The first total synthesis of gobichelin B: A mixed-ligand siderophore of: Streptomyces sp. NRRL F-4415

Article abstract of DOI:10.1039/c8ob00263k

The first total synthesis of a structurally diverse mixed ligand siderophore, gobichelin B produced by Streptomyces sp. NRRL F-4415, is reported. The systematic assembly of the building blocks to synthesize Gob-B 1^st half and Gob-B 2^nd half and subsequent coupling of these two fragments followed by global deprotection using Pearlman's catalyst led to the isolation of gobichelin B in excellent yield and purity.

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The First Total Synthesis of Gobichelin B: A Mixed-ligand
Siderophore of Streptomyces sp. NRRL F-4415
Received 00th January 20xx,
Accepted 00th January 20xx
Perali Ramu Sridhar,* Bhumireddy Chinnachennaiahgari Venkatesh, Shaik Kalesha and Chalapala
Sudharani
DOI: 10.1039/x0xx00000x
The first total synthesis of structurally diverse mixed ligand siderophore, Gobichelin B produced by Streptomyces sp. NRRL
F-4415 is reported. The systematic assembly of the building blocks to synthesize gob-B 1st half and gob-B 2nd half and
subsequent coupling of these two fragments followed by global deprotection using Pearlman's catalyst led to the isolation
of gobichelin B in excellent yield and purity.
www.rsc.org/
chelation therapy,10 treatment for iron poisoning and
thalassemia.11 Substantial work has been done on mycobactins
to understand and develop the novel antibiotics for the world
threat disease, tuberculosis.12Also, Miller’s13-15 and
Raymond’s16-19 pioneered work in the understanding of
siderophore-mediated iron transport in microbes led to the
discovery of innovative antibiotics and novel iron chelators.
Introduction
Iron is a vital trace element for almost all the life processes
of living systems.1 However, its bioavailability is very low as
most of the iron exists in the Fe+3 state in earth’s crust. The
solubility of Fe+3 is very low, ~10-18 M in water of biological pH,
whereas >1 µM of ferric ion concentration is required for the
growth of microbes.2 This scarcity of soluble iron enforces the
microbes to excrete siderophores, iron (Fe3+) chelating
molecules to acquire iron from the host.3 Iron acquisition is
one of the major challenges to the pathogenic bacteria during
infection and post-infection.4 Although hosts utilize effective
Iron-transport systems such as iron-loaded lactoferrin,
transferrin and heme proteins the bacterial siderophores have
much higher affinity towards iron than the host proteins. The
bacterial siderophores called “stealth siderophores” even can
strip iron from host iron-binding proteins and restore the
access to iron for bacteria.5 The battle for this coveted
transition metal at host and pathogen interface is a very
important task for the survival during infections. Apart from
this, recent studies revealed that siderophores play a major
role in bacterial virulence by acting as signals leading to a
robust host defence via inducing cytokine production, hypoxic
responses and mitophagy (a selective process where cells
degrade mitochondria). Bacterial siderophores are necessary
components in biofilm formation (extracellular polysaccharide
synthesis) which promotes the antibiotic resistance to the
organism.6,7 Thus, blocking of siderophore synthesis or its
Figure 1. Structures of Enterobactin, Desferrioxamine B and
Micobactins.
Typically, siderophores bind with ferric ions with three
bidentate ligand groups to form
a stable hexadentate,
octahedral (or distorted octahedral) complexes. Though
several types of siderophores are known to date, the
constitutional iron binding functional groups are almost similar
to hydroxamic acids, catechols, salicylate, and aryloxazoline
units.20
function is
a promising alternative approach, even for
multidrug-resistant, for the development of a new class of
antibacterial drugs.8 Fe+3-siderophore complexes are usually
taken up into bacterial cells by siderophore specific
extracellular receptors.9 Because of the strong iron binding
properties of siderophores viz., enterobactin, desferrioxamine
Recently, Kelleher et al., discovered two new mixed ligand
siderophores from Streptomyces sp. NRRL F-4415 using
proteomics-based approach “Proteomic Investigation of
B, mycobactins etc., (Figure 1), they have been using in metal
School of Chemistry, University of Hyderabad, Gachibowli, Hyderabad – 500 046,
INDIA
*Corresponding Author: p_ramu_sridhar@uohyd.ac.in
13
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J. Name., 2013, 00, 1-3 | 1
Electronic Supplementary Information (ESI) available: H, C, DEPT135 and HRMS.
COSY and HSQC NMR for compounds 25 and gobichelin B. See
DOI: 10.1039/x0xx00000x

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Secondary Metabolism (PrISM)” and named as Gobichelin A & lead to the three units, O-benzyl salicylic acid
B (Figure and His(Bzl)-OH . A similar type of disconnection of
).21 Distinct from most of the siderophores, serine
gobichelins possess one hydroxamate, one salicylate also leads to D-Boc-Lys(Z)-OH , O-benzyl serine and cyclic
oxazoline unit and two basic amine groups (imidazole of ornithine derivative as the required amino acid residues
3, O-benzyl
2
4
5
/
2
6
4
7
histidine unit and a terminal amine of lysine unit) which make
them interesting in iron chelation. Structurally, gobichelin A &
B slightly resembles with oxachelin22 and amychelin.23 The
(
Scheme 1).
The synthesis of Gob-B 1st half (
1) was planned to initiate
structure of gobichelin A (Gob-A) has fully elucidated by by procuring the required protected synthons. O-benzyl
spectroscopic techniques whereas the supporting data is not salicylic acid 324 and O-benzyl serine 425 have been synthesized
available for gobichelin B (Gob-B). The polar nature and the by following the literature procedures. However, our initial
combination of three different iron chelating units involving attempts to synthesize His(Bzl)-OH
5 by following the reported
hydroxamate, salicylate groups as well as infrequent basic method using Boc-His(Boc)-OMe under triflic anhydride
amino acid residues - histidine and D-lysine, serve Gob-B as an conditions26 was not successful in our hands. Therefore, an
inimitable mixed ligand iron-chelator. On the other hand, alternative protocol was designed starting from histidine.
biological profile of gobichelins is also not fully established, Thus, Boc-His-OMe
which may be due to the distinctive minute quantities, overall yield over 2 steps.27 Ester
difficult-to-reach, and often fleeting environments makes their benzyl bromide in the presence of anhydrous K2CO3 in DMF
total synthesis efficacious. in 66%
afforded Nα-boc-N1(im)-benzyl histidine methyl ester
yield and 15% of the regioisomeric Nα-boc-N3(im)-benzyl
histidine methyl ester 9a. Subsequent treatment of with
TMSCl in methanol at room temperature produced His(Bzl)-
OMe hydrochloride 10 in 95% yield (Scheme ).
8
was prepared from histidine in 80%
8
on further reaction with
9
9
2
Figure 2. Structure of Gobichelin A & B.
Scheme 2. Synthesis of His(Bzl)-OMe hydrochloride 10
.
Results and Discussion
In our retro synthetic approach, the disconnection was
Having all the required synthons for the construction of
Gob-B 1st half, we proceeded to couple the three fragments
using the N-hydroxysuccinimide (NHS) activation protocol.
made at 3rd peptide bond that leads to two fragments, Gob-B
1st half ( ) and Gob-B 2nd half (
1 2). Further disconnection of 1
Thus, activation of O-benzyl salicylic acid (
followed by addition of O-benzyl serine
3
) using DCC/NHS,
afforded N-((2-
4
benzyloxy)-benzoyl)-O-benzyl serine 11 in 85% yield. Acid 11
on further coupling with 10 produced Gob-B 1st half methyl
ester 12 as a mixture of rotamers.28 Subjecting 12 to LiOH
mediated ester hydrolysis afforded the gob-B 1st half (
(Scheme ).
1)
3
Scheme 3. Synthesis of Gob-B 1st half
1.
Scheme 1. Retrosynthetic approach of gobichelin B
2 | J. Name., 2012, 00, 1-3
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Considering the outlay of the D-lysine, initially, we decided HOBt/EDC and DMAP in dichloromethane proved to be the
to optimize the reaction conditions using L-lysine. Thus, in situ best conditions to achieve the pentapeptide 19 (Scheme ).
activation of L-Boc-Lys(Z)-OH (13) with DCC-NHS followed by Hence, equimolar amounts of and 18 were coupled under
addition of O-benzyl serine ( ) produced lys-ser dipeptide 15 in the above-said conditions. After the disappearance of the
5
1
4
good yield. Subsequent coupling of 15 with cyclic ornithine starting materials and normal workup, the crude compound
derivative 1629 afforded boc protected tripeptide 17 in 75% was directly purified by preparative HPLC using C18 reverse
yield.
A brief treatment of this tripeptide 17 with phase column eluting with 65% acetonitrile/water (isocratic)
trifluoroacetic acid in dichloromethane resulted in Gob-B 2nd under non-acidic buffer conditions to get the protected epi-
Half like peptide (epimeric at lysine) 18 (Scheme
4
).
gobichelin B (19) in 60% yield. Besides the NMR spectra, in
HRMS spectrum the base peak at m/z 1274.5926
corresponding to M+H of C73H80N9O12 (calculated for
1274.5926) confirmed the fully protected pentapeptide 19
.
The global deprotection of five benzyls and one Cbz group
was planned to intend by employing hydrogenolysis conditions
using Pd/C under hydrogen atmosphere. Regrettably, this
method didn’t work to afford the final compound, as the
compound underwent partial degradation to form a mixture of
products. To our fortunate, when the global deprotection was
attempted using Pearlman’s catalyst proved to be worthy of
achieving the transformation. Hence, a suspension of an
equimolar amount of Pearlman’s catalyst and compound 19
was stirred in anhydrous methanol under hydrogen
atmosphere (1 atm) at room temperature afforded epi-
gobichelin B 20 in 80% yield (Scheme 5). In the NMR spectra,
Scheme 4. Synthesis of epi-Gob-B 2nd half tripeptide 18
.
appearance of six amide bond peaks at δ 173.6, 172.3, 172.2,
171.7, 171.0, 169.7 ppm and phenolic carbon (C-OH) peak at
157.0 ppm, in addition to the presence of appropriate peaks in
1H and 13C NMR spectra, confirms the structure of epi-
gobichelin B. Additionally, this was supported by HRMS
spectrum of 20 showing base peak at m/z 690.3216 pertaining
to M+H of C30H44N9O10 (calc. 690.3211).
Having both of the required tri-peptide units 1 and 18 in
hand, we further proceeded to couple the two halves to obtain
the fully protected epi-gobichelin B. To accomplish the
coupling reaction of
1 and 18, various coupling reagents like
HOBt, EDC, DMAP, BOP, PyBOP, and DIPEA were chosen to
screen in varying combinations. Finally, the combination of
After accomplishing the synthesis of epi-gobichelin B (20),
in which L-lysine was placed instead of the D-lysine unit, we
applied the same protocol for the total synthesis of natural
siderophore gobichelin B. Thus, D-lysine containing tripeptide
2
(the Gob-B 2nd half) was synthesized by NHS-DCC activation
of D-Boc-Lys(Z)-OH (21), to give 22, followed by addition of O-
benzyl serine to afford D-lys-L-ser dipeptide 23 in excellent
yield. Further coupling with 16, to give compound 24, followed
4
by Boc deprotection produced gob-B 2nd half (
(Scheme ).
2
) in good yield
6
Scheme 6. Synthesis of gob-B 2nd half
2.
Similar to the synthesis of epi-gobichelin B (20), gob-B 1st
Scheme 5. Synthesis of epi-Gobichelin B (20).
half (
1
) and Gob-B 2nd half (
2) were coupled in the presence of
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HOBt-EDC and DMAP in DCM afforded the fully protected synthesis of congeners of gobichelin A and B. Amputation of
gobichelin 25 Finally, the global deprotection using five benzyls and -Cbz groups was achieved in an exceptional
Pearlman’s catalyst allowed us to achieve the first total way using Pearlman’s catalyst under hydrogen atmosphere to
B
.
synthesis of natural gobichelin B (Scheme
7).
get the final compound in excellent yield and high purity. The
biological importance of the synthesized intermediates and
siderophores and their iron chelating properties are in
progress.
Experimental Section
General Information
Chemicals and solvents were purchased from the local
suppliers and Sigma-Aldrich®, Carbosynth chemical company.
Solvents were used in the reactions after distilled over the
dehydrating agents. All the reactions were carried out under
appropriate conditions and monitored by the thin layer
chromatography (TLC) using silica gel on aluminum plates
(GF254) by charring with 3% (w/v) ninhydrin in n-butanol or by
3% (w/v) phosphomolybdic acid (PMA) stain or by ultra violet
(UV) detection. Silica-gel (100-200 or 230-400 mesh) was used
for column chromatography to purify the compounds. Optical
rotation was measured on Rudolph Autopol VI instrument.
HPLC purifications were carried out on Shimadzu 2100 Series
preparative HPLC system equipped with a UV detector (180-
Scheme 7. Final steps towards the synthesis of Gobichelin B.
1
700 nm). H, 13C, DEPT spectra were recorded on Bruker® 400
and 500 Avance MHz spectrometer in duterated solvents
(CDCl3/ DMSO-d6 / D2O). 1H NMR chemical shifts were
reported in parts per million (ppm) (δ) with TMS as an internal
standard (δ0.00), and13C NMR were reported in chemical shifts
with solvent reference (CDCl3, δ77.00, DMSO-d6δ39.52 ppm).
High-resolution mass spectra (HRMS) were recorded on Bruker
maXis ESI-TOF spectrometer.
In addition to the other relevant peaks in 1H, 13C NMR
spectrum of Gob-B, the presence of five peptide bond carbonyl
peaks at 173.8, 172.5, 171.8, 171.1, 171.0, one cyclic ornithine
amide peak at 169.7 and one salicylate C-OH peak at 156.9
ppm confirmed its structure. COSY, HSQC and HMBC NMR
spectra analyses were in agreement with the structure of
gobichelin B. Further, in HRMS, m/z peak at 690.3216
corresponding to M+H peak (calc. 690.3211 for C30H44N9O10
[M+H]) confirms the structure of gobichelin B.
Abbreviations
:
RT: Room temperature; DCM: Dichloromethane; EDC·HCl: 3-
Ethyl-1-(3-dimethylaminopropyl)carbodiimide hydrochloride;
DMF: Dimethyl formamide; TFA: Trifluoroacetic acid; TMSCl:
Trimethylsilyl chloride; NHS: N-hydroxysuccinimde; DCC: N,N'-
Dicyclohexylcarbodiimide; DCU: N,N'-Dicyclohexylurea; HOBt:
1-Hydroxybenzotriazole; DMAP: 4-Dimethylaminopyridine;
Boc: tert-Butoxycarbonyl; Cbz: Benzyloxycarbonyl.
To study the iron-binding properties, gobichelin B was
treated with 2 fold excess of FeCl3 in water at room
temperature and kept at 4 ºC for 10 hr and purified by
preparative HPLC to get the corresponding iron bound form of
gobichelin
B with m/z 743.2324 (calc. 743.2326 for
C30H41FeN9O10 [M-2H+Fe(III)]+).
Boc-His(Bzl)-OMe (9):
L-Boc-His-OMe (
8
) was prepared from L-histidine following the
literature procedure.27 To the solution of compound
8
(5.61 g,
20.85 mmol) in DMF (80 mL), anhydrous K2CO3 (1.98 g, 14.59
mmol) and benzyl bromide (3.56 g, 20.85 mmol) were added
and stirred at room temperature for 12 hr. After completion of
the reaction, DMF was removed under reduced pressure and
the residue was taken up in Ethyl acetate (200 mL) and water
(100 mL). The organic layer was separated and washed with
water, brine and dried over anhydrous Na2SO4 and
concentrated on rotavac followed by purification on column
Figure 3. Possible Fe-Gobichelin B complex.
Conclusions
In conclusion, the first total synthesis of gobichelin B and
epi-gobichelin B were achieved successfully. This is a robust
and scalable synthetic route for the total synthesis of
gobichelin B. The similar methodology can be adopted for the
chromatography, produced regioisomers,
9 (4.94 g, 66%) and
9a (1.12 g, 15%) as colourless gum which solidified slowly upon
cooling.
9:
+3.8° (c 1.0 in CHCl3); δH (400 MHz; CDCl3;
4 | J. Name., 2012, 00, 1-3
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Me4Si) 7.47 (s, 1H), 7.39-7.33 (m, 3H), 7.13 (d, 2H, J = 6.5 Hz), Gobichelin B 1st Half Methyl Ester:
ARTICLE
6.66 (s, 1H), 5.94 (d, 1H, J = 8.2 Hz), 5.05 (s, 2H), 4.57-4.53 (m, [(S)-methyl 3-(1-benzyl-1H-imidazol-5-yl)-2-((S)-3-(benzyloxy)-
1H), 3.65 (s, 3H), 3.09 (dd, 1H, J = 5.4, 14.7 Hz), 3.00 (dd, 1H, J 2-(2-(benzyloxy)benzamido)propanamido)propanoate] (12):
= 4.7, 14.6 Hz), 1.44 (s, 9H); δC (100 MHz; CDCl3; Me4Si) 172.5, O-Benzyl N-((2-benzyloxy)benzoyl)serine 11 (1.00 g, 2.46
155.6, 138.0, 137.2, 136.0, 128.9, 128.2, 127.2, 116.9, 79.5, mmol) and compound 10 (0.80 g, 2.71 mmol) were suspended
53.6, 52.0, 50.7, 30.3, 28.3; HRMS (ESI) calcd. for C19H26N3O4 in 20 mL of anhydrous CH2Cl2 and cooled to 0 °C. To this, HOBt
[M+H] 360.1923, found 360.1922.
(0.34 g, 2.46 mmol), DMAP (0.90 g, 7.38 mmol) were added
and stirred for 10 min. After that, EDC·HCl (0.71 g, 3.69 mmol)
9a
:
δH (400 MHz; CDCl3; Me4Si) 7.34-7.15 (m, 4H), 6.94 (bs, was added and stirred at 0 °C for 15 min and at room
2H), 6.73 (bs, 1H), 5.75 (bs, 1H), 5.02-4.93 (m, 2H), 4.36 (bs, temperature overnight.After completion of the reaction, the
1H), 3.57 (d, 3H, J = 5.2 Hz), 2.91-2.80 (m, 2H), 1.29 (s, 9H); δC reaction solutionwas concentrated and treated with EtOAc
(100 MHz; CDCl3; Me4Si) 171.7, 155.2, 138.0, 136.0, 128.9, (100 mL) and H2O (50 mL). After separation of the layers, the
128.3, 128.0, 126.7, 126.6, 79.8, 53.0, 52.3, 48.4, 28.2, 26.7; organic layer was washed with 5% NaHCO3 (2 x 50 mL), 10%
HRMS (ESI) calcd. for C19H26N3O4 [M+H] 360.1923, found aqueous citric acid (2 x 50 mL), and brine (100 mL), dried over
360.1922.
Na2SO4, filtered and concentrated in vacuo. The crude
compound was purified by column chromatography (50%
L-His(Bzl)-OMe hydrochloride (10):
To a solution of L-Histidine derivative
EtOAc/Hexanes) to give 12 as white foam (1.11 g, 70%).
=
9
(1.60 g, 4.45 mmol) in +17.0° (c 1.0 in CHCl3); δH (400 MHz; CDCl3; Me4Si) (Rotamers)
anhydrous MeOH (10 mL), TMSCl (2.82 mL, 22.25 mmol) was 8.90 (d, 1H, J = 7.4 Hz), 8.87 (d, 0.4 H, J = 7.4 Hz), 8.23-8.20 (m,
added dropwise at 0 °C and stirred at room temperature for 4 0.6 H), 8.15-8.11 (m, 1H), 7.43-7.36 (m, 3H), 7.30-7.20 (m,
hr. After completion of the reaction, solvent was removed, 11H), 7.04-6.99 (m, 4H), 6.93 (s, 0.6 H), 6.84 (s, 0.4 H), 6.62 (s,
triturated with distilled hexanes and dried in vacuo to get 0.4 H), 6.57 (s, 0.6 H), 5.21-5.10 (m, 2H), 4.95-4.88 (m, 1H),
compound 10 (1.25 g, 95%). The solid compound was used in 4.84-4.75 (m, 3H), 4.44-4.29 (m, 2H), 3.93-3.90 (m, 1H), 3.63-
the next step without further purification.
3.60 (m, 4H), 3.10-2.95 (m, 2H); δC (100 MHz; CDCl3;
Me4Si) (Rotamers) 171.7, 171.6, 170.0, 169.9, 165.3, 165.1,
157.0, 156.9, 137.9, 137.9, 137.5, 137.4, 136.9, 136.7, 136.1,
L-O-benzyl N-(2-Benzyloxybenzoyl)serine (11):
O-Benzyl salicylic acid
3 (1.00 g, 4.38 mmol) and NHS (0.53 g, 136.1, 135.8, 135.7, 133.0, 132.9, 132.3, 128.9, 128.8, 128.7,
4.64 mmol) were dissolved in dry THF (10 mL) and cooled to 0 128.7, 128.3, 128.2, 128.1, 128.1, 127.6, 127.5, 127.5, 127.4,
°C. To this reaction mixture, a solution of DCC (0.92 g, 4.46 127.2, 127.1, 121.7, 121.6, 121.3, 117.2, 117.0, 113.1, 113.0,
mmol) in dry THF (10 mL) was added and the reaction was 73.1, 72.9, 71.1, 71.0, 69.5, 69.3, 53.8, 53.3, 52.5, 52.3, 52.1,
stirred at room temperature for
6 hr. After complete 50.5, 29.9, 29.8; HRMS (ESI) calcd. for C38H39N4O6 [M+H]
conversion of acid to active ester, the resulting DCU was 647.2870, found 647.2864.
filtered off and washed with THF. The THF filtrate was used as
the active ester solution in the following reaction.
To a solution of L-O-benzyl serine (1.00 g, 5.16 mmol) and [(S)-3-(1-benzyl-1H-imidazol-5-yl)-2-((S)-3-(benzyloxy)-2-(2-
Gobichelin B 1st Half:
4
NaHCO3 (0.95 g, 11.40 mmol) in THF/H2O (40 mL, 4:3), the (benzyloxy)benzamido)propanamido)propanoic acid] (1):
active ester obtained from above was added at room To a compound 12 (0.50 g, 0.77 mmol) in THF/H2O (10 mL,
temperature and the reaction mixture was stirred for 8 h. The 9:1), LiOH·H2O (40 mg, 1.00 mmol) was added and stirred at
volatiles were evaporated, and the aqueous residue was room temperature for 2 hr. After completion of the reaction,
diluted with EtOAc (50 mL) and acidified to pH 2 with 10% solvent was removed and the residue was dissolved in MeOH
aqueous citric acid. The organic layer was separated, and the (15 mL) and Amberlite IR120 (hydrogen form, 300 mg) was
aqueous layer was extracted with EtOAc. The combined added and stirred at room temperature for 2 hr and filtered
extracts were washed with brine, dried with anhydrousNa2SO4, and concentrated to get acid compound
1 (0.389 g, 80%).
filtered and concentrated and purified by flash column HRMS (ESI) calcd. for C37H37N4O6 [M+H] 633.2713, found
chromatography on silica gel (5% MeOH/CHCl3) to give 11 633.2712.
(1.50 g, 85%) as a colourless gummy compound which
solidified slowly after several hours.
= -18.2° (c 1.0 in (L,L)-O-Benzyl (Nα-Boc-Nε-Cbz-lysinyl)serine (15):
CHCl3); δH (400 MHz; CDCl3; Me4Si) 10.51 (bs, 1H), 9.04 (d, 1H, L-Boc-Lys(Z)-OH 13 (1.70 g, 4.47 mmol) and NHS (0.54 g, 4.74
J = 7.5 Hz), 8.31 (dd, 1H, J = 1.6, 7.7 Hz), 7.48-7.44 (m, 3H), mmol) were dissolved in dry THF (10 mL) and cooled to 0 °C.
7.36-7.25 (m, 8H), 7.12-7.05 (m, 2H), 5.23 (d, 1H, J = 11.6 Hz), To this reaction mixture, a solution of DCC (0.97 g, 4.74 mmol)
5.18 (d, 1H, J = 11.8 Hz), 5.10-5.07 (m, 1H), 4.41 (s, 2H), 3.98 in dry THF (10 mL) was added and the reaction was stirred at
(dd, 1H, J = 3.7, 9.6 Hz), 3.82 (dd, 1H, J = 3.5, 9.7 Hz); δC (100 room temperature for 8 h. The resulting DCU was filtered off
MHz; CDCl3; Me4Si) 174.0, 165.8, 157.1, 137.6, 135.5, 133.3, and washed with THF. The THF filtrate was used as the active
132.4, 128.7, 128.4, 128.3, 127.8, 127.6, 127.5, 121.5, 120.9, ester solution in the following reaction.
113.0, 73.1, 71.3, 69.3, 53.4; HRMS (ESI) calcd. for C24H24NO5
To the solution of O-benzyl serine 4 (0.95 g, 4.91 mmol)
[M+H] 406.1654, found 406.1650.
and NaHCO3 (0.97 g, 11.60 mmol) in THF/H2O (40 mL/30 mL),
the active ester obtained from above was added at room
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temperature and the reaction mixture was stirred at room 16H), 7.18-6.91 (br d,1H), 5.26 (br d, 1H), 5.09 (d, 2H, J = 5.7
temperature for 6 h. The volatiles were evaporated, and the Hz), 4.95 (dd, 1H, J = 5.0, 10.5 Hz), 4.87 (t, 1H, J = 10.0 Hz),
aqueous residue was diluted with EtOAc (50 mL) and acidified 4.70-4.55 (m, 3H), 4.43-4.95 (m, 1H), 4.13-4.07 (m, 1H), 3.96
to pH 2 with 10% aqueous citric acid. The organic layer was (d, 1H, J = 6.2 Hz), 3.64-3.59 (m, 1H), 3.43-3.36 (m, 1H), 3.31
separated, and the aqueous layer was extracted with EtOAc. (bs, 1H), 3.24-3.16 (m, 2H), 2.35 (bs, 1H), 1.86 (bs, 4H), 1.2-
The combined extracts were washed with brine, dried over 1.64 (m, 1H), 1.55-1.36 (m, 14H); δC (100 MHz; CDCl3; Me4Si)
anhydrousNa2SO4, filtered and concentrated and purified by (Rotamers) 172.2, 170.0, 169.8, 167.4, 167.2, 156.7, 156.5,
flash column chromatography on silica gel using 10% 155.8, 137.6, 137.5, 136.7, 136.7, 135.2, 135.1, 129.5, 128.7,
MeOH/CHCl3 to give 15 (1.94 g, 78%) as a colourless gummy 128.4, 128.4, 128.0, 128.0, 127.8, 127.8, 127.7, 80.0, 75.9,
compound.
= +8.7° (c 2.0 in CHCl3); δH (500 MHz; CDCl3; 75.8, 73.4, 73.2,69.5, 69.3, 66.4, 54.8, 54.5, 52.8, 51.5, 51.1,
Me4Si) 8.54 (bs, 1H), 7.60 (br d, 1H), 7.34-7.24 (m, 10H), 5.74- 51.0, 40.4, 40.3, 31.9, 29.3, 28.3, 27.4, 27.4, 22.4, 20.8; HRMS
5.72 (bs, 1H), 5.23-5.08 (m, 3H), 4.77 (bs, 1H), 4.49 (s, 2H), 4.31 (ESI) calcd for C41H53N5NaO9 [M+Na] 782.3741, found
(bs, 1H), 3.90 (dd, 1H, J = 3.4, 9.6 Hz), 3.70 (dd, 1H, J = 2.6, 9.3 782.3744.
Hz), 3.10 (bs, 2H), 1.78-1.73(bs, 1H), 1.63-1.58 (bs, 1H), 1.43-
1.34 (m, 13H); δC (125 MHz; CDCl3; Me4Si) 172.8, 172.5, 156.7, Benzyl
((S)-5-amino-6-(((S)-3-(benzyloxy)-1-(((S)-1-
156.1, 137.5, 136.6, 128.5, 128.4, 128.1, 128.0, 127.7, 127.6, (benzyloxy)-2-oxopiperidin-3-yl)amino)-1-oxopropan-2-yl)-
80.2, 73.2, 69.6, 66.6, 54.1, 52.6, 40.5, 32.2, 29.2, 28.3, 22.4; amino)-6-oxohexyl)carbamate (18):
HRMS (ESI) calcd for C29H40N3O8 [M+H] 558.2815, found Tripeptide 17 (1.00 g, 1.32 mmol) was dissolved in 50 mL of
558.2820.
10% TFA/CH2Cl2and 0.2 mL of anisole was added and stirred
for 1 hr at 0 °C. After completion of the reaction, volatiles were
removed and theresidue was then dissolved in ethyl acetate
and washed twice with 5% aqueous NaHCO3 solution followed
(S)-3-Benzyloxycarbonylamino-1-benzyloxy-2-piperidinone
(A):
Compound
following literature procedure.2
lit:3
= +52.0° (c 1.45 in CH2Cl2); mp 87-89 °C; δH (400 then taken up in a small amount of CH2Cl2and applied to a
MHz; CDCl3; Me4Si) 7.45-7.33 (m, 10H), 5.73 (bs, 1H), 5.14 (s, silica plug. The deprotected tripeptide 18 (0.87 g, 80%) was
2H), 4.98 (d, 1H, J = 10.6 Hz), 4.90 (d, 1H, J = 10.6 Hz), 4.21- then eluted in 5% MeOH/CHCl3 as a colorless gum.
A
was prepared from L-ornithine hydrochloride by by water and brine solution. The organic layer was dried with
= +55.3° (c 1.0 in CH2Cl2; anhydrous Na2SO4 and concentrated in vacuo. The residue was
=
4.16 (m, 1H), 3.47-3.40 (m, 1H), 3.36-3.33 (m, 1H), 2.43 (bs, +10.0° (c 0.5 in CHCl3); δH (400 MHz; CDCl3; Me4Si) 7.98 (d, 1H,
1H), 1.91-1.82 (m, 2H), 1.56 (qd, 1H, J = 4.0, 12.5, 24.9 Hz); δC J = 7.2 Hz), 7.53 (d, 1H, J = 5.5 Hz), 7.39-7.30 (m, 15H), 5.16 (bs,
(100 MHz; CDCl3; Me4Si) 167.8, 156.4, 136.3, 135.1, 129.5, 1H), 5.08 (s, 2H), 4.94 (d, 1H, J = 10.4 Hz), 4.86 (d, 1H, J = 10.4
128.8, 128.5, 128.1, 75.9, 66.8, 52.7, 51.2, 28.1, 20.7; HRMS Hz), 4.68-4.62 (m, 1H), 4.58 (s, 2H), 4.38-4.32 (m, 1H), 3.91 (dd,
(ESI) calcd for C20H23N2O4 [M+H] 355.1658, found 355.1658.
1H, J = 4.2, 9.2 Hz), 3.63 (dd, 1H, J = 6.4, 8.9 Hz), 3.4 (t, 2H, J =
4.5 Hz), 3.30 (bs, 1H), 3.17 (d, 2H, J = 5.9 Hz), 2.35-2.33 (m,
1H), 2.08 (bs, 2H), 1.80 (bs, 3H), 1.50 (bs, 4H), 1.42-1.38 (m,
(S)-3-Amino-1-benzyloxypiperidin-2-one hydrobromide (16):
Following the literature procedure,2 brieftreatment of A with 2H); δC (100 MHz; CDCl3; Me4Si) 175.5, 170.3, 167.5, 156.5,
33% HBr in acetic acid afforded cyclic ornithine fragment 16 in 137.6, 136.7, 135.1, 129.5, 128.8, 128.5, 128.5, 128.4, 128.1,
95% yield.
128.0, 127.8, 127.8, 127.7, 76.0, 73.3, 69.6, 66.5, 55.0, 52.5,
51.5, 51.2, 40.6, 34.4, 29.6, 27.4, 22.7, 20.8; HRMS (ESI) calcd
for C36H46N5O7 [M+H] 660.3397, found 660.3397.
Protected Gob-B 2nd Half:
Benzyl
tert-butyl
((S)-6-(((S)-3-(benzyloxy)-1-(((S)-1-
(benzyloxy)-2-oxopiperidin-3-yl)-amino)-1-oxopropan-2-yl)-
amino)-6-oxohexane-1,5-diyl)dicarbamate (17):
[Benzyl
((S)-5-((S)-3-(1-benzyl-1H-imidazol-5-yl)-2-((S)-3-
(benzyloxy)-2-(2-(benzyloxy)-benzamido)propanamido)-
(L,L)-O-Benzyl (Nα-Boc-Nε-Cbz-lysinyl)serine 15 (1.86 g, 3.34 propanamido)-6-(((S)-3-(benzyloxy)-1-(((S)-1-(benzyloxy)-2-
mmol) and compound 16 (0.95 g, 3.17 mmol) were suspended oxopiperidin-3-yl)amino)-1-oxopropan-2-yl)amino)-6-
in 20 mL of anhydrous CH2Cl2and cooled to 0 °C. To this, HOBt oxohexyl)carbamate] (Protected epi-Gobichelin B)
(0.53 g, 3.40 mmol), DMAP (1.10 g, 9.02 mmol) were added (19):
and stirred for 10 min. After that, EDC·HCl (0.96 g, 5.01 mmol) Gob-B 1st Half acid
1 (400 mg, 0.63 mmol) and amine 18 (420
was added. After being stirred at 0 °C for 15 min and room mg, 0.63 mmol) were suspended in 10 mL of anhydrous
temperature overnight, the reaction mixture was concentrated CH2Cl2and cooled to 0°C. To this, HOBt (86 mg, 0.63 mmol),
and treated with EtOAc (100 mL) and H2O (50 mL). After DMAP (200 mg, 1.63 mmol) were added and stirred for 10 min.
separation of the layers, the organic layer was washed with 5% After that, EDC·HCl (181 mg, 0.95 mmol) were added and
aqueous NaHCO3 (2 x 50 mL), 10% aqueous citric acid (2 x 50 stirred at 0 °C for 15 min and at room temperature for 16
mL), and brine (100 mL), dried over anhydrous Na2SO4, h.After completion of the reaction, the reaction mixture was
filtered, and concentrated in vacuo. The crude compound was concentrated and treated with EtOAc (70 mL) and H2O (20 mL).
purified by column chromatography (50% EtOAc/Hexanes) to After separation of the layers, aqueous layer was discarded
give 17 (1.90 g, 75%) as a white foam.
= +41.9° (c 1.0 in and the organic layer was washed with 5% aqueous NaHCO3 (2
CHCl3); δH (400 MHz; CDCl3; Me4Si) (Rotamers) 7.41-7.29 (m, x 15 mL), 10% aqueous citric acid (2 x 15 mL), and brine, dried
6 | J. Name., 2012, 00, 1-3
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over anhydrous Na2SO4, filtered and concentrated in vacuo. 69.4, 66.5, 53.9, 52.4, 40.4, 32.1, 29.0, 28.1, 22.3; HRMS (ESI)
The crude compound was purified onpreparative HPLC using calcd for C29H39N3O8 [M+H] 558.2815, found 558.2816.
Phenomenex Luna C18 column (5 μm, 21.2 x250 mm)eluting
with an isocratic 65% acetonitrile/water (1% NH4CO3) gave 19 Benzyl
tert-butyl
((R)-6-(((S)-3-(benzyloxy)-1-(((S)-1-
(484 mg, 60%) as a white foam. δH (500 MHz; CDCl3; (benzyloxy)-2-oxopiperidin-3-yl)-amino)-1-oxopropan-2-yl)-
Me4Si) 9.45 (d, 1H, J = 6.3 Hz), 9.13 (d, 1H, J = 4.6 Hz), 8.0 (dd, amino)-6-oxohexane-1,5-diyl)dicarbamate (24):
1H, J = 1.6, 7.8 Hz), 7.59 (t, 2H, J = 9.0 Hz), 7.48-7.46 (m, 2H), Tripeptide 24 was obtained from (D,L)-O-Benzyl (D-Nα-Boc-Nε-
7.43-7.41 (m, 2H), 7.35-7.30 (m, 18H), 7.24-7.20 (m, 5H), 7.18 Cbz-lysinyl)serine 23 following the similar procedure used for
(d, 1H, J = 7.2 Hz), 7.14-7.12 (m, 2H), 7.08 (d, 1H, J = 8.4 Hz), the synthesis of tripeptide 17. Compound 24 was obtained as
7.02 (t, 1H, J = 7.4 Hz), 6.96 (bs, 2H), 6.58 (s, 1H), 5.97 (s, 1H), a white foam.
= +54.52° (c 1.0 in CHCl3). δH (400 MHz;
5.18 (d, 2H, J = 2.3 Hz), 5.16-5.14 (m, 1H), 5.05 (s, 2H), 4.92 (d, CDCl3; Me4Si) 7.48 (bs, 1H), 7.40-7.29 (m, 15H), 7.14 (bs,1H),
2H, J = 2.1 Hz), 4.76-4.71 (m, 3H), 4.57-4.51 (m, 4H), 4.35-4.26 5.39 (br d, 1H), 5.17 (br d, 1H), 5.09 (s, 2H), 4.95 (d, 1H, J = 10.3
(m, 2H), 4.21 (d, 1H, J = 12.1 Hz), 4.17 (d, 1H, J = 12.1 Hz), 3.95 Hz), 4.87 (d, 1H, J = 9.9 Hz), 4.67-4.65 (m, 1H), 4.59-4.55 (m,
(dd, 1H, J = 7.0, 10.0 Hz), 3.88 (dd, 1H, J = 4.7, 10.0 Hz), 3.55 2H), 4.35-4.32 (m, 1H), 4.14 (bs, 1H), 3.95 (bd, 1H, J = 8.7 Hz),
(dd, 1H, J = 3.7, 9.7 Hz), 3.40 (td, 1H, J = 4.4, 11.1 Hz), 3.28-3.26 3.65-3.61 (m, 1H), 3.42-3.38 (m, 1H), 3.31 (bs, 1H), 3.17 (bd,
(m, 1H), 3.16 (dd, 1H, J J = 3.1, 14.8 Hz), 3.06 (q, 2H, 6.3 Hz), 2H), 2.31 (s, 2H), 1.83 (bs, 3H), 1.73-69 (m, 1H), 1.52 (bs, 3H),
2.82 (dd, 1H, J = 4.7, 14.7 Hz), 2.11 (bs, 1H), 1.88-1.80 (m, 3H), 1.45-1.42 (m, 10H); δC (100 MHz; CDCl3; Me4Si) 172.3, 170.0,
1.76-1.64 (m, 4H), 1.44 (m, 2H); δC (125 MHz; CDCl3; 167.3, 156.6, 155.8, 137.5, 136.6, 135.2, 129.5, 128.8, 128.5,
Me4Si) 172.1, 171.6, 171.2, 169.8, 166.8, 166.7, 157.6, 156.3, 128.4, 128.1, 128.0, 127.9, 127.8, 80.2, 75.9, 73.4, 69.4, 66.5,
138.3, 137.8, 137.5, 136.7, 136.2, 135.7, 135.6, 135.4, 133.6, 54.7, 52.6, 51.6, 51.2, 40.5, 31.8, 29.4, 28.3, 28.3, 27.4, 22.4,
132.1, 129.5, 128.9, 128.7, 128.5, 128.4, 128.3, 128.2, 128.1, 20.9; HRMS (ESI) calcd for C41H54N5O9 [M+H] 760.3922, found
127.9, 127.8, 127.6, 127.3, 127.2, 121.2, 120.9, 117.2, 113.2, 760.3913.
75.6, 72.8, 72.8, 71.5, 69.5, 68.4, 66.4, 56.3, 54.8, 53.8, 53.4,
51.6, 50.9, 50.5, 40.6, 30.6, 29.0, 27.6, 27.2, 22.2, 21.1; HRMS Gob-B 2nd Half: Benzyl ((R)-5-amino-6-(((S)-3-(benzyloxy)-1-
(ESI) calcd for C73H80N9O12 [M+H] 1274.5926, found 1274.5926. (((S)-1-(benzyloxy)-2-oxopiperidin-3-yl)amino)-1-oxopropan-
2-yl)amino)-6-oxohexyl)carbamate (2):
Epi-Gobichelin B (20):
Protected Epi-Gobichelin B 19 (100 mg, 0.078 mmol) was described for the synthesis of tripeptide 18. Compound
dissolved in MeOH (8 mL) and Pd(OH)2/C (100 mg) was added obtained as a colourless gum (0.87 g, 80%). = +23.8° (c
Gob-B 2nd half
2
was prepared following the procedure
2
was
and stirred under hydrogen atmosphere (1 atm) for 6 hr. The 0.5 in CHCl3). δH (500 MHz; CDCl3; Me4Si) 8.41 (d, 1H, J = 5.8
progress of the reaction was monitored by HRMS. After Hz), 8.09 (d, 1H, J = 5.2 Hz), 7.36-7.24 (m, 15H), 5.66 (t, 1H, J =
observing single peak at 690.3220 [M+H], the reaction was 5.6 Hz), 5.40 (bs, 2H), 5.04 (s, 2H), 4.89 (d, 1H, J = 10.6 Hz),
filtered through a pad of celite and concentrated to get the 4.82 (d, 1H, J = 10.6 Hz), 4.75-4.72 (m, 1H), 4.50 (s, 2H), 4.40-
epi-Gobichelin B 20 (43.30 mg, 80%) as white solid. δH (500 4.35 (m, 1H), 3.89 (dd, 1H, J = 5.2, 9.5 Hz), 3.82 (t, 1H, J = 6.0
MHz; D2O) 8.28 (s, 1H), 7.65 (d, 1H, J = 7.1 Hz), 7.36 (bs, 1H), Hz), 3.67 (dd, 1H, J = 4.6, 9.5 Hz), 3.4 (bs, 1H), 3.20 (d, 1H, J =
7.12 (s, 1H), 6.90 (bs, 2H), 4.62 (bs, 2H), 4.45 (s, 1H), 4.31 (s, 8.0 Hz), 3.10 (d, 2H, J = 5.1 Hz), 2.35-2.33 (m, 1H), 2.09 (bs,
1H), 4.24 (s, 1H), 4.17 (s, 1H), 3.81 (s, 2H), 3.75 (bs, 2H), 3.21 1H), 1.76-1.69 (m, 5H), 1.43-1.38 (m, 4H); δC (100 MHz; CDCl3;
(s, 1H), 3.16 (bs, 2H), 3.05 (dd, 1H, J = 8.4, 14.5 Hz), 2.78 (t, 2H, Me4Si) 175.4, 170.3, 167.5, 156.4, 137.5, 136.6, 134.9, 129.4,
J = 7.1 Hz), 1.94 (bs, 1H), 1.77-1.59 (m, 5H), 1.50 (t, 2H, J = 6.8 128.7, 128.4, 128.3, 128.0, 127.9, 127.7, 127.7, 127.6, 75.9,
Hz), 1.24 (bs, 2H); δC (125 MHz; D2O) 173.6, 172.3, 172.2, 73.2, 69.5, 66.4, 54.9, 52.4, 51.4, 51.1, 40.5, 34.4, 29.5, 27.4,
171.7, 171.0, 169.7, 157.0, 134.6, 133.7, 129.4, 129.2, 129.0, 22.6, 20.7; HRMS (ESI) calcd for C36H46N5O7 [M+H] 660.3397,
120.3, 117.2, 116.5, 61.2, 60.9, 56.0, 55.6, 53.7, 52.6, 49.7, found 660.3386.
48.9, 41.6, 39.2, 30.4, 26.8, 26.7, 22.0, 20.3; HRMS (ESI) calcd
for C30H44N9O10 [M+H] 690.3211, found 690.3216.
[Benzyl
((S)-5-((S)-3-(1-benzyl-1H-imidazol-5-yl)-2-((S)-3-
(benzyloxy)-2-(2-(benzyloxy)-
(D,L)-O-Benzyl (Nα-Boc-Nε-Cbz-lysinyl)serine (23):
benzamido)propanamido)propanamido)-6-(((S)-3-
D-Lys+L-Ser dipeptide 23 was prepared from D-Boc-Lys(Z)-OH (benzyloxy)-1-(((S)-1-(benzyloxy)-2-oxopiperidin-3-yl)amino)-
21 following the similar procedure used for the synthesis of L- 1-oxopropan-2-yl)amino)-6-oxohexyl)carbamate] (Protected
Lys+L-Ser dipeptide 15. Compound 23 was obtained as a Gobichelin B) (25):
colourless gum.
= +19.3° (c 2.0 in CHCl3). δH (500 MHz; Protected gob-B 25 was synthesized following the similar
CDCl3; Me4Si) 7.44-7.25 (m, 12H), 5.55 (d, 1H, J = 7.3 Hz), 5.14- procedure depicted for the synthesis of pentapeptide 23, and
4.99 (m, 3H), 4.77 (bs, 1H), 4.51 (s, 2H), 4.34 (bs, 1H), 3.91 (dd, was obtained as a white foam in 60% yield. δH (500 MHz;
1H, J = 3.0, 9.5 Hz), 3.71 (dd, 1H, J = 2.5, 8.6 Hz), 3.12 (bs, 2H), CDCl3; Me4Si) 8.99 (d, 1H, J = 4.7 Hz), 8.79 (d, 1H, J = 7.4 Hz),
1.76-1.74 (m, 1H), 1.65-1.60 (m, 1H), 1.49-1.35 (m, 13H); δC 8.08 (dd, 1H, J = 1.3, 7.7 Hz), 7.62 (d, 1H, J = 7.7 Hz), 7.40-7.36
(100 MHz; CDCl3; Me4Si) 172.6, 172.4, 156.5, 155.9, 137.3, (m, 5H), 7.34-7.30 (m, 10H), 7.28-7.26 (m, 5H), 7.24 (s, 4H),
136.4, 128.3, 128.2, 127.9, 127.7, 127.6, 127.4, 80.0, 73.0, 7.21-7.20 (m, 3H), 7.11 (bs, 2H), 7.04-7.01 (m, 3H), 6.95 (t, 1H,
J = 7.4 Hz), 6.77 (s, 1H), 6.15 (s, 1H), 5.99 (t, 2H, J = 5.1 Hz),
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5.14 (s, 2H), 5.02 (d, 1H, J = 12.5 Hz), 4.99 (d, 1H, J = 12.1 Hz), Acknowledgements
4.92 (d, 1H, J = 10.5 Hz), 4.86 (d, 1H, J = 10.5 Hz), 4.76 (d, 1H, J This work was supported by UPE and PURSE grants. B.C.V
= 15.1 Hz), 4.64 (d, 1H, J = 15.1 Hz), 4.58 (d, 1H, J = 11.8 Hz), acknowledges the University Grants Commission (UGC), India
4.53 (d, 1H, J = 11.8 Hz), 4.50-4.45 (m, 3H), 4.16 (d, 1H, J = 12.1 for Dr. D. S. Kothari Post-Doctoral Fellowship (award No. F.4-
Hz), 4.11 (d, 1H, J = 12.1 Hz), 4.06-4.02 (m, 2H), 3.73-3.69 (m, 2/2006(BSR)/CH/14-15/0074).
2H), 3.45 (td, 1H, J = 4.6, 10.6 Hz), 3.36 (dd, 1H, J = 3.7, 9.8 Hz),
3.32-3.29 (m, 1H), 3.28-3.25 (m, 1H), 3.22-3.14 (m, 2H), 2.75 Notes and references
(dd, 1H, J = 4.9, 14.7 Hz), 2.54 (bs, 2H), 2.10-2.08 (m, 1H), 2.01-
1
2
3
4
M. P. Soares and G. Weiss, EMBO Rep., 2015, 16, 1482.
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1.95 (m, 1H), 1.90-1.88 (m, 1H), 1.80-1.64 (m, 5H), 1.48-1.43
(m, 2H); δC (125 MHz; CDCl3; Me4Si) 172.8, 172.0, 170.0,
169.7, 167.0, 166.7, 157.3, 156.6, 137.9, 137.5, 137.0, 136.6,
136.5, 136.2, 135.4, 135.4, 133.4, 132.6, 129.4, 128.7, 128.7,
128.6, 128.5, 128.4, 128.3, 128.3, 127.9, 127.8, 127.8, 127.7,
127.7, 127.6, 127.6, 127.3, 121.6, 120.9, 118.7, 113.0, 75.7,
73.3, 72.8, 71.4, 69.2, 68.4, 66.1, 56.1, 55.3, 53.4, 53.0, 51.0,
50.9, 50.4, 40.0, 29.7, 28.6, 28.3, 27.6, 22.8, 20.9; HRMS (ESI)
calcd for C73H80N9O12 [M+H] 1274.5926, found 1274.5926.
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Gobichelin B:
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Checkley, R. L. Edwards and H. I. Boshof, J. Am. Chem. Soc.,
2011, 133, 2076.
16 R. J. Abergel, A. M. Zawadzka and K. N. Raymond, J. Am.
Chem. Soc., 2008, 130, 2124.
17 R. J. Abergel, J. A. Warner, D. K. Shuh and K. N. Raymond, J.
Am. Chem. Soc., 2006, 128, 8920.
18 A. Stintzi, C. Barnes, J. Xu and K. N. Raymond, Proc. Natl.
Acad. Sci. USA, 2000, 97, 10691.
Adopting the procedure described for the synthesisof of epi-
gobichelin B 20, equal amounts of protected Gobichelin B 25
and 10% (w/w) Pd(OH)2/C were suspended in MeOH and
stirred under hydrogen atmosphere (1 atm) at room
temperature. The reaction was monitored by HRMS. After
observing single molecular ion peak [M+H] at 690.3209, the
reaction mass was filtered through a pad of celite and
concentrated to get the Gobichelin B in 80% yield as colourless
fluffy compound. δH (500 MHz; D2O) 8.46 (s, 1H), 7.66 (d, 1H, J
= 7.6 Hz), 7.36 (t, 1H, J = 7.3 Hz), 7.19 (s, 1H), 6.91-6.87 (m,
2H), 4.67 (bs merged in D2O, 2H), 4.44 (q, 1H, J = 5.13 Hz),
4.31-4.29 (m, 1H), 4.25-4.18 (m, 1H), 3.87-3.69 (m, 4H), 3.52
(bs, 1H), 3.22 (dd, 1H, J = 5.5, 15.4 Hz), 3.14-3.12 (m, 1H), 3.09-
3.04 (m, 1H), 2.85 (t, 2H, J = 7.2 Hz), 1.90-1.61 (m, 6H), 1.56
(quin, 2H, J = 7.1 Hz), 1.28-1.21 (m, 2H); δC (125 MHz;
D2O) 173.8, 172.5, 171.8, 171.1, 171.0, 169.7, 156.9, 134.6,
133.5, 129.4, 128.5, 120.3, 117.3, 117.1, 116.4, 61.1, 60.9,
56.1, 55.7, 54.0, 52.6, 49.7, 41.4, 39.2, 30.1, 26.7,26.3, 25.9,
22.0, 20.3; HRMS (ESI) calcd for C30H44N9O10 [M+H] 690.3211,
found 690.3216.
19 R. J. Abergel and K. N. Raymond, J. Biol. Inorg. Chem., 2008,
13, 229.
20 R. C. Hider and X. Kong, Nat. Prod. Rep., 2010, 27, 637.
21 Y. Chen, M. Unger, I. Ntai, R. A. McClure, J. C. Albright, R. J.
Thomson and N. L. Kelleher, Med. Chem. Commun., 2013, 4,
233.
22 B. Sontag, M. Gerlitz, T. Paululat, H. F. Rasser, I. G. Wollny
and F. G. Hansske, J. Antibiot., 2006, 59, 659.
23 M. R. Seyedsayamdost, M. F. Traxler, S. L. Zheng, R. Kolter
and J. Clardy, J. Am. Chem. Soc., 2011, 133, 11434.
24 K. Raghavendra, R. A. Yellin, H. Traboulsi, G. Melman, N.
Humbert, M. Elhabiri, E. G. Kontecka, A. M. Albrecht-Gary
and A. Shanzer, Inorg. Chem., 2007, 46, 2485.
25 S. Yamada, A. Sudo, M. Goto and T. Endo, RSC Adv., 2014, 4,
29890.
26 J. C. Hodges, Synthesis, 1987, 20.
Gob-B–Fe(III) Complex:
To a solution of gobichelin B (10 mg) in distilled water (4 mL),
aqueous solution of FeCl3 (2 fold excess) was added dropwise
and stirred for 10 min at room temperature and then kept at 4
°C for 10 h. The resulting solution was purified on
a
preparative Phenomenex Luna C18 column (5 μm, 21.2 x250
mm) operating at 15 mL/min using 5 x1 mL injections. Fe(III)-
Gobichelin B complex was eluted with a gradient of5-10% of
CH3CN/H2O. Elution of the metal complex was observed using
MLCT band at 435 nm and collected at RT 5.4 min. HRMS (ESI)
calcd for C30H41N9O10 [M-2H+Fe+3] 743.2326, found 743.2326.
27 J. M. Chalker, S. B. Gunnoo, O. Boutureira, S. C. Gerstberger,
M. F. González, G. J. L. Bernardes, L. H. GriffinHailu, C. J.
Schofield and B. G. Davis, Chem. Sci., 2011, 2, 1666.
28 The formation of the rotamers was confirmed by the EXSY
NMR. D. X. Hu, P. Grice and S. V. Lee, J. Org. Chem., 2012,
77, 5198.
Conflicts of interest
The authors declare no competing financial interest.
29 L. Dong and M. J. Miller, J. Org. Chem., 2002, 67, 4759.
8 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx

Page 9 of 9
Organic & Biomolecular Chemistry
View Article Online
DOI: 10.1039/C8OB00263K
Table of content entry:
The first total synthesis of structurally diverse mixed ligand siderophore, gobichelin B and an epi-gobichelin B is
reported.
Bn
OBn
H
N
N
O
O
N
H2N
OBn
OBn
H
N
R
S
S
N
N
O
+
H
O
OH
S
S
N
H
N
H
O
O
Cbz
OBn
Gob-B 1st Half
Gob-B 2nd Half
HN
N
OH
OH
O
O
O
H
H
H
N
N
N
OH
S
R
S
S
S
N
N
N
H
H
O
O
O
OH
NH2
Gobichelin B