Secondary structure of homo-thiopeptides based on a bridged β-proline analogue: Preferred formation of extended strand structures with trans-thioamide bonds

Article abstract of DOI:10.1016/j.tet.2012.01.018

Homo-thiopeptides of bicyclic 7-azabicyclo[2.2.1]heptane-2-endo-carboxylic acid (Ah2c), which is a conformationally constrained β-proline mimic, were synthesized and their structures were investigated. These homo-thiopeptides showed a preference for ordered secondary structures with trans-thioamide bonds in the solid state and in solution.

Full text of DOI:10.1016/j.tet.2012.01.018

Tetrahedron 68 (2012) 4418e4428
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Secondary structure of homo-thiopeptides based on a bridged b-proline analogue:
preferred formation of extended strand structures with trans-thioamide bonds
,
a
b
b
a,
Yuko Otani a , Tetsuharu Hori , Masatoshi Kawahata , Kentaro Yamaguchi , Tomohiko Ohwada
*
*
a Graduate School of Pharmaceutical Sciences, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
b Department of Pharmaceutical Sciences at Kagawa Campus Tokushima Bunri University, 1314-1 Shido, Sanuki, Kagawa 769-2193, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 24 August 2011
Received in revised form 27 December 2011
Accepted 10 January 2012
Available online 24 January 2012
Homo-thiopeptides of bicyclic 7-azabicyclo[2.2.1]heptane-2-endo-carboxylic acid (Ah2c), which is
a conformationally constrained -proline mimic, were synthesized and their structures were in-
vestigated. These homo-thiopeptides showed a preference for ordered secondary structures with trans-
thioamide bonds in the solid state and in solution.
b
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
Thioamides
Helix
b-Proline
Cisetrans equilibration
7-Azabicyclo[2.2.1]heptane
1. Introduction
Oligomers of
amines, such as
acid5 have also been studied. The circular dichroism (CD) spectra of
-proline oligomers have been reported,4 but the solution struc-
tures of -proline oligomers remain unclear, probably because of
b
-peptide derivatives containing cyclic secondary
b
-homoproline,3 nipecotic acid,4 and oxanipecotic
Several types of amino acid derivatives, such as a-proline de-
rivatives and N-substituted glycines (i.e., peptoids), contain a sec-
ondary amine in the main chain. Oligomers of proline1a and
peptoids1b have been found to adopt helical conformations. The
b
b
the presence of unbiased amide rotamers due to the small energy
differences with respect to cis and trans orientations of the tertiary
amide bonds (Fig. 1, X¼O) and ring puckering.
literature suggested that a stabilizing n➝
p
* interaction between Oi-
1(¼C) and Ci0 (¼O) of
a-proline oligomers has been shown to play
a substantial role in stabilizing the polyproline type II helix.1c Cis- or
trans-predominancy of the peptoid amide bond can be controlled
by incorporating appropriate side chains, such as alkyl and aryl
groups, on the N atoms.2
Several approaches to reducing the conformational freedom of
b-proline, particularly the cisetrans equilibration of the amide
linkage, have been reported. 2,2-Disubstituted -proline (A, Fig. 2)
b
oligomers have been synthesized to shift the cisetrans equilibrium
of the backbone amide group,6 and NOEs observed for the tetramer
showed a cis-conformation for each of the three inter-residue
amide linkages. Length-dependent induction of helical structure
has been demonstrated in the case of A.
On the other hand, development of organized structures of b-
peptides that do not involve hydrogen-bonding, most of which are
composed of tertiary amide bonds, is one of the challenging goals in
the areas of peptide engineering and medicinal chemistry. This is
because
resistance to enzymatic degradation. Structurally, however, the N-
amide linkage in a -amino acid is distal from the intraresidual
carboxylic acid functionality and direct interaction is unfavorable.
-Proline (Fig. 1), a cyclic -amino acid analogue of -proline also
has a secondary amine, and represents a potentially useful scaffold
for foldamers as examples of non-hydrogen-bonding -peptides.
b-peptides exhibit interesting biological features, such as
b
b
b
a
b
* Corresponding authors. E-mail addresses: otani@mol.f.u-tokyo.ac.jp (Y. Otani),
ohwada@mol.f.u-tokyo.ac.jp (T. Ohwada).
Fig. 1. Cisetrans isomerization of tertiary amides (X¼O)/thioamides (X¼S) of
b-
proline.
0040-4020/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2012.01.018

Y. Otani et al. / Tetrahedron 68 (2012) 4418e4428
4419
cisetrans equilibrium (Fig. 1, X¼S). Substitution of amide with
thioamide may cause changes in structural preference or stability of
ordered structure,10 because thioamides differ from amides in
several respects. For example, the C]S bond is longer than the C]
O bond and the thioamide unit has an enhanced preference for
planar structure,11 that is, thioamides have much higher rotational
barriers about the NeC(S) bond (i.e., enhanced double bond char-
acter) than amides do.12,13 It was also reported that the calculated
dipole moment of dimethyl thioacetamide was larger than that of
dimethyl acetamide.13
A
B
C
Fig. 2. Conformationally constrained
b
-proline analogues.
The structure and rotational barrier of N-thioaroyl-7-azabicyclo
[2.2.1]heptane have been investigated.14 X-ray crystallographic
analysis showed significant nonplanarity of the thioamide group in
this molecule.
Here, we describe synthesis of homo-thiopeptides of Ah2c
(Fig. 3), as well as b-proline thiopeptides. Elucidation of the solid
and solution structures of the dimer and the trimer, as well as CD
data of homo-thiopeptides up to the hexamer, indicates a prefer-
ence for ordered secondary structures with trans-thioamide bonds.
In the case of B (Fig. 2), a methano-bridge was introduced into
the
-proline skeleton to maintain an idealized ring pucker.7 X-ray
crystal structure and NMR solution structure analyses of the tet-
ramer of compound B showed that the major conformer takes an
all-trans amide structure in which adjacent carbonyl groups of the
amide groups are oriented in opposite directions.
b
The synthesis, crystallographic and CD/UV spectral analyses of
oligomers of a conformationally constrained b-proline mimic, 7-
azabicyclo[2.2.1]heptane-2-endo-carboxylic acid (Ah2c) (C, X¼H,
Fig. 2), have been studied.8 Although the existence of multiple
rotamers hampered detailed structural elucidation, the bicyclic
2. Results and discussion
2.1. Synthesis
skeleton was found to constrain the torsion angles (
along the main chain of -amino acids, causing the oligomers to
converge to CD-active ordered structures. Very recently
bridgehead-substituted 7-azabicyclo[2.2.1]heptane-2-endo-car-
4, q, and j)
Both enantiomers of bicyclic
were synthesized by means of modifications of the procedures de-
scribed previously. -Amino acid oligomers were synthesized by
b b
-amino acid8 and (S)- -proline15
b
b
boxylic acid derivatives have been synthesized (C, X¼CH2OMe, etc.
in Fig. 2). Bridgehead substitution was found to completely bias the
cisetrans equilibrium to the cis-amide structure (along the main
chain),9 and helical structures of homooligomers of C, based on the
cis-amide linkage, were reported to be generated.
using a solution-phase coupling procedure with N-(3-dimethyl-
aminopropyl)-N0-ethylcarbodiimide hydrochloride (EDCI$HCl),
Et3N, and N-hydroxybenzotriazole (HOBt) in CHCl3 for those up to
the tetramer, and by using O-(7-azabenzotriazol-1-yl)-N,N,N0,N0-
tetramethyluronium hexafluorophosphate (HATU), N-methyl-
morpholine (NMM), and 4-dimethylaminopyridine (DMAP) in DMF
Substitution of amide with thioamide is expected to be another
methodology to reduce molecular flexibility or to shift the
in the case of the hexamer (Schemes
1 and 2). Then
Fig. 3. Thiopeptides synthesized in this study.

4420
Y. Otani et al. / Tetrahedron 68 (2012) 4418e4428
Scheme 1. Synthesis of oligothiopeptides. (S)-Tet was synthesized in a similar manner to (R)-Tet. aYield after recycling of the recovered starting material.
thiocarbonylation of the amide carbonylgroupswas performed with
Lawesson’s reagent. While the thiocarbonylation proceeded in tol-
uene at 110 ꢀC (Table 1, entry3), many by-products werealso formed,
probably because deprotection of the Boc group occurred. Use of
K3PO4 as a base improved the yield (entry 5). Furthermore, the re-
action was accelerated by microwave irradiation (entry 6). Repeated
thiocarbonylation of the recycled unreacted or partially converted
amide oligomers gave the desired thiopeptides in good yields.
bicyclic skeleton, steric bulkiness of the sulfur atom, and probably
also dipoleedipole repulsion presumably contribute to the stabili-
zation of this well-defined structure.
2.3. 1H NMR
1
The H NMR signals of (R)-Di0, -Tri, and -Tet were sharper and
better resolved than those of bicyclic amide oligomers (Fig. 5 and
Fig. S1 in Supplementary data). Though some overlapping of the
signals was observed, we have successfully assigned all the 1H
peaks of (R)-Di0 and most of the 1H peaks of (R)-Tri using COSY,
TOCSY, and NOESY spectroscopy (Fig. S3e5 in Supplementary data).
The NOE correlations were diagnostic for assignment of the rota-
tional isomers with respect to cisetrans isomerization of thioamide
bonds: for example, in the case of the transetrans (tet) conformer
of (R)-Tri, cross peaks between bridgehead protons of adjacent
residues (H(1)eH(11) and H(8)eH(18)) and between the proton at
2.2. Crystal structures of thioamide oligomers
X-ray crystal structures of (R)-Di0 and (R)-Tri revealed that the
thioamide bonds are trans (Fig. 4). The dihedral angles 4, q, and j
are nearly identical in the two compounds (Table 2), clearly in-
dicating a tendency for regular structure. Further, the C]S bonds in
neighboring thioamide functionalities of (R)-Tri are in an almost
antiparallel arrangement (the dihedral angle between the neigh-
boring thiocarbonyl groups is ꢁ168ꢀ). The result shows a novel
the b-position and a bridgehead proton of the next residue (H(2)e
strand-like structure with a pitch of 8.1 A, whereas the natural
b
-
H(11) and H(9)eH(18)) were observed Fig. 5(c).16a As shown in
Table 3, (R)-Di0 exists as a mixture of trans (t)- and cis (c)-thioamide
ꢀ
ꢀ
strand has a pitch of 7.0 A. The restricted ring puckering in the

Y. Otani et al. / Tetrahedron 68 (2012) 4418e4428
4421
Scheme 2. Synthesis of
b
-proline oligothiopeptides. aYield after recycling of the recovered starting material.
conformers in a ratio of 61:39 in CDCl3 (Fig. S1 in Supplementary
data).17 (R)-Tri showed four (22) conformers: tet rotamer, ca.
54%; tec, ca. 33%; cet, ca. 11%; and cec, ca. 2%.16,18 However, as-
signments of thiopeptides longer than the tetramer was difficult
due to overlapped signals. Though (R)-Tet showed overlapped
signals of conformers, the major conformer could be assigned as the
tetet rotamer, corresponding to about 40% of the total integration
area (Fig. S6). These results indicate trans-predominancy of these
thioamide linkages.
Table 1
Reaction conditions for thiocarbonylation
The 1H NMR spectrum of
b-proline oligothiopeptides ((S)-bPro-
Di, -Tri, and -Tet) in CDCl3 could not be analyzed because of sig-
nificant overlapping of the signals even in the case of the dimer
(Figs. S2e11, S2e12, and S2e14 in Supporting Information).
2.4. CD spectra
The CD spectra of (R)-Tri and (R)-Tet at 100
mM in 2,2,2-
trifluoroethanol (TFE), (R)-Hex-O at 100 M in 2.8% (v/v) 1,2-
m
Entry
LR
(equiv)
Solvent
Base
Temp
Time
(h)
Yield
(%)
dichloroethaneeTFE solution, and (R)-Hex-S at 24 mM in 8.4% (v/
(ꢀC)
v) 1,2-dichloroethaneeTFE solution at 20 ꢀC showed characteristic
1
2
3
4
0.6
0.6
0.6
0.6
2.0
1.5
THF
d
50
80
110
130
110
105
20
48
18
3
24
1.5
No reaction
No reaction
15
Trace
46
bands at around 290 nm (positive) and 350 nm (negative),19 which
Benzene
Toluene
Xylene
Toluene
Toluene
d
were assigned as
pep*(C]S) and nep*(C]S) bands, respectively
d
Fig. 6(a).20 The intensity of the band at around 289 nm increased
in line with the number of thioamide units as the chain was
d
5
K3PO4 (3 equiv)
K3PO4 (3 equiv)
6a
56
elongated up to the tetramer (mean residue ellipticity
29,000 at 286 nm for (R)-Tri and ca. 53,000 at 289 nm for (R)-Tet),
q
¼ca.
a
The reaction was carried out under microwave irradiation.

4422
Y. Otani et al. / Tetrahedron 68 (2012) 4418e4428
Fig. 4. X-ray crystallographic structures of (R)-Di0 and (R)-Tri.
observation also showed that insertion of one carbonyl group does
Table 2
Main-chain torsional angles of thiopeptides in the crystal structure
not disturb the overall structure. The fact that the spectrum of (S)-
Tet, which is composed of enantiomeric (S)-b-amino acid, is a mir-
ror-image of that of (R)-Tet is consistent with the idea of chirality-
conserved secondary structures.
Compound
(R)-Di0
Residue
u
(ꢀ)
4
(ꢀ)
q
(ꢀ)
J
(ꢀ)
1
2
1
2
3
d
174.1a
d
(78.9)b
108.8
(75.1)b
102.8
115.8
161.1
165.9
160.0
165.7
160.6
80.5
57.2
75.0
73.4
72.1
On the other hand, the CD spectra of (S)-
((S)- Pro-Di, -Tri, and -Tet) at 100
M in TFE at 20 ꢀC showed weak
signals (
b-proline thiopeptides
(R)-Tri
ꢁ172.2a
b
m
ꢁ168.8a
q
¼ca. 10,000 at 268 nm), and the spectra did not show any
a
The values deviate from 180ꢀ because of nitrogen-pyramidalization of bicyclic
chain-length dependency Fig. 5(b).
thioamides.
b
The dihedral angle involves the N-Boc bond.
2.5. Calculations
while the hexamer (R)-Hex-S, which has five thioamide groups,
displayed a nearly identical spectrum to that of (R)-Tet. The cal-
culated CD spectrum of tet-conformer of (R)-Tri showed the best
fit with the experimental one and partially tec conformer (vide
infra). This tendency indicates that while a mixture of the several
conformers are present in solution, the CD-active conformers, i.e.,
most of them are extended strand-like structures (vide infra), are
favorable even in the trimer, and thiopeptides longer than the
tetramer have distinct preference for regular structures. Further-
more, the CD spectrum of (S)-Tri in the presence of water (TFE/
water¼80:20 solution) was practically the same as that of (S)-Tri
in TFE (Fig. S7), indicating that the preordered structure of the
DFT calculations showed that the minimum-energy structure of
a tet rotamer of (R)-Tri is very similar to the crystal structure
(Fig. S8 and Table S2 in Supplementary data). The calculated rela-
tive energies of possible rotamers (tet, tec, cet, and cec) in TFE
clearly showed that all-trans conformers were the lowest-energy
structures in the cases of (R)-Tri and (R)-Tet (Table S3). CD calcu-
lation was also performed for all the rotamers of (R)-Tri at the level
of TDDFT/B3LYP/aug-cc-pVDZ//B3LYP/6-31G(d). The calculated CD
spectrum of tet-conformer showed the best fit with the experi-
mental one (Fig. 7) and partially tec conformer, while those of the
other two conformers did not overlap well with it, indicating that
the CD spectrum is largely attributable to the tet and tec
conformers.
thiopeptides of the bicyclic
ence of water.
b-amino acid was stable in the pres-
In conclusion, we found that non-hydrogen-bonding thiopep-
(R)-Hex-O, the hexamer, containing one amide group flanked
with thioamide groups, showed a similar CD spectrum to those of
(R)-Tet and (R)-Hex-S,21 supporting the above idea. This
tides of bicyclic b-amino acid as short as the trimer predominantly
take highly organized structures, indicating at least that thio-
carbonylation did not inhibit the generation of ordered structures

Y. Otani et al. / Tetrahedron 68 (2012) 4418e4428
4423
Fig. 5. (a) Diagnostic NOE interactions characteristic of the four isomers of (R)-Tri. (b) The bridgehead hydrogen region of the 1H NMR spectrum of (R)-Tri in CDCl3. (c) Diagnostic
cross peaks in the NOESY spectra of the bridgehead region and observed NOE interactions for all-trans isomer of (R)-Tri in CDCl3: black, positive intensity levels; red, negative
intensity levels.

4424
Y. Otani et al. / Tetrahedron 68 (2012) 4418e4428
Table 3
based on the bicyclically constrained
b-proline derivative. The
a
Ratios of rotational isomers in CDCl3
generated ordered structures are proposed to be extended trans-
thioamide structures, both in the solid state and in solution. This is
in sharp contrast to the assignment that corresponding amide
oligomers prefer cis-amide helical structures. Rationalization of
these results is difficult at present, although many examples have
been reported that cis-/trans-conformational preference is dra-
matically changed upon thiocarbonylation of amides.10
(R)-Di
(R)-Tri
tet
tec
cet
cec
54%
33%
11%
2%
t
c
61%
39%
a
The measurement error of each ratio is ꢂ2.5%.
On the other hand, monocyclic b-proline thiopeptides are un-
structured even in the tetramer. This novel strand-like structure
may favor formation of sheet structures or may prove to be a useful
scaffold for peptideeprotein interactions.
Fig. 6. (a) CD spectra of bicyclic thiopeptides at 20 ꢀC. Thiopeptides up to the tetramer were measured in TFE, and (R)-Hex-O and (R)-Hex-S in 1,2-dichloroethaneeTFE. The intensity
is represented by molecular ellipticity, normalized for concentration and the number of thioamide units. (b) CD spectra of -proline thiopeptides at 100
M in TFE at 20 ꢀC.
b
m
3. Experimental section
3.1. Synthesis
3.1.1. General methods. All the reagents were commercially avail-
able and used without further purification, unless otherwise noted.
Reactions under microwave irradiation were carried out on a Biot-
age Initiator Robot Eight. HPLC data were obtained using a Hitachi
instrument with the following conditions: CHCl3/CH3OH¼100:1,
Senshu Pak Pegasil Silica SP-100, flow rate 3.0 mL/min, UV de-
tection at 280 nm. NMR data were recorded on a Bruker AV400
NMR spectrometer at 293 K, except for the 2D NMR spectra, which
were recorded on a Bruker AV600 NMR spectrometer at Bruker
Biospin, Japan, in 15 mM CHCl3 solutions at 300 K. For the NOESY,
a 600 ms mixing time was used, and the number of scans was 8. For
all the 2D spectra 2K time domain points and 512 increments were
applied. The processing was carried out by using a Qsine-bell
experimental (-)
t-t-(R)-Tri (
-
)
t-c-(R)-Tri (
-
)
window function for the NOESY. Chemical shifts (
parts per million and were referenced to undeuterated residual
solvent (i.e., CHCl3
d) are shown in
c-t-(R)-Tri (
-
)
(d
¼7.26 ppm)). Coupling constants are given in
c-c-(R)-Tri (
-
)
hertz. Electrospray ionization time-of-flight mass spectra (ESI-TOF
MS) were recorded on a Bruker micrOTOF. Flash column chroma-
tography was carried out on silica gel (silica gel 60N (40e50 mm),
Kanto). All the melting points were measured with a Yanaco Micro
Melting Point Apparatus and are uncorrected. The combustion
analyses were carried out in the microanalytical laboratory of this
faculty.
Bicyclic peptides (R)-1, (R)-2, (R)-3, (R)-4, (S)-3, and (R)-5 were
synthesized by means of modifications of the procedures described
previously.8
Fig. 7. Calculated CD spectra of (R)-Tri at the level of TDDFT/B3LYP/aug-cc-pVDZ//
B3LYP/6-31G(d). The intensity is represented by ε.
D

Y. Otani et al. / Tetrahedron 68 (2012) 4418e4428
4425
3.1.2. (R)-Di. A solution of (R)-1 (345 mg, 0.912 mmol), K3PO4
(586 mg, 2.76 mmol), and Lawesson’s reagent (370 mg,
0.915 mmol) in dry toluene (9 mL) was stirred at 105 ꢀC for 2 h
under microwave irradiation, then CHCl3 was added. The mixture
was washed with 0.5 N HCl aq and satd NaHCO3 aq, and dried over
MgSO4. The solvent was evaporated and the residue was flash-
chromatographed (CHCl3 only, then n-hexane/AcOEt¼3:1) to give
(R)-Di (215 mg, 60%) as a colorless solid.
3.1.5. (R)-Di0. A solution of (R)-4 (101 mg, 0.222 mmol), K3PO4
(143 mg, 0.674 mmol), and Lawesson’s reagent (92.4 mg,
0.228 mmol) in dry toluene (2.2 mL) was stirred at 105 ꢀC for 1.5 h
under microwave irradiation, then CHCl3 was added. The mixture
was washed with 0.5 N HCl aq and satd NaHCO3 aq, and dried over
MgSO4. The solvent was evaporated and the residue was flash-
chromatographed (n-hexane/AcOEt¼6:1) to give (R)-Di0 (63.2 mg,
60%) as a colorless solid.
Mp: 103.2e104.6 ꢀC (colorless powder, recrystallized from
Mp: 139.7e140.8 ꢀC (colorless needles, recrystallized from
CH2Cl2/n-hexane). 1H NMR (400 MHz, CDCl3) (mixture of cis- and
CH2Cl2/n-hexane). 1H NMR (400 MHz, CDCl3) (mixture of cis- and
trans-conformers):
d
5.57 (1H, m, trans-conformer), 5.38 (1H, m, cis-
trans-conformer): d 7.39e7.33 (5H, m), 5.58 (1H, m, trans-
conformer), 4.86 (1H, m, cis-conformer), 4.70 (1H, m, trans-
conformer), 4.18e4.13 (2H, m), 3.74e3.73 (3H, s, a mixture of cis-
and trans-conformer), 3.25e3.14 (2H, m), 2.73e2.65 (1H, m),
2.16e1.53 (11H, m), 1.46e1.43 (9H, m). 13C NMR (100 MHz, CDCl3)
conformer), 5.38 (1H, m, cis-conformer), 5.19e5.16 (2H, a mixture
of conformers), 4.84 (1H, m, cis-conformer), 4.71 (1H, m, trans-
conformer), 4.17e4.13 (2H, m), 3.25e3.08 (2H, m), 2.73e2.65 (1H,
m), 2.17e1.53 (11H, m), 1.46e1.43 (9H, m). 13C NMR (100 MHz,
(mixture of cis- and trans-conformers):
d
195.7, 195.6, 172.1, 171.7,
CDCl3) (mixture of conformers): d 195.7, 171.5, 171.1, 135.4, 135.3,
80.1, 61.3, 60.5, 59.3, 58.8, 58.3, 58.2, 58.1, 52.23, 52.20, 46.3, 44.1, 36.5,
128.7, 128.6, 128.5, 128.4, 80.1, 80.0, 67.2, 67.0, 61.3, 59.3, 58.9, 46.4,
44.3, 36.3, 33.5, 29.9, 29.7, 29.2, 28.28, 28.25, 27.5, 26.4, 23.8, 22.7.
HRMS (ESI, [MþH]þ) calcd for C26H35N2O4S: 471.2318. Found:
471.2315. Anal. Calcd for C26H34N2O4S$1/10H2O: C, 66.10; H, 7.30; N,
5.93. Found: C, 65.87; H, 7.23; N, 5.89.
33.6, 29.8, 29.2, 28.3, 27.6, 26.5, 23.9, 22.8. MS (ESI, [MþH]þ): 395.
3.1.3. (R)-Tri. A solution of (R)-2 (98.1 mg, 0.196 mmol), K3PO4
(253 mg, 1.19 mmol), and Lawesson’s reagent (160 mg, 0.396 mmol)
in dry toluene (3.5 mL) was stirred at 105 ꢀC for 2 h under micro-
wave irradiation, then CHCl3 was added. The mixture was washed
with 0.5 N HCl aq and satd NaHCO3 aq, and dried over MgSO4. The
solvent was evaporated and the residue was flash-
chromatographed (CHCl3 only, then n-hexane/AcOEt¼3:1) to give
(R)-Tri (45.6 mg, 44%) as a colorless solid.
Mp: 205.2e206.0 ꢀC (colorless plates, recrystallized from
CH2Cl2/n-hexane). 1H NMR (400 MHz, CDCl3) (mixture of con-
formers):
d 5.58e5.56 (0.6H, m), 5.40e5.32 (1.4H, m), 4.96e4.92
(0.3H, m), 4.87e4.81 (0.55H, m), 4.78e4.61 (1H, m), 4.52e4.48
(0.15H, m), 4.24e4.08 (2H, m), 3.77e3.74 (3H, m), 3.39e2.98 (4H,
m), 2.80e2.55 (1H, m), 2.23e1.55 (16H, m), 1.46e1.43 (9H, m). 13C
NMR (150 MHz, CDCl3) (mixture of conformers):
d 195.9, 193.5,
172.0, 171.7, 80.1, 62.4, 61.6, 61.5, 61.0, 60.8, 60.7, 59.9, 59.71, 59.66,
59.5, 59.0, 52.4, 52.3, 48.2, 46.3, 44.1, 37.5, 36.6, 36.0, 33.6, 33.4,
29.9, 29.8, 29.5, 29.3, 28.3, 28.2, 27.6, 27.4, 26.7, 23.9, 22.8, 21.5, 21.4.
HRMS (ESI, [MþH]þ) calcd for C27H40N3O4S2: 534.2460. Found:
534.2476. Anal. Calcd for C27H39N3O4S2$2/3H2O: C, 59.42; H, 7.45;
N, 7.70. Found: C, 59.37; H, 7.21; N, 7.49.
3.1.6. (S)-Tet.
A solution of (S)-3 (137 mg, 0.195 mmol), K3PO4
(248 mg, 1.17 mmol), and Lawesson’s reagent (191 mg, 0.473 mmol)
in dry toluene (2.5 mL) was stirred at 105 ꢀC for 2 h under micro-
wave irradiation, then CHCl3 was added. The mixture was washed
with 0.5 N HCl aq and satd NaHCO3 aq, and dried over MgSO4. The
solvent was evaporated and the residue was flash-
chromatographed (CHCl3 only, then n-hexane/AcOEt¼3:1) to give
(S)-Tet (30.7 mg, 21%) as a colorless solid.
3.1.4. (R)-Tet. A solution of (R)-3 (259 mg, 0.415 mmol), K3PO4
(537 mg, 2.53 mmol), and Lawesson’s reagent (336 mg,
0.831 mmol) in dry toluene (7.5 mL) was stirred at 105 ꢀC for 2 h
under microwave irradiation, then CHCl3 was added. The mixture
was washed with 0.5 N HCl aq and satd NaHCO3 aq, and dried over
MgSO4. The solvent was evaporated and the residue was flash-
chromatographed (CHCl3 only, then n-hexane/AcOEt¼3:1) to give
(R)-Tet (50.8 mg, 18%) as a colorless solid.
Mp: 247.2e248.5 ꢀC (colorless powder, recrystallized from
CHCl3/n-hexane). 1H NMR (400 MHz, CDCl3) (mixture of con-
formers):
d 7.39e7.33 (5H, m), 5.58e5.56 (0.7H, m), 5.41e5.26
(2.3H, m), 5.20e5.13 (2H, m), 5.03e4.88 (0.6H, m), 4.84e4.65 (2.1H,
m), 4.51e4.46 (0.3H, m), 4.24e4.08 (2H, m), 3.39e2.85 (6H, m),
2.71e2.47 (1H, m), 2.23e1.55 (21H, m), 1.46e1.43 (9H, m). 13C NMR
Mp: 243.4e244.8 ꢀC (colorless powder, recrystallized from
CHCl3/n-hexane). 1H NMR (400 MHz, CDCl3) (mixture of con-
(100 MHz, CDCl3) (mixture of conformers):
d 193.8, 193.7, 193.4,
formers):
d
5.58e5.56 (0.6H, m), 5.42e5.27 (2.4H, m), 4.96e4.92
171.4, 171.3, 170.9, 135.41, 135.36, 128.75, 128.65, 128.56, 128.52,
128.41, 128.28, 80.2, 80.1, 67.0, 62.7, 62.3, 61.8, 61.6, 61.3, 61.2, 61.0,
60.8, 60.5, 60.2, 60.1, 60.0, 59.7, 59.55, 59.46, 59.1, 50.4, 48.2, 48.1,
46.5, 44.2, 44.1, 38.0, 37.7, 37.6, 36.1, 33.9, 33.6, 30.0, 29.8, 29.6, 29.5,
29.3, 28.3, 28.2, 27.5, 27.2, 26.5, 25.2, 24.0, 22.8, 22.6, 21.6, 21.45,
21.40, 21.28. HRMS (ESI, [MþH]þ) calcd for C40H53N4O4S3:
749.3229. Found: 749.3236. Anal. Calcd for C40H52N4O4S3: C, 64.14;
H, 7.00; N, 7.48. Found: C, 63.86; H, 6.96; N, 7.35.
(0.6H, m), 4.87e4.62 (2.1H, m), 4.51e4.46 (0.3H, m), 4.24e4.08 (2H,
m), 3.77e3.74 (3H, m), 3.39e2.86 (6H, m), 2.75e2.44 (1H, m),
2.23e1.57 (21H, m), 1.46e1.43 (9H, m). 13C NMR (150 MHz, CDCl3)
(mixture of conformers):
d 193.69, 193.66, 193.43, 193.41, 191.9,
191.6, 172.04, 172.02, 171.7, 171.3, 80.3, 80.2, 80.1, 62.8, 62.7, 62.6,
62.3, 61.8, 61.6, 61.3, 61.1, 61.0, 60.8, 60.5, 60.3, 60.2, 60.0, 59.9, 59.8,
59.7, 59.5, 59.4, 59.0, 58.4, 52.4, 52.29, 52.27, 51.3, 50.8, 50.4, 50.3,
48.24, 48.20, 48.13, 48.10, 46.5, 46.4, 44.1, 44.0, 38.1, 38.0, 37.7, 37.6,
37.5, 36.8, 36.1, 33.9, 33.7, 33.6, 29.89, 29.87, 29.81, 29.7, 29.6, 29.5,
29.4, 28.3, 28.2, 27.5, 27.2, 27.1, 26.9, 26.7, 25.3, 25.2, 24.0, 22.9, 22.7,
21.6, 21.5, 21.4, 21.23, 21.17. HRMS (ESI, [MþNa]þ) calcd for
C34H48N4NaO4S3: 695.2735. Found: 695.2730. Anal. Calcd for
C27H39N3O4S2$1/10H2O: C, 60.52; H, 7.20; N, 8.30. Found: C, 60.34;
H, 7.13; N, 8.23.
3.1.7. (R)-Hex-O. (R)-Tet (88.2 mg, 0.131 mmol) was dissolved in
4 N HCl in 1,4-dioxane (1.5 mL) and stirred for 6 h. The solvent was
then removed in vacuo, and the residue was dried in vacuo. (R)-6
(53.2 mg, 0.140 mmol) in dry DMF (1.5 mL), N-methylmorpholine
(44
mL, 0.400 mmol) was added to the flask, which was then cooled
to 0 ꢀC. HATU (60.2 mg, 0.158 mmol) was added to the mixture, and

4426
Y. Otani et al. / Tetrahedron 68 (2012) 4418e4428
the whole was stirred for 26 h under Ar at rt. HATU (15.4 mg,
0.041 mmol, total 0.199 mmol) and DMAP (6.5 mg, 0.053 mmol)
were added and the mixture was further stirred for 23 h at rt. Then,
CHCl3 was added. The mixture was washed with 0.5 N HCl aq and
satd NaHCO3 aq, and dried over MgSO4. The solvent was evaporated
and the residue was flash-chromatographed (CHCl3/MeOH¼150:1)
to give (R)-Hex-O containing an unknown by-product as a colorless
solid (145.2 mg). The solid (62.8 mg) was washed with MeOH
(twice) and Et2O to give pure (R)-Hex-O (33.6 mg, 63% yield) as
a colorless solid (98.6% HPLC purity).
Senshu Pak PEGASIL Silica SP100, CHCl3/MeOH¼100:1, detection:
280 nm UV, flow rate 3.0 mL/min, tR¼18.4 min.
Mp: 270 ꢀC (decomp.) 1H NMR (400 MHz, CDCl3) (mixture of
conformers):
d 5.58e5.28 (4H, m), 4.95e4.36 (6H, m), 4.24e4.10
(2H, m), 3.77e3.74 (3H, m), 3.41e2.60 (10H, m), 2.37e1.56 (32H,
m), 1.46e1.43 (9H, m). 13C NMR (100 MHz, CDCl3) (mixture of
conformers):
d 195.8, 195.0, 193.6, 193.5, 172.0, 171.6, 171.2, 167.3,
156.0, 80.1, 62.74, 62.67, 62.58, 62.49, 61.8, 61.6, 60.8, 60.6, 60.5,
59.98, 59.86, 59.50, 59.1, 57.6, 56.7, 52.4, 52.2, 50.9, 50.4, 48.13,
48.09, 46.4, 44.1, 44.0, 43.2, 38.2, 37.8, 37.6, 33.8, 33.6, 30.2, 29.9,
29.7, 29.6, 29.4, 28.3, 27.5, 26.7, 23.2, 22.8, 22.7, 22.4, 21.5, 21.4, 21.1.
MS (ESI, [MþNa]þ): calcd for C48H66N6NaO5S4: 957.3875. Found:
957.3844. HPLC conditions: Senshu Pak PEGASIL Silica SP100,
CHCl3/MeOH¼100:1, detection: 280 nm UV, flow rate 3.0 mL/min,
tR¼21.8 min.
3.1.9. Compound 7. To a solution of 615 (863 mg, 4.01 mmol),
K2CO3 (829 mg, 6.00 mmol) in dry DMF (8 mL) was added MeI
(300 mL, 4.82 mmol). The mixture was stirred at rt for 4 h, water
was added and the whole was extracted with Et2O (three times).
The Et2O solution was dried over MgSO4 and evaporated, and the
residue was flash-chromatographed (n-hexane/AcOEt¼3:1) to give
7 (916 mg, 100%) as a colorless oil.
1H NMR (400 MHz, CDCl3):
d 3.70 (3H, s), 3.65e3.28 (4H, m),
3.10e2.98 (1H, m), 2.18e2.05 (2H, m), 1.45 (9H, s). LRMS (ESI,
[MþNa]þ) 252.1.
3.1.10. Compound 8. Compound 7 (872 mg, 3.80 mmol) was dis-
solved in 4 N HCl in 1,4-dioxane (10 mL) and stirred for 2 h. The
solvent was then removed in vacuo, and the residue was dried in
vacuo. Carboxylic acid 6 (821 mg, 3.81 mmol) in CHCl3 (20 mL),
Et3N (2.2 mL, 15.8 mmol), and HOBt (631 mg, 4.67 mmol) was
added to the flask, which was then cooled to 0 ꢀC. EDCI$HCl
(901 mg, 4.70 mmol) was added to the mixture, and the whole was
stirred for 20 h under argon at rt. Further CHCl3 was added, and the
solution was washed with 0.5 N HCl aq and satd NaHCO3 aq, and
dried over MgSO4. The solvent was evaporated and the residue was
flash-chromatographed (n-hexane/AcOEt¼1:3) to give 8 (637 mg,
51%) as a colorless oil.
3.1.8. (R)-Hex-S. A solution of (R)-Hex-O (82.4 mg, 0.088 mmol),
K3PO4 (56.0 mg, 0.264 mmol), and Lawesson’s reagent (36.0 mg,
0.089 mmol) in dry toluene (1.0 mL) was stirred at 105 ꢀC for 2 h
under microwave irradiation, then CHCl3 was added. The mixture
was washed with 0.5 N HCl aq and satd NaHCO3 aq, and dried over
MgSO4. The solvent was evaporated and the residue was flash-
1H NMR (400 MHz, CDCl3) (mixture of conformers):
(10H, m), 3.73e3.71 (3H, m, overlap), 2.33e1.95 (4H, m), 1.45 (9H,
s). 13C NMR (100 MHz, CDCl3) (mixture of conformers):
173.2,
d 3.79e3.00
chromatographed (CHCl3/MeOH¼150:1) to give
a
mixture of
d
product and starting material (52.8 mg), which was subjected to
HPLC (CHCl3/MeOH¼100:1) to give (R)-Hex-S (2.79 mg, 3.3%) as
a white solid (94% HPLC purity).
172.6, 170.8, 170.6, 154.1, 79.1, 52.1, 52.0, 48.2, 48.1, 48.0, 45.6, 45.4,
45.1, 43.3, 42.3, 42.1, 41.4, 41.2, 28.9, 28.6, 28.3, 28.2, 28.1, 27.8, 27.3.
HRMS (ESI, [MþK]þ) calcd for C16H26KN2O5: 365.1479. Found:
365.1478.
Mp: 255 ꢀC (decomp.) 1H NMR (400 MHz, CDCl3) (mixture of
conformers):
d 5.58 (0.6H, m), 5.41e5.29 (4.3H, m), 4.94 (1H, m),
4.83e4.82 (2.3H, m), 4.71 (1H, m), 4.50 (0.8H, m), 4.20 (2H, m),
3.78e3.69 (3H, mixture of singlets), 3.33e2.67 (10H, m), 2.18e1.55
(32H, m), 1.47 (9H, s). HRMS (ESI, [MþNa]þ) calcd for
C48H66N6NaO4S5: 973.3647. Found: 973.3604. HPLC conditions:
3.1.11. Compound 9. Compound 8 (133 mg, 0.407 mmol) was dis-
solved in 4 N HCl in 1,4-dioxane (1.2 mL) and the solution was
stirred for 3 h. The solvent was removed in vacuo, and the residue
was dried in vacuo. Carboxylic acid 6 (88.0 mg, 0.409 mmol) in

Y. Otani et al. / Tetrahedron 68 (2012) 4418e4428
4427
CHCl3 (2 mL), Et3N (230
m
L, 1.65 mmol) and HOBt (66.1 mg,
m),1.45 (9H, s). 13C NMR (100 MHz, CDCl3) (mixture of conformers):
200.0, 199.7, 172.8, 172.1, 171.8, 170.1, 154.2, 79.2, 56.3, 55.8, 53.5,
53.2, 52.5, 52.4, 52.30, 52.25, 52.0, 51.8, 51.7, 51.5, 51.4, 49.9, 49.61,
49.56, 49.48, 48.5, 47.6, 46.7, 46.6, 46.5, 46.1, 45.9, 45.5, 45.2, 43.4,
43.3, 42.5, 40.9, 40.1, 39.9, 32.4, 32.2, 31.8, 30.7, 29.01, 28.96, 28.4,
27.3, 27.1, 27.0. LRMS (ESI, [MþNa]þ): 575.
0.488 mmol) was added to the flask, which was then cooled to 0 ꢀC.
EDCI$HCl (94.4 mg, 0.492 mmol) was added to the mixture, and the
whole was stirred for 44 h under argon at rt. Further CHCl3 was
added and the solution was washed with 0.5 N HCl aq and satd
NaHCO3 aq, then dried over MgSO4. The solvent was evaporated
and the residue was flash-chromatographed (AcOEt/MeOH¼9:1) to
give 9 (121 mg, 70%) as a colorless oil.
d
3.1.15. (S)-bPro-Tet. A solution of 11 (106 mg, 0.192 mmol), K3PO4
1H NMR (400 MHz, CDCl3) (a mixture of conformers):
(126 mg, 0.594 mmol), and Lawesson’s reagent (80.6 mg,
0.199 mmol) in dry toluene (2 mL) was stirred at 105 ꢀC for 2 h
under microwave irradiation, then CHCl3 was added. The solution
was washed with 0.5 N HCl aq. and satd NaHCO3 aq, and dried over
MgSO4. The solvent was evaporated and the residue was flash-
chromatographed (CHCl3 only, then CHCl3/MeOH¼50:1) to give
d
3.83e2.99 (15H, m), 3.74e3.68 (3H, m, overlap), 2.36e1.93 (6H,
m), 1.44 (9H, s).
3.1.12. (S)-bPro-Di. A solution of 8 (95.9 mg, 0.294 mmol), K3PO4
(193 mg, 0.910 mmol), and Lawesson’s reagent (121 mg,
0.300 mmol) in dry toluene (3 mL) was stirred at 105 ꢀC for 2 h
under microwave irradiation, then CHCl3 was added. The mixture
was washed with 0.5 N HCl aq and satd NaHCO3 aq, and dried over
MgSO4. The solvent was evaporated and the residue was flash-
(S)-bPro-Tet (60.1 mg, 55%) as a colorless solid.
Mp: 168.0e169.6 ꢀC (colorless powder, recrystallized from
CH2Cl2/n-hexane). 1H NMR (400 MHz, CDCl3) (mixture of con-
formers):
2.67e2.03 (8H, m), 1.46 (9H, s). 13C NMR (100 MHz, CDCl3) (mixture
of conformers): 199.5, 199.44, 199.38, 198.9, 198.2, 197.6, 172.9,
d 4.60e3.16 (20H, m), 3.77e3.75 (3H, m, overlap),
chromatographed (n-hexane/AcOEt¼1:1) to give (S)-
bPro-Di
(57.3 mg, 57%) as a colorless solid.
d
Mp: 88.8e89.5 ꢀC (colorless powder, recrystallized from CH2Cl2/
172.7, 172.1, 171.8, 154.3, 79.3, 59.0, 58.9, 56.12, 56.06, 53.6, 53.4,
53.3, 53.2, 52.6, 52.43, 52.37, 52.1, 52.0, 51.9, 51.8, 51.2, 50.0, 49.82,
49.76, 49.6, 49.5, 48.6, 48.5, 47.8, 47.6, 46.4, 46.3, 46.2, 45.6, 45.4,
43.6, 43.5, 41.04, 41.03, 32.5, 32.4, 32.3, 31.9, 31.5, 30.4, 30.2, 29.1,
28.5, 27.2, 22.6, 14.1. HRMS (ESI, [MþNa]þ) calcd for
C26H40N4NaO4S3: 591.2109. Found: 591.2100.
n-hexane). 1H NMR (400 MHz, CDCl3) (mixture of conformers):
d
4.19e3.54 (7H, m), 3.74e3.73 (3H, m, overlap), 3.40e3.16 (3H, m),
2.50e2.18 (3H, m), 2.11e2.03 (1H, m), 1.45 (9H, s). 13C NMR
(100 MHz, CDCl3) (mixture of conformers): 200.5, 200.3, 172.9,
d
172.8, 172.1, 171.9, 154.2, 79.3, 55.9, 53.2, 53.1, 52.5, 52.3, 51.9, 51.8,
51.7, 51.1, 49.5, 48.7, 48.6, 47.9, 47.8, 45.6, 45.3, 43.5, 41.1, 32.6, 32.0,
29.0, 28.5, 27.2. HRMS (ESI, [MþNa]þ) calcd for C16H26N2NaO4S:
365.1511. Found: 365.1514. Anal. Calcd for C16H26N2O4S: C, 56.12; H,
7.65; N, 8.18. Found: C, 55.91; H, 7.53; N, 8.05.
3.2. X-ray crystallographic analysis
The data were measured using synchrotron radiation
ꢀ
(l
¼0.70000 A) (SPring-8 BL38B1). The structures were solved by
3.1.13. (S)-bPro-Tri. A solution of 9 (89.6 mg, 0.212 mmol), K3PO4
a direct method using the program SHELXS-97.22 The refinement
and all further calculations were carried out using SHELXL-97.22
The H atoms were included at calculated positions and treated as
riding atoms using the SHELXL default parameters. The non-H
atoms were refined anisotropically, using weighted full-matrix
least-squares on F2. Crystallographic data of the crystals have
been deposited at the Cambridge Crystallographic Data Center.
Copies of the data can be obtained free of charge on application to
CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (e-mail:
deposit@ccdc.cam.ac.uk).
(269 mg,1.27 mmol), and Lawesson’s reagent (175 mg, 0.433 mmol)
in dry toluene (3.5 mL) was stirred at 105 ꢀC for 2 h under micro-
wave irradiation, then CHCl3 was added. The solution was washed
with 0.5 N HCl aq and satd NaHCO3 aq, and dried over MgSO4. The
solvent was evaporated and the residue was flash-
chromatographed (CHCl3 only, then n-hexane/AcOEt¼1:3) to give
(S)-bPro-Tri (40.7 mg, 42%) as a colorless solid.
Mp: 102.1e104.5 ꢀC (colorless powder, recrystallized from
CH2Cl2/n-hexane). 1H NMR (400 MHz, CDCl3) (mixture of con-
formers):
d 4.49e3.46 (12H, m), 3.76e3.74 (3H, m, overlap),
Crystal data of (R)-Di0: 0.12ꢃ0.05ꢃ0.001 mm, C26H34N2O4S,
3.42e3.20 (3H, m), 2.50e2.03 (6H, m), 1.45 (9H, s). HRMS (ESI,
M¼470.61 g molꢁ1, monoclinic, ꢀP21, a¼11.9565(7) A, b¼6.2373(3) A,
ꢀ
ꢀ
[MþNa]þ) calcd for C21H33N3NaO4S2: 478.1810. Found: 478.1790.
3
ꢀ
ꢀ
c¼16.7804(11) A,
b
¼95.362(4) , V¼1245.94(12) A , T¼293(2) K,
13C NMR (100 MHz, CDCl3) (mixture of conformers):
d 199.9, 199.7,
Z¼2, Dc¼1.254 Mg/m3. Final goodness-of-fit on F2 was 1.060 and
172.9, 172.7, 171.8, 154.2, 79.2, 58.8, 56.0, 53.4, 53.3, 52.6, 52.40,
52.35, 51.9, 51.7, 51.1, 49.8, 49.7, 48.5, 48.4, 47.6, 46.4, 46.3, 45.6,
45.3, 43.5, 43.4, 41.0, 32.4, 31.9, 30.4, 29.6, 29.09, 29.06, 28.5, 27.2,
27.1. Anal. Calcd for C21H33N3O4S2$1/4H2O: C, 54.81; H, 7.34; N, 9.13.
Found: C, 54.64; H, 7.08; N, 8.86. MS (ESI, [MþNa]þ): 478.
final R indices were R1¼0.0706 and wR2¼0.1886 based on I>2
Flack parameter x was 0.1(2). CCDC 839192.
s(I).
Crystal data of (R)-Tri: 0.08ꢃ0.02ꢃ0.02 mm, C27H39N3O4S2,
ꢀ
M¼533.73
g
molꢁ1
,
orthorhombic, P212121, a¼7.1371(8) A,
3
ꢀ ꢀ
ꢀ
b¼13.8858(18) A, c¼27.990(3) A, V¼2774.0(6) A , T¼113(2) K, Z¼4,
Dc¼1.278 Mg/m3. Final goodness-of-fit on F2 was 1.115 and final R
3.1.14. Compound 11. (S)-bPro-Di (214 mg, 0.625 mmol) was dis-
indices were R1¼0.0509 and wR2¼0.1299 based on I>2
parameter x was 0.09(9). CCDC 839193.
s(I). Flack
solved in 4 N HCl in 1,4-dioxane (3 mL) and stirred for 5 h. The
solvent was then removed in vacuo, and the residue was dried in
vacuo. Carboxylic acid 10 (208 mg, 0.635 mmol) in CHCl3 (3.6 mL),
Et3N (350
mL, 2.51 mmol), and HOBt (102.8 mg, 0.761 mmol) was
Acknowledgements
added to the flask, which was then cooled to 0 ꢀC. EDCI$HCl
(146.2 mg, 0.763 mmol) was added to the mixture, and the whole
was stirred for 17 h under argon at rt. Further CHCl3 was added and
the solution was washed with 0.5 N HCl aq and satd NaHCO3 aq, and
dried over MgSO4. The solvent was evaporated and the residue was
flash-chromatographed (AcOEt only, then AcOEt/MeOH¼9:1) to
give 11 (249 mg, 72%) as a colorless amorphous solid.
The computations were performed at the Research Center for
Computational Science, Okazaki, Japan. We thank Dr. M. Waelchli,
Bruker Biospin, Japan, for measurement of the NMR spectra of (R)-Tri
and (R)-Tet. We also thank Drs. S. Baba and K. Miura (the Japan
Synchrotron Radiation Research Institute (JASRI)) for their help in
data collection during X-ray analysis of (R)-Di0 and (R)-Tri. The syn-
chrotron radiation experiment was performed at the BL38B1 station
in SPring-8 with the approval of JASRI (Proposal No. 2008B1981). We
thank Dr. S. Watanabe, the University of Tokyo, for valuable
Mp: 94.9e96.7 ꢀC (colorless powder, recrystallized from CH2Cl2/
n-hexane). 1H NMR (400 MHz, CDCl3) (mixture of conformers):
d
4.35e3.16 (20H, m), 3.76e3.74 (3H, m, overlap), 2.61e2.03 (8H,

4428
Y. Otani et al. / Tetrahedron 68 (2012) 4418e4428
9. Hosoya, M.; Otani, Y.; Kawahata, M.; Yamaguchi, K.; Ohwada, T. J. Am. Chem. Soc.
2010, 132, 14780e14789.
10. In fact, it has been reported that thiocarbonylation has a great impact on the
equilibrium of trans/cis conformers of tertiary amides. Review: Stewart, W. E.;
Siddall, T. H., III. Chem. Rev. 1970, 70, 517e551 See also Artis, D. R.; Lipton, M. A.,
J. Am. Chem. Soc. 1998, 120, pp 12200e12206.
comments. This work was supported by a Grant-in-Aid for Scientific
Research from the Japan Society for the Promotion of Science.
Supplementary data
11. Wiberg, K. B.; Rablen, P. R. J. Am. Chem. Soc. 1995, 117, 2201e2209.
12. (a) Loewenstein, A.; Melera, A.; Rigny, P.; Walter, W. J. Phys. Chem. 1964, 68,
Synthesis and spectral data of the thiopeptides and CD and UV
spectra of thiopeptides. Supplementary data related to this article
can be found online at doi:10.1016/j.tet.2012.01.018.
€
1597e1598; (b) Sandstrom, J. J. Phys. Chem. 1967, 71, 2318e2325; (c) Neuman,
R. C., Jr.; Young, L. B. J. Phys. Chem. 1965, 69, 2570e2576; (d) Neuman, R. C., Jr.;
Jonas, V. J. Phys. Chem. 1971, 75, 3532e3536; (e) Ref. 10 and references therein.
13. Wiberg, K. B.; Rush, D. J. J. Am. Chem. Soc. 2001, 123, 2038e2046.
14. Hori, T.; Otani, Y.; Kawahata, M.; Yamaguchi, K.; Ohwada, T. J. Org. Chem. 2008,
73, 9102e9108.
References and notes
15. (a) Cardillo, G.; Gentilucci, L.; Tolomelli, A.; Calienni, M.; Qasem, A. R.;
Spampinato, S. Org. Biomol. Chem. 2003, 1, 1498e1502; (b) Klein, S. I.;
Czekaj, M.; Molino, B. F.; Chu, V. Bioorg. Med. Chem. Lett. 1997, 7,
1773e1778.
16. (a) Isomer description starts from the N-terminus. (b) The isomer ratios of (R)-
Tri were determined based on the non-overlapped signals at 4.95, 4.84, and 4.
50 ppm, and cec H15 and cec H18 were not assigned.
1. (a) Beausoleil, E.; Lubell, W. D. Biopolymers 2000, 53, 249e256 and references
therein; (b) (i) Armand, P.; Kirshenbaum, K.; Goldsmith, R. A.; Farr-Jones, S.;
Barron, A. E.; Truong, K. T.; Dill, K. A.; Mierke, D. F.; Cohen, F. E.; Zuckermann, R. N.;
Bradley, E. K. Proc. Natl. Acad. Sci. U.S.A.1998, 95, 4309e4313; (ii) Kirshenbaum, K.;
Barron, A. E.; Goldsmith, R. A.; Armand, P.; Bradley, E. K.; Truong, K. T.; Dill, K. A.;
Cohen, F. E.; Zuckermann, R. N. Proc. Natl. Acad. Sci. U.S.A.1998, 95, 4303e4308; (c)
Jakobsche, C. E.; Choudhary, A.; Miller, S. J.; Raines, R. T. J. Am. Chem. Soc. 2010,132,
6651e6653 and references therein.
17. A mixture of cisetrans amide (trans/cis¼53:47) structures was observed with
the amide dimer of the bicyclic b
-amino acid.8 This ratio was slightly enhanced
2. (a) Wu, C. W.; Kirshenbaum, K.; Sanborn, T. J.; Patch, J. A.; Huang, K.; Dill, K. A.;
Zuckermann, R. N.; Barron, A. E. J. Am. Chem. Soc. 2003, 125, 13525e13530; (b)
Shah, N. H.; Butterfoss, G. L.; Nguyen, K.; Yoo, B.; Bonneau, R.; Rabenstein, D. L.;
Kirshenbaum, K. J. Am. Chem. Soc. 2008, 130, 16622e16632; (c) Stringer, J. R.;
Crapster, J. A.; Guzei, I. A.; Blackwell, H. E. J. Am. Chem. Soc. 2011, 133,
15559e15567 and references therein.
in the corresponding thioamide dimer (trans/cis¼61:39).
18. We have measured the NMR spectra of (R)-Tri of thiopeptides of bicyclic
b
-amino acid in 2,2,2-trifluoroethanol (TFE), a protic solvent. The ratios of
the rotamers (ca. 53% for transetrans, ca. 11% for cisetrans and ca. 36% for
a mixture of transecis and cisecis) were similar to those observed in
CDCl3.
€
3. Abele, S.; Vogtli, K.; Seebach, D. Helv. Chim. Acta 1999, 82, 1539e1558.
19. Since the solubility of (R)-Hex-O and (R)-Hex-S in TFE was insufficient, 1,2-
dichloroethane was used as a co-solvent. The concentration of (R)-Hex-S was
determined by interpolation of UV data (see Fig. S8 and Table S1 in
Supplementary data).
4. Huck, B. R.; Langenhan, J. M.; Gellman, S. H. Org. Lett. 1999, 1, 1717e1720.
5. Lee, M.; Kim, K.-Y.; Cho, U.-I.; Boo, D. W.; Shin, I. Chem. Commun. 2003, 968e969.
6. Huck, B. R.; Fisk, J. D.; Guzei, I. A.; Carlson, H. A.; Gellman, S. H. J. Am. Chem. Soc.
2003, 125, 9035e9037.
7. (a) Krow, G. R.; Liu, N.; Sender, M.; Lin, G.; Centafont, R.; Sonnet, P. E.; DeBrosse,
C.; Ross, C. W., III; Carroll, P. J.; Shoulders, M. D.; Raines, R. T. Org. Lett. 2010, 12,
5438e5441; (b) Krow, T. R.; Lin, G.; Herzon, S. B.; Thomas, A. M.; Moore, K. P.;
Huang, Q.; Carroll, P. J. J. Org. Chem. 2003, 68, 7562e7564.
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20. (a) Kajtar, M.; Hollosi, M.; Kajtar, J.; Majer, Z.; Kover, K. E. Tetrahedron 1986, 42,
3931e3942; (b) Sifferlen, T.; Rueping, M.; Gademann, K.; Jaun, B.; Seebach, D.
Helv. Chim. Acta 1999, 82, 2067e2093.
21. Normalization of the CD intensity was performed based on four thioamide
units in the case of (R)-Hex-O.
22. Sheldrick, G. M. Acta Crystallogr. 2008, A64, 112e122.
8. Otani, Y.; Futaki, S.; Kiwada, T.; Sugiura, Y.; Muranaka, A.; Kobayashi, N.;
Uchiyama, M.; Yamaguchi, K.; Ohwada, T. Tetrahedron 2006, 62, 11635e11644.