M. Tena-Solsona, C. A. Angulo-Pachón, B. Escuder, J. F. Miravet
FULL PAPER
sponding amine (3.5 mmol) was carried out in a similar way to that
described for ZPheNHPr. Subsequently, an extra deprotection step
was needed. Removal of the tBu protecting group was conducted
based on a reported procedure.[22] A solution tert-butyl ester
ZAsp(tBu)NHPr (1.09 g, 3.0 mmol) in a mixture of TFA (12 mL),
dichloromethane (12 mL) and water (0.9 mL) was stirred for
40 min at room temperature. Dichloromethane and TFA were evap-
orated under reduced pressure and the residue was co-distilled three
times with diethyl ether. The obtained product was dissolved in
dichloromethane (20 mL) and washed with HCl 0.1 m (3ϫ 25 mL)
and water (3ϫ 25 mL). The organic phase was dried with anhy-
drous magnesium sulfate and the solvent was evaporated under
vacuum. Global yield: 0.83 g (2.69 mmol, 85%); white solid. 1H
NMR (500 MHz, 30 °C, [D6]DMSO): δ = 7.81 (t, J = 5.1 Hz, 1 H),
7.46 (d, J = 8.2 Hz, 1 H), 7.41–7.22 (m, 5 H), 5.10–4.97 (m, 2 H),
4.37–4.26 (m, 1 H), 3.08–2.90 (m, 2 H), 2.71–2.56 (m, 1 H), 2.56–
2.42 (m, 1 H), 1.44–1.33 (m, 2 H), 0.81 (t, J = 7.4 Hz, 3 H) ppm.
13C NMR (126 MHz, 30 °C, [D6]DMSO): δ = 172.2, 170.9, 156.2,
137.4, 128.8, 128.2, 128.1, 65.9, 52.0, 40.9, 37.0, 22.7, 11.7 ppm.
HRMS (ESI-TOF, negative mode): m/z calcd. for C15H19N2O5
[M – H]– 307.1294; found 307.1295 [M – H]– (Δ = 0.3 ppm).
137.8, 137.4, 129.6, 128.7, 128.6, 128.4, 128.1, 127.8, 127.2, 126.9,
126.7, 65.6, 56.4, 53.9, 37.9, 37.2 ppm. HRMS (ESI-TOF, positive
mode): m/z calcd. for C26H26N2NaO5+ [M + Na]+ 469.1739; found
369.1737 [M + Na]+ (Δ = 0.4 ppm).
ZAsp(tBu)PheOH: A similar procedure to that described for
ZPhePheOH was used. The starting materials in this case were l-
phenylalanine methyl ester hydrochloride (518 mg, 2.4 mmol), tri-
ethylamine (0.36 mL, 2.6 mmol), and ZAsp(tBu)OSu (1.00 g,
1
2.4 mmol). Global yield: 0.89 g (1.89 mmol, 75%); white solid. H
NMR (300 MHz, 30 °C, [D6]DMSO): δ = 7.98 (d, J = 7.4 Hz, 1
H), 7.51 (d, J = 8.4 Hz, 1 H), 7.42–7.12 (m, 10 H), 5.12–4.91 (m,
2 H), 4.49–4.25 (m, 2 H), 3.07–2.85 (m, 2 H), 2.71–2.30 (m, 2 H),
1.36 (s, 9 H) ppm. 13C NMR (75 MHz, 30 °C, [D6]DMSO): δ =
173.0, 171.3, 171.0, 169.6, 156.1, 137.7, 137.3, 129.6, 128.8, 128.6,
128.2, 128.1, 126.9, 80.6, 65.9, 53.9, 51.8, 38.0, 37.0, 28.1 ppm.
+
HRMS (ESI-TOF, positive mode): m/z calcd. for C25H30N2NaO7
[M + Na]+ 493.1951; found 493.1947 [M + Na]+ (Δ = 0.8 ppm).
General Procedure for Peptide Coupling: The following peptides
were obtained by using conventional solution peptide chemistry.
See the Supporting Information for the synthetic scheme; general
procedures are shown below.
ZAspN(CH3)Pr:
A similar procedure to that described for
The N-hydroxysuccinimide ester of the Z-protected amino acid
(8.7 mmol) was dissolved in DME (50 mL), then the corresponding
peptide or amino acid with the C-terminal group as propylamide
(8.7 mmol) was dissolved in DME (15 mL) and added dropwise.
The resulting solution was stirred at room temperature for 18 h and
then warmed for 2 h at 40–50 °C. The solvent was evaporated un-
der vacuum and the resulting solid was washed with saturated so-
dium hydrogencarbonate solution, HCl 0.1 m aqueous solution,
and water. The final product was dried at 40 °C under vacuum.
ZAspNHPr was used. In this case, the reaction coupling was be-
tween ZAsp(tBu)OSu (1.35 g, 3.2 mmol) and N-methylpropan-1-
amine (0.36 mL, 3.5 mmol). The deprotection with TFA and
CH2Cl2 produced a transparent oil (0.73 g, 2.26 mmol, 71% global
1
yield). H NMR (300 MHz, 30 °C, [D6]DMSO): δ = 7.69 (dd, J =
11.9, 9.1 Hz, 1 H), 7.43–7.17 (m, 5 H), 5.03 (s, 2 H), 4.72 (m, 1 H),
3.27 (m, 2 H), 3.00–2.77 (ds, 3 H), 2.76–2.20 (m, 2 H), 1.47 (dt, J
= 29.1, 14.1, 7.1 Hz, 13 H), 0.78 (dt, J = 7.4 Hz, 3 H) ppm. 13C
NMR (75 MHz, 30 °C, [D6]DMSO): δ = 172.2, 170.5, 170.2, 156.0,
156.0, 137.4, 137.3, 128.8, 128.2, 128.1, 128.1, 66.0, 66.0, 55.3, 50.7,
49.3, 49.0, 48.0, 37.3, 36.9, 35.0, 33.7, 21.5, 20.2, 11.4, 11.3 ppm.
General Procedure for N-Benzyloxycarbonyl Deprotection: The cor-
responding N-benzyloxycarbonyl protected peptide or amino acide
derivative (7.8 mmol) and a catalytic amount of Pd over activated
carbon (5–10% w/w) were placed in a two-necked, round-bottomed
flask and suspended in MeOH (50 mL). The system was purged to
remove the air with N2 and connected to a H2 atmosphere. The
pasty grey suspension was stirred for several hours until it became
completely black [also checked with TLC, MeOH/CH2Cl2 (2:8) and
revealed with ninhydrin]. The black suspension was filtered through
Celite and the solvent was evaporated under reduced pressure. The
resulting oil was dried further in vacuo for 24 h.
–
HRMS (ESI-TOF, negative mode): m/z calcd. for C16H21N2O5
[M – H]– 321.1450; found 321.1451 [M – H]– (Δ = 0.3 ppm).
ZPhePheOH: In a first step, the corresponding methyl ester of the
dipeptide was obtained by following a reported procedure.[23] l-
Phenylalanine methyl ester hydrochloride (518 mg, 2.4 mmol) was
dissolved in dimethoxyethane (DME, 30 mL), then triethylamine
were added (0.36 mL, 2.6 mmol) and immediately a white precipi-
tate appeared. A solution of ZPheOSu (887 mg, 2.4 mmol) in DME
(30 mL) was added dropwise and the reaction mixture was stirred
at room temperature for 2 h, filtered, and the solvents were evapo-
rated. The residue was dissolved in dichloromethane (25 mL) and
washed with water (4ϫ 25 mL). The organic phase was dried with
anhydrous magnesium sulfate and the solvent was evaporated un-
der vacuum.
General Procedure for tert-Butyl Ester Group Deprotection: A solu-
tion of tert-butyl ester (3.0 mmol) in a mixture of TFA (12 mL),
dichloromethane (12 mL), and water (0.9 mL) was stirred for
40 min at room temperature. Dichloromethane and TFA were evap-
orated under reduced pressure and the residue was co-distilled three
times with diethyl ether. The solid was washed with water.
Following a reported procedure,[24] the corresponding methyl ester
(875 mg, 1.9 mmol) was dissolved in THF (20 mL) and cooled to
0 °C. A solution of lithium hydroxide (62.4 mg, 2.6 mmol) in water
(10 mL) was added dropwise and the reaction mixture was stirred
for 0.5 h. This mixture was then washed with diethyl ether (20 mL),
the aqueous layer was acidified with 1 m HCl to pH 2 and extracted
with diethyl ether (3ϫ 30 mL). The organic extracts were com-
bined, dried with anhydrous magnesium sulfate and the solvents
were evaporated.
ZPheAspAspPheNHPr: Global yield: 2.8 g (3.9 mmol, 51%); white
solid. 1H NMR (500 MHz, 30 °C, [D6]DMSO): δ = 8.40 (d, J =
7.5 Hz, 1 H), 8.15 (d, J = 7.5 Hz, 1 H), 7.77 (d, J = 8.2 Hz, 1 H),
7.62 (t, J = 5.5 Hz, 1 H), 7.46 (d, J = 8.5 Hz, 1 H), 7.39–6.96 (m,
15 H), 4.97–4.87 (m, 2 H), 4.58–4.54 (m, 1 H), 4.50–4.56 (m, 1 H),
4.38–4.34 (m, 1 H), 4.32–4.21 (m, 1 H), 3.08–2.88 (m, 4 H), 2.86–
2.81 (m, 1 H), 2.78–2.39 (m, 6 H), 1.43–1.27 (m, 2 H), 0.76 (t, J =
7.4 Hz, 3 H) ppm. 13C NMR (75 MHz, 30 °C, [D6]DMSO): δ =
172.3, 172.2, 172.2, 171.2, 170.6, 170.4, 156.3, 138.5, 138.2, 137.4,
129.7, 129.5, 128.7, 128.5, 128.4, 128.1, 127.9, 126.7, 65.7, 56.4,
54.7, 50.2, 50.0, 37.8, 36.5, 36.2, 22.6, 11.7 ppm. HRMS (ESI-TOF,
Global yield: 0.92 g (2.06 mmol, 82%); white solid. 1H NMR
(300 MHz, 30 °C, [D6]DMSO): δ = 8.23 (d, J = 7.8 Hz, 1 H), 7.42
(d, J = 8.8 Hz, 1 H), 7.36–6.94 (m, 15 H), 5.03–4.81 (s, 2 H), 4.57–
4.38 (m, 1 H), 4.29 (m, 1 H), 3.43 (dd, J = 41.4, 34.4 Hz, 2 H),
3.15–2.81 (m, 3 H), 2.70 (dd, J = 13.7, 10.9 Hz, 1 H) ppm. 13C
NMR (75 MHz, 30 °C, [D6]DMSO): δ = 173.2, 172.0, 156.2, 138.5,
negative mode): m/z calcd. for C37H42N5O10 [M – H]– 716.2932;
–
found 716.2924 [M – H]– (Δ = 1.1 ppm).
ZAspPhePheAspNHPr: Global yield: 1.5 g (2.09 mmol, 45%);
white solid. 1H NMR (300 MHz, 30 °C, [D6]DMSO): δ = 8.16 (dd,
3376
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Eur. J. Org. Chem. 2014, 3372–3378