Catalytic Synthesis and Antifungal Activity of New Polychlorinated Natural Terpenes

1Laboratory of Biomolecular and Medicinal Chemistry, URAC 16, Faculty of Sciences Semlalia, Marrakech, Morocco 2Laboratoire de Chimie et Modélisation Mathématique, Faculté Polydisciplinaire de Khouribga, Université Hassan 1er, BP 145, 25000 Khouribga, Morocco 3Equipe de Chimie de Coordination et Catalyse, Département de Chimie, Faculté des Sciences Semlalia, Université Cadi Ayyad, BP 2390, 40001 Marrakech, Morocco 4Laboratory of Phytopathology, Genetics and Integrated Control (LPGLI), National Institute of Agricultural Research (INRA), P.O. Box 533, Marrakesh, Morocco


Introduction
Terpenes are naturally occurring molecules derived from isoprene units that have demonstrated therapeutic activities with a wide spectrum of biological activity, including antiinflammatory, antitumor, and fungicidal properties [1,2].They are considered as convenient starting materials for the synthesis of a large and diverse collection of bioactive compounds [3].Furthermore, they are commercially available, inexpensive, and generally accessible in enantiomeric forms [4].
Catalytic functionalization is a well-established process in organic synthesis and an efficient method for the preparation of high added value products [5][6][7][8], among which is catalyzed Kharasch addition or atom transfer radical addition (TMC ATRA) which is an effective tool for carbon-carbon bond formation and has gained considerable interest in organic synthesis during the last 50-60 years [9][10][11].On the other hand, it is a synthetically useful process for functionalizing organic compounds by means of halogen derivatives with a high level of atom economy.Kharasch addition is typically catalyzed by transition metal complexes of Cu, Ru, Fe, or Ni [10,[12][13][14].However, although the Kharasch addition of CCl 4 to simple alkenes has been extensively studied [12][13][14], to our knowledge no works have been devoted to the corresponding reactions using natural terpenic alkenes.
On the other hand, by 2050 food request is expected to increase up to 98%; this is due to the global population that has quadrupled over the last century [15].Thus more attention is required to protect agricultural product from different deleterious factors such as the contamination caused by Verticillium dahliae Kleb.(Vd) which is a broadly distributed vascular soil-borne pathogen that causes Verticillium wilt and it has a large host range, with over 300 plant species known to be susceptible to this fungal pathogen [16,17].Fusarium oxysporum f. sp.albedinis (Foa) is the most injurious fungus species affecting date palm (Phoenix dactylifera) especially in Morocco and Algeria [18] and Fusarium oxysporum f. sp.canariensis (Foc) that infect date palm (Phoenix canariensis) has now spread worldwide [19].These phytopathogenic fungi 2 Advances in Materials Science and Engineering are hard to control.Thus there is growing interest for discovering and developing a new antifungal agent especially using natural compound for plant protection.
In the course of our studies on monoterpenes and their derivatives, we have recently reported an extremely efficient method for the preparation of various allylic terpenic chlorides [14,[20][21][22][23].Following our catalysis objective, we became interested in the preparation of a new class of polychlorinated natural product using Kharasch addition of CCl 4 catalyzed by iron acetylacetonate and to evaluate their inhibition capacity of the growth of the three pathogenic microorganisms.

Materials and Methods
NMR studies were performed on a Bruker Avance 300 spectrometer in CDCl 3 ; chemical shifts are given in ppm relative to external TMS and coupling constant () in Hz.Mass spectra were recorded on AMD 402 spectrometer (70 eV, EI).The reaction mixtures were analyzed on a Trace GC Thermo Finnigan chromatograph equipped with an FID detector (flame ionization detector).GC parameters for capillary columns BP (25 m × 0.25 mm, SGE) were as follows: injector 250 ∘ C; detector 250 ∘ C; oven 70 ∘ C for 5 minutes and then 3 ∘ C/minute until 250 ∘ C for 30 minutes; column pressure 20 kPa, column flow 6.3 mL/minute; linear velocity 53.1 cm/s; total flow 138 mL/minute.Liquid chromatography was performed on silica gel (Merck 60, 220-440 mesh; eluent: hexane).Analytical thin-layer chromatography (TLC) was conducted on Merck aluminium plates with 0.2 mm of silica gel 60F-254.All the reagents and solvents used in the experiments were purchased from commercial sources and used as received without further purification (Aldrich, Acros).

General Procedure for Atom Transfer Radical Addition.
In a 15 mL rotaflow Schlenk, we mixed 1 mol% of Fe(acac) 2 (0.0414 mmol), 5 mol% of NEt 3 (20.9mg, 0.207 mmol), olefin (4.14 mmol), and CCl 4 (2.55 g, 16.56 mmol).The solvent was added to bring the total volume to 5 mL.The resulting solution was stirred at 80 ∘ C, and the reaction was monitored by TLC.The reaction was quenched by the slow addition of saturated aqueous Na 2 SO 3 .The layers were separated and the aqueous layer was extracted with CH 2 Cl 2 (2 × 10 mL).The combined organic layer was dried over anhydrous Na 2 SO 4 .The solvent was removed under reduced pressure.Pure chlorinated products were obtained by column chromatography over silica gel.All isolated pure products were fully characterized by 1 H and 13 C NMR and MS.[22]. 1

Experimental Protocol.
In 250 mL Erlenmeyer flasks, we prepared various solution under aseptic and sterilized condition.1.25, 2.5, 5, and 7.5 g/mL of each compound were previously dissolved in 0.1% dimethyl sulfoxide (DMSO); the solution was added to 100 mL of Czapek solid medium at 45 ∘ C and then poured into Petri dishes.
Pelt (trademark: Procida Groups Roussel Uclaf) which is a systemic fungicide containing 70% methyl thiophanate was used as a standard antifungal compound and was also prepared with the same concentrations.The control water is used under the same conditions.
Mycelial discs of 5 mm diameter, from young cultures of Fusarium oxysporum f. sp.albedinis (Foa), Fusarium oxysporum f. sp.canariensis (Foc), or Verticillium dahliae (Vd) which were provided by Laboratory LPGLI of INRA Marrakech, Morocco (Centre Regional de Recherche Agronomique), and maintained in Czapek medium, were disposed in the middle of the Petri dishes.Incubation was done at 28 ∘ C under continuous light.Mycelial growth of colonies was estimated every 2 days for 8 days of incubation by the average of two perpendicular diameters [24,25].The values represent the average of 5 independent experiments.
For measuring sporulation on different media, a single block of 5 mm diameter was cut out from the fungal colony near the margin by sterilized cork borer and was transferred to 100 mL sterile distilled water in a test tube, where it was mixed thoroughly to make a uniform spore suspension.Two dilutions of 10 −5 and 10 −6 were prepared from the mother solution.The fungal suspension was then stirred using a vortex for 20 seconds to release spores conidiophores.For each dilution we took 1 mL of the solution being distributed in a Petri dish.These experiments were repeated five times.

Evaluation of Mycelial Growth.
The inhibition rate of mycelial growth of three fungi was calculated using the following formula [24,25]: where  co = colony diameter of the control (water) and   = colony diameter of the test plate.

Evaluation of Sporulation.
We proceeded to count the total number of spores using a Malassez cell.The inhibition percentage of the sporulation is calculated using the following formula [24,26]: where  co = estimated number of germinated spores of the control (water) and   = estimated number of germinated spores of the test plate.

Statistical Analysis.
Calculations and comparisons of treatment means for each experiment were conducted using analysis of variance (ANOVA) and the SPSS 20.0 software; means were separated by Tukey Honestly Significant Difference (Tukey HSD) test at  = 0.05.

Results and Discussion
3.1.Chemical Result.The synthesis of the target compounds was accomplished using our previously optimized catalytic system for the Kharasch addition of tetrachloromethane to olefins under mild reaction conditions [14].Using various transition metal acetylacetonates M(acac)  (M = Zn, Co, V, Ni, Mo, Mn, Fe, and Cr) and triethylamine as cocatalyst and iron complex highlighting its potential as an effective catalyst, the reactions offer the halogen derivatives in moderate to good yields.Encouraged by these results, we were interested to apply this catalytic system to some mono-and sesquiterpenes in order to synthesize a new series of polyorganochloride products.The Kharasch addition reaction requires either a free radical precursor as the promoter or a metal complex as the catalyst.The classical free radical chain mechanism is operative not only with initiators such as light, heat, and peroxides [27][28][29] but also with a variety of metal complexes (Scheme 1) [27].The addition of tetrachloromethane to unsaturated terpenes can be obtained by treatment with CCl 4 , Fe(acac) 2 as catalyst, and NEt 3 in dry acetonitrile at 80 ∘ C. A possible mechanism for Kharasch reaction is shown in Scheme 1.As proposed by Murai et al. [30] and Kameyama and Kamigata [31] in ruthenium catalyzed Kharasch reaction, we can assume that low-valent iron species probably react in a similar way as Ru II catalysts.At this stage, we cannot exclude a multiple-step process that involves radical intermediates as described by Oldroydg et al. [29].
In a typical experiment, NEt 3 as cocatalyst [14] was added to a solution of Fe(acac) 2 in toluene at 80 ∘ C and after 15 min CCl 4 was added.The reaction mixture was stirred for 30 min and then terpene 1a-f was added.The progress of the reaction was monitored by TLC and GC-MS.The final mixture was filtered through anhydrous K 2 CO 3 and the filtrate was evaporated under vacuum.The polyorganochlorides 2a-f were then purified by flash column chromatography.Reaction times, chemical yields, and conversions are reported in Table 1.The addition of tetrachloromethane to -pinene 1a and -pinene 1b led to the formation of the adduct 2a in excellent yields (86% and 85%, resp.) after opening the cyclobutane ring.Limonene oxide 1c reacted selectively with CCl 4 and led to the corresponding adduct 2c in 63% yield.
In order to investigate the competition reaction between two carbon-carbon double bonds present in 1d and 1e, Kharasch addition was then carried out on limonene 1e to lead selectively towards the monoadduct product 2e in 77% yield, corresponding to the activation of the exocyclic double bond.Likewise, the reaction with carvone 1d leads to 2d in 68% yield (Table 1).The reaction of sesquiterpene was tested using valencene 1f and led to the corresponding product 2f in 74% yield.We characterized the structures of all products by 1 H-NMR, 13 C-NMR, and mass spectrometry.In addition, the structure of 2d was confirmed by X-ray crystallographic analysis [22,23].

Screening of Antifungal Activity In Vitro.
The compounds 2a-f and the reference drug were screened for their potential in vitro antifungal activity against three plant pathogenic  fungi (Fusarium oxysporum f. sp.canariensis (Foc), Fusarium oxysporum f. sp albedinis (Foa), and Verticillium dahliae (Vd)).The results are listed in Tables 2 and 3.
As shown in Table 2, almost all the title compounds 2a-f displayed activities in varying degrees against the tested fungi.We noticed also that the compounds are more active against the Foc and Foa than Vd.The rate of the inhibition varied among the synthesized molecules.The first bioassay using mycelia growth rate method indicated the following result.
For the F.o. f. sp.canariensis (Foc), we observed that compounds 2c and 2d had the highest activities at the concentration of 7.5 g/mL, the inhibition rate was around 90% and 57% ( < 0.001), respectively, and at the concentration of 5 g/mL the inhibition rate was 75% and 41%, respectively, while we noticed that for the concentrations 2.50 g/mL and 1.25 g/mL some of the compounds did not have any effect.
On the other hand, it seems from the results reported in Table 2 that compounds 2c and 2d also showed the greater inhibition rates (86%, 73% and 100%, 85%) ( < 0.001) at 5 g/mL and 7.5 g/mL, respectively, against F.o. f. sp.albedinis (Foa).Compound 2c exhibited parallel activity as pelt which is in our case a positive control.As for the evaluation of the in vitro activity of the compounds against Verticillium dahliae (Vd), the results, therefore, suggest that compounds  2c and 2d at the concentration of 7.5 g/mL exhibited the highest inhibition.
The second bioassay was designed to measure fungistatic and fungicidal effects of the compounds by using a sporulation inhibition test.The results of the bioassay were summarized in the tables (Table 3).It confirmed that almost all the compounds exhibited fungistatic actions against the tested fungi, the range of inhibition was from −35 to 100%, and the most potent activity was demonstrated against (Foa).This result provided useful information to study the structureactivity relationship for these new structures, shown in Table 1.
Thus, the findings demonstrate that compound 2c exhibited the highest fungicidal effect against all the fungus followed by compound 2d (Table 3).This can be due to the presence of oxygen atom in common limonene oxide 2c as an epoxy group and carvone 2d as a carbonyl group; hence the nature of the group in the monoterpene could lead to highest interaction with the biological target.For instance, the evaluation of antifungal activity for diterpene 2f reveals that this structure significantly decreases antifungal activity and also the inhibition of sporulation.
A large number of studies have been done in recent years on terpenoids as active components showing antifungal activities against various fungi [32].For instance, polygodial was found to exhibit a fungicidal activity against a food spoilage yeast [33]; the diterpenoids 16-hydroxy-cleroda-3,13-( 14)-Zdiene-15,16-olide and 16-oxo-cleroda-3,13-( 14)-E-diene-15oic acid also demonstrated significant antifungal activity [34], while methyl angolensate and 1,3,7-trideacetylkhivorin displayed the highest antifungal activity, with 62.8 and 64% mycelial growth inhibition at 1000 mg/L, respectively, against the plant pathogenic fungus Botrytis cinerea [35,36].The results from the present study indicate the antifungal nature of new polychlorinated natural terpenes which showed significant inhibition in linear growth and sporulation against the three fungi (Foc, Foa, and Vd).The antifungal activity of these new compounds was observed for the first time; however, detailed studies regarding its mode of action and efficacy under field condition are needed to be carried out.

Conclusion
The catalytic Kharasch reaction of -pinene, -pinene, limonene, limonene oxide, and valencene was studied using the iron acetylacetonate as catalyst and triethylamine as cocatalyst.The catalytic system showed to be efficient for the addition of carbon tetrachloride to all terpenes used.Regarding biological control of plant disease management, the antifungal bioassays showed that compounds 2c and 2d could constitute new lead compounds for further studies to develop antifungal agent against Fusarium oxysporum f. sp.albedinis, Fusarium oxysporum f. sp.canariensis, and Verticillium dahliae.

Table 2 :
Linear growth inhibition of (Foc, Foa, and Vd) under the effect of different concentrations of the polyorganochlorine derivatives (2a-f) in comparison with the control, after 8 days of incubation at 28 ∘ C on Czapek medium.Note.Foc: Fusarium oxysporum f. sp.canariensis, Foa: Fusarium oxysporum f. sp albedinis, and Vd: Verticillium dahliae.* * Data in the table are the averages of five replicates.Data followed by different lower case letters show statistically significant differences at  < 0.05 according to Tukey's HSD.

Table 3 :
Inhibition of the spore germination of Foc, Foa, and Vd under the effect of different concentrations of the polyorganochlorine derivatives (2a-f) in comparison with the control, after 48 h of incubation at 28 ∘ C. Data in the table are the averages of five replicates.Data followed by different lower case letters show statistically significant differences at  < 0.05 according to Tukey's HSD.