L., Shi X.-W., Zhang W.-L. China Patent; 2009,
200910020963.7. 5.
Nandinsuren, T.; Shi, W.; Zhang, A.
L.; Bai, Y.
B.;
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1
H and 13C NMR spectra were recorded with a Bruker Avance 500 MHz. MS spectra was taken on
a Bruker Daltonics esquire 6000 (ESI-ION TRAP). Spectral data of representative compound 2g. Light yellow solid, yield 70%, m.p. 119.2-120.7 °C. 1H NMR (500 MHz, CD3OD) δ 7.71(s, 1 arom. H), 6.51 (s, 1 arom. H), 3.63-3.52 (m, 1H, CH), 2.07 (s, 3H, CH3), 1.17 (6H, d, J = 12.8 Hz, 2CH3); 13C NMR (125 MHz，CD3OD) δ210.1, 164.7, 163.0, 130.0, 112.1, 112.1, 107.8, 35.0, 19.9, 7.8; HRESI-MS: m/z [M-H]- 193.1834 (calcd. for C11H13O3, 193.1825).
11. Antifungal bioassay: The in vitro antifungal activity was determined by measuring half maximal inhibitory concentration (IC 50) according to the reported method.5,13 Briefly, after incubation with these compounds for 72 h, the mycelial growth of fungi (mm) in both treated (T) and control (C) Petri dishes was measured diametrically in three different directions (radial fungal-growth assay) till the fungal growth in the control dishes was almost complete. The percentage of growth inhibition (I) was calculated using the following formula. I (%) = [(C-T)/C] × 100%. 12. Ghose, A. K.; Crippen, G. M. J. Chem. Inf. Comput. Sci. 1987,27, 21. 13. Bai, Y. B.; Zhang, A. L.; Tang, J. J.; Gao, J. M. J. Agr. Food Chem.2013, 61, 2789.
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