Total synthesis and structure elucidation of JBIR-39: A linear hexapeptide possessing piperazic acid and γ-hydroxypiperazic acid residues

Article abstract of DOI:10.1002/chem.201406020

The total synthesis and stereochemical structural elucidation of JBIR-39, containing four nonproteinogenic piperazic acid (Piz) residues, is reported. The synthesis includes Sc(OTf)₃-catalyzed acylation of a Piz(γ-OTBS) derivative with piperazi

Full text of DOI:10.1002/chem.201406020

DOI: 10.1002/chem.201406020
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Natural Products
Total Synthesis and Structure Elucidation of JBIR-39:
A Linear Hexapeptide Possessing Piperazic Acid and
g-Hydroxypiperazic Acid Residues
Masahito Yoshida,[a] Naoki Sekioka,[a] Miho Izumikawa,[b] Ikuko Kozone,[b] Motoki Takagi,[b, c]
Kazuo Shin-ya,[d] and Takayuki Doi*[a]
Abstract: The total synthesis and stereochemical structural
elucidation of JBIR-39, containing four nonproteinogenic pi-
perazic acid (Piz) residues, is reported. The synthesis includes
Sc(OTf)3-catalyzed acylation of a Piz(g-OTBS) derivative with
piperazic acid chloride, providing the desired Piz-Piz(g-OTBS)
dipeptide in high yield without epimerization. After assem-
bling two additional Piz moieties and (S)-isoleucic acid at the
N-terminus, amidation with the (R)-a-methylserine ester at
the C-terminus, and deprotection afforded the desired
(2R,8S)-hexapeptide, which is the assumed structure of JBIR-
39. Although the spectral data of the (2R,8S)-hexapeptide
was not identical to JBIR-39, further synthesis of three ste-
reoisomers confirmed the stereochemical structure of JBIR-
39 to be (2S,6S,8S,11R,16S,21R,26S,27S).
Introduction
found in a cyclohexadepsipeptide, piperidamycins isolated
from mutant strains of a soil-isolated Streptomyces sp. by
Hosaka et al. in 2009 (Figure 1).[4] Although piperidamycins are
known to exhibit potent antibacterial activity, only their planar
structures were reported. Whereas JBIR-39 (1) did not display
Piperazic acid (Piz)-containing natural products are known to
exhibit antitumor and antimycobacterial activities.[1] Piz-con-
taining linear peptides and cyclic peptides are intriguing tar-
gets for total synthesis because of their unique structural fea-
tures and biological activities.[2] JBIR-39 (1), a linear hexapep-
tide isolated from Streptomyces sp. Sp080513SC-24 in 2011, is
composed of (S)-isoleucic acid and five rare nonproteinogenic
amino acid residues (two d-Piz, l-Piz, cis-Piz(g-OH), and (R)-a-
methylserine).[3] The planar structure of 1 has been elucidated
by 1D and 2D NMR spectroscopic analysis and by mass spec-
troscopy analysis, and the absolute configuration of each
amino acid residue was determined by the Marfey’s method;
except for cis-Piz(g-OH), the relative stereochemistry of which
was determined by NOE observation. This structure was also
Figure 1. Reported structure of JBIR-39 (1) and pipepidamycin F (2).
[a] Dr. M. Yoshida, N. Sekioka, Prof. Dr. T. Doi
Graduate School of Pharmaceutical Sciences
Tohoku University, 6-3 Aza-Aoba, Aramaki
Aoba-ku, Sendai 980-8578 (Japan)
antibacterial activity against Micrococcus luteus and Escherichia
coli, we initially planned the total synthesis of JBIR-39 (1),
which is speculated to be precursor of piperidamycin F (2), to
determine the unknown configurations toward elucidation of
their structure-activity relationships. The key issue in the syn-
thetic studies of 1 and 2 would be an efficient construction of
the unique sequence of the Piz-Piz-Piz-Piz(g-OH) moiety. In
2009, Ma et al. reported the total synthesis of a cyclohexadepsi-
peptide, piperazimycin A (4), possessing a (R,S)-Piz(g-Cl)-(S,S)-
Piz(g-OH) dipeptide unit (Figure 2). They were unable to install
the dipeptide by acylation of H-(S,S)-Piz(g-OH) ester with acid
chloride (R,S)-Piz(g-Cl)-Cl, so the piperazine ring of (R,S)-Piz(g-
Cl) residue was constructed after coupling its linear precursor.[5]
In 2010, Kennedy and Lindsley demonstrated direct acylation
E-mail: doi_taka@mail.pharm.tohoku.ac.jp
[b] Dr. M. Izumikawa, Dr. I. Kozone, Dr. M. Takagi
Japan Biological Informatics Consortium
2-4-7 Aomi, Koto-ku, Tokyo 135-0064 (Japan)
[c] Dr. M. Takagi
Present address: Translational Research Center
Fukushima Medical University
11-23 Sakaemachi, Fukushima, 960-8031 (Japan)
[d] Dr. K. Shin-ya
National Institute of Advanced Industrial Science
and Technology (AIST)
2-4-7 Aomi, Koto-ku, Tokyo 135-0064 (Japan)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201406020.
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cult because of the rather lower
nucleophilicity of the Piz(g-OH)
derivative. Therefore, develop-
ment of a concise method for
direct acylation of the Piz(g-OH)
derivative should be worthwhile
for the synthesis of piperazic
acid-containing natural products.
The reaction conditions for
acylation of H-Piz(Cbz)(g-OTBS)-
OAllyl (9)[11] with Fmoc-Piz(Cbz)-
Cl (10)[10a,12] were investigated
(Table 1). The reaction was per-
formed under conventional con-
Figure 2. Structures of piperazic acid-containing natural products, monamycin D1 (3) and piperazimycin A (4) iso-
lated from Streptomyces sp. and the assumed structure 1a as JBIR-39.
of H-(S,S)-Piz (g-OTBS)-OMe with Boc-(R,S)-Piz(Boc) (g-Cl)-OH
by using TCFH/CH2Cl2/collidine in 62% yield; however, ami-
Table 1. Acylation of piz(g-OTBS) derivative 9 with PizÀCl 10.
dation of H-(S,S)-Piz(Teoc)(g-OTBS)-OH with a-methylserine
ester (HATU/HOAt/CH2Cl2/collidine) resulted in low yield
(26%).[6] Thus, efficient methods for the formation of Piz-Piz-
Piz-Piz(g-OH) from Piz(g-OH) as well as the amidation at the
C-terminus of the Piz(g-OH) unit with a-methylserine ester
should be required in the synthesis of both 1 and 2. Herein,
we report a total synthesis and structure elucidation of JBIR-
39 (1b) utilizing Sc(OTf)3-catalyzed direct acylation of a H-
Piz(g-OH) derivative with hindered acid chlorides.
Entry Reagent
T [8C] t [h] Yield of 11 [%][a] Ratio of 11/12[b]
1[c]
2[c]
3[d]
4[d]
5[d]
6[d]
10% NaHCO3 (aq.)
2,6-lutidine
AgCN (15 mol%)
Sc(OTf)3 (10 mol%)
Y(OTf)3 (10 mol%)
La(OTf)3 (10 mol%)
RT 24
RT 13
23
30
>95:5
>95:5
80:20
>95:5
73:27
49:51
60
RT
RT
RT
2.5 69
1
1
1
86
53
60
Results and Discussion
Monamycin D1 (3)[7] and piperazimycin A (4),[8] isolated from
Streptomyces sp., are cyclodepsipeptides possessing a Piz-Piz
unit. Both are composed of the (R)- and (S)-configured
amino acid moieties in alternating order (Figure 2). Similar
to the structural features of 3
[a] Isolated yield. [b] The ratio was determined by HPLC analysis of the peak
area at UV 214 nm. [c] Performed in CH2Cl2. [d] Performed in toluene.
and 4, we assumed the structure
of 1a for JBIR-39.
Our synthetic plan for 1a in-
cluded i) acylation at the N-ter-
minus of tetrapiperazic acid de-
rivative 7 with (S)-isoleucic acid
chloride 8; ii) amidation at the C-
terminus of the Piz(g-OTBS)-OH
moiety in pentapeptide 5 with
(R)-a-methylserine ester 6, and
iii) removal of all protecting
groups (Figure 3). Synthesis of
Piz-Piz derivatives has been re-
ported;[5,6,9] however, there have
been no reports on the synthesis
of tri- or tetra-piperazic acid de-
rivatives. Given that the N1 atom
of Piz derivatives is relatively un-
reactive,[10] reactive acid chlor-
ides such as 10 are usually used
as reagents for the acylation of
Piz derivatives. The bond forma-
tion of a Piz-Piz(g-OH) sequence
is known to be particularly diffi- Figure 3. Retrosynthesis of 1a.
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[13]
ditions (i.e., 10% aqueous NaHCO3 or 2,6-lutidine[14] at room
Table 2. Scope and limitation in Sc(OTf)3-catalyzed acylation of 9 with
temperature) to provide the desired dipeptide 11 in low yield
(entries 1 and 2). The yield increased to 69% when the reac-
tion was performed in the presence of a catalytic amount of
AgCN at 608C in toluene, which is known to promote the acy-
lation of piperazic acid with acid chlorides.[10b,15] However, con-
siderable epimerization at the a-position of the Piz moiety was
observed (11/12=80:20, entry 3). To our delight, addition of
10 mol% scandium trifluoromethanesulfonate (Sc(OTf)3)[16] pro-
moted the acylation of 9 at room temperature, leading to the
formation of 11 in 86% yield without serious epimerization
(11/12= >95:5, entry 4). Other Lewis acids (Y(OTf)3 and
La(OTf)3)[17] gave poor results because the corresponding
ketene would be formed, leading to a mixture of 11 and its
epimer 12 (entries 5 and 6).
The scope of the substrates in this Sc(OTf)3-catalyzed acyla-
tion of 9 was investigated (Table 2). The reaction with Fmoc-
alanyl chloride (14a) in the presence of 10 mol% Sc(OTf)3
(Method A) proceeded at 08C, leading to the desired dipeptide
13a in 77% yield (entry 1). The conventional method (meth-
od B) using aqueous NaHCO3 gave a better result (98%,
entry 2). Interestingly, Method A was more effective than Meth-
od B for the acylation of 9 using bulky acid chlorides 14b–c to
afford 13b–c, respectively, as the sole product (entries 3 vs. 4
and 5 vs. 6). Reactions using acid chlorides 14d and ent-10
were completed within 4 h (Method A) to provide di-
peptides 13d and 12 in 70 and 82% yield, respec-
tively, without epimerization (entries 7 vs. 8 and 9 vs.
10). Notably, the Sc(OTf)3-catalyzed acylation of H-
Piz(g-OTBS) derivative 9 was remarkably effective for
coupling with sterically hindered acid chlorides.
various acid chlorides.
Entry RCOCl
Method t [h] T [8C] Product Yield [%][a]
1
2
A
B
4
18
0
RT
13a
13a
77
98
3
4
A
B
6
24
RT
RT
13b
13b
77
24
5
6
A
B
9
24
RT
RT
13c
13c
63
21
7
8
A
B
4
21
RT
RT
13d
13d
70
66
9
10
A
B
4
21
RT
RT
12
12
82
66
ent-10
[a] Isolated yield.
With the Piz-Piz(g-OTBS) derivative 11 in hand, the
synthesis of tetra-piperazic acid derivative 17 was in-
vestigated (Scheme 1). Removal of the Fmoc group
in 11, followed by acylation with ent-10 utilizing 2,6-
lutidine, quantitatively provided tri-piperazic acid de-
rivative 16, whereas the reaction using 10 mol%
Sc(OTf)3 afforded 16 in 76% yield. The coupling of
tri-piperazic acid derivative 16 with 10 was per-
formed in the same manner to afford the desired
tetra-piperazic acid derivative 17 in 72% yield.
Removal of the Fmoc group in 17, leading to 7,
followed by amidation with l-isoleucic chloride 8 uti-
lizing AgCN afforded pentapeptide 18 in 72% yield
without attaching the PMB group (Scheme 2).[18] Re-
Scheme 1. Synthesis of tetrapiperazic acid derivative 17 from 11.
moval of the allyl group in the presence of a catalytic
amount of Pd(PPh3)4 with morpholine provided the
corresponding acid 5 in 75% yield. However, amidation of 5
with (R)-a-methylserine derivative 6[19] using PyBroP/DIEA[20]
provided lactone 19 in 90% yield, rather than the desired
hexapeptide. Removal of the TBS group in acid 5 could occur;
consequently, intramolecular esterification led to lactone 19
prior to the intermolecular amidation with a-methylserine de-
rivative 6.
sponding acid 21 (cat. Pd(PPh3)4/PhSiH3), without attaching the
chloroacetyl group, in 63% yield. Amidation at the C-terminus
of the Piz(g-OClAc)-OH moiety in pentapeptide 21 with (R)-a-
methylserine derivative 6 utilizing PyBroP/DIEA or HATU/
DIEA[21] failed to yield the desired hexapeptide 22. However,
the use of AOI[22] as a coupling reagent provided 22 in 45%
yield.
To avoid lactone formation, the Piz(g-OTBS) moiety in 18
was transformed into a Piz(g-OClAc) moiety to yield 20
(Scheme 3). The allyl ester was then converted into the corre-
Removal of all protecting groups in hexapeptide 22 fur-
nished 1a in 26% overall yield as follows: i) removal of the
chloroacetyl group by treatment with thiourea at 508C, ii) re-
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configuration of the a-methylserine and/or adjacent
cis-g-hydroxypiperazic acid was incorrect.
To elucidate the absolute configuration of JBIR-39,
we synthesized diastereomers 1b, 1c, and 1d, in
a manner similar to the synthesis of 1a. Amidation of
21 with (S)-a-methylserine derivative ent-6 by using
the AOI reagent, followed by removal of all protect-
ing groups of 23 afforded (2S,6S,8S)-1b (Scheme 5).
The diastereomers (2R,6R,8R)-1c and (2S,6R,8R)-1d
were also prepared as follows. Amidation of 24[23]
with 6 and ent-6 was performed by using the AOI re-
agent to provide (2R,6R,8R)-25 and (2S,6R,8R)-26, re-
spectively, without formation of a lactone. Removal
of the TBS, tBu and PMB, and Cbz groups furnished
(2R,6R,8R)-1c and (2S,6R,8R)-1d in moderate yields.
Comparing the spectral data of 1b–d with those of
natural JBIR-39, we found that the spectral data
of 1b was in good agreement with those of
JBIR-39 (S115–S117).[23] Thus, the structure of
JBIR-39 was unequivocally determined to be
(2S,6S,8S,11R,16S,21R,26S,27S)-1b.[24]
Scheme 2. Formation of pentapeptide 5 and attempted amidation with a-methylserine
ester 6.
Conclusion
We have demonstrated the total
synthesis and structure elucida-
tion of JBIR-39 (1b). A catalytic
amount of Sc(OTf)3 efficiently
promoted direct acylation of g-
hydroxypiperazic acid derivative
9 with piperazic acid chloride 10
to afford Piz-Piz(g-OTBS) deriva-
tive 11 in high yield without epi-
merization. These reaction condi-
tions were applicable to the acy-
lation with other acid chlorides
14a–d and ent-10. Unprecedent-
ed tri- and tetra-piperazic acid
derivatives 16 and 17 were effi-
ciently synthesized from 11 by
using acid chlorides ent-10 and
10 with 2,6-lutidine. The AOI re-
agent was found to be effective
for amidation at the C-terminus
of the Piz(g-OTBS)-OH and Piz(g-
OClAc)-OH moieties in penta-
peptides 21 and 24 with both
(R)- and (S)-a-methylserine deriv-
atives 6 and ent-6, leading to
four diastereomeric hexapepti-
Scheme 3. Synthesis of hexapeptide 22.
moval of the tert-butyl ester under acidic conditions, and iii) re-
moval of the Cbz and PMB groups by hydrogenolysis
(Scheme 4). However, the spectral data of 1a was not identical
to those of natural JBIR-39. The main difference in the chemical
des 1a–d in moderate yields. Although (2R,6S,8S)-1a initially
synthesized was not identical to the natural JBIR-39, the
spectral data of (2S,6S,8S)-1b was in good agreement with
those reported for the natural product. Consequently, the
structure of JBIR-39 was unequivocally determined to be 1b,
and its absolute configurations were established to be
2S,6S,8S,11R,16S,21R,26S,27S. These results are expected to be
1
shifts of the H and 13C NMR spectra between the synthetic 1a
and natural JBIR-39 was in the methyl group in the (R)-a-meth-
ylserine moiety (S114);[23] thus, we supposed that the absolute
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Chemical Co.) were obtained by
passing commercially available
predried, oxygen-free formulations
through activated alumina column.
MeOH was distilled from iodine
and magnesium turnings. DMF
was purchased from Wako (for
peptide synthesis, grade: 99.5%).
All reactions in solution-phase
were monitored by thin-layer chro-
matography carried out on Merck
silica gel plates (0.2 mm, 60F-254)
with UV light, and visualized with
anisaldehyde, 10% ethanolic phos-
phomolybdic acid. Silica gel 60N
(Kanto Chemical Co. 100–210 mm)
was used for column chromatogra-
phy, and Merck silica gel plates
(2.0 mm, 60F-254) were used for
preparative thin-layer chromatog-
raphy. 1H NMR spectra (400 or
600 MHz) and 13C NMR spectra
(100 or 150 MHz) were recorded
with JEOL JNM-AL400 or JEOL
JNM-ECA600 or Varian NMR system
600NBCL spectrometers in the in-
dicated solvent. Chemical shifts (d)
are reported in units of parts per
million (ppm) relative to the signal
Scheme 4. Synthesis of (2R,6S,8S)-1a: the incorrect isomer of JBIR-39.
for
internal
tetramethylsilane
(0 ppm for 1H) for solutions in
CDCl3. NMR spectral data are re-
ported as follows: chloroform
(CHCl3; 7.26 ppm for 1H) or CDCl3
(77.0 ppm for 13C), dimethylsulfox-
ide (DMSO; 2.50 ppm for 1H) or
[D6]DMSO (39.5 ppm for 13C) when
internal standard is not indicated.
Multiplicities are reported by the
following abbreviations: s (singlet),
d (doublet), t (triplet), q (quartet),
m (multiplet), dd (double doublet),
dt (double triplet), ddd (double
double doublet), ddt (double
double triplet), dddd (double
double double doublet), brs
(broad singlet), brd (broad dou-
blet),
J (coupling constants in
Hertz). Gel permeation chromatog-
raphy was performed on LC-9201
(recycling preparative HPLC), with
RI-50 refractive index detector and
s-3740 ultra violet detector with
polystyrene gel columns (JAIGEL-
Scheme 5. Synthesis of the JBIR-39 (2S,6S,8S)-(1b) and its diastereomers (2R,6R,8R)-1c and (2S,6R,8R)-1d.
significant for further studies for the total synthesis of piperida-
mycin antibiotics and their structure-activity relationships.
1H and -2H, 20 mmꢁ600 mm), using CHCl3 as a solvent
(3.5 mLminÀ1). Mass spectra and high-resolution mass spectra were
measured with JEOL JMS-DX303 (EI), MS-AX500 (FAB), or Thermo
Scientific Exactive Plus Orbitrap Mass Spectrometer (ESI) instru-
ments. LC spectra were measured with a reversed-phase HPLC
(Waters LC/MS system ZQ2000) with Waters 2996 ultraviolet detec-
tor with X BridgeT C18 3.5 mm column using A: methanol and
B: H2O as solvent (flow rate: 1.1 mLminÀ1, gradient: 0–1 min, A 5%;
1–4 min, A 5–95%; 4–11 min, A 95%; 11–11.1 min, A 5%; 11.1–
15 min, A 5%). IR spectra were recorded with a Shimadzu FTIR-
Experimental Section
General techniques
All commercially available reagents including the substrates were
used as received. Anhydrous THF and dichloromethane (Kanto
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8400. Only the strongest and/or structurally important absorption
are reported as the IR data. Specific rotations were measured with
a JASCO P-1010 polarimeter. Melting points were measured with
a Yazawa Micro Melting Point BY-2 or a Round Science Inc. RFS-10
and are not corrected.
128.48, 128.43, 128.39, 128.31, 128.27, 128.22, 128.16, 127.98,
127.86, 127.76, 127.69, 127.57, 127.44, 127.35, 127.1, 125.3, 120.2,
119.92, 119.86, 118.5, 118.2, 68.9, 68.6, 68.1, 68.0, 67.9, 67.8, 67.5,
67.,4, 67.2, 66.2, 66.1, 66.0, 65.9, 63.5, 54.3, 50.6, 49.4, 49.3, 47.0,
32.7, 29.6, 25.71, 25.68, 25.63, 24.3, 24.1, 23.8, 18.1, 18.0, 17.9, À4.9,
À5.15, À5.19, À5.3, À5.4 ppm; IR (neat): n˜ =2954, 2928, 2856,
1713, 1451, 1410, 1358, 1296, 1253, 1195, 1124, 1089, 837,
741 cmÀ1; HRMS (FAB): m/z calcd for C50H59N4O10Si: 903.4000
[M+H]+; found: 903.4001 (See S21 in page S12 of the Supporting
Information).
Synthesis
Amine 9: Aqueous LiOH (1m, 2.0 mL, 0.200 mmol, 5.0 equiv) was
added to
a
solution of l-H-(Nd-Cbz)-Piz(g-OTBS)-OMe (S8)[23]
(169 mg, 0.414 mmol, 1 equiv) in anhydrous THF (2.0 mL,
5.0 mLmmolÀ1) at 08C. The mixture was stirred at RT for 2.5 h,
then the reaction was quenched with 1m aqueous HCl at 08C and
the aqueous layer was extracted with ethyl acetate. The organic
layer was washed with brine, dried over MgSO4, filtered, and con-
centrated in vacuo. The residue was used for the next reaction
without further purification. Allyl bromide (0.043 mL, 0.497 mmol,
1.2 equiv) was added to a solution of the above residue and DIEA
(0.216 mL, 1.24 mmol, 3.0 equiv) in anhydrous DMF (2.0 mL,
5.0 mLmmolÀ1) at 08C under argon. The mixture was stirred at RT
for 19 h, then the reaction was quenched with 1m aqueous HCl at
08C and the aqueous layer was extracted with ethyl acetate. The
organic layer was washed with saturated aqueous NaHCO3, brine,
dried over MgSO4, filtered, and concentrated in vacuo. The residue
was purified by silica gel column chromatography (hexane/ethyl
acetate=5:1) to afford amine 9 (132 mg, 0.304 mmol, 2 steps
73%) as a pale-yellow oil. [a]2D5 = +13.5 (c=1.00, CHCl3); 1H NMR
(400 MHz, CDCl3): d=7.26–7.36 (m, 5H), 5.91 (ddt, J=6.0, 10.4,
18.0 Hz, 1H), 5.33 (dd, J=1.2, 18.0 Hz, 1H), 5.26 (dd, J=1.2,
10.4 Hz, 1H), 5.22 (d, Jgem =12.0 Hz, 1H), 5.14 (d, Jgem =12.0 Hz, 1H),
4.63 (m, 2H), 4.15 (m, 1H), 3.69 (m, 1H), 3.58 (m, 1H), 2.77 (m, 1H),
2.25 (m, 1H), 1.62 (m, 1H), 0.86 (s, 9H), 0.05 ppm (s, 6H); 13C NMR
(100 MHz, CDCl3): d=169.8, 155.5, 136.2, 131.4, 128.5, 128.1, 127.9,
119.0, 67.7, 65.8, 65.6, 57.4, 51.5, 37.5, 25.6, 17.9, À4.82,
À4.85 ppm; IR (neat): n˜ =3298, 2954, 2929, 2886, 2858, 1742, 1703,
1254, 1170, 1122, 1095, 838, 778 cmÀ1; HRMS (ESI): m/z calcd for
C22H34N2O5SiNa: 457.2129 [M+Na]+; found: 457.2117.
Synthesis of acid chloride 14a, b, c, and d; general proce-
dure
DMF (1 drop) was added to a solution of the corresponding Fmoc-
amino acid (1.0 equiv) and oxalyl chloride (10 equiv) in anhydrous
CH2Cl2 (8.0 mLmmolÀ1) at 08C under argon. The mixture was
stirred at RT for 1 h, then concentrated in vacuo. The residue was
used for the next reaction immediately without further purification.
Synthesis of dipeptide (method A); general procedure
Scandium triflate (0.10 equiv) was added to a solution of amine 9
(1.0 equiv) and acid chloride 14 or ent-10 (2.0 equiv) in anhydrous
toluene (5.0 mLmmolÀ1) at 08C under argon. The mixture was
stirred at RT or 08C for 4–9 h, then the reaction was quenched
with saturated aqueous NaHCO3 at 08C and stirred at RT. The mix-
ture was stirred at the same temperature for 10 min, then extract-
ed with ethyl acetate. The organic layer was washed with brine,
dried over MgSO4, filtered, and concentrated in vacuo. The residue
was purified by silica gel column chromatography to afford dipep-
tide 13 or 12.
Synthesis of dipeptide (method B); general procedure
A solution of acid chloride 14 or ent-10 (2.0 equiv) in anhydrous
CH2Cl2 (5.0 mLmmolÀ1) was added to a suspension of amine 9
(1.0 equiv) in 10% aqueous NaHCO3 (5.0 mLmmolÀ1) at 08C. The
mixture was stirred at RT for 18–24 h, then diluted with ethyl ace-
tate. The organic layer was washed with saturated aqueous
NaHCO3, brine, dried over MgSO4, filtered, and concentrated in
vacuo. The residue was purified by silica gel column chromatogra-
phy to afford dipeptide 13 or 12.
Synthesis of acid chlorides 10 and ent-10; general proce-
dure
Oxalyl chloride (10 equiv) was added to a solution of Fmoc-l-(Nd-
Cbz)-Piz-OH (S11)[23] (1.0 equiv) and DMF (1 drop) in benzene
(5.0 mLmmolÀ1) at 08C under argon. The mixture was stirred at
808C for 3 h, then concentrated in vacuo. The resulting acid chlo-
ride 10 was used for the next reaction without further purification.
Dipeptide 13a: Method A: 08C, 4 h; Yield: 12.8 mg (77%,
17.6 mmol); Method B: 24 h; Yeld: 16.0 mg (98%, 22.0 mmol);
1
[a]3D1 =À9.81 (c=0.540, MeOH); Retention time: 11.3 min; H NMR
(600 MHz, CDCl3): d (mixture of rotamers)=7.76 (m, 2H), 7.57 (m,
2H), 7.08–7.43 (m, 10H), 5.65–5.99 (m, 1H), 5.02–5.40 (m, 5H),
3.46–4.76 (m, 8H), 1.95–2.83 (m, 2H), 1.05–1.78 (m, 4H), 0.83 (m,
9H), À0.10–0.08 ppm (m, 6H); 13C NMR (150 MHz, CDCl3): d (mix-
ture of rotamers)=169.2, 155.7, 155.6, 155.0, 143.9, 143.75, 143.72,
141.3, 132.0, 128.6, 128.5, 128.4, 128.2, 128.1, 127.75, 127.71, 127.6,
127.0, 125.25, 125.20, 125.1, 125.0, 120.0, 119.9, 67.0, 66.9, 66.2,
50.2, 47.1, 47.0, 34.1, 31.9, 29.7, 25.8, 25.7, 25.68, 25.63, 19.5, 18.3,
18.0, 17.9, 17.5, 14.1, À4.8, À5.1, À5.2, À5.3 ppm; IR (neat): n˜ =
2951, 2928, 2856, 1724, 1683, 1451, 1409, 1253 cmÀ1; HRMS (ESI):
m/z calcd for C40H49N3O8SiNa: 750.3181 [M+Na]+; found: 750.3160.
Dipeptide 11: Scandium triflate (35.0 mg, 0.144 mmol, 0.10 equiv)
was added to a solution of 9 (627 mg, 1.41 mmol, 1.0 equiv) and
acid chloride 10 (1.87 mmol, 1.3 equiv) in anhydrous toluene
(7.0 mL, 5.0 mLmmolÀ1) at 08C under argon. The mixture was
stirred at RT for 1.5 h, then the reaction was quenched with satu-
rated aqueous NaHCO3 at 08C and the mixture was stirred at RT.
The mixture was stirred at the same temperature for 10 min, then
extracted with ethyl acetate. The organic layer was washed with
brine, dried over MgSO4, filtered, and concentrated in vacuo. The
residue was purified by silica gel column chromatography (hexane/
ethyl acetate=3:1) to afford dipeptide 11 (1.13 g, 1.25 mmol, 89%)
as a pale-yellow amorphous solid. [a]2D4 =À9.68 (c=0.990, CHCl3);
1H NMR (400 MHz, [D6]DMSO): d (mixture of rotamers)=7.83 (m,
2H), 7.58 (m, 2H), 7.13–7.43 (m, 14H), 5.83 (m, 1H), 4.79–5.34 (m,
8H), 4.35–4.64 (m, 3H), 3.79–4.35 (m, 5H), 2.62–3.12 (m, 2H), 1.20–
2.08 (m, 6H), 0.76 (s, 9H), 0.01 (s, 3H), À0.01 ppm (s, 3H); 13C NMR
(100 MHz, CDCl3): d (mixture of rotamers)=169.0, 166.7, 155.0,
142.9, 141.4, 136.8, 136.2, 132.0, 129.3, 128.9, 128.6, 128.55, 128.51,
Dipeptide 13b: Method A: RT, 6 h; Yield: 16.9 mg(77%,
22.4 mmol); Method B: 24 h; Yield: 1.0 mg (24%, 1.32 mmol); [a]D31
=
+0.37 (c=0.40, MeOH); Retention time: 11.4 min; 1H NMR
(600 MHz, CDCl3): d (mixture of rotamers)=7.76 (m, 2H), 7.57 (m,
2H), 7.09–7.42 (m, 10H), 5.65–5.97 (m, 1H), 4.94–5.40 (m, 5H),
3.66–4.77 (m, 8H), 1.95–3.07 (m, 3H), 1.05–1.78 (m, 1H), 0.68–1.02
(m, 15H), 0.00–0.08 ppm (m, 6H); 13C NMR (150 MHz, CDCl3): d
&
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(mixture of rotamers)=169.1, 156.4, 143.8, 141.4, 131.8, 131.0,
128.5, 128.3, 127.7, 127.6, 127.0, 125.1, 125.0, 120.0, 119.9, 118.8,
118.5, 67.4, 66.9, 66.2, 65.9, 64.9, 55.0, 54.7, 47.1, 32.5, 31.9, 29.7,
29.5, 29.3, 25.8, 25.7, 25.6, 22.7, 19.9, 18.3, 18.1, 17.9, 17.0, 14.1,
À4.9, À5.2 ppm; IR (neat): n˜ =3344, 2928, 2855, 1734, 1685, 1451,
1407, 1254, 1126 cmÀ1; HRMS (ESI): m/z calcd for C42H53N3O8SiNa:
778.3494 [M+Na]+; found: 778.3474.
(1.0 mL, 6.0 mLmmolÀ1) at 08C under argon. The mixture was
stirred at RT for 18 h, then the reaction was quenched with 1.0m
aqueous HCl at 08C, and the aqueous layer was extracted with
ethyl acetate. The organic layer was washed with saturated aque-
ous NaHCO3, brine, dried over MgSO4, filtered, and concentrated in
vacuo. The residue was purified by silica gel column chromatogra-
phy (hexane/ethyl acetate=3:1) to afford tripeptide 16 (188 mg,
0.164 mmol, 2 steps quant) as a pale-yellow amorphous solid.
[a]2D4 =À0.251 (c=1.58, CHCl3); 1H NMR (400 MHz, CDCl3): d (mix-
ture of rotamers)=7.73 (m, 2H), 7.50 (m, 2H), 6.91–7.63 (m, 19H),
5.70–5.98 (m, 1H), 4.68–5.72 (m, 11H), 4.59 (m, 2H), 3.45–4.62 (m,
7H), 2.51–3.45 (m, 3H), 1.11–2.50 (m, 10H), 0.64–0.97 (m, 9H),
À0.23–0.16 ppm (m, 6H); 13C NMR (100 MHz, CDCl3): d (mixture of
rotamers)=169.5, 167.6, 155.0, 143.7, 143.0, 141.2, 136.8, 135.8,
135.1, 132.1, 131.9, 131.8, 128.4, 128.2, 128.1, 127.9, 127.8, 127.0,
125.2, 124.9, 119.8, 118.7, 118.0, 117.6, 70.0, 69.7, 68.9, 68.4, 67.9,
67.5, 67.4, 67.2, 67.1, 66.7, 66.1, 65.7, 65.6, 63.2, 62.1, 54.6, 53.9,
53.1, 50.1, 49.6, 49.2, 48.8, 46.8, 46.4, 45.9, 45.5, 45.2, 35.4, 34.7,
34.2, 33.9, 33.4, 33.0, 32.8, 32.3, 31.8, 29.5, 29.3, 25.6, 23.7, 23.5,
22.7, 22.4, 20.3, 19.1, 18.6, 18.4, 18.0, 17.9, 17.4, 16.6, 16.1, À4.5,
À5.1, À5.3 ppm; IR (neat): n˜ =2951, 2931, 2857, 1714, 1683, 1405,
1359, 1255, 1128, 979, 837, 740, 697 cmÀ1; HRMS (FAB): m/z calcd
for C63H72N6O13SiNa: 1171.4824 [M+Na]+; found: 1171.4860 (See
S22 in page S12 of the Supporting Information).
Dipeptide 13c: Method A: RT, 9 h; Yield: 21.9 mg (63%,
28.4 mmol); Method B: 24 h; Yield: 4.3 mg (21%, 5.58 mmol); [a]D33
=
À6.77 (c=0.235, MeOH); Retention time: 11.6 min; 1H NMR
(600 MHz, CDCl3): d (mixture of rotamers)=7.76 (m, 2H), 7.56 (m,
2H), 7.09–7.43 (m, 10H), 5.61–5.97 (m, 1H), 5.00–5.46 (m, 5H),
3.47–4.77 (m, 8H), 1.94–2.49 (m, 3H), 1.26–1.84 (m, 3H), 0.55–1.11
(m, 15H), 0.00–0.08 ppm (m, 6H); 13C NMR (150 MHz, CDCl3): d
(mixture of rotamers)=169.1, 156.3, 143.8, 141.3, 141.2, 132.0,
131.7, 128.6, 128.48, 128.44, 128.2, 127.7, 127.6, 127.0, 125.2, 125.1,
124.9, 120.0, 119.9, 118.7, 66.9, 66.2, 55.2, 47.1, 29.7, 28.1, 26.4, 25.7,
25.7, 25.7, 25.6, 22.7, 17.9, 16.3, 14.1, 1.0, À4.8, À5.2 ppm; IR (neat):
n˜ =2958, 2928, 1729, 1680, 1252, 1126, 740 cmÀ1; HRMS (ESI): m/z
calcd for C43H55N3O8SiNa: 792.3651 [M+Na]+; found: 792.3628.
Dipeptide 13d: Method A: RT, 4 h; Yield: 9.9 mg (70%, 13.1 mmol);
Method B: 21 h; Yield: 9.2 mg (66%, 12.2 mmol); [a]3D3 =À16.2 (c=
1
0.470, MeOH); Retention time: 11.9 min; H NMR (600 MHz, CDCl3):
d (mixture of rotamers)=7.75 (m, 2H), 7.58 (m, 2H), 7.26–7.43 (m,
9H), 5.88 (m, 1H), 5.05–5.44 (m, 5H), 3.95–4.69 (m, 8H), 3.47–3.82
(m, 3H), 1.43–2.33 (m, 6H), 0.80–0.88 (m, 9H), À0.05–0.08 ppm (m,
6H); 13C NMR (150 MHz, CDCl3): d (mixture of rotamers)=176.6,
176.0, 169.5, 169.7, 156.0, 154.9, 154.7, 144.0, 143.8, 141.2, 135.9,
132.0, 128.6, 128.4, 128.3, 128.1, 127.6, 127.0, 126.9, 125.2, 125.1,
119.95, 119.93, 118.4, 118.2, 68.2, 68.0, 67.4, 65.8, 65.6, 63.5, 63.3,
55.5, 54.9, 51.6, 50.0, 49.9, 47.1, 46.9, 32.9, 32.8, 31.4, 30.4, 30.2,
29.7, 29.3, 25.8, 25.7, 25.6, 24.6, 23.3, 22.7, 18.3, 14.1, 1.15, 1.02,
À5.0, À5.3 ppm; IR (neat): n˜ =2951, 2927, 2854, 1685, 1415, 1252,
1123 cmÀ1; HRMS (ESI): m/z calcd for C42H51N3O8SiNa: 776.3338
[M+Na]+; found: 776.3323.
Tetrapeptide 17: Diethylamine (75.0 mL, 0.725 mmol, 5.0 equiv)
was added to a solution of tripeptide 16 (166 mg, 0.145 mmol,
1.0 equiv) in anhydrous acetonitrile (1.5 mL, 10 mLmmolÀ1) at RT
under argon. The mixture was stirred at the same temperature for
15 h, then concentrated in vacuo. The residue was used for the
next reaction without further purification. A solution of acid chlo-
ride 10 (0.322 mmol, 2.0 equiv) in anhydrous CH2Cl2 (1.0 mL,
7.0 mLmmolÀ1) was added to a solution of the above residue and
2,6-lutidine (50.0 mL, 0.435 mmol, 3.0 equiv) in anhydrous CH2Cl2
(1.0 mL, 7.0 mLmmolÀ1) at 08C under argon. The mixture was
stirred at RT for 16 h, then the reaction was quenched with 1.0m
aqueous HCl at 08C, and the aqueous layer was extracted with
ethyl acetate. The organic layer was washed with saturated aque-
ous NaHCO3, brine, dried over MgSO4, filtered, and concentrated in
vacuo. The residue was purified by silica gel column chromatogra-
phy (hexane/ethyl acetate=2:1) to afford tetrapeptide 17 (147 mg,
Dipeptide 12: Method A: RT, 4 h; Yield: 12.4 mg(82%, 13.7 mmol);
Method B: 24 h; Yield: 9.4 mg (68%, 10.4 mmol); [a]3D3 = +12.0 (c=
0.994, CHCl3); Retention time: 12.1 min; 1H NMR (400 MHz,
[D6]DMSO): d (mixture of rotamers)=7.73 (m, 2H), 7.56 (m, 2H),
7.07–7.42 (m, 14H), 5.86 (m, 1H), 4.82–5.52 (m, 8H), 4.35–4.75 (m,
3H), 3.50–4.28 (m, 5H), 1.16–2.93 (m, 8H), 0.70–0.94 (m, 9H),
À0.21–0.06 ppm (m, 6H); 13C NMR (100 MHz, CDCl3): d (mixture of
rotamers)=172.1, 171.6, 170.5, 170.0, 156.2, 156.1, 155.9, 155.7,
155.3, 144.5, 144.1, 144.0, 143.9, 142.1, 142.0, 141.9, 136.8, 136.6,
136.3, 136.2, 132.8, 132.7, 132.6, 129.1, 129.0, 128.9, 128.8, 128.7,
128.5, 128.4, 128.3, 128.1, 127.7, 127.6, 127.5, 125.7, 125.4, 120.7,
120.4, 118.9, 118.8, 118.7, 118.5, 69.0, 68.7, 68.2, 68.1, 67.9, 66.0,
65.8, 63.8, 63.7, 63.0, 62.9, 52.0, 51.4, 50.6, 50.4, 50.0, 49.5, 49.3,
49.2, 49.1, 47.3, 46.9, 46.2, 45.9, 44.2, 32.7, 32.1, 25.85, 25.78, 25.2,
23.3, 23.2, 22.9, 19.7, 19.4, 18.31, 18.25, À5.2, À5.3, À5.4, À5.5,
À5.6 ppm; IR (neat): n˜ =3584, 3063, 3033, 2929, 2855, 1725, 1682,
1451, 1409, 1359, 1250, 1195, 1124, 1079, 1013, 872, 837, 741,
697 cmÀ1; HRMS (ESI): m/z calcd for C50H58N4O10SiNa: 925.3814
[M+Na]+; found: 925.3803.
0.105 mmol, 2 steps 72%) as a pale-yellow amorphous solid. [a]D26
=
1
À4.97 (c=1.27, CHCl3); H NMR (400 MHz, CDCl3): d (mixture of ro-
tamers)=7.74 (m, 2H), 7.53 (m, 2H), 7.04–7.51 (m, 24H), 5.71–5.98
(m, 1H), 4.78–5.70 (m, 14H), 3.45–4.70 (m, 10H), 2.25–3.42 (m, 4H),
1.12–2.25 (m, 14H), 0.65–0.93 (m, 9H), À0.22–0.13 ppm (m, 6H);
13C NMR (100 MHz, CDCl3): d (mixture of rotamers)=170.2, 169.1,
167.1, 157.2, 156.7, 155.8, 155.4, 154.2, 153.1, 142.7, 140.9, 135.4,
131.5, 128.2, 128.1, 127.9, 127.7, 127.5, 127.3, 127.2, 126.7, 124.8,
119.5, 117.7, 68.7, 67.8, 65.4, 62.9, 53.8, 53.0, 49.0, 48.6, 46.5, 45.6,
44.1, 25.3, 19.3, 18.8, 18.4, 17.7, À5.5, À5.6 ppm; IR (neat): n˜ =
3065, 3017, 2953, 2895, 2857, 1715, 1451, 1410, 1358, 1257, 1127,
1085, 837, 755, 698 cmÀ1
;
HRMS (ESI): m/z calcd for
C76H86N8O16SiNa: 1417.5823 [M+Na]+; found: 1417.5825 (See S23 in
page S12 of the Supporting Information).
Tripeptide 16: Diethylamine (83.0 mL, 0.803 mmol, 5.0 equiv) was
Pentapeptide 18: Diethylamine (16 mL, 0.158 mmol, 5.0 equiv) was
added to
a
solution of dipeptide 11 (145 mg, 0.161 mmol,
added to a solution of tetrapeptide 17 (44 mg, 31.5 mmol,
1.0 equiv) in anhydrous acetonitrile (1.6 mL, 10 mLmmolÀ1) at RT
under argon. The mixture was stirred at the same temperature for
2 h, then concentrated in vacuo. The residue was used for the next
reaction without further purification. A solution of acid chloride
ent-10 (0.322 mmol, 2.0 equiv) in anhydrous CH2Cl2 (1.0 mL,
6.0 mLmmolÀ1) was added to a solution of the above residue and
2,6-lutidine (56.0 mL, 0.483 mmol, 3.0 equiv) in anhydrous CH2Cl2
1.0 equiv) in anhydrous acetonitrile (1.0 mL, 32 mLmmolÀ1) at RT
under argon. The mixture was stirred at the same temperature for
5 h, then concentrated in vacuo. The residue was used for the next
reaction without further purification. Silver cyanide (4.0 mg,
31.5 mmol, 1.0 equiv) was added to a solution of the above residue
and acid chloride 8 (63.1 mmol, 2.0 equiv) in anhydrous toluene
(2.0 mL, 64 mLmmolÀ1) at 08C under argon. The mixture was
Chem. Eur. J. 2014, 20, 1 – 12
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stirred at 608C for 15 h, then the reaction was quenched with satu-
rated aqueous NaHCO3, and the aqueous layer was extracted with
ethyl acetate. The organic layer was washed with brine, dried over
MgSO4, filtered, and concentrated in vacuo. The residue was puri-
fied by gel permeation chromatography (eluted with CHCl3) to
afford pentapeptide 18 (35 mg, 24.7 mmol, 2 steps 78%) as a color-
Chloroacetate 20: A 1m solution of TBAF-AcOH (1:1) in THF
(0.450 mL, 0.450 mmol, 5.0 mLmmolÀ1) was added to a solution of
pentapeptide 18 (127 mg, 90.0 mmol, 1.0 equiv) in anhydrous THF
(1.8 mL, 20 mLmmolÀ1) at RT under argon. The mixture was stirred
at the same temperature for 24 h, then the reaction was quenched
with 1m aqueous HCl at 08C. The aqueous layer was extracted
with ethyl acetate. The organic layer was washed with saturated
aqueous NaHCO3, brine, dried over MgSO4, filtered, and concentrat-
ed in vacuo. The residue was used for the next reaction without
further purification. DMAP (11 mg, 90.0 mmol, 1.0 equiv) was added
to a solution of the above residue, DIEA (78.0 mL, 0.450 mmol,
5.0 equiv) and chloroacetyl chloride (14.3 mL, 0.180 mmol,
2.0 equiv) in anhydrous CH2Cl2 (1.8 mL, 20 mLmmolÀ1) at À408C
under argon. The mixture was stirred at RT for 17 h, then the reac-
tion was quenched with 1m aqueous HCl at 08C. The aqueous
layer was extracted with ethyl acetate. The organic layer was
washed with saturated aqueous NaHCO3, brine, dried over MgSO4,
filtered, and concentrated in vacuo. The residue was purified by
silica gel column chromatography (chloroform/methanol=80:1) to
afford chloroacetate 20 (93.4 mg, 68.2 mmol, 76%) as a yellow oil.
[a]2D0 = +2.34 (c=1.19, CHCl3); 1H NMR (400 MHz, CDCl3): d (mixture
of rotamers)=6.99–7.52 (m, 22H), 6.78 (m, 2H), 5.74–6.16 (m, 1H),
4.74–5.70 (m, 16H), 3.77 (m, 3H), 3.59–4.72 (m, 10H), 2.53–3.43 (m,
4H), 1.11–2.45 (m, 17H), 0.71–0.96 ppm (m, 6H); 13C NMR
(100 MHz, CDCl3): d (mixture of rotamers)=173.8, 173.1, 173.0,
172.9, 169.6, 168.9, 168.7, 168.5, 168.2, 166.4, 166.3, 166.1, 165.0,
158.85, 158.77, 157.2, 156.8, 156.2, 155.8, 154.9, 152.9, 136.2,
136.05, 135.25, 135.21, 135.1, 134.9, 131.75, 131.72, 131.69, 131.5,
130.2, 129.2, 128.3, 128.2, 127.9, 127.7, 127.6, 127.5, 127.4, 118.8,
118.5, 117.8, 113.4, 113.3, 113.2, 80.6, 71.7, 71.7, 71.6, 69.0, 68.9,
68.8, 68.7, 68.5, 68.4, 68.3, 68.0, 67.4, 67.34, 67.27, 66.7, 65.9, 65.62,
65.55, 66.3, 54.9, 54.0, 53.6, 53.4, 53.2, 47.7, 47.5, 47.1, 46.9, 46.6,
46.4, 46.1, 46.0, 45.9, 45.6, 45.5, 45.3, 45.3, 40.6, 40.3, 40.2, 37.2,
37.1, 37.0, 36.9, 36.8, 36.7, 36.3, 29.4, 29.0, 25.4, 25.1, 24.9, 24.5,
23.9, 23.25, 23.20, 23.13, 23.06, 19.0, 18.8, 18.6, 18.4, 18.3, 18.00,
17.96, 17.92, 17.7, 17.1, 16.2, 11.7, 11.5, 11.1 ppm; IR (neat): n˜ =
2961, 2929, 1719, 1680, 1406, 1353, 1250, 1196 cmÀ1; HRMS (FAB):
m/z calcd for C71H82N8O18ClNa: 1369.5436 [M+Na]+; found:
1369.5460.
1
less oil. [a]2D0 =À4.70 (c=1.01, CHCl3); H NMR (400 MHz, CDCl3): d
(mixture of rotamers)=6.93–7.62 (m, 22H), 6.78 (m, 2H), 5.68–6.15
(m, 1H), 4.85–5.64 (m, 15H), 3.77 (m, 3H), 3.35–4.72 (m, 9H), 2.70–
3.28 (m, 4H), 1.12–2.52 (m, 17H), 0.63–0.98 (m, 15H), À0.22–
0.08 ppm (m, 6H); 13C NMR (100 MHz, CDCl3): d (mixture of rotam-
ers)=173.3, 170.7, 169.1, 168.1, 159.0, 156.5, 156.38, 156.31, 155.1,
149.8, 148.1, 136.4, 136.3, 135.9, 135.6, 135.5, 132.1, 132.0, 131.95,
131.89, 130.6, 130.4, 129.7, 129.5, 129.4, 129.2, 129.0, 128.6, 128.54,
129.52, 128.42, 128.37, 128.33, 128.31, 128.27, 128.16, 128.0,
127.89, 127.85, 127.79, 127.7, 127.6, 127.43, 127.36, 127.29, 127.22,
127.1, 126.54, 126.49, 123.4, 118.9, 118.8, 118.0, 113.8, 113.7, 113.5,
113.39, 113.36, 113.2, 80.9, 80.2, 71.8, 69.2, 69.1, 68.9, 68.8, 68.7,
68.1, 67.9, 67.8, 67.6, 67.3, 66.5, 66.1, 65.94, 65.87, 65.6, 65.5, 63.3,
62.0, 55.2, 53.9, 51.9, 47.7, 47.5, 46.5, 46.3, 46.0, 45.9, 45.7, 43.1,
37.3, 31.9, 31.2, 29.65, 29.60, 29.5, 29.4, 29.3, 29.2, 29.1, 28.5, 28.4,
25.73, 25.67, 25.64, 25.58, 24.81, 24.76, 24.6, 24.5, 23.5, 23.4, 23.0,
22.6, 18.6, 18.5, 18.39, 18.33, 18.2, 18.10, 18.06, 18.03, 16.53, 16.47,
14.1, 12.02, 11.95, 11.8, 11.7, 0.99, À5.1, À5.16, À5.22, À5.3,
À5.4 ppm; IR (neat): n˜ =2954, 2932, 1720, 1680, 1455, 1407, 1353,
1253, 1197, 1127, 838, 775 cmÀ1; HRMS (FAB): m/z calcd for
C75H94N8O17SiNa: 1429.6404 [M+Na]+; found: 1429.6398 (See S27 in
page S14 of the Supporting Information).
Acid 5: Morpholine (0.9 mL, 10.5 mmol, 1.5 equiv) was added to a so-
lution of pentapeptide 18 (9.9 mg, 7.03 mmol, 1.0 equiv) and
Pd(PPh3)4 (0.81 mg, 0.703 mmol, 0.10 equiv) in anhydrous THF
(1.0 mL, 140 mLmmolÀ1) at RT under argon. The mixture was
stirred at the same temperature for 1 h, then concentrated in
vacuo. The residue was purified by gel permeation chromatogra-
phy (eluted with CHCl3) to afford acid 5 (7.2 mg, 5.24 mmol, 75%)
as a yellow oil. [a]2D7 =À4.44 (c=0.195, CHCl3); 1H NMR (600 MHz,
CDCl3): d (mixture of rotamers)=6.99–7.52 (m, 22H), 6.78 (m, 2H),
4.65–6.04 (m, 13H), 3.77 (m, 3H), 3.44–4.68 (m, 7H), 2.47–3.37 (m,
4H), 1.16–2.71 (m, 17H), 0.71–0.92 (m, 15H), À0.19–0.10 ppm (m,
6H); 13C NMR (150 MHz, CDCl3): d (mixture of rotamers)=176.1,
132.2, 130.1, 128.6, 128.5, 128.4, 127.9, 113.5, 68.8, 55.2, 31.9, 29.6,
29.3, 28.4, 25.6, 25.5, 22.6, 18.6, 18.0, 17.9, 16.4, 14.0, 11.8, 0.99,
À3.6, À5.4 ppm; IR (neat): n˜ =3363, 2926, 2359, 1720, 1676, 1406,
1252, 1123 cmÀ1; HRMS (FAB): m/z calcd for C72H90N8O17SiNa:
1389.6091 [M+Na]+; found: 1389.6074.
Acid 21: Phenylsilane (13 mL, 0.102 mmol, 1.5 equiv) was added to
a solution of allyl ester 20 (93.4 mg, 68.2 mmol, 1.0 equiv) and
Pd(PPh3)4 (7.9 mg, 6.82 mmol, 0.10 equiv) in anhydrous CH2Cl2
(2.0 mL, 30 mLmmolÀ1) at RT under argon. The mixture was stirred
at the same temperature for 1.5 h, then concentrated in vacuo.
The residue was purified by silica gel column chromatography
(chloroform/methanol=80:1) to afford acid 21 (57.5 mg,
43.2 mmol, 63%) as a yellow oil. [a]2D0 = +5.28 (c=0.800, CHCl3);
1H NMR (600 MHz, CDCl3): d (mixture of rotamers)=6.95–7.52 (m,
22H), 6.78 (m, 2H), 4.73–6.12 (m, 14H), 3.77 (m, 3H), 3.34–4.65 (m,
8H), 2.61–3.27 (m, 4H), 1.16–2.44 (m, 17H), 0.71–0.96 ppm (m, 6H);
IR (neat): n˜ =2958, 2935, 1719, 1676, 1405, 1353, 1253, 1196, 1122,
1035 cmÀ1; HRMS (ESI): m/z calcd for C68H77N8O18ClNa: 1351.4937
[M+Na]+; found: 1351.4967.
Lactone 19: PyBroP (3.0 mg, 6.36 mmol, 1.5 equiv) was added to
a solution of acid 5 (5.8 mg, 4.24 mmol, 1.0 equiv), amine 6
(8.48 mmol, 2.0 equiv), and DIEA (2.2 mL, 12.7 mmol, 3.0 equiv) in an-
hydrous DMF (1.0 mL, 235 mLmmolÀ1) at 08C under argon. The
mixture was stirred at RT for 7 h, then the reaction was quenched
with 1m aqueous HCl at 08C. The aqueous layer was extracted
with ethyl acetate. The organic layer was washed with saturated
aqueous NaHCO3, brine, dried over MgSO4, filtered, and concentrat-
ed in vacuo. The residue was purified by silica gel column chroma-
tography (chloroform/methanol=100:1) to afford lactone 19
(4.7 mg, 3.80 mmol, 90%) as a colorless oil. [a]2D7 = +7.4 (c=0.410,
CHCl3); 1H NMR (600 MHz, CDCl3): d (mixture of rotamers)=6.99–
7.57 (m, 22H), 6.82 (m, 2H), 4.69–5.70 (m, 9H), 3.48–4.65 (m, 14H),
2.57–3.20 (m, 4H), 1.16–2.31 (m, 17H), 0.64–0.98 ppm (m, 6H); IR
(neat): n˜ =2958, 2931, 1801, 1723, 1677, 1402, 1259, 1123,
1029 cmÀ1; HRMS (FAB): m/z calcd for C66H74N8O16Na: 1257.5120
[M+Na]+; found: 1257.5084.
Hexapeptide 22: AOI reagent (21.8 mg, 57.5 mmol, 3.0 equiv)
added to a solution of acid 21 (25.5 mg, 19.2 mmol, 1.0 equiv),
amine 6 (38.4 mmol, 2.0 equiv), and DIEA (16.7 mL, 96.0 mmol,
5.0 equiv) in anhydrous CH2Cl2 (2.0 mL, 100 mLmmolÀ1) at 08C
under argon. The mixture was stirred at RT for 24 h, then the reac-
tion was quenched with 1m aqueous HCl at 08C. The aqueous
layer was extracted with ethyl acetate. The organic layer was
washed with saturated aqueous NaHCO3, brine, dried over MgSO4,
filtered, and concentrated in vacuo. The residue was purified by
silica gel column chromatography (chloroform/methanol=100:1)
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to afford hexapeptide 22 (13.0 mg, 8.72 mmol, 45%) as a colorless
oil. [a]2D0 = +7.95 (c=1.12, CHCl3); 1H NMR (600 MHz, CDCl3): d
(mixture of rotamers)=6.95–7.72 (m, 23H), 6.81 (m, 2H), 4.71–6.10
(m, 14H), 3.77 (m, 3H), 3.45–4.72 (m, 10H), 2.49–3.38 (m, 4H),
1.05–2.46 (m, 29H), 0.72–1.04 ppm (m, 6H); IR (neat): n˜ =3345,
2965, 2935, 1724, 1676, 1412, 1250 cmÀ1; HRMS (ESI): m/z calcd for
C76H92N9O20ClNa: 1508.6039 [M+Na]+; found: 1508.6044.
added to a solution of the above residue in anhydrous CH2Cl2
(1.0 mL, 27 mLmmolÀ1) at RT under argon. The mixture was stirred
at the same temperature for 1 h, then concentrated in vacuo. The
residue was used for the next reaction without further purification.
NaHCO3 (15.3 mg, 0.182 mmol, 5.0 equiv) was added to a solution
of the above residue in anhydrous methanol (1.0 mL,
27 mLmmolÀ1) at RT under argon. The mixture was stirred at the
same temperature for 1 h, then filtered through a pad of Celite
and concentrated in vacuo. The residue was used for the next reac-
tion without further purification. Pd/C (10%, 10.8 mg, 20 wt%) was
added to a solution of the above residue in ethyl acetate–ethanol
(3:1, 2.0 mL, 54 mLmmolÀ1) under argon and the reaction mixture
was purged with hydrogen three times. The mixture was stirred at
RT for 30 min, then filtered through a pad of Celite and concentrat-
ed in vacuo. The residue was purified by reverse-phase HPLC
(MeOH–H2O) to afford (2S,6S,8S)-1b (10.1 mg, 14.4 mmol, 4 steps
Compound (2R,6S,8S)-1a: Thiourea (5.4 mg, 71.0 mmol, 3.0 equiv)
was added to a solution of hexapeptide 22 (35.2 mg, 23.6 mmol,
1.0 equiv) and NaHCO3 (9.9 mg, 0.118 mmol, 5.0 equiv) in ethanol
(1.0 mL, 40 mLmmolÀ1) at RT under argon. The mixture was stirred
at 508C for 2 h, then filtered through a pad of Celite and concen-
trated in vacuo. The residue was used in the next reaction without
further purification. Trifluoroacetic acid (1.0 mL, 40 mLmmolÀ1) was
added to a solution of the above residue in anhydrous CH2Cl2
(1.0 mL, 40 mLmmolÀ1) at RT under argon. The mixture was stirred
at the same temperature for 1 h, then concentrated in vacuo. The
residue was used for the next reaction without further purification.
Pd/C (10%, 7.0 mg, 20 wt%) was added to a solution of the above
residue in ethyl acetate–ethanol (3:1, 2.0 mL, 80 mLmmolÀ1) under
argon and the reaction mixture was purged with hydrogen three
times. The mixture was stirred at RT for 30 min, then filtered
through a pad of Celite and concentrated in vacuo. The residue
was purified by reverse-phase HPLC (MeOH–H2O) to afford
(2R,6S,8S)-1a (4.3 mg, 6.23 mmol, 3 steps 26%) as a colorless oil.
[a]3D3 =À14.2 (c=0.10, MeOH); 1H NMR (600 MHz, [D6]DMSO): d=
7.68 (s, 1H), 5.56–5.61 (m, 3H), 5.22 (dd, J=5.4, 9.6 Hz, 1H), 5.17
(d, J=13.2 Hz, 1H), 5.12 (d, J=12.6 Hz, 1H), 5.03 (d, J=12.6 Hz,
1H), 4.82 (m, 1H), 4.42 (d, J=4.2 Hz, 1H), 3.79 (d, Jgem =9.6 Hz, 1H),
3.67 (m, 1H), 3.56 (d, Jgem =9.6 Hz, 1H), 3.00 (m, 3H), 2.79 (m, 2H),
2.59 (m, 3H), 2.16 (m, 1H), 1.98–2.00 (m, 4H), 1.70–1.77 (m, 4H),
1.51 (m, 3H), 1.45 (m, 3H), 1.30 (s, 3H), 1.22–1.31 (m, 1H), 1.02 (m,
1H), 0.85 (d, J=6.6 Hz, 3H), 0.80 ppm (t, J=7.2 Hz, 3H); 13C NMR
(150 MHz, [D6]DMSO): d=174.9, 172.9, 172.8, 172.6, 172.3, 169.8,
71.7, 64.8, 61.1, 52.8, 48.7, 48.6, 48.3, 48.2, 46.9, 46.8, 37.4, 33.6,
29.2, 29.0, 25.9, 22.9, 22.3, 21.3, 20.2, 16.6, 12.1 ppm; IR (neat): n˜ =
3585, 2958, 2920, 2853, 1620 cmÀ1; HRMS (ESI): m/z calcd for
C30H51N9O10ClNa: 720.3651 [M+Na]+; found: 720.3644.
40%) as a colorless oil. [a]3D3 =À24.7 (c=0.210, MeOH) [lit. [a]D25
=
1
À11.0 (c=0.1, MeOH)]; H NMR (600 MHz, [D6]DMSO): d=7.89 (s, c,
1H), 5.53–5.61 (m, 3H), 5.17 (dd, J=9.6, 16.2 Hz, 1H), 5.14 (d, J=
13.2 Hz, 2H), 5.03 (d, J=12.0 Hz, 1H), 4.78 (d, J=5.4 Hz, 1H), 4.43
(d, J=4.2 Hz, 1H), 3.87 (d, Jgem =10.8 Hz, 1H), 3.68 (m, 1H), 3.42 (d,
J
gem =10.8 Hz, 1H), 3.00 (m, 3H), 2.81 (d, J=7.8 Hz, 2H), 2.60 (m,
3H), 2.27 (m, J=12.0 Hz, 1H), 1.90–2.04 (m, 4H), 1.71–1.79 (m, 4H),
1.51 (m, 3H), 1.47 (m, 3H), 1.30 (s, 3H), 1.25–1.34 (m, 1H), 1.03 (m,
1H), 0.87 (d, J=7.2 Hz, 3H), 0.81 ppm (t, J=7.5 Hz, 3H); 13C NMR
(150 MHz, [D6]DMSO): d=174.9, 172.9, 172.6, 172.2, 170.1, 71.8,
63.1, 61.2, 59.6, 52.7, 48.8, 48.5, 48.4, 48.2, 46.82, 46.8, 37.4, 33.4,
26.1, 26.0, 25.8, 22.7, 21.3, 21.26, 21.2, 20.5, 16.6, 12.1 ppm; IR
(neat): n˜ =3364, 3260, 2954, 2935, 2864, 1621, 1443, 1403, 1249,
915 cmÀ1; HRMS (FAB): m/z calcd for C30H51N9O10Na: 720.3657
[M+Na]+; found: 720.3671.
Acid 24: Phenylsilane (6.7 mL, 54.3 mmol, 1.5 equiv) was added to
a solution of pentapeptide S20[23] (51.0 mg, 36.2 mmol, 1.0 equiv)
and Pd(PPh3)4 (4.2 mg, 3.62 mmol, 0.10 equiv) in anhydrous CH2Cl2
(2.0 mL, 55 mLmmolÀ1) at RT under argon. The mixture was stirred
at the same temperature for 2.5 h, then concentrated in vacuo.
The residue was purified by gel permeation chromatography
(eluted with CHCl3) to afford acid 24 (44.9 mg, 32.8 mmol, 91%) as
a yellow oil. [a]2D7 =À5.65 (c=0.83, CHCl3); 1H NMR (600 MHz,
CDCl3): d (mixture of rotamers)=7.03–7.52 (m, 22H), 6.75 (m, 2H),
4.75–6.09 (m, 13H), 3.72 (m, 3H), 3.40–4.63 (m, 7H), 2.51–3.18 (m,
4H), 1.06–2.51 (m, 17H), 0.69–0.92 (m, 15H), À0.21–0.06 ppm (m,
6H); 13C NMR (150 MHz, CDCl3): d=173.3, 172.8, 171.4, 171.0,
160.2, 158.9, 156.4, 154.6, 136.3, 35.5, 135.4, 130.3, 129.5, 129.1,
128.8, 128.5, 128.48, 128.43, 128.35, 128.32, 128.0, 127.9, 127.7,
127.4, 127.2, 126.5, 113.5, 113.4, 113.2, 80.2, 79.9, 71.8, 70.1, 69.9,
69.2, 68.7, 68.8, 68.6, 67.7, 62.6, 60.3, 55.14, 55.11, 53.6, 52.8, 49.8,
47.9, 47.7, 46.1, 44.5, 44.2, 43.0, 37.3, 37.2, 30.7, 29.6, 25.7, 25.5,
23.5, 23.4, 20.9, 19.8, 18.7, 18.4, 17.9, 16.5, 16.4, 16.3, 14.1, 12.0,
11.9, 11.7, À5.45, À5.48, À5.5 ppm; IR (neat): n˜ =2959, 2933, 1725,
1676, 1456, 1410, 1351, 1252, 1122 cmÀ1; HRMS (FAB): m/z calcd for
C72H90N8O17SiNa: 1389.6091 [M+Na]+; found: 1389.6074.
Hexapeptide 23: AOI reagent (21 mg, 55.7 mmol, 3.0 equiv) was
added to a solution of acid 21 (24.7 mg, 18.6 mmol, 1.0 equiv),
amine ent-6 (crude, 37.1 mmol, 2.0 equiv), and DIEA (16 mL,
92.9 mmol, 5.0 equiv) in anhydrous CH2Cl2 (2.0 mL, 100 mLmmolÀ1
)
at 08C under argon. The mixture was stirred at RT for 19 h, then
the reaction was quenched with 1m aqueous HCl at 08C. The
aqueous layer was extracted with ethyl acetate. The organic layer
was washed with saturated aqueous NaHCO3, brine, dried over
MgSO4, filtered, and concentrated in vacuo. The residue was puri-
fied by silica gel column chromatography (chloroform/methanol=
100:1) to afford hexapeptide 23 (21.8 mg, 14.7 mmol, 79%) as a col-
orless oil. [a]2D0 = +4.09 (c=0.445, CHCl3); 1H NMR (600 MHz,
CDCl3): d (mixture of rotamers)=7.07–7.53 (m, 23H), 6.81 (m, 2H),
4.71–5.87 (m, 14H), 3.76 (m, 3H), 3.24–4.68 (m, 10H), 2.69–3.32 (m,
4H), 1.12–2.33 (m, 29H), 0.74–0.99 ppm (m, 6H); IR (neat): n˜ =
Hexapeptide 25: AOI reagent (14 mg, 37.2 mmol, 3.0 equiv) was
added to a solution of acid 24 (17 mg, 12.4 mmol, 1.0 equiv), amine
6 (24.9 mmol, 2.0 equiv), and DIEA (11 mL, 62.0 mmol, 5.0 equiv) in
anhydrous CH2Cl2 (2.0 mL, 160 mLmmolÀ1) at 08C under argon.
The mixture was stirred at RT for 24 h, then the reaction was
quenched with 1m aqueous HCl at 08C. The aqueous layer was ex-
tracted with ethyl acetate and the organic layer was washed with
saturated aqueous NaHCO3, brine, dried over MgSO4, filtered, and
concentrated in vacuo. The residue was purified by silica gel
column chromatography (chloroform/methanol=80:1) to afford
3346, 2961, 2928, 2857, 1724, 1676, 1457, 1405, 1353, 1251 cmÀ1
;
HRMS (ESI): m/z calcd for C76H92N9O20ClNa: 1508.6039 [M+Na]+;
found: 1508.6043.
Compound (2S,6S,8S)-1b: Thiourea (4.2 mg, 54.7 mmol, 1.5 equiv)
was added to a solution of hexapeptide 23 (54.2 mg, 36.4 mmol,
1.0 equiv) and NaHCO3 (6.1 mg, 72.9 mmol, 2.0 equiv) in ethanol
(1.0 mL, 27 mLmmolÀ1) at RT under argon. The mixture was stirred
at 508C for 6 h, then filtered through a pad of Celite and concen-
trated in vacuo. The residue was used next reaction without fur-
ther purification. Trifluoroacetic acid (1.0 mL, 27 mLmmolÀ1) was
hexapeptide 25 (15.0 mg, 9.81 mmol, 79%) as a colorless oil. [a]D28
=
Chem. Eur. J. 2014, 20, 1 – 12
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À2.29 (c=0.765, MeOH); 1H NMR (600 MHz, CDCl3): d (mixture of
rotamers)=7.67–7.96 (m, 1H), 7.07–7.50 (m, 22H), 6.77 (m, 2H),
4.64–6.12 (m, 13H), 3.73 (s, 3H), 3.43–4.63 (m, 9H), 2.58–3.23 (m,
4H), 1.16–2.48 (m, 29H), 0.69–0.93 (m, 15H), À0.19–0.05 ppm (m,
6H); 13C NMR (150 MHz, CDCl3): d (mixture of rotamers)=172.8,
171.6, 1693, 159.0, 156.5, 154.6, 136.4, 135.6, 129.5, 129.2, 128.6,
128.5, 128.4, 128.35, 128.31, 128.0, 127.9, 127.85, 127.81, 113.5,
81.5, 80.2, 71.8, 68.8, 68.7, 67.7, 63.8, 62.7, 55.2, 53.6, 52.1, 47.9,
47.7, 45.7, 44.2, 43.0, 37.2, 31.8, 29.6, 29.5, 29.4, 29.3, 28.6, 27.8,
27.0, 25.8, 25.5, 23.4, 22.6, 20.0, 19.9, 18.7, 18.5, 18.1, 16.4, 14.0,
11.7, 1.0, À5.0, À5.1 ppm; IR (neat): n˜ =3331, 2958, 2931, 1725,
(mixture of rotamers)=7.71–7.98 (m, 1H), 7.04–7.54 (m, 22H), 6.78
(m, 2H), 4.72–5.82 (m, 13H), 3.77 (s, 3H), 3.47–4.66 (m, 9H), 2.53–
3.28 (m, 4H), 1.15–2.47 (m, 29H), 0.72–0.95 (m, 15H), À0.15–
0.08 ppm (m, 6H); 13C NMR (150 MHz, CDCl3): d (mixture of rotam-
ers)=171.7, 170.3, 170.0, 159.0, 156.5, 136.4, 135.6, 135.5, 129.5,
129.2, 128.6, 128.5, 128.46, 128.43, 128.41, 128.3, 128.25, 128.23,
128.0, 127.9, 127.8, 126.7, 113.5, 113.4, 81.6, 81.5, 80.3, 71.8, 68.8,
67.8, 66.2, 63.7, 61.7, 55.2, 47.9, 47.7, 44.3, 37.4, 37.3, 29.7, 29.3,
27.95, 27.91, 25.8, 25.7, 25.6, 23.5, 23.4, 22.7, 21.3, 20.5, 19.8, 18.4,
16.6, 16.5, 14.1, 11.8, 1.0, À5.0, À5.1, À5.3 ppm; IR (neat): n˜ =2954,
2933, 1725, 1684, 1457, 1409, 1252, 1125, 755, 699 cmÀ1; HRMS
(ESI): m/z calcd for C80H105N9O19SiNa: 1546.7188 [M+Na]+; found:
1546.7187.
1676, 1406, 1251, 1123 cmÀ1
; HRMS (ESI): m/z calcd for
C80H105N9O19SiNa: 1546.7194 [M+Na]+; found: 1546.7177.
Compound (2S,6R,8R)-1d: Hydrogen fluoride-pyridine (3.3 mL,
36.7 mmol, 3.0 equiv) was added to a solution of hexapeptide 26
(18.2 mg, 12.2 mmol, 1.0 equiv) in anhydrous acetonitrile (2.0 mL,
160 mLmmolÀ1) at RT under argon. The mixture was stirred at the
same temperature for 10 h, then the reaction was quenched with
saturated aqueous NaHCO3 and the aqueous layer was extracted
with ethyl acetate. The organic layer was washed with brine, dried
over MgSO4, filtered, and concentrated in vacuo. The residue was
used in the next reaction without further purification. Trifluoroace-
tic acid (1.0 mL, 80 mLmmolÀ1) was added to a solution of the
above residue and triethylsilane (5.8 mL, 31.3 mmol, 3.0 equiv) in an-
hydrous CH2Cl2 (1.0 mL, 80 mLmmolÀ1) at RT under argon. The mix-
ture was stirred at the same temperature for 1 h, then concentrat-
ed in vacuo. The residue was used for the next reaction without
further purification. NaHCO3 (5.1 mg, 61.0 mmol, 5.0 equiv) was
added to a solution of the above residue in anhydrous methanol
(1.0 mL, 80 mLmmolÀ1) at RT under argon. The mixture was stirred
at the same temperature for 30 min, then filtered through a pad of
Celite and concentrated in vacuo. The residue was used for the
next reaction without further purification. Pd/C (10%, 2.4 mg,
20 wt%) was added to a solution of the above residue in ethyl ace-
tate–ethanol (3:1, 2.0 mL, 180 mLmmolÀ1) under argon, and the re-
action mixture was purged with hydrogen three times. The mixture
was stirred at RT for 5 h, then filtered through a pad of Celite and
concentrated in vacuo. The residue was purified by reverse-phase
HPLC (MeOH–H2O) to afford (2S,6R,8R)-1d (5.0 mg, 7.17 mmol,
4 steps 59%) as a colorless oil. [a]2D3 = +7.27 (c=0.205, MeOH);
1H NMR (600 MHz, [D6]DMSO): d=7.68 (s, 1H), 5.52–5.59 (m, 3H),
5.15 (m, 2H), 5.08 (d, J=12.6 Hz, 1H), 5.05 (d, J=13.2 Hz, 1H), 4.85
(d, J=5.4 Hz, 1H), 4.41 (m, 1H), 4.12 (d, J=7.2 Hz, 1H), 3.73 (d, J=
10.8 Hz, 1H), 3.65 (m, 1H), 3.54 (d, J=10.8 Hz, 1H), 2.99 (m, 3H),
2.72–2.88 (m, 2H), 2.37–2.62 (m, 3H), 2.16 (m, 2H), 2.01 (m, 3H),
1.72 (m, 4H), 1.22–1.54 (m, 10H), 1.03 (m, 1H), 0.85 (d, J=6.0 Hz,
3H), 0.81 ppm (t, J=7.2 Hz, 3H); 13C NMR (150 MHz, [D6]DMSO):
d=174.6, 174.2, 172.8, 172.52, 172.47, 172.3, 71.6, 65.0, 60.6, 60.5,
52.8, 48.3, 48.2, 48.1, 47.4, 46.64, 46.58, 37.2, 32.6, 25.7, 25.1, 22.7,
21.2, 21.1, 20.8, 19.1, 16.3, 11.8 ppm; IR (neat): n˜ =2924, 2853,
1653, 1202, 1136 cmÀ1; HRMS (ESI): m/z calcd for C30H51N9O10Na:
720.3657 [M+Na]+; found: 720.3640.
Compound (2R,6R,8R)-1c: Hydrogen fluoride-pyridine (2.8 mL,
30.9 mmol, 3.0 equiv) was added to a solution of hexapeptide 25
(15.3 mg, 10.3 mmol, 1.0 equiv) in anhydrous acetonitrile (2.0 mL,
200 mLmmolÀ1) at RT under argon. The mixture was stirred at the
same temperature for 13 h, then the reaction was quenched with
saturated aqueous NaHCO3, and the aqueous layer was extracted
with ethyl acetate. The organic layer was washed with brine, dried
over MgSO4, filtered, and concentrated in vacuo. The residue was
used in the next reaction without further purification. Trifluoroace-
tic acid (1.0 mL, 100 mLmmolÀ1) was added to a solution of the
above residue and triethylsilane (5.0 mL, 31.3 mmol, 3.0 equiv) in an-
hydrous CH2Cl2 (1.0 mL, 100 mLmmolÀ1) at RT under argon. The
mixture was stirred at the same temperature for 3 h, then concen-
trated in vacuo. The residue was used for the next reaction without
further purification. NaHCO3 (4.5 mg, 51.5 mmol, 5.0 equiv) was
added to a solution of the above residue in anhydrous methanol
(1.0 mL, 100 mLmmolÀ1) at RT under argon. The mixture was
stirred at the same temperature for 30 min, then filtered through
a pad of Celite and concentrated in vacuo. The residue was used
for the next reaction without further purification. Pd/C (10%,
2.1 mg, 20 wt%) was added to a solution of the above residue in
ethyl acetate–ethanol (3:1, 2.0 mL, 200 mLmmolÀ1) under argon
and the reaction mixture was purged with hydrogen three times.
The mixture was stirred at RT for 5 h, then filtered through a pad
of Celite and concentrated in vacuo. The residue was purified by
reverse-phase HPLC (MeOH-H2O) to afford (2R,6R,8R)-1c (3.3 mg,
4.73 mmol, 4 steps 45%) as a colorless oil. [a]2D8 = +12.2 (c=0.160,
MeOH); 1H NMR (600 MHz, [D6]DMSO): d=7.91 (s, 1H), 5.52–5.58
(m, 3H), 5.15 (m, 2H), 5.04 (m, 2H), 4.81 (m, 1H), 4.42 (m, 1H), 4.15
(m, 1H), 3.82 (m, 1H), 3.65 (m, 1H), 3.00 (m, 3H), 2.82 (m, 2H),
2.40–2.85 (m, 3H), 2.20 (m, 2H), 1.90–2.04 (m, 3H), 1.73 (m, 4H),
1.11–1.52 (m, 10H), 1.02 (m, 1H), 0.85 (d, J=6.6 Hz, 3H), 0.80 ppm
(t, J=7.5 Hz, 3H); 13C NMR (150 MHz, [D6]DMSO): d=174.6, 173.0,
172.9, 172.54, 172.51, 169.8, 71.6, 63.9, 60.55, 60.53, 55.0, 52.8, 48.4,
48.3, 48.2, 47.5, 46.7, 46.6, 37.2, 29.0, 25.7, 22.7, 21.2, 21.1, 20.9,
20.1, 16.3, 11.8 ppm; IR (neat): n˜ =3274, 2927, 1633, 1241 cmÀ1
;
HRMS (ESI): m/z calcd for C30H51N9O10Na: 720.3651 [M+Na]+;
found: 720.3644.
Hexapeptide 26: AOI reagent (18 mg, 46.8 mmol, 3.0 equiv) was
added to a solution of acid 24 (21.3 mg, 15.6 mmol, 1.0 equiv),
amine ent-6 (31.1 mmol, 2.0 equiv), and DIEA (8.0 mL, 46.8 mmol,
3.0 equiv) in anhydrous CH2Cl2 (2.0 mL, 130 mLmmolÀ1) at 08C
under argon. The mixture was stirred at RT for 24 h, then the reac-
tion was quenched with 1m aqueous HCl at 08C. The aqueous
layer was extracted with ethyl acetate. The organic layer was
washed with saturated aqueous NaHCO3, brine, dried over MgSO4,
filtered, and concentrated in vacuo. The residue was purified by
silica gel column chromatography (chloroform/methanol=80:1) to
afford hexapeptide 26 (20.2 mg, 13.2 mmol, 85%) as a pale-yellow
oil. [a]2D4 =À7.46 (c=0.700, CHCl3); 1H NMR (600 MHz, CDCl3): d
Acknowledgements
This work was supported by a grant from the New Energy and
Industrial Technology Development Organization (NEDO) of
Japan and by a Grant-in-Aid for Young Scientists (B) (No.
25750381) (M.Y.) from the Japan Society for the Promotion of
Science and partially supported by the Platform for Drug Dis-
covery, Informatics, and Structural Life Science from the Japa-
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Keywords: acylation · configuration determination · natural
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Received: November 7, 2014
Published online on && &&, 0000
Chem. Eur. J. 2014, 20, 1 – 12
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FULL PAPER
Structure solved! The total synthesis
and structure elucidation of linear pep-
tide JBIR-39 (1b) containing an unprece-
dented four nonproteinogenic piperazic
acid (Piz) residues, was achieved (see
scheme). The approach involved effi-
cient Sc(OTf)3-catalyzed direct acylation
of Piz(g-OTBS) derivative with piperazic
acid chloride, leading to the Piz-Piz(g-
OTBS) dipeptide in high yield without
epimerization.
&
Natural Products
M. Yoshida, N. Sekioka, M. Izumikawa,
I. Kozone, M. Takagi, K. Shin-ya, T. Doi*
&& – &&
Total Synthesis and Structure
Elucidation of JBIR-39: A Linear
Hexapeptide Possessing Piperazic Acid
and g-Hydroxypiperazic Acid Residues
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Chem. Eur. J. 2014, 20, 1 – 12
www.chemeurj.org
12
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!