Total Synthesis of the Thiopeptide Promothiocin A
J. Am. Chem. Soc., Vol. 122, No. 14, 2000 3311
aqueous layer was further extracted with ethyl acetate (20 mL) and the
organic extracts were combined, washed sequentially with saturated
aqueous sodium hydrogen carbonate solution (25 mL) and brine (25
mL), dried (Na2SO4), evaporated in vacuo, and purified by flash
chromatography on silica, eluting with ethyl acetate-light petroleum-
triethylamine (30:10:1) to give the title compound (263 mg, 69%) as a
colorless solid, mp 88-90 °C. [R]D23 -14.4 (c 1.09, CHCl3); IR (CHCl3)
3442, 3404, 3126, 2985, 2936, 2877, 1716, 1770, 1506, 1369, 1163,
1101, 1055 cm-1;1H NMR (400 MHz, CDCl3) δ 8.31 (1 H, d, J ) 8.2,
Py-4-H), 8.07 (1 H, s, SCH), 8.05 (1 H, s, SCH), 7.86 (1 H, d, J )
9.0, CHCHNH), 7.64 (1 H, d, J ) 8.2, Py-5-H), 7.55 (1 H, d, J ) 8.0,
MeCHNH), 7.41-7.26 (6 H, m, PhH and CH2NH), 5.52 (1 H, dq, J )
8.0, 7.0, CHMe), 5.27 (1 H, m, NHBoc), 4.90 (1 H, m, CHMe), 4.76
(2 H, s, PyCH2), 4.67 (2 H, s, PhCH2), 4.58 (1 H, dd, J ) 16.8, 5.8,
CHHNH), 4.54 (1 H, dd, J ) 9.0, 7.0, CHCHNH), 4.40 (1 H, dd, J )
16.8, 5.4, CHHNH), 4.37 (2 H, q, J ) 7.2, OCH2Me), 2.55 (3 H, s,
Me), 2.29 (1 H, m, Me2CH), 2.24 (3 H, s, Me), 1.71 (3 H, d, J ) 7.0,
CHMe), 1.47 (3 H, d, J ) 6.9, CHMe), 1.40 (9 H, s, CMe3), 1.36 (3 H,
t, J ) 7.2, CH2Me), 1.02 (3 H, d, J ) 7.0, MeCHMe), 1.00 (3 H, d, J
) 7.0, MeCHMe); 13C NMR (100 MHz, CDCl3) δ 173.6 (C), 171.3
(C), 164.9 (C), 163.2 (C), 161.2 (C), 161.13 (C), 161.07 (C), 160.4
(C), 157.5 (C), 154.9 (C), 154.2 (C), 149.3 (C), 148.5 (C), 148.2 (C),
147.1 (C), 138.5 (CH), 137.7 (C), 132.7 (C), 128.7 (C), 128.5 (CH),
127.9 (CH), 127.8 (CH), 127.5 (C), 127.3 (CH), 125.3 (CH), 120.5
(CH), 79.8 (C), 73.1 (CH2), 72.7 (CH2), 61.4 (CH2), 58.5 (CH), 46.9
(CH), 44.7 (CH), 36.6 (CH2), 31.2 (CH), 28.3 (Me), 21.2 (Me), 20.2
(Me), 19.4 (Me), 18.1 (Me), 14.3 (Me), 11.6 (Me), 11.0 (Me); m/z
(electrospray) 970 (MH+, 65%), 870 (12), 793 (8), 631 (18), 509 (11),
508 (10), 463 (40), 458 (29), 455 (15), 149 (54), 91 (30); HRMS calcd
for C47H56N9O10S2 (MH) 970.3592, found 970.3564.
aqueous layer was further extracted with ethyl acetate (20 mL) and the
organic extracts were combined, washed with brine (20 mL), dried
(Na2SO4), and evaporated in vacuo to afford the crude pentafluorophenyl
ester (240 mg, quant) that was used without further purification. A
solution of hydrogen chloride in dioxane (4.0 M; 8 mL) was added to
a stirred solution of the pentafluorophenyl ester (240 mg, 208 µmol)
in dry dioxane (15 mL) at room temperature. The mixture was stirred
for 3.5 h and dissolved in chloroform (150 mL). Aqueous potassium
hydrogen carbonate solution (1.0 M; 250 mL) was added and the
mixture was shaken vigorously for 5 min and then separated. The
aqueous layer was further extracted with chloroform (2 × 50 mL) and
the organic extracts were combined and dried (Na2SO4). Triethylamine
(0.60 mL, 4.2 mmol) was added and the solution was stirred at room
temperature for 4 days, evaporated in vacuo, and purified by flash
chromatography on silica, eluting with ethyl acetate-chloroform-
triethylamine (40:10:1), to give the title compound (94 mg, 55%) as a
colorless solid, mp 110-111 °C. [R]D25 +7.0 (c 1.2, CHCl3); IR (CHCl3)
3401, 3123, 2970, 2934, 2875, 1666, 1538, 1374, 1102, 1048, 998
cm-1;1H NMR (400 MHz, CDCl3) δ 8.43 (1 H, d, J ) 8.2, Py-4-H),
8.10 (1 H, s, SCH), 7.85 (1 H, s, SCH), 7.76 (1 H, d, J ) 8.5,
MeCHNH), 7.75 (1 H, d, J ) 8.5, CHCHNH), 7.67 (1 H, d, J ) 8.2,
Py-5-H), 7.44-7.29 (6 H, m, PhH and MeCHNH), 6.88 (1 H, t, J )
5.6, CH2NH), 5.43 (2 H, m, 2 CHMe), 4.81 (1 H, d, J ) 14.2, PyCHH),
4.75 (1 H, d, J ) 14.2, PyCHH), 4.70 (2 H, s, PhCH2), 4.55 (1 H, dd,
J ) 16.8, 5.6, CHHNH), 4.49 (1 H, dd, J ) 16.8, 5.6, CHHNH), 4.45
(1 H, dd, J ) 8.5, 5.8, CHCHNH), 2.59 (3 H, s, Me), 2.53 (1 H, m,
Me2CH), 2.46 (3 H, s, Me), 1.67 (3 H, d, J ) 6.9, CHMe), 1.60 (3 H,
d, J ) 7.0, CHMe), 1.08 (3 H, d, J ) 6.9, MeCHMe), 1.07 (3 H, d, J
) 6.7, MeCHMe); 13C NMR (100 MHz, CDCl3) δ 171.3 (C), 171.2
(C), 164.4 (C), 163.1 (C), 161.6 (C), 160.9 (C), 160.4 (C), 160.1 (C),
157.6 (C), 153.8 (C), 148.69 (C), 148.67 (C), 148.6 (C), 148.4 (C),
138.4 (CH), 137.7 (C), 132.9 (C), 128.8 (C), 128.5 (CH), 127.9 (CH),
127.8 (CH), 127.5 (C), 125.4 (CH), 124.7 (CH), 120.6 (CH), 73.2 (CH2),
72.7 (CH2), 59.0 (CH), 46.0 (CH), 43.2 (CH), 36.3 (CH2), 29.9 (CH),
20.2 (Me), 19.7 (Me), 19.5 (Me), 17.8 (Me), 11.6 (Me), 11.2 (Me);
MS (EI) m/z (relative intensity) 823 (M+, 20%), 731 (6), 717 (12),
362 (3), 256 (7), 138 (17), 105 (63), 91 (100), 77 (51); HRMS calcd
for C40H41N9O7S2 (M) 823.2570, found 823.2593.
(g) N-(tert-Butoxycarbonyl)-Protected Linear Peptide Carboxylic
Acid 42. Lithium hydroxide monohydrate (54 mg, 1.30 mmol) was
added to a solution of N-(tert-butoxycarbonyl)-protected linear peptide
ethyl ester 41 (208 mg, 0.22 mmol) in THF-water (5:1) (20 mL) at
room temperature. The reaction was stirred overnight, evaporated in
vacuo, and partitioned between water (30 mL) and ether (30 mL). The
aqueous layer was separated, acidified to pH 2.5 with aqueous citric
acid solution (10%), and extracted with ethyl acetate (3 × 25 mL).
The organic layers were combined, washed, with brine (25 mL), dried
(Na2SO4), and evaporated in vacuo to give the title compound (214
mg, 94%) as a colorless solid, mp 122-123 °C. [R]D25 -14.0 (c 1.15,
CHCl3); IR (CHCl3) 3439, 3401, 3125, 2984, 2932, 2875, 1730, 1715,
1667, 1637, 1510, 1455, 1377, 1165, 1099, 1045 cm-1;1H NMR (400
MHz, CDCl3) δ 8.34 (1 H, d, J ) 8.2, Py-4-H), 8.12 (1 H, s, SCH),
8.10 (1 H, s, SCH), 7.97 (1 H, d, J ) 8.9, CHCHNH), 7.73 (1 H, d, J
) 7.0, MeCHNH), 7.65 (1 H, m, CH2NH), 7.64 (1 H, d, J ) 8.2, Py-
5-H), 7.41-7.27 (5 H, m, PhH), 5.82 (1 H, br s, CO2H), 5.51 (1 H, m,
CHMe), 5.41 (1 H, br s, NHBoc), 4.93 (1 H, m, CHMe), 4.76 (2 H, s,
PyCH2), 4.68 (2 H, s, PhCH2), 4.57 (1 H, m, CHCHNH), 4.55 (1 H,
dd, J ) 16.8, 5.3, CHHNH), 4.47 (1 H, dd, J ) 16.8, 5.3, CHHNH),
2.54 (3 H, s, Me), 2.29 (1 H, m, Me2CH), 2.24 (3 H, s, Me), 1.68 (3
H, d, J ) 6.9, CHMe), 1.48 (3 H, d, J ) 6.9, CHMe), 1.39 (9 H, s,
CMe3), 1.01 (3 H, d, J ) 6.4, MeCHMe), 1.00 (3 H, d, J ) 6.4,
MeCHMe); 13C NMR (100 MHz, CDCl3) δ 173.4 (C), 171.5 (C), 164.8
(C), 163.5 (C), 162.7 (C), 161.3 (C), 161.1 (C), 160.3 (C), 157.9 (C),
155.1 (C), 154.2 (C), 149.0 (C), 148.3 (C), 146.7 (C), 138.5 (CH),
137.7 (C), 132.6 (C), 128.6 (C), 128.5 (CH), 128.1 (CH), 127.9 (CH),
127.8 (CH), 127.6 (C), 125.8 (CH), 120.6 (CH), 79.9 (C), 73.1 (CH2),
72.6 (CH2), 58.6 (CH), 46.8 (CH), 44.7 (CH), 36.5 (CH2), 31.1 (CH),
28.3 (Me), 21.3 (Me), 20.1 (Me), 19.3 (Me), 18.2 (Me), 11.6 (Me),
11.0 (Me), one C unobserved; MS (FAB) m/z (relative intensity) 941
(M+, 41%), 841 (30), 765 (6), 603 (28), 481 (18), 391 (48), 311 (11),
149 (100); HRMS calcd for C45H51N9O10S2 (M) 941.3200, found
941.3251.
Alternative Macrocyclization Strategy. (a) Valine-Oxazole Dipep-
tide Methyl Ester Hydrochloride 44. Acetyl chloride (0.49 mL, 6.90
mmol) was added dropwise to a stirred solution of the N-(tert-
butoxycarbonyl)-protected dipeptide 36 (127 mg, 0.35 mmol) in dry
methanol (10 mL) at room temperature. The mixture was stirred
overnight and evaporated in vacuo to give the title compound (ca. 100%)
as a pale yellow foam used directly in the next step.
(b) Pyridine 45. The oxazole-pyridine-thiazole carboxylic acid
40 (158 mg, 0.29 mmol) was dissolved in dry THF (5 mL) at 0 °C,
and treated successively with N-methylmorpholine (80 µL, 0.73 mmol)
and isobutyl chloroformate (4l µL, 0.32 mmol). The mixture was stirred
at 0 °C for 30 min, a solution of the amine hydrochloride salt 44 (ca.
0.35 mmol) in dry THF (2 mL) was added, and the mixture was stirred
for 1 h. Workup as above (for compound 41) and flash chromatography
on silica, eluting with ethyl acetate-light petroleum-triethylamine (30:
25
10:1), gave the title compound (110 mg, 48%) as a colorless oil. [R]D
-11.0 (c 1.06, CHCl3); IR (CH2Cl2) 3483, 3395, 3124, 2987, 2934,
2877, 1720, 1664, 1538, 1497, 1369, 1102 cm-1;1H NMR (400 MHz,
CDCl3) δ 8.31 (1 H, d, J ) 8.2, Py-4-H), 8.14 (1 H, s, SCH), 7.85 (1
H, d, J ) 8.5, MeCHNH), 7.72 (1 H, t, J ) 5.6, CH2NH), 7.63 (1 H,
d, J ) 8.2, Py-5-H), 7.36-7.26 (6 H, m, PhH and CHNH), 5.36 (1 H,
br, CHNH), 4.90 (1 H, br, CHNH), 4.68 (2 H, s, PyCH2), 4.64 (3 H,
s + m, PhCH2 and CHHNH), 4.48 (1 H, dd, J ) 16.8, 5.6, CHHNH),
3.82 (3 H, s, OMe), 2.52 (3 H, s, Me), 2.22 (1 H, m, Me2CH), 2.19 (3
H, s, Me), 1.46 (3 H, d, J ) 7.0, CHMe), 1.40 (9 H, s, CMe3), 0.99 (3
H, d, J ) 7.0, MeCHMe), 0.97 (3 H, d, J ) 7.0, MeCHMe); 13C NMR
(100 MHz, CDCl3) δ 171.2 (C), 164.8 (C), 163.2 (C), 162.4 (C), 160.9
(C), 160.3 (C), 158.7 (C), 156.8 (C), 154.9 (C), 149.2 (C), 148.5 (C),
148.1 (C), 138.5 (C), 137.7 (CH), 132.7 (C), 128.6 (CH), 127.9 (CH),
127.8 (CH), 127.5 (C), 127.4 (CH), 125.6 (C), 120.4 (CH), 79.7 (C),
73.1 (CH2), 72.6 (CH2), 58.1 (CH), 51.1 (Me), 44.7 (CH), 36.3 (CH2),
31.6 (CH), 28.3 (Me), 20.1 (Me), 19.3 (Me), 18.1 (Me), 11.9 (Me),
10.9 (Me); MS (FAB) m/z (relative intensity) 824 (M + Na+, 100%),
(h) Macrocyclic peptide 43. 1-(3-Dimethylaminopropyl)-3-ethyl
carbodiimide hydrochloride (46 mg, 240 µmol) was added to a stirred
solution of N-(tert-butoxycarbonyl)-protected linear peptide carboxylic
acid 42 (196 mg, 208 µmol) and pentafluorophenol (46 mg, 250 µmol)
in dry dichloromethane (5 mL) at 0 °C. The reaction mixture was
warmed slowly to room temperature over 16 h, evaporated in vacuo,
and partitioned between ethyl acetate (20 mL) and brine (20 mL). The