Amino equatorial effect of a six-membered ring amino acid on its peptide 310- and α-helices

Article abstract of DOI:10.1016/j.tet.2015.02.075

Abstract Two diastereomeric six-membered ring α,α-disubstituted α-amino acids (1R,3R)- and (1S,3R)-1-amino-3-methylcyclohexanecarboxylic acids (Ac₆c^3M); side-chain restricted leucine analogs, were stereoselectively synthesized from (3R)-3-methylcyclohexanone by a Bucherer-Bergs or Strecker reaction. Two series of homo-chiral homopeptides Cbz-[(1R,3R)- and (1S,3R)-Ac₆c^3M]_n-OMe, up to hexapeptides (n=6), were prepared, respectively, and the preferred conformations of cyclohexane rings of amino acid residues and the peptide-backbones were studied. In solution, these peptides formed helical structures, but the helical-screw control to one-handedness was not possible for the hexapeptide length. In the crystal state, all (1R,3R)-Ac₆c^3M residues formed cyclohexane chair form conformations with a 3-methyl substituent at equatorial orientation and an amino group at the axial position, whereas all (1S,3R)-Ac₆c^3M residues assumed cyclohexane chair forms with the 3-methyl and amino groups at equatorial orientations. The preferred peptide-backbone structure of (1R,3R)-Ac₆c^3M hexapeptide had (P) and (M) 3₁₀-helices, and that of (1S,3R)-Ac₆c^3M hexapeptide had (P) and (M) α-helices in the crystal state.

Full text of DOI:10.1016/j.tet.2015.02.075

Tetrahedron 71 (2015) 2409e2420
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Amino equatorial effect of a six-membered ring amino acid on its
peptide 310- and a-helices
Takayuki Hirata a, Atsushi Ueda b, Makoto Oba b, Mitsunobu Doi c, Yosuke Demizu d,
,
Masaaki Kurihara d, Masanobu Nagano a, Hiroshi Suemune a, Masakazu Tanaka b
*
a Graduate School of Pharmaceutical Sciences, Kyushu University, Fukuoka 812-8582, Japan
b Graduate School of Biomedical Sciences, Nagasaki University, Nagasaki 852-8521, Japan
c Osaka University of Pharmaceutical Sciences, Osaka 569-1094, Japan
d National Institute of Health Sciences, Tokyo 158-8501, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 26 January 2015
Received in revised form 20 February 2015
Accepted 21 February 2015
Available online 26 February 2015
Two diastereomeric six-membered ring a,a-disubstituted a-amino acids (1R,3R)- and (1S,3R)-1-amino-3-
methylcyclohexanecarboxylic acids (Ac6c3M); side-chain restricted leucine analogs, were stereo-
selectively synthesized from (3R)-3-methylcyclohexanone by a BucherereBergs or Strecker reaction. Two
series of homo-chiral homopeptides Cbz-[(1R,3R)- and (1S,3R)-Ac6c3M]n-OMe, up to hexapeptides (n¼6),
were prepared, respectively, and the preferred conformations of cyclohexane rings of amino acid resi-
dues and the peptide-backbones were studied. In solution, these peptides formed helical structures, but
the helical-screw control to one-handedness was not possible for the hexapeptide length. In the crystal
state, all (1R,3R)-Ac6c3M residues formed cyclohexane chair form conformations with a 3-methyl sub-
stituent at equatorial orientation and an amino group at the axial position, whereas all (1S,3R)-Ac6c3M
residues assumed cyclohexane chair forms with the 3-methyl and amino groups at equatorial orienta-
Keywords:
Amino acids
Chirality
Conformation analysis
Helical structures
Peptides
tions. The preferred peptide-backbone structure of (1R,3R)-Ac6c3M hexapeptide had (P) and (M) 310
helices, and that of (1S,3R)-Ac6c3M hexapeptide had (P) and (M)
-helices in the crystal state.
Ó 2015 Elsevier Ltd. All rights reserved.
-
a
1. Introduction
(tripeptide: type III b-turn, tetrapeptides: 310-helices) in the crystal
state, and the Ac6c had a chair form cyclohexane conformation. The
amino (amide) eNHe substituent usually showed axial orientation
and the carbonyl eCOe substituent occupied an equatorial orien-
tation at the chair conformation.5,6 The theoretical calculations of
AceAc6c-NHMe revealed that the amino eNHe axial minimum is
more stable than the eNHe equatorial minimum by w1.6 kcal/
mol.5b,7 Exceptionally, in the case of the free amino acid,5f,6b an
eNHe substituent equatorial occurred. Furthermore, the theoreti-
cal calculation indicated that the lowest energy conformation with
a
,
a
-Disubstituted
freedom of their oligopeptides.1,2 Achiral
-methylalanine, dimethylglycine; Aib, MeAla) is a prototype
disubstituted -amino acid, and is widely used to construct helical
secondary structures.3 The Aib-containing short peptides and Aib
homopeptides prefer to induce a 310-helix over an -helix. Besides
Aib, achiral cyclic -disubstituted
-amino acids (Acnc; n¼ring
size) such as 1-aminocyclopropanecarboxylic acid (Ac3c), 1-
aminocyclobutanecarboxylic acid (Ac4c), and 1-
aminocyclopentanecarboxylic acid (Ac5c) have been synthesized,
and these cyclic -disubstituted -amino acids-containing oligo-
peptides also induce 310
-helical secondary structures.4 Among
cyclic -disubstituted -amino acids, 1-aminocyclohexane-
a
-amino acids restrict the conformational-
a
-aminoisobutyric acid
(a
a
a,a-
a
a
a
,
a
a
torsion angles
f
ꢀ52ꢁ;
j
ꢀ46ꢁ occurred in the case of the eNHe
axial position, whereas that with torsion angles
f
ꢀ48ꢁ;
j
ꢀ60ꢁ5b
a
,
a
a
(or
f
ꢀ40ꢁ;
j
ꢀ80ꢁ5c) was preferred in the case of the eNHe
/
a
equatorial position. These results suggest that if the eNHe group of
Ac6c is controlled to an equatorial orientation (although it is un-
favorable in terms of energy), the peptide-backbone torsion angles
a
,a
a
carboxylic acid (Ac6c)-containing peptides have also been pre-
pared, and their preferred conformations of both the side-chain
cyclohexane ring of amino acid residues and peptide-backbone
structure have been studied by Toniolo, Balaram, Benedetti and
co-workers.5 The Ac6c homopeptides formed 310-helical structures
would prefer
f
ꢀ48ꢁ;
j
ꢀ60ꢁ,5b which are not those of the 310-helix,
but are closer to those of the a
-helix (Fig. 1a).8 However, no such
empirical studies on the relationship between the eNHe equatorial
orientation of the cyclohexane chair form or the peptide-backbone
structures have been reported so far. Toniolo, Benedetti and co-
workers reported X-ray crystallographic analysis of tetrapeptide
p-BrBz-(Ac6c)4-OtBu,5c where three of the eNHe groups of four
* Corresponding author. Tel./fax: þ81 95 819 2423; e-mail address: matanaka@
nagasaki-u.ac.jp (M. Tanaka).
http://dx.doi.org/10.1016/j.tet.2015.02.075
0040-4020/Ó 2015 Elsevier Ltd. All rights reserved.

2410
T. Hirata et al. / Tetrahedron 71 (2015) 2409e2420
2.1.1. Synthesis of (1R,3R)-Ac6c3M using
a BucherereBergs re-
action. Reaction of (3R)-1 with KCN and (NH4)2CO3 in 50% EtOH
aqueous solution at 60 ꢁC afforded a hydantoin product 2 (Scheme
1).10 Hydantoin 2 was a diastereomeric mixture of 2a and 2b at
a ratio of 4 to 1, and the re-crystallization of mixture 2 from hot
EtOH produced a diastereomerically pure 2a at a 67% yield. For
hydrolysis of the hydantoin moiety, once the amide functions in 2a
were protected as Boc-protecting groups (92%), hydrolysis with
aqueous LiOH gave a crude amino acid (1R,3R)-Ac6c3M 11
. The crude
(1R,3R)-Ac6c3M was protected with a Cbz-group at the N-terminus
and methyl ester at the C-terminus to give Cbz-[(1R,3R)-Ac6c3M]-
OMe 4a in 37% from Boc-protected hydantoin 3a. The relative ste-
reochemistry of 4a was assumed by the 2D correlation (COSY) and
nuclear Overhauser and exchange (NOESY) NMR spectra (Fig. S1).12
At first, the methine proton signal at the C(3)-position was de-
termined to have a peak at
d 1.60 based on the correlation with
methyl proton signals in the COSY NMR spectrum, and then, the
correlations among the methine proton, one proton (syn to the
methine) of methylene at the C(2)-position, and the NH amide
proton in the NOESY NMR spectrum suggested the stereochemistry
of 4a to be 1R,3R. Finally, the stereochemistry of 4a was un-
ambiguously confirmed by the X-ray crystallographic analysis of its
peptides (vide infra).
Fig. 1. Structures of achiral and chiral six-membered ring amino acids (Ac6c and
Ac6c3M), and their chair form conformations.
Ac6c residues occupied an equatorial orientation, and the related
j
torsion angles became slightly larger than those of Cbz-protected
Ac6c tetrapeptides with all axial eNHe groups. However, discus-
sions regarding the eNHe group equatorial effect on the peptide-
backbone structures were limited.5c
ꢀ
Jimenez, Cativiela and co-workers also reported the preferred
conformation
of
heteropeptides
having
chiral
1-
aminocyclohexanecarboxylic acids with a 2-phenyl group (c6Phe;
analogs of phenylalanine).9 Those c6Phe-containing dipeptides
preferentially formed
b-turn structures, which were stabilized by
an attractive NH to aromatic
p-orbitals interaction, and the eNHe
group oriented axial position, except for one crystal structure,
where the eNHe and the bulky 2-phenyl groups occupied an
equatorial orientation.9b However, the relationship between the
equatorial eNHe group and peptide-backbone structure was not
discussed because of phenyl group gauche and p-electronic effects.
We designed twodiastereomeric aminoacids(1S,3R)- and (1R,3R)-
1-amino-3-methylcyclohexanecarboxylic acids (Ac6c3M), which are
side-chain conformational-freedom restricted leucine analogs
(Fig. 1b). The 3-methyl substituent of side-chain cyclohexane should
have an equatorial orientation because if it takes an axial position,1,3-
diaxial repulsion between the methyl group and the carbonyl (or the
amino) group at the C(1)-position would arise. If the 3-methyl sub-
stituent of (1S,3R)-Ac6c3M occupies an equatorial position, the eNHe
substituent assumes an equatorial direction and the eCOe group an
axial direction. These orientations are opposite to those of the achiral
Ac6c, where the eNHe group occupied an axial orientation and the
eCOe group occupied an equatorial one. Thus, the equatorial orien-
tation of the eNHe group would prefer the peptide-backbone torsion
Scheme 1. Synthesis of (1R,3R)-Ac6c3M using the BucherereBergs reaction.
2.1.2. Synthesis of (1S,3R)-Ac6c3M using a Strecker reaction. Treat-
ment of (3R)-1 with KCN and NH4Cl in 35% EtOH aqueous solution at
65 ꢁC gave an aminonitrile 5b at a 96% yield (Scheme 2).13 Acetyla-
tion of the amino function in 5b with acetyl chloride (AcCl) afforded
acetylated nitrile 6b at an 89% yield. In the 1H NMR spectrum,
compound 6b seemed to be diastereomerically pure with one ste-
reoisomer. Hydrolysis of 6b under acidic conditions, followed by
esterification at C-terminus gave a methyl ester H-[(1S,3R)-Ac6c3M]-
OMe 7b at a 54% yield. Protection of the N-terminal amino group
with the Cbz-group produced N-terminal Cbz-protected amino acid
Cbz-[(1S,3R)-Ac6c3M]-OMe 4b at 91%. The 1H NMR, 13C NMR, and MS
spectra suggested that the amino acid 4b was a diastereomer of 4a.
Finally, X-ray crystallographic analysis of its peptides unambiguously
revealed the stereochemistry of 4b as 1S,3R (vide infra).
angles
f
ꢀ48ꢁ;
j
ꢀ60ꢁ, by the reported calculation.5b
We stereoselectively synthesized optically active (1R,3R)- and
(1S,3R)-Ac6c3M, prepared two series of diastereomeric homo-chiral
homopeptides, up to a hexapeptide, and studied the preferred
conformation of these peptides, including the side-chain cyclo-
hexane stereochemistry and peptide-backbone secondary struc-
tures, i.e., 310- or a-helix, and also right-handed (P) or left-handed
(M) helical-screw direction.
2. Results
2.1. Diastereoselective syntheses of two six-membered ring
a
,a
-disubstituted
a
-amino acids (1R,3R)- and (1S,3R)-Ac6c3M
2.2. Preparation of (1S,3R)- and (1R,3R)-Ac6c3M homopeptides
Two diastereomeric cyclic amino acids (1S,3R)- and (1R,3R)-
We synthesized C-terminal free amino acids 8a,b by alkaline
hydrolysis, and N-terminal free amino acids 7a,b by hydrogenolysis
using H2/PdeC, respectively (Schemes 1 and 2).
Ac6c3M were synthesized starting from an optically active (3R)-3-
methylcyclohexanone (3R)-1, respectively.

T. Hirata et al. / Tetrahedron 71 (2015) 2409e2420
2411
Scheme 2. Synthesis of (1S,3R)-Ac6c3M using the Strecker reaction.
The (1R,3R)-Ac6c3M homopeptides were prepared by solution
phase methods. Firstly, we prepared (1R,3R)-Ac6c3M dipeptide 9a at
a 62% yield by coupling between 7a and 8a using 1-ethyl-3-(3-
dimethylaminopropyl)carbodiimide (EDC) hydrochloride and 1-
hydroxybenzotriazole (HOBt) as coupling reagents. Next, we
attempted to prepare tetrapeptide 12a by coupling between di-
peptide amine 10a and dipeptide carboxylic acid 11a, which were
prepared from dipeptide 9a by hydrogenolysis or hydrolysis.
However, the coupling reaction proceeded only at a low yield (6%),
and attempts to improve chemical yield by examining several
coupling reagents such as O-(7-azabenzotriazol-1-yl)-1,1,3,3-
tetramethyluronium hexafluorophosphate (HATU), 1-[bis(dime-
thylamino)methylene]-1H-benzotriazolium
3-oxide
hexa-
fluorophosphate (HBTU), and (1H-benzotriazole-1-yloxy)
tris(pyrrolidino)phosphonium hexafluorophosphate (PyBOP) did
not work well.14 Therefore, we elongated the peptide chain by
stepwise one amino acid residue, up to a hexapeptide (Scheme 3).
Diastereomeric (1S,3R)-Ac6c3M homopeptides were prepared in
a manner similar to those of (1R,3R)-Ac6c3M peptides (Scheme 4).
The spectroscopic data of (1R,3R)- and (1S,3R)-Ac6c3M homopep-
tides supported their structures.
Scheme 3. Preparation of (1R,3R)-Ac6c3M homopeptides.
2.3. Conformational analysis in solution
The dominant conformations of two series of (1R,3R)- and
(1S,3R)-Ac6c3M homopeptides in solution were studied by using
FTIR, 1H NMR, and CD spectroscopies at room temperature. The
FTIR absorption spectra of Cbz-[(1R,3R)-Ac6c3M]n-OMe [n¼2 (9a), 3
(13a), 4 (12a), 5 (14a), 6 (15a)] and Cbz-[(1S,3R)-Ac6c3M]n-OMe
[n¼2 (9b), 3 (13b), 4 (12b), 5 (14b), 6 (15b)] in the 3500e3250 cmꢂ1
region at a peptide concentration of 5.0 mM in CDCl3 solution are
shown in Fig. 2. In both series of (1R,3R)- and (1S,3R)-Ac6c3M
homopeptides, weak bands at 3420e3440 cmꢂ1 region were
assigned to free (solvated) peptide NH groups and the strong bands
at the 3350e3370 cmꢂ1 region to peptide NH groups with
NeH/O]C intramolecular hydrogen bonds of different strengths.
With the peptide-chain length increase, the strong band shown at
3370 cmꢂ1 in tripeptides 13 shifted to slightly lower wavenumbers
of 3350 cmꢂ1 in hexapeptides 15, and the relative intensity of the
bands in the 3350e3370 cmꢂ1 region gradually increased. These
FTIR spectra are very similar to those of the 310-helical peptides,
such as Aib,3 Ac4c,4f,h Ac5c,4aec and Ac6c5d peptides, but different
from those of fully planar structure (S)-butylethylglycine,15 or
diethylglycine homopeptides.16
Scheme 4. Preparation of (1S,3R)-Ac6c3M homopeptides.
perturbation experiments by the addition of the strong H-bond
acceptor solvent DMSO-d6 (0e10% v/v) or the paramagnetic free
The 1H NMR spectra of (1R,3R)- and (1S,3R)-Ac6c3M hexapep-
tides 15a and 15b were measured in CDCl3 solution. In the 1H NMR
spectra, the N(1)H signals at the N-terminus could be un-
radical 2,2,6,6-tetramethylpiperidin-1-yloxyl (TEMPO; 0e5ꢃ10ꢂ2
%
w/v) are shown in Figs. 3 and 4. One NH chemical shift of 15a,b was
sensitive (solvent-exposed NH group), and one signal was slightly
sensitive, to the addition of the perturbing reagent DMSO-d6, al-
though one NH signal of both 15a,b could not be analyzed because
of overlap with the signals of the CBz-protecting group. Similarly,
the bandwidth of two NH signals of both 15a,b broadened with the
ambiguously determined by their high-field positions at
d 5.33 (br
s, 1H) in 15a, and 5.03 (br s, 1H) in 15b, due to their urethane
d
structure. However, the remaining five NH protons in 15a,b could
not be assigned at this stage (vide infra). The results of solvent

2412
T. Hirata et al. / Tetrahedron 71 (2015) 2409e2420
lowered below ꢂ30 ꢁC, the N(1)H and N(5)H signals of 15a split
with an intensity ratio of ca. 2: 1. However, the NH signals of 15b did
not split at ꢂ40 ꢁC (Fig. S3).12
The CD spectra of (1R,3R)- and (1S,3R)-Ac6c3M hexapeptides
15a,b in 2,2,2-trifluoroethanol (TFE) solution were measured
(Fig. S4).12 However, the CD spectra did not show any characteristic
maxima (222, 208 nm) for a helical conformation.
2.4. Structures of (1R,3R)- and (1S,3R)-Ac6c3M homopeptides
in the crystal state
Fig. 2. FTIR absorption spectra in the NeH stretching region of peptides. (A) Cbz-
[(1R,3R)-Ac6c3M]n-OMe (n¼1, 2, 3, 4, 5 and 6) (B) Cbz-[(1S,3R)-Ac6c3M]n-OMe (n¼1, 2, 3,
4, 5 and 6) in CDCl3. Peptide concentration 5.0 mM.
X-ray crystallographic analyses unambiguously revealed the
amino acid residue structures and the peptide-backbone secondary
structures of terminally protected (1R,3R)- and (1S,3R)-Ac6c3M
tetrapeptides 12a,b and hexapeptides 15a,b in the crystal state.18,19
The tetra- and hexapeptides became good crystals for X-ray crys-
tallographic analysis by slow evaporation of the solvent at room
temperature (12a: CH2Cl2/n-hexane, 15a: MeOH/H2O, 12b: MeOH/
EtOAc, 15b: MeOH/H2O). The crystal and diffraction parameters of
12a,b and 15a,b are summarized in Table S1.12 The molecular
structures are illustrated in Figs. 5e8. Relevant backbone and side-
chain torsion angles and the intra- and intermolecular hydrogen-
bond parameters are listed in Tables 1e4, respectively.
Fig. 3. Plots of NH chemical shifts in the 1H NMR spectra of peptides (A) Cbz-[(1R,3R)-
Ac6c3M]6-OMe 15a, (B) Cbz-[(1S,3R)-Ac6c3M]6-OMe 15b as a function of an increasing
percentage of DMSO-d6 (v/v) added to the CDCl3 solution. Peptides concentration:
1.0 mM.
Fig. 5. 310-Helical secondary structures of Cbz-[(1R,3R)-Ac6c3M]4-OMe (12a), de-
termined by X-ray crystallographic analysis.
Fig. 4. Plots of bandwidth of the NH protons of peptides (A) Cbz-[(1R,3R)- Ac6c3M]6-
OMe 15a, (B) Cbz-[(1S,3R)-Ac6c3M]6-OMe 15b as a function of increasing percentages of
TEMPO (w/v) added to the CDCl3 solution. Peptides concentration: 1.0 mM.
addition of the TEMPO radical, meaning two NH protons are
solvent-exposed, and suggesting two NH protons are not in-
tramolecularly hydrogen-bonded. These results are in accord with
a helical structure, in which two or three N-terminal NH groups are
freely solvated peptide NH groups (not intramolecularly hydrogen-
bonded).
The 2D rotating frame nuclear Overhauser and exchange
(ROESY) and NOESY NMR spectra of 15a and 15b were measured in
CDCl3 solution (Fig. S2).12 The ROESY NMR spectrum of (1R,3R)-
Ac6c3M hexapeptide 15a showed partial NH (i/iþ1) dipolar in-
teractions, from N(2)H to the C-terminal N(6)H, but the NOE con-
strain NH (1/2) was not observed. These partial series of
sequential NH correlations suggest helical structure formation of
Fig. 6. 310-Helical secondary structures of Cbz-[(1R,3R)-Ac6c3M]6-OMe (15a), de-
termined by X-ray crystallographic analysis.
15a, albeit whether a 310- or an a-helical conformation is unclear.
The NOESY NMR spectrum of (1S,3R)-Ac6c3M hexapeptide 15b
showed a complete series of sequential NH (i/iþ1) dipolar in-
teractions from the N-terminal N(1)H to the C-terminal N(6)H,
suggesting helical structure formation.17
We measured the 1H NMR spectra of hexapeptides 15a and 15b
at lower temperature. When the temperature was lowered, the NH
signal lines of 15a and 15b broadened, and the temperature was
Fig. 7. 310-Helical secondary structures of Cbz-[(1S,3R)-Ac6c3M]4-OMe (12b) de-
termined by X-ray crystallographic analysis.

T. Hirata et al. / Tetrahedron 71 (2015) 2409e2420
2413
Table 2
Selected torsion angles of (1S,3R)-Ac6c3M homo-chiral homopeptides 12b and 15ba
Torsion angle
(1S,3R)-Tetramer 12b
(1S,3R)-Hexamer 15b
E
F
G
H
I
u0
f1
j1
u1
f2
j2
u2
f3
j3
u3
f4
j4
u4
f5
j5
u5
f6
j6
u6
174.1
55.6
40.3
172.6
49.8
42.8
171.0
58.1
40.6
ꢂ180.0
54.9
40.2
165.8
55.8
38.6
172.5
52.8
44.7
179.9
ꢂ51.3
ꢂ45.9
179.0
d
ꢂ169.2
ꢂ54.5
ꢂ42.7
ꢂ168.1
ꢂ58.4
ꢂ29.9
ꢂ179.4
ꢂ54.3
ꢂ39.3
ꢂ175.4
42.8
ꢂ179.9
50.4
48.3
165.0
57.5
51.5
178.7
54.7
56.1
170.5
58.5
42.6
172.4
59.7
42.8
177.5
ꢂ47.1
ꢂ52.8
ꢂ174.8
ꢂ174.8
ꢂ51.2
ꢂ51.9
ꢂ167.9
ꢂ58.1
ꢂ49.0
ꢂ176.0
ꢂ60.0
ꢂ48.1
ꢂ171.9
ꢂ53.2
ꢂ40.1
ꢂ169.8
ꢂ55.2
ꢂ39.8
ꢂ168.1
49.8
173.1
ꢂ45.8
ꢂ51.8
Fig. 8.
a
-Helical secondary structures of Cbz-[(1S,3R)-Ac6c3M]6-OMe (15b) determined
56.0
by X-ray crystallographic analysis.
ꢂ177.1
d
ꢂ177.1
d
d
d
d
Table 1
d
d
d
Selected torsion angles of (1R,3R)-Ac6c3M homo-chiral homopeptides 12a and 15aa
d
d
d
d
d
d
53.0
178.2
Torsion angle
(1R,3R)-Tetramer 12a
(1R,3R)-Hexamer 15a
d
d
d
A
B
C
D
c1
ꢂ163.9
168.4
ꢂ164.5
170.8
ꢂ166.6
172.6
ꢂ176.5
171.0
d
ꢂ165.9
171.9
ꢂ163.1
169.2
ꢂ166.6
171.8
ꢂ174.0
167.9
d
ꢂ172.0
166.3
ꢂ168.3
163.7
ꢂ169.9
162.7
ꢂ170.5
173.3
d
ꢂ170.0
172.5
ꢂ175.1
168.9
u0
f1
j1
u1
f2
j2
u2
f3
j3
u3
f4
j4
u4
f5
j5
u5
f6
j6
u6
171.1
63.6
26.4
179.1
49.5
36.7
170.4
63.6
31.0
ꢂ178.9
54.4
51.2
ꢂ179.1
d
ꢂ173.8
ꢂ56.8
ꢂ29.5
ꢂ175.8
ꢂ49.8
ꢂ36.1
ꢂ170.2
ꢂ61.8
ꢂ32.4
ꢂ174.6
ꢂ56.4
ꢂ54.4
ꢂ177.6
d
171.2
63.0
31.0
168.8
56.0
36.0
171.7
56.0
35.0
168.7
59.0
27.0
172.1
68.0
22.0
151.0
ꢂ51.0
ꢂ42.0
174.0
ꢂ175.1
ꢂ56.0
ꢂ31.0
ꢂ176.2
ꢂ51.2
ꢂ37.8
ꢂ171.9
ꢂ55.5
ꢂ38.1
ꢂ171.3
ꢂ57.0
ꢂ30.0
ꢂ166.2
ꢂ68.0
ꢂ23.0
ꢂ170.8
50.0
0
c1
c2
ꢂ167.5
ꢂ172.2
0
c2
171.7
167.2
c3
ꢂ169.5
175.0
ꢂ170.7
165.4
0
c3
c4
ꢂ164.4
ꢂ172.5
0
c4
168.0
167.2
c5
ꢂ166.1
171.6
ꢂ169.6
161.1
0
c5
d
d
d
c6
d
d
d
ꢂ176.7
ꢂ170.5
0
c6
d
d
d
170.3
175.4
a
0
The torsion angles of cn and cn (n¼amino acid residue numbering); cn
:
NeC1eC2eC3, and cn : NeC eC eC5 (1, 2, 3, 5, and 6: cyclohexane ring numbering).
1
6
0
d
d
d
d
d
d
Table 3
d
d
41.0
179.0
Intra- and intermolecular H-bond parameters for (1R,3R)-Ac6c3M peptides 12a and
d
d
15a
Peptidea Donor DeH Acceptor A Distance [A] Angle [ꢁ] Symmetry
ꢁ
c1
69.0
ꢂ65.1
70.3
ꢂ68.4
67.6
ꢂ63.5
68.6
ꢂ65.9
d
66.0
ꢂ70.3
66.9
ꢂ69.7
66.1
ꢂ68.3
64.8
ꢂ69.6
d
67.0
ꢂ68.0
68.0
69.0
ꢂ74.0
65.1
0
D/A
DeH/A
operations
c1
Cbz-{(1R,3R)-Ac6c3M}4-OMe (12a)
c2
0
c2
ꢂ67.0
ꢂ68.8
A (M)
N3aeH
N4aeH
N3beH
N4beH
N1beH
N1aeH
O0a
O1a
O0b
O1b
O3a
O3b
3.22
3.12
3.11
3.12
2.97
2.99
165
147
167
155
165
171
x,y,z
x,y,z
x,y,z
x,y,z
x,y,z
x,y,zꢂ1
c3
71.5
66.0
0
c3
ꢂ67.0
70.0
ꢂ69.0
68.6
B (P)
c4
0
c4
ꢂ68.0
ꢂ72.8
c5
69.0
64.0
0
0
c5
d
d
ꢂ70.0
46.0
ꢂ70.0
ꢂ68.0
71.0
ꢂ76.0
c6
d
d
Cbz-{(1R,3R)-Ac6c3M}6-OMe (15a)b
0
c6
d
d
C (M)
N
N
N
N
N
N
N
N
N
4ceH
5ceH
6ceH
3deH
4deH
5deH
6deH
1deH
O1c
O2c
O3c
O0d
O1d
O2d
O3d
O5c
O5d
3.10
3.25
3.18
3.12
3.10
3.17
3.36c
2.85
2.93
159
164
158
159
151
162
170
162
165
x,y,z
x,y,z
x,y,z
x,y,z
x,y,z
x,y,z
x,y,z
x,y,z
a
0
The torsion angles of cn and cn (n¼amino acid residue numbering); cn
:
NeC1eC2eC3, and cn : NeC eC eC5 (1, 2, 3, 5, and 6: cyclohexane ring numbering).
1
6
0
D (P)
In the asymmetric unit of the (1R,3R)-Ac6c3M teltrapeptide 12a,
two crystallographically independent molecules A and B occurred,
along with a disordered solvent CH2Cl2. Molecule A is folded into
a left-handed (M) 310-helical structure, and thus, displays positive
signs of the
the other hand, molecule B is folded into a right-handed (P) 310
helical structure, showing negative signs of the and torsion
angles in each residue. The average and torsion angles of resi-
0
1c eH
1þx,y,ꢂ1þz
a
The number of amino acid residues begins at the N-terminus of the peptide
chain.
f and j torsion angles in each amino acid residue. On
b
c
ꢁ
The distance N3c/O0c (3.42 A) is too long for an intramolecular hydrogen bond.
The distance N/O is a little long for an intramolecular hydrogen bond.
-
f
j
f
j
dues (1e3) were þ58.9ꢁ, 31.4ꢁ in molecule A and ꢂ56.1ꢁ, ꢂ32.7ꢁ in
molecule B, although a slight distortion of j4 torsion angles of
residue (4) in both molecules A and B was observed. These average
torsion angles are in good agreement with the ideal torsion angles
(60ꢁ, 30ꢁ) of the 310-helical structure.8 The two molecules A and B
conformation, but showed small differences in the backbone
structure, and have the same configuration of chiral centers
(Fig. S5).12 Two intramolecular hydrogen bonds of the i)iþ3 type,
which correspond to the 310-helical structure, are found in each
molecule. Molecule A shows two intramolecular hydrogen bonds
between HeN(3a) and the C(0a)]O(0a) oxygen atom of the Cbz-
are generally mirror-image structures of
a peptide-backbone

2414
T. Hirata et al. / Tetrahedron 71 (2015) 2409e2420
related peptides.20 The average values of torsion angles
f and j of
Table 4
Intra- and intermolecular H-bond parameters for (1S,3R)-Ac6c3M peptides 12b and
residues (1e5) were þ60.4ꢁ, þ30.2ꢁ in molecule C, and ꢂ57.5ꢁ,
ꢂ32.0ꢁ in molecule D. Both molecules C and D are generally mirror-
image structures of the peptide-backbone, but show small back-
bone conformational differences, with the same configuration of
chiral centers, and are diastereomeric conformers (Fig. S6).12 Mol-
ecule C shows three intramolecular hydrogen bonds of the i)iþ3
15b
Peptidea Donor DeH Acceptor A Distance [A] Angle [ꢁ] Symmetry
ꢁ
D/A
DeH/A operations
Cbz-{(1S,3R)-Ac6c3M}4-OMe (12b)
E (M)
F (M)
G (P)
N
N
N
N
N
N
N
N
N
3eeH
4eeH
3eeH
3feH
4feH
3feH
3geH
4geH
3geH
O0e
O1e
O1e
O0f
O1f
O1f
O0g
O1g
O1g
O2e
OMeOH(ii)
O3f
3.12
3.01
3.03
3.33b
3.05
3.05
3.16
2.93
3.01
2.80
3.06
3.13
2.91
2.87
3.17
2.88
2.85
3.04
2.97
147
140
83c
146
132
84c
149
148
85c
175
152
175
177
160
139
171
173
179
146
x,y,z
x,y,z
x,y,z
x,y,z
x,y,z
x,y,z
x,y,z
x,y,z
x,y,z
x,y,z
x,y,z
x,y,z
x,y,z
x,y,z
2ꢂx,1/2þy,2ꢂz
2ꢂx,1/2þy,2ꢂz
x,y,z
ꢁ
type, between HeN(4c) and C(1c)]O(1c) (N(4c)/O(1c): 3.10 A),
between HeN(5c) and C(2c)]O(2c) (N(5c)/O(2c): 3.25 A), and
ꢁ
ꢁ
between HeN(6c) and C(3c)]O(3c) (N(6c)/O(3c): 3.18 A), al-
ꢁ
though the distance of N(3c)/O(0c) (3.42 A) was too long for an
intramolecular hydrogen bond. Molecule D forms four consecutive
intramolecular hydrogen bonds of the i)iþ3 type, between
ꢁ
MeOeH(i)
HeN(3d) and C(0d)]O(0d) (N(3d)/O(0d): 3.12 A), between
ꢁ
N
N
2eeH
1eeH
HeN(4d) and C(1d)]O(1d) (N(4d)/O(1d): 3.10 A), between
ꢁ
HeN(5d) and C(2d)¼O(2d) (N(5d)/O(2d): 3.17 A), and between
MeOeH(ii)
MeOeH(iii) O2f
O3f
ꢁ
HeN(6d) and C(3d)¼O(3d) (N(6d)/O(3d): 3.36 A), respectively.
Each residue in both molecules C and D assumes a chair form side-
chain cyclohexane ring, with the methyl substituent and the eCOe
group at the equatorial position, and with the eNHe group at an
0
0
N
N
1feH
2feH
O3g
O4g
O2g
O3e
O4e
MeOeH(iv)
0
N
N
1geH
2geH
2ꢂx,yꢂ1/2,1ꢂz
axial orientation, as the torsion angles (c, c
0) at each residue are
0
2ꢂx,yꢂ1/2,1ꢂz
close to þ60ꢁ, ꢂ60ꢁ. Molecule C and molecule D are connected by
one intermolecular hydrogen bond in the packing mode to form
a head-to-tail alignment of 310-helices of molecules C and D, i.e.,
/C(M)/D(P)/C(M)/D(P)/C(M)/D(P)/ chain.
Cbz-{(1S,3R)-Ac6c3M}6-OMe (15b)
H (M)d
N4h-H
O0h
O1h
O3h
O0i
O1i
O2i
O3i
O4h
3.27
2.95
3.20
3.18
3.30[b]
3.35[b]
3.16
2.73
2.72
2.99
2.98
2.70
2.70
3.00
3.00
172
157
127
164
145
136
151
157
173
156
170
162
172
170
156
x,y,z
x,y,z
x,y,z
x,y,z
x,y,z
x,y,z
x,y,z
N5heH
N6heH
N4ieH
N5ieH
N5ieH
N6ieH
Diastereomeric (1S,3R)-Ac6c3M tetrapeptide 12b crystallized in
the space group of P21 to adopt two left-handed (M) helical con-
formers (molecules E and F) and one right-handed (P) helical
conformer (molecule G) in the asymmetric unit, along with four
I (P)
0
MeOeH(v)
MeOeH(vi)
x,1þy,z
MeOH molecules. In each molecule, reversal of the C-terminal
and torsion angles to those of the preceding residues (1e3) is
observed.20 The average values of torsion angles
and of residues
f
OMeOH(v)
O5h
O6h
x,y,z
j
0
N
N
1ieH
2ieH
x,1þy,z
x,1þy,z
x,y,z
f
j
0
(1e3) are þ54.5ꢁ, þ41.2ꢁ in molecule E, þ54.5ꢁ, þ41.2ꢁ in molecule
F, and ꢂ55.7ꢁ, ꢂ37.3ꢁ in molecule G. Molecules E and F are almost
the same conformation both in terms of peptide-backbone and in
side-chain structures, but there are small conformational differ-
ences and crystallographically independent conformers. Molecule
G shows a similar mirror-image peptide-backbone structure to
molecules E and F, having the same 1S,3R configuration of the
residues (Fig. S7).12 Two intramolecular hydrogen bonds of the
i)iþ3 type, which correspond to the 310-helical conformation, are
shown in all three molecules EeG. That is to say, intramolecular
hydrogen bonds between HeN(3) and C(0)]O(0) [N(3e)/O(0e):
MeOeH(vii) O4i
MeOeH(viii) OMeOH(vii)
x,y,z
0
N
N
1heH
2heH
O5i
O6i
x,y,1þz
0
x,y,1þz
a
The number of amino acid residues begins at the N-terminus of the peptide
chain.
b
c
The distance of N/O is a little long for an intramolecular hydrogen bond.
The DeH/A angle is too small for a hydrogen bond.
The distances N5h/O2h (3.44 A) and N6h/O2h (3.41 A) are too long for intra-
d
ꢁ
ꢁ
molecular hydrogen bonds.
ꢁ
group (N(3a)/O(0a): 3.22 A) and between HeN(4a) and C(1a)]
O(1a) (N(4a)/O(1a): 3.12 A). Also, molecule B shows two intra-
molecular hydrogen bonds between HeN(3b) and C(0b)]O(0b)
(N(3b)/O(0b): 3.11 A) and between HeN(4b) and C(1b)]O(1b)
ꢁ
ꢁ
ꢁ
ꢁ
3.12 A; N(3f)/O(0f): 3.33 A; N(3g)/O(0g): 3.16 A], and between
ꢁ
HeN(4) and C(1)]O(1) [N(4e)/O(1e): 3.01 A; N(4f)/O(1f):
ꢁ
ꢁ
ꢁ
3.05 A; N(4g)/O(1g): 2.93 A] are observed. The distances between
HeN(3) and the O(1)-atom of C(1)]O(1) in molecules E, F, and G
ꢁ
(N(4b)/O(1b): 3.12 A). The side-chain cyclohexane ring of each
ꢁ
ꢁ
amino acid residue in both molecules A and B forms a chair con-
formation, with the 3-methyl substituent and the eCOe group at
an equatorial position, and with the eNHe group at an axial ori-
are N(3e)/O(1e): 3.03 A, N(3f)/O(1f): 3.05 A and N(3g)/O(1g):
ꢁ
3.01 A, which allow for the intramolecular hydrogen bonding dis-
tance, corresponding to the i)iþ2 type hydrogen bonds (2.27-
ribbon conformation). However, the angles for HeN(3)/O(1) are
83ꢁ (molecule E), 84ꢁ (molecule F) and 85ꢁ (molecule G), which are
too small for intramolecular hydrogen bonding angles. Thus, the O-
atom of C(1)]O(1) may not act as double-acceptor of hydrogen
bonds from HeN(3) and HeN(4). The conformation of a side-chain
cyclohexane ring at each residue in all three molecules EeG shows
a chain form, with the 3-methyl substituent and the eNHe group at
an equatorial orientation, and the eCOe group in an axial direction.
entation. torsion
All
angles
c
(NeC
a
eC
b
eC
g)
and
c
0(NeC eCb0eC 0) relating the cyclohexyl ring to the peptide
a
g
chain, namely NeC1eC2eC3 (numbering of cyclohexane) and
NeC1eC6eC5, show nearly þ60ꢁ, ꢂ60ꢁ, which means the eNHe
groups occupy the axial positions (Table 2).5c In the packing mode,
molecule A and molecule B are connected by one intermolecular
hydrogen bond, to form a head-to-tail alignment of 310-helices of
molecules A and B, i.e., /A(M)/B(P)/A(M)/B(P)/A(M)/B(P)/
chain.
All torsion angles (c,c
0) relating the cyclohexyl ring to the peptide
The (1R,3R)-Ac6c3M hexapeptide 15a crystallized in the space
group of P21 to form both left-handed (M) (molecule C) and right-
handed (P) (molecule D) 310-helices in the asymmetric unit. Both in
molecules C and D, reversals of the C-terminal torsion angles occur,
i.e., the signs of the
(residues 6) are opposite to those of the preceding residues (1e5).
chain show nearly ꢂ180ꢁ, þ180ꢁ, which means the eNHe groups
occupy the equatorial positions (Table 2).5c The O(2)-atom of car-
bonyl function C(2)]O(2) in all three molecules EeG forms hy-
drogen bonds with MeOH molecules. In the packing mode, an
intermolecular hydrogen bond between HeN(1e) and C(3f)]O(3f),
two intermolecular hydrogen bonds between HeN(1f) and C(3g0)]
O(3g0), and HeN(2f) and C(4g0)]O(4g0), and also two
f and j torsion angles of the C-terminal residue
These phenomena are frequently observed in 310-helical Aib and

T. Hirata et al. / Tetrahedron 71 (2015) 2409e2420
2415
intermolecular hydrogen bonds between HeN(1g) and C(3e0)]
O(3e0), and HeN(2g) and C(4e0)]O(4e0) are observed. Moreover,
intermolecular hydrogen bonds between HeN(2e) and the O-atom
of MeOH, and also between the MeOeH and the O-atom of C(3f)]
O(3f) are shown. Thus, three molecules E, F, and G form a head-to-
predominant (1R,3R)-product by the BucherereBergs reaction. The
diastereoselectivity of (3R)-3-(4-bromophenyl)cyclopentanone by
the Strecker reaction was reported to be 3 (1R,3R) to 1 (1S,3R) by
the Wallace group.22 Thus, the selectivity and predominant di-
astereoisomer are totally different between 3-substituted cyclo-
pentanone and 3-methylcyclohexanone 1. These results could be
attributed to the fact that 3-substituted cyclopentanones form
several envelop conformations and these conformations are
flexible.
tail alignment of 310/a-helices /E(M)/F(M)/G(P)/E(M)/F(M)/
G(P)/ chain.
X-ray crystallographic analysis of (1S,3R)-Ac6c3M hexapeptide
15b was solved in the P1 space group to show two helices (mole-
cules H and I) with four MeOH molecules in the asymmetric unit.
Molecule H has a left-handedness (M), and molecule I has a right-
In the reactions of 3-methylcyclohexanone, thermodynamically
stable various intermediates with the 3-methyl substituent at an
handedess (P). Reversals of the C-terminal
f
and
j
torsion angles to
equatorial position exist.23 Fig.
9 shows representative in-
those of the preceding residues (1e5) are shown both in molecules
termediates, which are inter-changeable very quickly, in the
Strecker and BucherereBergs reactions. The Strecker reaction gives
(1S,3R)-amino nitrile 5b as a result of the more thermodynamically
stable product than 5a. On the other hand, the BucherereBergs
reaction produces (1R,3R)-2a because in the intermediate (iii),
which leads to (1S,3R)-isomer 2b, severe steric hindrance between
the developing C]NH group and the 3,5-axial hydrogen atoms
exists, and this path is highly disfavored, as Jitrangsri and co-author
discussed the Bucherer-Bergs and Strecker reactions of 4-tert-
butylcyclohexanone.23
H and I. The average values of torsion angles
f and j of residues
(1e5) in molecule H are þ56.2ꢁ, þ48.3ꢁ, and those in molecule I are
ꢂ55.5ꢁ, ꢂ45.8ꢁ, near to the ideal
a
-helix (ꢀ57ꢁ, ꢀ47ꢁ).8b With
regard to the peptide-backbone structure, the right-handed (P) and
left-handed (M) helices (molecules H and I) have an almost enan-
tiomeric relationship, but the configuration of chiral centers is the
same 1S,3R, and so the relationship between molecules H and I is
diastereomeric (Fig. S8).12
In molecule H, three intramolecular hydrogen bonds are ob-
served. Two intramolecular hydrogen bonds of the i)iþ4 type,
which correspond to the a-helical structure, are observed between
ꢁ
HeN(4h) and C(0h)]O(0h) [N(4h)/O(0h): 3.27 A], and between
ꢁ
HeN(5h) and C(1h)]O(1h) [N(6h)/O(3h): 2.95 A], and one
intramolecular hydrogen bond of the i)iþ3 type was observed
ꢁ
between HeN(6h) and C(3h)]O(3h) [N(5h)/O(1h): 3.20 A].
Therefore, strictly speaking, the N-terminal amino acid residues
form an a-helical structure and the C-terminal residues form a 310-
helical structure. In molecule I, four intramolecular hydrogen
bonds, i. e., two i)iþ4 type hydrogen bonds between HeN(4i) and
ꢁ
C(0i)]O(0i) [N(4i)/O(0i): 3.18 A], and between HeN(5i) and
ꢁ
C(1i)]O(1i) [N(5i)/O(1i): 3.30 A], and two i)iþ3 type hydrogen
ꢁ
bonds between HeN(5i) and C(2i)]O(2i) [N(5i)/O(2i): 3.35 A],
ꢁ
and between HeN(6i) and C(3i)]O(3i) [N(6i)/O(3i): 3.16 A] are
observed. Similar to molecule H, molecule I forms an
a-helical
structure at the N-terminal residues, and a 310-helical structure at
the C-terminal residues. In both molecules H and I, the side-chain
cyclohexane rings of all amino acid residues assume chair forms,
with the 3-methyl and the eNHe groups in an equatorial, and the
eCOe group in an axial direction as all torsion angles (c,c
0) show
close to ꢂ180ꢁ, þ180ꢁ. The O(4)-atoms of carbonyl functions C(4)]
O(4) in both molecules H and I form hydrogen bonds with MeOH
molecules. In the packing mode, two intermolecular hydrogen
Fig. 9. Plausible reaction mechanisms for the Strecker and BucherereBergs reactions,
according to Jitrangsri’s report.
0
0
0
ꢁ
bonds between HeN(1i) and C(5h )]O(5h ) [N(1i)/O(5h ): 2.99 A]
0
0
0
ꢁ
and between HeN(2i) and C(6h )]O(6h ) [N(2i)/O(6h ): 2.98 A]
are observed. Also0, two intermolecular h0ydrogen bonds between
The FTIR and 1H NMR spectroscopies revealed that both series of
(1R,3R)- and (1S,3R)-Ac6c3M peptides preferentially formed helical
secondary structures in solution. The dynamic 1H NMR spectra of
hexapeptides suggested that several conformers existed in CDCl3
solution, and these conformers were in fast exchange at room
temperature. However, discrimination between a 310-helix and an
0
ꢁ
HeN(1h) and C(5i0)]O(5i0) [N(1h)/O(5i ): 3.00 A] and between
0
ꢁ
HeN(2h) and C(6i )]O(6i ) [N(2h)/O(6i ): 3.00 A] are observed.
Thus, the molecules H and I form a head-to-tail alignment of 310
helices /H(M)/I(P)/H(M)/I(P)/H(M)/I(P)/ chain.
/a-
3. Discussion
a
-helix of the Ac6c3M hexapeptides by the solvent perturbation
experiments was not possible because the N(3)-H of -helical
homopeptide was shielded and only two NH signals in -helix were
a
A BucherereBergs reaction of 1 produced two diastereomeric
hydantoins (1R,3R)-2a and (1S,3R)-2b at a ratio of 4 to 1. The
Azerad21 and Wallace22 groups reported that the BucherereBergs
reaction of 3-substituted cyclopentanones gave a 1 to 1 mixture of
diastereomeric products, and that of 3-carboxylcyclohexanone
a
sensitive to the addition of DMSO-d6 or TEMPO radical.24b Neither
the CD spectra of (1R,3R)- nor (1S,3R)-Ac6c3M homopeptides (up to
hexapeptides) showed characteristic maxima for a helical structure.
These results may be attributed the fact that the structure of chiral
amino acid Ac6c3M does not have a strong helical-screw bias for
right-handedness or left-handedness, and the length of hexapep-
tides was not long enough to control the helical-screw direction to
one-handedness.24e26
produced (1R
*
,3R
*
)- and (1R
*
,3S )-amino acid products at a ratio
*
of 9 to 1. Therefore, our selectivity of a BucherereBergs reaction of 1
is similar to those of the reported selectivities of 3-substituted
cyclohexanones. In contrast, a Strecker reaction of 1 produced di-
astereomeric (1S,3R)-5 as
astereoisomer by the Strecker reaction was different from the
a
sole product. The (1S,3R)-di-
The X-ray crystallographic analyses of (1R,3R)- and (1S,3R)-
Ac6c3M tetra- and hexapeptides revealed that all peptides formed

2416
T. Hirata et al. / Tetrahedron 71 (2015) 2409e2420
both right-handed (P) and left-handed (M) helices in the crystal
state. The right-handed (P) and left-handed (M) helices are a di-
astereomeric relationship because the configurations of chiral
centers are the same between right-handed and left-handed heli-
ces. In the asymmetric unit of (1S,3R)-Ac6c3M tetrapeptide 12b, two
left-handed (M) helical conformers and one right-handed (P) he-
lical conformer formed, and a head-to-tail alignment of 310-helices
/E(M)/F(M)/G(P)/E(M)/F(M)/G(P)/ chain was formed.
These results suggest that the left-handed (M) helix may be more
favorable than the right-handed (P) helix in the case of the (1S,3R)-
Ac6c3M peptide. However, crystal packing forces and nucleation
events may also be attributed to the left-handed (M) helical
structural preference in the crystal state of 12b because the elon-
gated (1S,3R)-Ac6c3M hexapeptide 15b formed both right-handed
(P) and left-handed (M) helices at a ratio of 1 to 1. Thus, the
structure of Ac6c3M does not have a strong bias to control helical-
screw direction to one-handedness,24 which is the same as the
bicyclic amino acid (R,R)-Ab5,6¼c or five-membered ring amino acid
between a 310-helix and an
regulation of the eNHe equatorial and axial orientations of a six-
a-helix in solution was difficult, the
membered ring
control 310- and
a
a
,
a
-disubstituted amino acid could be applied to
-helical structures of peptides longer than hex-
amer in the crystal state.
The achiral cyclic amino acid Ac6c and the related amino acid
have been reportedly used as peptide-catalysts.28,29 Also, the Ac6c
are reported to be useful for structureeactivity relationship studies
and for the design of potent and selective analogs of bioactive
peptides.5g,30 Therefore, the chiral Ac6c3M with a different torsion
angle propensity may be useful for precise modulation of peptide-
catalysts and peptide-drug candidates. Designing an
a-helical
peptide catalyst based on these results is currently under in-
vestigation by our group.31
5. Experimental section
5.1. General
(1S,3S)-Ac5cOM 25,26
.
The absolute average values of
f and j torsion angles of the
Optical rotations [a] were measured with a Jasco P-2200 or
D
(1R,3R)-Ac6c3M homopeptides are 57.5ꢁ, 32.1ꢁ (residues 1e3) in
tetrapeptide 12a, and 59.0ꢁ, 31.1ꢁ (residues 1e5) in hexapeptide
DIP-370 polarimeter using a 1.0 dm cell. Circular dichroism spectra
(CD) were measured with a Jasco J-720W spectropolarimeter using
1.0 mm path length cell. Infrared spectra (IR) were recorded on
a Jasco FT/IR-420 or Shimadzu IR Affinity-1 spectrometer for con-
ventional measurement (KBr), and the solution (CDCl3) method
using 0.1 mm path length of NaCl cell at room temperature. 1H NMR
spectra were determined at 400 or 500 MHz (Varian Unity or Joel
AL) at room temperature. FABMS spectra were taken on Jeol JMS-SX
102, or Jeol JMS-700N spectrometer. ESI-MS spectra were measured
on a Bruker microTOF II-SKF or Jeol JMS-T100TD. DART-MS spectra
were recorded on a Jeol JMS-T100TD.
15a, which are close to the ideal torsion angles (60ꢁ, 30ꢁ) of the 310
-
helical structure.8 Also, these two peptides showed intramolecular
hydrogen bonds of the i)iþ3 type, which correspond to a 310
helical structure. Thus, one property of (1R,3R)-Ac6c3M is to form
the 310-helical structure, as the Aib property is to form the 310
-
-
helix. In contrast, the absolute average
f and j torsion angles of the
(1S,3R)-Ac6c3M homopeptides are 54.9ꢁ, 39.9ꢁ (residues 1e3) in
tetrapeptide 12b, and 55.9ꢁ, 47.1ꢁ (residues 1e5) in hexapeptide
15b. The absolute values of
j
torsion angles of (1S,3R)-Ac6c3M be-
came larger than those of (1R,3R)-Ac6c3M, and the mean
f
and
j
torsion angles of 15b were especially close to the standard
a-helix
(57ꢁ, 47ꢁ).8b The tetrapeptide 12b formed two intramolecular hy-
drogen bonds of the i)iþ3 type (310-helix), and the hexapeptide
15b formed three or four intramolecular hydrogen bonds, i.e., two
5.2. Synthesis of amino acids
5.2.1. (5R,7R)-7-Methyl-1,3-diazaspiro[4.5]decane-2,4-dione (2a). A
mixture of (3R)-3-methylcyclohexanone (1, 30.0 g, 267 mmol),
(NH4)2CO3 (77.1 g, 802 mmol), and NaCN (26.2 g, 401 mmol) in 50%
aqueous EtOH (300 mL) was stirred at 65 ꢁC for 3 h. After removal of
EtOH, the solution was diluted with brine, and extracted with
EtOAc, and dried over MgSO4. Removal of the solvent afforded di-
astereomeric mixtures (2a/2b¼ca. 4:1) as a white solid. Re-crys-
i)iþ4 type (
drogen bonds. Crystals of helical hexapeptides 15a (310-helix) and
15b ( -helix) were obtained by re-crystallization from the same
solvents (MeOH/H2O). As a result, the property of (1S,3R)-Ac6c3M is
more
-helical than that of (1R,3R)-Ac6c3M, although the 310- and
a
-helix) and one or two i)iþ3 type (310-helix) hy-
a
a
a-
helices are dependent on peptide length and re-crystallizing sol-
vents.27 These results are in good accordance with the reported
theoretical calculation of AceAc6c-NHMe, where the torsion angles
tallization from EtOH gave a hydantoin 2a (32.9 g, 67%) as colorless
22
crystals: mp 234e236 ꢁC; [
a
]
D
ꢂ9.44 (c 1.00, MeOH); 1H NMR
f
ꢀ48ꢁ,
j
ꢀ60ꢁ were stable for the eNHe equatorial position.
(400 MHz, DMSO-d6) d 10.52 (s, 1H), 8.38 (s, 1H), 1.44e1.62 (m, 7H),
1.24 (t, J¼12.7 Hz, 1H), 0.845 (d, J¼6.2 Hz, 3H), 0.80e0.95 (m, 1H);
4. Conclusions
13C NMR (100 MHz, DMSO-d6)
d
178.4, 156.4, 62.8, 41.2, 33.3, 32.7,
27.0, 22.2, 20.8; IR (KBr)
n 3194 (br), 3065, 2945, 1775, 1732, 1458,
We synthesized two diastereomeric six-membered ring amino
acids starting from (3R)-3-methylcyclohexanone. A Bucherere-
Bergs reaction stereoselectively gave (1R,3R)- and (1S,3R)-Ac6c3M at
a ratio of 4 to 1, and a Strecker reaction produced (1S,3R)-Ac6c3M as
a sole diastereoisomer.
The X-ray crystallographic analysis of peptides 12a,b and 15a,b
unambiguously revealed the amino acid residue conformations and
peptide-backbone structures. All (1R,3R)-Ac6c3M residues formed
cyclohexane chair forms with the 3-methyl substituent and the
eCOe group at an equatorial position, and the eNHe group at an
axial position. On the other hand, the (1S,3R)-Ac6c3M residues
formed cyclohexane chair forms with the 3-methyl substituent and
the eNHe group at an equatorial position, and the eCOe group in
an axial direction. The secondary structure of (1R,3R)-Ac6c3M hex-
apeptide 15a involved the (P) and (M) 310-helices, but that of
(1S,3R)-Ac6c3M hexapeptide 15b with the eNHe equatorial position
1410 cmꢂ1; HRMS (ESI): [MþþNa], found 205.0983. C9H14N2O2Na
requires 205.0953.
5.2.2. (5R,7R)-7-Methyl-2,4-dioxo-1,3-diazaspiro[4.5]decane-1,3-
dicarboxyxlic acid di-tert-butyl ester (3a). A solution of hydantoin
(2a, 2.00 g, 10.9 mmol), 4,4-dimethylaminopyridine (4-DMAP;
0.1 g, 0.88 mmol), and di-tert-butyl dicarbonate [(Boc)2O; 7.20 g,
32.9 mmol] in THF (30 mL) was stirred at room temperature under
an Ar atmosphere for 2 h. After removal of THF, the solution was
diluted with brine, extracted with EtOAc, and dried over MgSO4.
After removal of the solvent, the residue was purified by column
chromatography on silica gel (10% EtOAc in n-hexane) to give 3a
(3.90 g, 92%) as colorless crystals: mp 90e92 ꢁC; [
a
]
23 þ3.93 (c 1.00,
D
CHCl3); 1H NMR (400 MHz, CDCl3)
d
2.14e2.21 (m, 2H), 2.06 (dd,
J¼4.5, 14.6 Hz, 1H), 1.60e1.88 (m, 5H), 1.58 (s, 9H), 1.56 (s, 9H), 1.15
(m, 1H), 0.95 (d, J¼6.7 Hz, 3H); 13C NMR (100 MHz, CDCl3)
d 171.8,
was nearly the same as (P) and (M)
a
-helices, as predicted by the
149.4, 147.8, 145.4, 86.4, 84.6, 65.3, 39.7, 30.43, 30.35, 27.9, 27.7,
theoretical calculation.5b Therefore, although the discrimination
26.3, 21.8, 18.8; IR (KBr)
n 2982, 2931, 1813, 1782, 1736, 1369,

T. Hirata et al. / Tetrahedron 71 (2015) 2409e2420
2417
1142 cmꢂ1; HRMS (ESI): [MþþNa], found 405.2023. C19H30N2O6Na
aqueous HCl (50 mL) was heated at 100 ꢁC for 24 h. After cooling to
room temperature, the solution was neutralized with NaHCO3,
washed with CH2Cl2, and then concentrated in vacuo to leave
a crude amino acid. The crude amino acid was dissolved in MeOH
(100 mL), and concentrated H2SO4 (5 mL) was added to the stirred
solution at 0 ꢁC. After stirring at 65 ꢁC for 48 h, the solution was
diluted with 5% aqueous NaHCO3. After removal of MeOH, the
aqueous solution was extracted with CHCl3, and dried over MgSO4.
Removal of the solvent gave an oily residue, which was purified by
requires 405.2002.
5 . 2 . 3 . ( 1 R , 3 R ) - 1 - B e n z y l o x y c a r b o n y l a m i n o - 3 -
methylcyclohexanecarboxylic acid methyl ester {Cbz-[(1R,3R)-
Ac6c3M]-OMe; 4a}. A solution of 3a (51.2 g, 134 mmol) and LiOH/
H2O (28.1 g, 669 mmol) in 50% aqueous MeOH (500 mL) was stirred
at room temperature for 5 days. After neutralization with 10%
aqueous HCl, MeOH was evaporated. Then, the aqueous solution
was washed with CHCl3. To the aqueous solution, K2CO3 (37.0 g,
268 mmol), benzyl chloroformate (CbzCl; 31 mL, 214 mmol), and
acetone (200 mL) were added, and the solution was stirred over-
night at room temperature. After removal of acetone, the solution
was acidified with 10% aqueous HCl, extracted with CHCl3, and
dried over Na2SO4. Removal of the solvent afforded a crude acid,
which was dissolved in MeOH (300 mL), and SOCl2 (31 mL,
429 mmol) was added at 0 ꢁC. After stirring at room temperature
for 24 h, the solution was diluted with 5% aqueous NaHCO3, and
then MeOH was evaporated. The aqueous solution was extracted
with CHCl3, dried over MgSO4, and evaporated. The oily residue was
purified by column chromatography on silica gel (20% EtOAc in n-
column chromatography on silica gel. The elution with 50% EtOAc
27
in n-hexane gave an amine 7b (3.60 g, 54%) as a colorless oil: [
a
]
D
ꢂ7.9 (c 1.05, CHCl3); 1H NMR (400 MHz, CDCl3)
d 3.71 (s, 3H),
2.10e2.20 (m, 2H), 1.65e1.72 (m, 2H), 1.46 (m, 1H), 1.34 (m, 1H), 1.19
(dt, J¼3.9, 12.6 Hz, 1H), 0.96 (d, J¼12.6 Hz, 1H), 0.91 (d, J¼6.4 Hz,
3H), 0.83 (dq, J¼3.7,12.6 Hz,1H); 13C NMR (100 MHz, CDCl3)
d 176.5,
n 3375, 2951,
57.9, 51.6, 44.7, 35.7, 33.9, 29.4, 23.0, 22.0.; IR (neat)
2932, 2847, 1732, 1454, 1207 cmꢂ1; FAB(þ)MS: m/z 172.1 [MþþH];
HRMS (DART): [MþþH], found 172.1347. C9H18NO2 requires
172.1332.
5 . 2 . 7 . ( 1 S , 3 R ) - 1 - B e n z y l o x y c a r b o n y l a m i n o - 3 -
methylcyclohexanecarboxylic acid methyl ester {Cbz-[(1S,3R)-
Ac6c3M]-OMe; 4b}. A mixture of amine 7b (1.90 g, 11.4 mmol),
K2CO3 (1.90 g, 13.7 mmol), and CbzCl (2.60 mL, 18.2 mmol) in ac-
etone (20 mL) and H2O (5 mL) was stirred at room temperature for
24 h. After removal of acetone, the aqueous solution was extracted
with CHCl3, dried over MgSO4. After evaporation of the solvent, the
residue was purified by column chromatography on silica gel (20%
hexane) to give cyclic amino acid 4a (15.2 g, 37%) as a colorless oil:
22
[
a]
ꢂ7.54 (c 1.00, CHCl3); 1H NMR (500 MHz, CDCl3)
d 7.29e7.39
D
(m, 5H), 5.09 (s, 2H), 4.92 (br s, 1H), 3.67 (br s, 3H), 2.11 (br t,
J¼15.0 Hz, 2H), 1.36e1.73 (m, 6H), 0.890 (d, J¼6.6 Hz, 3H),
0.85e0.95 (m, 1H); 13C NMR (100 MHz, CDCl3)
d 175.2, 155.5, 136.6,
128.68, 128.29, 128.23, 66.9, 60.0, 52.5, 40.7, 34.2, 32.0, 27.6, 22.3,
21.3; IR (CDCl3)
n ;
3445, 2953, 1734, 1502, 1456, 1265, 1239 cmꢂ1
FAB(þ)MS: m/z 306.1 [MþþH]; HRMS (ESI): [MþþNa], found
EtOAc in n-hexane) to afford a cyclic amino acid 4b (3.20 g, 91%) as
22
328.1491. C17H23NO4Na requires 328.1525.
colorless crystals: mp 54e56 ꢁC; [
a
]
D
ꢂ6.60 (c 1.00, CHCl3); 1H
NMR (500 MHz, CDCl3) d 7.29e7.38 (m, 5H), 5.06 (s, 2H), 4.99 (br s,
5.2.4. (1S,3R)-1-Amino-3-methylcyclohexanecarbonitrile
(5b). A
1H), 3.68 (br s, 3H), 2.41 (br t, J¼12.6 Hz, 2H),1.52e1.80 (m, 4H),1.30
mixture of 1 (3.50 g, 31.2 mmol), NH4Cl (3.40 g, 62.4 mmol), and
NaCN (4.10 g, 62.4 mmol) in EtOH (6 mL) and H2O (12 mL) was
heated at 60 ꢁC for 6 h. After removal of EtOH, the aqueous residue
was extracted with ether, and dried over MgSO4. Removal of the
solvent gave an oily residue, which was purified by short column
(dt, J¼4.4, 12.6 Hz, 1H), 1.03 (t, J¼12.6 Hz, 1H), 0.90 (d, J¼6.4 Hz, 3H),
0.84 (m, 1H); 13C NMR (100 MHz, CDCl3)
d 173.5, 154.8, 136.3, 128.4
(2C), 128.08 (2C), 128.06, 66.6, 59.4, 52.1, 43.1, 34.6, 33.9, 28.8, 22.4,
22.3; IR (CDCl3)
n 3442, 2954, 1744, 1732, 1717, 1506, 1456,
1226 cmꢂ1; FAB(þ)MS: m/z 306.2 [MþþH]; HRMS (ESI): [MþþNa],
chromatography on silica gel. Elution with 50% EtOAc in n-hexane
afforded a aminonitrile 5b (4.10 g, 96%) as a yellowish oil: [a]
found: 328.1512. C17H23NO4Na requires 328.1525.
22
D
ꢂ11.8 (c 1.00, CHCl3); 1H NMR (400 MHz, CDCl3)
d
1.95e2.04 (m,
5.2.8. (1R,3R)-1-Amino-3-methylcyclohexanecarboxylic acid methyl
ester (7a). A mixture of amino acid 4a (4.50 g, 14.7 mmol) and 10%
Pd/C (2.2 g) in MeOH (100 mL) was rigorously stirred under a H2
atmosphere at room temperature for 13 h. After the Pd/C catalyst
was filtered off, the filtrate was evaporated in vacuo. The residue
was purified by short column chromatography on silica gel to leave
2H), 1.57e1.85 (m, 6H), 1.35 (dt, J¼3.8, 12.6 Hz, 1H), 1.10 (t,
J¼12.6 Hz, 1H), 0.96 (d, J¼6.4 Hz, 3H), 0.85 (dq, J¼3.5, 12.6 Hz, 1H);
13C NMR (100 MHz, CDCl3)
d 124.1, 51.8, 46.0, 37.6, 33.2, 29.4, 22.6,
21.5; IR (neat)
n
3372, 2932, 2870, 2222, 1609, 1458 cmꢂ1; FAB(þ)
MS: m/z 139.2 [MþþH]; HRMS (ESI): [MþþH], found 139.1227.
C8H15N2 requires 139.1230.
amine 7a (2.50 g, quantitative) as an amorphous solid: [
a
]
26 ꢂ6.6 (c
D
1.02, CHCl3); 1H NMR (500 MHz, CDCl3)
d
3.80 (s, 3H), 2.21e2.09 (m,
5.2.5. (1S,3R)-1-Acetoamido-3-methylcyclohexanecarbonitrile
(6b). Acetyl chloride (AcCl; 0.77 mL, 11 mmol) and Et3N (0.75 mL,
5.4 mmol) were added successively to the stirred solution of ami-
nonitrile 5b (0.50 g, 3.60 mmol) in CH2Cl2 (3 mL) at 0 ꢁC. After
stirring at room temperature for 20 h, the solution was washed
with 10% aqueous HCl, 5% aqueous NaHCO3, brine, and dried over
MgSO4. Removal of the solvent gave a white solid, which was pu-
rified by column chromatography on silica gel (50% EtOAc in n-
hexane) to produce 6b (0.58 g, 89%) as colorless crystals: mp
3H), 2.05 (tt, J¼14.2, 3.3 Hz, 1H), 1.90e1.80 (m, 2H), 1.77e1.69 (m,
1H), 1.54 (t, J¼14.6, 13.3 Hz, 1H), 0.99e0.88 (m, 1H), 0.93 (d,
J¼6.2 Hz, 3H); 13C NMR (100 MHz, CDCl3)
d 171.7, 61.0, 52.9, 39.3,
n 3414, 2959, 2878, 1744, 1512,
33.2, 31.0, 25.9, 22.0, 19.8; IR (KBr)
1273 cmꢂ1; FAB(þ)MS: m/z 172.1 [MþþH]; HRMS (DART): [MþþH],
found 172.1358. C9H18NO2 requires 172.1332.
5 . 2 . 9 . ( 1 R , 3 R ) - 1 - B e n z y l o x y c a r b o n y l a m i n o - 3 -
methylcyclohexanecarboxylic acid (8a). Aqueous NaOH of 1 M
(16.0 mL, 16.0 mmol) was added to the stirred solution of amino
acid 4a (1.00 g, 3.27 mmol) in MeOH (20 mL) at 0 ꢁC, and the so-
lution was stirred at room temperature for 48 h. The solution was
neutralized with 10% aqueous HCl, and MeOH was evaporated. The
aqueous residue was extracted with CHCl3, and dried over Na2SO4.
111e113 ꢁC; [
a
]
D
24 ꢂ15.3 (c 1.00, CHCl3); 1H NMR (400 MHz, CDCl3)
d
5.90 (br s, 1H), 2.52 (d, J¼12.6 Hz, 2H), 2.01 (s, 3H), 1.65e1.90 (m,
4H), 1.35 (m, 1H), 1.08 (t, J¼12.6 Hz, 1H), 0.96 (d, J¼6.6 Hz, 3H), 0.89
(m, 1H); 13C NMR (100 MHz, CDCl3):
d
170.1, 119.8, 52.1, 43.0, 34.8,
33.3, 28.9, 22.9, 22.1, 21.4.; IR (KBr) n 3298, 2963, 2943, 2870, 2237,
1663, 1535 cmꢂ1
;
HRMS (ESI): [MþþNa], found 203.1189.
Removal of the solvent afforded a crude acid 8a (0.98 g, 98%) as
21
C
10H16N2ONa requires 203.1160.
colorless crystals: mp 89e91 ꢁC; [
NMR (400 MHz, CDCl3)
a
]
ꢂ5.88 (c 1.00, CHCl3); 1H
D
d 7.50 (br, 1H), 7.30e7.40 (m, 5H), 5.11 (s,
5.2.6. (1S,3R)-1-Amino-3-methylcyclohexanecarboxylic acid methyl
ester (7b). A mixture of 6b (7.00 g, 38.8 mmol) and concentrated
2H), 5.00 (br s,1H),1.96e2.29 (m, 2H),1.40e1.80 (m, 6H), 0.83e0.98
(m, 1H), 0.91 (d, J¼6.2 Hz, 3H); 13C NMR (100 MHz, CDCl3)
d 179.5,

2418
T. Hirata et al. / Tetrahedron 71 (2015) 2409e2420
155.7, 136.3, 128.6 (2C), 128.2, 128.1 (2C), 66.8, 59.7, 40.2, 33.9, 31.4,
NMR (500 MHz, CDCl3) d 3.69 (s, 3H), 2.28e2.10 (m, 4H), 2.07e1.53
27.2, 22.1, 20.9; IR (CDCl3)
n
3321, 2951, 1709, 1694, 1531, 1261 cmꢂ1
;
(m, 10H), 1.47 (dd, J¼14.3, 12.6 Hz, 1H), 1.35 (dd, J¼13.7, 12.2 Hz, 1H),
FAB(þ)MS: m/z 292.1 [MþþH]; HRMS (DART): [MþþH], found
0.95 (d, J¼6.4 Hz, 3H), 0.97e0.86 (m, 2H), 0.90 (d, J¼6.3 Hz, 3H); IR
292.1551. C16H22NO4 requires 292.1543.
(KBr)
CDCl3)
n ;
3279, 2989, 2681, 1721, 1655 cmꢂ1 13C NMR (125 MHz,
d
174.7, 170.8, 61.1, 60.5, 52.5, 40.0, 39.8, 33.9, 33.4, 31.7, 31.0,
5 . 2 . 1 0 . ( 1 S , 3 R ) - 1 - B e n z y l o x y c a r b o n y l a m i n o - 3 -
methylcyclohexanecarboxylic acid (8b). Compound 8b was prepared
27.5, 26.6, 22.2 (2C), 21.4, 20.3; HRMS (ESI): [MþþNa], found
333.2134. C17H30N2O3Na requires 333.2148. A solution of acid 8a
(2.62 g, 9.00 mmol), EDC hydrochloride (1.73 g, 9.00 mmol), 1-
hydroxy-7-azabenzotriazole (HOAt; 1.23 g, 9.00 mmol), and
iPr2EtN (1.6 mL, 9.0 mmol) in MeCN (10 mL) was stirred at room
temperature for 1 h. Then, dipeptide amine 10a (2.79 g, 9.00 mmol)
in MeCN (20 mL) was added to the stirred solution, and the solution
was stirred at 65 ꢁC for 24 h. After evaporation of the solvent, the
residue was diluted with EtOAc, washed with 10% aqueous HCl, 5%
aqueous NaHCO3, brine, and dried over MgSO4. Removal of the
solvent afforded a white solid, which was purified by column
from 4b in a manner similar to that described for the preparation of
22
8a: >99%; colorless crystals; mp 85e86 ꢁC; [
a
]
D
ꢂ3.76 (c 1.00,
CHCl3); 1H NMR (400 MHz, CDCl3)
d 7.30e7.40 (m, 5H), 5.10 (br s,
3H), 2.46 (br t, J¼12.3 Hz, 2H), 1.80 (m, 1H), 1.60e1.74 (m, 3H), 1.32
(dt, J¼5.1, 12.6 Hz, 1H), 1.05 (t, J¼12.6 Hz, 1H), 0.91 (d, J¼6.6 Hz, 3H),
0.85 (m, 1H); 13C NMR (100 MHz, CDCl3)
d 177.8, 155.5, 136.2, 128.5
(2C), 128.2, 128.1 (2C), 66.9, 59.4, 42.5, 34.1, 33.8, 28.6, 22.23, 22.19;
IR (KBr)
n 3387, 3090, 2955, 2928, 2839, 1724, 1701, 1531,
1254 cmꢂ1; HRMS (ESI): [MþþNa], found 314.1363. C16H21NO4Na
requires 314.1374.
chromatography on silica gel (50% EtOAc in n-hexane) to give tri-
21
peptide 13a (2.84 g, 54%) as colorless crystals: mp 94e96 ꢁC; [
a]
D
5.3. Preparation of peptides
ꢂ6.14 (c 1.00, CHCl3); 1H NMR (400 MHz, CDCl3)
d 7.30e7.40 (m,
6H), 6.35 (br s, 1H), 5.13 (d, J¼12.0 Hz, 1H), 5.11 (d, J¼12.0 Hz, 1H),
5.06 (br s, 1H), 3.67 (s, 3H), 1.98e2.30 (m, 6H), 1.20e1.85 (m, 18H),
0.92 (d, J¼6.0 Hz, 3H), 0.88 (d, J¼6.4 Hz, 3H), 0.85 (d, J¼6.2 Hz, 4H),
5.3.1. Cbz-[(1R,3R)-Ac6c3M]2-OMe [(1R,3R)-dipeptide; 9a]. A solu-
tion of 8a (4.72 g, 16.2 mmol), 1-ethyl-3-(3-dimethylaminopropyl)
carbodiimide hydrochloride (EDC hydrochloride; 3.10 g,
16.2 mmol), and 1-hydroxybenzotriazole (HOBt; 2.20 g, 16.2 mmol)
in MeCN (15 mL) was stirred at room temperature for 1 h. Then,
a solution of 7a (2.52 g, 14.7 mmol) in MeCN (5 mL) was added to
the stirred solution, and the solution was stirred at 65 ꢁC for 24 h.
After evaporation of the solvent, the residue was diluted with
EtOAc, washed with 10% aqueous HCl, 5% aqueous NaHCO3, brine,
and dried over MgSO4. Removal of the solvent gave a white solid,
which was purified by column chromatography on silica gel (20%
0.80e1.00 (m, 3H); 13C NMR (100 MHz, CDCl3)
d 175.3, 173.6, 173.4,
155.2, 136.0, 128.8 (2C), 128.6, 128.3 (2C), 67.1, 61.1, 60.6, 59.6, 52.0,
40.1, 39.7, 39.4, 34.2, 33.9, 33.6, 31.4, 31.1 (2C), 27.51, 27.46, 27.1,
22.3, 22.2, 22.1, 21.15, 21.10, 21.0; IR (CDCl3)
n 3431, 3377, 2952,
2928, 2868, 1730, 1720, 1670, 1506, 1456, 1249 cmꢂ1; FAB(þ)MS: m/
z
584.6 [MþþH]; HRMS (ESI): [MþþNa], found 606.3520.
C33H49N3O6Na requires 606.3513.
5.3.4. Cbz-[(1R,3R)-Ac6c3M]4-OMe [(1R,3R)-tetrapeptide; 12a]. A
mixture of 13a (258 mg, 0.440 mmol) and 10% Pd/C (100 mg) in
MeOH (10 mL) was rigorously stirred under a H2 atmosphere at
room temperature for 24 h. After filtration, the filtrate was con-
centrated in vacuo to give a crude tripeptide amine (199 mg,
quantitative). A solution of acid 8a (150 mg, 0.530 mmol), O-(7-
EtOAc in n-hexane) to leave dipeptide 9a (4.05 g, 62%) as colorless
21
crystals: mp 148e150 ꢁC; [
(400 MHz, CDCl3)
a]
ꢂ6.26 (c 1.00, CHCl3); 1H NMR
D
d 7.32e7.40 (m, 5H), 7.12 (br s, 1H), 5.13 (d,
J¼12.3 Hz, 1H), 5.10 (d, J¼12.3 Hz, 1H), 4.85 (br s, 1H), 3.66 (s, 3H),
2.04e2.21 (m, 4H), 1.25e1.72 (m, 12H), 0.90 (d, J¼6.0 Hz, 3H), 0.85
(d, J¼6.2 Hz, 3H), 0.80e1.00 (m, 2H); 13C NMR (100 MHz, CDCl3)
azabenzotriazol-1-yl)-N,N,N0,N0tetramethyluronium
hexa-
d
174.7, 173.7, 155.6, 136.2, 128.5 (2C), 128.2, 128.0 (2C), 66.7, 60.2,
fluorophosphate (HATU; 200 mg, 0.53 mmol), and HOAt (70 mg,
0.53 mmol), and iPr2EtN (0.15 mL, 0.90 mmol) in MeCN (2 mL) was
stirred at room temperature for 1 h. Then, tripeptide amine
(199 mg, 0.440 mmol) in MeCN (5 mL) was added, and the solution
was stirred at 65 ꢁC for 5 days. Removal of the solvent gave a resi-
due, which was diluted with EtOAc, washed with 10% aqueous HCl,
5% aqueous NaHCO3, brine, and dried over MgSO4. After removal of
the solvent, the white solid was purified by column chromatogra-
59.0, 52.0, 40.0, 39.3, 33.9, 33.7, 31.2, 31.1, 27.3, 27.2, 22.2, 22.0, 21.0,
20.9.; IR (CDCl3)
n 3432, 2953, 2928, 2868, 1733, 1669, 1507, 1456,
1244 cmꢂ1; FAB(þ)MS: m/z 445.5 [MþþH]; HRMS (DART): [MþþH],
found 445.2711. C25H37N2O5 requires 445.2697.
5.3.2. Cbz-[(1R,3R)-Ac6c3M]2-OH (11a). Aqueous NaOH solution of
1 M (0.23 mL, 0.23 mmol) was added to a stirred solution of di-
peptide 9a (20.0 mg) in MeOH (0.23 mL) at 0 ꢁC, and the resultant
mixture was stirred for 4 days at room temperature. The reaction
mixture was cooled to 0 ꢁC and acidified to ca. pH 3 by adding 1 M
aqueous HCl. After removal of MeOH, the aqueous solution was
extracted with CHCl3, and dried over MgSO4. Removal of the sol-
phy on silica gel (50% EtOAc in n-hexane) to give tripeptide 12a
21
(213 mg, 66%) as colorless crystals: mp 164e166 ꢁC; [
a]
ꢂ1.90 (c
D
1.00, CHCl3); 1H NMR (400 MHz, CDCl3)
d 7.32e7.40 (m, 6H), 6.80
(br s, 1H), 6.48 (br s, 1H), 5.30 (br s, 1H), 5.15 (d, J¼12.4 Hz, 1H), 5.12
(d, J¼12.4 Hz, 1H), 3.64 (s, 3H), 2.30e2.40 (m, 3H), 2.20 (m, 1H),
1.20e2.03 (m, 28H), 0.83e0.97 (m, 16H); 13C NMR (100 MHz, CDCl3)
d 175.7, 174.6, 173.4, 172.8, 155.4, 136.1, 128.6 (2C), 128.4, 127.5 (2C),
66.9, 60.9, 60.5, 60.3, 59.7, 51.7, 40.0, 39.9 (2C), 39.5, 34.3, 34.0, 33.7,
33.6, 31.3, 30.7, 30.5 (2C), 27.8, 27.5, 27.4, 26.9, 22.3 (2C), 22.2, 22.1,
vent gave acid 11a (19.3 mg, 99%) as a white solid: mp 208e209 ꢁC;
28
[
a]
ꢂ10.3 (c 1.00, CHCl3); 1H NMR (400 MHz, CDCl3)
d
7.46e7.27
D
(m, 5H), 7.18 (br s, 1H), 5.17 (br s, 1H), 5.15e5.03 (m, 2H), 2.27e1.97
(m, 4H), 1.71e1.21 (m, 12H), 0.95e0.78 (m, 8H); IR (KBr)
2947, 2677, 1720, 1686, 1655, 1501 cmꢂ1 13C NMR (100 MHz, CDCl3)
177.3, 175.0, 155.9, 136.1, 128.6 (2C), 128.3, 128.0 (2C), 66.9, 60.3,
n 3310,
;
21.4 (2C), 21.1, 21.0; IR (CDCl3) n 3430, 3366, 2952, 2928, 2867, 1730,
d
1718, 1618, 1507, 1456, 1250 cmꢂ1; FAB(þ)MS: m/z 723.5 [MþþH];
HRMS (ESI): [MþþNa], found 745.4505. C41H62N4O7Na requires
745.4511.
59.5, 39.7, 39.3, 33.8, 33.7, 31.0 (2C), 27.3 (2C), 22.2, 22.1, 20.9 (2C);
HRMS (ESI): [MþþNa], found 453.2375. C24H34N2O5Na requires
453.2360.
5.3.5. Cbz-[(1R,3R)-Ac6c3M]5-OMe
[(1R,3R)-pentapeptide;
5.3.3. Cbz-[(1R,3R)-Ac6c3M]3-OMe [(1R,3R)-tripeptide; 13a]. A mix-
ture of 9a (4.00 g, 9.00 mmol) and 10% Pd/C (2.0 g) in MeOH
(100 mL) was rigorously stirred under a H2 atmosphere at room
temperature for 4 h. After filtration, the filtrate was concentrated in
vacuo to give a crude amine 10a (2.79 g, quantitative). H-[(1R,3R)-
14a]. Pentapeptide 14a was prepared from 12a and 8a in a manner
similar to that described for the preparation of 13a: 30%; colorless
22
crystals; mp 265e267 ꢁC; [
a]
ꢂ12.4 (c 0.50, CHCl3); 1H NMR
D
(400 MHz, CDCl3) d 7.30e7.40 (m, 6H), 6.98 (br s,1H), 6.93 (br s,1H),
6.48 (br s, 1H), 5.30 (br s, 1H), 5.17 (d, J¼12.5 Hz, 1H), 5.11 (d,
Ac6c3M]2-OMe (10a): mp 140e141 ꢁC; [
a
]
28 ꢂ6.30 (c 1.00, CHCl3); 1H
J¼12.5 Hz, 1H), 3.64 (s, 3H), 1.10e2.50 (m, 40H), 0.80e0.98 (m,
D

T. Hirata et al. / Tetrahedron 71 (2015) 2409e2420
2419
20H); 13C NMR (100 MHz, CDCl3)
155.6,136.1,128.7 (2C),128.5,127.2 (2C), 66.9, 61.2, 60.8, 60.4, 59.74,
59.67, 51.8, 39.8 (5C), 34.4, 34.3, 34.0, 33.8, 33.6, 31.5 (5C), 27.9, 27.7,
27.6, 27.4, 26.8, 22.5, 22.4 (2C), 22.3, 22.1, 21.7, 21.5, 21.4, 21.2, 21.0;
d
175.9, 175.1, 173.9, 173.6, 173.5,
155.4, 136.2, 128.6 (2C), 128.3, 127.7 (2C), 66.9, 59.3, 58.7, 58.6, 58.5,
51.5, 43.1, 43.0, 42.7, 42.3, 35.3, 35.2, 35.0, 34.3 (2C), 34.0, 33.9, 33.4,
28.8, 28.7, 28.2, 28.1, 22.7, 22.6, 22.53, 22.50 (2C), 22.2, 21.9, 21.8;
FAB(þ)MS: m/z 745.5 [MþþNa], HRMS (DART): [MþþH], found
723.4728. C41H63N4O7 requires 723.4691.
IR (CDCl3)
n 3431, 3359, 2951, 2927, 2867, 1720, 1674, 1517, 1457,
1252 cmꢂ1; HRMS (DART): [MþþH], found 862.5719. C49H76N5O8
requires 862.5689.
5.3.10. Cbz-[(1S,3R)-Ac6c3M]5-OMe
[(1S,3R)-pentapeptide;
14b]. Pentapeptide 14b was prepared from 12b and 8b in a manner
5.3.6. Cbz-[(1R,3R)-Ac6c3M]6-OMe
15a]. Hexapeptide 15a was prepared from 14a and 8a in a manner
[(1R,3R)-hexapeptide;
similar to that described for the preparation of 9b: 27%; colorless
22
crystals; mp 261e263 ꢁC; [
a
]
D
ꢂ35.2 (c 0.20, CHCl3); 1H NMR
similar to that described for the preparation of 13a: 21%; colorless
(400 MHz, CDCl3) d 7.32e7.38 (m, 5H), 7.23 (br s, 1H), 6.93 (br s, 1H),
22
crystals; mp 234e236 ꢁC; [
a
]
ꢂ16.2 (c 0.10, CHCl3); 1H NMR
6.66 (br s, 1H), 6.30 (br s, 1H), 5.18 (br s, 1H), 5.11 (d, J¼12.4 Hz, 1H),
D
(500 MHz, CDCl3) d 7.43e7.29 (m, 6H), 7.14 (br s, 1H), 7.05 (br s, 1H),
5.08 (d, J¼12.4 Hz, 1H), 3.66 (s, 3H), 1.97e2.62 (m, 12H), 0.65e1.80
6.98 (br s,1H), 6.51 (br s,1H), 5.40 (br s,1H), 5.25e5.05 (m, 2H), 3.65
(s, 3H), 2.50e1.13 (m, 48H), 1.07e0.62 (m, 24H); IR (CDCl3) 3431,
3353, 2951, 2927, 2866, 2359, 1718, 1671, 1518, 1456, 1215 cmꢂ1 13C
NMR (100 MHz, CDCl3) 176.1, 175.4, 174.8, 174.4, 173.92, 173.87,
(m, 48H); IR (CDCl3)
n 3454, 3354, 2953, 2928, 1732, 1716, 1682,
; d 174.6, 174.1,
n
1508, 1456, 1219 cmꢂ1 13C NMR (125 MHz, CDCl3)
;
172.7 (2C), 172.5, 155.5, 136.3, 128.7 (2C), 128.3, 127.5 (2C), 66.9,
59.3, 58.7, 58.45, 58.41, 58.3, 51.5, 43.3, 43.2, 42.54, 42.50, 42.2, 35.7,
35.5, 35.4, 35.2, 34.5, 34.4, 34.3, 34.1, 34.0, 33.2, 28.8 (2C), 28.3,
28.12, 28.09, 22.8, 22.7, 22.61, 22.60 (2C), 22.3, 22.2, 21.9, 21.8 (2C);
HRMS (ESI): [MþþNa], found 884.5487. C49H75N5O8Na requires
884.5513.
d
155.6, 136.1, 128.7 (2C), 128.5, 127.1 (2C), 66.9, 61.2, 60.9, 60.5, 60.2,
59.7, 59.6, 51.8, 39.9 (6C), 34.4, 34.3, 34.1, 34.0, 33.7, 33.6, 31.4 (6C),
27.9, 27.7 (3C), 27.3, 26.8, 22.52, 22.47, 22.4, 22.3, 22.2, 22.1, 21.8,
21.6, 21.44, 21.37, 21.2, 21.0; HRMS (FAB): [MþþH], found
1001.6663. C57H89N6O9 requires 1001.6686.
5.3.11. Cbz-[(1S,3R)-Ac6c3M]6-OMe
[(1S,3R)-hexapeptide;
5.3.7. Cbz-[(1S,3R)-Ac6c3M]2-OMe [(1S,3R)-dipeptide; 9b]. A solu-
tion of 8b (2.03 g, 7.00 mmol), HATU (2.70 g, 7.00 mmol), HOAt
(1.00 g, 7.35 mmol), and iPr2EtN (2.5 mL, 14 mmol) in MeCN (10 mL)
was stirred at room temperature for 1 h. Then, a solution of 7b
(1.20 g, 7.00 mmol) in MeCN (5 mL) was added to the stirred so-
lution, and the solution was stirred at 70 ꢁC for 24 h. After evapo-
ration of the solvent, the residue was diluted with EtOAc, washed
with 10% aqueous HCl, 5% aqueous NaHCO3, brine, and dried over
MgSO4. Removal of the solvent gave a white solid, which was pu-
rified by column chromatography on silica gel (25% EtOAc in n-
hexane) to give dipeptide 9b (2.61 g, 84%) as colorless crystals: mp
15b]. Hexapeptide 15b was prepared from 14b and 8b in a manner
similar to that described for the preparation of 9b: 12%; colorless
22
crystals; mp 246e248 ꢁC; [
a
]
ꢂ27.1 (c 0.10, CHCl3); 1H NMR
D
(400 MHz, CDCl3) d 7.32e7.40 (m, 6H), 7.05 (br s, 1H), 6.89 (br s, 1H),
6.60 (br s, 1H), 6.20 (br s, 1H), 5.13 (d, J¼12.5 Hz, 1H), 5.12 (br s, 1H),
5.09 (d, J¼12.5 Hz, 1H), 3.67 (s, 3H), 1.92e2.70 (m, 18H), 0.60e1.80
(m, 54H); IR (CDCl3)
n 3447, 3350, 2953, 2928, 1718, 1673, 1518,
;
1455, 1244, 1220 cmꢂ1
13C NMR (100 MHz, CDCl3)
d 175.1, 174.4,
173.9, 173.4, 173.1, 172.8, 155.7, 136.7, 128.7 (2C), 128.3, 127.2 (2C),
66.7, 59.2, 58.8, 58.3 (2C), 58.2, 58.1, 51.5, 43.2 (2C), 42.6, 42.4 (2C),
42.1, 35.6, 35.4 (2C), 35.2, 35.0, 34.6, 34.43, 34.38, 34.30, 34.1, 34.0,
33.1, 28.8 (2C), 28.3, 28.15, 28.09 (2C), 22.9, 22.7 (2C), 22.6, 22.5,
22.4, 22.2 (2C), 21.9 (3C), 21.8; HRMS (ESI): [MþþNa], found
1023.6525. C57H88N6O9Na requires 1023.6510.
122e124 ꢁC; [
a
]
D
21 ꢂ14.6 (c 1.00, CHCl3); 1H NMR (400 MHz, CDCl3)
d
7.25e7.40 (m, 6H), 5.08 (s, 2H), 4.83 (br s, 1H), 3.64 (s, 3H),
2.30e2.60 (m, 4H), 1.50e1.80 (m, 8H), 1.15e1.30 (m, 2H), 0.80e1.00
(m, 10H); IR (CDCl3) 3425, 2953, 2930, 1733, 1699, 1507,
1218 cmꢂ1 13C NMR (100 MHz, CDCl3)
173.6, 171.4, 155.5, 136.3,
n
;
d
Acknowledgements
128.6 (2C), 128.2, 128.1 (2C), 66.7, 59.2, 58.8, 51.7, 42.7, 42.6, 34.3,
34.01, 33.97, 33.89, 28.7, 28.5, 22.3, 22.24, 22.22, 22.1; HRMS (ESI):
[MþþNa], found 467.2559. C25H36N2O5Na requires 467.2522.
This work was supported in part by a Grant-in-Aid for Young
Scientists (A) (25713008) from Japan Society for the Promotion of
Science (M.O.), a Grant-in-Aid for Scientific Research on Innovative
Areas ‘Advanced Molecular Transformations by Organocatalysts’
(26105745) from Ministry of Education, Culture, Sports, Science,
and Technology (M.T.), and Special Coordination Funds for Pro-
moting Science and Technology from Japan Science and Technology
Agency (A.U.).
5.3.8. Cbz-[(1S,3R)-Ac6c3M]3-OMe
[(1S,3R)-tripeptide;
13b]. Tripeptide 13b was prepared from 9b and 8b in a manner
similar to that described for the preparation of 9b: 82%; colorless
22
crystals; mp 160e162 ꢁC; [
a]
ꢂ13.2 (c 1.00, CHCl3); 1H NMR
D
(400 MHz, CDCl3) d 7.52 (br s, 1H), 7.30e7.38 (m, 5H), 6.61 (br s, 1H),
5.08 (s, 2H), 4.90 (br s, 1H), 3.66 (s, 3H), 2.38e2.52 (m, 6H),
1.51e1.77 (m, 12H), 1.20e1.30 (m, 2H), 1.10 (m, 1H), 0.79e1.02 (m,
15H); IR (CDCl3)
NMR (100 MHz, CDCl3)
n ;
3421, 2954, 2929, 1732, 1716, 1507, 1218 cmꢂ1 13C
Supplementary data
d
173.8, 172.1, 171.9, 155.3, 136.1, 128.6 (2C),
128.4, 128.1 (2C), 66.8, 59.5 (2C), 58.7, 51.6, 43.0, 42.8, 42.5, 34.8,
34.5, 34.2, 34.0, 33.93, 33.89, 28.7, 28.5, 28.4, 22.37 (2C), 22.30,
22.25, 22.22, 22.0; HRMS (DART): [MþþH], found 584.3712.
Supplementary data associated with this article can be found in
the online version, at http://dx.doi.org/10.1016/j.tet.2015.02.075.
C
33H50N3O6 requires 584.3694.
References and notes
5.3.9. Cbz-[(1S,3R)-Ac6c3M]4-OMe
12b]. Tetrapeptide 12b was prepared from 13b and 8b in a manner
[(1S,3R)-tetrapeptide;
1. (a) Cativiela, C.; Díaz-de-Villegas, M. D. Tetrahedron: Asymmetry 1998, 9,
3517e3599; (b) Cativiela, C.; Díaz-de-Villegas, M. D. Tetrahedron: Asymmetry
2000, 11, 645e732; (c) Cativiela, C.; Ordonez, M. Tetrahedron: Asymmetry 2009,
20, 1e63.
2. (a) Heimgartner, H. Angew. Chem., Int. Ed. Engl. 1991, 30, 238e264; (b) Toniolo,
C.; Crisma, M.; Formaggio, F.; Valle, G.; Cavicchioni, G.; Precigoux, G.; Aubry, A.;
Kamphuis, J. Biopolymers 1993, 33, 1061e1072; (c) Benedetti, E. Biopolymers
(Pept. Sci.) 1996, 40, 3e44; (d) Wysong, C. L.; Yokum, T. S.; McLaughlin, M. L.;
Hammer, R. P. Chemtech 1997, 26e33; (e) Toniolo, C.; Crisma, M.; Formaggio, F.;
Peggion, C. Biopolymers (Pept. Sci.) 2001, 90, 396e419; (f) Tanaka, M. Chem.
Pharm. Bull. 2007, 55, 349e358.
similar to that described for the preparation of 9b: 79%; colorless
22
crystals; mp 217e219 ꢁC; [
a]
ꢂ21.5 (c 1.00, CHCl3); 1H NMR
D
(400 MHz, CDCl3) d 7.30e7.40 (m, 6H), 6.72 (br s,1H), 6.46 (br s,1H),
5.10 (d, J¼12.3 Hz, 1H), 5.08 (d, J¼12.3 Hz, 1H), 4.97 (br s, 1H), 3.66
(s, 3H), 2.10e2.52 (m, 8H), 1.50e2.10 (m, 16H), 0.70e1.40 (m, 24H);
IR (CDCl3)
1217 cmꢂ1
n
;
3420, 3357, 2953, 2928, 1732, 1716, 1683, 1507, 1456,
13C NMR (125 MHz, CDCl3)
174.2, 173.3, 172.5, 171.8,
d

2420
T. Hirata et al. / Tetrahedron 71 (2015) 2409e2420
3. (a) Karle, I. L.; Balaram, P. Biochemistry 1990, 29, 6747e6756; (b) Toniolo, C.;
Benedetti, E. Macromolecules 1991, 24, 4004e4009; (c) Improta, R.; Rega, N.;
Aleman, C.; Barone, V. Macromolecules 2001, 34, 7550e7557.
16. (a) Benedetti, E.; Barone, V.; Bavoso, A.; Di Blasio, B.; Lelj, F.; Pavone, V.; Pedone,
C.; Bonora, G. M.; Toniolo, C.; Leplawy, M. T.; Kaczmarek, K.; Redlinski, A. Bio-
polymers 1988, 27, 357e371; (b) Toniolo, C.; Bonora, G. M.; Bavoso, A.; Bene-
detti, E.; Di Blasio, B.; Pavone, V.; Pedone, C.; Barone, V.; Lelj, F.; Leplawy, M. T.;
Kaczmarek, K.; Redlinski, A. Biopolymers 1988, 27, 373e379; (c) Peggion, C.;
Crisma, M.; Toniolo, C.; Formaggio, F. Tetrahedron 2012, 68, 4429e4433; (d)
Formaggio, F.; Crisma, M.; Ballano, G.; Peggion, C.; Venanzi, M.; Toniolo, C. Org.
Biomol. Chem. 2012, 10, 2413e2421.
4. (a) Bardi, R.; Piazzesi, A. M.; Toniolo, C.; Sukumar, M.; Balaram, P. Biopolymers
1986, 25, 1635e1644; (b) Santini, A.; Barone, V.; Bavoso, A.; Benedetti, E.; Di
Blasio, B.; Fraternali, F.; Lelj, F.; Pavone, V.; Pedone, C.; Crisma, M.; Bonora, G.
M.; Toniolo, C. Int. J. Biol. Macromol. 1988, 10, 292e299; (c) Crisma, M.; Bonora,
G. M.; Toniolo, C.; Benedetti, E.; Bavoso, A.; Di Blasio, B.; Pavone, V.; Pedone, C.
Int. J. Biol. Macromol. 1988, 10, 300e304; (d) Benedetti, E.; Blasio, B. D.; Pavone,
V.; Pedone, C.; Santini, A.; Crisma, M.; Valle, G.; Toniolo, C. Biopolymers 1989, 28,
175e184; (e) Toniolo, C. Int. J. Pept. Protein Res. 1990, 35, 287e300; (f) Toniolo,
C.; Crisma, M.; Formaggio, F.; Benedetti, E.; Santini, A.; Iacovino, R.; Saviano, M.;
Blasio, B. D.; Pedone, C.; Kamphuis, J. Biopolymers (Pept. Sci.) 1996, 40, 519e522;
(g) Benedetti, E.; Blasio, B. D.; 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, 2023e2032; (h) Gatos, M.; For-
maggio, F.; Crisma, M.; Toniolo, C.; Bonora, G. M.; Benedetti, Z.; Blasio, B. D.;
Iacovino, R.; Santini, A.; Saviano, M.; Kamphuis, J. J. Pept. Sci. 1997, 3, 110e122.
5. (a) Bardi, R.; Piazzesi, A. M.; Toniolo, C.; Sukumar, M.; Raj, P. A.; Balaram, P. Int. J.
Pept. Protein Res. 1985, 25, 628e639; (b) Paul, P. K. C.; Sukumar, M.; Bardi, R.;
Piazzesi, A. M.; Valle, G.; Toniolo, C.; Balaram, P. J. Am. Chem. Soc. 1986, 108,
6363e6370; (c) Pavone, V.; Benedetti, E.; Barone, V.; Di Blasio, B.; Lelj, F.; Pe-
done, C.; Santini, A.; Crisma, M.; Bonora, G. M.; Toniolo, C. Macromolecules 1988,
21, 2064e2071; (d) Crisma, M.; Bonora, G. M.; Toniolo, C.; Bavoso, A.; Benedetti,
E.; Di Blasio, B.; Pavone, V.; Pedone, C. Macromolecules 1988, 21, 2071e2074; (e)
Valle, G.; Crisma, M.; Toniolo, C.; Sen, N.; Sukumar, M.; Balaram, P. J. Chem. Soc.,
Perkin Trans. 2 1988, 393e398; (f) Benedetti, E.; Di Blasio, B.; Pavone, V.; Pe-
done, C.; Santini, A.; Crisma, M.; Toniolo, C. Acta Crystallogr., Sect. C 1989, 45,
634e638; (g) Fabiano, N.; Valle, G.; Crisma, M.; Toniolo, C.; Saviano, M.; Lom-
bardi, A.; Isernia, C.; Pavone, V.; Di Blasio, B.; Pedone, C.; Benedetti, E. Int. J. Pept.
Protein Res. 1993, 42, 459e465.
€
17. (a) Wuthrich, K. NMR of Proteins and Nucleic Acids; John Wiley & Sons: New
€
€
€
York, NY, 1986; (b) Wagner, G.; Neuhaus, D.; Worgotter, E.; Vasak, M.; Kagi, J. H.
€
R.; Wuthrich, K. J. Mol. Biol. 1986, 187, 131e135.
18. (a) Sheldrick, G. M. Program for Crystal Structure Refinement (SHELXL 97); Uni-
€
€
versity of Gottingen: Gottingen, Germany, 1997; (b) Beurskens, P. T.; Admiraal,
G.; Beurskens, G.; Bosman, W. P.; de Gelder, R.; Israel, R.; Smits, J. M. M. The
DIRDIF-99 Program System Technical Report of the Crystallography Laboratory;
University of Nijmegen: The Netherlands, 1994.
19. CCDC-1036629 (12a), 1036630 (15a), 1036631 (12b), and 1036632 (15a) contain
the supplementary crystallographic data for this paper. These data can be ob-
tained free of charge from The Cambridge Crystallographic Data Centre via:
www.ccdc.cam.ac.uk/data_request/cif
20. (a) Shamala, N.; Nagaraj, R.; Balaram, P. J. Chem. Soc., Chem. Commun. 1978,
996e997; (b) Benedetti, E.; Bavoso, A.; Di Blasio, B.; Pavone, V.; Pedone, C.;
Crisma, M.; Bonora, G. M.; Toniolo, C. J. Am. Chem. Soc. 1982, 104, 2437e2444.
21. Trigalo, F.; Buisson, D.; Azerad, R. Tetrahedron Lett. 1988, 29, 6109e6112.
22. Wallace, G. A.; Gordon, T. D.; Hayes, M. E.; Konopacki, D. B.; Fix-Stenzel, S. R.;
Zhang, X.; Grongsaard, P.; Cusack, K. P.; Schaffter, L. M.; Henry, R. F.; Stoffel, R. H.
J. Org. Chem. 2009, 74, 4886e4889.
23. (a) Edward, J. T.; Jitrangsri, C. Can. J. Chem. 1975, 53, 3339e3350; (b) Casabona,
D.; Cativiela, C. Tetrahedron 2006, 62, 10000e10004.
24. (a) Tanaka, M.; Demizu, Y.; Doi, M.; Kurihara, M.; Suemune, H. Angew. Chem.,
Int. Ed. 2004, 43, 5360e5363; (b) Demizu, Y.; Doi, M.; Kurihara, M.; Maruyama,
T.; Suemune, H.; Tanaka, M. Chem.dEur. J. 2012, 18, 2430e2439; (c) Oba, M.;
Takazaki, H.; Kawabe, N.; Doi, M.; Demizu, Y.; Kurihara, M.; Kawakubo, H.;
Nagano, M.; Suemune, H.; Tanaka, M. J. Org. Chem. 2014, 79, 9125e9140.
25. (a) Tanaka, M.; Anan, K.; Demizu, Y.; Kurihara, M.; Doi, M.; Suemune, H. J. Am.
Chem. Soc. 2005, 127, 11570e11571; (b) Anan, K.; Demizu, Y.; Oba, M.; Kurihara,
M.; Doi, M.; Suemune, H.; Tanaka, M. Helv. Chim. Acta 2012, 95, 1694e1713; (c)
Oba, M.; Ishikawa, N.; Demizu, Y.; Kurihara, M.; Suemune, H.; Tanaka, M. Eur. J.
Org. Chem. 2013, 7679e7682.
6. (a) Chacko, K. K.; Srinivasan, R.; Zand, R. J. Cryst. Mol. Struct. 1971, 1, 261e269; (b)
Varughese, K. I.; Chacko, K. K.; Zand, R. Acta Crystallogr., Sect. B 1975, 31, 866e868;
€
(c) Strassler, C.; Linden, A.; Heimgartner, H. Helv. Chim. Acta 1997, 80, 1528e1554.
ꢀ
7. The calculations by Aleman and co-workers estimated the amino axial is more
stable than the equatorial by 3.0 kcal/mol. Also, they suggested that the
peptide-backbone structures influence the equilibrium between the amino
axial and equatorial conformations. See: Rodríguez-Ropero, F.; Zanuy, D.; Ca-
ꢀ
sanovas, J.; Nussinov, R.; Aleman, C. J. Chem. Inf. Model. 2008, 48, 333e343.
8. (a) Toniolo, C.; Benedetti, E. Trends Biochem. Sci. 1991, 16, 350e353; (b) Liskamp,
R. M. J. Recl. Trav. Chim. Pays-Bas. 1994, 113, 1e19; (c) Pal, L.; Basu, G.; Chakra-
barti, P. Proteins: Struct. Funct. Genet. 2002, 48, 571e579; (d) Toniolo, C.; Crisma,
M.; Formaggio, F.; Peggion, C.; Broxterman, Q. B.; Kaptein, B. Biopolymers (Pept.
Sci.) 2004, 76, 162e176.
26. Nagano, M.; Tanaka, M.; Doi, M.; Demizu, Y.; Kurihara, M.; Suemune, H. Org.
Lett. 2009, 11, 1135e1137.
27. (a) Crisma, M.; Saviano, M.; Moretto, A.; Broxterman, Q. B.; Kaptein, K.; Toniolo,
C. J. Am. Chem. Soc. 2007, 129, 15471e15473; (b) Bellanda, M.; Mammi, S.;
Geremia, S.; Demitri, N.; Randaccio, L.; Broxterman, Q. B.; Kaptein, B.; Pengo, P.;
Pasquato, L.; Scrimin, P. Chem.dEur. J. 2007, 13, 407e416; (c) Moretto, A.; For-
maggio, F.; Kaptein, B.; Broxterman, Q. B.; Wu, L.; Keiderling, T. A.; Toniolo, C.
Biopolymers (Pept. Sci.) 2008, 90, 567e574.
28. (a) Horstmann, T. E.; Guerin, D. J.; Miller, S. J. Angew. Chem., Int. Ed. 2010, 39,
3635e3638; (b) Mbofana, C. T.; Miller, S. J. J. Am. Chem. Soc. 2014, 135,
3285e3292.
ꢀ
ꢀ
9. (a) Jimenez, A. I.; Cativiela, C.; Gomez-Catalan, J.; Perez, J. J.; Aubry, A.; Paris, M.;
ꢀ
Marraud, M. J. Am. Chem. Soc. 2000, 122, 5811e5821; (b) Lasa, M.; Jimenez, A. I.;
ꢀ
Zurbano, M. M.; Cativiela, C. Tetrahedron Lett. 2005, 46, 8377e8380; (c) Aleman,
ꢀ
C.; Jimenez, A. I.; Cativiela, C.; Nussinov, R.; Casanovas, J. J. Org. Chem. 2009, 74,
7834e7843.
10. (a) Bergs, H. German Patent 566,094, May 26, 1929; Chem. Abstr. 1933, 27, 1001;
(b) Bucherer, H. T.; Steiner, W. J. Prakt. Chem. 1934, 140, 291e316; (c) Bucherer,
H. T.; Lieb, V. A. J. Prakt. Chem. 1934, 141, 5e43; (d) Ware, E. Chem. Rev. 1950, 46,
403e470; (e) Lopez, C. A.; Trigo, G. G. Adv. Heterocycl. Chem. 1985, 38, 177e228;
29. (a) Akagawa, K.; Kudo, K. Adv. Synth. Catal. 2011, 353, 843e847; (b) Akagawa, K.;
Sen, J.; Kudo, K. Angew. Chem., Int. Ed. 2013, 52, 11585e11588.
ꢀ
30. (a) Kowalczyk, W.; Prahl, A.; Derdowska, I.; Dawidowska, O.; Slaninova, J.;
€
(f) Kruger, G. In Methoden der Organischen Chemie; Falbe, J., Ed.; Thieme:
Lammek, B. J. Med. Chem. 2004, 47, 6020e6024; (b) Labudda-Dawidowska, O.;
ꢀ
ꢀ
Stuttgart, Germany, 1986; Band E5, p 536.
Wierzba, T. H.; Prahl, A.; Kowalczyk, W.; Gawinski, L.; Plackova, M.; Slaninova,
J.; Lammek, B. J. Med. Chem. 2005, 48, 8055e8059; (c) Fedi, V.; Altamura, M.;
Catalioto, R.-M.; Giannotti, D.; Giolitti, A.; Giuliani, S.; Guidi, A.; Harmat, N. J. S.;
Lecci, A.; Meini, S.; Nannicini, R.; Pasqui, F.; Tramontana, M.; Triolo, A.; Maggi,
C. A. J. Med. Chem. 2007, 50, 4793e4807; (d) Long, Y.-Q.; Xue, T.; Song, Y.-L.; Liu,
Z.-L.; Huang, S.-X.; Yu, Q. J. Med. Chem. 2008, 51, 6371e6380.
11. Wysong, C. L.; Yokum, T. S.; Morales, G. A.; Gundry, R. L.; McLaughlin, M. L.;
Hammer, R. P. J. Org. Chem. 1996, 61, 7650e7651.
12. See Supplementary data.
13. Strecker, A. Liebigs Ann. 1850, 75, 27e45.
14. (a) Han, S.-Y.; Kim, Y.-A. Tetrahedron 2004, 60, 2447e2467; (b) ChemFiles 2007,
7;
http://www.sigmaaldrich.com/content/dam/sigma-aldrich/docs/Aldrich/
31. (a) Nagano, M.; Doi, M.; Kurihara, M.; Suemune, H.; Tanaka, M. Org. Lett. 2010,
12, 3564e3566; (b) Demizu, Y.; Yamagata, N.; Nagoya, S.; Sato, Y.; Doi, M.;
Tanaka, M.; Nagasawa, K.; Okuda, H.; Kurihara, M. Tetrahedron 2011, 67,
6155e6165.
Brochure/al_chemfile_v7_n2.pdf.
15. (a) Imawaka, N.; Tanaka, M.; Suemune, H. Helv. Chim. Acta 2000, 83,
2823e2835; (b) Tanaka, M.; Nishimura, S.; Oba, M.; Demizu, Y.; Kurihara, M.;
Suemune, H. Chem.dEur. J. 2003, 9, 3082e3090.