Isothiazole ring formation with substituted 2-alkylthio-3-acyl-4-quinolinone using O-(mesitylenesulfonyl)hydroxylamine (MSH)

Article abstract of DOI:10.1055/s-2003-36802

Isothiazole ring skeleton was formed by the treatment of substituted 2-alkylthio-3-acyl-4-quinolinone with O-(mesitylenesulfonyl)hydroxylamine(MSH). A mixture of alkyl transferred 3-methyl-9-alkyl-4,9-dihydroisothiazolo[5,4-b]quinolin-4-one as a major product and dealkylated 3-methyl-4,9-dihydroisothiazolo[5,4-b]quinolin-4-one as a minor product was obtained from unsubstituted 2-alkylthio-3-acyl-4-quinolinone in the presence of K₂CO₃. When 2 equivalents of MSH were used in the absence of K₂CO₃ only dealkylated product was obtained in quantitative yield.

Full text of DOI:10.1055/s-2003-36802

1
66
LETTER
Isothiazole Ring Formation with Substituted 2-Alkylthio-3-acyl-4-quinolinone
Using O-(Mesitylenesulfonyl)hydroxylamine (MSH)
I
J
sothiazole
o
Ring Forma
n
tion with Su
g
bstituted 2-Alkylth
H
io-3-acyl-4-quino
el inone e Choi, Eun Bok Choi, Chwang Siek Pak*
Korea Research Institute of Chemical Technology, Yusung-Ku Jang-Dong 100, Taejeon, 305-600, Korea
Fax +82(42)8600307; E-mail: cspak@krict.re.kr
Received 9 December 2002
aldehyde or ester. An alkylthio or thiol group is first con-
Abstract: Isothiazole ring skeleton was formed by the treatment of
substituted 2-alkylthio-3-acyl-4-quinolinone with O-(mesitylene-
sulfonyl)hydroxylamine(MSH). A mixture of alkyl transferred 3-
verted into sulfenyl chloride with sodium hypochlorite,
thionyl chloride or sulfuryl chloride as an acceptor for ni-
methyl-9-alkyl-4,9-dihydroisothiazolo[5,4-b]quinolin-4-one as a trogen.8d,11,12b To the best of our knowledge, the only pub-
major product and dealkylated 3-methyl-4,9-dihydroisothiazo- lished example of the addition of a sulfur atom to nitrogen
lo[5,4-b]quinolin-4-one as a minor product was obtained from un-
substituted 2-alkylthio-3-acyl-4-quinolinone in the presence of
as an alkylthio group was the cyclization of 2-ethylthio-3-
O-acetylaldoxime quinoline to yield isothiazoloquino-
K CO . When 2 equivalents of MSH were used in the absence of
9c
2
3
line. Cleavage of the ethyl group from the ethylthio
K CO only dealkylated product was obtained in quantitative yield.
2
3
group was explained by the intramolecular transfer of the
ethyl group to an acetate group in the oxime acetate dipo-
lar complex, to ultimately producing ethyl acetate.
Key words: imine, electrophilic addition, cyclizations, isothiazole,
O-(mesitylenesulfonyl)hydroxyl amine
Here we report direct fused isothiazole ring formation by
O-(mesitylenesulfonyl)hydroxylamine (MSH) which has
never been employed as a condensing agent to yield an
isothiazole ring. In the literature MSH has been known to
deliver its amino moiety as an electrophile to a number of
Isothiazole moiety either fused or isolated form has been
frequently found to play an important role in various bio-
1
logical activities such as an anti-inflammatory, antifun-
2
3
4
5
gal, antibiotics, antiviral, cholesterol lowering agent,
13
14
6
7
different atoms such as nitrogen, sulfur, or phenolic
microbicide, and agrochemical. Numerous methods of
preparation of fused isothiazoles such as benzoiso-
thiazoles, quinolinoisothiazoles, and other heterocylic
15
oxygen and to undergo condensation with carbonyl
group.16
8
9
1
0
11
isothiazoles or of isolated isothiazoles have been very In the beginning we attempted N-amination of 2-meth-
well documented. Among others, cyclizations of aliphatic ylthio-3-acylquinolin-4-one (1a) with MSH as described
substrates or aromatic substrates containing sulfenamide, in the literature17 to prepare N-aminoquinolinone which,
thiocyanato, alkylthio or thiol group adjacent (ortho posi- in turn, was needed for our ongoing program to prepare
tion) to aldehydes, or esters group are common in the biologically active molecules for a new agrochemical
presence of a condensing agent such as hydroxylamine, purpose (Scheme 1). Instead of N-aminoquinolinone, we
hydroxylamine-O-sulfonic acid as a nitrogen source for unexpectedly obtained a mixture of dihydroisothiazolo-
1
0–12
isothiazole.
Mostly, it is necessary for sulfur atom to quinolinones 2 and 3 of which structures were identified
bear good leaving group like NH , CN, or Cl to accommo- unambiguously by X-ray crystallography as shown in
2
date nucleophilic attack by nitrogen atom condensed with Figure 1 and Figure 2.18
O
O
S
O
ONH2
R
O
O
S
O S
N
X
DMF/K2CO3
NH2
O
O
+
R
N
H
X
N
+
N
1
a-l
S
N
R
S
X
N
H
X
2
a-l
3a-l
Scheme 1
Synlett 2003, No. 2, Print: 31 01 2003.
Art Id.1437-2096,E;2003,0,02,0166,0172,ftx,en;U09802ST.pdf.
Georg Thieme Verlag Stuttgart · New York
ISSN 0936-5214
©

LETTER
Isothiazole Ring Formation with Substituted 2-Alkylthio-3-acyl-4-quinolinone
167
Figure 1 X-ray structure of 2a at the 50% probability level
Results for various isothiazole formation are summarized
Figure 2 X-ray structure of 3f at the 50% probability level
1
9
in Table 1.
It is obvious from entry 1a–d that these substrates provide
the same dealkylated isothiazole product 3 regardless of 0 °C to prevent decomposition. The only product isolated
the character of R group.
was the corresponding dealkylated isothiazole 3j without
any trace of N-methylated product. In addition MSOMe
was always isolated in minute quantity (ca. 8%). Drastic
changes in the product ratio of 1j and ik can be explained
Yield of the minor product 3 varies from 10% to 40% de-
pending on the type of alkyl group. Yield of the phenyl
group is the lowest. In the case of 1j, evaporation of the
solvent under vacuum at room temperature resulted in the
decomposition of residue. The product was isolated at
by difference in σ where Br exhibits more electron
p
withdrawing power so that the proton on nitrogen atom
Table 1 Product Yields of Dihydroisothiazoloquinolinones 2 and 3a
Substrate (1)
X
R
Product yield (%)
Mp (°C)
2
3
2
3
1
1
1
1
1
1
1
1
1
1
a
b
c
d
e
f
H
Me
n-Pr
Bn
55
45
57
55
20
60
22
63
61
0
30
40
22
10
40
10
42
11
19
70b
199–200
125–126
181–182
177–179
211–212
189–190
183–185
208–209
241–243
–
342–344
342–344
342–344
342–344
262–263
284–285
291–293
330–331
339–340
258–259
H
H
H
Ph
8-Et
8-Ph
6-t-Bu
6-MeO
6-C1
6-C1
Me
Me
Me
Me
Me
Me
g
h
i
j
8
-CF3
6-Br
-CF3
1k
Me
55
0c
145–147
–
8
a
b
c
Reaction condition: see ref.19
Work-up was performed at 0 °C due to instability of the product during isolation.
Significant amount (22%) of N-amination product was observed.
Synlett 2003, No. 2, 166–172 ISSN 0936-5214 © Thieme Stuttgart · New York

1
68
J. H. Choi et al.
LETTER
become more acidic. Efforts have been made to optimize Table 3 Product Yields of Dihydroisothiazoloquinolinones 3a
reaction condition as can be seen in Table 2. Depending
O
O
O
upon the amount of K CO , product yields changed dra-
2
3
2
equiv MSH
DMF
matically.
N
/
S
Although more than 70% of the starting material was re-
covered, demethylated product 3a was obtained as a sin-
gle product in a high conversion rate when less than 1
N
H
S
N
H
3
X
X
1
equivalent of K CO is was used. Even without it the re-
b
2
3
Entry
Substrate 1 (X =)
Yield (%) of 3
action proceeded to give only product 3a. To complete the
1
2
3
4
5
6
7
8
H
99
98
reaction 2.5 equivalents of K CO was required. Addi-
2
3
tional amount did not affect the yield and ratio of the prod-
uct mixture. Surprisingly, when an excess amount of
MSH (2 equiv) was employed in the absence of K CO the
6-OMe
8-Et
100
99
2
3
reaction was completed within 5 minutes furnishing 3a
quantitatively as a single product (entry 5). However, the
amount of MSOMe isolated was approximately the same
as before in 8% yield. Addition of K CO almost did not
8-Ph
6-Cl
98
2
3
6-Cl, 8-CF3
98
affect yield of 3a except that minute amount of O-methyl-
ated product (2–3%) instead of MSOMe was obtained.
Various substrates are subjected under these reaction con-
ditions and the results are summarized in Table 3.
6-Br, 8-CF
S-Bnd
52c
100
0
3
9
8-MeO, 3-CN
The reaction of quinolinones proceeded so rapidly that the
substrate disappeared completely at room temperature
and the product 3 was precipitated within 5 minutes. Ap-
proximately the same amount (ca. 8%) of MSOMe was
obtained regardless of substituent of quinolinone. Assum-
ing the only fate of methyl group is to form MSOMe, far
less MSOMe was isolated than expected. In the case of
entry 7, a mixture of 3 and N-aminated product was ob-
tained in 52% and 32% yield respectively.
a
2
Equiv of MSH was used in the absence of K CO .
2 3
b
c
Isolated yield.
Mp138–140 °C, significant amount (32%) of N-amination product
was obtained.
d
X = H, S-Bn instead of S-Me.
1
In order to clarify the fate of the methyl group H NMR
500 MHz) was run at a concentration diluted by one fifth
of the real reaction condition in DMF-d in order to avoid
(
When cyano group instead of acyl group is substituted
like entry 9, it was inert to the reaction condition.8a,9a We
can isolate MSOH in an aqueous layer from hydrolysis of
an ion pair or/and from the remaining MSH during the
work up. This was verified by isolating MSOH after treat-
7
20
1
precipitate of product. According to the analysis of H
NMR, MeOH and MSOH peaks appeared early in the re-
action at δ = 3.284 ppm (OMe) and δ = 6.824 ppm (aro-
matic) respectively. As the reaction proceeded, a new
peak at δ = 3.706 ppm (OMe) gradually increased and
ment of MSH in aqueous DMF (10 equiv H O).
2
Table 2 Reaction of 2-Thiomethyl-3-acylquinolin-4-one (1a)a
Entry
MSH (equiv)
K CO (equiv)
Reaction time
Yield (%)b
2
3
1
a
2a
0
3a
24c
20
30
30
98
97
98
MSOMe
1
2
3
4
5
6
1
0
1
4 h
4 h
70
72
0
7
1
0
9
1.1
1.1
2
2.5
3.5
0
4 h
55
55
0
8
4 h
0
8
5 min
15 min
15 min
0
8
2
1
0
0
0d
0d
7
2
2.5
0
0
a
Reaction was run in DMF at r.t.
Isolated yield.
Addition of 1 equiv and 10 equiv of H O did not change product yield.
b
c
d
2
Small amount (2–3%) of 4-methoxy-3-methylisothiazoloquinoline was obtained.
Synlett 2003, No. 2, 166–172 ISSN 0936-5214 © Thieme Stuttgart · New York

LETTER
Isothiazole Ring Formation with Substituted 2-Alkylthio-3-acyl-4-quinolinone
169
1
reached its maximum after 2 hours. By comparing its H ion pair 7′ undergoes hydrolysis by H O generated from
2
2
1
NMR in DMF-d and GCMS spectra to an authentic cyclized adduct 6′ to produce MeOH and MSOH. Once
7
sample, it was characterized as methylformate. In addi- MeOH is generated, it is trapped by the solvent to form
22
tion, the presence of a small amount of MSOMe was con- dimethylformamide hemiacetal, which was consequent-
firmed by an authentic sample prepared separately by ly split into dimethylamine and methylformate as indicat-
reacting MSCl with MeOH as solvent in the presence of ed in Scheme 3.
Et N. Corresponding peaks of MSOMe (OMe: δ = 3.711
3
ppm, 2 H aromatic: δ = 7.011 ppm) appeared at the early
stage of the reaction (within 5 min) in a small amount and
remained during the whole reaction period. Possibility to
transfer methyl group from MSOMe to 3a has been ex-
cluded after control experiment treating 3a with MSOMe
OH
O
MSOH
H
N
H
OH
H
+
MeOH
N
OMe
N
H
OMe
OMS
O
NH
+
+
MSOH
in the presence of K CO . Reaction did not proceed at all.
2
3
H
O
While all of the aromatic protons (δ = 7.116 ppm) of MSH
disappeared, those of MSOH (δ = 6.824 ppm) protons
kept increasing. Even after the substrate peaks disappear,
further decrease of MSH peaks was observed.
Scheme 3
A control experiment verified in DMF reacting MSH with
MeOH in the presence of catalytic amount of MSOH that
disappearance of the MSH aromatic peak was followed by
appearance of methylformate peak. Since MSH is known
1
Based upon a time dependant stack plot analysis of H
NMR, a plausible mechanism can be depicted as dis-
played in Scheme 2. It can be explained that the incipient
O
O
N
N
S
N
N
S
OMS
2
K
7
-H2O
O
O
OH
N
NH
S
N
H
3
S
N
OMS
K
6
+
MeOH + MSOH
- MSOMe
K CO
2
3
O
O
S
H
O
HO
O
N
OMS
H O
O
O
-
N
S
2
+
MSONH2
O
N
H
1
N
H
4
S
minor
N
H
S
5
major
-
MSOMe
O
O
S
O
OH
NH
O
MS
S
N
S
N
H
OMS
N
H
3
6'
-
H O
2
+
MSOH +MeOH
O
N
S
N
H
OMS
7'
Scheme 2 Mechanism for the formation of dihydroisothiazoloquinolinone
Synlett 2003, No. 2, 166–172 ISSN 0936-5214 © Thieme Stuttgart · New York

1
70
J. H. Choi et al.
LETTER
to decompose by itself to nitrogen, hydrazine and Table 4 Product Yields of Isothiazoles with Various Substratesa
2
3
MSOH, an excess amount of MSH is required to com-
plete the reaction. Since MSOMe was produced in small
quantities and stayed constant during the reaction a dual
reaction pathway seemed to be operating. A major path-
way is forming the ion pair 7′ from tetrahedral intermedi-
ate 4 and a minor pathway is the oxime 5 formation, which
Entry Substrate
1
Product
Yield (%)b
O
100c
N
N
S
S
N
N
S
9
c,d
undergoes cyclization to provide MSOMe.
Thus the
Cl
Cl
O
Cl
Cl
methyl group of thiomethyl ended up as MeOH, methyl-
formate, and MSOMe via the competitive reaction of
forming ion pair 7′ and oxime 5. Thus, once MSOH and/
2
88c
N
O
S
N
or H O are generated, the remaining MSH will be rapidly
2
destroyed unless equilibrium is shifted to the far right. The
equilibrium shift forms tetrahedral intermediate 4 so that
one equivalent of MSH should be used up quickly to form
product. In comparison with the equimolar reaction, the
excess amount of MSH must have played a role shifting
equilibrium to tetrahedral intermediate so that equivalent
amount of MSH was used up quickly before it decom-
posed. Remaining MSH must have been decomposed ei-
ther by itself or by reacting with other species as
O
3
4
5
–
–
–
0d
0d
0d
O
S
O
S
O
S
mentioned above. Depletion of H O peak (δ = 3.299
2
ppm), which was appeared at the beginning of the reaction
1
was observed in H NMR indicating that H O was used up
2
in the course of reaction. When K CO is present, potassi-
2
3
a
b
c
d
2
Equiv of MSH was used in the absence of K CO .
2 3
um salt of 7′ can be assumed to be 7. In addition, the same
reaction as intemediate 7′ intramolecular methyl transfer
to nitrogen anion in ion pair 7 can occur via four mem-
bered [1,3]thiazeto transition state, of which its existence
can be supported by recent examples of the synthesis of
Isolated yield.
Starting material was recovered after 86 h.
Strong fluorescent.
[
1,3]thiazeto[3,2a]quinoline-3-carboxylic acids.24 The
distinguish reactivity between nucleophilic oxime forma-
tion and electrophilic sulfilimine was made without suc-
cess. A reaction of quinolinone 8 and 9 with MSH to
isolate an imine adduct or sulfilimine did not proceed at
all (Figure 3).27 Thus it is difficult to determine which
route is preferable at the moment.
fact that the use of one equivalent of K CO did not pro-
2
3
vide 2 means that it is not sufficient large enough to
generate a nitrogen anion. The optimal amount of K CO3
2
was 2.5 equivalent, which in turn, obtained at maximum
yield of 2.
In order to expand the scope, the reaction was attempted
with a few different skeletons. Substrates were prepared
O
O
O
2
5
according to the known procedures. The results are sum-
marized in Table 4. Quinoline (entry 1–2) also furnished
the corresponding isothiazole in excellent yields which
exhibited strong fluorescent under UV lamp. However,
reaction did not take place with similar skeletone like
chromone (entry 3) and naphthalene (entry 4). Even after
H
S
N
H
H
N
H
8
9
Figure 3
8
hours of prolonged stirring, starting material is recov-
ered completely. According to MOPAC PM3 calculation
2
6
for electrophilic superdelocalizability of carbonyl car- In conclusion an isothiazole ring can be constructed easily
bon it looks similar for entry 1, 3 and 4. However, that of with 2-alkylthio-3-acyl-4-quinolinones and with 2-alkyl-
N, O and C in the aromatic region is quite different from thio-3-acyl-quinolines under simple and mild conditions
each other. Although transition state calculation between employing O-(mesitylenesulfonyl)hydroxylamine (MSH)
MSH and the substrate adduct has not been performed, as a condensing agent.
richer electron density might be developed at carbonyl
carbon of the substrate due to larger value of superdelo-
calizability of N compared to O or C so as to attract MSH
The financial support of Ministry of Science and Technology (in
more favorably. In spite of being contradictory to the nor-
mal nucleophilic attack to carbonyl carbon, it might be
due to amphiphilic nature of MSH nitrogen. Efforts to
Acknowledgment
Korea) is greatly appreciated. We thank Dr. Jong Hyuck Kim of
KRICT for X-ray crystallography.
Synlett 2003, No. 2, 166–172 ISSN 0936-5214 © Thieme Stuttgart · New York

LETTER
Isothiazole Ring Formation with Substituted 2-Alkylthio-3-acyl-4-quinolinone
(17) (a) Chenault, J.; Plisson, C.; Treillard, P.; Buisson, Y.;
171
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(
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4
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(
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2
10
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1 1
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(
c) Hull, R.; van den Broek, P. J.; Swain, M. L. J. Chem.
1
f
Soc., Perkin Trans. 1 1975, 2271. (d) Hull, R. J. Chem. Soc.,
Perkin Trans. 1 1973, 2911. (e) Daniel, T. W. C.; Claiborne,
A. K. J. Heterocycl. Chem. 1990, 27, 1191.
1
(
hexanes:EtOAC = 3:1), mp 342–344 °C. H NMR (200
MHz, DMSO-d ): δ = 12.75 (s, 1 H), 8.25–8.22 (m, 1 H),
6
7
2
1
1
1
2
.78–7.71 (m, 1 H), 7.53–7.49 (m, 1 H), 7.40–7.33 (m, 1 H),
(
(
(
(
10) (a) Cecchetti, V.; Fravolini, A.; Fringuelli, R.; Schiaffella, F.
J. Heterocycl. Chem. 1993, 30, 11431. (b) Daniel, T. W. C.;
Claiborne, A. K. J. Heterocycl. Chem. 1990, 27, 1191.
13
.72 (s, 3 H). C NMR (125 MHz, DMSO-d ): δ = 173.75,
6
65.97, 165.24, 140.07, 133.20, 125.93, 123.55, 122.93,
+
18.64, 117.60, 20.35. MS: m/z = 216 (M ), 199, 183, 170,
11) (a) Dieter, R. K.; Chang, H. J. J. Org. Chem. 1989, 54, 1088.
61. HRMS (EI): calcd for C H N O S : 216.0360. Found:
1
1
8
2
1 1
(
b) Laaman, S. M.; Meth-Cohn, O.; Rees, C. W. Synthesis
999, 757.
12) (a) Iijima, I.; Rice, K. C. J. Heterocycl. Chem. 1978, 15,
527. (b) Rahman, L. K. A.; Scrowston, R. M. J. Chem. Soc.,
16.0357. 2,4,6-Trimethyl-benzenesufonic Acid Methyl
1
Ester: Yield: 8%, Yellow oil, R = 0.76 (hexanes:EtOAc
f
1
=
(
3:1). H NMR (200 MHz, CDCl ): δ = 6.99 (s, 2 H), 3.70
s, 3 H), 2.63 (s, 6 H), 2.32 (s, 3 H). MS: m/z = 214 (M ), 196,
3
1
+
Perkin Trans. 1. 1983, 2973.
1
82, 174, 165, 149.
13) (a) Stumpf, M.; Hall, H. Liebigs Ann. Chem. 1994, 1049.
1
(
(
20) H NMR was run in Bruker AMX 500MHz in DMF-d . A
7
(
1
b) Liu, K. C.; Shin, B. J.; Hu, M. K. J. Heterocycl. Chem.
987, 24, 1729. (c) Gunzenhauser, S.; Ball, H. Helv. Chim.
mixture of MSH (22 mg) and 1a (12 mg) in 1mL of DMF-d7
was used and the spectrum was run at an interval of 5 min up
Acta 1985, 68, 56.
th
to 1 h. After 12 run 3 h, 1 d, 3 d interval spectrum was
(
14) (a) Anthony, D. M.; Curtis, R. M.; Christoper, A.; Ramsden
Muhammad, R. R. Chem. Commun. 1999, 189.
obtained.
21) In order to prepare GCMS sample for low boiling material,
reaction was run under the standard condition. After 2 h, low
boiling cut was collected using dry ice-acetone trap. The
sample was analyzed in HP 5890 series II (GC) and HP5971
series MSD (mass detector). Conditions: capillary column
DB-wax 30m × 0.25 mm (0.25 µm film thickness), ion
source temperature 200 °C, injection temperature 250 °C:
oven temperature 35 °C to ca. 240 °C programmed 10 °C/
(
5
b) William, L. M.; Joyce, Z. Z. J. Org. Chem. 1990, 55,
791. (c) Tamura, Y.; Sumoto, K.; Minamikawa, J.; Ikeda,
M. Tetrahedron Lett. 1972, 4137.
(
15) (a) Haga, N.; Endo, Y.; Kataoka, K.; Yamaguchi, K.; Shudo,
K. J.Am. Chem. Soc. 1992, 114, 9795. (b) Kagechika, H.;
Himi, T.; Namikawa, K.; Kawaki, E.; Hasimoto, Y.; Shudo,
K. J. Med. Chem. 1989, 32, 1098.
(16) (a) Keana, J. W.; Heo, G. S.; Gaughan, G. T. J. Org. Chem.
min, retention time: 2.08 min HCO Me, 3.47 min MeOH.
2
1985, 50, 2346. (b) Luh, T. Y.; Chow, H. F.; Leung, W. Y.;
(
22) Guthrie, J. P. J. Am. Chem. Soc. 1974, 96, 3608.
Tam, S. W. Tetrahedron 1985, 41, 519.
Synlett 2003, No. 2, 166–172 ISSN 0936-5214 © Thieme Stuttgart · New York

1
72
J. H. Choi et al.
LETTER
(
(
23) Johnson, C. R.; Corkins, H. G. J. Org. Chem. 1978, 43, 4136.
24) (a) Segawa, J.; Kitano, M.; Kazuno, K.; Matsuoka, M.;
Shirahase, I.; Ozaki, M.; Matsuda, M.; Tomii, Y.; Kise, M.
J. Med. Chem. 1992, 35, 4727. (b) Matsuoka, M.; Segawa,
J.; Makita, Y.; Ohmachi, S.; Kashima, T.; Nakamura, K. I.;
Hattori, M.; Kitano, M.; Kise, M. J. Heterocycl. Chem. 1997,
(26) (a) Electrophilic superdelocalizability was calculated with
CacheWorkSystem version 3.2 program. (b) Rogers, K. S.;
Cammarata, A. J. Med. Chem. 1969, 12, 692.
(27) Methylthio group of quinolinone 1a was removed by Raney
nickel to provide 3-acyl-4-quinolinone (8) and acyl group of
1a was removed by treatment with trifluoro acetic acid as
solvent to give 2-methylthio-4-quinolinone (9). Reaction of
8 or 9 with MSH did not proceed after overnight stirring
under the same condition.
3
4, 1773.
25) Entry 1: (a) Bhat, B.; Bhaduri, A. P. Synthesis 1984, 673.
b) See also: Meth-Cohn, O.; Narine, B.; Tarnowski, B.
(
(
Tetrahedron Lett. 1979, 3111. (c) Entry 3: Eiden, F.;
Rademacher, G. Arch. Pharm. (Weinheim) 1983, 316, 34.
(
d) Entry 4: Smith, J. G.; Dibble, P. W.; Sandborn, R. E. J.
Org. Chem. 1986, 51, 3762.
Synlett 2003, No. 2, 166–172 ISSN 0936-5214 © Thieme Stuttgart · New York