S. Kar et al. / Journal of Molecular Structure 963 (2010) 160–167
161
O
O
with diethyl ether. The aqueous layer was cooled in ice-bath, neu-
tralized by using 2 N HCl and extracted with ethyl acetate. The sol-
vent was evaporated in vacuo to give a waxy colorless solid. Yield:
2.2 g (85.93%).
H
N
H
N
O
N
H
OMe
O
O
O
PeptideI
O
O
2.1.3. Boc-Gly-Aib-m-ABA-OMe (peptide I)
θ1
φ
ψ
H
N
Peptide 2 (2.2 g, 8.46 mmol) was dissolved in dichloromethane
(DCM, 5 ml). H-mABA-OMe obtained from its hydrochloride
(3.17 g, 16.92 mmol) was added to former solution, followed by
addition of DCC (2.61 g, 12.69 mmol) and HOBT (1.14 g,
8.46 mmol) in ice-cold condition. The reaction mixture was stirred
at room temperature for 2 days. The precipitated DCU was filtered
off. The organic layer was diluted with 30 ml ethyl acetate and
washed with 1 N HCl (3 ꢀ 30 ml), brine, 1 M Na2CO3 solution
(3 ꢀ 30 ml) and then again with brine. The solvent was then dried
over anhydrous Na2SO4 and evaporated in vacuo, to give a colorless
solid compound. Purification was done using silica gel as stationary
phase and ethyl acetate–petroleum ether mixture (1:4) as eluent.
Yield: 2.8 g (84.33%). Mp = 168 °C; IR (KBr): 3352, 3284.6,
O
N
N
H
OMe
H
O
II
Peptide
O
O
θ1
φ
ψ
θ2
H
H
N
O
N
N
H
OMe
O
O
PeptideIII
Fig. 1. Schematic diagram of peptides I–III.
of flexible and rigid
x-amino acids simultaneously as in peptides II
and III can lead to the creation of unusual turns and novel helical
folds through intramolecular hydrogenbonding. There are examples
of constrained cyclic peptides in which substituted benzenes have
been inserted to mimic the turn region of the neurotrophin, a nerve
growth factor [48]. Peptides I–III will be examples of acyclic ana-
logues if they adopt turn structures. All the peptides were prepared
by conventional solution phase synthesis and their conformational
studies have been carried out by single crystal X-ray diffraction
and NMR studies.
2973.1, 2930.7, 1719.8, 1676.3, 1541.5 cmꢁ1 1H NMR 300 MHz
;
(CDCl3, d ppm): 9.16 (m-ABA(3)-NH, 1H, s), 8.23 (Ha m-ABA(3),
1H, s), 7.97(Hd m-ABA(3), 1H, d, J = 7.8 Hz), 7.75 (Hb m-ABA(3),
1H, d, J = 7.9 Hz), 7.37 (Hc m-ABA(3), 1H, m), 6.60 (Aib(2)-NH, 1H,
a
s), 5.34 (Gly(1)-NH, 1H, s), 4.30 (C H of Gly(1), 2H, t, J = 6.6 Hz),
3.89 (AOCH3, 3H, s), 1.62 and 1.25 (CbH of Aib(2), 6H, s), 1.46
(Boc-CH3s, 9H, s); 13C NMR 75 MHz (CDCl3, d ppm): 172.48,
169.61, 166.87, 138.69, 130.89, 130.63, 128.85, 126.34, 125.07,
124.53, 128.85, 126.34, 125.07, 124.53, 121.03, 81.14, 58.08,
52.06, 28.18, 25.50; Anal. Calcd for C19H27N3O6 (393.43): C,
57.99; H, 6.91; N, 10.68%; Found: C, 57.91; H, 6.84; N, 10.60%.
2. Experimental
2.1. Synthesis of the peptides
2.1.4. Boc-bAla-Aib-OMe (3)
Peptide 3 was synthesized following the same procedure as that
of peptide 1, starting with Boc-bAla-OH (2.0 g, 10.57 mmol). Yield:
2.6 g (85.52%).
The peptides were synthesized by conventional solution phase
methods. The tert-butyloxycarbonyl (Boc) group was used for N-
terminal protection and the C-terminus was protected as methyl
ester. Couplings were mediated by N,N0-dicyclohexylcarbodiimide
(DCC)/1-hydroxybenzotriazol (HOBT) [49]. All intermediates were
characterized by thin layer chromatography on silica gel and used
without further purification. Final peptides were purified by col-
umn chromatography using silica gel (100–200 mesh) as the sta-
tionary phase. Ethyl acetate and petroleum ether mixture was
used as the eluent. The peptides II and III were fully characterized
by X-ray crystallography and NMR studies and peptide I with NMR
spectroscopy.
2.1.5. Boc-bAla-Aib-OH (4)
Peptide 4 was synthesized following the same procedure as that
of peptide 2. Yield: 2.1 g (85.02%).
2.1.6. Boc-bAla-Aib-m-ABA-OMe (peptide II)
Synthesis and purification of peptide II were done following the
same procedure as in the case of peptide I starting with peptide 4.
Single crystals of peptide II were grown from CHCl3–petroleum
ether by slow evaporation and were stable at room temperature.
Yield: 2.7 g (86.53%). Mp = 108 °C; IR (KBr): 3357.9, 3322.6,
2981.9, 1710.3, 1668.5, 1532.2 cmꢁ1 1H NMR 300 MHz (CDCl3, d
;
2.1.1. Boc-Gly-Aib-OMe (1)
Boc-Gly-OH (2.0 g, 11.42 mmol) was dissolved in a mixture of
dichloromethane (DCM, 6 ml) and dimethylformamide (DMF,
5 ml). H-Aib-OMe obtained from its hydrochloride (3.5 g,
22.84 mmol) was added to the former solution followed by addi-
tion of DCC (3.52 g, 17.13 mmol) and HOBT (1.54 g, 11.42 mmol)
in ice-cold condition. The reaction mixture was stirred at room
temperature for 1 day. The precipitated N,N0-dicyclohexylurea
(DCU) was filtered off. The organic layer was diluted with 30 ml
ethyl acetate and washed with 1 N HCl (3 ꢀ 30 ml), brine, 1 M
Na2CO3 solution (3 ꢀ 30 ml) and then again with brine. The solvent
was dried over anhydrous Na2SO4 and evaporated in vacuo to give a
waxy colorless solid. Yield: 2.7 g (86.26%).
ppm): 9.32 (m-ABA(3)-NH, 1H, s), 8.14 (Ha m-ABA(3), 1H, s), 7.77
(Hd m-ABA(3), 1H, d, J = 8.1 Hz), 7.75 (Hb m-ABA(3), 1H, d,
J = 7.9 Hz), 7.37 (Hc m-ABA(3), 1H, m), 6.4 (Aib(2)-NH, 1H, s), 5.10
(bAla(1)-NH, 1H, s), 3.90 (AOCH3, 3H, s), 3.39–3.45 (CbH of bAla(1),
a
2H, m), 2.35–2.49 (C H of bAla(1), 2H, m), 1.62 (CbH of Aib(2), 6H,
s), 1.42 (Boc-CH3s, 9H, s); 13C NMR 75 MHz (CDCl3, d ppm): 173.02,
172.45, 166.80, 156.42, 138.53, 130.78, 128.94, 125.16, 124.61,
121.04, 79.83, 58.57, 52.11, 37.48, 36.96, 28.34, 25.36; Anal. Calcd
for C20H29N3O6 (407.47): C, 58.90; H, 7.17; N, 10.31%; Found: C,
58.82; H, 7.08; N, 10.25%.
2.1.7. Boc-cAbu-Aib-OMe (5)
Peptide 5 was synthesized following the same procedure as that
2.1.2. Boc-Gly-Aib-OH (2)
of peptide 1, starting with Boc-cAbu-OH (2.0 g, 9.84 mmol). Yield:
Peptide 1 (2.7 g, 9.85 mmol) was dissolved in methanol (20 ml)
and 2 N NaOH (10 ml) was added to it. The reaction mixture was
stirred for 1 day at room temperature. The progress of reaction
was monitored by TLC. After completion of reaction the methanol
was evaporated. The residue was diluted with water and washed
2.6 g (87.54%).
2.1.8. Boc-cAbu-Aib-OH (6)
Peptide 6 was synthesized following the same procedure as that
of peptide 2. Yield: 2.2 g (89.06%).