Jasmonates are signaling molecules that are ubiquitous in higher plants. They are key regulators of plant growth and development, as well as of responses to both biotic and abiotic stresses , . Jasmonates are biosynthesized from α-linolenic acid, which, after several enzymatic steps, yields jasmonic acid [JA (1)] as the (3R,7S)-isomer (Scheme 1). However, some degree of epimerization at C7 may occur, and, moreover, the 7R epimer can be found within the cell . Like JA (1), JA-derivatives exist in planta as a mixture of the 7S and 7R epimers. For instance, compound JKL (7) has been described as the 7R epimer , . In response to environmental and developmental cues, JA (1) is activated by its conjugation to L-isoleucine (Ile) . This reaction produces the most active jasmonate (3R,7S)-N-jasmonoyl-L-isoleucine (JA-Ile, 4). The ω-oxidation of active JA-Ile (4) is carried out by members of the CYP94 family. This reaction produces the ω-hydroxylated acid 12-OH-JA-Ile (8) that is further converted into other ω-modified (12-modified) jasmonates (Scheme 1). Many 12-modified jasmonates (e.g. compounds 5–10, Scheme 1) exist in relatively high abundance (compared to other jasmonates) in various plant species ; however, little is known about their physiological roles. These chemical species may be stored in cell compartments such as vacuoles and may contribute to maintaining the homeostasis of jasmonates within plant tissues, which is of crucial importance for plant fitness and defense . On the one hand, naturally occurring 12-modified jasmonates are believed to be partially or completely inactive with regard to the canonical jasmonate pathway , . However, 12-OGlc-JA (5) was found to activate leaf-closing movement in Samaneasaman . The action mechanism of 12-OGlc-JA (5) is independent of the COI1-JAZ complex, the canonical jasmonate receptor. This suggests that endogenous 12-modified jasmonates may have biological roles beyond the classical jasmonate functions known to date. On the other hand, some synthetic 12-modified jasmonate derivatives have been described as inducers of plant defenses similarly to bioactive endogenous jasmonates . Furthermore, these derivatives were recently employed to uncouple growth and defense in Nicotiana attenuata plants, opening a whole new field of research with potential application in agriculture . Nonetheless, the production costs of 12-modified jasmonates (and jasmonates in general) must be sufficiently low if these compounds are to be employed in agricultural systems. The above-mentioned observations highlight the need to develop new synthetic strategies to prepare 12-modified jasmonates from a commercial product, such as methyljasmonate, in order to evaluate their mode of action and eventually to employ these compounds in agriculture.
The jasmonates’ biological relevance made them the target of several synthetic approaches aiming to obtain not only naturally occurring compounds but also synthetic analogs; all of these contributed greatly to our understanding of how these phytohormones act , , . A few studies have reported on the synthesis of 12-modified jasmonates , . However, because the procedures for synthesis are lengthy and provide low overall yield, access to these compounds for both bioassays and agricultural applications may be limited . To circumvent these limitations, a synthetic route to ω-modified jasmonates based on the cross-metathesis (CM) of alkenes was explored (Scheme 2). The preferential formation of the more stable E over the Z isomer and the generation of complex mixtures disadvantaged CM for years. Nowadays, at least the stereoselectivity in favor of the Z isomer is no longer an issue, thanks to the recent development of molybdenum- , , tungsten- , and ruthenium-based (commercially available, inexpensive, and versatile) Z-selective catalysts , .
Jasmonates (commercially available or synthetic) are commonly employed in bioassays as mixtures of all possible diastereoisomers – in particular, with respect to the stereocenters at the cyclopentanone ring , , . This is not at all disadvantageous, as cross-inhibition by the non-natural isomers or by the natural but less active (3R,7R) form does not occur. Therefore, inexpensive, commercially available MeJA (2) [a mixture of the (3R,7R), (3S,7S), (3S,7R), and (3R,7S) isomers in a ratio of approximately 45:45:5:5, respectively] was employed throughout this study in order to develop and optimize the CM method. And hence, all ω-modified jasmonates were prepared as a diastereomeric mixture unless otherwise stated.
2 Results and discussion
2.1 Optimization of the reaction conditions for CM
First, the CM reaction of MeJA (2) with 14a was investigated employing the Grubbs second-generation catalyst 11 (Scheme 3). Despite the broad functional group tolerance of 11 , the alcohol 14a barely reacted with 2, which meant that 2 could be recovered almost completely from the reaction mixture. This result may be explained by the reported low reactivity of homoallylic free alcohols in metathesis reactions . Therefore, the protected alcohol 14b was evaluated in the CM with 2 (Scheme 3). Catalyst 11 was active at loadings as low as 0.5 mol% (relative to 2) in the CM. Product 15b was obtained in 61% yield after 1 h using 1 mol% of 11 (Table 1, entry 2). To further optimize the reaction, solvents commonly employed in metathesis were screened (Table 1, entries 2, 4, and 5). All tested solvents performed similarly, but THF gave higher yields of 15b (Table 1, entry 2). Conveniently, the reaction slowly proceeded under solvent-free conditions, so the process was environmentally friendly (Table 1, entry 6). The performance of catalysts 12 (Hoveyda–Grubbs second generation) and 13 (Grubbs Z-selective) was also evaluated. Reactions were run in THF at 27°C with substrates at a concentration of approximately 0.2 M and 1 mol% catalyst loading in order to better assess differences between catalysts . Catalysts 11 and 12 produced similar yield towards 15b after 1 h of reaction (Table 1, entries 2 and 7). However, the Z-selective catalyst 13 needed a longer induction time (compared to 11 and 12), and for up to 1 h, no CM product was detected. This was unexpected, as 13 has been reported active in the homodimerization of monosubstituted alkenes at catalyst loadings as low as 0.1 mol% . The low reactivity of 13 may be explained by the nature of both substrate 2 and catalyst 13. MeJA (2) is a 1,2-disubstituted olefin with a bulky cyclopentanone ring that could favor the homodimerization of 14b over the CM product due to steric hindrance with the bulky adamantyl group of 13. Following this reasoning, both catalyst loadings of 13 and different ratios of the starting substrates 2 and 14b were screened (Table 2). It must be noted that under solvent-free conditions and at room temperature, 13 was active from a catalyst loading of 5 mol%, giving an excellent 95% of Z-selectivity (Table 2, entry 2). On the other hand, a six-molar excess of the terminal alkene 14b improved the yield toward 15b significantly, while the amount of 16 (homodimerization product of 2) did not exceed 5% (Table 2, entry 6). While increasing the temperature may speed up a CM reaction, it may also negatively influence the yield and/or the Z-selectivity of the desired CM product . Strikingly, increasing the temperature in the CM of 2 with 14b shortened reaction times from 24 h to 1 h and slightly improved the Z-selectivity for 15b (Table 2, entries 8–10). It is worth mentioning that at short reaction times, the self-metathesis product 16 was not observed.
2.2 Influence of the -R group of 14
The reaction of 2 with substrates 14b–d was studied with complexes 11 and 12 in order to generalize the method for these unselective but widely employed catalysts. As expected, 11 and 12 preferentially afforded the E-15b–d alkene (Table 3). Interestingly, the Z/E ratio of the CM product 15 was influenced by the nature of the protecting group. The amount of Z-15 increased about two-fold with 14d as the substrate (THP as the protecting group; Table 3, entry 3) compared to 14b (Table 3, entry 1). Therefore, the E/Z ratio of the CM-product 15 can be fine-tuned by choosing the best protective group in substrate 14. This observation may be useful in other CM reactions when the protection of alcohol groups is needed. Envisioning the possibility of directly installing more complex substituents at C12 (e.g. a sugar moiety as in jasmonate 5, Scheme 1), the CM of 2 and the more challenging substrate 14e (sterically hindered and more likely to chelate Ru) was explored. Under the employed conditions [13 (20 mol%), not optimized], the CM product 15e was obtained in 40% yield after purification and 60% of Z-selectivity. This result suggests that it is possible to install complex moieties at C12, such as a glycoside, by employing CM. Nonetheless, the preparation of a glycoside such as 12-OGlc-JA (5) might be made more efficient by the glycosylation of Z-3, which can be readily obtained from Z-15b (more than 90% Z-selectivity). Furthermore, it is reasonable to speculate that the CM of 2 and terminal alkenes bearing a protected sulfonic acid, aldehyde, or carboxylic acid group may offer an efficient way to prepare compounds 6, 9, or 10, respectively.
2.3 Gram scale preparation of Z-15b
Previous reports have shown that CM reactions perform better under a static vacuum, which helps to remove undesired volatile olefins from the reaction mixture . Following this idea, the CM of 2 and 14b was carried out under a dynamic vacuum in order to remove both 1-butene and ethylene formed during the CM (Scheme 4). The versatile jasmonate 15b (1.05 g, >80% after column purification) was obtained with more than 90% Z-selectivity. It is worth mentioning that 17b (a valuable 1,2-disubstituted, structurally symmetric alkene, as no ethylene is produced in self-metathesis ) was recovered and may be employed in further reactions. This CM allows for the gram-scale preparation of the valuable scaffold 15b, a scaffold which could be used as the substrate to prepare many other 12-modified jasmonates.
2.4 Preparation of 12-modified derivatives of JA-Ile (4)
Several 12-modified jasmonates exist in nature as conjugates with amino acids , , for example, compound 8 (Scheme 1). Therefore, it was briefly explored whether the preparation of such derivatives would be more efficient with the CM reaction preceding the conjugation to the amino acid (Scheme 2, ia–iia) or vice versa (Scheme 2, ib–iib). The Z-selective catalyst 13 (catalyst loading up to 7 mol%) failed to promote the CM reaction of JA-Ile (4) [mixture of diastereoisomers] and 14b. This result is consistent with the low activity reported for 13 catalyzing the CM of substrates bearing aldehyde or carboxylic acid groups . However, catalyst 11 promoted CM at a catalyst loading of 3 mol% (Figure S1). These results indicate that the CM reaction should precede the conjugation to amino acids when aiming the synthesis of Z-enriched 12-modified JA-amino acid conjugates.
The CM reaction described here can be conducted on the lab bench employing inexpensive, commercially available catalysts 11, 12, or 13, and the substrates MeJA (2) and 14b. The Z-selective catalyst 13 should be used to prepare the (Z)-jasmonates resembling the natural signals. In addition, (E)-enriched 12-modified derivatives can be prepared by employing catalyst 11 or 12. This feature may be of interest when conducting structure activity relationship (SAR) studies. The MeJA (2) employed in this study was a mixture of all possible diastereoisomers containing the (3R,7R)- and (3S,7S)-diastereoisomers (both trans- relative to the cyclopentanone ring) in a roughly 1:1 ratio. A similar 1:1 ratio of (3R,7R)- and (3S,7S)-diastereoisomers was found for two diastereomeric macrolactones prepared by using the CM method presented here as a key step . This result indicates that the CM reaction of substrates 2 and 14b is not stereocontrolled by the preexisting stereocenters on MeJA (2). Epimerization at C7 may occur both in vivo and when handling solutions of synthetic jasmonates ; therefore, our synthetic approach starting with inexpensive MeJA (2) is extremely useful.
In conclusion, an efficient synthetic approach which allows for the preparation of many 12-modified jasmonates has been developed based on olefin CM. Due to its efficiency and versatility, this method will likely be widely employed to prepare both naturally occurring and rationally designed 12-modified jasmonates of biological interest. The availability of such compounds (grams if needed) makes it possible to study their biological properties and, eventually, to apply them in agriculture. Additionally, the described method allows for the preparation of molecular probes in order to better understand a major signaling pathway in plants.
4 Experimental section
4.1 General experimental methods and materials
Reactions were carried out in oven-dried glassware. Flash-column chromatography was performed on silica gel 60 (230–400 mesh). Plates of silica gel 60 F254 were used for thin-layer chromatography. Compounds were visualized by soaking the plates in a potassium permanganate staining agent. All solvents, catalysts, and substrates 2, 14a, and 14b were commercially available and were used as received. Compounds 1 , 4 , 14c , 14d , and 14e  were prepared according to procedures described in the literature. Spectroscopic data from 15c and 15d matched those previously described , . Nuclear magnetic resonance (NMR) spectra were acquired at 400 MHz for 1H NMR and 100 MHz for 13C NMR. Chemical shifts were assigned relative to the residual solvent peak, which served as an internal standard. Gas chromatography-mass spectrometry (GC-MS) spectra were measured using electron impact (70 eV) as the ionization method in positive mode (EI+). High-resolution mass spectrometry (HRMS) electrospray ionization in positive mode (ESI+) was performed on high-resolution time-of-flight (TOF) equipment.
4.2 General procedure for CM reactions
CM reactions were carried out following the methodology described in . Briefly, the desired amount of catalyst was added to a 4 mL vial. The vial was sealed with a screw cap to which a rubber septum was attached and purged with argon for 5 min. A syringe was used to add first the solvent, then a mixture of the substrates. The reaction was allowed to stir in a heating block at the desired temperature. Aliquots of the reaction mixture were taken at the specified time periods. Samples for GC-MS analysis were obtained by adding 20 μL of the reaction to 0.5 mL of a solution of ethyl vinyl ether (3 mol L−1 in CH2Cl2); reactions under solvent-free conditions were stopped in a similar manner, but 1 μL aliquots were taken unless otherwise stated. The sample was shaken for 5 min and then analyzed via GC-MS. All reactions were performed in duplicate. Conditions for each particular experiment were as follows:
Screening catalyst loadings of 11, Table 1: 11 (0.9, 1.8, 3.7, or 9.0 mg; 1.1, 2.2, 4.4, or 11 μmol, respectively); THF (1 mL); 2 (50.6 mg; 0.22 mmol); 14b (24.9 mg; 0.22 mmol); 27°C.
Screening solvents, Table 1: 11 (1.8 mg; 2.2 μmol, 1 mol%); benzene, THF, toluene, or dichloromethane (1 mL); 2 (50.6 mg; 0.22 mmol); 14b (24.9 mg; 0.22 mmol); 27°C. For solvent free; 2 (101.2 mg; 0.44 mmol); 14b (49.8 mg; 0.44 mmol).
Screening catalysts 11, 12, and 13 at 1 mol%, Table 1: 11, 12, or 13 (2.2 μmol; 1 mol%; 1.8, 1.4, or 1.4 mg, respectively); THF (1 mL); 2 (50.6 mg; 0.22 mmol); 14b (24.9 mg; 0.22 mmol); 27°C.
Screening catalyst loadings of 13, Table 2: 13 (2.8, 13.9, or 27.8 mg; 4.4, 22, or 44 μmol, respectively); 2 (101.2 mg; 0.44 mmol); 14b (49.8 mg; 0.44 mmol); Neat; 27°C.
Screening substrate ratios, Table 2: 13 (6.0 mg, 9.5 μmol, 5 mol%); 14b/2 (0.1/0.2, 0.2/0.2, 0.4/0.2, or 1.2/0.2, mmol/mmol); Neat; 27°C. Experiments were conducted in 1.5 mL vials.
Screening temperatures, Table 2: 13 (6 mg, 9.5 μmol, 5 mol%); 2 (46.0 mg; 0.2 mmol); 14b (135.6 mg; 1.2 mmol); Neat; 27, 40, 60, or 80°C. Experiments were conducted in 1.5 mL vials.
Screening protecting groups, Table 3: 11 (2.0 mg, 2.4 μmol, 1 mol%); 2 (55.2 mg; 0.24 mmol); 14b, 14c, or 14d (27.1, 44.0, or 37.5 mg, respectively; 0.24 mmol). 12 (2.0 mg, 3.2 μmol); 2 (73.6 mg; 0.32 mmol); 14b, 14c, or 14d (36.2, 59.6, or 50.5 mg, respectively; 0.32 mmol). Reactions were carried out under solvent-free conditions in 1.5 mL vials. Aliquots of 2 μL were taken after 1 h for GC-MS analysis.
CM of JA-Ile (4) and 14b, Figure S1: 11 (6.0 mg, 7 μmol, 3 mol% relative to 4) or 13 (17 mg, 27 μmol, 7 mol% relative to 4); 4 (74.4 mg, 0.23 mmol); 14b (259.0 mg, 2.3 mmol); Neat; 60°C. The reaction was followed by thin-layer chromatography. Samples for GC-MS analysis were obtained by adding 2 μL of the reaction mixture to 500 μL of ethyl vinyl ether (3 mol L−1 in CH2Cl2), shaking for 5 min and derivatizing with trimethylsilyldiazomethane.
4.3 Synthetic procedures
4.3.1 Methyl (Z)-2-(2-(5-acetoxypent-2-en-1-yl)-3-oxocyclopentyl)acetate (Z-15b); gram-scale
A 10 mL Schlenk flask was charged with 2 (1.02 g, 4.5 mmol), 14b (3.05 g, 27 mmol, 6.0 equiv.) and catalyst 13 (0.14 g, 0.23 mmol, 5 mol%). The flask was connected to a vacuum line (400 mbar, dynamic vacuum), and gentle warming (45°C) was applied while the contents were being stirred. After 3 h, the reaction was quenched by the addition of 5 mL of a solution of ethyl vinyl ether (3 mol L−1 in CH2Cl2). Stirring continued for 30 min, at which time Pb(AcO)4 (1.5 equivalents relative to the catalyst) was added and the mixture stirred overnight. The reaction mixture was then filtered through a short pad of silica (approximately 10 g/0.05 mmol of catalyst) and eluted with four times 55 mL of CH2Cl2:AcOEt (10:1, v/v). The combined organic fractions were concentrated at reduced pressure and purified by flash chromatography on SiO2 (150 g) eluted with AcOEt:n-pentane (3:7, v/v). Product 15b (1.05 g; 83%, Z:E=93:7) was obtained as a pale yellow oil. GC-MS (EI+): m/z (%): 43 (100), 67 (43), 83 (52), 130 (57), 149 (83), 193 (20), 282 (3) [M]+; HRMS (ESI+-TOF): m/z=305.1354 [M + Na]+ (calc. for C15H22NaO5=305.1359); 1H NMR (CDCl3, 400 MHz): δ 5.39 (m; 2H), 4.00 (td; J=6.9, 6.9, 1.3 Hz; 2H), 3.63 (s; 3H), 2.63 (m; 1H), 2.40–2.12 (m; 8H), 2.12–1.9 (m; 1H), 1.98 (s; 3H), 1.86 (dt; J=10.3, 5.4, 5.4 Hz; 1H), 1.45 ppm (m; 1H); 13C NMR (CDCl3, 100 MHz): δ 218.7, 172.5, 171.1, 128.6, 127.0, 63.7, 53.9, 51.7, 38.7, 38.0, 37.7, 27.2, 26.9, 25.5, 20.9 ppm.
4.3.2 (2R,3R,4S,5R)-2-(acetoxymethyl)-6-(((Z)-5-(2-(2-methoxy-2-oxoethyl)-5-oxocyclopentyl)pent-3-en-1-yl)oxy)tetrahydro-2H-pyran-3,4,5-triyl triacetate (15e)
A two-neck flask fitted with a condenser was charged with 2 (59 mg, 0.26 mmol, 2 equiv.), 14e (53 mg, 0.13 mmol, α:β=1:1), THF (1 mL) and catalyst 13 (16.6 mg, 0.03 mmol, 20 mol% relative to 14e). A gentle stream of argon was passed through the solution, and the mixture was warmed to 45°C. The reaction was stirred overnight (approximately 18 h) and worked up as described for 15b [flash chromatography using AcOEt:n-hexane (1:1, (v/v)]. Product 15e (30 mg; 40%, Z:E=60:40) was obtained as a viscous oil. HRMS (ESI+-TOF): m/z=593.2181 [M + Na]+ (calc. for C27H38NaO13=593.2205); 1H NMR (CDCl3, 400 MHz): δ 5.44 (m; 2H), 5.19 (m; 1H), 5.03 (m; 2H),4.50 (d; J=2.9 Hz; 1H), 4.26 (m; 1H), 4.05 (m; 2H), 3.87 (m; 1H), 3.70 (m; 4H), 3.46 (m; 1H), 2.66 (m; 1H), 2.32 (m; 7H), 2.07 (m; 13H), 1.91 (m; 1H), 1.66 ppm (m; 1H); 13C NMR (CDCl3, 100 MHz): δ 218.9, 172.6, 170.8, 170.4, 169.6, 169.5, 128.4, 127.5, 100.9, 73.0, 71.9, 71.4, 69.6, 68.6, 62.1, 54.1, 51.8, 38.6, 38.1, 37.9, 27.4, 25.7, 25.7, 20.9, 20.8, 20.8, 20.8 ppm (β-anomer).
All experimental work of this study was supported by the Max Planck Society and the DFG (ChemBioSys, CRC 1127). Special thanks go to Kerstin Ploss for HRMS measurements and to Emily Wheeler for editorial assistance.
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Published Online: 2017-06-29
Published in Print: 2017-07-14