C. Quirin, U. Kazmaier
FULL PAPER
was complete (monitored by TLC). The solvent was removed in
vacuo, and the resulting hydrochloride salt was added to a stirred
emulsion of sat. NaHCO3 solution and CH2Cl2. The aqueous layer
was extracted thrice with CH2Cl2 and dried with Na2SO4, and the
solvent was removed in vacuo. The obtained amine (0.60 g,
1.65 mmol) and the acid (0.45 g, 1.50 mmol) were dissolved in dry
CH2Cl2, and TBTU (0.53 g, 1.65 mmol) was added at 0 °C. After
the addition of Et3N (0.17 g, 1.65 mmol), the mixture was allowed
to warm to room temperature overnight. The reaction mixture was
washed with KHSO4 solution (1 ), sat. NaHCO3 solution, and
water. The organic layer was dried with Na2SO4 and the solvent
was removed in vacuo. The crude product was purified by column
chromatography (silica gel, hexanes/EtOAc, 30:70 Ǟ 10:90) to yield
tetrapeptide 10 (0.91 g, 1.42 mmol, 95%) as a viscous oil. Rf = 0.09
(hexanes/EtOAc, 30:70). [α]2D0 = +17 (c = 1.0, CHCl3). Major rot-
amer/diastereomer: 1H NMR (400 MHz): δ = 1.38 (s, 9 H), 1.41 (s,
3 H), 1.47 (s, 3 H), 1.53 (m, 3 H), 1.81 (m, 3 H), 2.09 (m, 2 H),
2.36 (m, 2 H), 2.47 (br. s, 1 H), 2.65 (m, 1 H), 2.88 (m, 1 H), 2.98
(dd, J = 13.0, 5.2 Hz, 1 H), 3.49 (m, 1 H), 3.61 (s, 3 H), 3.98 (br.
s, 1 H), 4.05 (m, 1 H), 4.24 (m, 1 H), 4.84 (m, 1 H), 5.02 (m, 1 H),
5.15 (m, 2 H), 5.34 (m, 1 H), 5.50 (m, 1 H), 5.80 (m, 1 H), 6.76 (s,
1 H), 6.91 (d, J = 8.0 Hz, 1 H), 7.15 (m, 5 H) ppm. 13C NMR
(100 MHz): δ = 24.3, 25.5, 28.2, 28.4, 28.9, 35.2, 36.2, 39.2, 46.7,
52.1, 52.5, 54.4, 56.9, 58.7, 72.0, 80.1, 114.2, 125.0, 126.8, 128.3,
129.5, 134.5, 136.3, 141.2, 155.6, 169.8, 170.9, 172.1, 173.5 ppm.
Minor rotamer and diastereomer: 1H NMR (400 MHz, selected
signals): δ = 1.37, 1.39 (2ϫs, 9 H), 1.43, 1.44 (2ϫs, 6 H), 2.81 (m,
1 H), 3.05 (m, 1 H), 3.64, 3.66 (2ϫs, 3 H), 3.89 (br. s, 1 H), 4.39
(m, 1 H), 4.62 (m, 1 H), 4.98 (m, 1 H), 6.63, 6.70, 6.74 (3ϫs, 1 H),
6.82, 6.98 (d, J = 8.0 Hz, 1 H) ppm. 13C NMR (CDCl3, 100 MHz,
selected signals): δ = 22.4, 24.8, 24.9, 31.0, 36.2, 39.1, 46.6, 46.9,
52.5, 52.6, 56.8, 59.0, 59.2, 72.1, 114.1, 114.3, 126.5, 126.8, 128.2,
129.2, 129.7, 136.4, 136.9, 169.7, 170.6, 170.8, 172.2, 172.7,
was stirred for 30 min at the same temperature. The aqueous layer
was extracted thrice with CH2Cl2, and the combined organic layers
were dried with Na2SO4. The solvent was removed in vacuo, and
after purification by column chromatography (silica gel, hexanes/
EtOAc, 30:70), cyclic tetrapeptide 11 (46 mg, 0.09 mmol, 65%) was
obtained as a white foam. Rf = 0.12 (hexanes/EtOAc, 30:70). [α]2D0
= –84 (c = 1.0, CHCl3). 1H NMR (400 MHz): δ = 1.22 (s, 3 H),
1.47 (m, 2 H), 1.64 (m, 5 H), 2.00 (m, 2 H), 2.07 (m, 1 H), 2.21
(m, 2 H), 2.41 (m, 1 H), 2.84 (dd, J = 13.5, 5.7 Hz, 1 H), 3.13 (m,
2 H), 3.74 (m, 1 H), 3.99 (m, 1 H), 4.14 (dt, J = 10.2, 7.5 Hz, 1 H),
4.55 (m, 1 H), 4.99 (m, 1 H), 5.04 (m, 1 H), 5.11 (m, 1 H), 5.27
(m, 1 H), 5.46 (m, 1 H), 5.75 (m, 1 H), 6.08 (s, 1 H), 7.06 (d, J =
10.5 Hz, 1 H), 7.14 (m, 5 H), 7.41 (d, J = 10.2 Hz, 1 H) ppm. 13C
NMR (100 MHz): δ = 23.5, 24.7, 24.9, 26.3, 28.4, 32.2, 35.8, 36.3,
46.9, 53.4, 54.2, 57.7, 58.7, 72.2, 114.5, 124.8, 126.6, 128.5, 129.0,
133.8, 137.0, 141.0, 171.2, 172.8, 174.0, 175.6 ppm. HRMS (CI)
calcd. for C28H39N4O5 [M + H]+: 511.2920; found 511.2945.
Epoxy Alcohol 12: Cyclic tetrapeptide 11 (64 mg, 125 µmol) was
dissolved in dry CH2Cl2 (1 mL), and VO(acac)2 (496 µL,
1.88 µmol) was added dropwise as a CH2Cl2 solution [1 mg
VO(acac)2/1 mL dry CH2Cl2]. After 5 min a solution of tert-
BuOOH in decane (5.5 , 30.2 µL, 163 µmol) was added, and the
solution was allowed to stir overnight at room temperature. The
solvent was removed in vacuo, and the residue was purified by col-
umn chromatography (silica gel, CH2Cl2/MeOH, 97:3) to afford
epoxy alcohol 12 (60 mg, 114 µmol, 90%) as a yellowish oil. Rf =
0.11 (CH2Cl2/MeOH, 97:3). 1H NMR (400 MHz): δ = 1.31 (s, 3
H), 1.60 (m, 3 H), 1.74 (m, 2 H) 1.74 (s, 3 H), 2.15 (m, 2 H), 2.31
(m, 2 H), 2.49 (m, 1 H), 2.66 (dd, J = 4.9, J = 2.8 Hz, 0.3 H), 2.70
(dd, J = 5.0, J = 4.0 Hz, 0.7 H), 2.77 (m, 1 H), 2.94 (m, 2 H), 3.21
(m, 2 H), 3.81 (m, 2 H), 4.21 (dt, J = 10.2, 7.4 Hz, 1 H), 4.63 (m,
1 H), 5.13 (td, J = 10.1, 5.8 Hz, 1 H), 3.39 (m, 1 H), 5.54 (m, 1 H),
5.97 (br. s, 1 H), 7.11 (d, J = 10.3 Hz, 1 H), 7.22 (m, 5 H), 7.45 (d,
J = 10.2 Hz, 1 H) ppm. 13C NMR (100 MHz): δ = 23.5, 24.7, 25.0,
26.4, 28.3, 32.2, 32.8, 35.8, 43.5, 45.0, 47.0, 53.4, 54.2, 54.4, 57.9,
58.8, 67.8, 125.1, 126.7, 128.6, 129.0, 133.6, 137.0, 171.7, 172.8,
174.0, 175.6 ppm. HRMS (CI) calcd. for C28H38N4O6 [M]+:
526.2791; found 526.2772.
174.1 ppm. HRMS (CI) calcd. for C34H51N4O8 [M
643.3707; found 643.3519.
+
H]+:
Cyclotetrapeptide 11
Preparation of the Pentafluorophenol Ester: Tetrapeptide 10
(500 mg, 0.78 mmol) was dissolved in 1,4-dioxane (10 mL), and
NaOH (1 , 0.85 mL, 0.85 mmol) was added at 0 °C. After com-
plete saponification (monitored by TLC), the solvent was removed
in vacuo and the residue was dissolved in water. The aqueous solu-
tion was extracted thrice with Et2O and then acidified to pH 2 with
KHSO4 solution (1 ). The resulting emulsion was extracted thrice
with EtOAc, and the combined organic layers were dried with
Na2SO4. The solvent was removed in vacuo to afford the crude
acid (442 mg, 0.70 mmol, 90%). The acid (315 mg, 0.50 mmol) was
dissolved in Et2O, pentafluorophenol (92.0 mg, 0.50 mmol) was
added, and the solution was cooled to 0 °C. After addition of DCC
(104 mg, 0.50 mmol) the reaction mixture was allowed to warm up
to room temperature overnight. The precipitated DCU was filtered
through a pad of celite, washed with Et2O, and the combined ethe-
real layers were concentrated in vacuo. Filtration through a small
pad of silica gel (hexanes/EtOAc, 40:60) yielded the pentafluoro-
phenol ester of 10 (290 mg, 0.36 mmol, 73%), which was directly
subjected to the cyclization step.
Chlamydocin (1): Epoxy alcohol 12 (20 mg, 38.0 µmol) was dissolved
in MeOH (5 mL), and the system was stirred under hydrogen in the
presence of Pd on charcoal (5%, 1.08 mg). After the hydrogenation
was complete (monitored by TLC), the catalyst was filtered off and
washed with EtOAc, and the solvent was removed in vacuo. The
residue was dissolved in dry CH2Cl2 (1 mL) and added to a solution
of DMP (32.2 mg, 76.0 µmmol) in dry CH2Cl2 (1 mL). After 3 h the
precipitated solid was filtered off, and the residue was filtered
through a short pad of silica gel (CHCl3/MeOH, 99:1) to provide 1
(14 mg, 26.6 µmol, 70%). Rf = 0.20 (CHCl3/MeOH, 99:1). Major
diastereomer: 1H NMR (400 MHz): δ = 1.28 (m, 4 H), 1.32 (s, 3 H),
1.67 (m, 6 H), 1.75 (s, 3 H), 2.15 (m, 1 H), 2.33 (m, 3 H), 2.84 (dd,
J = 5.8, 2.5 Hz, 1 H), 2.93 (dd, J = 13.8, 6.0 Hz, 1 H), 2.97 (dd, J
= 5.8, 4.7 Hz, 1 H), 3.22 (m, 2 H), 3.40 (dd, J = 4.6, 2.5 Hz, 1 H),
3.83 (m, 1 H), 4.17 (m, 1 H), 4.64 (m, 1 H), 5.14 (td, J = 10.1,
5.8 Hz, 1 H), 5.95 (s, 1 H), 7.08 (d, J = 9.4 Hz, 1 H), 7.22 (m, 5 H),
7.49 (d, J = 10.5 Hz, 1 H) ppm. 13C NMR (100 MHz): δ = 22.7,
23.5, 24.8, 25.0, 25.3, 26.5, 28.9, 29.7, 35.8, 35.9, 46.1, 47.0, 53.4,
54.3, 57.8, 58.8, 126.7, 128.6, 129.0, 137.0, 171.8, 172.8, 174.4, 175.6,
207.6 ppm. Minor diastereomer: 13C NMR (100 MHz, selected sig-
nals): δ = 22.8, 23.6, 29.4, 36.3, 54.3, 174.3 ppm. No signals could
be detected in 1H NMR; only a slight splitting of the amide protons
was observed. HRMS (CI) calcd. for C28H39N4O6 [M + H]+:
527.2870; found 527.2867.
Cyclization: The active ester (111 mg, 0.14 mmol) was dissolved in
dry CH2Cl2, and trifluoroacetic acid (0.14 mL) was added at 0 °C.
After Boc removal was complete (monitored by TLC), the solvent
was removed in vacuo. The residue was dissolved in CH3CN
(10 mL) and added by syringe pump at 50 °C over 5 h to a rigor-
ously stirred emulsion of saturated NaHCO3 solution (12.5 mL)
and CHCl3 (87.5 mL). After the addition was complete the mixture
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Eur. J. Org. Chem. 2009, 371–377