Synthesis and Fungicidal Activity of Hydrated Geranylated Phenols against Botrytis cinerea

Botrytis cinerea is a ubiquitous fungus that affects hundreds of plants, resulting in economic losses to the horticulture and fruit industry. The search for new antifungal agents is a matter of current interest. Thus, in this work a series of geranylated phenols in which the side alkyl chain has been hydrated have been synthesized, and their activity against B. cinerea has been evaluated. The coupling of phenol and geraniol has been accomplished under microwave irradiation obtaining the highest reaction yields in the shortest reaction times. Hydration of the side chain was carried out in dioxane with p-toluenesulfonic acid polymer-bound as the catalyst. All synthesized compounds were tested against B. cinerea using the growth inhibition assay and EC50 values were determined. The results show that activity depends on the number and nature of functional groups in the phenol ring and hydration degree of the geranyl chain. The most active compound is 1,4-dihydroquinone with one hydroxyl group attached at the end of the alkyl chain. Results from a molecular docking study suggest that hydroxyl groups in the phenol ring and alkyl chain are important in the binding of compounds to the active site, and that the experimental antifungal activity correlates with the number of H-bond that can be formed in the binding site.

De Rosa et al. [1] have shown that biological activity of 2-prenyl-1,4-hydroquinones depends on the size of the prenyl chain, and the optimal length is in the range of five to fifteen carbon atoms. In previous work, we reported the synthesis of a series of geranylated phenols with different chemical substituents on the aromatic ring (see Figure 1). These compounds exhibited interesting biological activities, such as in vitro cytotoxicity on cancer cell lines [9,10], mycelial growth inhibition of Botrytis cinerea [11][12][13][14] and antifungal activity against Phytophthora cinnamomi [15]. The results indicate that the activity is enhanced mainly by the presence of −OH, -OCH 3 and -OAc functions in the aromatic ring [11][12][13].
enhanced mainly by the presence of −OH, -OCH3 and -OAc functions in the aromatic ring [11][12][13]. On the other hand, the effect of chemical modification of the side chain on antifungal activity has received much less attention, i.e., it has been shown that the hydrated derivative of 2-allylphenol exhibits antifungal activity against P. aphanidermatum, V. mali, B. cinerea and R. cereals [16], whereas some hydrated derivatives of geranylated phenols (Figure 2) inhibit the mycelial growth of B. cinerea [14]. Thus, the aim of this work is to carry out a systematic study of the effect of ring substitution and hydration of side chain on the activity of geranylated phenols against B. cinerea. Therefore, a series of geranylated phenols with and without hydroxyl groups in the side chain were synthesized, and their mycelial growth inhibition activity on B. cinerea was determined. The obtained data were used to establish a structure-activity relationship, whereas a molecular docking study was made to visualize the main interactions of these molecules into the active site.

Results
To establish a structure-activity relationship, between mycelial-growth inhibition of B. cinerea and chemical substitution on phenol ring and hydration in the side alkyl chain, four series of geranylated phenols have been synthesized. Antifungal activities against B. cinerea of all synthesized compounds were evaluated. On the other hand, the effect of chemical modification of the side chain on antifungal activity has received much less attention, i.e., it has been shown that the hydrated derivative of 2-allylphenol exhibits antifungal activity against P. aphanidermatum, V. mali, B. cinerea and R. cereals [16], whereas some hydrated derivatives of geranylated phenols ( Figure 2) inhibit the mycelial growth of B. cinerea [14].
Molecules 2021, 26, x FOR PEER REVIEW 2 of 19 enhanced mainly by the presence of −OH, -OCH3 and -OAc functions in the aromatic ring [11][12][13]. On the other hand, the effect of chemical modification of the side chain on antifungal activity has received much less attention, i.e., it has been shown that the hydrated derivative of 2-allylphenol exhibits antifungal activity against P. aphanidermatum, V. mali, B. cinerea and R. cereals [16], whereas some hydrated derivatives of geranylated phenols (Figure 2) inhibit the mycelial growth of B. cinerea [14]. Thus, the aim of this work is to carry out a systematic study of the effect of ring substitution and hydration of side chain on the activity of geranylated phenols against B. cinerea. Therefore, a series of geranylated phenols with and without hydroxyl groups in the side chain were synthesized, and their mycelial growth inhibition activity on B. cinerea was determined. The obtained data were used to establish a structure-activity relationship, whereas a molecular docking study was made to visualize the main interactions of these molecules into the active site.

Results
To establish a structure-activity relationship, between mycelial-growth inhibition of B. cinerea and chemical substitution on phenol ring and hydration in the side alkyl chain, four series of geranylated phenols have been synthesized. Antifungal activities against B. cinerea of all synthesized compounds were evaluated. Thus, the aim of this work is to carry out a systematic study of the effect of ring substitution and hydration of side chain on the activity of geranylated phenols against B. cinerea. Therefore, a series of geranylated phenols with and without hydroxyl groups in the side chain were synthesized, and their mycelial growth inhibition activity on B. cinerea was determined. The obtained data were used to establish a structure-activity relationship, whereas a molecular docking study was made to visualize the main interactions of these molecules into the active site.

Results
To establish a structure-activity relationship, between mycelial-growth inhibition of B. cinerea and chemical substitution on phenol ring and hydration in the side alkyl chain, four series of geranylated phenols have been synthesized. Antifungal activities against B. cinerea of all synthesized compounds were evaluated.

Synthesis
The synthesis of geranylated phenols is well described and generally it proceeds by direct coupling of geraniol with the proper phenol in the presence of BF 3 ·OEt 2 as a catalyst and dioxane as solvent [1,9,14,[17][18][19]. Alternatively, BF 3 ·OEt 2 /AgNO 3 has been used as catalyst and acetonitrile as solvent [11,12]. The time reaction varies between 24 and 72 h and the yields are in the range of 3 to 12%. In this work, this coupling reaction was carried out under microwave irradiation to obtain a series of geranylated phenols with different substituents on the phenolic ring (see Figure 3).

Synthesis
The synthesis of geranylated phenols is well described and generally it proceeds by direct coupling of geraniol with the proper phenol in the presence of BF3·OEt2 as a catalyst and dioxane as solvent [1,9,14,[17][18][19]. Alternatively, BF3·OEt2/AgNO3 has been used as catalyst and acetonitrile as solvent [11,12]. The time reaction varies between 24 and 72 h and the yields are in the range of 3 to 12%. In this work, this coupling reaction was carried out under microwave irradiation to obtain a series of geranylated phenols with different substituents on the phenolic ring (see Figure 3). Compounds 3, 18, 16, 17, 21 and 22 have been described previously, whereas derivatives 19 and 20 are new compounds. Interestingly, coupling reactions under microwave irradiation proceed with higher yields (21-32%) and reduced reaction time (10 min). For example, Scheme 1 shows the coupling reaction between o-cresol and geraniol giving compound 3. Under microwave irradiation, three is obtained with 22% yield after 10 min, while in absence of irradiation the reaction yield is 3% after 48 h of reaction [14].

Scheme 1. Synthesis of geranylated phenol 3 under microwave irradiation.
On the other hand, there are just a few reports of hydration reaction of the side chain in geranylated phenols. Hydrated geranylated orcinol derivatives were obtained by a coupling reaction of geraniol and orcinol in the presence of oxalic acid (1%) at 80 °C [20]. No reaction yields were reported. Compounds 13 and 14 were synthesized by electrophilic addition of geraniol in dioxane and using BF3 as catalyst in the presence of water [14]. Herein, geranylated phenols were hydrated in dioxane containing water and using p-toluenesulfonic acid bound to a polymer matrix as catalyst. In this way, two series of new mono-and di-hydrated geranylated phenols (Figures 4 and 5, respectively) were obtained. For example, Scheme 1 shows the coupling reaction between o-cresol and geraniol giving compound 3. Under microwave irradiation, three is obtained with 22% yield after 10 min, while in absence of irradiation the reaction yield is 3% after 48 h of reaction [14].

Synthesis
The synthesis of geranylated phenols is well described and generally it proceeds by direct coupling of geraniol with the proper phenol in the presence of BF3·OEt2 as a catalyst and dioxane as solvent [1,9,14,17,18,19]. Alternatively, BF3·OEt2/AgNO3 has been used as catalyst and acetonitrile as solvent [11,12]. The time reaction varies between 24 and 72 h and the yields are in the range of 3 to 12%. In this work, this coupling reaction was carried out under microwave irradiation to obtain a series of geranylated phenols with different substituents on the phenolic ring (see Figure 3). For example, Scheme 1 shows the coupling reaction between o-cresol and geraniol giving compound 3. Under microwave irradiation, three is obtained with 22% yield after 10 min, while in absence of irradiation the reaction yield is 3% after 48 h of reaction [14].

Scheme 1. Synthesis of geranylated phenol 3 under microwave irradiation.
On the other hand, there are just a few reports of hydration reaction of the side chain in geranylated phenols. Hydrated geranylated orcinol derivatives were obtained by a coupling reaction of geraniol and orcinol in the presence of oxalic acid (1%) at 80 °C [20]. No reaction yields were reported. Compounds 13 and 14 were synthesized by electrophilic addition of geraniol in dioxane and using BF3 as catalyst in the presence of water [14]. Herein, geranylated phenols were hydrated in dioxane containing water and using p-toluenesulfonic acid bound to a polymer matrix as catalyst. In this way, two series of new mono-and di-hydrated geranylated phenols (Figures 4 and 5, respectively) were obtained. On the other hand, there are just a few reports of hydration reaction of the side chain in geranylated phenols. Hydrated geranylated orcinol derivatives were obtained by a coupling reaction of geraniol and orcinol in the presence of oxalic acid (1%) at 80 • C [20]. No reaction yields were reported. Compounds 13 and 14 were synthesized by electrophilic addition of geraniol in dioxane and using BF 3 as catalyst in the presence of water [14]. Herein, geranylated phenols were hydrated in dioxane containing water and using ptoluenesulfonic acid bound to a polymer matrix as catalyst. In this way, two series of new mono-and di-hydrated geranylated phenols (Figures 4 and 5, respectively) were obtained.    The product of this reaction is a mixture of hydrated compounds carrying one (26 and 27) or two hydroxyl groups (36). As can be seen, the product reaction is a mixture of hydrated compounds carrying one or two hydroxyl groups.
Structural Determination of Geranylated Orcinol Derivatives 26, 27 and 36    The product of this reaction is a mixture of hydrated compounds carrying one (26 and 27) or two hydroxyl groups (36). As can be seen, the product reaction is a mixture of hydrated compounds carrying one or two hydroxyl groups.
Structural Determination of Geranylated Orcinol Derivatives 26, 27 and 36    The product of this reaction is a mixture of hydrated compounds carrying one (26 and 27) or two hydroxyl groups (36). As can be seen, the product reaction is a mixture of hydrated compounds carrying one or two hydroxyl groups.
Structural The product of this reaction is a mixture of hydrated compounds carrying one (26 and 27) or two hydroxyl groups (36). As can be seen, the product reaction is a mixture of hydrated compounds carrying one or two hydroxyl groups.

In Vitro Antifungal Activity against B. cinerea
The inhibition of mycelial growth of B. cinerea by all geranylated phenols, with and without hydrated side chain, was evaluated by measuring colony diameters in the presence and absence of tested compounds at different concentrations. Typical results obtained for derivative 30 are shown in Figure 8. The mycelial growth in the presence of three different concentrations was measured and compared with those observed in the absence of any growth inhibitor (negative control) and in presence of a commercial fungicide (BC-1000, positive control). From these results the percentage of inhibition was calculated and plotted as a function of concentration for each compound. Finally, fitting data to the response-activity equation provides the EC50 values that are tabulated in Tables 1-3   Table 1. Effect of chemical structure on the growth inhibition activity of geranylated phenols with no hydroxyl groups in the side chain.   From these results the percentage of inhibition was calculated and plotted as a function of concentration for each compound. Finally, fitting data to the response-activity equation provides the EC 50 values that are tabulated in Tables 1-3   Table 1. Effect of chemical structure on the growth inhibition activity of geranylated phenols with no hydroxyl groups in the side chain. The biological activity of geranylated phenols with no hydroxyl functions in the side chain ( Figure 3) are listed in Table 1.

Geranylated
The data in Table 1 indicate that the highest activity against B. cinerea is exhibited by  compounds 17, 18 and 19. In all of them the geranyl chain is in the ortho position to the phenol group. Changing the hydroxyl group by a methyl group or moving the geranyl chain to the para position (compounds 16 and 3, respectively), reduces the inhibition activity. In a previous work it was shown that 2-geranylhydroquinone (compound 18) was the most active of a series of geranylated phenols with two or three hydroxyl groups, and that the activity decreases slightly by acetylation of hydroxyl groups [11]. Herein, compound 19, with two methoxy groups, exhibits almost the same activity, suggesting that methoxy groups have no effect on activity.  On the other hand, EC 50 values obtained for geranylated phenols with hydrated side chain are listed in Tables 2 and 3.
The data show that the effect of side chain hydration on bioactivity depends strongly on the structure of geranylated phenols. For example, the position of -OH in monohydrated compounds 23 and 24 has almost no effect on EC 50 values, whereas compound 27 (-OH in C-3 ) is three times more active than 26 (-OH in C-7 ). On the other hand, hydration in the C-3 slightly diminishes the bioactivity from 84 (compound 18) to 108 (compound 25), whereas hydration in C-7 of compound 19 leads to compound 30 which is the most active geranyl derivative reported in this work. This structural change induces a five-fold change in the EC 50 value. Finally, addition of a second hydroxyl group decreases the activity at a level that some of them are completely inactive (compares EC 50 values of compounds 23, 24 and 34; 26, 27 and 36). The exception is compound 35 that exhibits higher activity than 25, i.e., EC 50 values of 54 and 108, respectively.
To get a better insight in the way that growth inhibition activity depends on the chemical structure of the geranylated phenols, and specially the hydration of the side alkyl chain, we have performed a docking molecular study.

Molecular Docking Study
Recently, it has been shown that the antifungal activity of a variety of compounds is mainly due to inhibition of succinate dehydrogenase (SDH) which is a component of the mitochondrial respiratory chain and the tricarboxylic acid cycle [21][22][23]. Thus, the interactions of antifungal compounds with the ubiquinone binding site have been evaluated to understand the experimental activity shown by different series of related compounds [24][25][26][27][28]. In this work, we have performed molecular docking studies of geranylated phenols carrying one (23-33), two (34-38) or no (14, 18, 19) hydroxyl groups in the side alkyl chain. Molecular coupling of these compounds to a single crystal structure of SDH (PDB code 2FBW) was carried out by using Glide software. The obtained data indicate that binding energies of these compounds to the allosteric binding site of SDH are in the range of −9.2 to −5.4 kcal mol −1 (Table 4). Interestingly, the most active compounds (lowest EC 50 values) give the highest docking scores. For example, the most active geranylated phenols with one -OH in the geranyl chain are compounds 30, 32 and 27, which give the highest binding energies, namely −9.2 to −7.6. The same trend is obtained for di-hydrated compounds, 37 and 35, and compounds with no -OH in the geranyl chain, 19 and 18. In a previous study, we evaluated the antifungal activity of a series of geranylated phenols, geranylated quinones and hydrated-geranyl phenols. The results indicated that compound 14 was the most active compound and at least two times more active than the dihydrated derivative 13 [14]. Therefore, 14 was included in the molecular docking study and a docking score of −8.9 kcal mol −1 was obtained, which is very close to that obtained for compound 30. Table 4. Binding energies, obtained from molecular docking studies and antifungal activities against B. cinerea (EC 50 values) of geranylated phenols with one (14, 23-33), two (34)(35)(36)(37)(38) or none (18,19,21) hydroxyl groups in the side alkyl chain. CBE is used for comparison 1 . In the predicted pose of 30, the most active compound, five H-bonds were identified. Two of them are formed by the hydroxyl group in position 4 with Tyr 58 and Trp 173 at 1.98 Å and 2.17 Å, respectively; the others are formed by interaction between the hydroxyl group in position 1 and Ser 39 at 1.91 Å; the methoxy group at position 5 and Trp 173 at 2.05 Å; the hydroxyl group at C7 and Tyr 58. The latter is mediated by a water molecule located at 1.84 Å from the hydroxyl group and 2.18 Å from the Tyr 58 residue (see Figure 9). On the other hand, the side alkyl chain participates in van der Waals interactions with Pro 169, Ile 27, Ile 40, Ile 218, Trp 32, Trp 172, Hie 216, Arg 43 and Met 36.

One Hydroxyl Group
It seems that hydroxyl groups in positions 1 and 4 and in the alkyl side chain play a main role in the binding of compounds to the active site, and consequently in the antifungal activity [29]. For example, compounds 14 and 29 differ only in the nature of substituent at position 4, i.e., -OH in compound 14 and -OCH 3 in 29 and the docking scores are completely different: −8.9 and −6.5, respectively. To address the binding mode, three compounds exhibiting different antifungal activities were tested and the interactions with amino acid residues in the SDH active site were determined ( Figure 10). It seems that hydroxyl groups in positions 1 and 4 and in the alkyl side chain play a main role in the binding of compounds to the active site, and consequently in the antifungal activity [29]. For example, compounds 14 and 29 differ only in the nature of substituent at position 4, i.e., -OH in compound 14 and -OCH3 in 29 and the docking scores are completely different: −8.9 and −6.5, respectively. To address the binding mode, three compounds exhibiting different antifungal activities were tested and the interactions with amino acid residues in the SDH active site were determined ( Figure 10). The results indicate that the experimental antifungal activity correlates with the number of H-bonds formed, i.e., compound 20 (low activity, EC50 = 224 µ g/mL) forms two Hbonds, Tyr 58 and Trp 173; compound 23 (medium activity, EC50 = 130 µ g/mL) forms three H-bonds, Trp 173 and two with Tyr 58; compound 27 (high activity, EC50 = 59 µ g/mL)  It seems that hydroxyl groups in positions 1 and 4 and in the alkyl side chain play a main role in the binding of compounds to the active site, and consequently in the antifungal activity [29]. For example, compounds 14 and 29 differ only in the nature of substituent at position 4, i.e., -OH in compound 14 and -OCH3 in 29 and the docking scores are completely different: −8.9 and −6.5, respectively. To address the binding mode, three compounds exhibiting different antifungal activities were tested and the interactions with amino acid residues in the SDH active site were determined ( Figure 10). The results indicate that the experimental antifungal activity correlates with the number of H-bonds formed, i.e., compound 20 (low activity, EC50 = 224 µ g/mL) forms two Hbonds, Tyr 58 and Trp 173; compound 23 (medium activity, EC50 = 130 µ g/mL) forms three H-bonds, Trp 173 and two with Tyr 58; compound 27 (high activity, EC50 = 59 µ g/mL) The results indicate that the experimental antifungal activity correlates with the number of H-bonds formed, i.e., compound 20 (low activity, EC 50 = 224 µg/mL) forms two H-bonds, Tyr 58 and Trp 173; compound 23 (medium activity, EC 50 = 130 µg/mL) forms three H-bonds, Trp 173 and two with Tyr 58; compound 27 (high activity, EC 50 = 59 µg/mL) forms four H-bonds, Ser 39, Trp 173 and two with Tyr 58. Thus, it can be concluded that functional groups with H-bond forming atoms, both in the aromatic ring and side alkyl chain, favor the binding of compounds to the allosteric site of SDH and consequently enhance their antifungal activity.

Chemistry
All chemicals were obtained from Merck (Darmstad, Germany) or Aldrich (St. Louis, MO, USA) and used without further purification. The 1 H and 13 C NMR spectra were recorded on a Bruker Avance 400 Digital NMR spectrometer, operating at 400.1 MHz for 1 H and 100.6 MHz for 13 C. Samples were prepared in CDCl 3 and chemical shifts are reported in δ ppm, whereas coupling constants (J) are given in Hz. All signals were referenced to the residual peak of CHCl 3 at δ = 7.26 ppm and δ = 77.00 ppm for 1 H and 13 C, respectively. Lowresolution mass spectra were obtained on a VG Micromass ZAB-2F and high-resolution mass spectra on a VG Micromass ZAB-2F at 70 eV. Pure compounds were isolated from product reaction by column chromatography (CC), using silica gel (Merck 200-300 mesh). On the other hand, the degree of reaction was tested by thin layer chromatography (TLC) and using silica gel plates (HF-254). Spots were detected on TLC by heating after spraying with H 2 SO 4 (25% in H 2 O). Microwave syntheses were carried out in a microwave reactor (Monowave 200, Anton Parr, Graz, Austria) using sealed glass microwave containers with polyether ether ketone (PEEK) lids (capacity 10 or 30 mL).

Microwave-Assisted Electrophilic Aromatic Substitution Reaction
Coupling of geraniol and substituted phenols was carried out in presence of BF 3 ·OEt 2 as catalyst, acetonitrile as solvent and under microwave radiation. In a typical reaction, a phenol is dissolved in acetonitrile and then an excess of geraniol is added under constant stirring. Once the mixture is homogeneous, it is located into a microwave reactor at 75 • C and 40 W of power is applied for 10 min. At the end of this period, the power is switched off and the temperature decreased to 40 • C. The reacted mixture is slowly added to water (20 mL) and then extracted with ethyl acetate (EtOAc). The organic phase was washed with aqueous NaHCO 3 solution (10%, 20 mL) and water (3 times, 20 mL), dried over MgSO 4 and concentrated by reduced pressure distillation. Pure compounds were obtained by column chromatography using hexane/EtOAc mixtures of increasing polarity.

Antifungal Assays
Antifungal activity of all synthesized compounds was evaluated by determining mycelial growth inhibition of B. cinerea in the radial growth test. Assayed compounds in ethanol solution were added to potato dextrose agar (PDA) at 50 • C, reaching final concentrations in the range 10 to 250 ppm. The medium with dissolved geranylated phenols was poured into 50 mm diameter Petri dishes. After solidification, Petri dishes were inoculated with 4 mm diameter agar discs with thin mycelium of B. cinerea. Negative control containing only PDA culture medium, and positive control with BC-1000 at the same concentrations of tested compounds were prepared. Three replicates were made for each treatment. All dishes were incubated at 23 • C under 16 h light/8 h dark photoperiod. After 3 days, the mycelial growth diameter was measured and the percentage of mycelial inhibition (%I) was calculated. These values were plotted against fungicide concentration and fitted to a dose-response equation. This fitting gives EC 50 , the concentration at which mycelial growth is inhibited to 50% as compared to the negative control. Plotting of the data, fitting and EC 50 calculation were carried out with Origin 8.0. Significant differences were evaluated with a two-way analysis of variance (Tukey's test; p < 0.05).

Molecular Docking
The X-ray coordinates of SDH enzyme in complex with carboxin were obtained from the RCSB PDB database (PDB code 2FBW). PDB structures for docking were prepared using the Protein Preparation Wizard (Schrodinger, LLC, New York, NY, USA, 2018) that is accessible within the Maestro program (Maestro, version 11.8; Schrodinger, LLC: New York, NY, USA, 2018). Three-dimensional structures of ligands to be docked were generated and prepared using LigPrep as implemented in Maestro 11.5 (LigPrep, Schrodinger, LLC: New York, NY, USA, 2018). Docking of ligands was carried out using Glide program (Schrödinger, LLC, New York, NY, USA, 2021). Details used for sampling of group orientations, optimization of receptor, minimization on the ligand-protein complexes, docking of compounds and ligand poses have been given elsewhere [33,34]. Briefly, GlideScore-a modified version of ChemScore-was used to estimate binding affinity and rank ligands.
Ligands were docked using the extra precision mode (Glide XP), whereas XP Pose Rank was used to select the best-docked pose for each ligand.

Conclusions
Geranylated phenols with different functional groups in the aromatic ring were synthesized by direct coupling of phenol and geraniol under microwave irradiation. Yields of 21-32% were obtained in reduced reaction time (10 min). Hydration of geranylated phenols, using p-toluenesulfonic acid bound to a polymer matrix as catalyst, leads to compounds with one or two hydroxyl groups in the side alkyl chain with yields ranging from 10 to 36%.
The mycelium growth inhibition of B. cinerea induced by all synthesized geranylated phenols has been studied and EC 50 values have determined for all tested compounds. The data indicate that activity depends on the structure of the phenol ring and hydration degree of the side alkyl chain. The most active compound is compound 30, which has two hydroxyl groups in positions 1 and 4 and a mono hydrated side alkyl chain. A molecular docking study shows that binding energies of these compounds to the allosteric binding site of SDH are in the range of −9.2 to −5.4 kcal mol −1 . The results suggest that hydroxyl groups in positions 1 and 4 and in the side alkyl chain are important in the binding of compounds to the active site, and that the experimental antifungal activity correlates with the number of H-bonds that can be formed in the binding site.