Efficient synthesis of bicyclic α-hydroxy-α-trifluoromethyl- γ-lactams

Article abstract of DOI:10.1055/s-2006-956479

A facile method for the synthesis of bicyclic 3-hydroxy-3-trifluoromethyl- γ-lactams using a variety of enamines and ethyl trifluoropyruvate with or without a catalyst was examined. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-2006-956479

LETTER
3484
Efficient Synthesis of Bicyclic a-Hydroxy-a-trifluoromethyl-g-lactams
S
ynthesis of Bicyc
o
lic
a
-Hydrox
r
y-
a
-
triflu
i
orome
o
thyl-
g
-lactams Shibata,* Hiroyuki Fujimoto, Satoshi Mizuta, Shinichi Ogawa, Yumi Ishiuchi, Shuichi Nakamura,
Takeshi Toru*
Department of Applied Chemistry, Graduate School of Engineering, Nagoya Institute of Technology, Gokiso, Showa, Nagoya 466-8555,
Japan
Fax +81(52)7355442; E-mail: nozshiba@nitech.ac.jp; E-mail: toru@nitech.ac.jp
Received 25 September 2006
O
O
O
HO
Abstract: A facile method for the synthesis of bicyclic 3-hydroxy-
3-trifluoromethyl-g-lactams using a variety of enamines and ethyl
trifluoropyruvate with or without a catalyst was examined.
3
CF3
F3C
CO2Et
O
R1
R1
R1
R1
N
CH2Cl2, r.t.
NHR
R
Key words: fluorine, tandem reactions, bicyclic compounds, het-
erocycle, green chemistry
1
2
O
O
CF3
H
O
R1
OEt
Heterocycles containing a trifluoromethyl group consti-
tute a major contributing nucleus in agricultural and me-
dicinal chemistry,1 and thus the development of a simple
and flexible method to generate a novel trifluoromethylat-
ed heterocyclic system has received much attention.2 Bi-
cyclic g-lactams with an a-trifluoromethyl alcohol moiety
should be the molecules of considerable interest for po-
tential application in drug synthesis, not only because
the a-hydroxy-a-trifluoromethyl amide derivatives are
frequently used in pharmaceuticals3 but also because the
g-lactam skeleton such as oxindoles is present in numer-
ous biological compounds.4 As a part of our continuing in-
terest in the design and synthesis of fluorine-containing
biologically active molecules,5 we required the bicyclic 3-
hydroxy-3-trifluoromethyl-g-lactams, dihydro-4-oxo-3-
hydroxy-3-trifluoromethyl-2-oxindoles 1. In 1985, the
non-fluorinated dihydro-4-oxo-3-hydroxy-2-oxindoles
were synthesized from enamines with a-keto acids in low
yields (30–40%).6 Sviridov et al. reported the reaction of
isoquinolines with trifluoropyruvate to give tricyclic g-
lactams.7 Indeed, trifluoropyruvate is now one of the most
versatile building blocks for the synthesis of trifluoro-
methylated compounds. Many reports have appeared con-
cerning the nucleophilic addition to trifluoropyruvate,2g,8,9
whereas there are few reactions using enamines as nucleo-
philes.10
N
R1
H
R
Scheme 1
Table 1 Synthesis of 1 from Enamines 2 with 3a
Run
1
2
R1
H
R
Time (h) 1
Yield (%)
1a 89
1b 99
2a
2b
2c
2d
2e
2f
PhCH2
Ph
1
2
H
0.5
3b
4
H
PhCH2CH2
PhCH2
Ph
1 + 2
0.5
1c 99
1d 99
1e 95
1f 86
1g 39
1g 94
1g 94
1h 93
1h 86
Me
Me
Me
H
5
0.5
6
PhCH2CH2
Ph2CH
Ph2CH
Ph2CH
Ph2CH
Ph2CH
0.5
7b
8c
2g
2g
2g
2h
2h
2i
1 + 20
8
H
9d
10b
11d
12b
13b
H
1 + 1
1 + 30
1 + 1
Me
Me
H
3,5-(CF3)2C6H3CH2 1 + 46
2-CF3C6H4CH2 1 + 46
1i
71
In this paper, we wish to report a facile method for the
synthesis of dihydro-4-oxo-3-hydroxy-3-trifluoromethyl-
2-oxindoles 111 (Scheme 1) and related bicyclic a-hy-
droxy-a-trifluoromethyl-g-lactams from a variety of
enamines 2 and ethyl trifluoropyruvate (3) with or without
catalyst. The reaction proceeds via a tandem aldol con-
densation–cyclization pathway. The multicomponent ap-
proach of this procedure towards green chemistry is also
attempted.
2j
H
1j 84
a Compounds 2 (1.0 equiv) and 3 (2.0 equiv) were reacted in CH2Cl2
at r.t. unless otherwise noted.
b After 1 h stirring at r.t., dichloroethane was added as a co-solvent.
The mixture was then refluxed for the time indicated in Table 1.
c The reaction was carried out at r.t. in the presence of DABCO (0.1
equiv).
d After 1 h stirring at r.t., dichloroethane was added as a co-solvent.
The mixture was then refluxed for 1 h in the presence of DABCO (0.1
equiv).
SYNLETT 2006, No. 20, pp 3484–3488
1
8
.1
2
.2
0
0
6
Advanced online publication: 08.12.2006
DOI: 10.1055/s-2006-956479; Art ID: U11406ST
© Georg Thieme Verlag Stuttgart · New York

LETTER
Synthesis of Bicyclic a-Hydroxy-a-trifluoromethyl-g-lactams
3485
O
O
O
HO
HO
CF3
CF3
3 (2.0 equiv)
CO2Et
NH
O
NHBn
CH2Cl2, r.t., 0.5 h
90%
N
Bn
1k
HO
2k
2l
5g
Ph
Ph
CF3
NC
NC
Me
3 (2.0 equiv)
Figure 1
O
DABCO (0.1 equiv)
CH2Cl2, r.t., 4 d
N
Me
O
NHBn
Bn
1l
We first examined the reaction of enamine 2a with ethyl
trifluoropyruvate (3) in CH2Cl2 in the absence of a cata-
lyst. A tandem reaction sequence composed of aldol con-
densation and cyclization nicely proceeded at room
temperature within one hour to furnish the target dihydro-
4-oxo-3-hydroxy-3-trifluoromethyl-2-oxindole (1a) in
excellent yield (Table 1, run 1). As the reaction conditions
are extremely mild, the carbonyl group can be tolerated
and a variety of bicyclic a-hydroxy-a-trifluoromethyl-g-
lactams 1a–f was obtained in high to excellent yields
(runs 1–6). When the diphenylmethylated enamine 2g was
used as a substrate, an aldol product 5g was provided in-
stead of the desired bicyclic g-lactam 1g (Figure 1). We
speculated that the steric hindrance of N-substituent pre-
vented the subsequent cyclization after aldol condensa-
tion to afford the initially produced intermediate 5g. This
limitation was easily overcome by refluxing dichloroet-
hane for 20 hours, and the corresponding bicyclic a-hy-
droxy-a-trifluoromethyl-g-lactam 1g was isolated in 39%
yield (run 7). The tandem reaction sequence leading to 1g
more smoothly proceeded in the presence of a catalytic
amount of DABCO, in CH2Cl2 at room temperature for
eight hours (94% yield, run 8), or in refluxing dichloroet-
hane for one hour (94% yield, run 9). Reaction of 2h was
also accelerated by a catalytic amount of DABCO with
heating to give 1h in high yield (runs 10 and 11). The role
of DABCO is apparently to abstract a proton from the NH
group of the intermediate 5, therefore the following cy-
clization proceeds smoothly. Long reaction periods were
required for the reactions of 2i and 2j, which have a CF3
substituent on the aromatic ring, to proceed to completion
(runs 12 and 13). This is presumably due to the lower
nucleophilicity of the nitrogen atom of the enamines; thus,
the nucleophilicity of the nitrogen was also found to be
critical for this tandem reaction. All the new compounds
were fully characterized by 1H NMR, 19F NMR, IR spec-
troscopy, and mass spectrometry.12
80%
HO
CF3
CO2Et
O
3 (2.0 equiv)
CH2Cl2, r.t., 0.5 h
99%
NHBn
CF3
NHBn
5m
2m
OH
O
HO
HO
CF3
Hg(OAc)2 (1.0 equiv)
O
O
mesitylene, 100 °C, 7 h
70%
N
N
Bn
Bn
1a
4
Scheme 2
der the oxidation condition using Hg(OAc)2 in mesitylene
at 100 °C14 in 70% yield (Scheme 2).
Finally, a multicomponent strategy for the construction
of the bicyclic a-hydroxy-a-trifluoromethyl-g-lactam
nucleus was attempted. Three-component condensation
of benzylamine, cyclohexane-1,3-dione (6), and ethyl
trifluoropyruvate (3) in CH2Cl2 at room temperature
failed to give the desired 1a. Fortunately, an efficient one-
pot method for converting 1,3-diketone to the correspond-
ing dihydro-4-oxo-3-hydroxy-3-trifluoromethyl-2-oxin-
dole via an enamine was clearly demonstrated. Thus, the
diketone 6 was reacted with 1.1 equivalents of benzyl-
amine in refluxing benzene in a Dean–Stark apparatus,
where water that formed was removed continuously for
three hours. After cooling to room temperature, the enam-
ine derivative was not isolated; instead, 2.0 equivalents of
ethyl trifluoropyruvate (3) was added, and stirring was
continued for one additional hour at room temperature to
furnish the corresponding product 1a in 85% yield.
O
O
HO
1) NH2CH2Ph (1.1 equiv)
benzene, reflux , 3 h
CF3
O
2) 3 (2.0 equiv), r.t., 1 h
A variant of the tandem sequence provided an access to a
novel bicyclic 3-hydroxy-3-trifluoromethyl-g-lactam
fused with seven-membered ring 1k from enamine 2k in
90% yield. The construction of a monocyclic g-lactam
framework 1l from an acyclic precursor 2l was also possi-
ble through the cascade reaction in the presence of a cata-
lytic amount of DABCO. The enamine 2m with a five-
membered ring could not afford the subsequent cycliza-
tion that establishes a structurally inaccessible unsaturat-
ed [3.3.0]-bicyclic g-lactam; therefore, it consequently
provided the corresponding aldol adduct 5m with high
yield of 99%. It should be noted that 1a was converted to
biologically important 3-hydroxy-2-oxindole4a–4d,13 4 un-
N
O
Bn
85%
1a
6
Scheme 3
In conclusion, we have disclosed a simple and facile strat-
egy for the synthesis of dehydro-4-oxo-3-hydroxy-3-tri-
fluoromethyl-2-oxindoles 1 and related a-hydroxy-a-
trifluoromethyl-g-lactams using enamines and ethyl tri-
fluoropyruvate featuring a tandem aldol condensation–cy-
clization reaction. The products can be derivatized easily
to other heterocyclic compounds. It should be noted that
this work serves to highlight the intrinsic power of tandem
reactions for building complexes rapidly without help of a
Synlett 2006, No. 20, 3484–3488 © Thieme Stuttgart · New York

3486
N. Shibata et al.
LETTER
Heterocycl. Chem. 1994, 60, 1. (g) Lin, P.; Jiang, J.
catalyst, in an atom-economical fashion. The one-pot pro-
tocol shown in Scheme 3 stresses the advantage of this
transformation that has only one molar equivalent each of
water and ethanol as by-products. Applications of this
strategy to the synthesis of structurally related natural
products of biological interest are in progress. An asym-
metric version of this transformation will also be reported
in due course.
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General Procedure for Tandem Reaction: 1-Benzyl-3-hydroxy-
3-(trifluoromethyl)-6,7-dihydroindole-2,4(3H,5H)-dione (1a)
To a stirred solution of enamine 2a (30 mg, 0.15 mmol) in CH2Cl2
(1.0 mL) was added ethyl trifluoropyruvate (40 mL, 0.30 mmol) at
r.t. After 1 h stirring, the solvent was removed under reduced pres-
sure. The residue was purified by column chromatography on silica
gel (hexane–EtOAc, 1:1) to furnish 1a (43 mg, 89%) as colorless
crystals.
One-Pot Procedure for the Preparation of 1a
A mixture of cyclohexane-1,3-dione (22 mg, 0.20 mmol) and ben-
zylamine (24 mL, 0.22 mmol) in benzene (2 mL) was refluxed for
3 h in a Dean–Stark apparatus. After cooling to r.t., to the solution
was added ethyl trifluoropyruvate (53 mL, 0.40 mmol). After 1 h
stirring at the temperature, the solvent was removed under reduced
pressure. The residue was purified by column chromatography on
silica gel (hexane–EtOAc, 3:2) to furnish 1a (56 mg, 85%) as
colorless crystals.
Oxidation of 1a to 4
A mixture of 1a (25 mg, 0.077 mmol) and Hg(OAc)2 (25 mg, 0.077
mmol) in mesitylene (0.5 mL) was heated at 100 °C for 7 h under
N2 atmosphere. Then, the mixture was allowed to cool at r.t. After
cooling, the precipitate was filtrated and washed with EtOAc. The
filtrate was concentrated under reduced pressure. The residue was
chromatographed on silica gel (hexane–EtOAc, 1:2) to afford 4 (17
mg, 70%).
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Acknowledgment
This research was supported in part by a Grant-in-Aid for Scientific
Research from the Ministry of Education, Culture, Sports, Science
and Technology, Japan (17350047, 17590087) and by Japan Sci-
ence & Technology (JST) Agency. We are grateful to Dr. Akihiro
Ishii of Central Glass Co., Ltd. for a gift of 3.
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Synlett 2006, No. 20, 3484–3488 © Thieme Stuttgart · New York

LETTER
Synthesis of Bicyclic a-Hydroxy-a-trifluoromethyl-g-lactams
Compound 1c: colorless crystals. 1H NMR (200 MHz,
3487
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Compound 1d: colorless crystals. 1H NMR (200 MHz,
CDCl3): d = 1.05 (s, 3 H), 1.07 (s, 3 H), 2.24 (d, J = 16.6 Hz,
1 H), 2.35 (s, 2 H), 2.38 (d, J = 16.6 Hz, 1 H), 4.26 (br s, 1
H), 4.66 (d, J = 16.0 Hz, 1 H), 4.89 (d, J = 16.0 Hz, 1 H),
7.13–7.18 (m, 2 H), 7.31–7.41 (m, 3 H). 19F NMR (188
MHz, CDCl3): d = –78.0 (s, 3 F). IR (KBr): 3201, 2961,
1756, 1625, 1599, 1421, 1267, 1191, 1173, 1062, 956, 701
cm–1. MS (EI): m/z = 353 [M+].
Compound 1e: colorless crystals. 1H NMR (200 MHz,
CDCl3): d = 1.12 (s, 3 H), 1.14 (s, 3 H), 2.29 (d, J = 18.6 Hz,
1 H), 2.23 (d, J = 16.4 Hz, 1 H), 2.41 (d, J = 18.6 Hz, 1 H),
2.45 (d, J = 16.4 Hz, 1 H), 4.44 (br s, 1 H), 7.17–7.22 (m, 2
H), 7.47–7.53 (m, 3 H). 19F NMR (188 MHz, CDCl3): d =
–78.0 (s, 3 F). IR (KBr): 3398, 3222, 2962, 1766, 1647,
1615, 1496, 1400, 1244, 1192, 1163, 1074, 1048, 897, 742,
715, 571, 505, 449 cm–1. MS (EI): m/z = 339 [M+].
Compound 1f: colorless crystals. 1H NMR (200 MHz,
CDCl3): d = 0.85 (s, 3 H), 0.95 (s, 3 H), 1.67 (d, J = 18.2 Hz,
1 H), 1.80 (d, J = 18.2 Hz, 1 H), 2.10 (d, J = 16.4 Hz, 1 H),
2.22 (d, J = 16.4 Hz, 1 H), 2.80–3.03 (m, 2 H), 3.60 (ddd,
J = 14.0, 7.8, 5.6 Hz, 1 H), 3.89 (dt, J = 14.2, 6.0 Hz, 1 H),
4.28 (br s, 1 H), 7.10–7.14 (m, 2 H), 7.31–7.34 (m, 3 H). 19
F
NMR (188 MHz, CDCl3): d = –77.8 (s, 3 F). IR (KBr): 3200,
2962, 1755, 1642, 1605, 1422, 1263, 1189, 1167, 1068, 701,
497 cm–1. MS (EI): m/z = 367 [M+].
Compound 5g: colorless crystals. 1H NMR (200 MHz,
CDCl3): d = 1.31 (t, J = 7.2 Hz, 3 H), 1.75–1.93 (m, 2 H),
2.27–2.37 (m, 2 H), 2.47–2.62 (m, 2 H), 4.17–4.34 (m, 2 H),
5.76 (d, J = 6.4 Hz, 1 H), 6.06 (s, 1 H), 7.21–7.42 (m, 10 H),
7.69 (d, J = 6.4 Hz, 1 H). 19F NMR (188 MHz, CDCl3): d =
–76.1 (s, 3 F). IR (KBr): 3321, 2953, 1747, 1565, 1432,
1373, 1281, 1193, 1034, 991, 946, 744, 704, 550 cm–1. MS
(EI): m/z = 447 [M+].
Compound 1g: colorless crystals. 1H NMR (200 MHz,
CDCl3): d = 1.86–2.06 (m, 3 H), 2.18–2.54 (m, 3 H), 4.20 (br
s, 1 H), 6.81 (s, 1 H), 7.15–7.20 (m, 4 H), 7.33–7.45 (m, 6
H). 19F NMR (188 MHz, CDCl3): d = –78.1 (s, 3 F). IR
(KBr): 3400, 3064, 3033, 2954, 1759, 1647, 1603, 1243,
1191, 1165, 1076, 1013, 741, 715, 700, 612, 512, 418 cm–1.
MS (EI): m/z = 401 [M+].
Compound 1h: colorless crystals. 1H NMR (200 MHz,
CDCl3): d = 0.93 (s, 3 H), 1.01 (s, 3 H), 1.89 (d, J = 18.2 Hz,
1 H), 2.09 (d, J = 18.2 Hz, 1 H), 2.16 (d, J = 16.4 Hz, 1 H),
2.31 (d, J = 16.4 Hz, 1 H), 4.29 (br s, 1 H), 6.83 (s, 1 H),
7.17–7.21 (m, 4 H), 7.34–7.41 (m, 6 H). 19F NMR (188
MHz, CDCl3): d = –77.9 (s, 3 F). IR (KBr): 3390, 2962,
1765, 1646, 1600, 1424, 1405, 1356, 1266, 1225, 1193,
1161, 1078, 1067, 904, 743, 702, 567, 447 cm–1. MS (EI):
m/z = 429 [M+].
(11) Although a literature survey on SciFinder® reveals the types
of compounds 1 as being registered, surprisingly, neither
references nor the synthetic information on them is
available.
(12) Compound 1a: colorless crystals. 1H NMR (200 MHz,
CDCl3): d = 2.02–2.18 (m, 2 H), 2.26–2.66 (m, 4 H), 4.37 (br
s, 1 H), 4.65 (d, J = 15.8 Hz, 1 H), 4.89 (d, J = 15.8 Hz, 1 H),
7.15–7.20 (m, 2 H), 7.31–7.42 (m, 3 H). 19F NMR (188
MHz, CDCl3): d = –78.1 (s, 3 F). IR (KBr): 3128, 1763,
1632, 1601, 1413, 1359, 1263, 1183, 1158, 1075, 1026, 960,
945, 737, 712, 568, 499 cm–1. MS (EI): m/z = 325 [M+].
Compound 1b: colorless crystals. 1H NMR (200 MHz,
CDCl3): d = 2.09–2.22 (m, 2 H), 2.36–2.68 (m, 4 H), 4.48 (br
s, 1 H), 7.19–7.25 (m, 2 H), 7.45–7.56 (m, 3 H). 19F NMR
(188 MHz, CDCl3): d = –78.4 (s, 3 F). IR (KBr): 3424, 1767,
1651, 1613, 1496, 1399, 1255, 1191, 1086, 1015, 849, 744,
715, 552 cm–1. MS (EI): m/z = 311 [M+].
Compound 1i: colorless crystals. 1H NMR (200 MHz,
CDCl3): d = 2.13–2.24 (m, 2 H), 2.32–2.62 (m, 4 H), 4.41 (br
s, 1 H), 4.68 (d, J = 16.4 Hz, 1 H), 5.09 (d, J = 16.4 Hz, 1 H),
7.64 (s, 2 H), 7.87 (s, 1 H). 19F NMR (188 MHz, CDCl3):
d = –63.0 (s, 6 F), –78.3 (s, 3 F). IR (KBr): 3545, 1764, 1645,
1612, 1408, 1353, 1281, 1184, 1133, 1081, 957, 708, 684
cm–1. MS (EI): m/z = 461 [M+].
Compound 1j: colorless crystals. 1H NMR (200 MHz,
CDCl3): d = 2.02–2.16 (m, 2 H), 2.29–2.59 (m, 4 H), 4.39 (br
Synlett 2006, No. 20, 3484–3488 © Thieme Stuttgart · New York

3488
N. Shibata et al.
LETTER
742, 716, 614, 557 cm–1. MS (EI): m/z = 296 [M+].
s, 1 H), 4.88 (d, J = 16.8 Hz, 1 H), 5.12 (d, J = 16.8 Hz, 1 H),
7.09 (d, J = 7.6 Hz, 1 H), 7.44 (t, J = 7.6 Hz, 1 H), 7.56 (t,
J = 7.6 Hz, 1 H), 7.72 (t, J = 7.6 Hz, 1 H). 19F NMR (188
MHz, CDCl3): d = –60.1 (s, 3 F), –78.0 (s, 3 F). IR (KBr):
3126, 2813, 1760, 1645, 1612, 1409, 1319, 1263, 1157,
1110, 1080, 1039, 967, 946, 878, 770, 709, 657, 426 cm–1.
MS (EI): m/z = 393 [M+].
Compound 4: colorless crystals. 1H NMR (200 MHz,
CDCl3): d = 1.80 (br s, 1 H), 4.65 (d, J = 15.8 Hz, 1 H), 5.10
(d, J = 15.8 Hz, 1 H), 5.65 (br s, 1 H), 6.34 (d, J = 7.6 Hz, 1
H), 6.65 (d, J = 8.4 Hz, 1 H) 7.15–7.40 (m, 6 H). 19F NMR
(188 MHz, CDCl3): d = –78.1 (s, 3 F). IR (KBr): 3592, 3471,
3338, 3239, 1721, 1618, 1471, 1388, 1352, 1281, 1174,
1123, 1038, 948, 788, 702 cm–1. MS (EI): m/z = 323 [M+].
Compound 5m: colorless crystals. 1H NMR (200 MHz,
CDCl3): d = 1.34 (t, J = 7.2 Hz, 3 H), 2.43–2.48 (m, 1 H),
2.58–2.65 (m, 2 H), 4.25–4.38 (m, 2 H), 4.51 (d, J = 6.0 Hz,
2 H), 7.20–7.25 (m, 2 H), 7.31–7.43 (m, 3 H), 8.01 (br d, 1
H), 9.76 (s, 1 H). 19F NMR (188 MHz, CDCl3): d = –78.7
(s, 3 F). IR (KBr): 3375, 2986, 2715, 1759, 1648, 1558,
1453, 1433, 1341, 1240, 1192, 1158, 1038, 970, 737, 665,
461 cm–1. MS (EI): m/z = 357 [M+].
Compound 1k: colorless crystals. 1H NMR (200 MHz,
CDCl3): d = 1.79–1.98 (m, 4 H), 2.63–2.71 (m, 4 H), 4.65 (d,
J = 16.2 Hz, 1 H), 4.97 (d, J = 16.2 Hz, 1 H), 5.99 (s, 1 H),
7.13–7.17 (m, 2 H), 7.29–7.40 (m, 3 H). 19F NMR (188
MHz, CDCl3): d = –78.1 (s, 3 F). IR (KBr): 3384, 3011,
2948, 2883, 1761, 1590, 1403, 1320, 1259, 1207, 1171,
1039, 944, 743, 710, 627, 525, 463 cm–1. MS (EI): m/z = 339
[M+].
Compound 1l: brown oil. 1H NMR (200 MHz, CDCl3): d =
2.23 (s, 3 H), 4.79 (d, J = 16.0 Hz, 1 H), 4.87 (d, J = 16.0 Hz,
1 H), 7.10–7.17 (m, 2 H), 7.30–7.36 (m, 3 H). 19F NMR (188
MHz, CDCl3): d = –78.6 (s, 3 F). IR (neat): 3341, 2220,
1748, 1633, 1402, 1352, 1267, 1181, 1139, 1083, 1032, 953,
(13) Furukawa, Y.; Kishimoto, E. Jpn. Kokai Tokkyo Koho JP
226351A, 2001.
(14) Shipchandler, M. T.; Mitscher, L. A. J. Heterocycl. Chem.
1971, 8, 695.
Synlett 2006, No. 20, 3484–3488 © Thieme Stuttgart · New York