Nitration of dimethyl 1-substituted indole-2,3-dicarboxylates: synthesis of nitro- and aminoindole derivatives

Article abstract of DOI:10.3987/COM-07-11145

The treatment of dimethyl indole-2,3-dicarboxylate with nitronium tetrafluoroborate in the presence of tin (IV) chloride produced dimethyl 5-nitroindole-2,3-dicarboxylate as the major product. In a similar manner, the dimethyl 1-benzyl- and 1-benzenesulfo

Full text of DOI:10.3987/COM-07-11145

HETEROCYCLES, Vol. 71, No. 11, 2007
2457
HETEROCYCLES, Vol. 71, No. 11, 2007, pp. 2457 - 2463. © The Japan Institute of Heterocyclic Chemistry
Received, 20th June, 2007, Accepted, 20th July, 2007, Published online, 3rd August, 2007. COM-07-11145
NITRATION
OF
DIMETHYL
1-SUBSTITUTED
INDOLE-2,3-DICARBOXYLATES: SYNTHESIS OF NITRO-
AND AMINOINDOLE DERIVATIVES
Yasuyoshi Miki,* Misako Umemoto, Hiroko Maruyama, Makoto Kuromatsu, and
Hiromi Hamamoto
Faculty of Pharmaceutical Sciences, Kinki University, 3-4-1, Kowakae,
Higashi-Osaka 577-8502, Japan
Abstract – The treatment of dimethyl indole-2,3-dicarboxylate with nitronium
tetrafluoroborate in the presence of tin (IV) chloride produced dimethyl
5-nitroindole-2,3-dicarboxylate as the major product. In a similar manner, the
dimethyl 1-benzyl- and 1-benzenesulfonylindole-2,3-dicarboxylates provided a
mixture of the corresponding 4-nitro-, 5-nitro-, 6-nitro- and 7-nitroindole
derivatives. However, dimethyl 5-bromoindole-2,3- dicarboxylate gave dimethyl
5-bromo-4-nitroindole-2,3-dicarboxylate as the sole product, which was converted
to dimethyl 4-aminoindole-2,3-dicarboxylate.
The nitration of indoles is one of the useful reactions for introducing the nitrogen atom directly into
indole rings, but indoles having an electron-donating group are unstable under the usual nitration
conditions and also undergo undesirable oxidation and polymerization. However, indoles possessing an
electron-withdrawing group such as acyl or ester group are relatively stable under severe nitration
conditions. The aromatic nitro group is a useful functional group, which could be converted into various
groups via an amino group by reduction. The nitration of ethyl indole-2-carboxylate afforded
4-nitroindole derivative in low yield as reported by Norland1, and from methyl indole-3-carboxylate, a
mixture of methyl 4-nitroindole-3-carboxylate (30%) and ethyl 6-nitroindole-3-carboxylate (30%) was
obtained by Nakatsuka.2 Ottoni showed that 3-acetyl-5-nitroindole was obtained at low temperature by
the nitration of 3-acetylindole with nitronium tetrafluoroborate (NO2BF4), but at 60° C,
3-acetyl-6-nitroindole was isolated as the sole product.3 Tobinaga reported the synthesis of
chuangxinmycin from 3-acetyl-4-nitroindole, which was prepared by the nitration of 3-acetylindole in the
presence of metal in low yield.4 We reported that dimethyl indole-2,3-dicarboxylates and
indole-2,3-dicarboxylic anhydrides were useful synthons for the synthesis of pratosine,5 hippadine,5
murrayaquinone-A,6 ellipticine,7-9 olivacine,10 and caulersin.11 Recently, we showed the selective
bromination of the dimethyl indole-2,3-dicarboxylates and the synthesis of the dimethyl 5-bromo-,

2458
HETEROCYCLES, Vol. 71, No. 11, 2007
6-bromo-, and 5,6-dibromoindole-2,3-dicarboxylates because many bromoindole alkaloids have been
isolated from various sources.12 In this study, we examine the nitration of the dimethyl 1-substituted
indole-2,3-dicarboxylates (1) using NO2BF4 and trifluoroacetyl nitrate (TFAN, CF3COONO2)13 to
enhance their utility as a synthon for the synthesis of the indole alkaloids.
The reaction of dimethyl indole-2,3-dicarboxylate (1a) (R = H) with NO2BF4 in the presence of tin (IV)
chloride in dichloromethane at -20 °C gave dimethyl 5-nitroindole-2,3-dicarboxylate (3a) in 79% yield as
a major product with a mixture of dimethyl 6-nitro- (4a) and 7-nitroindole-2,3-dicarboxylate (5a), in 4%
and 13% yields, respectively, but with TFAN, 1a gave
a
mixture of dimethyl
4-nitroindole-2,3-dicarboxylate (2a), 3a, 4a, and 5a in 9%, 30%, 13%, and 9% yields, respectively.
(Entries 1, 2) The nitration of dimethyl 1-benzylindole-2,3-dicarboxylate (1b) (R = CH2Ph) with
NO2BF4 resulted in a complex mixture, but with TFAN, a mixture of 2b, 3b, and 4b was obtained.
(Entries 3, 4) The treatment of dimethyl 1-benzenesulfonylindole-2,3-dicarboxylate (1c) (R = SO2Ph)
with NO2BF4 or TFAN afforded an inseparable mixture of 2c, 3c, 4c, and 5c. (Entries 5, 6) (Table 1)
NO2
Scheme 1
O2N
CO2Me
CO2Me
CO2Me
CO2Me
+
+
CO2Me
CO2Me
N
R
2
N
R
NO2BF4 (Method A)
or
TFAN (Method B)
N
R
1
3
CO2Me
CO2Me
CO2Me
CO2Me
+
a : R = H
O2N
N
R
N
R
b: R = CH2Ph
c : R = SO2Ph
NO2
4
5
Table 1
Yield (%)
Method
Entry
1
Condition
-20 °C
2
-
3
Total
99
84
-
4
5
1
A
B
A
B
A
B
a
a
b
b
c
c
79
30
-
1
2
3
4
5 h
4
13
9
-
23
-
-78 °C
rt
0.5 h
3 h
9
13
-
-
-
-78 °C
rt
2 h
16
16
30
35
26
28
86
68
97
19
26
24
-
16
-
1)
5
8 h
-
1)
-78 °C
6
3 h
15
-
1) SO2Ph derivatives (2c, 3c, 4c, 5c) were isolated as NH derivatives (2a, 3a, 4a, 5a) by treatment with
tetrabutylammonium fluoride.
Method A: NO2BF4 (2.2 eq) and Sn (IV)Cl4 (5 eq) in CH2Cl2.
Method B: NH4NO3 (1.2 eq) and (CF3COO)2O (10eq) in CH2Cl2.
We also examined the nitration of the dimethyl 5-bromoindole-2,3-dicarboxylates (6a and 6b) because 6a
and 6b were easily obtained by the bromination of dimethyl indole-2,3-dicarboxylate (1a) and (1b),

HETEROCYCLES, Vol. 71, No. 11, 2007
2459
respectively.12
A
complex mixture was obtained from the reaction of dimethyl
5-bromoindole-2,3-dicarboxylate (6a) (R = H) with TFAN, but dimethyl 5-bromo-4-nitroindole-2,3-
dicarboxylate (7a) was isolated as the sole product in 83% yield by treatment with NO2BF4. (Entries 1, 2)
However, the treatment of 6b with TFAN provided an inseparable mixture of dimethyl 5-bromo-4-nitro-
(7b) and 5-bromo-4-nitroindole-2,3-dicarboxylate (8b), which were isolated as 7a and 8a by treatment of
tetrabutylammonium fluoride in 60% and 35% yields, respectively. (Entry 3) (Table 2)
Scheme 2
NO2
Br
Br
Br
CO2Me
CO2Me
CO2Me
CO2Me
CO2Me
CO2Me
nitration
CH2Cl2
+
O2N
N
R
N
R
N
R
8
6
7
a : R = H
b : R = SO2Ph
Table 2
Yield(%)
Nitrating agent
Condition
-20 °C 0.5 h
0°C 0.5 h
7
-
8
-
6
a
a
b
R
H
Entry
1
TFAN (3 eq)
NO2BF4 (2.2 eq)
TFAN (3 eq)
83
60
-
H
2
3 1)
-20 °C 0.5 h
35
SO2Ph
1) SO2Ph derivatives (7b and 8b) were isolated as NH derivatives (7a and 8a) by treatment with
tetrabutylammonium fluoride.
Finally, we examined the conversion of the nitro group in dimethyl 5-nitroindole-2,3-dicarboxylate (3a)
and dimethyl 5-bromo-4-nitroindole-2,3-dicarboxylate (7a) to an amino group in them. (3a) was treated
with ammonium formate in the presence of 10% Pd-C in hot MeOH to give a corresponding dimethyl 5-
aminoindole-2,3-dicarboxylate (9) in 88% yield. (Scheme 3)
Scheme 3
H2N
O2N
CO2Me
CO2Me
CO2Me
CO2Me
10% Pd-C / HCO2NH4
MeOH
(88%)
N
H
N
H
3a
9
The reduction of 7a with ammonium formate in the presence of 10% Pd-C in hot MeOH provided
dimethyl 4-aminoindole-2,3-dicarboxylate (10) in 85% yield, but dimethyl 5-bromo-4-aminoindole-2,3-
dicarboxylate (11) was not isolated. (Entry 1) The treatment of 7a with sodium borohydride in the

2460
HETEROCYCLES, Vol. 71, No. 11, 2007
presence of tin (II) chloride resulted in a low yield. (Entry 2) However, 7a was treated with
tetra-n-butylammonium borohydride (3 eq) in the presence of tin (II) chloride in tetrahydrofuran to give
dimethyl 4-amino-5-bromoindole-2,3-dicarboxylate (11) in 45% yield and 7a was also recovered in 42%
yield, but in the presence of excess tetra-n-butylammonium borohydride (6 eq), a mixture of 10 and 11
was obtained in 42% and 58% yields, respectively. (Entries 3, 4) (Scheme 4) (Table 3)
Scheme 4
NH2
NO2
NH2
NO2
Br
Br
CO2Me
CO2Me
CO2Me
CO2Me
CO2Me
CO2Me
CO2Me
CO2Me
+
N
N
H
N
H
N
H
H
12
7a
10
11
Yield (%)
Table 3
Entry
Reducing agent
Recovered
Solvent
MeOH
MeOH
THF
Condition
10
85
30
-
11
-
-
1
2
3
4
HCO2NH4 (3 eq) / 10% Pd-C
reflux 2 h
reflux 9 h
-
NaBH4 (1.5 eq) / SnCl2 (5 eq)
n-Bu4NBH4 (3 eq) / SnCl2 (5 eq)
n-Bu4NBH4 (6 eq) / SnCl2 (5 eq)
-
42
-
rt
rt
3 d
45
58
THF
10 min
42
EXPERIMENTAL
Melting points were determined using a Yanagimoto micromelting point apparatus and are uncorrected.
The 1H-NMR spectra were determined by a JEOL JNM-GSX 270 spectrometer using tetramethylsilane as
the internal standard. The IR spectra were recorded using a JASCO FT/IR-7000 spectrophotometer.
The high MS were recorded by a JOEL JMS-HX100 spectrometer. Column chromatography was
performed on E. Merck silica gel 60 (70-230 mesh or 230-400 mesh).
Nitration of Dimethyl Indole-2,3-dicarboxylates (1 ): General Procedure
By Using Nitronium Tetrafluoroborate (Method A)
To a mixture of dimethyl indole-2,3-dicarboxylates (1) (1 mmol) in CH2Cl2 (1 mL) was added 1M tin
(IV) chloride in a CH2Cl2 solution, then the nitronium tetrafluoroborate (1–3 mmol) and the reaction
mixture was stirred at rt. Water was added to the mixture and the mixture was extracted with CHCl3 :
MeOH (10 : 1). The extracts were washed with water, dried over Na2SO4, and concentrated under
reduced pressure to afford a residue, which was purified by preparative thin-layer chromatography on
silica gel (n-hexane : AcOEt = 3 : 1 - 2 : 3) to give the dimethyl 4-nitro- (2), 5-nitro (3), 6-nitro- (4), and

HETEROCYCLES, Vol. 71, No. 11, 2007
2461
7-nitroindole-2,3-dicarboxylate (5). These reaction conditions and results are shown in Tables 1 and 2.
Using Trifluoroacetyl Nitrate (TFAN) (Method B)
The dimethyl indole-2,3-dicarboxylates (1) (1 mmol) were added to trifluoroacetyl nitrate13 (prepared
from ammonium nitrate (1-3 mmol) and trifluoroacetic anhydride (5-10 mmol) in CH2Cl2 (1 mL)),
stirring for 1 h at rt) and the mixture was stirred. The reaction mixture was added to water and the mixture
was extracted with CH2Cl2. The extracts were washed with water, dried over Na2SO4, and concentrated
under reduced pressure to afford a residue, which was purified by preparative thin-layer chromatography
on silica gel (n-hexane : AcOEt = 3 : 1 - 2 : 3) to give the dimethyl 4-nitro- (2), 5-nitro (3), 6-nitro- (4),
and 7-nitroindole-2,3-dicarboxylate (5). These reaction conditions and results are shown in Table 1 and
2.
Debenzensulfonylation of Dimethyl 1-Benzenesulfonylnitroindole-2,3-dicarboxylates (2c, 3c, 4c, 5c)
and Dimethyl 1-Benzenesulfonyl-5-bromo-nitroindole-2,3-dicarboxylates (7, 8) : General Procedure
for Preparation of Dimethyl Nitroindole-2,3-dicarboxylates
To a solution of an inseparable mixture of dimethyl nitroindole-2,3-dicarboxylates (2c, 3c, 4c, 5c) (40 mg,
0.1 mmol) in THF (1 mL), a 1.0 M solution of tetrabutylammonium fluoride in THF (0.1 mL, 0.1 mmol)
was added at -20 °C, and the mixture was stirred for 30 min. The reaction mixture was neutralized with
1% hydrochloric acid, and the aqueous mixture was extracted with CHCl3. The extracts were washed with
water, dried over Na2SO4, and concentrated under reduced pressure to afford a residue, which was
purified by preparative thin-layer chromatography on silica gel.
Dimethyl 4-Nitroindole-2,3-dicarboxylate (2a); mp 241 ˚C (MeOH).ꢀ IR (Nujol) cm-1: 1679, 1519.
1H-NMR (CDCl3) ꢀ: 3.99, 4.05 (6H, s, 2xCO2CH3), 7.60 (1H, t, J = 8 Hz, H-6), 7.78 (1H, d, J = 8.5 Hz,
H-7), 8.05 (1H, d, J = 8.5 Hz, H-5), 9.30 (1H, br s, H-1). Anal. Calcd for C12H10N2O6: C, 51.80; H, 3.62;
N, 10.07. Found: C, 51.70; H, 3.70; N, 10.11.
Dimethyl 5-Nitroindole-2,3-dicarboxylate (3a); mp 213-214 ˚C (MeOH).ꢀ IR (Nujol) cm-1: 1737, 1525.
1H-NMR (CDCl3) ꢀ: 4.04 (6H, s, 2xCO2CH3), 7.54 (1H, d, J = 9 Hz, H-7), 8.27 (1H, dd, J = 9, 2 Hz, H-6),
9.02 (1H, d, J = 2 Hz, H-4), 9.64 (1H, br s, H-1). Anal. Calcd for C12H10N2O6: C, 51.80; H, 3.62; N,
10.07. Found: C, 51.89; H, 3.67; N, 10.10.
Dimethyl 6-Nitroindole-2,3-dicarboxylate (4a); mp 214 ˚C (MeOH).ꢀ IR (Nujol) cm-1: 1678, 1518.ꢀ
1H-NMR (DMSO-d6)ꢀꢀ: 3.87, 3.95 (6H, s, 2xCO2CH3), 8.07 (1H, dd, J = 8, 1.5 Hz, H-5), 8.12 (1H, d, J =
8 Hz, H-4), 8.38 (1H, d, J = 1.5 Hz, H-7), 13.30 (1H, br s, H-1). HRMS (EI) m/z: Calcd for C12H10N2O6:
278.0564. Found: 278.0439. Anal. Calcd for C12H10N2O6: C, 51.80; H, 3.62; N, 10.07. Found: C, 51.82; H,
3.60; N, 10.16.
ꢀ
Dimethyl 7-Nitroindole-2,3-dicarboxylate (5a); mp 120 ˚C (n-hexane). IR (Nujol) cm-1: 1707, 1544.ꢀ
1H-NMR (CDCl3) ꢀ: 4.01, 4.05 (6H, s, 2xCO2CH3), 7.41 (1H, d, J = 8 Hz, H-5), 8.34 (1H, dd, J = 8, 1 Hz,
H-6 or H-4), 8.47 (1H, d, J = 8 Hz, H-4 or H-6), 10.60 (1H, br s, H-1). Anal. Calcd for C12H10N2O6: C,
51.80; H, 3.62; N, 10.07. Found: C, 51.86; H, 3.67; N, 10.13.
ꢀ
Dimethyl 1-Benzyl-4-nitroindole-2,3-dicarboxylate (2b); mp 114 ˚C (EtOH). IR (CHCl3) cm-1: 1724,
1532. 1H-NMR (CDCl3) ꢀ: 3.91, 4.00 (6H, s, 2xCO2CH3), 5.85 (2H, s, CH2), 7.00-7.08 (2H, m, arom),
7.23-7.32 (3H, m, arom), 7.42 (1H, t, J = 8 Hz, H-6), 7.69 (1H, d, J = 8 Hz, H-7 or 5), 8.11 (1H, d, J = 8

2462
HETEROCYCLES, Vol. 71, No. 11, 2007
Hz, H-5 or 7). Anal. Calcd for C19H16N2O6: C, 61.95; H, 4.38; N, 7.61. Found: C, 61.89; H, 4.43; N,
7.61.
Dimethyl 1-Benzyl-5-nitroindole-2,3-dicarboxylate (3b); mp 147 ˚C (AcOEt). IR (CHCl3) cm-1: 1714,
1524. 1H-NMR (CDCl3) ꢀ: 3.94, 3.99 (6H, s, 2xCO2CH3), 5.48 (2H, s, CH2), 7.07-7.13 (2H, m, arom),
7.28-7.33 (3H, m, arom), 7.38 (1H, d, J = 9 Hz, H-7), 8.18 (1H, dd, J = 9, 2 Hz, H-6), 9.07 (1H, d, J = 2
Hz, H-4). Anal. Calcd for C19H16N2O6: C, 61.95; H, 4.38; N, 7.61. Found: C, 61.91; H, 4.39; N, 7.60.
Dimethyl 1-Benzyl-6-nitroindole-2,3-dicarboxylate (4b); mp 167 ˚C (AcOEt). IR (CHCl3) cm-1: 1713,
1
1522. H-NMR (CDCl3) ꢀ: 3.94, 3.96 (6H, s, 2xCO2CH3), 5.50 (2H, s, CH2), 7.10-7.16 (2H, m, arom),
7.28-7.34 (3H, m, arom), 8.16 (1H, dd, J = 9, 2 Hz, H-5), 8.27 (1H, d, J = 9 Hz, H-4), 8.29 (1H, d, J = 2
Hz, H-7). Anal. Calcd for C19H16N2O6: C, 61.95; H, 4.38; N, 7.61. Found: C, 62.10; H, 4.39; N, 7.63.
Dimethyl 5-Bromo-4-nitroindole-2,3-dicarboxylate (7a); mp 230-232 °C (MeOH). IR (CHCl3) cm-1:
1719, 1543. 1H-NMR (CDCl3) ꢀ: 3.91, 3.99 (6H, s, 2xCO2CH3), 7.48 (1H, d, J = 9 Hz, H-6 or H-7), 7.61
(1H, d, J = 9 Hz, H-7 or H-6), 9.48 (1H, br s, H-1). Anal. Calcd for C12H9N2O6Br: C, 40.36; H, 2.54; N,
7.85. Found: C, 40.26; H, 2.59; N, 7.90.
Dimethyl 5-Bromo-6-nitroindole-2,3-dicarboxylate (8a); mp 224-226 °C (MeOH). IR (CHCl3) cm-1:
1
1716. H-NMR (CDCl3) ꢀ: 4.01, 4.04 (6H, s, 2xCO2CH3), 8.04 (1H, s, H-4 or H-7), 8.47 (1H, s, H-7 or
H-4). Anal. Calcd for C12H9N2O6Br: C, 40.36; H, 2.54; N, 7.85. Found: C, 40.37; H, 2.55; N, 7.87.
Preparation of Dimethyl 5-Aminoindole-2,3-dicarboxylate (9), Dimethyl 4-Aminoindole-2,3-
dicarboxylate (10), and Dimethyl 5-Bromo-4-aminoindole-2,3-dicarboxylate (11) by Reduction of
Dimethyl Nitroindole-2,3-dicarboxylate (3a and 7a): General Procedure
a) Using Ammonium Formate in the Presence of 10% Pd/C
A mixture of dimethyl 5-bromo-4-nitroindole-2,3-dicarboxylate (3a) (56 mg, 0.2 mmol), ammonium
formate (76 mg, 1.2 mmol), and 10% Pd/C (6 mg) in MeOH (2 mL) was refluxed for 2 h. The catalyst
was removed by filtration through Cerite, then the filtrate was evaporated. The residue was purified by
preparative thin-layer chromatography (n-hexane : AcOEt = 1 : 1) to give dimethyl 5-aminoindole-2,3-
dicarboxylate (9) (44 mg, 88%) as a yellow solid. These reaction conditions and results are shown in
Table 3.
b) Using Sodium Borohydride or Tetrabutylammonium Borohydride
A suspension of dimethyl 5-bromo-4-nitroindole-2,3-dicarboxylate (7a) (36 mg, 0.1 mmol), tin (II)
.
chloride (SnCl2 2H2O), sodium borohydride or tetrabutylammonium borohydride in MeOH or THF (1
mL) was stirred or refluxed. Water was added to the reaction mixture and the mixture was extracted with
CHCl3. The extracts were washed with water, dried over Na2SO4, and concentrated under reduced
pressure to afford a residue, which was purified by chromatography on silica gel (n-hexane : AcOEt = 1 :
1) to afford dimethyl 4-amino-indole-2,3-dicarboxylate (10) and dimethyl 5-bromo-4-aminoindole-
2,3-dicarboxylate (11). These reaction conditions and results are shown in Table 3.
Dimethyl 5-Aminoindole-2,3-dicarboxylate (9); mp 74-77 ˚C (n-hexane-CH2Cl2). IR (CHCl3) cm-1:

HETEROCYCLES, Vol. 71, No. 11, 2007
2463
3403, 3318, 1716, 1683. 1H-NMR (CDCl3) ꢀ: 3.69 (2H, br s, NH2), 3.97 (6H, s, 2xCO2CH3), 6.83 (1H,
dd, J = 8, 1.5 Hz, H-6), 7.23 (1H, d, J = 8 Hz, H-7), 7.33 (1H, d, J = 1.5 Hz, H-6), 9.08 (1H, br s, H-1).
HRMS (FAB) m/z: Calcd for C12H11O4N2Br: 325.9902. Found: 325.9883.
Dimethyl 4-Aminoindole-2,3-dicarboxylate (10); mp 128-130 ˚C (n-hexane-ether). IR (CHCl3) cm-1:
1
3449, 3360, 1730, 1698. H-NMR (CDCl3) ꢀ: 3.95, 3.96 (6H, s, 2xCO2CH3), 5.49 (2H, br s, NH2), 6.66
(1H, d, J = 9 Hz, H-6 or H-7), 7.37 (1H, d, J = 9 Hz, H-7 or H-6), 8.97 (1H, br s, H-1). Anal. Calcd for
C12H12N2O4: C, 58.06; H, 4.87; N, 11.29. Found: C, 57.96; H, 4.85; N, 11.17.
Dimethyl 4-Amino-5-bromoindole-2,3-dicarboxylate (11); mp 172-173 °C (MeOH). IR (KCl) cm-1:
3445, 3303, 1698. 1H-NMR (CDCl3) ꢀ: 3.95, 3.96 (6H, s, 2xCO2CH3), 5.49 (2H, br s, NH2), 6.66 (1H, d, J
= 9 Hz, H-6 or H-7), 7.37 (1H, d, J = 9 Hz, H-7 or H-6), 8.97 (1H, br s, H-1). Anal. Calcd for
C12H11N2O4Br: C, 43.94; H, 3.34; N, 8.46. Found: C, 44.06; H, 3.39; N, 8.57.
ACKNOWLEDGEMENT
This work was supported by “High-Tech Research Center Project” for Private Universities: matching
fund subsidy from MEXT (2007) and Kinki University.
REFERENCES
1. W. E. Noland and K. R. Rush, J. Org. Chem., 1966, 31, 70.
2. S. Nakatsuka, T. Masuda, O. Asano, T. Teramae, and T. Goto, Tetrahedron Lett., 1986, 27, 4327.
3. O. Ottoni, R. Cruz, and N. H. Krammer, Tetrahedron Lett., 1999, 40, 1117.
4. M. Murase, T. Koike, Y. Moriya, and S. Tobinaga, Chem. Pharm. Bull., 1987, 35, 2656.
5. Y. Miki, H. Shirokoshi, and K. Matsushita, Tetrahedron Lett., 1999, 40, 4347.
6. Y. Miki and H. Hachiken, Synlett, 1993, 333.
7. Y. Miki, Y. Tada, N. Yanase, H. Hachiken, and K. Matsushita, Tetrahedron Lett., 1996, 37, 7753.
8. Y. Miki, Y. Tada, and K Matsushita, Heterocycles, 1998, 48, 1593.
9. Y. Miki, Y. Aoki, Y. Tsuzaki, M. Umemoto, and H. Hibino, Heterocycles, 2005, 65, 2693.
10. Y. Miki, Y. Tsuzaki, H. Hibino, and Y. Aoki, Synlett, 2004, 2206.
11. Y. Miki, Y. Aoki, H. Miyatake, T. Minematsu, and H. Hibino, Tetrahedron Lett., 2006, 47, 5215.
12. Y. Miki, M. Umemoto, M. Nakamura, H. Hibino, N. Ohkita, A. Kato, and Y. Aoki, Heterocycles,
2006, 68, 1893.
13. J. V. Crivello, J. Org. Chem., 1981, 46, 3056.