Synthetic studies of thiazoline and thiazolidine-containing natural products. Part 3: Total synthesis and absolute configuration of the siderophore yersiniabactin

Article abstract of DOI:10.1016/S0040-4020(01)00012-6

Total synthesis of yersiniabactin, a siderophore from cultures of the bacterium Yersinia enterocolitica, was accomplished. Chirality at the readily racemizable C-9 carbon was preserved during cyclization of β-hydroxythioamide by means of Burgess reagent leading to thiazoline. Based on its synthesis, the absolute configuration of natural yersiniabactin has been determined as 9R, 10RS, 12R, 13S and 19S.

Full text of DOI:10.1016/S0040-4020(01)00012-6

TETRAHEDRON
Pergamon
Tetrahedron 57 (2001) 1897±1902
Synthetic studies of thiazoline and thiazolidine-containing natural
products. Part 3: Total synthesis and absolute con®guration
of the siderophore yersiniabactin
Akira Inop and Akira Murabayashi
Aburahi Laboratories, Shionogi and Co. Ltd, Koka, Shiga 520-3423, Japan
Received 20 November 2000; accepted 18 December 2000
AbstractÐTotal synthesis of yersiniabactin, a siderophore from cultures of the bacterium Yersinia enterocolitica, was accomplished.
Chirality at the readily racemizable C-9 carbon was preserved during cyclization of b-hydroxythioamide by means of Burgess reagent
leading to thiazoline. Based on its synthesis, the absolute con®guration of natural yersiniabactin has been determined as 9R, 10RS, 12R, 13S
and 19S. q 2001 Elsevier Science Ltd. All rights reserved.
In our previous papers,1 we reported the total synthesis of
micacocidin (1),2 which comprises two thiazoline and one
thiazolidine moieties. In our continuing studies, we have
accomplished total synthesis of yersiniabactin3a (yersinio-
phore3b), a siderophore produced by the gram-negative
coccoid bacterium Yersinia enterocolitica.
that the C-10 isomer of 1 could be readily isomerized to the
natural C-10R form through metal-chelation.1 This ready
complex-formation of yersiniabactin and micacocidin (1)
suggested that both compounds might have common stereo-
chemical characteristics.
We inferred the absolute con®guration of yersiniabactin to
be identical with that of micacocidin (1) as shown by
structure 3, except the C-10 con®guration. Consequently,
a retrosynthetic analysis for yersiniabactin (3) was designed
in which the product was constructed from two segments A
and B, and for the synthesis of segment B, an intermediary
compound prepared in the synthesis of 1 could be utilized.
On the other hand, stereocontrol at C-9 was most critical in
our micacocidin synthesis.1 However, lack of the 5-pentyl
moiety in yersiniabactin suggested that stereocontrol at C-9
in the thiazoline moiety might be achieved more easily. So,
the method for synthesis of segment A was designed starting
from d-serine via thioamide 4 (Scheme 1).
Yersiniabactin was initially isolated as a mixture of two
diastereomers (isomers I and II) in regard to its C-10
con®guration, and the proposed plane structure closely
resembled micacocidin (1) as well as in pyochelin I (2),
another siderophore (Fig. 1). Through gallium and alu-
minum complex formation, yersiniabactin can be readily
uni®ed into a single stereoisomer.3 As reported previously,
micacocidin (1) also readily formed complexes with some
metal ions such as zinc, iron and copper. From the result of
molecular modeling of 1, we assumed that a molecular
structure with natural stereochemistry was most favorable
for forming these metal complexes. In addition, we found
Figure 1.
Keywords: siderophores; thiazolines; thiazolidines; con®guration.
Corresponding author. Tel.: 1748-88-3281; fax: 1748-88-2783; e-mail: akira.ino@shionogi.co.jp
p
0040±4020/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4020(01)00012-6

1898
A. Ino, A. Murabayashi / Tetrahedron 57 (2001) 1897±1902
Scheme 1. Retrosynthetic analysis.
Scheme 2.
Scheme 3.
1. Synthesis of segment A
2. Stereocontrolled synthesis of pyochelin I
Ester 8, which was prepared by condensation of Weinreb
amide 74 and 2-methoxybenzoyl chloride, was treated
with TFA and then subjected to alkaline-mediated acyl-
migration to afford amide 9 in quantitative yield. After
demethylation by treatment with boron trichloride giving
10, the amide was converted to thioamide 4 via an oxazoline
intermediate by treatment ®rst with Burgess reagent (11)
and then with H2S and Et3N. Treatment of 4 again with 11
gave thiazoline 12 in high enantiomerical purity (73±
89% ee).5 Finally, the phenol residue of 12 was protected
with a t-butyldimethylsilyl (TBDPS) group to give the
protected segment A 13, which was provided to the
following reactions without separation of the enantiomer
(Scheme 2).
Since protected segment A (13) was available, we next
performed stereoselective synthesis of another siderophore,
pyochelin I (2),6 which is also known to exist in nature as a
mixture of two diastereomers in regard to the C-10 con®gu-
ration, in order to clarify the stereochemical preference at
C-9 and C-10 chiral centers, which were crucial in our
synthetic route for yersiniabactin (3). In accordance with
our method for micacocidin synthesis and with that for
pyochelin synthesis reported by Cox et al.,6b,² the conden-
sation of labile aldehyde 14, which was prepared by reduc-
tion of Weinreb amide 13, with N-methyl-l-cysteine
²
In this synthesis, the C-9 chiral center of pyochelin was not de®ned
stereoselectively.

A. Ino, A. Murabayashi / Tetrahedron 57 (2001) 1897±1902
1899
Scheme 4.
relative stereochemistry as micacocidin (1), except for the
C-10 con®guration which is an R,S mixture in nature, as
shown by 3.
Furthermore, the CD spectrum of synthesized 3 shown in
Fig. 2 was identical to that of natural yersiniabactin.3,§ Thus,
the absolute con®guration of yersiniabactin was determined
as 9R, 10RS, 12R, 13S and 19S as shown 3.
5. Experimental
5.1. General procedure
Figure 2. CD spectrum of synthesized 3. cˆ9.24£1025 M in H2O.
Melting points were determined on a Yanagimoto micro
melting point apparatus. IR spectra were recorded on a
JASCO FT/IR-300E spectrometer. The samples were
hydrochloride7 and subsequent deprotection of the TBDPS
group gave pyochelins (a mixture of four diastereomers).
Treatment of the mixture with zinc chloride was shown to
unify the C-10 chiral center into R con®guration to yield
pyochelin I (2) and neopyochelin II (17) (as a ca. 5:1
mixture) (Scheme 3).
1
prepared as KBr pellets. H NMR spectra were recorded
on a JEOL GSX-270 or JMN A-400 spectrometer. Tetra-
methylsilane was used as an internal standard for the spectra
taken in CDCl3, and DMF-d7. All solvents were dried over
3A or 4A molecular sieves before use. Chromatography was
carried out on Merck silica gel 60. Preparative thin-layer
chromatography (p.TLC) was carried out on 2.00 mm
Merck silica gel 60F254 plates.
3. Total synthesis of yersiniabactin
Favorable condensation reaction of two segments for syn-
thesis of yersiniabactin was achieved through a procedure as
carried out in our total synthesis of micacocidin (1).1 Thus,
the protecting groups of thiol and amino groups in alcohol
18,1b which was an intermediary compound in our previous
synthesis of micacocidin, were removed via three succes-
sive reactions to provide 20. Condensation reaction of 20
with 14 through the same procedure afforded thiazolidine
21 as a mixture with the C-9 isomers. Finally, alkaline
hydrolysis of the terminal ester moiety and the puri®cation
by HPLC furnished 3 (Scheme 4).
5.1.1. (2R)-2-tert-Butoxycarbonylamino-N-methoxy-3-
(2-methoxybenzoyloxy)-N-methylpropionamide (8). To
an ice-cold solution of 7 (1.24 g, 5.00 mmol), Et3N
(1.53 ml, 2.20 equiv.) and DMAP (31.0 mg, 0.05 equiv.)
in THF (15.0 ml) was added o-methoxybenzoyl chloride
(0.82 ml, 1.10 equiv.), and the mixture was stirred at room
temperature overnight. The reaction mixture was diluted
with AcOEt and washed with sat. aq. NH4Cl, sat. aq.
NaHCO3 and brine, dried over Na2SO4 and then con-
centrated in vacuo. Chromatography (SiO2 100 g, AcOEt/
hexaneˆ2:3) of the residue afforded 8 (1.92 g, 100%) as a
colorless caramel. [a]24 ˆ212.8 (c 1.00, CHCl3); IR nmax
D
3342, 2975, 1717, 1669, 1491, 1250 cm21
;
1H NMR
4. Absolute stereochemistry of yersiniabactin
(270 MHz, CDCl3) d 1.45 (9H, s), 3.23 (3H, s), 3.80 (3H,
s), 3.93 (3H, s), 4.46 (1H, dd, Jˆ11.0, 3.7 Hz), 4.63 (1H, dd,
Jˆ11.0, 4.3 Hz), 5.01 (1H, m), 5.65 (1H, br-d, Jˆ7.9 Hz),
6.97 (2H, m), 7.47 (1H, br-t, Jˆ7.9 Hz), 7.80 (1H, dd,
1
The H NMR spectrum of synthesized 3 in DMF-d7 was
shown to be identical to that of natural yersiniabactin.3,8,³
So, we concluded that yersiniabactin possesses the same
§
We considered the subtle difference observed around 250 nm in the CD
³
The C-9 isomer of 3, which was synthesized separately from an S-isomer
of Weinreb amide 7 through the same procedure, did not show a spectrum
identical to that of natural yersiniabactin.
spectra of natural yersiniabactin and synthesized 3 to be due to the noni-
dentical composition of the two diastereomers with regard to the C-10
con®guration.

1900
A. Ino, A. Murabayashi / Tetrahedron 57 (2001) 1897±1902
Jˆ7.9, 1.8 Hz); LSIMS m/z 765 [2M1H]1, 383 [M1H]1,
327, 135; HR-LSIMS m/z 383.1821 [M1H]1 (calcd
383.1818 for C18H27N2O7).
(6.60 ml) and Et3N (3.30 ml) was saturated with H2S gas
and stirred at room temperature for 5 days. The reaction
mixture was evaporated in vacuo. Chromatography (SiO2
14 g, AcOEt/hexaneˆ5:1 to AcOEt only) of the residue
afforded 4 (305 mg, 85% from 10) as a yellow caramel.
5.1.2. (2R)-3-Hydroxy-N-methoxy-2-(2-methoxybenzoyl-
amino)-N-methylpropionamide (9). To an ice-cold solu-
tion of 8 (1.90 g, 4.97 mmol) in CH2Cl2 (20.0 ml) was added
TFA (5.00 ml), and the mixture was stirred at room
temperature for 3.5 h and then concentrated in vacuo. The
residue was taken up into AcOEt and the extract was treated
with sat. aq. NaHCO3. The aqueous layer was saturated with
NaCl and extracted with AcOEt. The combined organic
layer was dried over Na2SO4 and then concentrated in
vacuo. Chromatography (SiO2 20 g, AcOEt only) of the
residue afforded pure 9 (1.40 g, 100%) as a colorless
[a]26 ˆ24.6 (c 1.00, CHCl3); IR nmax 3278, 2940, 1653,
D
1
1539, 1461, 1360, cm21; H NMR (270 MHz, CDCl3) d
3.31 (3H, s), 3.88 (3H, s), 4.14 (2H, m), 5.76 (1H, br-s),
6.89 (1H, t, Jˆ7.9 Hz), 7.01 (1H, d, Jˆ7.3 Hz), 7.36 (1H, t,
Jˆ7.3 Hz), 7.52 (1H, d, Jˆ7.9 Hz), 8.90 (1H, br-s); FABMS
m/z 569 [2M1H]1, 285 [M1H]1, 224, 137; HR-FABMS
m/z 285.0900 [M1H]1 (calcd 285.0909 for C12H17N2O4S).
5.1.5. (4S)-2-(2-Hydroxyphenyl)-4,5-dihydrothiazole-4-
carboxylic acid N-methoxy-N-methylamide (12). To a
solution of 4 (260 mg, 0.91 mmol) in THF (4.60 ml) was
added Burgess reagent 11 (261 mg, 1.20 equiv.), and the
mixture was stirred at room temperature for 40 min and
then re¯uxed for 30 min. After cooling to room temperature,
the mixture was concentrated in vacuo. Chromatography
(SiO2 15 g, AcOEt/hexaneˆ1:2) of the residue afforded 12
(170 mg, 70%). IR nmax 2973, 2938, 1669, 1621, 1593,
caramel. [a]26 ˆ240.8 (c 1.00, CHCl3); IR nmax 3372,
D
2945, 1640, 1600, 1523, 1483, 1241 cm21
;
1H NMR
(270 MHz, CDCl3) d 3.19 (1H, br-s) 3.28 (3H, s), 3.83
(3H, s), 3.85±4.02 (2H, m), 4.03 (3H, s), 5.30 (1H, br-s),
7.00 (1H, d, Jˆ7.9 Hz), 7.06 (1H, t, Jˆ7.9 Hz), 7.47 (1H, td,
Jˆ7.9, 1.8 Hz), 8.19 (1H, dd, Jˆ7.9, 1.8 Hz), 9.12 (1H,
br-d); FABMS m/z 565 [2M1H1], 283 [M1H]1, 222,
194, 135; HR-FABMS m/z 283.1292 [M1H]1 (calcd
283.1294 for C13H19N2O5).
1
1491 cm21; H NMR (270 MHz, CDCl3) d 3.30 (3H, s),
3.48 (1H, dd, Jˆ10.8, 9.2 Hz), 3.79 (1H, br-t, Jˆ10.4 Hz),
3.84 (3H, s), 5.70 (1H, br-t, Jˆ9.2 Hz), 6.88 (1H, t, Jˆ
8.5 Hz), 6.98 (1H, d, Jˆ7.9 Hz), 7.36 (1H, td, Jˆ8.5,
1.2 Hz), 7.43 (1H, dd, Jˆ7.9, 1.2 HZ), 12.32 (1H, br-s);
LSIMS m/z 267 [M1H]1, 251, 178; HR-LSIMS m/z
267.0797 [M1H]1 (calcd 267.0803 for C12H15N2O3S).
5.1.3. (2R)-3-Hydroxy-2-(2-hydroxybenzoylamino)-N-
methoxy-N-methylpropionamide (10). To a solution of 9
(1.20 g, 4.25 mmol) in CH2Cl2 (13.0 ml) was added BCl3
(1.00 M in hexane, 5.10 ml, 1.20 equiv.) at 2788C. After
stirring at room temperature overnight, further BCl3
(1.00 M in hexane, 4.25 ml, 1.00 equiv.) was added and
the mixture was stirred at the same temperature for 1 h.
After addition of ice, the mixture was poured into brine
and extracted with AcOEt. The combined AcOEt layer
was dried over Na2SO4 and then concentrated in vacuo.
Chromatography (SiO250 g, AcOEt/hexaneˆ5:1) of the
residue afforded 10 (926 mg, 81%) as a colorless caramel.
5.1.6. (4S)-2-[2-(tert-Butyldiphenylsilyloxy)phenyl]-4,5-
dihydrothiazole-4-carboxylic acid N-methoxy-N-methyl-
amide (13). To an ice-cold solution of 12 (50.0 mg,
0.19 mmol) in DMF (1.50 ml) were added TBDPSCl
(0.10 ml, 2.10 equiv.) and imidazole (55.0 mg, 4.30
equiv.). After stirring at room temperature for 2 h, the
mixture was diluted with AcOEt and washed with water
and brine, dried over Na2SO4, then concentrated in vacuo.
Chromatography (SiO2 5 g, AcOEt/hexaneˆ1:3) of the resi-
due afforded 13 (79.8 mg, 84%) as a pale yellow caramel.
Due to lability of the O±Si bond, obtained 13 was used
immediately for the next reaction. IR nmax 2932, 2858,
[a]26 ˆ236.0 (c 1.00, CHCl3); IR nmax 3349, 2942, 1637,
D
1601, 1536, 1492 cm21, 1H NMR (270 MHz, CDCl3) d 2.66
(1H, br-s), 3.29 (3H, s), 3.84 (3H, s), 3.98 (2H, br-s), 5.25
(1H, dt, Jˆ7.3, 3.7 Hz), 6.87 (1H, t, Jˆ7.9 Hz), 6.98 (1H, d,
Jˆ7.9 Hz), 7.41 (1H, br-t, Jˆ7.9 Hz), 7.51 (2H, m), 12.02
(1H, s); FABMS m/z 537 [2M1H]1, 269 [M1H]1, 208,
180, 121; HR-FABMS m/z 269.1132 [M1H]1 (calcd
269.1137 for C12H17N2O5).
1
1668, 1590, 1486, 1113, 702 cm21; H NMR (270 MHz,
CDCl3) d 1.11 (9H, s), 3.32 (3H, s), 3.50 (1H, dd, Jˆ
11.0, 9.2 Hz), 3.84 (1H, m), 3.87 (3H, s), 5.57 (1H, br-s),
6.40 (1H, d, Jˆ7.9 Hz), 6.85 (2H, m), 7.39 (6H, m), 7.75
(5H, m); FABMS m/z 505 [M1H]1, 447; HR-FABMS m/z
505.1966 [M1H]1 (calcd 505.1981 for C28H33N2O3SSi).
5.1.4. (2R)-3-Hydroxy-2-(2-hydroxythiobenzoylamino)-
N-methoxy-N-methylpropionamide (4). To a solution of
10 (338 mg, 1.26 mmol) in THF (6.30 ml) was added
Burgess reagent 11 (360 mg 1.20 equiv.), and the mixture
was stirred at room temperature for 30 min and then
re¯uxed for 1 h. After cooling to room temperature, the
mixture was concentrated in vacuo. Chromatography
(SiO2 15 g, AcOEt/hexaneˆ2:1) of the residue afforded a
colorless oil (oxazoline, 281 mg, 89%). IR nmax 2974, 1668,
1639, 1492, 1261 cm21; 1H NMR (270 MHz, CDCl3) d 3.28
(3H, s), 3.86 (3H, s), 4.51 (1H, dd, Jˆ10.4, 8.5 Hz), 4.84
(1H, br-t, Jˆ7.9 Hz), 5.40 (1H, br-t, Jˆ8.5 Hz), 6.88 (1H,
td, Jˆ7.9, 1.2 Hz), 6.99 (1H, dd, Jˆ8.5, 1.2 Hz), 7.38 (1H,
td, Jˆ8.5, 1.2 Hz), 7.68 (1H, dd, Jˆ7.9, 1.2 Hz), 11.76 (1H,
s).
5.2. Determination of the optical purity
Enantiomeric excess of the intermediate was determined by
HPLC with a chiral column (Chiralcel OD for amide 9, 10 or
OJ for thiazoline 12); 9,10.95% ee, 12ˆ73% ee. (9-epi-
12ˆ89% ee).
5.2.1. Pyochelin I (2). To an ice-cold solution of freshly
prepared TBDPS ether 13 (64.0 mg, 0.13 mmol) in THF
(1.50 ml) was added LiAlH4 (5.30 mg, 1.10 equiv.), and
the mixture was stirred at the same temperature for
30 min. The reaction was quenched with AcOEt, and then
the mixture was poured into sat. aq. NH4Cl and extracted
with AcOEt. The AcOEt extract was washed with aq.
A solution of the oil (281 mg, 1.12 mmol) in MeOH

A. Ino, A. Murabayashi / Tetrahedron 57 (2001) 1897±1902
1901
Seignette salt and brine, dried over Na2SO4, then concen-
trated in vacuo to give crude aldehyde 14 as a bright yellow
amorphous solid.
3.82 (1H, t, Jˆ6.9 Hz), 4.37 (1H, d, Jˆ5.4 Hz), 4.98 (1H,
ddd, Jˆ8.9, 7.1, 5.4 Hz), 6.89 (1H, t, Jˆ7.8 Hz), 7.01 (1H,
d, Jˆ8.2 Hz), 7.37 (1H, t, Jˆ8.2 Hz), 7.41 (1H, dd, Jˆ7.8,
1.2 Hz).
Without puri®cation, the 14 was dissolved in CH2Cl2
(3.00 ml) and MeOH (0.60 ml) and the solution was treated
with AcOK (125 mg, 10.0 equiv.) and N-Me-l-Cys´HCl (ca.
50%, 109 mg, 2.50 equiv.). After stirring at room tempera-
ture overnight, the reaction mixture was diluted with
AcOEt, washed with sat. aq. NH4Cl and brine, dried over
Na2SO4, then concentrated in vacuo.
5.2.4. (4S)-2-[(2S,3R)-3-tert-Butoxycarbonylamino-2-hy-
droxy-4-mercapto-1,1-dimethyl]butyl-4-methyl-4,5-dihydro-
thiazole-4-carboxylic acid methyl ester (19). To an ice-
cold solution of 18 (113 mg, 0.22 mmol) in CH2Cl2
(4.30 ml) was added freshly prepared Npys-Cl (49.0 mg,
1.20 equiv.), and the mixture was stirred at the same
temperature for 30 min, and then concentrated in vacuo.
Puri®cation of the residue with p.TLC (AcOEt/hexaneˆ3:1)
afforded a yellow caramel (120 mg).
To an ice-cold solution of the residue in THF (1.50 ml) was
added TBAF (1.00 M in THF, 0.13 ml, 1.00 equiv.), and the
mixture was stirred at room temperature for 30 min. The
reaction mixture was diluted with AcOEt and washed with
5% aq. KHSO4 and then extracted with sat. aq. NaHCO3.
The combined aqueous layer was acidi®ed (pHˆ4) with
conc. HCl and extracted again with AcOEt. The AcOEt
layer was washed with brine, dried over Na2SO4, and then
concentrated in vacuo to give pyochelins 15 (25.0 mg, 61%
from 13) as a mixture of four diastereomers.
To a solution of the caramel (120 mg, ca. 0.21 mmol) in
acetone (4.00 ml) and water (1.00 ml) was added n-Bu3P
(0.06 ml, ca. 1.05 equiv.), and the mixture was stirred at
room temperature for 20 min. The acetone was removed
in vacuo and the residue was taken up in AcOEt. The
AcOEt extract was washed with 10% aq. citric acid, water
and brine, dried over Na2SO4, then concentrated in vacuo.
Puri®cation of the residue by p.TLC (AcOEt/hexaneˆ3:2)
To a solution of 15 (25.0 mg, 7.70£1025 mol) in MeOH
(2.00 ml) was added ZnCl2 (26.0 mg, 2.5 equiv.), and the
mixture was stirred at room temperature overnight. The
reaction mixture was diluted with AcOEt, washed with
5% aq. KHSO4 and brine, dried over Na2SO4, then con-
centrated in vacuo to give a mixture of pyochelin I (2)
afforded thiol 19 (64.0 mg, 73% from 18). [a]25 ˆ275.9 (c
D
1.00, CHCl3); IR nmax 3416, 3370, 2977, 2556, 1741, 1710,
1601, 1505 cm21; 1H NMR (270 MHz, CDCl3) d 1.25 (3H,
s), 1.38 (3H, s), 1.41 (9H, s), 1.51 (1H, t, Jˆ9.2 Hz), 1.58
(3H, s), 2.71 (2H, m), 3.09 (1H, d, Jˆ11.6 Hz), 3.61 (1H, d,
Jˆ11.6 Hz), 3.80 (3H, s), 3.81 (1H, m), 3.96 (1H, br-d,
Jˆ6.7 Hz), 5.28 (1H, br-d, Jˆ9.2 Hz), 5.52 (1H, br-d, Jˆ
6.7 Hz); LSIMS m/z 813 [2M1H]1, 407 [M1H]1, 333,
230; HR-LSIMS m/z 407.1676 [M1H]1 (calcd 407.1674
for C17H31N2O5S2).
1
and neopyochelin II (17) (ca. 5:1 by H NMR, 19.0 mg,
76%) as a yellow caramel. 2: IR nmax 3012, 2940, 1718,
1592, 1490, 1221 cm21 1H NMR (270 MHz, CDCl3)
;
(major isomer) d 2.71 (3H, s), 3.26 (1H, dd, Jˆ11.4,
7.9 Hz), 3.27±3.47 (2H, m), 3.48 (1H, dd, Jˆ11.4,
8.6 Hz), 3.85 (1H, t, Jˆ6.9 Hz), 4.36 (1H, d, Jˆ7.9 Hz),
4.87 (1H, q, Jˆ8.1 Hz), 6.90 (1H, t, Jˆ7.9 Hz), 7.01 (1H,
d, Jˆ8.2 Hz), 7.38 (1H, t, Jˆ8.2 Hz), 7.41 (1H, dd, Jˆ7.9,
1.5 Hz); FABMS m/z 649 [2M1H]1, 325 [M1H]1, 178,
146; HR-FABMS m/z 325.0676 [M1H]1 (calcd 325.0681
for C14H17N2O3S2).
5.2.5. Yersiniabactin methyl ester (21). To an ice-cold
solution of 19 (64.0 mg, 0.16 mmol) in CH2Cl2 (2.00 ml)
was added TFA (0.40 ml), and the mixture was stirred at
the same temperature for 15 min and at room temperature
for a further 2 h. The mixture was concentrated in vacuo to
give Segment B TFA salt 20 (quant.) as a pale yellow oil.
Compound 20 thus obtained was used without further puri-
1
5.2.2. Pyochelin I methyl ester (16). A solution of 2
(19.0 mg, 5.85£1025 mol, containing 17 ca. 20%) in
CH2Cl2 (3.00 ml) and MeOH (1.00 ml) was treated with
TMSCHN2 (1.00 M in THF) until generation of N2 gas
ceased. The reaction was quenched with AcOH and the
mixture was concentrated in vacuo. The residue was puri-
®ed by p.TLC (AcOEt/hexaneˆ2:3) to give pure pyochelin
I methyl ester 16) (11.0 mg). IR nmax 2950, 2858, 1747,
®cation for next reaction. H NMR (270 MHz, CDCl3) d
1.34 (3H, s), 1.50 (3H, s), 1.63 (1H, t, Jˆ9.2 Hz), 2.88
(1H, dd, Jˆ14.0, 6.7 Hz), 2.98 (1H, dd, Jˆ14.0, 7.3 Hz),
3.26 (1H, d, Jˆ11.6 Hz), 3.56 (1H, m), 3.68 (1H, d, Jˆ
11.6 Hz), 3.79 (1H, m), 3.81 (3H, s).
To an ice-cold solution of freshly prepared TBDPS ether 13
(79.8 mg, 0.16 mmol) in THF (2.00 ml) was added LiAlH4
(6.60 mg, 1.10 equiv.), and the mixture was stirred at the
same temperature for 30 min. The reaction was quenched
with AcOEt and then the whole mixture was poured into sat.
aq. NH4Cl and extracted with AcOEt. The AcOEt extract
was washed with aq. Seignette salt and brine, dried over
Na2SO4, then concentrated in vacuo to give crude aldehyde
as a bright yellow amorphous solid.
1
1592, 1490, 1220 cm21; H NMR (270 MHz, CDCl3) d
2.59 (3H, s), 3.11 (1H, dd, Jˆ10.6, 6.4 Hz), 3.18 (1H, dd,
Jˆ10.6, 9.1 Hz), 3.40 (1H, dd, Jˆ11.4, 9.1 Hz), 3.47 (1H,
dd, Jˆ11.4, 8.6 Hz), 3.65 (1H, dd, Jˆ9.1, 6.4 Hz), 3.76 (3H,
s), 4.52 (1H, d, Jˆ5.1 Hz), 5.07 (1H, td, Jˆ8.6, 5.1 Hz),
6.87 (1H, td, Jˆ8.4, 1.2 Hz), 6.98 (1H, dd, Jˆ8.4,
1.2 Hz), 7.35 (1H, td, Jˆ8.4, 1.5 Hz), 7.41 (1H, dd, Jˆ
7.9, 1.5 Hz).
Under a nitrogen atmosphere, to a suspension of the amor-
phous product and AcOK (155 mg, 10.0 equiv.) in CH2Cl2
(2.50 ml), was added a solution of TFA salt 20 (1.00 equiv.)
in CH2Cl2 (1.50 ml) dropwise over 30 min. After stirring at
room temperature for 18 h, the reaction mixture was diluted
with AcOEt, washed with water and brine, dried over
5.2.3. Neopyochelin II (17). Through the same procedure
for 2 from epi-7, was obtained a mixture of 2 and neopyo-
chelin II (17) (ca. 1:5 by 1H NMR). 17; 1H NMR (270 MHz,
CDCl3) (major isomer) d 2.65 (3H, s), 3.29 (1H, dd, Jˆ11.9,
7.1 Hz), 3.30±3.48 (2H, m), 3.54 (1H, dd, Jˆ11.4, 8.9 Hz),

1902
A. Ino, A. Murabayashi / Tetrahedron 57 (2001) 1897±1902
Na2SO4, then concentrated in vacuo to give a bright yellow
caramel.
FABMS m/z 482 [M1H]1, 303, 295; HR-FABMS m/z
482. 1245 [M1H]1 (calcd 482.1242 for C21H28N3O4S3).
To an ice-cold solution of the above caramel in THF
(3.00 ml) was added TBAF (1.00 ml in THF, 0.19 ml,
1.20 equiv.), and the mixture was stirred at room tempera-
ture for 20 min. The reaction mixture was diluted with
AcOEt, washed with sat. aq. NH4Cl and brine, dried over
Na2SO4, then concentrated in vacuo. Chromatography (SiO2
6 g, AcOEt/hexaneˆ1:3) of the residue afforded yersinia-
bactin methyl ester (21) (34.0 mg, 43% from 13, ca. 6:1
mixture of diastereomers) as a bright yellow caramel. IR
nmax 3287, 2965, 2932, 1736, 1593, 1489, 1456, 1221,
Acknowledgements
We thank Professor Isao Kitagawa (Kinki University,
Osaka) for discussions and helpful support. We are also
grateful to Dr C. Chen and Mr K. Kikuchi (Marine Bio-
technology Institute, Shimizu Laboratories) for providing
us with a natural sample and NMR data for yersiniabactin.
References
1
755 cm21; H NMR (400 MHz, CDCl3) (major isomer) d
1.35 (3H, s), 1.36 (3H, s), 1.57 (3H, s), 2.97 (1H, dd, Jˆ9.6,
5.6 Hz), 3.08 (1H, d, Jˆ11.6 Hz), 3.11 (1H, m), 3.29 (1H,
dd, Jˆ10.8, 9.2 Hz), 3.35 (1H, m), 3.50 (1H, dd, Jˆ10.8,
8.4 Hz), 3.67 (3H, s), 3.73 (1H, d, Jˆ11.6 Hz), 3.87 (1H, d,
Jˆ1.2 Hz), 4.88 (1H, d, Jˆ6.8 Hz), 4.94 (1H, td, Jˆ8.8,
6.8 Hz), 5.30 (1H, brs), 6.86 (1H, t, Jˆ7.6 Hz), 6.97 (1H,
d, Jˆ8.4 Hz), 7.34 (1H, t, Jˆ8.4 Hz), 7.39 (1H, d, Jˆ
7.6 Hz), 12.38 (1H, brs); FABMS m/z 496 [M1H]1, 317,
295, 518 [M1Na]1; HR-FABMS m/z 518.1218 [M1Na]1
(calcd 518.1218 for C22H29N3O4S3Na).
1. (a) Ino, A.; Hasegawa, Y.; Murabayashi, A. Tetrahedron Lett.
1998, 39, 3509±3512. (b) Ino, A.; Murabayashi, A.
Tetrahedron 1999, 55, 10271±10282. (c) Ino, A.; Hasegawa,
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(b) Kobayashi, S.; Hidaka, S.; Kawamura, Y.; Ozaki, M.;
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5.2.6. Yersiniabactin (3). To a solution of yersiniabactin
methyl ester (21) (a mixture of two diastereomers, 31.0 mg,
6.25£1025 mol) in DMF (1.50 ml) and water (0.50 ml) was
added LiOH´H2O (5.50 mg, 2.10 equiv.), and the mixture
was stirred at room temperature for 2.5 h. The reaction
mixture was diluted with AcOEt, washed with sat. aq.
NH4Cl, water and brine, dried over Na2SO4, then concen-
trated in vacuo to give crude yersiniabactin (26.0 mg).
4. Ageno, G.; Ban®, L.; Cascio, G.; Giuseppe, G.; Manghisi, E.;
Riva, R.; Rocca, V. Tetrahedron 1995, 51, 8121±8134.
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The synthesized crude yersiniabactin (19.0 mg) was puri®ed
by HPLC (ODS HG-5 (50£250 nm), 33% MeCN11 mM
phosphate buffer (pHˆ7), 10.0 ml/min, det. UV 254 nm:
rt; 16.6 min) to give pure yersiniabactin (3) (Isomer I/IIˆ
ca. 6:1, 7.00 mg). IR nmax 3098 (broad), 2971, 2931, 1592,
1483, 1455, 1389, 1299, 1221 cm21; CD (c 9.24£1025 M,
H2O) shown in Fig. 2; 1H NMR (400 MHz, DMF-d7) (major
isomer) d 1.31 (6H, s), 1.48 (3H, s), 2.87 (1H, t, Jˆ9.6 Hz),
2.99 (1H, dd, Jˆ9.6, 5.6 Hz), 3.21 (1H, d, Jˆ11.2 Hz), 3.35
(1H, m), 3.40 (1H, dd, Jˆ10.8, 9.6 Hz), 3.67 (1H, m), 3.71
(1H, d, Jˆ11.2 Hz), 3.97 (1H, d, Jˆ1.2 Hz), 5.00 (1H, d,
Jˆ4.8 Hz), 5.15 (1H, td, Jˆ9.2, 4.8 Hz), 6.97 (1H, t, Jˆ
8.0 Hz), 6.99 (1H, dd, Jˆ8.8, 1.2 Hz), 7.45 (2H, m);
7. Blondeau, P.; Berse, C.; Gravel, D.; Can J. Chem. 1967, 45, 49.
8. Kikuchi, K.; Chen, C.; Adachi, K.; Nishijima, M.; Araki, M.;
Sano, H. The 38th Symposium on the Chemistry of Natural
Products Symposium Papers 1996, 427±432.