Azepane quaternary amino acids as effective inducers of 310 helix conformations

Article abstract of DOI:10.1021/jo301379r

A simple method for the synthesis of an azepane quaternary amino acid in enantiopure form is described. Theoretical, NMR, and X-ray studies indicated that this azepane-derived amino acid is an effective stabilizer of 3 ₁₀ helical structures in short peptides.

Full text of DOI:10.1021/jo301379r

Note
pubs.acs.org/joc
Azepane Quaternary Amino Acids As Effective Inducers of 310 Helix
Conformations
Diego Nunez-Villanueva,† Lourdes Infantes,‡ M. Teresa García-Lopez,† Rosario Gonzalez-Muniz,†
́
́
́
̃
̃
and Mercedes Martín-Martínez*,†
†Instituto de Química Med
́
ica (IQM-CSIC), Juan de la Cierva 2, 28006 Madrid, Spain
‡Instituto de Química Física Rocasolano (IQFR-CSIC), Serrano 119, 28006 Madrid, Spain
S
* Supporting Information
ABSTRACT: A simple method for the synthesis of an azepane quaternary
amino acid in enantiopure form is described. Theoretical, NMR, and X-ray
studies indicated that this azepane-derived amino acid is an effective stabilizer
of 310 helical structures in short peptides.
he relevant role of secondary peptide structure in
Tpeptide−protein or protein−protein interactions has
prompted the research for scaffolds able to induce or mimic
these motifs.1−3 In this sense, Cα-tetrasubstituted amino acids
are useful tools to reduce the conformational space available to
peptides, especially when the α,α substituents are incorporated
within a ring structure. Studies on peptides incorporating cyclic
quaternary α-amino acids indicated their general preference for
adopting turn or helical structures.4,5 However, quite
frequently, these studies have been carried out with peptides
that also contain another restricted amino acid, as Pro or Aib,
that by themselves have a marked tendency toward particular
secondary structure elements.6,7 Only a few reports deal with
conformationally constrained peptides in which the restriction
is exclusively imposed by cyclic quaternary amino acids. For
instance, the seven member cycloaliphatic derivative, 1-
aminocycloheptane-1-carboxylic acid, led to the adoption of
β-turns in tetrapeptide models (Figure 1, I).8 Related to
heterocyclic tetrasubstituted amino acids, the achiral 1,2-
dithiolane derivative (II) was also able to induce β-turns,9
while the chiral tetrahydrofuran-derived amino acid RSS-TAA
(III), prepared as racemic, stabilized either β-turns or distorted
10 helix structures.10 The achiral 4-piperidine, and specially its
3
N-oxide analogue TOAC (IV), are by far the most widely
studied heterocyclic quaternary amino acids, which have been
described as effective β-turn and 310/α-helix inducers in
peptides, especially in combination with Aib residues.6,11,12 In
absence of any other structuring element, the incorporation of
two TOAC residues in medium-size peptides favors the
adoption of either 310 or α-helical structures,13 while just one
residue induces a right-handed helix, starting by a 310 helix and
changes to a two consecutive α-turn motif near the C-
terminus.14
Last year, we have described quaternary α,α-2-oxoazepane α-
amino acid derivatives, like 1, that when incorporated into the i
+ 1 position of tetrapeptide models are able to induce β-turn
secondary structures.15 Compound 1 can be synthesized from
commertially available H-Orn(Z)-OMe in seven steps with an
overall yield of 32%.15 In these models, the 2-CO group of the
heterocyclic ring participates in an intramolecular H-bond with
the oxoazepane 4-NH proton, thus contributing to the global
3D arrangement. However, this intraresidue H-bond could be a
handicap when the α,α-quaternary amino acid occupies other
positions within the peptide, since it could hamper the
participation of this NH proton in hydrogen bonds with
other residues, like in helix conformations. To circumvent this
issue, here we describe the selective transformation of
compound 1 to the orthogonally protected azepane amino
acid derivative 3, and its influence on the adoption of stable
Figure 1. Structure of some cyclic and heterocyclic tetrasubstituted
amino acids.
Received: July 6, 2012
© XXXX American Chemical Society
A
dx.doi.org/10.1021/jo301379r | J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Note
Scheme 1. Synthesis of the Azepane-Derived Quaternary Amino Acid 3
peptide secondary structures when incorporated into pentapep-
tide models. In addition, compound 3 could be suitable for
further derivatization to allow side-chain cross-linking in
peptides, as previously described for the lower piperidine
homologue.16
The convenient synthetic method implied the selective
reduction of lactam 1 with diphenylsilane in the presence of a
rhodium catalyst,17 followed by catalytic hydrogenolysis to
remove the benzyl group (Scheme 1). Next, treatment of the
resulting azepane 2 with Alloc-chloride and the subsequent
hydrolysis of the ester gave compound 3, suitably protected for
the incorporation into peptide sequences (64% overall yield
from 1). All the steps proceeded with complete preservation of
the configurational integrity of the stereocenters, leading to
enantiopure 3.
To characterize the conformational preferences of small
peptides that incorporate our azepane derived amino acid, we
decided to study their ability to induce turns or helical
structures. To this aim, we selected model systems able to
mimic pentapeptides, Ac-Xaa-Yaa-Zaa-NHMe18 in which two
of the residues are Ala, and the third one is our template (Table
1, A−C). In these models, any bias toward a particular 3D
Figure 2. Representative minimal energy conformers of tripeptides
that incorporate the azepane amino acid at i +1 (A), i + 2 (B) and i + 3
(C).
Table 1. Theoretical Percentage of Conformers with Reverse
Turns
On the whole, our molecular modeling studies suggest that
the azepane residue is an efficient inducer of reverse turns, with
higher structure degree when the restricted amino acid moves
toward the N-terminal part of the model peptides (A and B).
To get additional knowledge about the conformation
induced by the azepane-derived amino acid into peptides, we
selected a few pentapeptide models to be synthesized and
further studied. Thus, tripeptide derivatives 4 and 5 were
obtained from amino acid 3 by coupling with the
corresponding H-Ala-Ala-NHR3 dipeptide amides (Scheme
2). Removal of the Alloc group from compound 5 yielded
azepane-deprotected analogue 6, while the N-Boc deprotection,
followed by acetylation, gave the Ac-tripeptide derivative 7. In a
Scheme 2. Synthesis of Azepane-Containing Tripeptide
Derivatives
a
Within a 3 kcal/mol window from the global minimum.
structure could be solely attributed to the azepane derivative
contribution. Theoretical studies, carried out by molecular
dynamic simulations using Amber as the force field, indicated
that the azepane containing peptides A−C have a marked
tendency to adopt either α or β-turns (Table 1). Analysis of the
dihedral angles of the minimum energy families showed that
either the global minimum in B and C, or a family over 0.2
kcal/mol from the global minimum in A, is able to adopt two
consecutive type III β-turns, therefore constituting two turns of
a 310 helix,19 in which the characteristic hydrogen bonds are
observed (Figure 2, and Supporting Information Table S1).
Additionally, the global minimum of peptide A and the second
family of low energy in B are a combination of a type I-αRS and
type I or III β-turns.
B
dx.doi.org/10.1021/jo301379r | J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Note
Table 2. Chemical Shifts of NH Protons in CDCl3 and DMSO-d6
a
ab
,
δ (CDCl3)
δ (DMSO)
compd NHi+1
NHi+2
NHi+3
NHi+4
6.89
NHi+1
NHi+2
8.00 (1.52)
NHi+3
NHi+4
4
4.82,
5.00
6.48
7.57
6.95, 7.02 (1.95−2.20)
7.85 (0.28)
7.41, 7.48 (0.52−0.59)
5
5.03,
5.13
6.50
7.59
6.53,
6.57
7.01, 7.07 (1.98−2.04)
8.09 (1.59)
7.83 (0.24)
7.15, 7.18 (0.58−0.65)
c
6
7
7.30
8.23
7.84
7.34
6.45
5.32
6.53,
6.70
7.56,
7.63
6.67,
6.73
7.80, 7.82 (1.35−1.37)
8.13 (1.43−1.60)
7.73 (0.10−0.17)
7.07, 7.10 (0.34−0.43)
9
6.55,
6.57
7.45,
7.47
7.16
7.31, 7.32 (1.99−2.00)
7.46, 7.48 (0.93−0.89)
7.58, 7.59 (0.11−0.14)
7.46 (0.30)
a
NH were assigned on the basis of COSY and ROESY experiments; see the Supporting Information. When there are two NH signals, each
b
c
corresponds to one of the rotamers. ΔδDMSO−CDCl3 values are indicated in parentheses. Not soluble in CDCl3.
similar way, the incorporation of the constrained amino acid at
the i + 2 position of the model peptide was carried out by
sequential deprotection and coupling, first at the C-terminal
and then at the N-terminal part, affording the corresponding
tripeptide derivative 9.
The conformational preferences of tripeptides 4−7 and 9
were analyzed by 1H NMR; in particular, we studied the
chemical shift of amide protons in CDCl3 and their variation
when the solvent was changed to DMSO-d6 (Table 2). As well,
we evaluated the temperature coefficient values (Δδ/ΔT, Table
3). As expected, the presence of an Alloc substituent on the
and NHi+4 amide protons are pinpointing to their involvement
in hydrogen bonds. On the contrary, the variation of the
chemical shift when the solvent is changed from CDCl3 to
DMSO-d6 for NHi+1 and NHi+2 is between 0.9 and 2.2 pm,
which is indicative of solvent accessible NHs. It is interesting to
mention that different acyl groups at N-terminus (4 vs 7) or
variable alkyl amides at C-terminal (4 vs 5) were always
compatible with the existence of these hydrogen bonds. In a
similar manner, the heterocyclic amino group can be either
protected or deprotected without significant influence on the
ability of the peptides to be structured (5 vs 6). When the
quaternary amino acid is at the i + 2 position, as in compound
9, the i + 4 amide proton is also solvent protected, whereas the
temperature coefficient for the NHi+3 is in the uncertainty
range, suggesting a slightly lower stabilization of folded
structures. However, the small chemical shift variation of the
NHi+3 proton in 9 when the solvent is changed from CDCl3 to
DMSO-d6 indicated once more that both amide protons
(NHi+3 and NHi+4) are involved in H-bonds. The experimental
temperature coefficients measured for peptides 4−7 and 9 can
be correlated with the two main secondary structures predicted
by the theoretical studies, either a 310 helix or a combination of
an α- and a β-turn. Unfortunately, no conclusive data toward
any of these conformations could be obtained by examining the
NOESY and ROESY 2D-spectra.
Further insights into the conformational preference of these
derivatives were obtained from the crystal structure of
compound 7.21 Tripeptide derivative 7 crystallizes with an
EtOAc molecule that participates in stabilizing the crystal
packing (see Supporting Information for details). In the solid
state, peptide 7 assumes a 310 helix conformation, having the
characteristic hydrogen bonds of this structure. The first H-
bond is formed between the CO group of the acetyl moiety and
the Ala NHi+3 amide proton (distance N···O 2.9 Å and angle
NH···O 160.8°), while the second is formed between the
carbonyl of the azepane residue and the C-terminal NHi+4
proton (distance N···O 3.1 Å and angle NH···O 158.7°).
Additionally, the dihedral angles match very well to those
Table 3. Temperature Coefficients for NH Protons
ab
,
Δδ/ΔT (ppb/K)
compd
NHi+1
NHi+2
NHi+3
NHi+4
4
5
6
7
9
−7.6, −9.6
−7.6, −8.2
−6.6
−4.3, −4.3
−4.7
−8.6
−7.7
−6.2
−6.0
−4.3
−2.7
−2.6
−2.5
−2.0
−3.7
−0.4, −1.6
−0.2, −2.1
−1.4
−1.6, −1.5
−2.0
a
NH were assigned on the basis of COSY and ROESY experiments.
When there are two NH coefficients, each corresponds to one of the
b
rotamers. Δδ measured in DMSO-d6, 30−45 °C (each 5 °C for a
total of 4 points), except for compound 9 measured in DMSO-d6, 45−
60 °C (each 5 °C for a total of 4 points); samples at 7−10 mM
concentration.
heterocyclic NH give rise to two sets of signals corresponding
to the rotamers of the urethane moiety, which coalesce when
increasing the temperature. For the conformational studies,
whenever possible both sets of signals were considered.
Values of δ (CDCl3) higher than 7 ppm and small variation
of ΔδDMSO−CDCl are indicative of NHs shielded from solvent.20
3
Taking into consideration this criteria, NHi+3 is solvent-
shielded, whereas the others NH have a chemical shift in
CDCl3 below 7 ppm. However, the chemical shift of NHi+4 is
close to 7.00 ppm, and its variation when the solvent is changed
to DMSO-d6 is small (0.3−0.6 ppm), which might be
suggestive of this proton also being involved in a hydrogen
bond. On the other hand, Δδ/ΔT coefficients below 3 ppb·K−1
(in absolute value) are indicative of solvent-shielded NH
protons, which in small peptides suggest their participation in
intramolecular hydrogen bonds, whereas values above 4
ppb·K−1 indicate NH protons accessible to solvent.20 If we
consider variable temperature studies, when the restricted
amino acid is incorporated at the i + 1 position, as in peptide
derivatives 4−7, the temperature coefficient values for NHi+3
expected for this type of helical structure (ϕi+1 = −59.9; ψi+1
=
−27.3; ϕi+2 = −61.5; ψi+2 = −21.9; ϕi+3 = −78.3; ψi+3 = −18.0)
(Figure 3). A good overall superposition is observed between
the X-ray structure and one of the minimum energy families,
A2.
In conclusion, we have presented a facile synthesis of an
orthogonally protected azepane-containing α,α-tetrasubstituted
amino acid derivative (3R,4S-configuration). We have also
shown that when incorporated into model pentapeptides, this
quaternary amino acid is able to efficiently induce the adoption
C
dx.doi.org/10.1021/jo301379r | J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Note
A solution of this N-benzyl azepane derivative (0.070 g, 0.19 mmol)
in MeOH (10 mL) was treated with 37% HCl (0.016 mL, 0.19 mmol)
and Pd(OH)2 (0.021 g, 30% w/w). The suspension was hydrogenated
at 50 °C and 45 psi for 12 h. After filtration of the catalyst, the solvent
was evaporated. Product 2 was used for the following reaction without
further purification. Syrup: 0.061 g, quantitative yield; [α]D20 = +5.5 (c
0.9, CHCl3); HPLC tR = 1.24 min (gradient A/B from 2:98 to 5:95
over 20 min); 1H NMR (300 MHz, DMSO-d6) δ 0.91 (d, 3H, J = 7.1,
3-CH3), 1.36 (s, 9H, CH3, Boc), 1.68 (m, 2H, 6-H), 2.16 (m, 1H, 5-
H), 2.27 (m, 1H, 5-H), 2.54 (m, 1H, 3-H), 2.92 (m, 2H, 2-H, 7-H),
3.12 (m, 2H, 2-H, 7-H), 3.60 (s, 3H, OCH3), 7.06 (s, 1H, 4-NH). 13
C
NMR (75 MHz, DMSO-d6) δ 14.6 (3-CH3), 19.0 (6-C), 28.1 (CH3,
Boc), 32.3 (5-C), 37.3 (3-C), 45.0 (7-C), 46.5 (2-C), 52.0 (OCH3),
63.3 (4-C), 78.4 (C, Boc), 155.0 (CO, Boc), 173.5 (COO); MS (ESI)
m/z = 287.3 [M − HCl + H]+. Elemental analysis calcd (%) for
C14H27N2O4Cl: C 52.09, H 8.43, N 8.68. Found (%): C 52.04, H 8.61,
N 8.30.
Figure 3. X-ray structure of derivative 7 [Ac-Aze(Alloc)-Ala-Ala-
NHiPr]. In the ORTEP view, ellipsoids are drawn at 30% probability
level.
(3R,4S)-1-Allyloxycarbonyl-4-(tert-butoxycarbonyl)amino-4-
carboxy-3-methylazepane (3). Compound 2 (0.060 g, 0.19 mmol)
was dissolved in dry CH2Cl2 (6 mL) and TEA (0.026 mL, 0.19 mmol),
propylene oxide (0.195 mL, 2.79 mmol) and Alloc-Cl (0.040 mL, 0.37
mmol) were successively added. The solution was stirred at room
temperature for 12 h. After evaporation of the solvent, the residue was
purified on a silica gel column, using EtOAc/hexane (1:3), leading to
of 310 helical structures, both in solution and in solid state, thus
constituting a new tool to stabilize the conformational
preferences of short peptides.22 Since the incorporation of
substituents at the NH group of the azepane ring seems a priori
straightforward, the preparation of diverse peptide derivatives
for use as biological probes could be easily envisaged.
(3R,4S)-1-allyloxycarbonyl-4-(tert-butoxycarbonylamino)-4-methoxy-
20
carbonyl-3-methylazepane as a syrup: yield, 0.052 g, 76%; [α]D
=
EXPERIMENTAL SECTION
+12.8 (c 1.0, CHCl3); HPLC tR = 14.68 min (major rotamer) and
■
Molecular Modeling Studies. Molecular dynamic simulations
were carried out using the Amber 10 suite of programs with the ff99SB
force field. Antechamber was used to assign atom types to model
tripeptides and to calculate a set of point charges using the AM1-BCC
charge model. The Hawkins, Cramer, Truhlar pairwise generalized
Born model was used to simulate implicit waters. The molecules were
relaxed by energy minimization, and then a molecular dynamic
simulation was carried out during 40 ps at constant temperature (300
K). Then the system was heated to 1000 K during 350 ps and allowed
to stay at this temperature for 100 ps. The structures were
subsequently cooled slowly to 300 K in steps; in each step the
temperature was lowered by 100 K, and the system was allowed to stay
at the new temperature for 100 ps, with 200 ps extra at 300 K. The
final conformation obtained was energy-refined using steepest descent
followed by conjugate gradient algorithm with a final gradient of 0.001
kcal/mol as the convergence criteria. The conformers were stored and
used to start a new simulation at high temperature. This procedure
afforded samples of 100 energy-minimized conformations, which were
compared to each other to eliminate the identical ones. The resulting
structures were visually inspected using the computer program VMD
and analyzed using the ptraj analysis program within Amber.
15.92 (minor rotamer) (gradient A/B from 5:95 to 80:20 over 20
1
min); H NMR (300 MHz, CDCl3, two rotamers, Mr/mr = 1.2:1) δ
0.86 (d, 3H, J = 7.1, 3-CH3, mr), 0.87 (d, 3H, J = 7.1, 3-CH3, Mr), 1.44
(s, 9H, CH3, Boc), 1.76 (m, 2H, 6-H), 1.88 (m, 1H, 5-H), 2.33 (m,
1H, 3-H), 2.61 (m, 1H, 5-H), 3.02 (m, 1H, 2-H), 3.21 (m, 1H, 7-H),
3.63 (m, 2H, 2-H, 7-H), 3.71 (s, 3H, OCH3, Mr), 3.72 (s, 3H, OCH3,
mr), 4.56 (s, 1H, 4-NH), 4.59 (m, 2H, 1′-H, Alloc), 5.21 (dq, 1H, J =
10.6 and 1.4, 3′-H, Alloc), 5.30 (ddt, 1H, J = 17.2, 2.4 and 1.4, 3′-H,
Alloc), 5.94 (ddt, 1H, J = 17.2, 10.6 and 5.4, 2′-H, Alloc); 13C NMR
(75 MHz, CDCl3) δ 13.9 (3-CH3), 21.3 (6-C, mr), 21.9 (6-C, Mr),
28.5 (CH3, Boc), 31.8 (5-C), 41.2 (3-C, Mr), 41.5 (3-C, mr), 46.3 (7-
C, Mr), 46.5 (7-C, mr), 47.7 (2-C, Mr), 48.5 (2-C, mr), 52.5 (OCH3,
Mr), 52.6 (OCH3, mr), 64.4 (4-C, mr), 64.5 (4-C, Mr), 66.2 (1′-C,
Alloc), 80.1 (C, Boc, mr), 80.2 (C, Boc, Mr), 117.4 (3′-C, Alloc,
mr),117.5 (3′-C, Alloc, Mr), 133.3 (2′-C, Alloc), 155.5 and 156.1
(CO, Alloc and Boc, mr), 155.7 and 156.5 (CO, Alloc and Boc, Mr),
174.4 (COO, Mr), 174.5 (COO, mr); MS (ESI) m/z = 393.2 [M +
Na]+. Elemental analysis calcd (%) for C18H30N2O6: C 58.36, H 8.16,
N 7.56. Found (%): C 58.11, H 8.17, N 7.49.
A solution of that resulting 1-allyloxycarbonyl-azepane derivative
(0.052 g, 0.14 mmol) in MeOH (2 mL) was treated with 2N NaOH
(0.702 mL, 1.40 mmol) and stirred at 65 °C for 48 h. Then, the
solvent was evaporated to dryness, and the residue was dissolved in
H2O and washed with EtOAc. The aqueous phase was acidified with 1
M HCl to pH 3 and extracted with EtOAc. The organic phase was
separated, dried over Na2SO4 and evaporated to dryness. Product 3
was used for the following reaction without further purification. Solid:
(3R,4S)-4-(tert-Butoxycarbonylamino)-4-methoxycarbonyl-
3-methylazepane hydrochloride (2). A solution of compound 1
(0.137 g, 0.35 mmol) in dry THF (8 mL) under Ar atmosphere was
treated with RhH(CO)(PPh3)3 (0.010 g, 0.01 mmol) and Ph2SiH2
(0.163 mL, 0.88 mmol), and stirred at room temperature for 40 h.
After evaporation of the solvent, the residue was purified on a silica gel
column, using MeOH/CH2Cl2 (1:90), yielding (3R,4S)-1-Benzyl-4-
(tert-butoxycarbonyl)amino-4-methoxycarbonyl-3-methylazepane as a
syrup: yield, 0.119 g, 90%; [α]D20 = +14.8 (c 0.9, CHCl3); HPLC tR =
3.43 min (protonated amine) and 3.54 min (deprotonated amine)
(gradient A/B from 10:90 to 100:0 over 5 min); 1H NMR (300 MHz,
CDCl3) δ 0.72 (d, 3H, J = 7.1, 3-CH3), 1.39 (s, 9H, CH3, Boc), 1.56
(m, 2H, 6-H), 2.06 (m, 1H, 5-H), 2.21 (m, 1H, 3-H), 2.43 (m, 1H, 5-
H), 2.54 (m, 4H, 7-H, 2-H), 3.50 (d, 1H, J = 13.4, 1-CH2), 3.57 (d,
1H, J = 13.4, 1-CH2), 3.64 (s, 3H, OCH3), 5.21 (s, 1H, 4-NH), 7.15−
7.30 (m, 5H, CH, Bn); 13C NMR (75 MHz, CDCl3) δ 15.8 (3-CH3),
24.1 (6-C), 28.5 (CH3, Boc), 33.4 (5-C), 40.4 (3-C), 52.2 (OCH3),
55.4 (7-C), 59.1 (2-C), 63.6 (1-CH2), 64.6 (4-C), 79.5 (C, Boc),
127.2, 128.5, 128.9, and 139.7 (1-C and CH, Bn), 155.7 (CO, Boc),
174.6 (COO); MS (ESI) m/z = 377.5 [M + H]+, 378.5 [M + 2H]+.
Elemental analysis calcd (%) for C21H32N2O4: C 66.99, H 8.57, N
7.44. Found (%): C 66.81, H 8.62, N 7.30.
20
mp 83−86 °C (EtOAc); yield, 0.046 g, 93%; [α]D = +14.5 (c 0.7,
CHCl3); HPLC tR = 4.94 min (gradient A/B from 10:90 to 100:0 over
5 min); 1H-RMN (300 MHz, CDCl3, two rotamers, Mr/mr = 1.1:1) δ
0.93 (d, 3H, J = 6.9, 3-CH3, mr), 0.94 (d, 3H, J = 7.0, 3-CH3, Mr), 1.44
(s, 9H, CH3, Boc), 1.73 (m, 1H, 5-H), 1.86 (m, 2H, 6-H), 2.38 (m,
1H, 3-H), 2.63 (m, 1H, 5-H), 3.04 (m, 1H, 2-H), 3.18 (m, 1H, 7-H),
3.70 (m, 2H, 2-H, 7-H), 4.60 (m, 2H, 1′-H, Alloc), 4.65 (s, 1H, 4-
NH), 5.21 (dq, 1H, J = 10.6 and 1.5, 3′-H, Alloc), 5.29 (dq, 1H, J =
17.1 and 1.5, 3′-H, Alloc), 5.92 (ddt, 1H, J = 17.1, 10.6 and 5.4, 2′-H,
Alloc); 13C-RMN (75 MHz, CDCl3) δ 13.9 (3-CH3), 21.2 (6-C, mr),
21.9 (6-C, Mr), 28.5 (CH3, Boc), 31.5 (5-C, mr), 31.7 (5-C, Mr), 41.1
(3-C, Mr), 41.3 (3-C, mr), 46.3 (7-C, mr), 46.4 (7-C, Mr), 47.6 (2-C,
mr), 48.4 (2-C, Mr), 64.3 (4-C), 66.2 (1′-C, Alloc), 80.7 (C, Boc),
117.5 (3′-C, Alloc), 133.2 (2′-C, Alloc), 156.2 and 156.5 (CO, Alloc
and Boc), 177.7 (COO, Mr), 177.8 (COO, mr); MS (ESI) m/z =
D
dx.doi.org/10.1021/jo301379r | J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Note
379.2 [M + Na]+. Elemental analysis calcd (%) for C17H28N2O6: C
57.29, H 7.92, N 7.86. Found (%): C 57.00, H 8.23, N 7.81.
Boc-Aze-Ala-Ala-NHiPr (6). A solution of tripeptide 5 (0.048 g,
0.09 mmol) in dry CH2Cl2 (3 mL) under Ar atmosphere was treated
with Pd(PPh3)4 (0.010 g, 0.009 mmol) and PhSiH3 (0.110 mL, 0.89
mmol). After being stirred at room temperature for 1 h, the solvent
was evaporated to dryness, and the residue was dissolved in H2O and
purified by reverse-phase flash chromatography using a gradient
CH3CN/H2O (0.05% TFA) from 0:100 to 100:0. Solid: mp 82−85 °C
(MeOH); yield, 0.035 g, 69%; [α]D20 = −1.2 (c 1.0, MeOH); HPLC tR
General Procedure for the Synthesis of Boc-Aze(Alloc)-Ala-
Ala-NHR Tripeptides. A solution of the azepane-derived amino acid
3 (0.233 g, 0.65 mmol) in dry CH2Cl2 (10 mL) was treated with BOP
(0.434 g, 0.98 mmol), H-Ala-Ala-NHR·HCl (0.98 mmol) and TEA
(0.273 mL, 1.96 mmol). After being stirred at room temperature for 15
h, the solution was washed successively with citric acid (10%),
NaHCO3 (10%), H2O and brine. The organic phase was dried over
Na2SO4 and evaporated to dryness. Then, the residue was purified on
a silica gel column, using the solvent system specified in each case.
Boc-Aze(Alloc)-Ala-Ala-NHMe (4). Solid: mp 92−95 °C
(EtOAc/hexane); yield, 0.170 g, 51%; eluent, MeOH/CH2Cl2
1
= 1.33 min (gradient A/B from 2:98 to 5:95 over 20 min); H NMR
(400 MHz, DMSO-d6) δ 0.92 (d, 3H, J = 7.1, 3-CH3), 1.03 (d, 6H, J =
7.0, CH3, iPr), 1.20 (d, 3H, J = 6.9, α-CH3, Alai+2), 1.24 (d, 3H, J = 7.3,
α-CH3, Alai+3), 1.41 (s, 9H, CH3, Boc), 1.68 (m, 2H, 6-H), 2.14 (m,
2H, 5-H), 2.54 (m, 1H, 3-H), 2.91 (m, 1H, 7-H), 3.12 (m, 2H, 2-H),
3.16 (m, 1H, 7-H), 3.78 (oct, 1H, J = 7.0, CH, iPr), 4.12 (quint., 1H, J
= 7.3, α-H, Alai+3), 4.15 (quint., 1H, J = 6.9, α-H, Alai+2), 7.30 (s, 1H,
4-NH), 7.34 (bs, 1H, NHiPr), 7.84 (d, 1H, J = 6.9, α-NH, Alai+3), 8.23
(bs, 1H, α-NH, Alai+2), 8.40 (bs, 1H, 1-H), 8.84 (bs, 1H, 1-H). 13C
NMR (75 MHz, DMSO-d6) δ 14.4 (3-CH3), 17.4 (α-CH3, Alai+3),
20
(1:40); [α]D = +18.2 (c 0.8, CHCl3); HPLC tR = 10.95 min
(gradient A/B from 5:95 to 80:20 over 20 min); 1H NMR (300 MHz,
DMSO-d6, two rotamers, Mr/mr = 1.2:1) δ 0.83 (d, 3H, J = 5.6, 3-
CH3, mr), 0.84 (d, 3H, J = 6.1, 3-CH3, Mr), 1.21 (d, 3H, J = 6.6, α-
CH3, Ala), 1.23 (d, 3H, J = 6.6, α-CH3, Ala), 1.41 (s, 9H, CH3, Boc),
1.58 (m, 2H, 6-H), 1.72 (m, 1H, 5-H), 2.08 (m, 1H, 3-H), 2.30 (m,
1H, 5-H), 2.56 (d, 3H, J = 4.6, NCH3), 3.03 (td, 1H, J = 13.0 and 6.0,
7-H, mr), 3.13 (td, 1H, J = 13.2 and 6.5, 7-H, Mr), 3.36 (m, 1H, 2-H),
3.44 (m, 1H, 2-H), 3.53 (m, 1H, 7-H), 4.15 (quint., 2H, J = 6.6, α-H,
Ala), 4.53 (m, 2H, 1′-H, Alloc), 5.18 (ddt, 1H, J = 10.5, 3.0 and 1.7, 3′-
H, Alloc), 5.26 (ddt, 1H, J = 17.2, 3.4 and 1.7, 3′-H, Alloc), 5.92 (ddt,
1H, J = 17.2, 10.5 and 5.0, 2′-H, Alloc), 6.95 (s, 1H, 4-NH, Mr), 7.02
(s, 1H, 4-NH, mr), 7.41 (m, 1H, NHCH3, Mr), 7.48 (m, 1H, NHCH3,
mr), 7.85 (d, 1H, J = 6.6, α-NH, Alai+3), 8.00 (m, 1H, α-NH, Alai+2);
13C NMR (75 MHz, DMSO-d6) δ 13.3 (3-CH3), 17.3 (α-CH3, Ala),
17.7 (α-CH3, Ala), 20.1 (6-C, mr), 20.8 (6-C, Mr), 25.5 (NCH3, Mr),
25.6 (NCH3, mr), 28.1 (CH3, Boc), 29.6 (5-C, mr), 29.9 (5-C, Mr),
40.6 (3-C), 45.3 (7-C, Mr), 45.4 (7-C, mr), 47.0 (2-C, mr), 47.6 (2-C,
Mr), 48.2 (α-CH, Alai+3, Mr), 48.3 (α-CH, Alai+3, mr), 49.5 (α-CH,
Alai+2), 63.3 (4-C, mr), 63.6 (4-C, Mr), 65.0 (1′-C, Alloc, Mr), 65.1
(1′-C, Alloc, mr), 78.8 (C, Boc, mr), 78.9 (C, Boc, Mr), 116.5 (3′-C,
Alloc, Mr), 116.6 (3′-C, Alloc, mr), 133.6 (2′-C, Alloc), 155.1 and
155.6 (CO, Alloc and Boc, mr), 155.3 and 155.5 (CO, Alloc and Boc,
Mr), 171.9 (CONH), 172.1 (CONH, Mr), 172.2 (CONH), 172.3
(CONH, mr); MS (ESI) m/z = 512.3 [M + H]+, 1045.5 [2M + Na]+.
Elemental analysis calcd (%) for C24H41N5O7: C 56.34, H 8.08, N
13.69. Found (%): C 56.22, H 7.94, N 13.43.
i
i
18.0 (α-CH3, Alai+2), 19.4 (6-C), 22.2 (CH3, Pr), 22.3 (CH3, Pr),
i
28.1 (CH3, Boc), 31.4 (5-C), 36.5 (3-C), 40.4 (CH, Pr), 46.0 (7-C),
47.5 (2-C), 48.4 (α-CH, Alai+3), 49.5 (α-CH, Alai+2), 63.4 (4-C), 79.2
(C, Boc), 155.5 (CO, Boc), 170.9, 171.8, and 174.3 (CONH); MS
(ESI) m/z = 456.3 [M − TFA + H]+, 911.7 [2M − TFA + Na]+.
Elemental analysis calcd (%) for C24H42N5O7F3: C 50.61, H 7.43, N
12.29. Found (%): C 50.72, H 7.14, N 12.17.
Ac-Aze(Alloc)-Ala-Ala-NHiPr (7). Tripeptide derivative 5 (0.045
g, 0.08 mmol) was dissolved in a solution of HCl/EtOAc (4 mL, 3.1
M), and the solution was stirred at room temperature for 4 h. After
evaporation of the solvent, the crude mixture was dissolved in dry
CH2Cl2 (4 mL) and TEA (0.012 mL, 0.08 mmol), and propylene
oxide (0.088 mL, 1.25 mmol) and Ac−Cl (0.007 mL, 0.10 mmol)
were added. The solution was stirred at room temperature for 18 h,
the solvent was evaporated to dryness, and the residue was purified by
chromatography on silica gel using MeOH:CH2Cl2 (1:10). Solid: mp
20
118−121 °C, EtOAc:hexane); yield, 0.031 g, 78%; [α]D = +25.6 (c
0.8, MeOH); HPLC tR = 8.56 min (gradient A/B from 5:95 to 80:20
1
over 20 min); H NMR (400 MHz, DMSO-d6, two rotamers, Mr/mr
= 1.1:1) δ 0.88 (d, 3H, J = 6.9, 3-CH3, Mr), 0.90 (d, 3H, J = 6.8, 3-
i
CH3, mr), 1.00 (d, 6H, J = 6.8, CH3, Pr, Mr), 1.02 (d, 6H, J = 6.8,
CH3, iPr, mr), 1.22 (d, 3H, J = 7.3, α-CH3, Alai+3), 1.24 (d, 3H, J = 7.3,
α-CH3, Alai+2), 1.39 (m, 1H, 6-H), 1.67 (m, 2H, 5-H, 6-H), 1.96 (m,
1H, 3-H), 2.01 (s, 3H, CH3, Ac), 2.46 (m, 1H, 5-H), 2.97 (td, 1H, J =
12.8 and 6.0, 7-H, mr), 3.06 (td, 1H, J = 13.0 and 6.6, 7-H, Mr), 3.23
(m, 1H, 2-H), 3.52 (m, 1H, 2-H), 3.64 (m, 1H, 7-H), 3.77 (oct, 1H, J
= 6.8, CH, iPr), 4.04 (quint., 1H, J = 7.3, α-H, Alai+3), 4.11 (quint., 1H,
J = 7.3, α-H, Alai+2, Mr), 4.12 (quint., 1H, J = 7.3, α-H, Alai+2, mr),
4.53 (m, 2H, 1′-H, Alloc), 5.16 (dq, 1H, J = 10.3 and 1.7, 3′-H, Alloc,
Mr), 5.17 (dq, 1H, J = 10.3 and 1.7, 3′-H, Alloc, mr), 5.24 (dq, 1H, J =
17.3 and 1.7, 3′-H, Alloc, Mr), 5.26 (dq, 1H, J = 17.3 and 1.7, 3′-H,
Alloc, mr),5.93 (ddt, 1H, J = 17.3, 10.3 and 5.0, 2′-H, Alloc), 7.07 (d,
1H, J = 6.8, NHiPr, mr), 7.10 (d, 1H, J = 6.8, NHiPr, Mr), 7.73 (d, 1H,
J = 7.3, α-NH, Alai+3), 7.80 (s, 1H, 4-NH, Mr), 7.82 (s, 1H, 4-NH,
mr), 8.13 (d, 1H, J = 7.1, α-NH, Alai+2); 13C NMR (75 MHz, DMSO-
d6) δ 13.4 (3-CH3, Mr), 13.5 (3-CH3, mr), 16.8 (α-CH3, Alai+2), 17.4
(α-CH3, Alai+3), 20.3 (6-C, mr), 20.9 (6-C, Mr), 22.1 (CH3, iPr), 22.2
Boc-Aze(Alloc)-Ala-Ala-NHiPr (5). Solid: mp 160−162 °C
(EtOAc/hexane); yield, 0.312g, 88%; eluent, EtOAc:hexane (8:1);
20
[α]D = +27.3 (c 0.9, CHCl3); HPLC tR = 12.50 min (gradient A/B
from 5:95 to 80:20 over 20 min); 1H NMR (400 MHz, DMSO-d6, two
rotamers, Mr/mr = 1.3:1) δ 0.84 (d, 3H, J = 6.6, 3-CH3, Mr), 0.86 (d,
3H, J = 6.6, 3-CH3, mr), 1.02 (d, 6H, J = 7.3, CH3, iPr), 1.21 (d, 3H, J
= 7.2, α-CH3, Ala), 1.24 (d, 3H, J = 7.2, α-CH3, Ala), 1.42 (s, 9H, CH3,
Boc), 1.48 (m, 1H, 6-H), 1.66 (m, 1H, 6-H), 1.73 (m, 1H, 5-H), 2.06
(m, 1H, 3-H), 2.34 (m, 1H, 5-H), 3.02 (td, 1H, J = 12.8 and 6.3, 7-H,
mr), 3.12 (td, 1H, J = 13.8 and 6.7, 7-H, Mr), 3.29 (m, 1H, 2-H), 3.46
(d, 1H, J = 13.6, 2-H, Mr), 3.49 (d, 1H, J = 12.3, 2-H, mr), 3.59 (m,
1H, 7-H), 3.78 (oct, 1H, J = 7.3, CH, iPr), 4.10 (quint., 2H, J = 7.2, α-
H, Ala), 4.52 (m, 2H, 1′-H, Alloc), 5.17 (ddt, 1H, J = 10.4, 2.5 and 1.6,
3′-H, Alloc), 5.25 (ddt, 1H, J = 17.5, 4.8 and 1.6, 3′-H, Alloc), 5.91
(ddt, 1H, J = 17.5, 10.4 and 4.8, 2′-H, Alloc), 7.01 (s, 1H, 4-NH, Mr),
7.07 (s, 1H, 4-NH, mr), 7.15 (d, 1H, J = 7.3, NHiPr, Mr), 7.18 (d, 1H,
J = 7.3, NHiPr, mr), 7.83 (d, 1H, J = 7.2, α-NH, Alai+3), 8.09 (m, 1H,
α-NH, Alai+2); 13C NMR (75 MHz, DMSO-d6) δ 13.3 (3-CH3, mr),
13.4 (3-CH3, Mr), 17.3 (α-CH3, Ala), 17.7 (α-CH3, Ala), 20.1 (6-C,
i
(CH3, Pr), 23.4 (CH3, Ac, Mr), 23.5 (CH3, Ac, mr), 29.4 (5-C, mr),
29.7 (5-C, Mr), 40.4 (CH, iPr), 41.1 (3-C), 45.2 (7-C, Mr), 45.4 (7-C,
mr), 47.4 (2-C, mr), 48.1 (2-C, Mr), 48.6 (α-CH, Alai+3), 49.6 (α-CH,
Alai+2), 64.0 (4-C), 65.0 (1′-C, Alloc), 116.5 (3′-C, Alloc), 133.6 (2′-
C, Alloc), 155.1 (CO, Alloc, mr), 155.3 (CO, Alloc, Mr), 171.0, 171.8
(mr), 171.9 (Mr), 172.1 and 173.9 (CONH); MS (ESI) m/z = 482.3
[M + H]+, 985.7 [2M + Na]+. Elemental analysis calcd (%) for
C23H39N5O6: C 57.36, H 8.16, N 14.54. Found (%): C 57.45, H 8.01,
N 14.64.
i
i
mr), 20.8 (6-C, Mr), 22.1 (CH3, Pr), 22.2 (CH3, Pr), 28.1 (CH3,
Boc), 29.8 (5-C, mr), 30.1 (5-C, Mr), 40.0 (3-C), 40.6 (CH, iPr), 45.2
(7-C, Mr), 45.4 (7-C, mr), 47.2 (2-C, mr), 47.8 (2-C, Mr), 48.4 (α-
CH, Alai+3), 49.6 (α-CH, Alai+2), 63.6 (4-C), 64.9 (1′-C, Alloc, Mr),
65.0 (1′-C, Alloc, mr), 78.9 (C, Boc, mr), 80.0 (C, Boc, Mr), 116.5
(3′-C, Alloc), 133.6 (2′-C, Alloc), 155.1 and 155.7 (CO, Alloc and
Boc, Mr), 155.3 and 155.7 (CO, Alloc and Boc, mr), 170.8, 172.1, and
174.4 (CONH); MS (ESI) m/z = 540.3 [M + H]+, 1101.7 [2M +
Na]+. Elemental analysis calcd (%) for C26H45N5O7: C 57.87, H 8.40,
N 12.98. Found (%): C 57.85, H 8.76, N 12.85.
Boc-Aze(Alloc)-Ala-NHMe (8). A solution of the azepane-derived
amino acid 3 (0.070 g, 0.20 mmol) in dry CH2Cl2 (7 mL) was treated
with BOP (0.133 g, 0.30 mmol), H-Ala-NHMe·HCl (0.041 g, 0.30
mmol) and TEA (0.083 mL, 0.60 mmol). After being stirred at room
temperature for 15 h, the solution was washed successively with citric
acid (10%), NaHCO3 (10%), H2O and brine. The organic phase was
E
dx.doi.org/10.1021/jo301379r | J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Note
dried over Na2SO4 and evaporated to dryness. Then, the residue was
crystallization after 7 days at room temperature in a closed jar,
saturated with hexane.
purified on a silica gel column, using EtOAc:hexane (8:1). Solid: mp
20
141−144 °C (MeOH); yield, 0.067 g, 78%. [α]D = +11.0 (c 1.3,
CHCl3); HPLC tR = 10.99 min (gradient A/B from 5:95 to 80:20 over
ASSOCIATED CONTENT
1
■
20 min); H NMR (400 MHz, DMSO-d6, two rotamers, Mr/mr =
S
* Supporting Information
1.1:1) δ 0.80 (d, 3H, J = 6.8, 3-CH3, Mr), 0.82 (d, 3H, J = 6.8, 3-CH3,
mr), 1.21 (d, 3H, J = 7.0, α-CH3, Ala), 1.42 (s, 9H, CH3, Boc), 1.46
(m, 1H, 6-H), 1.63 (m, 1H, 6-H), 1.68 (m, 1H, 5-H), 2.07 (m, 1H, 3-
H), 2.30 (m, 1H, 5-H), 2.56 (d, 3H, J = 4.5, NCH3), 3.01 (td, 1H, J =
13.2 and 6.8, 7-H, mr), 3.11 (td, 1H, J = 13.5 and 6.5, 7-H, Mr), 3.29
(m, 1H, 2-H), 3.45 (m, 1H, 2-H), 3.58 (m, 1H, 7-H), 4.23 (quint., 1H,
J = 7.0, α-H, Ala, Mr), 4.24 (quint., 1H, J = 7.0, α-H, Ala, mr), 4.52 (m,
2H, 1′-H, Alloc), 5.18 (dq, 1H, J = 10.5 and 1.5, 3′-H, Alloc), 5.26
(ddt, 1H, J = 17.2, 3.5 and 1.5, 3′-H, Alloc), 5.92 (ddt, 1H, J = 17.2,
10.5 and 5.1, 2′-H, Alloc), 6.94 (s, 1H, 4-NH, Mr), 7.00 (s, 1H, 4-NH,
mr), 7.62 (bs, 1H, NHCH3), 7.80 (d, 1H, J = 7.0, α-NH, Ala, Mr),
7.83 (d, 1H, J = 7.0, α-NH, Ala, mr); 13C NMR (75 MHz, DMSO-d6)
δ 13.2 (3-CH3, mr), 13.3 (3-CH3, Mr), 17.6 (α-CH3, Ala), 20.1 (6-C,
Mr), 20.8 (6-C, mr), 25.5 (NCH3), 28.2 (CH3, Boc), 30.0 (5-C), 40.8
(3-C), 45.3 (7-C, Mr), 45.5 (7-C, mr), 47.1 (2-C, Mr), 47.7 (2-C, mr),
48.2 (α-CH, Ala), 63.7 (4-C), 65.0 (1′-C, Alloc), 78.8 (C, Boc, Mr),
78.9 (C, Boc, mr), 116.5 (3′-C, Alloc, mr), 116.7 (3′-C, Alloc, Mr),
133.6 (2′-C, Alloc), 155.4 and 155.8 (CO, Alloc and Boc, Mr), 155.2
and 155.7 (CO, Alloc and Boc, mr), 172.5 and 173.1 (CONH); MS
(ESI) m/z = 441.2 [M + H]+, 463.3 [M + Na]+, 881.7 [2M + H]+,
903.5 [2M + Na]+. Elemental analysis calcd (%) for C21H36N4O6: C
57.25, H 8.24, N 12.72. Found (%): C 57.08, H 8.07, N 12.80.
Boc-Ala-Aze(Alloc)-Ala-NHMe (9). Dipeptide derivative 5 (0.035
g, 0.08 mmol) was dissolved in a solution of HCl/EtOAc (4 mL, 3.1
M), and the solution was stirred at room temperature for 4 h. After
evaporation of the solvent, the crude mixture was dissolved in dry
THF (6 mL), and TEA (0.030 mL, 0.22 mmol), Boc-Ala-OH (0.025 g,
0.13 mmol) and BOP (0.059 g, 0.13 mmol) were added. After being
stirred at 65 °C for 48 h, the solvent was evaporated to dryness, and
the residue was dissolved in EtOAc. Then, the solution was washed
successively with citric acid (10%), NaHCO3 (10%), H2O and brine.
The organic phase was dried over Na2SO4 and evaporated to dryness.
Crystallographic data (CIF) of derivative 7, computational
1
results and copies of the H and 13C NMR spectra of all
compounds. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*Fax: + 34 915644853. Tel: +34 915622900. E-mail:
iqmm317@iqm.csic.es.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This research was supported by SAF2009-09323, and
Consolider-Ingenio CSD2008-00005 (SICI) and CSD2006-
00015 (Crystallization Factory). D.N.-V. thanks the CSIC for a
JAE-predoc fellowship from the Program “Junta para la
́
Ampliacion de Estudios”, cofinanced by the ESF.
REFERENCES
■
(1) Stites, W. E. Chem. Rev. 1997, 97, 1233.
(2) Perez de Vega, M. J.; Martín-Martínez, M.; Gonzal
Curr. Top. Med. Chem. 2007, 7, 33.
(3) Meireles, L. M. C.; Mustata, G. Curr. Top. Med. Chem. 2011, 11,
248.
́
́
ez-Muniz, R.
̃
(4) Toniolo, C.; Crisma, M.; Formaggio, F.; Peggion, C. Biopolymers
2001, 60, 396.
(5) Maity, P.; Koenig, B. Biopolymers 2008, 90, 8.
(6) Yokum, T. S.; Gauthier, T. J.; Hammer, R. P.; McLaughlin, M. L.
J. Am. Chem. Soc. 1997, 119, 1167.
The residue was purified on a silica gel column, using EtOAc:hexane
20
(10:1). Solid: mp 73−76 °C (MeOH); yield, 0.019 g, 47%; [α]D
=
+27.6 (c 0.9, CHCl3); HPLC tR = 11.28 min (gradient A/B from 5:95
to 80:20 over 20 min); 1H NMR (400 MHz, DMSO-d6, two rotamers,
A/B = 1:1) δ 0.82 (d, 3H, J = 6.3, 3-CH3, A), 0.83 (d, 3H, J = 6.6, 3-
CH3, B), 1.25 (d, 3H, J = 7.6, α-CH3, Alai+1), 1.26 (d, 3H, J = 6.9, α-
CH3, Alai+3. A), 1.27 (d, 3H, J = 6.9, α-CH3, Alai+3, B), 1.39 (s, 9H,
CH3, Boc), 1.43 (m, 1H, 6-H), 1.64 (m, 1H, 6-H), 1.71 (m, 1H, 5-H),
2.03 (m, 1H, 3-H), 2.48 (m, 1H, 5-H), 2.57 (d, 3H, J = 4.7, NCH3),
2.97 (m, 1H, 7-H, A), 3.08 (td, 1H, J = 12.9 and 6.1, 7-H, B), 3.23 (m,
1H, 2-H), 3.52 (m, 1H, 2-H), 3.60 (m, 1H, 7-H), 3.97 (m, 1H, α-H,
Alai+1), 4.17 (quint., 1H, J = 6.9, α-H, Alai+3), 4.53 (m, 2H, 1′-H,
Alloc), 5.18 (ddt, 1H, J = 10.5, 3.2 and 1.7, 3′-H, Alloc), 5.26 (ddt, 1H,
J = 17.3, 3.2 and 1.7, 3′-H, Alloc), 5.92 (ddt, 1H, J = 17.3, 10.5 and 5.1,
2′-H, Alloc), 7.31 (d, 1H, J = 5.2, α-NH, Alai+1, A), 7.32 (d, 1H, J =
5.0, α-NH, Alai+1, B), 7.46 (q, 1H, J = 4.7, NHCH3), 7.48 (s, 1H, 4-
NH, A), 7.49 (s, 1H, 4-NH, B), 7.58 (d, 1H, J = 6.9, α-NH, Alai+3, A),
7.59 (d, 1H, J = 6.9, α-NH, Alai+3, B); 13C NMR (75 MHz, DMSO-d6)
δ 13.3 (3-CH3, A), 13.4 (3-CH3, B), 16.4 (α-CH3, Ala), 16.9 (α-CH3,
Ala, A), 17.5 (α-CH3, Ala, B), 20.0 (6-C, A), 20.7 (6-C, B), 25.6
(NCH3), 28.1 (CH3, Boc), 29.5 (5-C, A), 29.9 (5-C, B), 41.2 (3-C),
45.2 (7-C, A), 45.4 (7-C, B), 47.4 (2-C, A), 48.0 (2-C, B), 48.7 (α-CH,
Alai+3), 51.6 (α-CH, Alai+1, A), 51.7 (α-CH, Alai+1, B), 64.1 (4-C), 65.1
(1′-C, Alloc), 79.0 (C, Boc), 116.6 (3′-C, Alloc, A), 116.8 (3′-C, Alloc,
B), 133.5 (2′-C, Alloc, A), 133.6 (2′-C, Alloc, B), 155.0 and 156.3
(CO, Alloc and Boc, A), 155.2 and 156.3 (CO, Alloc and Boc, B),
172.3, 172.4, and 174.9 (CONH); MS (ESI) m/z = 512.3 [M + H]+,
1045.7 [2M + Na]+. Elemental analysis calcd (%) for C24H41N5O7: C
56.34, H 8.08, N 13.69. Found (%): C 56.26, H 8.21, N 13.54.
Preparation of Single Crystals for X-ray Diffraction Analysis.
Pure compound 7 (8 mg) was dissolved in EtOAc (4 mL), and the
mixture was put in a crystallizing dish, resulting in spontaneous
́ ́ ́ ́
(7) Jimenez, A. I.; Cativiela, C.; Gomez-Catalan, J.; Perez, J. J.; Aubry,
A.; París, M.; Marraud, M. J. Am. Chem. Soc. 2000, 122, 5811.
(8) Benedetti, E.; DiBlasio, B.; Iacovino, R.; Menchise, V.; Saviano,
M.; Pedone, C.; Bonora, G. M.; Ettorre, A.; Graci, L.; Formaggio, F.;
Crisma, M.; Valle, G.; Toniolo, C. J. Chem. Soc., Perkin Trans. 2 1997,
2023.
(9) Morera, E.; Lucente, G.; Ortar, G.; Nalli, M.; Mazza, F.; Gavuzzo,
E.; Spisani, S. Bioorg. Med. Chem. 2002, 10, 147.
(10) (a) Maity, P.; Zabel, M.; Konig, B. J. Org. Chem. 2007, 72, 8046.
̈
(b) Grauer, A. A.; Cabrele, C.; Zabel, M.; Konig, B. J. Org. Chem. 2009,
74, 3718.
̈
(11) Ousaka, N.; Tani, N.; Sekiya, R.; Kuroda, R. Chem. Commun.
2008, 2894.
(12) (a) Toniolo, C.; Crisma, M.; Formaggio, F. Biopolymers 1998,
47, 153. (b) Peggion, C.; Jost, M.; De Borggraeve, W. M.; Crisma, M.;
Formaggio, F.; Toniolo, C. Chem. Biodiversity 2007, 4, 1256.
(c) Schreier, S.; Bozelli, J.; Marín, N.; Vieira, R.; Nakaie, C. Biophys.
Rev 2012, 4, 45.
(13) (a) Toniolo, C.; Valente, E.; Formaggio, F.; Crisma, M.; Pilloni,
G.; Corvaja, C.; Toffoletti, A.; Martinez, G. V.; Hanson, M. P.;
Millhauser, G. L.; George, C.; Flippen-Anderson, J. L. J. Pept. Sci. 1995,
1, 45. (b) Smythe, M. L.; Nakaie, C. R.; Marshall, G. R. J. Am. Chem.
Soc. 1995, 117, 10555. (c) Flippen-Anderson, J. L.; George, C.; Valle,
G.; Valente, E.; Bianco, A.; Formaggio, F.; Crisma, M.; Toniolo, C. Int.
J. Pept. Protein Res. 1996, 47, 231. (d) Hanson, P.; Martinez, G.;
Millhauser, G.; Formaggio, F.; Crisma, M.; Toniolo, C.; Vita, C. J. Am.
Chem. Soc. 1996, 118, 271. (e) Hanson, P.; Anderson, D. J.; Martinez,
G.; Millhauser, G.; Formaggio, F.; Crisma, M.; Toniolo, C.; Vita, C.
Mol. Phys. 1998, 95, 957.
F
dx.doi.org/10.1021/jo301379r | J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Note
(14) Crisma, M.; Deschamps, J. R.; George, C.; Flippen-Anderson, J.
L.; Kaptein, B.; Broxterman, Q. B.; Moretto, A.; Oancea, S.; Jost, M.;
Formaggio, F.; Toniolo, C. J. Pept. Res. 2005, 65, 564.
(15) Nunez-Villanueva, D.; Bonache, M. A.; Infantes, L.; García-
́
̃
Lop
́
ez, M. T.; Martín-Martínez, M.; Gonzalez-Muniz, R. J. Org. Chem.
́
̃
2011, 76, 6592.
(16) Ousaka, N.; Sato, T.; Kuroda, R. J. Am. Chem. Soc. 2009, 131,
3820.
(17) (a) Kuwano, R.; Takahashi, M.; Ito, Y. Tetrahedron Lett. 1998,
39, 1017. (b) Gerona-Navarro, G.; Bonache, M. A.; Alías, M.; Per
Vega, M. J.; García-Lopez, M. T.; Lopez, P.; Cativiela, C.; Gonzal
Muniz, R. Tetrahedron Lett. 2004, 45, 2193.
́
ez de
́
́
́
ez-
̃
(18) Ac indicates acetyl group; Xaa, Yaa and Zaa indicates an amino
acid. Aze indicates (3R,4S)-4-amino-4-carbonyl-3-methylazepane.
(19) For clarity, type III β-turn is indicated: Venkatachalam, C. M.
Biopolymers 1968, 6, 1425. This type of turn is now considered to
correspond to a turn of a 310 helix: Richardson, J. S. Adv. Protein Chem.
1981, 34, 167.
(20) (a) Kessler, H. Angew. Chem., Int. Ed. 1982, 21, 512. (b) Belvisi,
L.; Gennari, C.; Mielgo, A.; Potenza, D.; Scolastico, C. Eur. J. Org.
Chem. 1999, 1999, 389. (c) Baeza, J. L.; Gerona-Navarro, G.;
Thompson, K.; Per
T.; Gonzalez-Muniz, R.; Martín-Martínez, M. J. Org. Chem. 2009, 74,
8203.
́ ́
ez de Vega, M. J.; aInfantes, L.; García-Lopez, M.
́
̃
(21) CCDC 887956 contains the supplementary crystallographic
data for compound 7. This data can be obtained free of charge from
The Cambridge Crystallographic Data Center via http://www.ccdc.
cam.ac.uk/data_request/cif.
(22) A marked helicogenic effect in short Ala-Aib pentapeptides was
recently described for a related 1H-Azepine-4-amino-4-carboxylic
residue: Pellegrino, S.; Contini, A.; Clerici, F.; Gori, A.; Nava, D.;
Gelmi, M. L. Chem.−Eur. J. 2012, 18, 8705.
G
dx.doi.org/10.1021/jo301379r | J. Org. Chem. XXXX, XXX, XXX−XXX