Rice blast disease, caused by infection of rice blast fungus, Magnaporthe grisea (Hebert) Barr, is one of the most harmful diseases for rice . Several salicylaldehyde derivatives, such as pyriculol (2) , pyriculariol (3) , pyriculone (4) , and pyricuol (5) , have been isolated from the fungus as suspicious compounds responsible for the disease; they induce dark necrotic spot, when being applied to wounded rice leaves. In addition, o-formyl-m-hydroxycinnamic acid (6), probably further oxidized compound derived from 4, has also been found in the culture extract of the fungus  (Scheme 1). We have reported the synthesis of the derivatives 1–3, 5 using a common intermediate, 5-(2,2-dimethyl-4H-1,3-benzodioxin)methanol (7) (Scheme 2) [7–10]. In continuation of our synthetic studies of these compounds [7–13], the most oxidized derivative 6 was prepared for the first time from 7. Isolation and synthesis of o-carboxy-m-hydroxycinnamic acid, the related phytotoxin from other sources, has been reported [14–16]. Details of the synthesis are described in this report.
Results and discussion
We have already reported the preparation of compounds with the same carbon skeleton as 6, as intermediates towards the synthesis of pyricuol (5) [9, 10]. Thus, we chose the intermediate 7 as the starting material. Partial oxidation of 7 and the Horner-Wadsworth-Emmons reaction afforded ester 8, which was then reduced to give aldehyde 9 according to our procedure  (Scheme 3). At first, the aldehyde 9 was oxidized with Jones reagent to give acid 10, and then the acetonide protecting group was removed under acidic conditions. However, the resulting dihydroxy acid 11 could barely be purified because of its high hydrophilicity. Thus, we restarted the synthesis from the ester 8, and the acetonide group was removed under acidic conditions. The desired diol 12 was obtained as a colorless oil after silica gel purification in 45% yield. Then, the alcoholic hydroxy group was oxidized using MnO2 in DMSO/CHCl3  to give aldehyde 13 in 97% yield. The alkaline hydrolysis of the ester group was examined. The use of K2 CO3 or KOH in EtOH/H2 O resulted in a complex mixture. Finally, we found that LiOH in EtOH/H2 O was the best choice that afforded the target compound 6 as colorless needles (mp 132–133°C) in 59% yield. The overall yield was 26% from 8. The 1H NMR spectra of the natural product 6 and synthetic compound 6 were virtually identical.
o-Formyl-m-hydroxycinnamic acid, the most oxidized salicylaldehyde-type phytotoxin isolated from rice blast fungus, Magnaporthe grisea, was successfully synthesized for the first time using 5-(2,2-dimethyl-4H-1,3-benzodioxin)methanol as the starting material.
Melting point was measured on a Yanako MP-J3 instrument and is uncorrected. FT-IR spectra were recorded as films by a Jasco 4100 spectrometer (ATR, Zn-Se). 1H NMR spectra were recorded with a Varian 400 MR (400 MHz) spectrometer in CDCl3 with CHCl3 (δH 7.26 ppm) or CD3 OD with CD3 OH (δH 3.30 ppm) as internal standard. Mass spectra were recorded with a Jeol JMS-700 spectrometer. Merck silica gel 60 (70–230 mesh) was used for column chromatography. Merck silica gel 60 F254 (0.25 mm thickness) was used for TLC analysis.
Ethyl (E)-3-(3′-hydroxy-2′-hydroxymethylphenyl)ethenoate (12)
A solution of 8 [9, 10] (82.0 mg, 0.31 mmol) and p-TsOH×H2 O (21.0 mg, mmol) in THF/H2 O (ca. 1 mL) was stirred at room temperature for 3 days and then treated with a saturated aqueous solution of NaHCO3. The resulting mixture was extracted with EtOAc. The organic layer was washed with brine, dried (MgSO4), and concentrated in vacuo. The residue was chromatographed on silica gel (hexane/EtOAc, 3:1) to give 12 (31.1 mg, 0.14 mmol, 45%) as a colorless oil; Rf = 0.16 (hexane/EtOAc, 1:1); IR: ν 3450 (br. s, O–H), 2924 (m), 2854 (w), 1701 (w, C=O), 1640 (w), 1019 (s, C–O), 953 (m) cm-1; 1H NMR (CDCl3, 400 MHz): δ 7.90 (1H, d, J = 15.6 Hz, H-3), 7.70 (1H, s, ArOH), 7.22 (1H, pseudo t, J = 8.0 Hz, H-5′), 7.08 (1H, d, J = 8 Hz), 6.93 (1H, d, J = 8 Hz), 6.30 (1H, d, J = 15.6 Hz, H-2), 5.08 (2H, d, J = 5 Hz, CH2 OH), 4.26 (2H, q, J = 7 Hz, CH2 CH3), 2.24 (1H, br., CH2 OH), 1.34 (3H, t, J = 7 Hz, CH2 CH3). HR-FABMS. Calcd for C12 H12 O4 Na ([M+Na]+): m/z 245.0789. Found: m/z 245.0792.
Ethyl (E)-3-(2′-formyl-3′-hydroxyphenyl)ethenoate (13)
A suspension of 12 (31.1 mg, 0.14 mmol) and MnO2 (500 mg) in DMSO/CHCl3 (7:3, 10 mL) was stirred at room temperature for 5 h. The mixture was filtered through a Celite pad and the filtrate was concentrated in vacuo. The residue was chromatographed on silica gel (hexane/EtOAc, 2:1) to give 13 (30.0 mg, 0.14 mmol, 97%) as a pale yellow oil; Rf = 0.69 (hexane/EtOAc, 1:1); IR: ν 2982 (w), 2957 (w), 2925 (w), 1717 (s, C=O), 1651 (s), 1456 (m), 1335 (m), 1265 (m), 1184 (m), 1161 (m) cm-1; 1H NMR (CDCl3, 400 MHz): δ 11.92 (1H, s, HC=O), 10.38 (1H, s, OH), 8.22 (1H, d, J = 15.8 Hz, H-3′), 7.52 (1H, t, J = 8.0 Hz, H-5′), 7.06 (1H, d, J = 7.5 Hz), 7.02 (1H, d, J = 8.8 Hz), 6.37 (1H, d, J = 15.8 Hz), 4.26 (2H, q, J = 7.2 Hz, CH2 CH3), 1.34 (3H, t, J = 7.2 Hz, CH2 CH3). HR-EIMS. Calcd for C12 H12 O4 (M+.): m/z 220.0736. Found: m/z 220.0736.
(E)-3-(2′-Formyl-3′-hydroxyphenyl)ethenoic acid [(E)-o-formyl-m-hydroxycinnnamic acid] 5
A solution of 13 (20.0 mg, 0.091 mmol) and LiOH×H2 O (40.8 mg, 0.972 mmol) in THF/H2 O (3:1, 2 mL) was stirred at 0°C for 5 h. The solution was neutralized with citric acid and the mixture extracted with CH2 Cl2. The combined organic layer was concentrated in vacuo. The residue was chromatographed on silica gel (CHCl3/MeOH, 15:1) and crystallized from hexane/EtOAc to give 6 (10.2 mg, 0.053 mmol, 59%) as colorless needles; mp 132–133°C; Rf = 0.18 (CHCl3/MeOH, 15:1); IR: ν 3400 (br. s, O–H), 2948 (m), 2833 (w), 1653 (w), 1449 (w), 1021 (s) cm-1; 1H NMR (CD3 OD, 400 MHz): δ 10.46 (1H, s, HC=O), 8.31 (1H, d, J = 15.7 Hz, H-2), 7.54 (1H, t, J = 8 Hz, H-5′), 7.17 (1H, d, J = 8 Hz), 6.99 (1H, d, J = 8 Hz), 6.39 (1H, d, J = 15.7 Hz, H-3); 1H NMR (CDCl3, 400 MHz): δ 11.92 (1H, s, HC=O), 10.39 (1H, s, OH), 8.31 (1H, d, J = 15.8 Hz, H-3′), 7.54 (1H, t, J = 8 Hz, H-5′), 7.09 (1H, d, J = 8 Hz), 7.05 (1H, d, J = 8 Hz), 6.40 (1H, d, J = 15.7 Hz). HR-FABMS. Calcd for C10 H7 O4 ([M–H]–): m/z 191.0344. Found: m/z 191.0341.
Financial support by grant-in-aid from JSPS KAKENHI (numbers 17580092, 19580120, 22560112, and 25450144), the Agricultural Chemical Research Foundation, Intelligent Cosmos Foundation, and the Naito Foundation are gratefully acknowledged.
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Published Online: 2014-05-12
Published in Print: 2014-06-01