Stereoselective Synthesis of Protected l - Allo -Enduracididine and l -Enduracididine via Asymmetric Nitroaldol Reaction

Article abstract of DOI:10.1055/s-0039-1691522

The diastereoselecetive and scalable synthesis of cyclic guanidine-containing nonproteinoginic amino acids, enduracididines, has been achieved. Both diastereomers, l - allo -enduracididine and l -enduracididine, were prepared via catalyst-controlled asymmetric nitroaldol reaction with the aldehyde precursor derived from l -aspartic acid. The cyclic guanidine of di-Cbz-protected l - allo -enduracididine was fully protected with an allyl group to suppress nucleophilic side reactions. Introduced allyl group was efficiently removed via π-allylpalladium chemistry without attaching the Cbz group on the cyclic guanidine moiety.

Full text of DOI:10.1055/s-0039-1691522

SYNTHESIS
0
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1
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© Georg Thieme Verlag Stuttgart · New York
2019, 51, A–G
paper
A
en
K. Ohsawa et al.
Paper
Synthesis
Stereoselective Synthesis of Protected L-allo-Enduracididine and
L-Enduracididine via Asymmetric Nitroaldol Reaction
Kosuke Ohsawaa
BocHN
OtBu
asymmetric
BocHN
OtBu
BocHN
OtBu
Hongbin Zhaoa
Takuya Tokunagaa
Carys Thomasb
A. Ganesanb
nitroaldol reaction
O2N
O2N
O
O
O
HO
HO
O
common
intermediate
91%, dr 96:4
92%, dr 96:4
Yuichi Masudaa,c
Takayuki Doi*a
BocHN
OtBu
BocHN
OtBu
0
0
0
0-0
0
0
2-8
3
0
6-
6
8
1
9
high diastereoselectivity
multi-gram scale synthesis
HN
HN
O
O
a Graduate School of Pharmaceutical Sciences, Tohoku University,
6-3 Aza-aoba, Aramaki, Aoba-ku, Sendai 980-8578, Japan
doi_taka@mail.pharm.tohoku.ac.jp
N
N
CbzN
CbzN
Cbz
Cbz
L-allo-enduracididine
(61%, 4 steps)
L-enduracididine
(71%, 4 steps)
b School of Pharmacy, University of East Anglia, Norwich Research
Park, Norwich NR4 7TJ, UK
c Graduate School of Bioresources, Mie university, 1577 Kuri-
mamachiya-cho, Tsu 514-8507, Japan
Received: 09.11.2019
Accepted after revision: 14.11.2019
Published online: 26.11.2019
relationship (SAR) studies using various teixobactin ana-
logues have been reported.6e,7 In many cases, the syntheti-
cally challenging L-allo-enduracididine residue at the 10 po-
sition is replaced by commercially available cationic amino
acids such as Arg, Orn, and Lys, reducing antibacterial activ-
ity. Several analogues bearing a hydrophobic side chain in-
stead of the cyclic guanidine moiety suggested cationic
amino acids at the 10 position are not essential for Lipid II
binding and antibacterial activity,8 though it is unclear
whether these hydrophobic analogues show the same dual
targeting mechanism as positively charged teixobactin by
binding to both Lipid II and Lipid III. Thus, development of
an efficient synthetic route to both L-allo-enduracididine
and L-enduracididine is essential for obtaining sufficient
quantities toward a library synthesis of enduracididine-
containing teixobactin analogues. Starting with the first dia-
stereoselective synthesis by Shiba et al,9a several stereose-
lective and scalable synthesis of L-allo-enduracididine have
been accomplished as a building block for teixobactin to
date. However, these target-oriented approaches are not ac-
cessible for the preparation of its -epimer, L-enduracidi-
dine, because the chiral center at the  position was in-
stalled by the SN2 reaction of the hydroxy group in com-
mercially available 4-trans-hydroxyproline derivatives,6d,9b
substrate-controlled reduction of a nitroketone,6a,9c or in-
tramolecular double guanidinylation of a terminal olefin in
an allylglycine derivative.6e Liu et al. has recently reported
the synthesis of both L-allo-enduracididine and L-enduraci-
didine via the nonselective reduction of the nitroketone fol-
lowed by chromatographic separation of the resulting two
diastereomers in low yields.6c Therefore, it is valuable to es-
tablish a highly stereocontrolled approach from a common
synthetic intermediate to enduracididine and its diastereo-
DOI: 10.1055/s-0039-1691522; Art ID: ss-2019-z0568-op
Abstract The diastereoselecetive and scalable synthesis of cyclic guan-
idine-containing nonproteinoginic amino acids, enduracididines, has
been achieved. Both diastereomers, L-allo-enduracididine and L-endura-
cididine, were prepared via catalyst-controlled asymmetric nitroaldol
reaction with the aldehyde precursor derived from L-aspartic acid. The
cyclic guanidine of di-Cbz-protected L-allo-enduracididine was fully pro-
tected with an allyl group to suppress nucleophilic side reactions. Intro-
duced allyl group was efficiently removed via -allylpalladium chemistry
without attaching the Cbz group on the cyclic guanidine moiety.
Key words enduracididines, nonproteinogenic amino acids, cyclic
guanidines, asymmetric nitroaldol reactions, guanidine functionaliza-
tion
Enduracididines,1 which are one of the unique nonpro-
teinogenic amino acids that contain a five-membered cyclic
guanidine moiety, are a component of cyclic peptide and
depsipeptide natural products: enduracidins,2 mannopepti-
mycins,3 and teixobactin4 (Figure 1). These enduracididine-
containing macrocycles exhibit potent antibacterial activity
against a wide-range of Gram-positive bacteria including
multi-drug resistant strains. Among them, teixobactin, iso-
lated from the uncultivable beta-proteobacterium Elefthe-
ria terrae by using a new iChip technology, inhibits cell wall
biosynthesis by binding to a highly conserved pyrophos-
phate and the first sugar moieties of both Lipid II (precursor
of peptidoglycan) and Lipid III (precursor of wall teichoic
acid).4,5 This dual targeting of cell-wall precursors makes it
difficult to develop drug-resistance and therefore, teixobac-
tin is expected to herald a new class of antibiotics. Along
with the total synthesis of teixobactin,6 structure-activity
© 2019. Thieme. All rights reserved. Synthesis 2019, 51, A–G
Georg Thieme Verlag KG, Rüdigerstraße 14, 70469 Stuttgart, Germany

B
K. Ohsawa et al.
Paper
Synthesis
mer. Herein, we report the stereoselective synthesis of suit-
ably protected L-allo-enduracididine 1a and its epimer, L-
enduracididine 1b, via asymmetric nitroaldol reaction.
not complete in 72 hours, providing the desired product
(R)-5a in 53% yield and in high diastereoselectivity (dr
95:5, Table 1, entry 1). At –40 °C, the yield of 5a increased
up to 88%, whereas the selectivity decreased (dr 82:18, en-
try 2). Thus, we utilized 10 mol% catalyst to complete the
reaction at –78 °C in 48 hours, leading to 5a in a high yield
and good selectivity (91%, dr 96:4, entry 3). These reaction
conditions were applied to the synthesis of (S)-5b using
the catalyst (R,R)-4 instead of (S,S)-4. The substrate was
consumed within 24 hours, and the corresponding 5b was
obtained in high yield and high selectivity (92%, dr 96:4, en-
try 4). The results suggest that the catalyst-controlled
asymmetric nitroaldol reaction was achieved using the
cobalt-salen catalyst 4.
O
NH2
O
O
O
OH
O
H
N
H
N
H
N
HN
N
H
N
H
N
H
O
O
O
OH
HN
O
NH
O
O
O
NH HN
HN
O
N
H
HN
L-allo-End10
teixobactin
Table 1 Asymmetric Nitroaldol Reaction of Aldehyde 3 Catalyzed by
Cobalt-Salen Complexes (S,S)-4 and (R,R)-4
H2N
OH
O
H2N
OH
O
BocHN
OtBu
γ
γ
HN
HN
O2N
γ
cobalt-salen
complex 4
MeNO2 (40 equiv)
DIPEA (2.5 equiv)
N
N
H
O
HN
L-allo-enduracididine
HN
H
HO
L-enduracididine (End)
BocHN
O
OtBu
(γR)-5a
Figure 1 Chemical structures of teixobactin, L-allo-enduracididine, and
L-enduracididine
+
O
CH2Cl2 (0.1 M)
–78 °C
BocHN
OtBu
O2N
3
γ
O
HO
Our retrosynthetic analysis for 1 is illustrated in Scheme
1. The desired cyclic guanidines 1 can be obtained by guan-
idinylation followed by intramolecular N-alkylation of -hy-
droxyornithine 2, which would be prepared by asymmetric
nitroaldol reaction of 3,10 followed by reduction of the nitro
group. The catalyst-controlled asymmetric nitroaldol reac-
tion could provide both diastereomers corresponding to L-
allo-enduracididine and L-enduracididine.
(γS)-5b
S
S
H
H
N
O
N
Co
tBu
O
tBu
tBu
tBu
(S,S)-4
Entry
Cat. (mol%)
Time (h)
Product yield (%)a Ratio of 5a:5bb
1
(S,S)-4 (5)
(S,S)-4 (5)
(S,S)-4 (10)
(R,R)-4 (5)
72
18
48
24
5a (53)c
5a (88)
5a (91)
5b (92)
95:5
82:18
96:4
4:96
BocHN
OtBu
BocHN
OtBu
guanidinylation
cyclization
2d
3
H2N
HN
γ
γ
O
O
N
CbzN
HO
Cbz
(γS)-1a
(γR)-1b
(γR)-2a
(γS)-2b
4
a Isolated yield of a mixture of 5a and 5b.
b The ratio was determined by chiral HPLC analysis.
c Compound 3 was recovered in 13% yield.
d The reaction was performed at –40 °C.
asymmetric
nitroaldol reaction
BocHN
O
OtBu
O
reduction
3
With both 5a and 5b in hand, cyclic guanidine forma-
tion was performed as shown in Scheme 2. The hydrogena-
tion of the nitro group in 5 followed by the guanidinylation
of the resulting amine using Cbz-protected Goodman’s re-
agent 613 afforded acyclic guanidines 7. After short-column
chromatography, trace amount of minor diastereomer was
removed by recrystallization to isolate 7 in 77–88% yield
over two steps. Although formation of the cyclic guanidine
1 from 7 was precedented (70–79% yield for 1a6a,c,9c and 31%
yield for 1b6c), we investigated various reaction conditions
for improvement of the total yield. After several attempts, a
reported12 two-step procedure via mesylates was found to
Scheme 1 Retrosynthetic analysis of protected enduracididines 1
Our study began with the asymmetric nitroaldol reac-
tion of the aldehyde 3 readily prepared from L-aspartic ac-
id.10 Yamada’s conditions11 using commercially available co-
balt-salen complex 4, which has been adopted for the syn-
thesis of (+)-K01-0509 B and its epimer,12 were surveyed as
depicted in Table 1. We initially performed the reactions in
the presence of 5 mol% of the catalyst (S,S)-4 under various
reaction temperatures, which was found to be critical for
the yield and stereoselectivity. At –78 °C, the reaction did
© 2019. Thieme. All rights reserved. Synthesis 2019, 51, A–G

C
K. Ohsawa et al.
Paper
Synthesis
be optimal. Mesylation of 7 with MsCl/NEt3 followed by the
intramolecular SN2 reaction using DBU provided desired 1
in 79–81% over two steps. Notably, the yield for 1b dramati-
cally improved and we have established the multi-gram
scale synthesis of protected L-allo-enduracididine 1a and L-
enduracididine 1b.
tively. No migration of the allyl group on the cyclic guani-
dine moiety in both 8 and 9 was observed in the presence
of Pd catalyst.
Boc
HMBC
H
HN
OtBu
H
N
*
O
N
1) H2, 10% Pd/C
AcOH, MeOH
rt, 1 h
CbzN
Cbz
(γS)-8a (47%)
(γR)-8b (50%)
BocHN
*
OtBu
BocHN
*
OtBu
cat. Pd(PPh3)4
diallyl carbonate
O2N
HN
CbzN
(γS)-1a
(γR)-1b
+
O
O
2) 6, NEt3
1,4-dioxane/H2O
rt, 3 h
CH2Cl2
rt, 20 h
CbzHN
HO
HO
BocHN
OtBu
(γR)-5a
(γS)-5b
(γR)-7a: 77% (2 steps)
(γS)-7b: 88% (2 steps)
N
*
O
NTf
N
N
Cbz
Cbz
CbzHN
NHCbz
(γS)-9a (50%)
(γR)-9b (50%)
6
Scheme 3 Synthesis of N-allyl-protected L-allo-enduracididines 8 and 9
1) MsCl, NEt3
THF, rt, 9 h
BocHN
*
OtBu
HN
O
2) DBU, CH2Cl2
rt, 2 h
N
The allyl group in 8 and 9 introduced on the cyclic guan-
idine moiety was simply removed in the presence of a pal-
ladium catalyst by treatment with N,N-dimethylbarbituric
acid (NDMBA), which is a superior scavenger of a -allylpal-
ladium complex, preventing the resulting free guanidine
from retrapping the -allylpalladium complex.16 As sum-
marized in Table 2, deprotected 1 was obtained by optimal
conditions in 68–88% yields (Table 2, entries 1–4).
CbzN
Cbz
(γS)-1a: 79% (2 steps)
(γR)-1b: 81% (2 steps)
Scheme 2 Synthesis of protected L-allo-enduracididine 1a and
L-enduracididine 1b
Next, suitable protection of the nucleophilic nitrogen
atoms on the cyclic guanidine moiety in 1a was investigat-
ed because fully N-protected cyclic guanidine will be valu-
able to avoid undesired reaction with electrophiles.14 Dhara
et al. have reported partial removal of the Cbz groups in tri-
Cbz-protected cyclic guanidine substrate during basic hy-
drolysis and unsuccessful removal of the remaining Cbz and
COOH groups under hydrogenolysis conditions.9c It is con-
ceivable that this unexpected instability is due to poor elec-
tron density of the guanidinyl group by protection with
three electron-deficient alkoxycarbonyl groups. Given se-
lective removal in the presence of Cbz groups, we chose an
electron-donating allyl group to protect free nitrogen on
cyclic guanidine moiety in 1a. N-Allylation of 1a was per-
formed at 50 °C using allyl iodide under basic conditions
(NaH, K2CO3, and Cs2CO3). The substrate was consumed,
however, partial removal of the Cbz group in allylated prod-
ucts was observed. After several attempts, we found -allyl-
palladium chemistry enabled the allylation of 1a without
removal of the Cbz group.15 Treatment of 1a with diallyl
carbonate in the presence of a catalytic amount of Pd(PPh3)4
provided unexpectedly two regioisomers 8a and 9a, which
were separated by column chromatography in 47% and 50%
yield, respectively (Scheme 3). The structure of 8a was de-
termined by observation of the correlation between the al-
lylic carbon and two protons on the cyclic guanidine moi-
ety in the heteronuclear multiple bond correlation (HMBC)
spectrum as illustrated in Scheme 3. Allylation of 1b was
also successful, providing 8b and 9b in 50% yield, respec-
Table 2 Removal of the Allyl Group in 8 and 9
10 mol% Pd(PPh3)4
NDMBA (3.0 equiv)
1
8, 9
THF
rt, 9 h
Entry
Substrate
Product yield (%)a
1
2
3
4
(S)-8a
(S)-9a
(R)-8b
(R)-9b
(S)-1a (70)
(S)-1a (75)
(R)-1b (68)
(R)-1b (88)
a Isolated yield.
In summary, we have demonstrated the diastereoselec-
tive and scalable synthesis of suitably protected L-enduraci-
didine and L-allo-enduracididine derivatives. Stereochemis-
try at the  position of ornithine was constructed by cata-
lyst-controlled asymmetric nitroaldol reaction with
aldehyde 3, providing both diastereomers 5a and 5b in good
yields and high selectivities. During the investigation of cy-
clic guanidine formation for 7a and 7b, a two-step proce-
dure, O-mesylation followed by the intramolecular SN2 re-
action using DBU, provided both 1a and 1b. Further N-pro-
tection of the cyclic guanidines of 1 with an allyl group was
accomplished via -allylpalladium chemistry, and depro-
tection was efficiently carried out without attaching the
© 2019. Thieme. All rights reserved. Synthesis 2019, 51, A–G

D
K. Ohsawa et al.
Paper
Synthesis
Cbz group on the cyclic guanidine moiety. This allyl protec-
tion–deprotection strategy should be valuable not only to
suppress nucleophilic side reactions but also for late-stage
functionalization of the cyclic guanidine moiety toward a
SAR study. Synthesis of teixobactin and enduracididine-
containing analogues is underway.
1H NMR (400 Hz, CDCl3):  = 5.42 (br s, 1 H), 4.50–4.58 (m, 1 H), 4.45
(d, J = 5.9 Hz, 2 H), 4.25–4.29 (m, 1 H), 3.38 (br s, 1 H), 2.06–2.10 (m, 1
H), 1.86–1.93 (m, 1 H), 1.48 (s, 9 H), 1.45 (s, 9 H).
13C{1H} NMR (100 Hz, CDCl3):  = 170.9, 155.7, 82.9, 80.5, 80.2, 66.0,
51.3, 36.8, 28.3, 27.9.
HRMS-ESI: m/z [M + Na]+ calcd for C14H26N2O7Na: 357.1632; found:
357.1628.
tert-Butyl (2S,4S)-[(tert-Butoxycarbonyl)amino]-4-hydroxy-5-
nitropentanoate (5b) (Table 1, entry 4)
All commercially available reagents were purchased from commercial
suppliers and used as received. Anhyd THF and CH2Cl2 (Kanto Chemi-
cal Co.) were obtained by passing commercially available pre-dried,
oxygen-free formulations through activated alumina column. All re-
actions in the solution phase were monitored by TLC carried out on
Merck silica gel plates (0.2 mm, 60F-254) with UV light, and visual-
ized by p-anisaldehyde/H2SO4/EtOH solution, phosphomolybdic acid–
EtOH solution or ninhydrin/AcOH/BuOH solution. Column chroma-
tography was carried out with silica gel 60 N (Kanto Chemical Co.
100–210 m). 1H NMR spectra (400 and 600 MHz) and 13C NMR spec-
tra (100 and 150 MHz) were recorded on JEOL JNM-AL400 and JEOL
JNM-ECA600 spectrometers in the indicated solvent. Chemical shifts
() are reported in units parts per million (ppm) relative to the signal
for internal TMS (0.00 ppm for 1H) for solutions in CDCl3. NMR spec-
tral data are reported as follows: CHCl3 (7.26 ppm for 1H) or CDCl3
(77.0 ppm for 13C), when internal standard is not indicated. Multiplic-
ities are reported by using standard abbreviations and coupling con-
stants are given in hertz. High-resolution mass spectra were recorded
on Thermo Scientific Exactive Plus Orbitrap Mass Spectrometer (for
ESI). IR spectra were recorded on a JASCO FTIR-4100 spectrophotom-
eter. Only the strongest and/or structurally important absorption are
reported as the IR data afforded in wavenumbers (cm–1). Optical rota-
tions were measured on a JASCO P-1010 polarimeter. Melting points
were measured with Round Science Inc. RFS-10, and are not correct-
ed. SHIMAZU LC-10AT and Shodex RI-101 were used for normal-
phase chiral HPLC analysis.
By following GP-A, the nitroaldol reaction of the aldehyde 3 (1.81 g,
6.63 mmol) was performed using (R,R)-cobalt-salen complex 4 (400
mg, 0.663 mmol) for 24 h. The nitroalcohol 5b was obtained in 92%
yield (2.03 g, 6.08 mmol, dr 96:4) as a white solid; mp 102–103 °C;
[]D25 +2.9 (c 1.0, CHCl3); Rf = 0.24 (toluene/THF/EtOAc 18:1:1).
IR (neat): 3397, 2980, 1691, 1558, 1509, 1368, 1252, 1155 cm–1
.
1H NMR (400 Hz, CDCl3):  = 5.45 (br d, J = 6.8 Hz, 1 H), 4.72 (brs, 1 H),
4.32–4.53 (m, 4 H), 1.92–1.99 (m, 1 H), 1.49–1.56 (m, 1 H), 1.48 (s, 9
H), 1.46 (s, 9H ).
13C{1H} NMR (100 Hz, CDCl3):  = 170.9, 157.1, 83.0, 81.1, 79.8, 65.0,
50.5, 38.4, 28.2, 27.9.
HRMS-ESI: m/z [M + Na]+ calcd for C14H26N2O7Na: 357.1632; found:
357.1628.
Introduction of the Guanidinyl Group in the Nitroalcohols 5; Gen-
eral Procedure B (GP-B)
To a solution of nitroalcohol 5 (1.0 equiv) in anhyd MeOH (10
mL/mmol) were added AcOH (1.0 equiv) and 10% Pd/C (30 wt%) at rt
under an argon atmosphere, and the flask was purged three times
with H2. After stirring at rt for 1 h, the reaction mixture was filtered
through a pad of Celite. The filtrate was concentrated in vacuo, and
the resulting crude amine was used for the next reaction without fur-
ther purification.
To a solution of the above crude amine in 1,4-dioxane (10 mL/mmol)
and H2O (2 mL/mmol) were added NEt3 (2.1 equiv) and Goodman’s
reagent 6 (1.5 equiv) at rt under an argon atmosphere. After stirring at
the same temperature for 3 h, the reaction mixture was diluted with
EtOAc, and quenched with 10% aq citric acid. The organic layer was
separated, and the aqueous layer was extracted with EtOAc (2 ×). The
combined organic layers were washed with sat. aq NaHCO3 and brine,
dried (MgSO4), and filtered. The filtrate was concentrated in vacuo,
and the resulting residue was passed through a short pad of silica gel
(eluent: hexane/EtOAc 2:1). The resulting product was recrystallized
from CH2Cl2/hexane to remove small amount of minor diastereomer
to afford guanidines 7 as a white solid.
Asymmetric Nitroaldol Reaction of the Aldehyde 3; General Proce-
dure A (GP-A)
To a solution of the aldehyde 310 (1.0 equiv) in anhyd CH2Cl2 (2.5
mL/mmol) were added MeNO2 (40 equiv) and cobalt-salen complex 4
(0.10 equiv) at rt under an argon atmosphere. The mixture was cooled
to –78 °C, and N,N-diisopropylethylamine (DIPEA) (2.5 equiv) was
added to the mixture dropwise. After stirring at the same tempera-
ture, the reaction mixture was quenched with sat. aq NH4Cl. The or-
ganic layer was separated, and the aqueous layer was extracted with
CH2Cl2. The combined organic layers were washed with brine, dried
(MgSO4), and filtered. The filtrate was concentrated in vacuo, and the
resulting residue was purified by column chromatography on silica
gel (eluent: hexane/EtOAc 3:1) to afford nitro alcohols 5. The diaste-
reomeric ratio of 5 was determined by chiral HPLC analysis [column:
AD-H; elution rate: hexane/iPrOH = 9:1 (isocratic); flow rate: 1.0
mL/min; retention time: 13.5 min for 5a, 20.3 min for 5b].
tert-Butyl (2S,4R)-5-{(E)-2,3-Bis[(benzyloxy)carbonyl]guanidino}-
2-[(tert-butoxycarbonyl)amino]-4-hydroxypentanoate (7a)
By following GP-B, the introduction of the guanidinyl group in the ni-
troalcohol 5a (4.62 g, 13.8 mmol) was performed. The guanidine 7a
was obtained in 77% yield over 2 steps (6.50 g, 10.6 mmol) as a white
solid; mp 125–126 °C; []D25 +11 (c 1.0, CHCl3); Rf = 0.56 (toluene/EtOAc
3:1).
tert-Butyl (2S,4R)-[(tert-Butoxycarbonyl)amino]-4-hydroxy-5-
nitropentanoate (5a) (Table 1, entry 3)
By following GP-A, the nitroaldol reaction of the aldehyde 3 (5.00 g,
18.3 mmol) was performed using (S,S)-cobalt-salen complex 4 (1.10 g,
1.83 mmol) for 48 h. The nitroalcohol 5a was obtained in 91% yield
IR (neat): 3338, 2977, 1732, 1643, 1570, 1455, 1367, 1215, 1152 cm–1
.
1H NMR (600 Hz, CDCl3):  = 11.7 (s, 1 H), 8.69 (t, J = 5.2 Hz, 1 H),
7.26–7.39 (m, 10 H), 5.40 (br d, J = 5.8 Hz, 1 H), 5.19 (s, 2 H), 5.10 (s, 2
H), 4.54 (br s, 1 H), 4.21–4.22 (m, 1 H), 3.95 (br s, 1 H), 3.62–3.65 (m, 1
H), 3.38–3.43 (m, 1 H), 1.95–1.98 (m, 1 H), 1.79–1.84 (m, 1 H), 1.43 (s,
18 H).
25
(6.11 g, 18.2 mmol, dr 96:4) as a white solid; mp 129–130 °C; []D
–26 (c 1.0, CHCl3); Rf = 0.18 (toluene/THF/EtOAc 18:1:1).
IR (neat): 3423, 2979, 2934, 1689, 1556, 1507, 1368, 1253, 1156 cm–1
.
© 2019. Thieme. All rights reserved. Synthesis 2019, 51, A–G

E
K. Ohsawa et al.
Paper
Synthesis
13C{1H} NMR (150 Hz, CDCl3):  = 171.9, 163.2, 157.0, 155.7, 153.6,
136.5, 134.5, 128.8, 128.7, 128.5, 128.4, 128.0, 127.9, 82.3, 80.0, 68.7,
68.3, 67.1, 52.1, 47.2, 37.8, 28.3, 27.9.
HRMS-ESI: m/z [M + H]+ calcd for C31H41N4O8: 597.2919; found:
597.2912.
HRMS-ESI: m/z [M + H]+ calcd for C31H43N4O9: 615.3025; found:
Boc-End(Cbz)2-OtBu (1b)
615.3015.
By following GP-C, the cyclic guanidine formation for the acyclic
guanidine 7b (3.39 g, 4.67 mmol) was performed. The cyclic guani-
dine 1b was obtained in 81% yield over 2 steps (2.25 g, 3.78 mmol) as
a yellowish oil; []D25 +7.4 (c 1.0, CHCl3); Rf = 0.48 (hexane/EtOAc 1:1).
tert-Butyl (2S,4S)-5-{(E)-2,3-Bis[(benzyloxy)carbonyl]guanidino}-
2-[(tert-butoxycarbonyl)amino]-4-hydroxypentanoate (7b)
By following GP-B, the introduction of the guanidinyl group for the
nitroalcohol 5b (1.56 g, 4.67 mmol) was performed. The guanidine 7b
was obtained in 88% yield over 2 steps (2.52 g, 4.11 mmol) as a white
solid; mp 124–125 °C; []D25 +10 (c 1.0, CHCl3); Rf = 0.58 (toluene/EtOAc
3:1).
IR (neat): 3345, 2973, 1712, 1654, 1616, 1368, 1307, 1257, 1153 cm–1
.
1H NMR (600 Hz, CDCl3):  = 8.65 (br s, 1 H), 7.28–7.45 (m, 10 H),
5.24–5.27 (m, 2 H), 5.07–5.21 (m, 3 H), 4.37 (br s, 1 H), 4.13–4.16 (m,
1 H), 3.80 (br s, 1 H), 3.55 (br s, 1 H), 1.98 (br s, 2 H), 1.44 (s, 9 H), 1.42
(s, 9 H).
13C{1H} NMR (150 Hz, CDCl3):  = 170.6, 155.8, 136.7, 135.1, 128.6,
128.3, 128.0, 127.9, 82.9, 80.2, 68.3, 67.4, 53.5, 50.6, 45.0, 36.7, 28.2,
27.9.
IR (neat): 3339, 2977, 1732, 1644, 1570, 1498, 1455, 1276, 1051 cm–1
.
1H NMR (600 Hz, CDCl3):  = 11.7 (s, 1 H), 8.73 (br s, 1 H), 7.27–7.39
(m, 10 H), 5.42 (br d, J = 7.9 Hz, 1 H), 5.19 (s, 2 H), 5.11 (s, 2 H), 4.73
(br d, J = 2.8 Hz, 1 H), 4.37 (t, J = 8.3 Hz, 1 H), 3.72–3.78 (m, 2 H), 3.25–
3.30 (m, 1 H), 1.86–1.90 (m, 1 H), 1.48–1.56 (m, 1 H), 1.45 (s, 9 H),
1.44 (s, 9 H).
HRMS-ESI: m/z [M + H]+ calcd for C31H41N4O8: 597.2919; found:
597.2910.
13C{1H} NMR (150 Hz, CDCl3):  = 171.5, 163.5, 156.9, 156.3, 153.6,
136.7, 134.6, 128.8, 128.7, 128.5, 128.4, 128.1, 127.9, 82.6, 80.6, 68.2,
67.1, 66.2, 50.9, 46.3, 39.2, 28.3, 28.0.
HRMS-ESI: m/z [M + H]+ calcd for C31H43N4O9: 615.3025; found:
615.3018.
N-Allylation of the Cyclic Guanidines 1; General Procedure D (GP-
D)
To a solution of guanidine 1 (1.0 equiv) in anhyd CH2Cl2 (20
mL/mmol) were added diallyl carbonate (1.2 equiv) and Pd(PPh3)4
(0.10 equiv) at rt under an argon atmosphere. After stirring at the
same temperature for 20 h, the reaction mixture was quenched with
sat. aq NH4Cl. The organic layer was separated, and the aqueous layer
was extracted with CH2Cl2. The combined organic layers were washed
with brine, dried (MgSO4), and filtered. The filtrate was concentrated
in vacuo, and the resulting residue was purified by column chroma-
tography on silica gel (eluent: hexane/EtOAc 3:2 to 1:1) to afford the
N-allylated guanidines 8 and 9.
Cyclic Guanidine Formation from 7; General Procedure C (GP-C)
To a solution of alcohol 7 (1.0 equiv) in anhyd THF (12 mL/mmol)
were added NEt3 (2.5 equiv) and MsCl (2.5 equiv) at 0 °C under an ar-
gon atmosphere. After stirring at rt for 9 h, the reaction mixture was
quenched with 1 M aq HCl. The organic layer was separated, and the
aqueous layer was extracted with EtOAc (2 ×). The combined organic
layers were washed with brine, dried (MgSO4), and filtered. The fil-
trate was concentrated in vacuo, and the resulting crude mesylate
was used in the next reaction without further purification.
N-Allylated allo-Enduracididines 8a and 9a
By following GP-D, the N-allylation of the cyclic guanidine 1a (300
mg, 500 mol) was performed. The N-allylated guanidines 8a and 9a
were obtained in 47% yield (150 mg, 235 mol) as a colorless oil and
50% yield (159 mg, 250 mol) as a colorless oil, respectively.
To a solution of the crude mesylate in anhyd CH2Cl2 (12 mL/mmol)
was added DBU (2.5 equiv) at 0 °C under an argon atmosphere. After
stirring at rt for 2 h, the reaction mixture was quenched with 1 M aq
HCl. The organic layer was separated, and the aqueous layer was ex-
tracted with EtOAc (2 ×). The combined organic layers were washed
with brine, dried (MgSO4), and filtered. The filtrate was concentrated
in vacuo and the resulting residue was purified by column chroma-
tography on silica gel (eluted with hexane/EtOAc 2:1) to afford the cy-
clic guanidines 1.
8a
[]D25 +4.3 (c 1.0, CHCl3); Rf = 0.42 (hexane/EtOAc 1:1).
IR (neat): 3349, 2977, 1716, 1620, 1392, 1366, 1262, 1153, 1024 cm–1
.
1H NMR (600 Hz, CDCl3):  = 7.28–7.37 (m, 10 H), 5.73–5.78 (m, 1 H),
5.31 (br d, J = 7.1 Hz, 1 H), 5.25–5.27 (m, 1 H), 5.23 (br s, 1 H), 5.18 (d,
J = 12.2 Hz, 1 H), 5.02 (d, J = 12.4 Hz, 1 H), 4.99 (d, J = 12.2 Hz, 1 H),
4.94 (d, J = 12.4 Hz, 1 H), 4.50 (br s, 1 H), 4.06 (q, J = 7.1 Hz, 1 H), 4.00
(dd, J = 15.2, 6.9 Hz, 1 H), 3.96 (dd, J = 15.2, 6.4 Hz, 1 H), 3.59 (dd, J =
10.3, 8.2 Hz, 1 H), 3.30 (d, J = 10.3 Hz, 1 H), 2.25–2.30 (m, 1 H), 1.96
(dt, J = 1.57, 7.1 Hz, 1 H), 1.42 (s, 18 H).
13C{1H} NMR (150 Hz, CDCl3):  = 170.5, 160.0, 155.2, 151.5, 137.0,
135.0, 131.3, 128.6, 128.52, 128.47, 128.3, 128.2, 127.7, 119.6, 82.6,
80.0, 68.5, 67.2, 54.1, 51.4, 47.7, 47.6, 37.0, 28.3, 27.9.
Boc-allo-End(Cbz)2-OtBu (1a)
By following GP-C, the cyclic guanidine formation for the acyclic
guanidine 7a (3.00 g, 4.88 mmol) was performed. The cyclic guani-
dine 1a was obtained in 79% yield over 2 steps (2.30 g, 3.85 mmol) as
25
a yellowish oil; []D +26.7 (c 1.0, CHCl3); Rf = 0.46 (hexane/EtOAc
1:1).
IR (neat): 3342, 2977, 1713, 1654, 1616, 1367, 1306, 1256, 1152 cm–1
.
1H NMR (600 Hz, CDCl3):  = 8.63 (br s, 1 H), 7.29–7.46 (m, 10 H),
5.15–5.28 (m, 5 H), 4.56 (br s, 1 H), 4.10–4.14 (m, 1 H), 3.76 (br s, 1 H),
3.60 (br s, 1 H), 2.27 (br s, 1 H), 1.90 (br s, 1 H), 1.44 (s, 9 H), 1.43 (s, 9
H).
13C{1H} NMR (150 Hz, CDCl3):  = 170.5, 164.7, 159.6, 155.1, 150.7,
136.8, 135.2, 128.7, 128.6, 128.3, 128.02, 127.98, 127.93, 127.8, 127.7,
82.9, 80.2, 68.4, 67.3, 53.9, 51.5, 45.2, 37.0, 28.3, 27.9.
HRMS-ESI: m/z [M + Na]+ calcd for C34H44N4O8Na: 659.3051; found:
659.3045.
9a
[]D25 +37.1 (c 1.0, CHCl3); Rf = 0.65 (hexane/EtOAc 1:1).
IR (neat): 3360, 2977, 1731, 1637, 1394, 1354, 1270, 1153, 1021 cm–1
.
© 2019. Thieme. All rights reserved. Synthesis 2019, 51, A–G

F
K. Ohsawa et al.
Paper
Synthesis
1H NMR (600 Hz, CDCl3):  = 7.31–7.37 (m, 8 H), 7.20 (br s, 2 H), 5.83–
5.87 (m, 1 H), 5.22 (d, J = 16.9 Hz, 1 H), 5.10 (d, J = 10.3 Hz, 1 H), 4.96–
5.02 (m, 4 H), 4.40 (br s, 1 H), 4.25 (br s, 2 H), 4.03 (br s, 1 H), 3.75 (br
s, 1 H), 3.63–3.66 (m, 1 H), 1.58 (br s, 2 H), 1.44 (s, 9 H), 1.43 (s, 9 H).
13C{1H} NMR (150 Hz, CDCl3):  = 170.9, 155.0, 153.6, 151.1, 150.3,
135.2, 132.9, 128.8, 128.6, 128.5, 128.4, 117.8, 82.3, 79.9, 68.3, 67.8,
56.6, 55.2, 51.5, 51.3, 36.0, 28.3, 27.9.
Boc-allo-End(Cbz)2-OtBu (1a)
By following GP-E, the removal of the allyl group in 8a (50.0 mg, 78.5
mol) and 9a (50.0 mg, 78.5 mol) was performed. The deprotected
guanidines 1a were obtained in 70% yield (33.0 mg, 55.3 mol, from
8a) and 75% yield (35.0 mg, 58.7 mol, from 9a), respectively.
Boc-End(Cbz)2-OtBu (1b)
HRMS-ESI: m/z [M + Na]+ calcd for C34H44N4O8Na: 659.3051; found:
659.3046.
By following GP-E, the removal of the allyl group in 8b (25.0 mg, 39.2
mol) and 9b (50.0 mg, 78.5 μmol) was performed. The deprotected
guanidines 1b were obtained in 68% yield (15.5 mg, 26.0 mmol, from
8b) and 88% yield (41.0 mg, 68.7 mol, from 9b), respectively.
N-Allylated Enduracididines 8b and 9b
By following GP-D, the N-allylation of the cyclic guanidine 1b (200
mg, 340 mol) was performed. The N-allylated guanidines 8b and 9b
were obtained in 50% yield (110 mg, 170 mol) as a colorless oil and
50% yield (110 mg, 170 μmol) as a colorless oil, respectively.
Funding Information
This work was supported by JSPS KAKENHI, Grant no. JP15H05837
(Grant-in-Aid for Scientific Research on Innovative Areas: Middle Mo-
lecular Strategy). This work was partially supported by the Platform
Project for Supporting in Drug Discovery and Life Science Research
from AMED under Grant Number JP19am0101095 and
JP19am0101100. Carys Thomas thanks the Royal Society of Chemis-
8b
[]D25 +5.7 (c 1.0, CHCl3); Rf = 0.16 (hexane/EtOAc 3:1).
IR (neat): 2978, 1738, 1718, 1393, 1366, 1253, 1153 cm–1
.
1H NMR (600 Hz, CDCl3):  = 7.24–7.36 (m, 10 H), 5.72–5.77 (m, 1 H),
5.23–5.26 (m, 2 H), 5.15 (d, J = 12.0 Hz, 1 H), 5.10 (br d, J = 8.4 Hz, 1 H),
5.06–5.10 (m, 2 H), 4.97 (d, J = 12.4 Hz, 1 H), 4.35 (t, J = 8.6 Hz, 1 H),
4.07–4.12 (m, 2 H), 3.90 (dd, J = 15.1, 6.2 Hz, 1 H), 3.63 (dd, J = 10.6,
8.6 Hz, 1 H), 3.26 (d, J = 10.6 Hz, 1 H), 1.99–2.06 (m, 2 H), 1.45 (s, 9 H),
1.40 (s, 9 H).
13C{1H} NMR (150 Hz, CDCl3):  = 170.5, 159.9, 155.8, 151.2, 151.1,
137.0, 135.0, 131.3, 128.6, 128.5, 128.24, 128.19, 127.6, 119.4, 82.8,
80.1, 68.4, 67.3, 53.5, 50.7, 47.5, 47.4, 37.3, 28.2, 27.9.
try for a Researcher Mobility Grant.
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Supporting information for this article is available online at
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References
HRMS-ESI: m/z [M + H]+ calcd for C34H45N4O8: 637.3232; found:
637.3231.
(1) Atkinson, D. J.; Naysmith, B. J.; Furkert, D. P.; Brimble, M. A. Beil-
stein J. Org. Chem. 2016, 12, 2325.
(2) Horii, S.; Kameda, Y. J. Antibiot. 1968, 21, 665.
9b
[]D25 –35 (c 1.0, CHCl3); Rf = 0.21 (hexane/EtOAc 3:1).
IR (neat): 2977, 1738, 1717, 1637, 1394, 1155 cm–1
(3) He, H.; Williamson, R. T.; Shen, B.; Graziani, E. I.; Yang, H. Y.;
Sakya, S. M.; Petersen, P. J.; Carter, G. T. J. Am. Chem. Soc. 2002,
124, 9729.
(4) Ling, L. L.; Schneider, T.; Peoples, A. J.; Spoering, A. L.; Engels, I.;
Conlon, B. P.; Mueller, A.; Schäberle, T. F.; Hughes, D. E.; Epstein,
S.; Jones, M.; Lazarides, L.; Steadman, V. A.; Cohen, D. R.; Felix, C.
R.; Fetterman, K. A.; Millett, W. P.; Nitti, A. G.; Zullo, A. M.; Chen,
C.; Lewis, K. Nature 2015, 517, 455.
.
1H NMR (600 Hz, CDCl3):  = 7.19–7.38 (m, 10 H), 5.87 (br s, 1 H), 5.24
(d, J = 16.1 Hz, 1 H), 5.00–5.12 (m, 6 H), 4.28 (br s, 3 H), 4.07 (br s, 1 H),
3.81 (br s, 1 H), 3.64 (d, J = 14.5 Hz, 1 H), 1.76 (br s, 2 H), 1.42 (s, 9 H),
1.41 (s, 9 H).
13C{1H} NMR (150 Hz, CDCl3):  = 171.1, 155.7, 153.7, 150.1, 135.2,
132.9, 128.7, 128.6, 128.4, 128.1, 117.8, 82.3, 79.9, 68.4, 67.8, 56.2,
55.2, 51.7, 50.6, 36.2, 28.3, 27.9.
(5) Homma, T.; Nuxoll, A.; Gandt, A. B.; Ebner, P.; Engels, I.;
Schneider, T.; Götz, F.; Lewis, K.; Conlon, B. P. Antimicrob. Agents
Chemother. 2016, 60, 6510.
HRMS-ESI: m/z [M + H]+ calcd for C34H45N4O8: 637.3232; found:
(6) (a) Giltrap, A. M.; Dowman, L. J.; Nagalingam, G.; Ochoa, J. L.;
Linington, R. G.; Britton, W. J.; Payne, R. J. Org. Lett. 2016, 18,
2788. (b) Jin, K.; Sam, I. H.; Po, K. H. L.; Lin, D.; Ghazvin, Z.;
Ebrahim, H.; Chen, S.; Yuan, Y.; Li, X. Nat. Commun. 2016, 7,
12394. (c) Liu, L.; Wu, S.; Wang, Q.; Zhang, M.; Wang, B.; He, G.;
Chen, G. Org. Chem. Front. 2018, 5, 1431. (d) Gao, B.; Chen, S.;
Hou, Y. N.; Zhao, Y. J.; Ye, T.; Xu, Z. Org. Biomol. Chem. 2019, 17,
1141. (e) Zong, Y.; Fang, F.; Meyer, K. J.; Wang, L.; Ni, Z.; Gao, H.;
Lewis, K.; Zhang, J.; Rao, Y. Nat. Commun. 2019, 10, 3268.
(7) (a) Abdel Monaim, S. A. H.; Jad, Y. E.; El-Faham, A.; de la Torre, B.
G.; Albericio, F. Bioorg. Med. Chem. 2018, 26, 2788. (b) Guo, C.;
Mandalapu, D.; Ji, X.; Gao, J.; Zhang, Q. Chem. Eur. J. 2018, 24,
5406; and references cited therein.
637.3228.
Removal of the Allyl group in 8 and 9; General Procedure E (GP-E)
To a solution of N-allylguanidine 8 and 9 (1.0 equiv), respectively, in
anhyd THF (12 mL/mmol) were added NDMBA (3.0 equiv) and
Pd(PPh3)4 (0.10 equiv) at 0 °C under an argon atmosphere. After stir-
ring at rt for 9 h, the reaction mixture was diluted with EtOAc and
quenched with sat. aq NaHCO3. The organic layer was separated, and
the aqueous layer was extracted with EtOAc (2 ×). The combined or-
ganic layers were washed with brine, dried (MgSO4), and filtered. The
filtrate was concentrated in vacuo, and the resulting residue was puri-
fied by column chromatography on silica gel (eluent: hexane/EtOAc
3:1) to afford the guanidines 1 as a yellowish oil.
(8) (a) Jin, K.; Po, K. H. L.; Kong, W. Y.; Lo, C. H.; Lo, C. W.; Lam, H. Y.;
Sirinimal, A.; Reuven, J. A.; Chen, S.; Li, X. Bioorg. Med. Chem.
2018, 26, 1062. (b) Parmar, A.; Iyer, A.; Prior, S. H.; Lloyd, D. G.;
© 2019. Thieme. All rights reserved. Synthesis 2019, 51, A–G

G
K. Ohsawa et al.
Paper
Synthesis
Leng Goh, E. T.; Vincent, C. S.; Palmai-Pallag, T.; Bachrati, C. Z.;
Breukink, E.; Madder, A.; Lakshminarayanan, R.; Taylor, E. J.;
Singh, I. Chem. Sci. 2017, 8, 8183.
(13) Feichtinger, K.; Zapf, C.; Sings, H. L.; Goodman, M. J. Org. Chem.
1998, 63, 3804.
(14) Possibility of the reaction of the di-Cbz-protected guanidine with
Fmoc-OSu has been mentioned by Giltrap; Giltrap, A., Ed.; Total
Synthesis of Natural Products with Antimicrobial Activity,
Springer Theses Recognizing Outstanding Ph.D. Research;
Springer: Heidelberg, 2018, Chap. 2.
(9) (a) Tsuji, S.; Kusumoto, S.; Shiba, T. Chem. Lett. 1975, 1281.
(b) Craig, W.; Chen, J.; Richardson, D.; Thorpe, R.; Yuan, Y. Org.
Lett. 2015, 17, 4620. (c) Dhara, S.; Gunjal, V. B.; Handore, K. L.;
Reddy, D. S. Eur. J. Org. Chem. 2016, 4289.
(10) Detail of the synthesis of the aldehyde 3 is described in the Sup-
porting Information.
(15) Miyabe, H.; Yoshida, K.; Reddy, V. K.; Takemoto, Y. J. Org. Chem.
2009, 74, 305.
(11) (a) Kogami, Y.; Nakajima, T.; Ashizawa, T.; Kezuka, S.; Ikeno, T.;
Yamada, T. Chem. Lett. 2004, 33, 614. (b) Kogami, Y.; Nakajima,
T.; Ikeno, T.; Yamada, T. Synthesis 2004, 1947.
(16) (a) Gomez-Martinez, P.; Dessolin, F.; Guibé, M.; Albericio, F.
J. Chem. Soc., Perkin Trans. 1 1999, 2871. (b) Kunz, H.; März, J.
Angew. Chem., Int. Ed. Engl. 1988, 27, 1375.
(12) Tsuchiya, S.; Sunazuka, T.; Hirose, T.; Mori, R.; Tanaka, T.;
Iwatsuki, M.; Ōmura, S. Org. Lett. 2006, 8, 5577.
© 2019. Thieme. All rights reserved. Synthesis 2019, 51, A–G