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Synlett 2009(8): 1307-1310  
DOI: 10.1055/s-0029-1217158
LETTER
© Georg Thieme Verlag Stuttgart ˙ New York

Total Synthesis of Emericellamide A: A Secondary Metabolite of Marine Cyclic Depsipeptide with Antimicrobial Properties

Jing-Yi Maa, Long-Fei Xua, Wen-Feng Huanga, Bang-Guo Wei*a, Guo-Qiang Lin*a,b
a Department of Chemistry, Fudan University, 220 Handan Road, Shanghai 200433, P. R. of China
Fax: +86(21)54237757; e-Mail: bgwei1974@fudan.edu.cn.;
b Institute of Biomedical Sciences, Fudan University, 138 Yixueyuan Road, Shanghai 200032, P. R. of China
Further Information

Publication History

Received 11 December 2008
Publication Date:
07 May 2009 (online)

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Abstract

Emericellamide A is a secondary metabolite of marine cyclic depsipeptide from the co-culture of the marine-derived fungus Emericella sp. and actinomycete Salinispora arenicola. A general method for the total synthesis of emericellamide A is depicted in this report.

Key words

cyclic depsipeptide - macrolactamization - antifungal agents - emericellamide - total synthesis

9

Preparation of Compound 8
To a suspension of pentyltriphenylphosphonium bromide (11.2 g, 27.18 mmol) in dry THF (70 mL) was dropped n-BuLi (15.9 mL, 1.6 M in hexane, 25.5 mmol) at r.t. under argon atmosphere. After the red mixture was stirred for 30 min, a solution of aldehyde 7 (1.80 g, 5.14 mmol) in dry THF (10 mL) was dropped and stirred for 2 h. The mixture was diluted with EtOAc and washed with sat. aq NaHCO3 and brine. Dried, filtered, and concentrated, the residue was purified by chromatography on SiO2 (EtOAc-PE = 1:150) to give major-8 (1.79 g, 86.5%) and minor-8 (135 mg, 6.5%) both as a colorless oil.
Analytical and Spectral Data of Polypeptide major-8 [α]D ²5 +9.5 (c 1.2, CHCl3). IR (film): νmax = 3000, 2928, 2856, 1464, 1406, 1252, 1097 cm-¹. ¹H NMR (400 MHz, CDCl3): δ = 7.36-7.27 (m, 5 H), 5.29-5.17 (m, 2 H), 4.47 (s, 2 H), 3.57 (dd, J = 5.09, 9.39 Hz, 1 H), 3.43 (dd, J = 3.88, 6.26 Hz, 1 H), 3.27 (dd, J = 7.82, 9.40 Hz, 1 H), 2.70-2.59 (m, 1 H), 2.08-1.92 (m, 3 H), 1.35-1.25 (m, 4 H), 0.99 (d, J = 7.04 Hz, 3 H), 0.92 (d, J = 7.05 Hz, 3 H), 0.90-0.87 (m, 12 H), 0.04 (s, 3 H), 0.03 (s, 3 H) pp.; ¹³C NMR (100 MHz, CDCl3, 25 ˚C, TMS): δ = 138.8, 133.8, 128.8, 128.2 (2 C), 127.5 (2 C), 127.3, 78.5, 72.9, 72.7, 38.4, 35.3, 31.9, 27.2, 26.2 (3 C), 22.5, 18.4, 17.4, 14.9, 14.0, -3.7, -3.9 ppm. ESI-MS: 427.3 [M + Na+]. HRMS (ESI): m/z calcd for [C25H44O2Si + Na+]: 427.3010; found: 427.3020.

13

Preparation of Ester 11
To a solution of alcohol 10 (1.50 g, 5.17 mmol), EDCI (4.93 g, 25.83 mmol) in dry CH2Cl2 (20 mL) was added N-Boc-Ala-OH (2.93 g, 15.50 mmol) at 0 ˚C under argon atmo-sphere and stirred for 0.5 h, then DMAP (315 mg, 2.58 mmol) was added in one portion. After being stirred for 3 h, the mixture was warmed to r.t. and stirred for 6 h at this temperature. The resulting mixture was diluted with CH2Cl2 (30 mL), the organic layer was separated, and the aqueous layer was extracted with CH2Cl2 (2 × 30 mL). The combined organic layers were washed with HCl (5%), sat. aq NaHCO3 and brine, then dried and concentrated. The residue was purified by chromatography on SiO2 (Et2O-PE = 1:50 to 1:30) to give ester 11 (1.35 g, yield 57%; 87% based on the recovered starting material) as a light yellow oil.
Analytical and Spectral Data of Ester 11
[α]D ²5 +8.25 (c 3.50, CHCl3). IR (film): νmax = 3373, 2966, 2930, 1716, 1497, 1454, 1366, 1167 cm-¹. ¹H NMR (600 MHz, CDCl3): δ = 7.36-7.26 (m, 5 H), 5.37-5.31 (m, 1 H), 5.14-5.08 (m, 2 H), 4.90-4.84 (m, 1 H), 4.46 (s, 2 H), 4.29 (dd, J = 6.74, 7.54 Hz, 1 H), 3.51 (dd, J = 4.56, 9.26 Hz, 1 H), 3.24 (dd, J = 7.28, 9.26 Hz, 1 H), 2.85-2.81 (m, 1 H), 2.24-2.08 (m, 1 H), 2.04-1.87 (m, 2 H), 1.44 (s, 9 H), 1.39 (d, J = 6.90 Hz, 3 H), 1.36-1.25 (m, 4 H), 0.99 (d, J = 6.72 Hz, 3 H), 0.91-0.85 (m, 6 H) ppm. ¹³C NMR (150 MHz, CDCl3): δ = 171.6, 155.1, 138.6, 131.0, 130.6, 128.3 (2 C), 127.6 (2 C), 127.5, 80.4, 79.7, 73.1, 71.5, 49.5, 35.5, 35.7, 33.3, 31.7, 28.3 (3 C), 27.3, 22.4, 16.3, 15.0, 14.0 ppm. ESI-MS: 484.3 [M + Na+]. HRMS (MALDI): m/z calcd for [C27H43NO5 + Na+]: 484.3039; found: 484.3035.

14

Preparation of Peptide 13
To a suspension of Pd/C (10%, 50 mg) in MeOH (10 mL) was added a solution of 11 (495 mg, 1.074 mmol) in MeOH (2 mL) at r.t. under H2 atmosphere (1.013 bar). After being stirred for 4 h at the same conditions, the catalyst was filtered carefully, and the filtrate was concentrated in vacuo to give the crude alcohol (400 mg, quant.) as a colorless oil.
[α]D ²5 -27.0 (c 1.31, CHCl3). IR (film): νmax = 3460, 3368, 2965, 2929, 1715, 1518, 1456, 1169 cm -¹. ¹H NMR (600 MHz, CDCl3): δ = 5.04 (d, J = 6.9 Hz, 1 H), 4.90 (d, J = 10.02 Hz, 1 H), 4.32-4.46 (m, 1 H), 3.60-3.56 (m, 1 H), 3.48-3.44 (m, 1 H), 2.47 (br s, 1 H), 1.88-1.83 (m, 1 H), 1.78-1.73 (m, 1 H), 1.45 (s, 9 H), 1.41 (d, J = 5.10 Hz, 3 H), 1.32-1.21 (m, 9 H), 1.01 (d, J = 6.96 Hz, 3 H), 0.92 (d, J = 6.84 Hz, 3 H), 0.88 (t, J = 7.00 Hz, 3 H) ppm. ¹³C NMR (150 MHz, CDCl3): δ = 174.8, 155.4, 80.1, 78.4, 64.1, 49.7, 36.9, 34.2, 33.7, 31.8, 29.4, 28.3 (3 C), 27.1, 22.6, 14.0 (2 C), 13.0 (2 C) ppm. ESI-MS: 396.3 [M + Na+]. HRMS (MALDI): m/z calcd for [C20H39NO5 + Na+]: 396.2726; found: 396.2730.
To a stirred solution of the above alcohol (400 mg, 1.07 mmol) in CCl4 (5 mL) were added MeCN (5 mL) and H2O (7.5 mL), then NaIO4 (916 mg, 4.28 mmol) and RuCl3 (10.8 mg, 0.04 mmol) were added sequentially at r.t. After being stirred for 4 h, the reaction was diluted with Et2O (20 mL) and stirred for 20 min to precipitate black RuO2. Then the mixture was dried and filtered through Celite, the solid residue was washed with Et2O, and the combined organic phases were concentrated in vacuo to give the carboxylic acid 12 without further purification. The crude acid 12 (415 mg, 1.07 mmol) and HOBt (159 mg, 1.18 mmol) were added in dry DMF (8 mL) and stirred for 15 min at r.t., then the reaction mixture was cooled to -15 ˚C, and EDCI (225 mg, 1.18 mmol) was added in one portion. After being stirred for 1.5 h at -15 ˚C, N-Gly-OBn (194 mg, 1.18 mmol) was added and stirred overnight at r.t. The mixture was quenched with H2O (20 mL) and extracted with Et2O (3 × 30 mL). The combined organic layers were washed with H2O, sat. aq NaHCO3 and brine. Dried and concentrated, the residue was purified by chromatography on SiO2 (EtOAc-PE = 1:5) to give 13 (458 mg, 80%) as a colorless oil.
Analytical and Spectral Data of Peptide 13 [α]D ²5 -9.56 (c 0.68, CHCl3). IR (film): νmax = 3323, 2929, 2929, 1744, 1716, 1525, 1456, 1174 cm-¹. ¹H NMR (600 MHz, CDCl3): δ = 7.40-7.33 (m, 5 H), 6.40 (m, 1 H), 5.21 (d, J = 12.24 Hz, 1 H), 5.17 (d, J = 12.24 Hz, 1 H), 5.09-5.06 (m, 1 H), 4.34-4.27 (m, 1 H), 4.09 (dd, J = 2.40, 4.74 Hz, 2 H), 2.71-2.67 (m, 1 H), 1.81-1.77 (m, 1 H), 1.42 (s, 9 H), 1.39 (d, J = 7.20 Hz, 3 H), 1.36-1.21 (m, 9 H), 1.16 (d, J = 7.08 Hz, 3 H), 1.15-1.10 (m, 1 H), 0.90-0.86 (m, 6 H) ppm. ¹³C NMR (150 MHz, CDCl3): δ = 173.7, 172.7, 170.1, 155.3, 135.2, 128.6 (2 C), 128.5 (2 C), 128.3, 79.8, 78.6, 67.2, 49.4, 43.6, 41.4, 34.3, 33.5, 31.8, 29.4, 28.3 (3 C), 26.8, 22.6, 14.7, 14.1 (2 C), 13.6 (2 C) ppm. ESI-MS: 557.3 [M + Na+]. HRMS (MALDI): m/z calcd for [C29H46N2O7 + Na+]: 557.3203; found: 557.3193.

15

Preparation of Peptide 15
To a solution of 13 (200 mg, 0.374 mmol) in CH2Cl2 (3 mL) was added TFA (3 mL) and stirred for 2 h at r.t. The solvent was removed in vacuo, and the residue was dissolved in DCE (3 mL) and removed. This process repeated for three times to give crude amide 14. HOBt (50.5 mg, 0.374 mmol) was added to a solution of N-Cbz-Val-Leu-Ala-OH (148 mg, 0.339 mmol) in dry DMF (5 mL) at -15 ˚C. After 30 min of stirring, EDCI (71.2 mg, 0.374 mmol) was added and stirred at -15 ˚C for 1.5 h, then NMM (82 µL, 0.745 mmol) and a solution of salt 14 in DMF was added. After being stirred for overnight at r.t., the reaction was quenched with H2O (15 mL) and extracted with EtOAc (3 × 30 mL). The combined organic extracts were washed with H2O, sat. aq NaHCO3 and brine. Dried and concentrated, the residue was purified by chromatography on SiO2 (MeOH-CH2Cl2 = 1:60) to give 15 (225 mg, 71%) as a white solid.
Analytical and Spectral Data of Polypeptide 15
[α]D ²5 -34.2 (c 0.32, CHCl3). IR (film): νmax = 3279, 2959, 2928, 1741, 1699, 1634, 1538, 1210 cm -¹. ¹H NMR (600 MHz, DMSO-d 6): δ = 8.24 (m, 1 H), 8.03 (m, 1 H), 7.92-7.87 (m, 2 H), 7.38-7.28 (m, 11 H), 5.11 (d, J = 12.72 Hz, 1 H), 5.09 (d, J = 12.72 Hz, 1 H), 5.03 (d, J = 14.49 Hz, 1 H), 5.01 (d, J = 14.49 Hz, 1 H), 4.94 (dd, J = 2.28, 9.48 Hz, 1 H), 4.33-4.23 (m, 3 H), 3.89-3.82 (m, 2 H), 3.77 (m, 1 H), 2.66 (ddd, J = 6.84, 13.58, 16.60 Hz, 1 H), 1.96-1.91 (m, 1 H), 1.71-1.65 (m, 1 H), 1.62-1.56 (m, 1 H), 1.43-1.38 (m, 2 H), 1.27-1.14 (m, 15 H), 1.04-0.99 (m, 1 H), 0.95 (d, J = 6.96 Hz, 3 H), 0.86-0.77 (m, 18 H) ppm. ¹³C NMR (150 MHz, DMSO-d 6): δ = 174.0, 172.0, 171.9, 171.6, 171.5, 170.3, 156.6, 137.6, 136.4, 128.9 (2 C), 128.8 (2 C), 128.5 (2 C), 128.4 (2 C), 128.2, 128.1, 79.6, 77.5, 66.3, 65.8, 60.7, 51.2, 48.2, 48.0, 42.1, 41.1, 33.8, 33.6, 31.7, 30.7, 29.3, 27.0, 24.5, 23.5, 22.5, 21.9, 19.7, 18.7, 18.6, 17.8, 14.6, 14.4, 13.6 ppm. ESI-MS: 874.5 [M + Na+]. HRMS (MALDI): m/z calcd for [C46H69N5O10 + Na+]: 874.4942; found: 874.4931.

17

To a suspension of Pd/C (10%, 15 mg) in MeOH was added a solution of 15 (105 mg, 0.123 mmol) in MeOH (3 mL) at r.t. under H2 atmosphere (1.013 bar). After being stirred for 3 h at the same conditions, the catalyst was filtered carefully, and the filtrate was concentrated in vacuo to give the crude product (73 mg, 95%) without further purification. The crude product was suspended in anhyd MeCN (120 mL, 1.0˙10-³ M) and cooled to 0 ˚C, pentafluorophenyl-diphenylphosphinate (FDPP) reagent (90 mg, 0.234 mmol) was added in one portion and stirred for 20 min at the same temperature. Then DIPEA (82 µL, 0.468 mmol) was slowly dropped over a period of 30 min. After beeing stirred for 26 h at r.t., the mixture was concentrated in vacuo. The residue was purified by RP C18 HPLC with 70% aq MeCN (Sepax-tech Amethyst C18 semipreparative column, 10 mm × 150 mm, 2 mL/min, refractive index detection) to give emericellamide A (37 mg, 51%).
Analytical and Spectral Data of Emericellamide A
[α]D ²5 -42.89 (c 0.26, MeOH) {lit.¹ [α]D ²5 -43 (c 0.23, MeOH); lit.4 [α]D ²5 -42.99 (c 0.2, MeOH)}. IR (KBr): νmax = 3323, 2931, 1755, 1668, 1650, 1538 cm-¹. ¹H NMR (600 MHz, DMSO-d 6): δ = 8.07 (d, J = 6.24 Hz, 1 H), 8.01 (br s, 1 H), 7.94 (d, J = 7.74 Hz, 1 H), 7.51 (br s, 1 H), 7.39 (d, J = 6.18 Hz, 1 H), 4.92 (dd, J = 2.40, 10.02 Hz, 1 H), 4.31 (dd, J = 5.73, 17.52 Hz, 1 H), 4.09-4.00 (m, 3 H), 3.97 (dd, J = 8.22, 8.28 Hz, 1 H), 3.61 (dd, J = 2.40, 17.52 Hz, 1 H), 2.85 (dq, J = 6.96, 9.81 Hz, 1 H), 1.91-1.84 (m, 1 H), 1.69-1.63 (m, 1 H), 1.59-1.52 (m, 2 H), 1.31-1.16 (m, 8 H), 1.23 (d, J = 7.38 Hz, 3 H), 1.21 (d, J = 6.96 Hz, 3 H), 1.15-1.08 (m, 1 H), 1.05-0.98 (m, 1 H), 0.89 (d, J = 6.78 Hz, 3 H), 0.88 (d, J = 6.84 Hz, 3 H), 0.87 (d, J = 7.08 Hz, 3 H), 0.86 (d, J = 7.03 Hz, 3 H), 0.83 (t, J = 6.94 Hz, 3 H), 0.82 (d, J = 7.08 Hz, 3 H), 0.80 (d, J = 6.48 Hz, 3 H) ppm. ¹³C NMR (150 MHz, DMSO-d 6): δ = 173.3, 171.9, 171.7, 171.6, 171.3, 169.2, 77.1, 60.5, 52.3, 48.7. 47.8, 42.9, 41.5, 33.9, 33.7, 31.6, 30.6, 29.3, 27.0, 25.0, 23.6, 22.5, 21.2, 19.5, 19.2, 18.7, 16.8, 14.8, 14.4, 13.4 ppm. ESI-MS: 610.4 [M + H+]. HRMS (MALDI): m/z calcd for [C31H55N5O7 + Na+]: 632.3999; found: 632.4004.