Alternative synthesis of cystothiazole A

Article abstract of DOI:10.1016/j.tet.2006.09.064

Palladium-catalyzed methoxycarbonylation of (-)-(2R,3S)-1-tert-butyldimethylsiloxy-3-methyl-2-methoxypenta-4-yne 9 derived from (2R,3S)-epoxy butanoate 5 gave the acetylenic ester 10, which was treated with MeOH in the presence of Bu₃P to affor

Full text of DOI:10.1016/j.tet.2006.09.064

Tetrahedron 62 (2006) 11592–11598
Alternative synthesis of cystothiazole A
*
Hiroyuki Akita, Noriyuki Sutou, Takamitsu Sasaki and Keisuke Kato
School of Pharmaceutical Sciences, Toho University, 2-2-1 Miyama, Funabashi, Chiba 274-8510, Japan
Received 23 August 2006; revised 15 September 2006; accepted 19 September 2006
Available online 16 October 2006
Abstract—Palladium-catalyzed methoxycarbonylation of (ꢀ)-(2R,3S)-1-tert-butyldimethylsiloxy-3-methyl-2-methoxypenta-4-yne
9
derived from (2R,3S)-epoxy butanoate 5 gave the acetylenic ester 10, which was treated with MeOH in the presence of Bu3P to afford selec-
tively (Z)-b-methoxy acrylate congener 11 in 86% yield. Treatment of (Z)-11 with 99.8% enrichment of CDCl3 followed by consecutive
desilylation and oxidation afforded the left-half aldehyde (+)-2. The overall yield (10 steps from 5; 23%) of (+)-2 via the present route
was improved in comparison to that (10 steps from 5; 10%) of the previously reported route. By applying the modified Julia’s coupling
method, selectivity (E/Z¼14:1) of the (E)-form (cystothiazole A 1) against the (Z)-form was improved in comparison to the Wittig method
(E/Z¼4:1 to 6.9:1).
Ó 2006 Elsevier Ltd. All rights reserved.
1. Introduction
methodology for the development of the C(4)/C(5) vicinal
stereochemistry.5 After our synthesis of cystothiazoles A
16a,b and B6c was reported, syntheses of cystothiazoles A
1,4,7–9 B,9 C,5,7 and E10 were reported. In this paper, we
describe a new chiral synthesis of cystothiazole A 1 for
the purpose of improvement of the overall yield and the
(E)-selectivity of the C(6)/C(7) double bond. Our retro-
synthetic strategy of cystothiazole A 1 is illustrated in
Scheme 1. Retrosynthetically, the synthesis of 1 can be
achieved by the modified Julia’s coupling11 of the left-half
aldehyde 2 and the right-half sulfone 3 or 4. The synthesis
of the left-half aldehyde 2 is shown in Scheme 2.
Antifungal substances, myxothiazole A1 and cystothiazole
A 1,2 were isolated from different strains of myxobacterium
Myxococcus fulvus and Cystobacter fuscus, respectively.
These antibiotics possessing a bis-thiazole skeleton as well
as a b-methoxy acrylate moiety and cystothiazole A 1
have indicated potent antifungal activity against the phyto-
pathogenic fungus, Phytophthora capsici (2 mg/disk), and
have shown activity against a broad range of additional fungi
with no effect on bacterial growth.2 Furthermore, cystothia-
zole A 1 was examined for in vitro cytotoxicity using human
colon carcinoma HCT-116 and human leukemia K562 cells.
The IC50 values of cystothiazole A 1 were 110–130 ng/ml,
which were significantly higher than those of myxothiazole
A. The fungicidal activity of these b-methoxy acrylate
(MOA) inhibitors has been shown to be due to their ability
to inhibit mitochondrial respiration by blocking electron
transfer between cytochrome b and cytochrome c.3 The
absolute structure of cystothiazole A 1 was established by
a combination of spectroscopic analysis and chemical degra-
dation of the natural product.2 The left-half structure of
cystothiazole A 1 has been reported to be responsible for
antifungal activity.4 In future, for the purpose of structure–
biological activity relationship study of cystothiazole A
congeners, efficient synthesis of the left-half aldehyde 2 is
considered to be highly important.
By applying the previously reported procedure,6b the reac-
tion of (2R,3S)-epoxy butanoate 512 and lithium silyl-acetyl-
ide in the presence of Et2AlCl gave 6 in 71% yield.
Methylation of 6 followed by consecutive reduction and
silylation afforded the silyl ether 9 possessing the terminal
acetylene group in 52% overall yield from 6. By applying
Tsuji’s procedure,13 the acetylenic compound 9 was con-
verted to acetylenecarboxylate 10 in 87% yield under atmo-
spheric pressure (balloon) of carbon monoxide at room
temperature using 5 mol % of PdCl2 and a stoichiometric
amount of CuCl2 in MeOH. By applying the reported proce-
dures,14 conjugate addition of MeOH to acetylenecarboxyl-
ate 10 in the presence of a catalytic amount of Bu3P afforded
the corresponding (Z)-b-methoxy-a,b-unsaturated ester 11
in 86% yield. The (Z)-geometry of 11 was confirmed by
the NOE enhancement for the olefinic proton and the
methine proton (8.6%). Isomerization of (Z)-11 to (E)-12
was carried out by the following procedure. When a solution
of (Z)-11 in CDCl3 (chloroform-d+1% v/v TMS (D,
99.8%)+silver foil) from Cambridge Isotope Laboratories,
Inc.) was stood for 3 days at room temperature, (E)-12 was
The first enantiocontrolled synthesis of cystothiazole A 1
was achieved based on the preparation of a bis-thiazole
core for the application of the asymmetric Evans aldol
* Corresponding author. Tel.: +81 474 72 1805; e-mail: akita@phar.toho-u.
ac.jp
0040–4020/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2006.09.064

H. Akita et al. / Tetrahedron 62 (2006) 11592–11598
11593
OMe OMe
R
R
OMe OMe
CHO
5
R
(+)-2
MeOOC
R1O2S
Me
S
N
10
1
N
S
9
11
MeOOC
Me
R1
R1
=
=
+
N
S
13
left half
right half
Cystothiazole A (1)
Me
Me
4
3
4
S
N
N
N
2
4'
N
N
S
N
2'
Me
Ph
S
Me
Scheme 1.
exclusively obtained. The crude (E)-12 was treated with
Et3N$(HF)3 in CH2Cl2 to give the primary alcohol (+)-13
([a]D +91.4 (c 0.64, CHCl3)) in 85% overall yield from
(Z)-11, which was identical with the reported (+)-13 ([a]D
+76.1 (c 0.7, CHCl3)).6b Another CDCl3 (99.8% atom % D
from Aldrich) was also effective for this selective isomeriza-
tion. On the other hand, the distillated CHCl3, CH2Cl2,
(CF3SO3)Yb/CH2Cl2, and silica gel/MeOH were inactive for
this selective isomerization. Treatment of (Z)-11 with pyridi-
nium p-toluenesulfonate (PPTS)/MeOH/PhH or (CF3SO3)Sc/
CH2Cl2 or InCl3/CH2Cl2 gave a complex mixture including
a mixture of (Z)-12 and (E)-12. The same type of isomeriza-
tion as conversion of (Z)-11 to (E)-12 in 99.8% CDCl3 was
observed in our previous case.15 In the case of b-methoxy
acrylate, it was reported that a trace amount of acidic impu-
rities often was sufficient to bring about rapid equilibration
toward the (E)-form from the (Z)-form at ordinary tempera-
tures.16 The thus-obtained (+)-13 was subjected to Dess–
Martin periodinane oxidation to provide the desired aldehyde
(+)-2 ([a]D +104.7 (c 0.55, CHCl3)) in 89% yield. Then, the
synthesis of the right-half sulfone 3 or 4 is carried out as
shown in Scheme 3.
afforded a bithiazole ester 19 in 94% yield. LiBH4 reduc-
tion (20, 91% yield) of 19 followed by treatment with 2-mer-
captobenzothiazole (BTSH)/Ph3P/diethylazodicarboxylate
(DEAD) provided a sulfide 21 in 90% yield. The sulfide
21 was subjected to oxidation with 30% H2O2 in the pres-
ence of Mo7O24(NH4)6$4H2O to give the corresponding
sulfone 3 in 88% yield. Moreover, an alcohol 20 was treated
with 1-phenyl-1H-tetrazole-5-thiol (PTSH)/Ph3P/diethyl-
azodicarboxylate (DEAD) to provide a sulfide 22 in 96%
yield. The sulfide 22 was subjected to oxidation with 30%
H2O2 in the presence of Mo7O24(NH4)6$4H2O to give the
corresponding sulfone 4 in 91% yield. Then, the modified
Julia’s coupling of the aldehyde 2 with sulfones (3 or 4)
was carried out. The reaction of 2 and 3 in the presence
of lithium bis(trimethylsilyl)amide in THF gave a mixture
(E/Z¼14:1) of cystothiazole A 1 and (Z)-cystothiazole A
23 in 64% yield. A part of this mixture was again subjected
to chromatography to provide (E)-1, whose spectral data
(1H and 13C NMR) were identical with those of the reported
data.6b The reaction of 2 and 4 in the presence of lithium
bis(trimethylsilyl)amide in THF gave a mixture (E/Z¼4:1)
of cystothiazole A 1 and (Z)-cystothiazole A 23 in 66%
yield.
Treatment of isopropylamide 14 with P4S10 gave an isopro-
pylthioamide 15, which was reacted with a-bromopyruvate
to afford a mono-thiazole ester 16 in 82% overall yield
from 14. Treatment of 16 with NH3/MeOH followed by thio-
amidation with P4S10 yielded a thioamide 18 in 67% overall
yield from 16. The reaction of 18 with a-bromopyruvate
In conclusion, palladium-catalyzed methoxycarbonyla-
tion of (–)-(2R,3S)-1-tert-butyldimethylsiloxy-3-methyl-
2-methoxypenta-4-yne 9 derived from (2R,3S)-epoxy
butanoate 5 gave the acetylenic ester 10, which was treated
with MeOH in the presence of Bu3P to afford selectively
OR2
OMe
HC
Me3Si
O
2R
a
c
C
OR3
Me
S
R
COOMe
COOMe
3S
Me
R2= H
Me
R3= H
R3= SiMe2Bu 9 (93%)
(2R,3S)-5
8 (71%)
6 (71%)
b
d
R2= Me 7 (81%)
t
MeOOC
OMe
OMe OMe
C
t
t
e
f
C
OSiMe2Bu
MeOOC
OSiMe2Bu
87%
86%
H Me
10
(Z)-11
Me
H
OMe OMe
n.O.e. 8.4%
OR3
i
g
(+)-2
89%
(E)-12
13 (89% from 11)
SiMe3 / Et2AlCl; (b) MeI / Ag2O; (c) (1) Bu4N+F-, (2) LiBH4;
Me
MeOOC
h
R3= SiMe2Bu
t
R3= H
(a) Li+
-C
C
t
(d) BuMe2SiCl / imidazole / DMF; (e) PdCl2 (5 mol %) / CuCl2 / NaOAc / CO (balloon) / CH2Cl2;
(f) Bu3P/MeOH; (g) CDCl3 (D; 99.8%); (h) Et3N·(HF)3 / CH2Cl2; (i) Dess-Martin periodinane
Scheme 2.

11594
H. Akita et al. / Tetrahedron 62 (2006) 11592–11598
R5
R4
N
N
b
a
b
H2NCCHMe2
Me
H2NCCHMe2
N
S
S
O
14
S
15
Me
Me
Me
S
R4= COOEt 16 (82% from 14)
c
R4= CONH2 17
R5= COOEt
R5= CH2OH
R5= CH2SBT
19 (94%)
20 (91%)
21 (90%)
a
d
R4= CSNH2 18 (67% from 16)
e
f
(a) P2S5; (b) BrCH2COCOOEt / EtOH;
(c) NH3 / MeOH;
R5= CH2SO2BT 3 (88%)
(d) LiBH4 / THF;
(e) 2-mercaptobenzothiazole (BTSH) / DEAD / Ph3P;
R5= CH2OH
20
g
f
(f) H2O2 / Mo7O24(NH4)6·4H2O / EtOH;
(g) 1-phenyl-1H-tetrazole-5-thiol (PTSH) / DEAD / Ph3P
R5= CH2SPT
22 (96%)
R5= CH2SO2PT 4 (91%)
N
N
N
N
S
( = PTSH)
SH
( = BTSH)
SH
Me
N
Me
S
N
Ph
N
S
LiN(SiMe3)2
(+)-2
(+)-2
3
4
OMe OMe
R
S
+
+
64 % (E : Z = 14:1)
cystothiazole A (1)
+
H
LiN(SiMe3)2
66 % (E : Z = 4:1)
Me
H
MeO
O
(Z)-cystothiazole A (23)
Scheme 3.
(Z)-b-methoxy acrylate congener 11 in 86% yield. Treat-
ment of (Z)-11 with 99.8% enrichment of CDCl3 followed
by consecutive desilylation and oxidation gave the left-half
aldehyde (+)-2. The overall yield (10 steps from 5, 23%)
of (+)-2 via the present route was improved in comparison
to that (10 steps from 5, 10%) of the previously reported
route.6a,b By applying the modified Julia’s coupling method,
selectivity (E/Z¼14:1) of the (E)-form (cystothiazole A 1)
against the (Z)-form was improved in comparison to the
Wittig method (E/Z¼4:16b to 6.9:18).
1 h at 0 ꢁC. A solution of (2R,3S)-5 (3.0 g, 25.8 mmol) in
toluene (10 ml) was added to the above reaction mixture
and the whole mixture was stirred for 3 h at 5–10 ꢁC. The
reaction mixture was diluted with H2O (20 ml) at 0 ꢁC and
the generated white precipitate was filtered off with the aid
of Celite. The precipitate was washed with AcOEt and the
washing and filtrate were combined. The extracted organic
layer was dried over MgSO4 and evaporated to give an
oily product, which was chromatographed on silica gel
(60 g, n-hexane–AcOEt 30:1) to afford methyl (2R,3S)-2-
hydroxy-3-methyl-5-trimethylsilyl-4-pentynoate 6 (3.91 g,
71% yield) as a colorless oil. Compound (ꢀ)-6: [a]D25
1H NMR: d 0.15 (9H, s), 1.21 (3H, d, J¼7.2 Hz), 2.86–
2.93 (1H, m), 3.81 (3H, s), 4.21 (1H, d, J¼4.4 Hz);
13C NMR: d ꢀ0.07, 15.8, 32.3, 52.4, 73.5, 86.9, 106.1,
173.1; HRMS (FAB) calcd for C10H19O3Si (M++1):
215.1104. Found: m/z: 215.1106.
2. Experimental
2.1. General
ꢀ24.1 (c 1.07, CHCl3); IR (neat): 3482, 2168, 1741 cmꢀ1
;
All melting points were measured on a Yanaco MP-3S
1
micro melting point apparatus and are uncorrected. H and
13C NMR spectra were recorded on JEOL AL 400 spectro-
meter in CDCl3. Carbon substitution degrees were estab-
lished by DEPT pulse sequence. High-resolution mass
spectra (HRMS) and the fast atom bombardment mass spec-
tra (FABMS) were obtained with a JEOL JMS 600H
spectrometer. IR spectra were recorded with a JASCO FT/
IR-300 spectrometer. Optical rotations were measured with
a JASCO DIP-370 digital polarimeter. All evaporations
were performed under reduced pressure. For column chro-
matography, silica gel (Kieselgel 60) was employed.
A mixture of 6 (2.105 g, 9.8 mmol), methyl iodide (18 g,
127 mmol), and Ag2O (3.2 g, 13.8 mmol) in DMF (6 ml)
was stirred for 48 h at room temperature. The reaction mix-
ture was diluted with AcOEt (30 ml) and filtered off with the
aid of Celite. The filtrate was diluted with H2O and extracted
with n-hexane. The organic layer was dried over MgSO4.
Concentration of the organic layer gave a crude residue,
which was chromatographed on silica gel (50 g, n-hexane–
AcOEt 60:1) to provide methyl (2R,3S)-2-methoxy-3-methyl-
5-trimethylsilyl-4-pentynoate 7 (1.819 g, 81% yield) as a
colorless oil. Compound (ꢀ)-7: [a]2D3 ꢀ0.32 (c 1.225,
CHCl3); IR (neat): 1752, 2170 cmꢀ1; 1H NMR: d 0.14 (9H,
s), 1.23 (3H, d, J¼6.8 Hz), 2.89 (1H, dq, J¼6.4, 6.8 Hz),
3.43 (3H, s), 3.71 (1H, d, J¼6.4 Hz), 3.77 (3H, s);
13C NMR: d ꢀ0.02, 16.4, 30.5, 51.8, 58.9, 84.0, 86.6,
106.3, 171.4; HRMS (FAB) calcd for C11H21O3Si (M++1):
229.1260. Found: 229.1289.
2.2. (L)-(2R,3S)-1-tert-Butyldimethylsiloxy-3-methyl-2-
methoxypenta-4-yne (9)
n-Butyllithium (n-BuLi, 1.6 M in hexane, 24.5 ml,
39.2 mmol) was added to a stirred solution of trimethylsilyl-
acetylene (3.8 g, 38.7 mmol) in toluene (50 ml) at ꢀ40 ꢁC
under an argon atmosphere and the mixture was stirred for

H. Akita et al. / Tetrahedron 62 (2006) 11592–11598
11595
To a solution of (2R,3S)-7 (1.66 g, 7.3 mmol) in THF (20 ml)
was added 1 M tetrabutylammonium fluoride (TBAF) in
THF solution (3.7 ml, 3.7 mmol) at 0 ꢁC and the whole mix-
ture was stirred for 10 min at 0 ꢁC. The reaction mixture was
diluted with H2O and extracted with AcOEt. The organic
layer was dried over MgSO4 and evaporated to give a crude
residue, which was used for the next reaction without further
purification. To a solution of the above 2-methoxy ester in
THF (20 ml) was added LiBH4 (0.641 g, 29.4 mmol) at
0 ꢁC and the whole mixture was stirred for 60 min at room
temperature. The reaction mixture was diluted with MeOH
(2 ml), H2O (100 ml), and Et2O (100 ml), and the whole
mixture was stirred for 12 h at the same temperature. The
generated precipitate was filtered off with the aid of Celite
to afford the filtrate. The filtrate was extracted with Et2O.
The organic layer was dried over MgSO4. Evaporation of
the filtrate gave a crude residue, which was chromato-
graphed on silica gel (20 g, n-hexane–AcOEt 5:1) to provide
(ꢀ)-8 as a colorless oil (0.66 g, 71%). Compound (ꢀ)-8: [a]D27
1H NMR: d 0.06 (6H, s), 0.89 (9H, s), 1.25 (3H, d,
J¼7.2 Hz), 2.86 (1H, dq, J¼7.2, 6.8 Hz), 3.23–3.27
(1H, m), 3.47 (3H, s), 3.68–3.77 (2H, m), 3.74 (3H, s);
13C NMR: d ꢀ5.52, ꢀ5.47, 15.0, 18.2, 25.8 (3C), 27.5,
52.5, 59.0, 62.7, 73.7, 83.6, 91.3, 154.1; HRMS (FAB) calcd
for C15H29O4Si (M++1): 301.1836. Found: 301.1837.
2.4. (L)-Methyl (4R,5R)-6-tert-butyldimethylsiloxy-
3,5-dimethoxy-4-methyl-2(Z)-hexenoate (11)
A mixture of (ꢀ)-10 (1.25 g, 4.2 mmol), Bu3P (0.254 g,
1.25 mmol), and MeOH (0.335 g, 10.5 mmol) in CH2Cl2
(25 ml) was stirred for 4 h at room temperature. The reaction
mixture was evaporated to give a crude residue, which was
chromatographed on silica gel (30 g, n-hexane/AcOEt
50:1) to afford (ꢀ)-11 (1.90 g, 86%) as a colorless oil. Com-
pound (ꢀ)-11: [a]2D4 ꢀ15.6 (c 0.96, CHCl3); IR (neat): 1717,
1630 cmꢀ1; 1H NMR: d 0.05 (6H, s), 0.89 (9H, s), 1.11 (3H,
d, J¼7.2 Hz), 2.59 (1H, dq, J¼6.8, 6.8 Hz), 3.30 (1H, dd,
J¼5.6, 6.8 Hz), 3.41 (3H, s), 3.61 (2H, d, J¼5.6 Hz), 3.66
(3H, s), 3.91 (3H, s), 5.07 (1H, s); 13C NMR: d ꢀ5.45,
ꢀ5.38, 13.4, 18.3, 25.9 (3C), 40.5, 50.9, 59.3, 60.2, 62.9,
83.1, 95.8, 165.9, 174.7; HRMS (FAB) calcd for
C16H33O5Si (M++1): 333.2097. Found: 332.2120.
ꢀ27.6 (c 1.05, CHCl3); IR (neat): 3429, 3296, 2113 cmꢀ1
;
1H NMR: d 1.25 (3H, d, J¼6.8 Hz), 2.11 (1H, d,
J¼2.4 Hz), 2.38 (1H, br s), 2.76 (1H, ddq, J¼2.4, 6.8,
6.8 Hz), 3.16 (1H, ddd, J¼3.2, 5.6, 6.8 Hz), 3.80 (2H, dq,
J¼12, 5.6 Hz); 13C NMR: d 17.1, 27.3, 58.4, 61.9, 70.3,
84.5, 85.5; HRMS (FAB) calcd for C7H13O2 (M++1):
129.0916. Found: 129.0919.
2.5. (D)-Methyl (4R,5R)-6-hydroxy-3,5-dimethoxy-
4-methyl-2(E)-hexenoate (13)
A solution of (ꢀ)-8 (0.452 g, 3.5 mmol), imidazole (0.492 g,
7.1 mmol), and tert-butyldimethylsilyl chloride (TBDMSCl,
0.797 g, 5.3 mmol) in DMF (2 ml) was stirred for 1 h at
room temperature. The reaction mixture was diluted with
brine and extracted with AcOEt/n-hexane (1:1). The organic
layer was dried over MgSO4 and evaporated to give a crude
9, which was chromatographed on silica gel (30 g, n-hexane/
AcOEt 30:1) to afford 9 (0.801 g, 93%) as a colorless oil.
Compound (ꢀ)-9: [a]2D5 ꢀ4.74 (c 1.06, CHCl3); IR (neat):
A solution of (ꢀ)-11 (0.103 g, 0.45 mmol) in CDCl3 (2 ml;
D, 99.8% including 1% v/v TMS, silver foil, available from
Cambridge Isotope Laboratories Inc.) was stood for 2 weeks
at room temperature. Evaporation of the solvent gave 12 as
a colorless oil, which was used for the next reaction without
further purification. Compound 12: 1H NMR: d 0.03 (6H, s),
0.87 (9H, s), 1.15 (3H, d, J¼7.2 Hz), 3.33 (1H, dq, J¼3.0,
7.2 Hz), 3.46 (3H, s), 3.54 (1H, dd, J¼7.2, 11.2 Hz), 3.61
(3H, s), 3.61–3.65 (1H, m), 3.65 (3H, s), 3.97–4.04 (1H,
m), 4.96 (1H, s).
1
2115 cmꢀ1; H NMR: d 0.06 (6H, s), 0.89 (9H, s), 1.21
(3H, d, J¼7.2 Hz), 2.05 (1H, d, J¼2.4 Hz), 2.68 (1H, ddq,
J¼2.4, 7.2, 6.4 Hz), 3.16 (1H, ddq, J¼4.4, 5.6, 6.4 Hz),
3.71 (1H, dd, J¼5.6, 10.8 Hz), 3.82 (1H, dd, J¼4.4,
10.8 Hz); 13C NMR: d ꢀ5.46, ꢀ5.41, 16.4, 18.3, 25.9
(3C), 27.4, 59.0, 63.3, 69.4, 84.6, 86.5; HRMS (FAB) calcd
for C13H27O2Si (M++1): 243.1781. Found: 243.1785.
A mixture of 12 and Et3N$(HF)3 (0.6 g, 3.7 mmol) in
CH2Cl2 (4 ml) was stirred for 12 h at room temperature.
The reaction mixture was diluted with CH2Cl2 and washed
with 7% aqueous NaHCO3. The organic layer was dried
over MgSO4. Evaporation of the organic solvent gave a crude
residue, which was chromatographed on silica gel (5 g,
n-hexane/AcOEt 3:1) to afford (+)-13 (0.06 g, 89% from
(ꢀ)-11) as a colorless oil. Compound (+)-13: [a]2D3 +91.4
2.3. (L)-Methyl (4S,5R)-6-tert-butyldimethylsiloxy-
4-methyl-5-methoxy-2-hexynoate (10)
A 100-ml two-necked round-bottomed flask, containing
a magnetic stirring bar, PdCl2 (47 mg, 0.27 mmol), anhy-
drous CuCl2 (1.79 g, 13.2 mmol), and anhydrous NaOAc
(1.09 g, 13.3 mmol) in MeOH (25 ml), was fitted with a rub-
ber septum and three-way stopcock connected to a balloon
filled with carbon monoxide. The apparatus was purged
with carbon monoxide by pumping–filling via the three-
way stopcock. A solution of (ꢀ)-9 (1.289 g, 5.33 mmol) in
MeOH (3.5 ml) was added to the above stirred mixture via
a syringe at 0 ꢁC. After being stirred for 4 h, the reaction
mixture was diluted with AcOEt (100 ml)/H2O (100 ml).
The organic layer was dried over MgSO4. Evaporation
of the organic solvent gave a crude residue, which was chro-
matographed on silica gel (30 g, n-hexane/AcOEt 50:1) to
provide (ꢀ)-10 as a homogeneous oil (1.399 g, 87%). [a]D24
(c 0.64, CHCl3); IR (KBr): 3450, 1711, 1623, 1150 cmꢀ1
;
1H NMR: d 1.19 (3H, d, J¼6.8 Hz), 3.24–3.32 (1H, m),
3.44–3.48 (1H, m), 3.45 (3H, s), 3.63 (3H, s), 3.67–3.73
(1H, m), 3.69 (3H, s), 4.07–4.15 (1H, m), 5.05 (1H, s);
13C NMR: d 14.5, 36.0, 51.2, 55.6, 58.1, 61.4, 83.6, 91.4,
168.6, 176.6; HRMS (FAB) calcd for C10H17O5 (M++1):
217.1076. Found: 217.1079.
2.6. (D)-Methyl (4R,5R)-3,5-dimethoxy-4-methyl-
2(E)-hexenoate (2)
A mixture of (+)-14 (0.142 g, 0.65 mmol) and Dess–Martin
reagent (0.708 g, 1.67 mmol) in CH2Cl2 (6 ml) was stirred
for 2 h at room temperature. The reaction mixture was
diluted with ether. The organic layer was washed with 7%
aqueous NaHCO3 and dried over MgSO4. Evaporation of
ꢀ15.1 (c 0.86, CHCl3); IR (neat): 2236, 1717 cmꢀ1
;

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H. Akita et al. / Tetrahedron 62 (2006) 11592–11598
the organic solvent gave a crude residue, which was chroma-
tographed on silica gel (5 g, n-hexane/AcOEt 9:1) to provide
(+)-2 as a colorless oil (0.125 g, 89%). Compound (+)-2:
[a]2D3 +104.7 (c 0.55, CHCl3); IR (KBr): 1712, 1628, 1441,
layer was washed with brine and dried over MgSO4. The
organic layer was evaporated to give a crude residue, which
was chromatographed on silica gel (30 g, n-hexane/AcOEt
30:1) to afford 19 (1.41 g, 94%). Recrystallization of 19
from n-hexane provided 19 as colorless needles. Compound
1149, 1095 cmꢀ1
;
1H NMR: d 1.20 (3H, d, J¼7.2 Hz),
1
3.43 (3H, s), 3.56 (1H, dd, J¼2.4, 6.8 Hz), 3.64 (3H, s),
3.68 (3H, s), 4.48 (1H, dq, J¼6.8, 7.2 Hz), 5.07 (1H, s),
9.59 (1H, q, J¼2.4 Hz); 13C NMR: d 13.9, 36.5, 51.0,
55.7, 58.6, 87.4, 91.9, 167.6, 174.4, 202.1; HRMS (FAB)
calcd for C10H17O5 (M++1): 217.1076. Found: 217.1079.
19: mp 76–77 ꢁC; IR (KBr): 1729 cmꢀ1; H NMR: d 1.43
(3H, d, J¼7.1 Hz), 1.44 (6H, d, J¼6.9 Hz), 3.36 (1H, sept,
J¼6.9 Hz), 4.44 (2H, q, J¼7.1 Hz), 8.02 (1H, s), 8.16 (1H,
s); 13C NMR: d 14.4, 23.1, 33.4, 61.5, 116.1, 127.4, 147.5,
147.6, 161.3, 163.5, 178.4. Anal. Calcd for C12H14N2O2S2:
C, 51.02; H, 5.00; N, 9.92%. Found: C, 50.89; H, 5.02; N,
9.96. FABMS m/z: 283 (M++1).
2.7. Ethyl 2-isopropylthiazole-4-carboxylate (16)
To a solution of phosphorus pentasulfide (P4S10; 5.1 g,
11.47 mmol) in ether (40 ml) was added isobutylamide 14
(10 g, 114.7 mmol) and the whole mixture was stirred for
2 h at room temperature. The reaction mixture was diluted
with brine and extracted with ether. The organic layer was
dried over MgSO4 and evaporated to give a crude 15. A mix-
ture of the crude 15 and ethyl a-bromopyruvate (22.39 g,
114.5 mmol) in EtOH (100 ml) was stirred at reflux for
15 min. The reaction mixture was evaporated, diluted with
AcOEt, and washed with 7% aqueous NaHCO3. The organic
layer was dried over MgSO4 and evaporated to give a crude
oil, which was chromatographed on silica gel (200 g,
n-hexane/AcOEt 20:1) to afford 16 as a colorless compound
(18.92 g, 82% overall yield from 14). Compound 16: IR
2.10. 20-Isopropyl[2,40]bithiazolyl-4-methyl alcohol (20)
A mixture of 19 (1.787 g, 6.3 mmol) and LiBH4 (0.69 g,
31.6 mmol) in THF (20 ml) was stirred for 2 h at room tem-
perature. The reaction mixture was diluted with H2O (10 ml)
and the whole was stirred for 5 h at the same temperature.
The reaction mixture was extracted with AcOEt and washed
with brine, and dried over MgSO4. The organic layer was
evaporated to give a crude residue, which was chromato-
graphed on silica gel (10 g, n-hexane/AcOEt 5:1) to afford
20 (1.506 g, 91%). Recrystallization of 20 from n-hexane
provided 20 as colorless needles. Compound 20: mp 56–
58 ꢁC; IR (KBr): 3247, 1536, 1447 cmꢀ1; 1H NMR: d 1.44
(6H, d, J¼7.2 Hz), 3.25–3.40 (1H, br s), 3.37 (1H, sept,
J¼7.2 Hz), 4.81 (2H, s), 7.19 (1H, t, J¼0.8 Hz), 7.84 (1H,
s); 13C NMR: d 23.1, 23.1, 33.4, 60.9, 115.0, 115.2, 148.4,
157.2, 163.7, 178.8. Anal. Calcd for C10H12N2OS2: C,
49.97; H, 5.03; N, 11.66%. Found: C, 50.27; H, 5.19; N,
11.43. FABMS m/z: 241 (M++1).
(neat): 1726 cmꢀ1
;
1H NMR: d 1.40 (3H, t, J¼7.2 Hz),
1.42 (6H, t, J¼7.0 Hz), 3.43 (1H, sept, J¼7.0 Hz), 4.42
(2H, q, J¼7.2 Hz), 8.06 (1H, s); 13C NMR: d 14.4, 23.3
(C2), 33.6, 61.4, 126.4, 146.6, 161.6, 179.0; MS (FAB)
m/z: 200 (M++1).
2.8. 2-Isopropylthiazole-4-thioamide (18)
2.11. 20-Isopropyl[2,40]bithiazolyl-4-methylenethio-
2-benzothiazole (21) and 20-isopropyl[2,40]bithiazolyl-
4-methylenesulfonyl-2-benzothiazole (3)
The thiazole ester 16 (20.01 g, 100.1 mmol) was treated with
saturated NH3 in MeOH (10 ml) in a sealed tube and the
whole mixture was left to stand for 3 days at room tempera-
ture. After cooling, the reaction mixture was concentrated to
afford a crude amide 17. To a solution of crude 17 in benzene
(300 ml) was added phosphorus pentasulfide (P4S10; 36.5 g,
82.1 mmol) and the whole mixture was stirred for 2 h at
reflux. The reaction mixture was diluted with H2O and ex-
tracted with AcOEt. The organic layer was washed with brine
and dried over MgSO4. The organic layer was evaporated to
give a crude residue, which was chromatographed on silica
gel (300 g, n-hexane/AcOEt 10:1) to afford 18 as a pale yel-
low compound (12.53 g, 67% from 16). Recrystallization of
18 from n-hexane afforded 18 as pale yellow needles. Coꢀm1-
To a mixture of 20 (0.1 g, 0.42 mmol), triphenylphosphine
(0.218 g, 0.83 mmol), and diethylazocarboxylate (0.27 g,
0.62 mmol) in THF (5 ml) was added 2-mercaptobenzothia-
zole (0.14 g, 0.84 mmol) under argon atmosphere and the
whole mixture was stirred for 60 min at room temperature.
The reaction mixture was diluted with 2 M aqueous NaOH
and extracted with Et2O. The organic layer was washed
with brine and dried over MgSO4. The organic layer was
evaporated to give a crude residue, which was chromato-
graphed on silica gel (20 g, n-hexane/AcOEt 30:1) to afford
22 (0.146 g, 90%). Recrystallization of 21 from n-hexane
provided 21 as colorless needles. Compound 21: mp 74–
1
pound 18: mp 83–85 ꢁC; IR (KBr): 3290, 3155, 1634 cm
;
75 ꢁC; IR (KBr): 3119, 2962, 1430, 997 cmꢀ1; H NMR:
1H NMR: d 1.40 (6H, d, J¼6.9 Hz), 3.28 (1H, sept, J¼
6.9 Hz), 7.79 (1H, br s), 8.32 (1H, s), 8.70 (1H, br s);
13C NMR: d 22.9, 33.3, 85.1, 126.4, 152.5, 177.7, 190.9.
Anal. Calcd for C7H10N2S2: C, 45.13; H, 5.41; N, 15.04%.
Found:C, 45.13;H, 5.36;N, 15.01. FABMS m/z: 187 (M++1).
d 1.42 (6H, d, J¼7.0 Hz), 3.36 (1H, sept, J¼7.0 Hz), 4.76
(2H, s), 7.27–7.31 (1H, m), 7.33 (1H, s), 7.40–7.44 (1H,
m), 7.73–7.75 (1H, m), 7.82 (1H, s), 7.89–7.91 (1H, m);
13C NMR: d 23.1, 23.1, 33.0, 33.3, 115.0, 117.5, 121.0,
121.6, 124.3, 126.0, 135.4, 148.4, 152.1, 153.1, 163.2,
166.0, 178.7. Anal. Calcd for C17H15N3S4: C, 52.41; H,
3.88; N, 10.79%. Found: C, 52.44; H, 3.96; N, 10.33.
FABMS m/z: 390 (M++1).
2.9. Ethyl 20-isopropyl[2,40]bithiazolyl-4-carboxylate
(19)
A solution of 19 (0.99 g, 5.3 mmol) and ethyl bromopyru-
vate (1.14 g, 5.8 mmol) in absolute EtOH (30 ml) was stirred
for 1 h at reflux. The reaction mixture was diluted with 7%
aqueous NaHCO3 and extracted with Et2O. The organic
To a solution of 21 (0.094 g, 0.24 mmol) in EtOH (2 ml)
were added Mo7O24(NH4)6$4H2O (0.03 g, 0.024 mmol)
and 30% H2O2 (0.5 ml) at 0 ꢁC, and the whole mixture was
stirred for 60 min at room temperature. The reaction mixture

H. Akita et al. / Tetrahedron 62 (2006) 11592–11598
11597
was diluted with brine and extracted with AcOEt. The
organic layer was dried over MgSO4. The organic layer was
evaporated to give a residue, which was chromatographed on
silica gel (15 g, n-hexane/AcOEt 3:1) to afford 3 (0.089 g,
88%). Recrystallization of 3 from n-hexane/AcOEt provided
3 as colorless needles. Compound 3: mp 129–130 ꢁC; IR
THF, 0.4 ml, 0.4 mmol) at 0 ꢁC under argon atmosphere
and the whole mixture was stirred for 20 min at the same
temperature. A solution of (+)-2 (0.073 g, 0.34 mmol) in
THF (2 ml) was added to the above reaction mixture at
0 ꢁC and the whole mixture was stirred for 20 min at the
same temperature. The reaction mixture was diluted with
H2O and extracted with AcOEt. The organic layer was dried
over MgSO4 and evaporated to afford a crude product, which
was chromatographed on silica gel (12 g, n-hexane/AcOEt
20:1) to give a 14:1 mixture (0.091 g, 64%) of (E)-1 and
(Z)-cystothiazole 23. A part of this mixture was again sub-
jected to chromatography on silica gel (5 g, n-hexane/AcOEt
(KBr): 1332, 1150 cmꢀ1
;
1H NMR: d 1.38 (6H, d,
J¼6.8 Hz), 3.31 (1H, sept, J¼6.8 Hz), 5.15 (2H, s), 7.19
(1H, s), 7.68 (1H, s), 7.67–7.71 (1H, m), 7.74–7.78 (1H, m),
8.21–8.23 (1H, m), 8.29–8.31 (1H, m); 13C NMR (acetone-
d6): d 23.1, 23.1, 33.8, 57.4, 115.7, 122.9, 123.8, 126.1,
128.7, 128.9, 138.2, 144.7, 148.9, 153.8, 163.6, 167.1, 179.4.
Anal. Calcd for C17H15N3O2S4: C, 48.43; H, 3.59; N, 9.97%.
Found: C, 48.30; H, 3.61; N, 9.80. FABMS m/z: 422 (M++1).
1
20:1) to afford (E)-1 (0.005 g). H and 13C NMR data of
the present (E)-1 were identical with those of the reported
data.2,5
2.12. 20-Isopropyl[2,40]bithiazolyl-4-methylenethio-
5-(1-phenyl-1H)-tetrazole (22) and 20-isopropyl[2,40]-
bithiazolyl-4-methylenesulfonyl-5-(1-phenyl-1H)-
tetrazole (4)
2.14. Modified Julia’s coupling of (D)-2 and 4 (synthesis
of cystothiazole A (E)-1)
To a solution of 4 (0.216 g, 0.50 mmol) in THF (5 ml) was
added lithium bis(trimethylsilyl)amide (1 M solution in
THF, 0.5 ml, 0.5 mmol) at 0 ꢁC under argon atmosphere
and the whole mixture was stirred for 20 min at the same
temperature. A solution of (+)-2 (0.09 g, 0.42 mmol) in
THF (2 ml) was added to the above reaction mixture at
0 ꢁC and the whole mixture was stirred for 20 min at the
same temperature. The reaction mixture was diluted with
H2O and extracted with AcOEt. The organic layer was dried
over MgSO4 and evaporated to afford a crude product, which
was chromatographed on silica gel (15 g, n-hexane/AcOEt
20:1) to give a 4:1 mixture (0.116 g, 66%) of (E)-1 and
(Z)-cystothiazole 23.
To a mixture of 20 (0.1 g, 0.42 mmol), triphenylphosphine
(0.218 g, 0.83 mmol), and diethylazocarboxylate (0.27 g,
0.62 mmol) in THF (5 ml) was added 1-phenyl-1H-tetra-
zole-5-thiol (0.148 g, 0.83 mmol) under argon atmosphere
and the whole mixture was stirred for 60 min at room tem-
perature. The reaction mixture was diluted with 2 M aqueous
NaOH and extracted with Et2O. The organic layer was
washed with brine and dried over MgSO4. The organic layer
was evaporated to give a crude residue, which was chroma-
tographed on silica gel (20 g, n-hexane/AcOEt 6:1) to afford
22 (0.16 g, 96%). Recrystallization of 22 from n-hexane
provided 22 as colorless needles. Compound 22: mp 102–
1
104 ꢁC; IR (KBr): 3114, 2957, 1498, 1393 cmꢀ1; H NMR:
d 1.43 (6H, d, J¼6.8 Hz), 3.36 (1H, sept, J¼6.8 Hz), 4.77
(2H, s), 7.43 (1H, s), 7.51–7.55 (5H, m), 7.79 (1H, s); 13C
NMR: d 23.1, 23.1, 33.0, 33.3, 115.1, 118.4, 123.9, 123.9,
129.8, 129.8, 130.1, 133.6, 148.3, 150.9, 153.8, 163.5, 178.8.
Anal. Calcd for C17H16N6S3: C, 50.98; H, 4.03; N, 20.98%.
Found: C, 51.00; H, 4.00; N, 20.89. FABMS m/z: 401 (M++1).
References and notes
1. (a) Gerth, K.; Irschik, H.; Reichenbach, H.; Trowitzsch, W.
J. Antibiot. 1980, 33, 1474–1479; (b) Trowitzsch, W.;
Reifenstahl, G.; Wray, V.; Gerth, K. J. Antibiot. 1980, 33,
€
To a solution of 22 (0.34 g, 0.85 mmol) in EtOH (1 ml) were
added Mo7O24(NH4)6$4H2O (0.105 g, 0.085 mmol) and
30% H2O2 (1 ml) at 0 ꢁC, and the whole mixture was stirred
for 60 min at room temperature. The reaction mixture was
diluted with brine and extracted with AcOEt. The organic
layer was dried over MgSO4. The organic layer was evapo-
rated to give a crude residue, which was chromatographed on
silica gel (20 g, n-hexane/AcOEt 8:1) to afford 4 (0.333 g,
91%). Recrystallization of 4 from n-hexane/AcOEt provided
4 as colorless needles. Compound 4: mp 121–122 ꢁC;
1480–1490; (c) Trowitzsch, W.; Hofle, G.; Sheldrick, W. S.
Tetrahedron Lett. 1981, 22, 3829–3832.
2. Ojika, M.; Suzuki, Y.; Tsukamoto, A.; Sakagami, Y.; Fudou,
R.; Yoshimura, T.; Yamanaka, S. J. Antibiot. 1998, 51, 275–
281.
3. Thierbach, G.; Reichenbach, H. Biochim. Biophys. Acta 1981,
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4. Ojika, M.; Watanabe, T.; Qi, J.; Tanino, T.; Sakagami, Y.
Tetrahedron 2004, 60, 187–194.
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IR (KBr): 1338, 1160 cmꢀ1 1H NMR: d 1.43 (6H, d,
;
J¼6.8 Hz), 3.35 (1H, sept, J¼6.8 Hz), 5.02 (2H, s), 7.44
(1H, s), 7.50–7.60 (5H, m), 7.66 (1H, s); 13C NMR: d 23.1,
23.1, 33.3, 58.1, 115.7, 122.4, 125.6, 125.6, 129.5, 129.5,
131.5, 132.9, 141.3, 147.7, 153.2, 164.2, 179.1. Anal. Calcd
for C17H16N6O2S3: C, 47.21; H, 3.73; N, 19.43%. Found: C,
47.24; H, 3.77; N, 19.17. FABMS m/z: 433 (M++1).
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2.13. Modified Julia’s coupling of (D)-2 and 3 (synthesis
of cystothiazole A (E)-1)
To a solution of 3 (0.17 g, 0.40 mmol) in THF (3 ml) was
added lithium bis(trimethylsilyl)amide (1 M solution in
11. Blakemore, P. R. J. Chem. Soc., Perkin Trans. 1 2002, 2563–
2585.

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H. Akita et al. / Tetrahedron 62 (2006) 11592–11598
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