Synthetic applications of 3H-1,2,4-dithiazoles: Structure and reactivity studies

Article abstract of DOI:10.1002/hc.20764

Synthetic applications of 3H-1,2,4-dithiazoles have been studied extensively. The facile synthesis of hitherto unknown 3H-1,2,4-dithiazole S-oxides was realized by the mCPBA oxidation of 3H-1,2,4-dithiazoles. The structural information of the 1,2,4-dichalcogenazole ring system was revealed by the ORTEP analysis of 3-tert-butyl-5-4-chloro-phenyl-3H-1,2,4-dithiazole S-oxide. The reactivities of 3H-1,2,4-dithiazoles are also discussed.

Full text of DOI:10.1002/hc.20764

Heteroatom Chemistry
Volume 23, Number 2, 2012
S
ynthetic Applications of
3
H-1,2,4-Dithiazoles: Structure
and Reactivity Studies
1
2
1
Md. Rafiqul Islam, Yuji Takikawa, and Kwon Taek Lim
1
Department of Imaging System Engineering, Pukyong National University, Busan,
608-737, South Korea
2
Department of Chemistry and Bioengineering, Iwate University, Morioka, Iwate 020-8551, Japan
Received 26 July 2011; revised 21 August 2011
variants (Chart 1) because of their heterocumulene-
like structures and intriguing reactivity for the syn-
thesis of various heterocycles. However, there have
only been a few studies on the synthesis of thiocar-
bonyl S-sulfides (thiosulfines) possessing higher π-
conjugation systems. In spite of their huge synthetic
potentialities, this area remains unexplored. There-
fore, the generation and synthetic applications of
thiosulfoxides leading to heterocycles with multifar-
ious purposes are being considered as a pragmatic
area for current research [5–11]. Our strategy is to
synthesize heterocycles having nitrogen and chalco-
gen moieties through generating highly reactive in-
termediates possessing chalcogen and nitrogen func-
tionalities and their subsequent ring closure or
trapping.
ABSTRACT: Synthetic applications of 3H-1,2,4-
dithiazoles have been studied extensively. The facile
synthesis of hitherto unknown 3H-1,2,4-dithiazoleS-
oxides was realized by the mCPBA oxidation of 3H-
1
1
,2,4-dithiazoles. The structural information of the
,2,4-dichalcogenazole ring system was revealed by the
ORTEP analysis of 3-tert-butyl-5-4-chloro-phenyl-3H-
,2,4-dithiazole S-oxide. The reactivities of 3H-1,2,4-
1
dithiazoles are also discussed. ꢀC 2011 Wiley Periodicals,
Inc. Heteroatom Chem 23:154–159, 2012; View this article
online at wileyonlinelibrary.com. DOI 10.1002/hc.20764
INTRODUCTION
Many heterocyclic rings are found to be key com-
ponents in biological systems. Heterocyclic scaf-
folds having nitrogen, sulfur, and oxygen atoms are
playing a pivotal role in pharmaceutical chemistry
and other fields due to their recognized biologi-
cal profiles and unique multifunctional characteris-
tics [1–4]. Reactive intermediates containing thio-
carbonyl functionalities conjugated with nitrogen
moiety have long been studied as versatile build-
ing blocks for the synthesis of useful heterocyclic
compounds. A great deal of attention has been fo-
cused on the generation and synthetic application
of α,β-unsaturated thione S-oxides and their thio
In this way, we have already developed a few
methods for generating highly reactive building
blocks (sulfine or thiosulfine) through the thermal
cycloreversion of appropriate precursors [12–14].
In addition, a series of 3H-1,2,4-dichalcogenazoles
3–7 were synthesized by the reaction of 6H-1,3,5-
oxachalcogenazines 1 or 2 with respective chalco-
gens, as shown in Scheme 1 [15].
As 1,2,4-dichalcogenzoles 3–7 are new com-
pounds, detailed structural information on this new
ring system is valuable for scientific purposes. In
this regard, we tried to prepare a crystalline com-
pound of this series with different of our meth-
ods, but all efforts were unsuccessful. We then
turned our efforts to the preparation of any crys-
talline derivative of 1,2,4-dichalcogenzoles in this
Correspondence to: Kwon Taek Lim; e-mail: ktlim@pknu.ac.kr.
ꢀ
c 2011 Wiley Periodicals, Inc.
154

Synthetic Applications of 3H-1,2,4-Dithiazoles: Structure and Reactivity Studies 155
tween heretodiene A, generated through the thermal
cycloreversion of 1, and elemental sulfur [15].
Synthesis, Characterization, and Structure of
3H-1,2,4-Dithiazole S-oxides 8
Since all of the 1,2,4-dichalcogenazoles 3–7 were
oily in nature, obtaining clear structural informa-
tion on the compounds was not possible from the
X-ray analysis. Therefore, we attempted to synthe-
size a crystalline derivative of this series. In addi-
tion, it is a matter of great interest to find out the
preferential oxidation site (sulfur atom) of hetero-
cycles having a disulfide moiety. The oxidation of
CHART 1 Representative building blocks.
3
having two sulfur atoms in the 1,2-positions by
◦
mCPBA (1.1 molar amount) at 0 C for 1 h afforded
the corresponding sulfoxides 8 as a single epimer
in nearly quantitative yields. Fortunately, the crys-
talline product 8b was obtained with this approach.
study, and finally, the crystalline compound of 3H-
1
1,2,4-dithiazole-S-oxide 8b. In addition to getting
The spectral data, including MS, IR, H NMR, and
13
a structural overview of the 1,2,4-dichalcogenazole
ring system, we were also interested in the syn-
thetic applications of the 1,2,4-dichalcogenazole
ring system. Herein, we report on the synthetic
applications, characterization, and reactivity of the
C NMR, as well as the elemental analysis were fully
consistent with the structures of 3-aryl-5-alkyl-3H-
1,2,4-dithiazole S-oxides 8a–d. The results for the
synthesis of 3 and 8 are summarized in Table 1.
1
In the H NMR spectrum, the peak ascribed to
3
H-1,2,4-dithizoles ring system.
the methine proton of compound 3b appeared at
RESULTS AND DISCUSSION
TABLE 1 Synthesis of 3H-1,2,4-Dithiazoles 3 and 3H-1,2,4-
Dithiazole-S-oxides 8
Synthesis of 3H-1,2,4-Dithiazoles 3
In the course of our studies on reactive building
blocks for the synthesis of heterocyclic compounds,
we developed a facile method for the synthesis of
compounds 3 by the thermal reaction of 6H-1,3,5-
oxathiazines 1 with elemental sulfur, as shown in
Scheme 1 [15]. All of the compounds 3 were oily
in nature. Characterization of 3H-1,2,4-dithiazoles
3
was carried out by measuring physical data, in-
1
13
cluding MS, IR, H NMR, C NMR, and elemental
analysis. The formation mechanism of 3 was sug-
gested by the speculating [4 + 1]-type reaction be-
SCHEME 1 Synthesis of 1,2,4-dichalcozenazoles 3–7.
Heteroatom Chemistry DOI 10.1002/hc

156 Islam, Takikawa, and Lim
◦
◦
˚
FIGURE 1 ORTEP drawing of 8b. Selected bond lengths (A), bond angles (. ), and torsion angles (. ): S1–O2 = 1.484(6);
S1–S2 = 2.090(3); N1–C9 = 1.262(8); S2–S1-C8 = 91.5(2); S2–S1– O2=110.2(3); O2–S1–C8 = 104.4(3); C8–S1–S2–C9 =
–
16.9(2); O2–S1–S2–C9 = 89.1(2).
δ6.28, which shifted downfield by 0.32 ppm com-
pared to that of the corresponding methine proton
of 8b (δ5.96). This might be due to the anisotropic
effect of S O groups. Therefore, the position of
the newly introduced oxygen atom was assumed
to be the S-2 atom of 8b [16–18]. Finally, this
speculation was unequivocally confirmed by the
X-ray crystallographic analysis. The ORTEP draw-
ing of 8b, as shown in Fig. 1, demonstrates that
the relative stereochemistry of the sulfinyl group at
the S-2 atom is in a trans position to that of the
nearby t-butyl group at the C-3 position. The delo-
calization of the electron pair on the sulfur atom of
the 1-position might occur toward the C N moiety,
which possibly exposed the S-2 atom to oxidation.
The final structural elucidation of compound 8b by
X-ray analysis would absolutely provide a reliable
platform for exploring the core structure of the 1,2,4-
dichalcogenazole ring system.
ture was solved by using the direct method
(SIR97).
Crystallographic data (excluding structure fac-
tors) have been sent to the Cambridge Crys-
tallographic Data Centre as supplementary pub-
lication no. CCDC 219595. Copies of the data
can be obtained, free of charge, via the Inter-
net at http://www.ccdc.cam.ac.uk, or on applica-
tion to the director at CCDC 219595, 12 Union
Road, Cambridge CB2 1EZ, UK; tel.: (+44)1223-336-
408; fax: (+44)1223-336-033; e- mail: deposit@ccdc.
cam.ac.uk.
Synthetic Applications of 3H-1,2,4-Dithiazoles 3
The salient feature of the reactivity of 3 is shown in
Scheme 2. It was observed that compounds 3 were
relatively stable for a long time. It was already men-
tioned in the preceding section that the mCPBA (1.1
molar amount) oxidation of 3 afforded 8 in nearly
quantitative yields. When a toluene solution of 3a
was heated with p-toluenesulfonic acid (1.1 molar
amount) at the refluxing temperature, 10 was pro-
duced in a 43% yield, along with some unidentified
Colorless plates of 8b suitable for X-ray in-
vestigation were obtained from hexane. Crystal
data for 8b: C12
H
14ClNOS
2
, FW = 287.02, crystal
3
size 0.30 × 0.20 × 0.10 mm , monoclinic, space
group P2
1
/n (#14), a = 15.086(1), b = 6.0722(6),
˚
◦
c = 15.711(1) A, β = 110.651(4) , V = 1346.8(2)
˚
3
3
−1
A , Z = 4, Dcalc = 1.419 g/cm , μ = 5.76 cm .
compounds. The reaction of 3a with Ph P in the
3
From 25,338 reflections measured, 3362 were unique
presence of EtOH at room temperature gave 9 (1,4-
adduct of heterodiene A) in a 98% yield, along with
(
R
int = 0.023). R = 0.070, R
w
= 0.084, Mo
˚
◦
Kα (λ = 0.71075 A), T = 23 C. The struc-
Ph
3
P S in a 72% yield. On the other hand, when this
Heteroatom Chemistry DOI 10.1002/hc

Synthetic Applications of 3H-1,2,4-Dithiazoles: Structure and Reactivity Studies 157
ionization at 20 or 70 eV using a direct inlet sys-
tem. IR spectra were recorded for thin film (neat)
or KBr disks on a Jasco FT/IR-7300 spectrometer.
Elemental analyses were performed using a Yanagi-
moto CHN recorder MT-5. Column chromatography
was performed using silica gel (Merck, cat. no. 7734)
without pretreatment. All substrates and reagents
were commercially available in a reagent grade and
were used without further pretreatment.
General Procedure for the Synthesis of
3
H-1,2,4-Dithiazoles (3)
A toluene solution of 1a–d (1.0 mmol) was treated
with a 5 molar amount of elemental sulfur at a re-
fluxing temperature for 12 h. The solvent was evapo-
rated in vacuo, sulfur was removed from the result-
ing crude product by filtration, and the filtrate was
SCHEME 2 Reactivity of 3H-1,2,4-dithiazoles 3.
subjected to chromatographic separation on Al
to afford 3H-1,2,4-dithiazoles 3 in moderate to ex-
cellent yields.
2
O
3
reaction was conducted in the absence of EtOH, a
complex mixture was obtained. It is speculated that a
thiophilic attack of Ph
in 2-position) of 3a, which subsequently extruded
the sulfur atom from the ring, resulting in 1,3-thiaza-
,3-butadiene A at room temperature. This is a new
3
P occurred at the sulfur atom
(
3
-tert-Butyl-5-phenyl-3H-[1,2,4]dithiazole, 3a
+
[
(
1
14]. Pale yellow oil; MS m/z (%) 237 (M ; 18), 205
1
+
+
M – S, 1), 180 (M – C
4
9
H ; bp); IR (neat): 2961,
approach for the generation of heterodiene A at a
low temperature by using 3 as the precursor.
−1
1
633, 1448, 1363, 1046, 928, 689, 602 cm ; H
NMR (400 MHz, CDCl ): δ1.13 (9H, s), 6.31 (1H, s),
3
13
7
.40–7.48 (3H, m), 7.83–7.85 (2H, m); C NMR
CONCLUSION
(CDCl ): δ26.7 (q), 38.9 (s), 104.1 (d), 128.7 (d),
3
1
6
6
28.9 (d), 131.7 (d), 132.1 (s), 165.1 (s); Found: C,
The synthetic applications of the 3H-1,2,4-dithiazole
ring system have been investigated. A facile syn-
thesis of novel 3H-1,2,4-dithiazoleS-oxides 8 was
realized by the simple mCPBA oxidation of 3H-1,2,4-
dithiazoles 3. The structural feature of 3H-1,2,4-
dithiazole-S-oxide 8b was uncovered by an X-ray
analysis. The reactivity of 3H-1,2,4-dithiazoles 3 was
studied. Further studies for the extensive utiliza-
tion of the 1,2,4-dichalcogenazole ring system are
in progress.
0.91; H, 6.56; N, 5.89%. Calcd. for C12
0.71; H, 6.37; N, 5.90%.
H
15NS : C,
2
3
-tert-Butyl-5-(4-chloro-phenyl)-3H-[1,2,4]dithia-
+
zole,3b [14] . Pale yellow oil; MS m/z (%) 271 (M ;
5), 239 (M – S, 2), 214 (M – C
+
+
1
4
9
H ; bp); IR (neat):
2
961, 1623, 1488, 1363, 1253, 1093, 927, 832, 590
−
1 1
cm ; H NMR (400 MHz, CDCl ): δ1.10 (9H, s), 6.29
3
(
8
1H, s), 7.39 (2H, d, J = 8.4 Hz), 7.77 (2H, d, J =
13
.5 Hz); C NMR (CDCl ): δ26.7 (q) 38.9 (s), 104.0
3
(
d), 128.4 (s), 128.5 (s), 128.9 (d), 130.2 (d), 69.6
(s); Found: C, 53.28; H, 5.33; N, 4.62%. Calcd. for
: C, 53.02; H, 5.19; N, 5.15%.
EXPERIMENTAL
General
C
12
H
14ClNS
2
Melting points were measured in open capil-
lary tubes with a Buchi 535 micromelting point
apparatus and are uncorrected. H NMR spec-
3-tert-Butyl-5-(4-fluoro-phenyl)-3H-[1,2,4]dithia-
+
zole, 3c [14] . Pale yellow oil; MS m/z (%) 255 (M ;
1
+
+
18), (M – S, 223), 198 (M – C
4
H
9
; bp); IR (neat):
-
1
tra were determined at 400 MHz (Bruker AC-
2962, 1629, 1507, 1236, 1046, 929, 840, 601 cm ;
13
1
4
00P spectrometer), and C NMR spectra were
H NMR (400 MHz, CDCl
s), 7.10 (2H, t, J = 8.5 Hz), 7.84 (2H, dd = 5.3, 5.4
Hz); C NMR (CDCl ): δ26.6 (q), 38.8 (s), 104.0 (d),
3
): δ1.10 (9H, s), 6.28 (1H,
determined at 100 MHz (Bruker AC-400P spec-
trometer). Chemical shifts are expressed in parts
13
3
per million (δ units) downfield from
tetram-
115.7 (d), 115.9 (d), 131.0 (d), 131.1 (d), 163.8 (s),
ethylsilane which was used as an internal refer-
ence. Mass spectra were recorded on a Hitachi
M-2000 mass spectrometer with electronimpact
164.8 (d, J = 251.4 Hz); Found: C, 56.89; H, 5.48; N,
5.17%. Calcd. for C12
5.48%.
H
14FNS : C, 56.44; H, 5.53; N,
2
Heteroatom Chemistry DOI 10.1002/hc

158 Islam, Takikawa, and Lim
3
-tert-Butyl-5-(4-methoxy-phenyl)-3H-[1,2,4]dit-
165.0 (d, J = 251.8 Hz); Calcd. for C12
H
14FNOS : C,
2
hiazole, 3d [14] . Pale yellow oil; MS m/z (%) 267
53.11; H, 5.20; N, 5.16%; Found: C, 53.28; H, 5.22;
N, 4.95%.
+
+
+
(
M ; 11), 235(M – S, 3), 210 (M – C
4
9
H ; bp); IR
(
neat): 2961, 1606, 1509, 1257, 1173, 926, 835, 600
−
1
1
cm ; H NMR (400 MHz, CDCl
3
7
3
): δ1.10 (9H, s),
.84 (3H, s), 6.26 (1H, s), 6.91 (2H, d, J = 8.8 Hz),
.80 (2H, d, J = 8.8 Hz); C NMR (CDCl ): δ26.7
3
-tert-Butyl-5-(4-methoxy-phenyl)-3H-[1,2,4]dit-
hiazole S-oxide, 8d. Colorless oil; MS m/z (%) 267
13
+
+
3
(
2
M – O; 7), 203 (M – S O, 13), 188 (bp); IR (neat):
2
(q), 38.8 (s), 55.4 (q), 104.0 (d), 113.9 (d), 124.8 (s),
-
1
1
962, 1604, 1509, 1464, 1079, 1057, 603 cm ; H
1
6
6
30.7 (d), 139.1 (s), 164.3 (s); Found: C, 58.67; H,
NMR (400 MHz, CDCl ): δ1.14 (9H, s), 3.87 (3H,
3
.23; N, 4.96%. Calcd. for C13
.41; N, 5.24%.
H
17NOS : C, 58.39; H,
2
s), 5.96 (1H, s), 6.96 (2H, d, J = 8.8 Hz), 7.88 (2H,
13
d, J = 8.8 Hz); C NMR (CDCl
3
): δ 27.5 (q), 37.4
(
s), 55.5 (q), 114.3 (d), 124.4 (s), 129.7 (d), 131.6
d), 136.4 (s), 162.8 (s); Calcd. for C13 : C,
5.09; H, 6.05; N, 4.94%; Found: C, 55.15; H, 6.02;
N, 4.36%.
(
5
H17NO
2
S
2
Synthesis of 3H-1,2,4-Dithiazoles S-oxides 8 by
mCPBA Oxidation of 3H-1,2,4 Dithiazoles 3
A
chloroform solution (20 mL) of 3H-1,2,4-
dithiazoles (3, 1.0 mmol) was treated with mCPBA
N-(1-Ethoxy-2,2-dimethyl-propyl)-thiobenzamide,
◦
(1.1 molar amount) at 0 C for 1 h in the presence
1
9
(
5
(
. Pale yellow oil, (lit. [8]); H NMR (CDCl
3
): δ1.04
of NaHCO
was quenched with aqueous Na
3
(2 molar amount). The reaction mixture
SO solution and
9H, s), 1.19 (3H, t, J= 5.5 Hz), 3.64 (2H, dq, J =
2
3
.3 Hz), 5.81 (1H, d, J = 9.5 Hz), 7.39 (3H, m), 7.74
was extracted with chloroform. The mixture was
then subjected to the usual workup. The solvent was
evaporated in vacuo, and the crude product was sub-
jected to chromatographic separation on silica gel.
The products 3H-1,2,4-dithiazole S-oxides 8 were
obtained in a high yield.
2H, d, J = 7.1 Hz).
Benzonitrile, 10. Colorless oil; MS m/z (%) 103
+
+
+
(
M , bp), 104.05 (M , 7.9%), 76 (M – CN, 28); IR
1
(
neat): 2962, 1604, 1509, 1464, 1079, 1057, 603 cm- ;
1
H NMR (400 MHz, CDCl
.38 (3H, m); C NMR (CDCl ): δ 112.5 (s), 118.9 (s),
3
): δ7.34(2H, d, J = 8.1 Hz),
13
7
3
3
-tert-Butyl-5-phenyl-3H-[1,2,4]dithiazole S-oxi-
+
129.3 (d), 132.3 (d), 132.9 (d); Calcd. for C H N: C,
7
5
de, 8a. Colorless oil; MS m/z (%) 237 (M – O; 2),
21 (M – S, 1), 158 (bp); IR (neat): 2963, 1634, 1474,
+
81.53; H, 4.89; N, 13.58%; Found: C, 81.65; H, 4.91;
N, 13.47%.
2
1
-
1
1
148, 1084, 1056, 689 cm ; H NMR (400 MHz,
): δ1.15 (9H, s), 5.98 (1H, s), 7.46–7.55 (3H,
CDCl
m), 7.92–7.95 (2H, m); C NMR (CDCl
3
13
3
): δ27.5 (q),
7.4 (s), 128.9 (d), 129.5 (d), 12987 (d), 131.0 (s),
32.3 (d), 160.6 (s); Calcd. for C12 : C, 56.88;
ACKNOWLEDGMENTS
3
1
H15NOS
2
Dr. Rafiqul gratefully acknowledges MEXT,
Japanese government, for the pre-doctoral scholar-
ship and the second stage of the BK21 program of
the South Korean government for the postdoctoral
fellowship during this research.
H, 5.97; N, 5.53%; Found: C, 57.02; H, 5.98; N, 5.39%.
3
-tert-Butyl-5-(4-chloro-phenyl)-3H-[1,2,4]dithia-
◦
zole S-oxide, 8b. Colorless needles, mp 88–89 C;
MS m/z (%) 271 (M – O; 3), 207 (M – S
+
+
2
O, 12), 192
(
9
bp); IR (KBr): 2962, 1624, 1488, 1399, 1083, 1056,
-
1
1
REFERENCES
20, 618 cm ; H NMR (400 MHz, CDCl ): δ1.16
3
(
(
(
(
9H, s), 5.96 (1H, s), 7.45 (2H, d, J = 8.6 Hz), 7.87
2H, d, J = 8.6 Hz); C NMR (CDCl ): δg g27.5
q), 37.5 (s), 129.2 (s), 129.3 (d), 129.5 (d), 130.4
s), 131.0 (d), 159.5 (s); Calcd. for C12 : C,
0.07; H, 4.90; N, 4.87%; Found: C, 50.23; H, 4.91;
N, 4.74%.
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Heteroatom Chemistry DOI 10.1002/hc