380.1798. [α]2D5 = +7.4 (c 0.71, MeOH). [α]2D0 = +16.8 (c 1.0,
Me2CO).
7:2:1) yielded title compound 31 as dark blue crystals (242 mg,
372 µmol, 81% yield over two steps). Rf = 0.58 (CH2Cl2–
MeOH, 9:1, visible without stain, stains immediately purple with
vanillin). 1H NMR (400 MHz, DMSO-d6) δ 10.70 (s, 1H),
8.27 (t, J = 10.8 Hz, 2H), 7.99 (d, J = 8.1 Hz, 1H), 7.91 (t, J =
5.9 Hz, 1H), 7.69 (d, J = 3.8 Hz, 1H), 7.62 (t, J = 9.9 Hz, 1H),
7.27 (d, J = 3.9 Hz, 1H), 7.13 (td, J = 9.7, 4.9 Hz, 2H), 6.88 (t,
J = 7.8 Hz, 1H), 6.83 (d, J = 7.9 Hz, 1H), 6.74 (d, J = 8.3 Hz,
1H), 6.43 (d, J = 7.3 Hz, 1H), 4.54 (q, J = 7.6 Hz, 1H), 4.19
(td, J = 8.5, 4.5 Hz, 1H), 3.86 (s, 3H), 3.79 (dd, J = 5.9,
2.8 Hz, 2H), 3.61 (s, 3H), 3.29 – 2.95 (m, 4H), 2.29 (s, 3H),
1.22 (s, 9H). 13C NMR (101 MHz, DMSO-d6) δ 172.0, 171.0,
170.1, 155.1, 153.1, 140.3, 137.8, 137.6, 136.6, 136.3, 136.1,
133.7, 131.5, 126.0, 122.3, 121.7, 120.6, 117.9, 116.6, 105.7,
104.1, 98.9, 78.1, 56.2, 55.0, 54.8, 51.7, 40.7, 29.6, 28.3, 28.0,
11.1. HRMS (ESI): calcd. for C34H40N4O7 [M+H]+ 639.2795,
found 639.2793. [α]2D0: not measurable due to dark color of
solution even at very low concentrations.
5.7.4. Ethyl (R)-N-(2-(2-
aminopropanamido)acryloyl)-N-methylglycinate,
TFA salt (38)
Boc amide 43 (85.0 mg, 238 µmol) was stirred in CH2Cl2–
TFA (2:1, 6.8 mL) at 0 °C for 10 min, then additional 30 min at
ambient temperature. Toluene was added and all volatiles were
removed under reduced pressure to obtain the crude TFA salt
which was dried under high vacuum for 2 h and used directly for
the coupling.
5.8. Total synthesis of AzuAla1 argyrin C
5.8.1. Methyl (S)-(2-((tert-butoxycarbonyl)amino)-
3-(4-methoxy-2-methyl-1H-indol-3-
yl)propanoyl)glycinate (30)
4-Methoxytryptophan 23 (1.60 g, 3.18 mmol) was refluxed in
3 N NaOH–MeOH (1:2) for 36 h, cooled to ambient temperature
and acidified to pH 2 with 1 N HCl. EtOAc was added and the
layers were separated. The aqueous layer was extracted one
additional time with EtOAc. The combined organic layers were
dried over MgSO4 and concentrated in vacuo to obtain a light
brown foam which was dissolved in anhydrous CH2Cl2 (32 mL).
HOBt (645 mg, 4.78 mmol, 1.5 eq.) and glycine methyl ester
hydrochloride (600 mg, 4.78 mmol, 1.5 eq.) were added and the
reaction mixture was cooled to 0 °C. EDC·HCl (763 mg,
3.98 mmol, 1.25 eq.) was added in one portion followed by the
dropwise addition of DIPEA (1.23 g, 1.6 mL, 9.55 mmol,
3.0 eq.). The reaction mixture was stirred for 18 h with the
cooling bath slowly warming up to ambient temperature. 0.5 N
HCl were added, the layers were separated and the aqueous layer
was extracted twice with CH2Cl2. The combined organic layers
were dried over MgSO4 and concentrated under reduced pressure
to yield crude dipeptide 30 which was purified by flash column
chromatography (cyclohexane–EtOAc, 1:1.2 → 1:1.5) to obtain
title compound 30 as white solid (1.05 g, 2.51 mmol, 79% yield
over two steps). Rf = 0.20 (cyclohexane–EtOAc, 1:1.2, stains
5.8.3. Methyl ((S)-2-((S)-3-(azulen-1-yl)-2-(2-((R)-
1-((tert-butoxycarbonyl)amino)ethyl)thiazole-4-
carboxamido)propanamido)-3-(4-methoxy-2-methyl-
1H-indol-3-yl)propanoyl)glycinate (32)
Boc amide 31 (123 mg, 199 µmol) was stirred in CH2Cl2–TFA
(3:1, 4.0 mL) at 0 °C for 20 min, then additional 20 min at
ambient temperature. Sat. aq. K2CO3 was added and the layers
were separated. The aqueous layer was extracted two times with
CH2Cl2 (until organic layer became colorless) and the combined
organic layers were dried over Na2SO4 and concentrated in vacuo
to obtain the crude amine as TFA salt as dark blue solid which
was dissolved in anhydrous CH2Cl2 (7.7 mL). HOBt (39.2 mg,
290 µmol, 1.50 eq.) and Boc-Thiaz-OH (36, 67.0 mg, 246 µmol,
1.3 eq.) were added and the reaction mixture was cooled to 0 °C.
EDC·HCl (46.4 mg, 242 mg, 1.25 eq.) was added in one portion
followed by the dropwise addition of DIPEA (75.1 mg, 99 µL,
3.0 eq.). The reaction mixture was stirred for 14 h with the
cooling bath slowly warming up to ambient temperature. 1 N HCl
was added and the layers were separated. The aqueous layer was
extracted one additional time with CH2Cl2, the combined organic
layers were dried over Na2SO4 and concentrated in vacuo. The
residue was subjected to flash column chromatography (CH2Cl2–
MeOH, 33:1) to obtain title compound 32 as dark blue solid
(133 mg, 172 µmol, 86% yield over two steps). Rf = 0.4 (pure
EtOAc, visible without stain, stains immediately purple with
1
very strong blue with vanillin). H NMR (400 MHz, DMSO-
d6) δ 10.71 (s, 1H), 8.05 (t, J = 5.8 Hz, 1H), 6.90 – 6.81 (m,
2H), 6.58 (d, J = 8.4 Hz, 1H), 6.44 (dd, J = 7.5, 1.1 Hz, 1H),
4.12 (td, J = 9.0, 4.7 Hz, 1H), 3.92 – 3.82 (m, 2H), 3.86 (s,
3H), 3.63 (s, 3H), 3.11 (dd, J = 14.0, 4.7 Hz, 1H), 2.90 (dd,
J = 14.0, 9.5 Hz, 1H), 2.26 (s, 3H), 1.21 (s, 9H). 13C NMR
(101 MHz, DMSO-d6) δ 172.6, 170.3, 154.9, 152.9, 136.6,
131.4, 120.5, 117.8, 105.8, 104.2, 98.9, 77.8, 56.5, 54.9, 51.7,
40.7, 28.2, 28.0, 11.0. HRMS (ESI): calcd. for C21H29N3O6
[M+Na]+ 442.1954, found 442.1956. [α]2D5 = +11.8 (c 0.75,
CHCl3).
1
vanillin). H NMR (400 MHz, CDCl3) δ 8.37 (s, 1H), 8.22 (t,
J = 9.2 Hz, 2H), 7.86 (s, 1H), 7.70 (d, J = 3.9 Hz, 1H), 7.58 (t,
J = 9.9 Hz, 1H), 7.46 (d, J = 5.3 Hz, 1H), 7.25 (d, J = 3.9 Hz,
1H), 7.17 (t, J = 9.7 Hz, 1H), 7.11 (t, J = 9.7 Hz, 2H), 6.96 (t,
J = 7.9 Hz, 1H), 6.82 (br s, 1H), 6.80 (d, J = 8.1 Hz, 1H), 6.45
(d, J = 7.8 Hz, 1H), 5.08 (d, J = 7.3 Hz, 1H), 4.95 (dt, J =
12.5, 6.2 Hz, 1H), 4.62 – 4.43 (m, 2H), 4.06 (dd, J = 17.8,
6.2 Hz, 1H), 3.83 (dd, J = 17.9, 5.1 Hz, 1H), 3.70 (s, 3H),
3.66 (s, 3H), 3.45 (dd, J = 14.5, 6.1 Hz, 1H), 3.27 (dd, J =
14.7, 5.4 Hz, 1H), 3.20 (dd, J = 14.6, 9.4 Hz, 1H), 3.10 (dd,
J = 14.5, 7.8 Hz, 1H), 2.06 (br s, 3H), 1.47 (s, 9H), 1.42 (d,
J = 6.9 Hz, 3H). 13C NMR (101 MHz, CDCl3) δ 175.0, 172.0,
171.3, 170.1, 161.6, 155.2, 152.8, 148.9, 141.0, 137.9, 137.8,
136.9, 136.9, 136.9, 133.9, 131.8, 124.1, 123.6, 123.0, 122.6,
121.6, 118.6, 117.4, 105.7, 104.7, 100.2, 80.9, 56.2, 55.4,
55.0, 52.2, 49.2, 41.2, 29.5, 28.5, 26.5, 21.7, 11.1. HRMS
(ESI): calcd. for C40H46N6O8S [M+Na]+ 793.2996, found
793.2997. [α]20: not measurable due to dark color of solution
even at very lDow concentrations.
5.8.2. Methyl ((S)-2-((S)-3-(azulen-1-yl)-2-((tert-
butoxycarbonyl)amino)propanamido)-3-(4-methoxy-
2-methyl-1H-indol-3-yl)propanoyl)glycinate (31)
Boc amide 30 (240 mg, 572 µmol) was stirred in CH2Cl2–TFA
(3:1, 12 mL) at 0 °C for 45 min. Toluene was added and all
volatiles were removed under reduced pressure to obtain the
crude TFA salt which was dried under high vacuum for 2 h. A
solution of Boc-AzuAla-OH (13) in anhydrous CH2Cl2 (12 mL)
was added to the crude amine salt followed by the addition of
HOBt (116 mg, 858 µmol, 1.5 eq.). The reaction mixture was
cooled to 0 °C and EDC·HCl (137 mg, 715 µmol, 1.25 eq.) was
added in one portion followed by the dropwise addition of
DIPEA (222 mg, 292 µL, 1.72 mmol, 3.0 eq.). The reaction
mixture was stirred for 14 h with the cooling bath slowly
warming up to ambient temperature. Silica was added and the
solvent was removed in vacuo to complete dryness. Purification
by flash column chromatography (CH2Cl2–acetone–cyclohexane,
5.8.4. Ethyl N-(2-((R)-2-(2-((S)-2-((S)-3-(azulen-1-
yl)-2-(2-((R)-1-((tert-
butoxycarbonyl)amino)ethyl)thiazole-4-