Synlett 2010(16): 2498-2502  
DOI: 10.1055/s-0030-1258547
LETTER
© Georg Thieme Verlag Stuttgart ˙ New York

Practical Total Synthesis of Fuligocandins A and B

Midori A. Arai*, Junya Seto, Firoj Ahmed, Kento Uchiyama, Masami Ishibashi*
Graduate School of Pharmaceutical Sciences, Chiba University, Inage, Chiba 263-8522, Japan
e-Mail: mish@p.chiba-u.ac.jp; e-Mail: marai@p.chiba-u.ac.jp;
Further Information

Publication History

Received 15 July 2010
Publication Date:
26 August 2010 (online)

Abstract

Fuligocandins A and B, cycloanthranilylproline derivatives isolated from the myxomycete Fuligo candida, were synthesized efficiently by a Meyer-Schuster rearrangement, which gave Z-isomer with high selectivity. The absolute stereochemistry at C-11a of the natural products was determined to be S by comparison of the optical rotation of the natural products with synthetic counterparts derived from l-proline. This report is the first total synthesis of optically active fuligocandin B.

    References and Notes

  • 1 Nakatani S. Yamamoto Y. Hayashi M. Komiyama K. Ishibashi M. Chem. Pharm. Bull.  2004,  52:  368 
  • 2 Hasegawa H. Yamada Y. Komiyama K. Hayashi M. Ishibashi M. Sunazuka T. Izuhara T. Sugahara K. Tsuruda K. Masuda M. Takasu N. Tsukasaki K. Tomonaga M. Kamihira S. Blood  2007,  110:  1664 
  • 3 For a reported synthesis of fuligocandin A, see: More SS. Shanmughapriya D. Lingam Y. Patel NB. Synth. Commun.  2009,  39:  2058 
  • 4 For the synthesis of racemic fuligocandins A and B, see: Pettersson B. Hasimbegovic V. Bergman J. Tetrahedron Lett.  2010,  51:  238 
  • 5 For a reported Meyer-Schuster rearrangement of N-Boc lactam, see: Sun C. Lin X. Weinreb S. J. Org. Chem.  2006,  71:  3159 
  • 7 A Meyer-Schuster rearrangement of N-alkylated succinimide gave E-isomer as a main product: Mamouni A. Daïch A. Decroix B. Synth. Commun.  1998,  28:  1839 
6

NOE (5.0%) was observed between α-proton of α,β-unsaturated ketone and a proton at C-1 position in five-membered ring.

8

The ee was determined by a chiral phase HPLC; CHIRALPAK AD-RH, MeOH-H2O = 75:25, flow rate: 1 mL/min, λ = 254 nm detection, t R = 11.5 min [(ent)-1)], t R = 14.3 min (1) {or CHIRALPAK IA, hexane-i-PrOH = 4:1, flow rate: 0.5 mL/min, λ = 254 nm detection, t R = 18.6 min [(ent)-1)], tR = 29.7 min (1)}. Natural product 1: >99% ee.

9

Racemization was checked by comparing retention times of 8 with 9, 10 with 11 by CHIRALPAK AD-RH.

10

Optical rotation of (ent)-1 (52% ee): [α]¹8 D -224 (c = 1.4, MeOH).

11

Compound 1 which was synthesized at 0 ˚C was used for the synthesis of 2. Therefore, the ee of 2 was not decreased by this reaction. The ee was determined by a chiral phase HPLC; CHIRALPAK AD-RH, MeOH-H2O = 1:1, flow rate: 1 mL/min, λ = 370 nm detection, t R = 56.3 min [(ent)-2], t R = 68.1 min (2) {or CHIRALPAK IA, hexane- i-PrOH = 4:1, flow rate: 0.5 mL/min, λ = 370 nm detection, t R = 58.7 min [(ent)-2)], t R =79.1 min (2)}. Natural product 2: >99% ee.

12

Optical rotation of (ent)-2 (57% ee): [α]¹8 D -181 (c = 0.6, MeOH).

13

( S )-1-(2- tert -Butoxycarbonylaminobenzoyl)pyrrolidine-2-carboxylic Acid Methyl Ester (6): A solution of 3 (167 mg, 0.70 mmol), PyBOP (474 mg, 0.91 mmol), and DIPEA (0.48 mL, 2.80 mmol) in CH2Cl2 (7.0 mL) was cooled to 0 ˚C under an argon atmosphere. The mixture was stirred for 10 min, and then l-proline methyl ester hydrochloride (4, 151 mg, 0.91 mmol) was added. The reaction mixture was allowed to warm to r.t. and stirred for 1 h. The reaction mixture was quenched with 5% aq KHSO4 solution and extracted with EtOAc. The combined organic layers were washed with sat. aq NaHCO3 solution, brine, dried over Na2SO4, filtered and concentrated in vacuo. The residue was chromatographed on silica gel (hexane-EtOAc, 1:1) to afford 6 (210 mg, 0.60 mmol, 86%) as a white amorphous solid. Data for 6: ¹H NMR (400 MHz, CDCl3): δ = 8.37 (br s, 1 H), 8.18 (d, 1 H, J = 8.3 Hz), 7.36-7.40 (m, 2 H), 7.01 (t, 1 H, J = 7.6 Hz), 4.67-4.71 (m, 1 H), 3.79 (s, 3 H), 3.58-3.64 (m, 1 H), 3.48-3.54 (m, 1 H), 2.32-2.41 (m, 1 H), 1.95-2.08 (m, 2 H), 1.85-1.92 (m, 1 H), 1.50 (s, 9 H). ¹³C NMR (100 MHz, CDCl3): δ = 172.6, 168.8, 153.0, 137.3, 131.0, 127.2, 123.5, 121.6, 120.2, 80.3, 59.1, 52.5, 50.0, 29.4, 28.3, 25.3. IR (ATR): 3336, 2978, 1730, 1625, 1588, 1522, 1456, 1416, 1242, 1199, 1158 cm. EIMS: m/z = 348 [M+]. HRMS (EI): m/z [M+] calcd for C17H24N2O5: 348.1685; found: 348.1689.
(11a S )-10- tert -Butoxycarbonyl-2,3-dihydro-1 H -pyrrolo-[2,1- c ][1,4]benzodiazepine-5,11 (10H,11aH)dione (8):
To a stirred solution of 6 (210 mg, 0.60 mmol) in solvent (THF-MeOH-H2O = 3:1:1, 2.0 mL) was added LiOH×H2O (50 mg, 1.2 mmol) at 0 ˚C. The solution was stirred for 5 h at r.t. The reaction mixture was acidified with 1 M aq HCl to pH 2, then extracted with EtOAc. The organic layer was washed with brine, dried over Na2SO4, filtered and concentrated in vacuo to afford (S)-1-(2-tert-butoxycarbonyl-­aminobenzoyl)pyrrolidine-2-carboxylic acid (195 mg) as a white powder. To a solution of the carboxylic acid and PyBOP (406 mg, 0.75 mmol) in CH2Cl2 (20 mL) was added DIPEA (0.42 mL, 2.32 mmol) at 0 ˚C under an argon atmosphere. The reaction mixture was allowed to warm to r.t. and stirred for 72 h. The reaction mixture was quenched with 5% aq KHSO4 and extracted with EtOAc. The organic layer was washed with sat. aq NaHCO3 solution, brine, dried over Na2SO4, filtered and concentrated in vacuo. The residue was chromatographed on silica gel (hexane-EtOAc = 1:5) to afford 8 (148 mg, 0.47 mmol, 79% in 2 steps) as a white amorphous solid. Data for 8: [α]¹4 D +157 (c = 1.0, MeOH). ¹H NMR (400 MHz, CDCl3): δ = 7.91 (dd, 1 H, J = 1.5, 7.8 Hz), 7.51 (dt, 1 H, J = 1.5, 7.8 Hz), 7.41 (dt, 1 H, J = 1.2, 7.8 Hz), 7.23 (dd, 1 H, J = 1.2, 7.8 Hz), 4.06-4.15 (m, 1 H), 3.82-3.87 (m, 1 H), 3.52-3.59 (m, 1 H), 2.69-2.73 (m, 1 H), 2.09-2.16 (m, 1 H), 1.94-2.07 (m, 2 H), 1.46 (s, 9 H). ¹³C NMR (100 MHz, CDCl3): δ = 169.1, 164.9, 150.6, 135.3, 131.2, 130.9, 129.7, 127.4, 126.1, 84.6, 58.8, 46.6, 27.5, 26.3, 23.5. IR (ATR): 2979, 1773, 1748, 1716, 1646, 1457, 1416, 1238, 1144 cm. EIMS: m/z = 316 [M+]. HRMS: (EI): m/z [M+] calcd for C17H20N2O4: 316.1423; found: 316.1431.
(11a S )-10- tert -Butoxycarbonyl-2,3-dihydro-11-hydroxy-11-prop-1-ynyl-1 H -pyrrolo[2,1- c ][1,4]benzodiazepine-5,11 (10H,11aH)dione (10): To a solution of prepared LDA (9.0 mmol) in THF (90 mL) was added 1,2-dibromopropane (0.31 mL, 3.0 mmol) at -78 ˚C under an argon atmosphere. The solution was stirred for 10 min at this temperature, and another 20 min at 0 ˚C. The resulting lithium acetylide solution was then transferred to a THF solution of 8 (624 mg, 2.0 mmol) via cannula at -78 ˚C. The mixture was then stirred at -78 ˚C for 40 min and sat. aq NH4Cl solution was added slowly. The mixture was extracted with EtOAc and the organic extracts were dried over Na2SO4, filtered and concentrated in vacuo. The residue was chromatographed on silica gel (hexane-EtOAc, 2:1) to afford a single isomer 10 (329 mg, 0.92 mmol, 46%) as a white amorphous solid. Data for 10: ¹H NMR (400 MHz, CDCl3): δ = 8.44 (br s, 1 H), 8.21 (d, 1 H, J = 7.6 Hz), 7.38 (t, 1 H, J = 7.6 Hz), 7.02 (t, 1 H, J = 7.6 Hz), 4.80-4.84 (m, 1 H), 3.52-3.66 (m, 2 H), 2.30-2.40 (m, 1 H), 2.06 (s, 3 H), 1.83-2.11 (m, 3 H), 1.51 (s, 9 H). ¹³C NMR (100 MHz, CDCl3): δ = 185.6, 168.9, 153.0, 137.6, 131.2, 127.3, 123.1, 121.5, 120.2, 92.9, 80.4, 66.5, 29.7, 28.5, 28.3, 25.2, 4.3. IR (ATR): 3336, 2926, 2220, 1727, 1521, 1409, 1340, 1157, 756 cm. EIMS: m/z = 356 [M+]. HRMS (EI): m/z [M+] calcd for C20H24N2O4: 356.1736; found: 356.1730.
Fuligocandin A (1): To a solution of 10 (10 mg, 33.0 mmol) in CH2Cl2 (0.8 mL) was added TFA (0.8 mL) at -20 ˚C. The reaction mixture was stirred for 4 h at -20 ˚C. After addition of toluene, TFA was carefully evaporated in vacuo at r.t. To this mixture was added sat. aq NaHCO3 solution, then the mixture was stirred at r.t. The mixture was extracted with CH2Cl2 and the combined organic layers were dried over Na2SO4, filtered and concentrated in vacuo. The residue was chromatographed on silica gel (hexane-EtOAc = 1:1) to afford 1 (7.6 mg, 29.7 µmol, 90%, 70% ee). The ee value was determined by CHIRALPAK AD-RH (46 × 150 mm); eluent: 75% MeOH; flow rate: 1.0 mL/min; t R: (ent)-1 (14.4 min), 1 (22.6 min). Data for 1: [α]¹8 D +225 (c = 1.4, MeOH). ¹H NMR (400 MHz, CDCl3): δ = 12.6 (br s, 1 H), 7.97 (dd, 1 H, J = 1.5, 7.7 Hz), 7.45 (dt, 1 H, J = 1.5, 7.9 Hz), 7.21 (dt, 1 H, J = 1.5, 7.9 Hz), 7.02 (d, 1 H, J = 7.9 Hz), 5.29 (s, 1 H), 4.30 (d, 1 H, J = 7.9 Hz), 3.78-3.85 (m, 1 H), 3.61-3.69 (m, 1 H), 2.39-2.41 (m, 1 H), 2.20 (s, 3 H), 2.06-2.16 (m, 3 H). ¹³C NMR (100 MHz, CDCl3): δ = 198.4, 165.7, 158.8, 137.1, 132.4, 131.1, 127.2, 124.4, 122.1, 91.1, 55.3, 46.9, 30.0, 27.0, 23.4. IR (ATR): 2987, 2877, 1629, 1591, 1557, 1406, 1262, 1250, 754, 733, 696, 683 cm. EIMS: m/z = 256 [M+]. HRMS (EI): m/z [M+] calcd for C15H16N2O2: 256.1212; found: 256.1211.
Fuligocandin B (2): To a solution of prepared LDA (0.79 mmol) in THF (4.0 mL) was added 1 (51 mg, 0.20 mmol) in THF via cannula at -78 ˚C under an argon atmosphere. The solution was stirred for 30 min at this temperature. The resulting solution was then transferred to a THF solution of 20 (298 mg, 1.00 mmol) via cannula at -78 ˚C. The solution was stirred for 20 min at this temperature. The reaction mixture was allowed to warm to 0 ˚C. The reaction mixture was stirred for 1 h at 0 ˚C and H2O was then added. The mixture was extracted with EtOAc. The organic layer was washed with brine, dried over Na2SO4, filtered and concentrated in vacuo. The residue was chromatographed on silica gel (hexane-EtOAc = 10:1 → 1:1) to afford a white solid. The solution of white solid in solvent (THF-1 N aq HCl = 1:1, 5.0 mL) was stirred for 17 h at r.t. and sat. aq NaHCO3 was then added to it and the mixture was then extracted with EtOAc. The organic layer was washed with H2O, brine, dried over Na2SO4, filtered and concentrated
in vacuo. The residue was chromatographed on silica gel (hexane-EtOAc = 1:2 → 0:1) to afford 2 (48 mg, 0.12
mmol, 63%, 61% ee). The ee value was determined by CHIRALPAK AD-RH (46 × 150 mm); eluent: 50% MeCN; flow rate: 1.0 mL/min; t R: (ent)-2 (56.3 min), 2 (68.1 min). Data for 2: [α]¹8 D +248 (c = 0.6, MeOH). ¹H NMR (400 MHz, acetone-d 6): δ = 13.4 (br s, 1 H), 10.9 (br s, 1 H), 8.02 (d, 1 H, J = 8.3 Hz), 7.91 (d, 1 H, J = 15.8 Hz), 7.89 (dd, 1 H, J = 1.7, 7.8 Hz), 7.83 (d, 1 H, J = 2.7 Hz), 7.49-7.54 (m, 2 H), 7.17-7.24 (m, 3 H), 7.13 (d, 1 H, J = 8.1 Hz), 7.02 (d, 1 H, J = 15.8 Hz), 5.81 (s, 1 H), 4.44 (d, 1 H, J = 7.6 Hz), 3.67-3.72 (m, 1 H), 3.55-3.62 (m, 1 H), 2.56-2.63 (m, 1 H), 2.22-2.32 (m, 1 H), 2.05-2.16 (m, 2 H). ¹³C NMR (100 MHz, acetone-d 6): δ = 190.2, 165.8, 160.4, 138.7, 138.6, 134.9, 133.1, 131.9, 128.2, 126.4, 124.3, 123.8, 123.4, 122.6, 121.6, 121.2, 114.4, 113.0, 93.4, 56.1, 47.4, 27.6, 24.1. IR (ATR): 3250, 1590, 1567, 1270, 1247, 1137, 1106, 754, 738 cm. HRMS (ESI): m/z [M + Na]+ calcd for C24H21O2N3Na: 406.1514; found: 406.1526.
TRAIL-Resistance-Overcoming Activity: To assess the sensitivity to TRAIL, cell viability was evaluated using a fluorometric microculture cytotoxicity assay (FMCA). Briefly, AGS cells were seeded in a 96-well plate (6 × 10³ cells per well) in 200 µL of RPMI medium containing 10% FBS. Cells were cultured in 24 h and then treated with test samples in the absence or presence of TRAIL (100 ng/mL), and control cells were treated with culture medium containing 0.1% DMSO. After 24 h incubation, the cells were washed with PBS, and 200 µL of PBS containing fluorescein diacetate (10 µg/mL) was added to each well. After 1 h incubation at 37 ˚C, fluorescence was measured in a 96-well scanning spectrofluorometer (Thermo Electron Corporation) at 538 nm, following excitation at 485 nm.