HETEROCYCLES

Note | Special issue | Vol. 82, No. 2, 2011, pp. 1675-1683
Received, 29th June, 2010, Accepted, 4th August, 2010, Published online, 6th August, 2010.
DOI: 10.3987/COM-10-S(E)80

■ An Efficient Synthesis of Antibiotic SF-2312 (3-Dihydroxyphosphoryl-1,5-dihydroxy-2-pyrrolidone)

Tadashi Hanaya* and Chika Itoh

Graduate School of Natural Science and Technology, Okayama University, 3-1-1 Tsushima-naka, Okayama 700-8530, Japan

Abstract

N-Benzyloxy-2-(diethoxyphosphoryl)pent-4-enamide (6) was prepared from ethyl diethoxyphosphorylacetate in a 3-step sequence. Oxidative cleavage of the terminal olefin of 6 with osmium tetroxide and sodium periodate afforded 1-benzyloxy-3-diethoxyphosphoryl-5-hydroxy-2-pyrrolidone (7). The first synthesis of racemic SF-2312 was achieved by treatment of 7 with trimethylsilyl bromide, followed by hydrogenolysis.

Phosphonic acid antibiotics containing a hydroxamic acid function have attracted considerable interest in medicinal chemistry because of their antimicrobial activities. For example, fosmidomycin (1)1 and its N-acetyl analog FR-900098 (2),2 isolated from Streptomyces lavendulae and S. rubellomurinus sp., respectively, were found to be potent inhibitors for the 1-deoxy-D-xylulose 5-phosphate reductoisomerase.3 In recent years, a number of analogs of these compounds have been synthesized owing to the investigation of structure-activity relationships and development of antimalarial agents.4,5

Meanwhile, SF-2312 (3), a phosphonic acid antibiotic active against Gram-positive and Gram-negative bacteria, was isolated from Micromonospora sp.6,7 Despite its unique structure in having a 1,5-dihydroxy-2-pyrrrolidone ring, attempts at preparation of 3 and the assignment of the stereochemistry have not been made so far. We describe herein the first, efficient synthesis of racemic SF-2312 (3) as a preliminary study for asymmetric synthesis of 3.
Ethyl diethoxyphosphorylacetate served as the starting material for preparation of the 2-phosphorylpent-4-enohydroxamate derivative (6), the key precursor for the 1,5-dihydroxy- 2-pyrrolidone ring formation (Scheme 1). The reported procedures8 for preparation of ethyl 2-(diethoxyphosphoryl)pent-4-enoate (4a) from ethyl diethoxyphosphorylacetate was slightly modified and thus 4a was obtained in an improved yield (72%) together with the diallyl-substituted compound (4b) (11%). Chemoselective hydrolysis of 4a in aqueous ethanol containing potassium hydroxide provided the corresponding pent-4-enoic acid (5) in a quantitative yield.

Condensation of 5 with O-benzylhydroxylamine in the presence of 1-ethyl-3-(3’-dimethylaminopropyl)¬carbodiimide (EDC) hydrochloride and 4-dimethylaminopyridine (DMAP) afforded the O-benzyl hydroxamate (6) in 88% yield. The same condensation in the presence of EDC hydrochloride, 1-hydroxybenzotriazole (HOBt), and N-methylmorpholine, provided 6 in a similar yield.

The 1,5-dihydroxy-2-pyrrolidone ring formation of 6 was carried out by the intramolecular hemiacetalization of the hydroxamate with the terminal aldehyde. Namely, the oxidative cleavage of the terminal olefin of 6 with osmium tetroxide and sodium periodate afforded the aldehyde intermediate, which was immediately cyclized to give the 5-hydroxy-2-pyrrolidone derivative (7) as a 65:35 diastereomeric mixture. The structural assignments of these isomers were made by 1H-NMR with the aid of 2D NOESY measurement and thus the cis form of the major isomer and the trans form of the minor one were confirmed (Table 1, Figure 1).
As the hemiacetal (hydroxylactam) moiety of 7 seemed to be susceptible to the conditions required for the following cleavage of the benzyl and phosphonic ester moieties, the 5-hydroxy group of 7 was protected as a methyl acetal (methoxylactam) by way of precaution. Namely, 7 was treated with trifluoroacetic acid (TFA) in methanol at 40 oC to give the 5-methoxy-2-pyrrolidone derivative (8) in 82% yield. The product was confirmed by NOE experiments to be a 28:72 mixture of cis and trans isomers in the reverse ratio to that of 7 (Table 1, Figure 1).

Hydrogenolysis of 8 in the presence of 10% Pd-C afforded the debenzylated product (9) and the same treatment of the hemiacetal compound (7) also yielded the deprotected compound (10) without formation of byproducts. Therefore protection of the hemiacetal moiety seems to be unnecessary for such a reductive condition. However cleavage of the phosphonic ester of 9 and 10 with trimethylsilyl bromide in dichloromethane resulted in the formation of an inseparable mixture of unidentified products and a minor amount of the desired product 3.
We thus attempted removal of protecting groups of 7 and 8 in the alternative sequence: the phosphonic ester of 7 was first cleaved by the treatment with trimethylsilyl bromide to afford 11 quantitatively. It is noteworthy that the similar treatment of 8 also provided 11 quantitatively, as a result of simultaneous cleavage of the methyl acetal. Judging from these results, it seems that the functional group necessary for protection of the 1,5-dihydroxy-2-pyrrolidone moiety against the action of trimethylsilyl bromide is not the 5-hydroxy but 1-hydroxy group.
The final removal of the benzyl group of 11 by hydrogenolysis furnished 3-dihydroxyphosphoryl-1,5-dihydroxy-2-pyrrolidone (3) as a diasteromeric mixture (cis/trans = 41:59).9 The NMR data of 3 were found to be identical with those reported for the natural product.6,7
The present work thus demonstrates the first synthesis of racemic SF-2322 (3) by a short and efficient route. Extension of this work including applications of these findings in synthesizing optically active SF-2322 is in progress.

EXPERIMENTAL
All reactions were monitored by TLC (Merck silica gel 60F, 0.25 mm) with an appropriate solvent system. Column chromatography was performed with Daiso Silica Gel IR-60/210w. Components were detected by spraying them with 20% sulfuric acid-ethanol or 20% phosphomolybdic acid-ethanol (with subsequent heating). The NMR spectra were meas­ured in CDCl3 with Varian Unity Inova AS600 (600 MHz for 1H, 151 MHz for 13C) and Mercury 300 (121 MHz for 31P) spectrometers at 23 °C, unless otherwise stated. Chemical shifts are reported as δ values relative to CHCl3 (7.26 ppm in CDCl3) and DOH (4.79 ppm in D2O) as an internal standard for 1H NMR, CDCl3 (77.0 ppm in CDCl3) and 1,4-dioxane (67.2 ppm in D2O) as an internal standard for 13C NMR, and 85% phosphoric acid (0 ppm) as an external standard for 31P NMR. The IR spectra were recorded on a Thermo Nicolet Avatar360. The MS spectra were measured on a VG-70SE instrument.

Ethyl 2-(diethoxyphosphoryl)pent-4-enoate (4a) and ethyl 2-diethoxyphosphoryl-2-(prop-2-enyl)pent-4-enoate (4b)
Modification of the literature procedures8 was made as follows. To a solution of ethyl diethoxyphophorylacetate (500 mg, 2.23 mmol) in dry THF (5 mL) was added sodium hydride (60% in oil, 98 mg, 2.45 mmol) at 0 oC. After stirring at the same temperature for 1 h, allyl bromide (0.210 mL, 2.40 mmol) was added and the mixture was stirred at rt for 12 h. The mixture was treated with saturated aqueous NH4Cl at 0 oC and then most of THF was distilled off in vacuo. The resulting solution was diluted with water and extracted with CHCl3 (3 x 10 mL). The organic layer was dried (Na2SO4) and evaporated in vacuo. The residue was separated by column chromatography with 3:1 AcOEt-hexane to give 4a [423 mg, 72% (lit.,8 47% yield by distillation)] and 4b (75 mg, 11%).
4a: Colorless oil; Rf = 0.43 (1:1 AcOEt-hexane); 1H NMR δ = 1.27 (3H, t, JCH2CH3 = 7.2 Hz, COCH2CH3) 1.33, 1.34 (3H each, 2t, JCH2CH3 = 7.1 Hz, POCH2CH3), 2.58, 2.70 (1H each, 2m, Ha,Hb-3), 3.01 (1H, ddd, J2,P = 22.3, J2,3a = 11.4, J2,3b = 4.0 Hz, H-2), 4.12–4.22 (4H, m, POCH2), 4.17 (2H, q, COCH2), 5.04 [1H, dq, J4,5E = 10.2, J3,5E = J5E,5Z = 1.5 Hz, H(E)-5], 5.11 [1H, dq, J4,5Z = 17.0, J3,5Z = 1.5 Hz, H(Z)-5], 5.76 (1H, ddt, J3a,4 = J3b,4 = 6.7 Hz, H-4); 31P NMR δ = 22.46.
4b: Colorless oil; Rf = 0.55 (1:1 AcOEt-hexane); 1H NMR δ = 1.28 (3H, t, JCH2CH3 = 7.1 Hz, COCH2CH3) 1.32 (6H, t, JCH2CH3 = 7.1 Hz, POCH2CH3), 2.67 (4H, ddt, J3,P = 14.8, J3,4 = 7.1, J3,5E = J3,5Z = 1.3 Hz, H-3), 4.12–4.17 (4H, m, POCH2), 4.21 (2H, q, COCH2), 5.09 [2H, ddt, J4,5E = 10.2, J5E,5Z = 2.3 Hz, H(E)-5], 5.11 [2H, dqt, J4,5Z = 17.1 Hz, H(Z)-5], 5.86 (2H, ddtd, 4J4,P = 0.6 Hz, H-4); 31P NMR δ = 22.91.

2-(Diethoxyphosphoryl)pent-4-enoic acid (5).
To a solution of 4 (500 mg, 1.89 mmol) in EtOH (5.0 mL) was added 10 M aq. KOH (0.28 mL, 2.8 mmol). The mixture was stirred at rt for 24 h and then neutralized with Amberlite IR-120 (H+). The resin was filtered off and the filtrate was evaporated in vacuo to give 5 (447 mg, quant) as a colorless syrup, which was used for the next step without further purification: Rf = 0.08 (1:1 AcOEt-hexane).
1H NMR δ = 1.33, 1.335 (3H each, 2t, JCH2CH3 = 7.1 Hz, CH2CH3), 2.52, 2.68 (1H each, 2m, Ha,Hb-3), 3.03 (1H, ddd, J2,P = 22.5, J2,3a = 11.0, J2,3b = 3.9 Hz, H-2), 4.16, 4.20 (2H each, 2m, CH2CH3), 5.04 [1H, dq, J4,5E = 10.3, J3,5E = J5E,5Z = 1.2 Hz, H(E)-5], 5.12 [1H, dq, J4,5Z = 17.0, J3,5Z = 1.2 Hz, H(Z)-5], 5.78 (1H, ddt, J3a,4 = J3b,4 = 6.6 Hz, H-4), 5.90 (1H, br s, CO2H); 31P NMR δ = 23.57

N-Benzyloxy-2-(diethoxyphosphoryl)pent-4-enamide (6).
A. With EDC-DMAP. To a solution of 5 (200 mg, 0.847 mmol) and O-benzylhydroxylamine ydrochloride (170 mg, 1.07 mmol) dissolved in dry CH2Cl2 (10 mL) were added DMAP (265 mg, 2.17 mmol) and EDC hydrochloride (200 mg, 1.04 mmol). After stirring for 36 h at rt, the mixture was diluted with CHCl3 (20 mL) and washed with 1M HCl (10 mL) and then brine (2 x 10 mL). The organic layer was dried (Na2SO4) and evaporated in vacuo. The residue was purified by column chromatography with 3:1 AcOEt-hexane to give 6 (253 mg, 88%) as a colorless syrup: Rf = 0.35 (AcOEt).
1H NMR δ = 1.31 (6H, t, JCH2CH3 = 7.1 Hz, CH2CH3), 2.52 (1H, m, Hb-3), 2.70 (2H, m, H-2, Ha-3), 4.11 (4H, m, CH2CH3), 4.90 (2H, s, CH2Ph), 5.04 [1H, dt, J4,5E = 10.3, J3,5E = J5E,5Z = 1.2 Hz, H(E)-5], 5.10 [1H, dt, J4,5Z = 16.9, J3,5Z = 1.2 Hz, H(Z)-5], 5.75 (1H, ddt, J3a,4 = J3b,4 = 6.4 Hz, H-4), 7.36 [3H, m, Ph(m,p)], 7.40 [2H, m, Ph(o)], 9.09 (1H, br s, NH); 13C NMR δ = 16.30 (d, 3JC,P = 4.5 Hz, CH2CH3), 16.33 (d, 3JC,P = 5.8 Hz, CH2CH3), 30.64 (C-3), 43.58 (d, 1J2,P = 130.1 Hz, C-2), 62.73 (d, 2JC,P = 5.8 Hz, CH2CH3), 63.32 (d, 2JC,P = 6.9 Hz, CH2CH3), 78.24 (CH2Ph), 117.35 (C-5), 128.45 [Ph(m)], 128.58 [Ph(p)], 129.12 [Ph(o)], 134.37 (d, 3J4,P = 15.0 Hz, C-4), 135.30 [Ph(ipso)], 165.20 (C-1); 31P NMR δ = 24.07. Anal. Calcd for C16H24NO5P: C, 56.30; H, 7.09. Found: C, 56.21; H, 7.20.
B. With EDC-HOBt. To a solution of 5 (120 mg, 0.508 mmol) and O-benzylhydroxylamine hydrochloride (100 mg, 0.627 mmol) dissolved in dry CH2Cl2 (5 mL) were added HOBt (83.0 mg, 0.614 mmol), N-methylmorpholine (140 mL, 1.27 mmol) and EDC hydrochloride (120 mg, 0.626 mmol). After stirring at rt for 24 h, the reaction mixture was purified by use of same procedures described above, giving 6 (147 mg, 85%).

1-Benzyloxy-3-diethoxyphosphoryl-5-hydroxy-2-pyrrolidone (7).
To a solution of 6 (57.6 mg, 0.169 mmol) in 50% aqueous 1,4-dioxane (5.0 mL) was added osmium tetroxide (4.4 mg, 0.017 mmol). The mixture was stirred at rt for 30 min and then sodium periodate (110 mg, 0.514 mmol) was added gradually. The mixture was stirred at rt for 1 h, diluted with water (15 mL), and extracted with CHCl3 (3 x 10 mL). The combined organic layers were washed with water (15 mL), dried (Na2SO4) and evaporated in vacuo. The residue was purified by column chromatography with 1:19 MeOH-CHCl3 to give a diastereomeric mixture (cis/trans = 65:35) of 7 (55.2 mg, 95%) as a colorless solid: Rf = 0.50, 0.43 (1:9 MeOH-CHCl3). Anal. Calcd for C15H22NO6P: C, 52.48; H, 6.46. Found: C, 52.60; H, 6.55.
cis-7: 1H NMR, see Table 1; 13C NMR δ = 16.28, 16.37 (2d, 3JC,P = 6.3 Hz, CH2CH3), 28.69 (d, 2J4,P = 4.0 Hz, C-4), 37.24 (d, 1J3,P = 135.9 Hz, C-3), 62.72, 65.16 (2d, 2JC,P = 6.3 Hz, CH2CH3), 78.31 (CH2Ph), 81.79 (3J5,P = 0 Hz, C-5), 128.46 [Ph(m)], 128.88 [Ph(p)], 129.56 [Ph(o)], 134.79 [Ph(ipso)], 164.19 (d, 2J2,P = 3.5 Hz, C-2); 31P NMR δ = 25.70.
trans-7: 1H NMR, see Table 1; 13C NMR δ = 16.32, 16.41 (2d, 3JC,P = 6.3 Hz, CH2CH3), 26.86 (d, 2J4,P = 3.5 Hz, C-4), 36.11 (d, 1J3,P = 148.6 Hz, C-3), 62.54, 63.33 (2d, 2JC,P = 6.3 Hz, CH2CH3), 77.94 (CH2Ph), 80.46 (d, 3J5,P = 9.2 Hz, C-5), 128.59 [Ph(m)], 129.00 [Ph(p)], 129.59 [Ph(o)], 134.98 [Ph(ipso)], 166.27 (d, 2J2,P = 3.5 Hz, C-2); 31P NMR δ = 23.26.

1-Benzyloxy-3-diethoxyphosphoryl-5-methoxy-2-pyrrolidone (8).
To a solution of 7 (50.6 mg, 0.147 mmol) in dry MeOH (4.0 mL) was added TFA (0.010 mL, 0.13 mmol). The mixture was stirred at 40 oC for 36 h and then triethylamine (0.20 mL) was added at rt. The mixture was evaporated in vacuo and the residue was purified by column chromatography with 1:19 MeOH-CHCl3 to give a diastereomeric mixture (cis/trans = 28:72) of 8 (43.2 mg, 82%) as a colorless syrup: Rf = 0.55, 0.49 (1:9 MeOH-CHCl3). Anal. Calcd for C16H24NO6P: C, 53.78; H, 6.77. Found: C, 53.89; H, 6.59.
cis-8: 1H NMR, see Table 1; 13C NMR δ = 16.37, 16.46 (2d, 3JC,P = 6.3 Hz, CH2CH3), 24.95 (d, 2J4,P = 3.5 Hz, C-4), 36.62 (d, 1J3,P = 144.5 Hz, C-3), 56.46 (MeO), 62.74, 63.40 (2d, 2JC,P = 6.3 Hz, CH2CH3), 78.02 (CH2Ph), 88.25 (d, 3J5,P = 4.7 Hz, C-5), 128.53 [Ph(m)], 128.96 [Ph(p)], 129.55 [Ph(o)], 134.77 [Ph(ipso)], 164.99 (d, 2J2,P = 4.8 Hz, C-2); 31P NMR δ = 21.92.
trans-8: 1H NMR, see Table 1; 13C NMR δ = 16.37, 16.46 (2d, 3JC,P = 6.3 Hz, CH2CH3), 25.70 (d, 2J4,P = 3.5 Hz, C-4), 36.20 (d, 1J3,P = 149.1 Hz, C-3), 56.82 (MeO), 62.44, 63.35 (2d, 2JC,P = 6.3 Hz, CH2CH3), 77.82 (CH2Ph), 87.99 (d, 3J5,P = 9.2 Hz, C-5), 128.56 [Ph(m)], 128.99 [Ph(p)], 129.76 [Ph(o)], 134.94 [Ph(ipso)], 165.43 (d, 2J2,P = 4.5 Hz, C-2); 31P NMR δ = 23.32.

3-Diethoxyphosphoryl-1-hydroxy-5-methoxy-2-pyrrolidone (9).
Compound 8 (24.3 mg, 0.0680 mmol) was dissolved in 50% aqueous 1,4-dioxane (5.0 mL) and 10% Pd-C (8.5 mg, 0.0080 mmol) was added. The mixture was stirred at rt for 3 h under atmospheric pressure of hydrogen. The catalyst was filtered off and the filtrate was evaporated in vacuo. The residue was purified by short-path column chromatography with 1:19 MeOH-CHCl3 to give a diastereomeric mixture (cis/trans = 30:70) of 9 (17.1 mg, 94%) as a colorless syrup: Rf = 0.46, 0.40 (1:9 MeOH-CHCl3). Anal. Calcd for C9H18NO6P: C, 40.45; H, 6.79. Found: C, 40.28; H, 6.88.
cis-9: 1H NMR, see Table 1; 13C NMR δ = 16.27, 16.33 (2d, 3JC,P = 6.3 Hz, CH2CH3), 24.75 (d, 2J4,P = 4.0 Hz, C-4), 36.77 (d, 1J3,P = 145.7 Hz, C-3), 56.57 (MeO), 62.93, 63.50 (2d, 2JC,P = 6.3 Hz, CH2CH3), 88.79 (d, 3J5,P = 5.2 Hz, C-5), 165.12 (d, 2J2,P = 4.5 Hz, C-2); 31P NMR δ = 22.65.
trans-9: 1H NMR, see Table 1; 13C NMR δ = 16.29, 16.36 (2d, 3JC,P = 6.3 Hz, CH2CH3), 25.59 (d, 2J4,P = 3.5 Hz, C-4), 36.58 (d, 1J3,P = 150.3 Hz, C-3), 57.10 (MeO), 62.55, 63.86 (2d, 2JC,P = 6.3 Hz, CH2CH3), 89.02 (d, 3J5,P = 9.2 Hz, C-5), 165.09 (d, 2J2,P = 4.0 Hz, C-2); 31P NMR δ = 24.15.

3-Diethoxyphosphoryl-1,5-dihydroxy-2-pyrrolidone (10).
By use of same procedures described for 9 from 8, compound 7 (26.6 mg, 0.0775 mmol) was treated with 10% Pd-C (8.7 mg, 0.0082 mmol) to give a diastereomeric mixture (cis/trans = 38:62) of 10 (17.9 mg, 91%) as a colorless syrup: Rf = 0.26, 0.19 (1:9 MeOH-CHCl3). Anal. Calcd for C8H16NO6P: C, 37.95; H, 6.37. Found: C, 37.79; H, 6.53.
cis-10: 1H NMR, see Table 1; 13C NMR δ = 16.26, 16.30 (2d, 3JC,P = 5.8 Hz, CH2CH3), 27.59 (d, 2J4,P = 4.5 Hz, C-4), 37.32 (d, 1J3,P = 145.1 Hz, C-3), 62.83, 64.60 (2d, 2JC,P = 6.3 Hz, CH2CH3), 82.11 (d, 3J5,P = 4.6 Hz, C-5), 164.74 (d, 2J2,P = 3.8 Hz, C-2); 31P NMR δ = 24.82.
trans-10: 1H NMR, see Table 1; 13C NMR δ = 16.30, 16.32 (2d, 3JC,P = 5.8 Hz, CH2CH3), 27.33 (d, 2J4,P = 4.1 Hz, C-4), 36.60 (d, 1J3,P = 149.1 Hz, C-3), 63.18, 63.45 (2d, 2JC,P = 6.3 Hz, CH2CH3), 81.76 (d, 3J5,P = 8.6 Hz, C-5), 165.53 (d, 2J2,P = 4.0 Hz, C-2); 31P NMR δ = 24.16.

1-Benzyloxy-3-dihydroxyphosphoryl-5-hydroxy-2-pyrrolidone (11).
A. From 7. Compound 7 (24.0 mg, 0.0699 mmol) was dissolved in dry CH2Cl2 (1.0 mL) and trimethylsilyl bromide (0.140 mL, 1.07 mmol) was added. The mixture was stirred at rt for 24 h and then evaporated in vacuo. The residue was dissolved in water (5 mL) and washed with AcOEt (10 mL). The aqueous layer was evaporated in vacuo to give a diastereomeric mixture (cis/trans = 39:61) of 11 (19.7 mg, 98%) as a pale yellow syrup, which was spectrometrically pure and was used directly for the next step: Rf = 0.55 (3:7 H2O-EtOH); 1H NMR (D2O), see Table 1. HRMS(FAB): m/z calcd for C11H15NO6P [M + H]+ 288.0637, found 288.0650.
B. From 8. By use of same procedures describe above, compound 8 (23.5 mg, 0.0657 mmol) was treated with rimethylsilyl bromide (0.140 mL, 1.07 mmol) at rt for 24 h, giving 11 (18.5 mg, 98%).

3-Dihydroxyphosphoryl-1,5-dihydroxy-2-pyrrolidone (3).
To a solution of 11 (19.7 mg, 0.0685 mmol) in water (1.5 mL) was added 10% Pd-C (9.0 mg, 0.0085 mmol). The mixture was stirred at rt for 2.5 h under atmospheric pressure of hydrogen. The catalyst was filtered off and the filtrate was diluted with water. The mixture was treated with active charcoal and filtered. The filtrate was evaporated in vacuo to give a diastereomeric mixture (cis/trans = 41:59) of 3 (13.0 mg, 96%) as a colorless hygroscopic syrup: Rf = 0.18 (3:7 H2O-EtOH); IR (neat) 3390 (OH), 1685 cm-1 (C=O). HRMS(FAB): m/z calcd for C4H9NO6P [M + H]+ 198.0168, found 198.0175. Anal. Calcd for C4H8NO6P·H2O: C, 22.34; H, 4.69. Found: C, 22.06; H, 4.82.
cis-3: 1H NMR (D2O), see Table 1; 13C NMR (D2O) δ = 27.70 (d, 2J4,P = 3.5 Hz, C-4), 38.84 (d, 1J3,P = 133.0 Hz, C-3), 82.44 (d, 3J5,P = 3.7 Hz, C-5), 169.35 (d, 2J2,P = 8.2 Hz, C-2); 31P NMR (D2O) δ = 17.70.
trans-3: 1H NMR (D2O), see Table 1; 13C NMR (D2O) δ = 26.64 (d, 2J4,P = 2.9 Hz, C-4), 38.20 (d, 1J3,P = 135.9 Hz, C-3), 82.01 (d, 3J5,P = 6.0 Hz, C-5), 169.52 (d, 2J2,P = 8.2 Hz, C-2); 31P NMR (D2O) δ = 18.19.

ACKNOWLEDGEMENTS
We are grateful to the SC-NMR Laboratory of Okayama University for the NMR measurements.

References

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9. Although 1H NMR spectrum of an isomeric mixture (ca. 2:3) of the natural 3 is shown in Ref. 6, the cis- and trans-structures of the isomers are not assigned. The isomeric structures of the synthetic 3 and 9–11 were assigned by comparison to the 1H NMR data for the isomers of 7 and 8 having the similar characteristic tendency of the corresponding coupling constants and chemical shifts (Table 1). The precise conformations of these compounds are currently under consideration.