InBr3-Catalyzed Conjugate Addition of Indoles to p-Quinones: An Efficient Synthesis of 3-Indolylquinones

Article abstract of DOI:10.1055/s-2003-44364

Indium(III) bromide catalyzes efficiently the conjugate addition of indoles to p-benzoquinones under mild conditions to afford the corresponding 3-indolylquinones in high yields with high selectivity. 1,4-Naphthoquinones also underwent Michael addition with indoles under similar conditions to give 3-indolylnaphthoquinones.

Full text of DOI:10.1055/s-2003-44364

106
PAPER
InBr3-Catalyzed Conjugate Addition of Indoles to p-Quinones: An Efficient
Synthesis of 3-Indolylquinones
Synthesis
.
of 3-Indolylq
S
uinones . Yadav,* B. V. S. Reddy, T. Swamy
Division of Organic Chemistry, Indian Institute of Chemical Technology, Hyderabad-500 007, India
Fax +91(40)27160512; E-mail: yadav@iict.ap.nic.in
Received 31 July 2003; revised 11 November 2003
the ability of asterriquinones to either promote or prevent
protein-protein interactions.
Abstract: Indium(III) bromide catalyzes efficiently the conjugate
addition of indoles to p-benzoquinones under mild conditions to af-
ford the corresponding 3-indolylquinones in high yields with high
selectivity. 1,4-Naphthoquinones also underwent Michael addition
with indoles under similar conditions to give 3-indolylnaphtho-
quinones.
Generally, 3-indolylbenzoquinones are prepared by the
condensation of indoles with quinones under acidic con-
ditions.6,7 Despite their wide range of pharmacological ac-
tivities, the synthesis of indol-3-ylbenzoquinones has
received little attention. Therefore, the development of an
efficient and versatile catalytic method for the construc-
tion of indol-3-ylbenzoquinone core structure would wid-
en the scope of this methodology.
Key words: p-quinones, indoles, indium compounds, indol-3-yl-
benzoquinones
The 3-indolylbenzoquinone is a core structure in various
natural products such as asterriquinones.1 Bis(in-
dolyl)quinones (Figure 1) have been isolated from a wide
range of fungi, including Aspergillus terreus, Chaetomi-
um sp., and Pseudomassaria sp.2 The asterriquinones ex-
hibit a wide spectrum of biological activities including
antitumor properties and function as inhibitors of HIV re-
verse transcriptase.3 Asterriquinone A1, has been shown
to arrest the cell cycle in G1 and promote apoptotic cell
death.4 Recently, asterriquinone has been reported as an
orally active nonpeptidyl mimetic of insulin with antidia-
betic activity.5 All these properties apparently stem from
Recently, indium halides have emerged as mild and wa-
ter-tolerant Lewis acids imparting high regio-, chemo-,
and stereoselectivity in various organic transformations.8
Compared to conventional Lewis acids, particularly indi-
um tribromide has advantages of low catalyst loading,
moisture stability and catalyst recycling. Thus, indi-
um(III) bromide has been shown to be a more efficient
catalyst over conventional Lewis acids in promoting vari-
ous transformations including glycosidation, thioacetal-
ization, cyanation of ketones and conjugate addition
reactions.9
In this report, we wish to describe a simple, convenient
and efficient protocol for the synthesis of indol-3-ylben-
zoquinones using indium tribromide as a novel catalyst.
Thus, 2,5-dichloro-p-benzoquinone on treatment with in-
dole in the presence of 5 mol% of indium(III) bromide at
room temperature gave 2,5-dichloro-3-indolylhydro-
quinone 3a in 85% yield (Scheme 1).
These 2,5-dihalo-3-indolylquinones are useful precursors
in the total synthesis of asterriquinone natural products.10
The 2,5-dichloro-3-indolylquinone can be easily convert-
ed to the corresponding 2,5-dihydroxy-3-indolylquinone
Figure 1 Asterriquinone A1
Scheme 1
SYNTHESIS 2004, No. 1, pp 0106–0110
x
x
.
x
x
.2
0
0
3
Advanced online publication: 09.12.2003
DOI: 10.1055/s-2003-44364; Art ID: Z11703SS
© Georg Thieme Verlag Stuttgart · New York

PAPER
Synthesis of 3-Indolylquinones
107
Scheme 2
which is a core structure in asterriquinone.7a Similarly, 2- This method is clean and free from chlorinated side prod-
methyl-, N-methyl-, 5-bromo-, and 7-ethyl substituted in- ucts, which are normally observed under protic acid
doles reacted smoothly with 2,5-dichlorobenzoquinone (conc. HCl in THF) conditions. This method also works
under the reaction conditions to give the corresponding well with electron-deficient, 2-ethoxycarbonylindole to
2,5-dichloro-3-indolylhydroquinones (Table 1, entries give the corresponding indol-3-ylhydroquinone in 78%
3b–e). However, treatment of parent benzoquinone with yield (entry 3g). However, most of the reported methods
indole under similar conditions gave bis(indolyl)hydro- fail to produce 3-indolylquinones with electron-deficient
quinone 3h1 in 80% yield. Furthermore, substituted indoles. Thus, this method is an efficient and very useful
quinones such as 2-methyl-, 2-methoxy-, 2,6-dimethyl-p- synthetic procedure for the synthesis of natural product
benzoquinones and indole or 2-methylindole gave the cor- core structures. Among various Lewis acids such as InCl3,
responding indol-3-ylbenzoquinones in high yields CeCl3·7H2O and YCl3 studied for this reaction, indi-
(Scheme 2, Table 1, entries 4i–l).
um(III) bromide was found to be the most effective in
terms of conversion and reaction rates. However, similar
yields and selectivity were also obtained using 5 mol% of
BiCl3 under these reaction conditions. The scope and gen-
erality of this process is illustrated with respect to various
indoles and a wide range of quinones and the results are
presented in Table 1.
In a similar fashion, 1,4-naphthoquinone (2, R = H) and 2-
methyl-1,4-naphthoquinone (2, R = Me) afforded 2-(3-in-
dolyl)-1,4-naphthoquinones 5 under identical conditions
(Scheme 3, Table 1, entries 5m–o).
In summary, we have described a simple, convenient and
high yielding protocol for the preparation of 3-in-
dolylquinones via the nucleophilic addition of indoles to
quinones using InBr3 as a novel catalyst. This method is
applicable to both electron-rich as well as electron-defi-
cient indoles and is useful for the total synthesis of natu-
rally occurring asterriquinones. The attractive features of
this process are mild reaction conditions, short reaction
times, cleaner reactions with improved yields, no forma-
tion of by-products such as chloroquinones, ready avail-
ability of starting materials, high regioselectivity, and
simplicity in experimental procedure. The use of air and
moisture stable indium reagent makes this process useful
and attractive for the synthesis of 3-indolylquinone core
structures.
Scheme 3
In all cases, the reaction proceeds rapidly at room temper-
ature with high regioselectivity. As solvent, dichlo-
romethane appears to give the best results. The products
were characterized by NMR, IR and mass spectroscopic
data and also by comparison with authentic samples.7b
The probable mechanism seems to be the addition of in-
dole to the unsaturated position of the quinone, which is
activated by indium tribromide. The initial addition prod-
uct tautomerizes to the hydroquinone, which subsequently
undergoes rapid oxidation with another equivalent of p-
quinone resulting in the formation of the indol-3-ylquin-
one (Scheme 4).
Melting points were recorded on Büchi R-535 apparatus and are un-
corrected. IR spectra were recorded on a Perkin-Elmer FT-IR 240-
1
c spectrophotometer using KBr optics. H NMR spectra were re-
corded on Gemini-200 spectrometer in CDCl3 using TMS as inter-
Scheme 4
Synthesis 2004, No. 1, 106–110 © Thieme Stuttgart · New York

108
J. S. Yadav et al.
PAPER
Table 1 Indium(III) Bromide-Catalyzed Addition of Indoles to p-Quinones
Entry
Indole
Quinone
Producta
Time (min)
35
Yield (%)b
85
a
3a
b
c
3b
3c
25
45
90
83
d
e
f
3d
3e
3f
50
40
30
79
84
86
g
3g
30
45
78
h
80c
3h
4i
i
30
82
j
4j
35
30
85
88
k
4k
l
4l
35
50
84
80
m
5m
n
o
5n
5o
35
50
85
83
a All products were characterized by 1H NMR, IR and mass spectroscopy.
b Isolated and unoptimized yields.
c Bis(indolyl)hydroquinone was isolated.
Synthesis 2004, No. 1, 106–110 © Thieme Stuttgart · New York

PAPER
Synthesis of 3-Indolylquinones
109
nal standard. Mass spectra were recorded on a Finnigan MAT 1020
mass spectrometer operating at 70 eV.
1H NMR (200 MHz, CDCl3): = 1.35 (t, J = 6.9 Hz, 3 H), 2.80 (q,
J = 6.9 Hz, 2 H), 5.15 (br s, 1 H, OH), 5.25 (br s, 1 H, OH), 7.10–
7.45 (m, 5 H), 8.40 (br s, 1 H, NH).
Indium(III) Bromide-Catalyzed Addition of Indoles to p-
Quinones and 1,4-Naphthoquinones; General Procedure
A mixture of p-quinone 2 (2.5 mmol), indole 1 (1 mmol) and InBr3
(5 mol%) in CH2Cl2 (10 mL) was stirred at r.t. for the specified time
(see Table 1). After completion of the reaction as indicated by TLC,
the reaction mixture was quenched with H2O (15 mL) and extracted
with CH2Cl2 (2 × 10 mL). Evaporation of the solvent followed by
purification on silica gel (Merck, 100–200 mesh, EtOAc–hexane,
0.5–9.5) afforded pure indol-3ylquinone.
EIMS: m/z (%) = 321 (M+, 100), 306 (60), 195 (10), 156 (20), 97
(50), 71 (80), 57 (80), 43 (50).
2-(2-Methyl-1H-indol-3-yl)-1,4-hydroquinone (3f)
Solid, mp 106–108 °C.
IR (KBr): 3279, 2923, 1636, 1565, 1457, 1294, 1010, 774 cm–1.
1H NMR (200 MHz, CDCl3): = 2.40 (s, 3 H), 4.40 (br s, 1 H, OH),
4.60 (br s, 1 H, OH), 6.70–6.80 (m, 2 H), 6.90 (d, J = 8.0 Hz, 1 H),
7.05–7.20 (m, 2 H), 7.30–7.45 (m, 2 H), 8.05 (br s, 1 H, NH).
13C NMR (50 MHz, CDCl3): = 12.9, 109.9, 110.8, 114.3, 116.1,
116.8, 118.3, 118.9, 120.5, 122.9, 128.3, 133.3, 135.6, 147.8, 149.8.
EIMS: m/z (%) = 239 (M+, 30), 155 (10), 141 (20), 199 (100), 82
(95), 47 (80).
2,5-Dichloro-3-(1H-indol-3-yl)-1,4-benzenediol (3a)
Solid, mp 81–82 °C.
IR (KBr): 3425, 2923, 2852, 1642, 1561, 1420, 1244, 1008, 878,
744, 592 cm–1.
1H NMR (200 MHz, CDCl3): = 5.15 (br s, 1 H, OH), 5.30 (br s, 1
H, OH), 7.05 (d, J = 1.9 Hz, 1 H), 7.10–7.35 (m, 5 H), 8.70 (br s, 1
H, NH).
Ethyl 3-(2,5-Dihydroxyphenyl)-2-indolecarboxylate (3g)
Solid, mp 129–130 °C.
13C NMR (50 MHz, CDCl3): = 113.0, 116.5, 118.0, 119.6, 120.4,
120.8, 122.3, 124.0, 126.4, 127.8, 128.4,130.9, 138.2, 147.8.
IR (KBr): 3412, 2964, 2929, 2360, 1562, 1436, 1334, 1219, 1123,
854, 770 cm–1.
EIMS: m/z (%) = 293 (M+, 10), 256 (20), 119 (100), 82 (30), 43
(70), 29 (20).
1H NMR (200 MHz, CDCl3): = 1.25 (t, J = 7.0 Hz, 3 H), 4.30 (q,
J = 7.0 Hz, 2 H), 5.25 (br s, 2 H, OH), 6.75–6.81 (m, 1 H), 6.85–
6.90 (m, 1 H), 7.10–7.20 (m, 2 H), 7.30–7.45 (m, 2 H), 7.60 (d,
J = 8.0 Hz, 1 H), 9.05 (br s, 1 H NH).
2,5-Dichloro-3-(2-methyl-1H-indol-3-yl)-1,4-benzenediol (3b)
Solid, mp 182–183 °C.
IR (KBr): 3405, 2922, 1653, 1446, 1247,1219, 1136, 854, 771 cm–1.
EIMS: m/z (%) = 297 (10), 252 (60), 168 (20),139 (20), 110 (20),
68 (25), 42 (100).
1H NMR (200 MHz, CDCl3): = 2.38 (s, 3 H), 5.10 (br s, 1 H, OH),
5.35 (br s, 1 H, OH), 7.05–7.38 (m, 5 H), 8.25 (br s, 1 H, NH).
13C NMR (50 MHz, CDCl3): = 13.2, 106.7, 111.4, 116.0, 119.2,
119.4, 119.7, 120.9, 121.6, 125.0, 128.5, 135.3, 136.3, 145.6, 147.2.
2,5-Bis(1H-indol-3-yl)-1,4-hydroquinone (3h)1
Solid, mp 116–118 °C.
IR (KBr): 3398, 1618, 1457, 1337, 1096, 743 cm–1.
1H NMR (200 MHz, CDCl3): = 4.80 (br s, 2 H, OH), 7.05 (d,
J = 1.7 Hz, 2 H), 7.10–7.45 (m, 8 H), 7.80 (d, J = 8.1 Hz, 2 H), 8.30
(br s, 2 H, NH).
EIMS: m/z (%) = 307 (M+, 100), 273 (15), 237 (10), 154 (35), 137
(45), 95 (50), 69 (100), 55 (100).
13C NMR (50 MHz, CDCl3): = 112.8, 113.9, 116.9, 117.6, 120.2,
122.2, 124.2, 126.5, 127.2, 137.3, 147.9.
2,5-Dichloro-3-(1-methyl-1H-indol-3-yl)-1,4-benzenediol (3c)
Solid, mp 146 °C.
EIMS: m/z (%) = 341 (M+, 100), 257 (10), 228 (12), 156 (15), 142
(70), 84 (80), 47 (20).
IR (KBr): 3459, 2921, 2852, 1615, 1433, 1378, 1129, 853, 736
cm–1.
1H NMR (200 MHz, CDCl3): = 3.80 (s, 3 H), 5.30 (br s, 2 H OH),
7.05 (d, J = 1.8 Hz, 1 H), 7.10–7.45 (m, 5 H).
13C NMR (50 MHz, CDCl3): = 11.6, 107.8, 110.2, 115.4, 118.7,
119.7, 120.2, 120.5, 121.4, 124.7, 127.3, 130.4, 136.8, 146.3, 146.9.
EIMS: m/z (%) = 307 (M+, 20), 273 (15), 207 (10), 144 (15), 121
2-(1H-Indol-3-yl)-5-methylbenzo-1,4-quinone (4i)
Solid, mp 182 °C.
IR (KBr): 3276, 1739, 1667, 1635, 1565, 1295, 1253, 1119, 714
cm–1.
1H NMR (200 MHz, CDCl3): = 2.10 (s, 3 H), 6.58–6.61 (m, 1 H),
7.15 (d, J = 1.8 Hz, 1 H), 7.20–7.30 (m, 3 H), 7.38–7.42 (m, 1 H),
7.88–7.90 (m, 1 H), 8.50 (br s, 1 H NH).
(20), 109 (30), 95 (50), 83 (50), 69 (100), 55 (50).
3-(5-Bromo-1H-indol-3-yl)-2,5-dichloro-1,4-benzenediol (3d)
13C NMR (50 MHz, CDCl3): = 15.9, 108.0, 112.9, 120.3, 121.5,
122.9, 125.8, 132.0, 133.5, 134.0, 137.3, 140.8, 145.7, 187.6, 188.4.
EIMS: m/z (%) = 237 (M+, 50), 209 (10), 141 (50), 120 (60), 88
Solid, mp 220 °C.
IR (KBr): 3458, 2923, 2853, 1653, 1460, 1377, 1128, 852, 767
cm–1.
1H NMR (200 MHz, CDCl3): = 7.20 (d, J = 1.8 Hz, 1 H), 7.25–
7.30 (m, 2 H), 7.35–7.40 (m, 1 H), 7.56–7.60 (m, 1 H), 8.70 (br s, 1
H, NH).
(100), 43 (15).
2-Methyl-5-(2-methyl-1H-indol-3-yl)benzo-1,4-quinone (4j)
Solid, mp 196–197 °C.
IR (KBr): 3398, 2362, 1616, 1457, 1219 cm–1.
EIMS: m/z (%) = 373 (20), 237 (15), 154 (60), 137 (100), 123 (50),
115 (20), 109 (80).
1H NMR (200 MHz, CDCl3): = 2.15 (s, 3 H), 2.40 (s, 3 H), 6.65–
6.70 (m, 1 H), 6.75–6.80 (m, 1 H), 7.05–7.25 (m, 3 H), 7.40–7.48
(m, 1 H), 8.10 (br s, 1 H, NH).
2,5-Dichloro-3-(7-ethyl-1H-indol-3-yl)-1,4-benzenediol (3e)
Solid, mp 73–75 °C.
13C NMR (50 MHz, CDCl3): = 13.6, 15.4, 105.9, 111.2, 119.2,
120.1, 121.5, 127.6, 132.9, 133.8, 138.3, 142.3, 145.4, 146.3, 187.2,
187.8.
IR (KBr): 3391, 2926, 2855, 2359, 1675, 1454, 1218, 1012, 770
cm–1.
Synthesis 2004, No. 1, 106–110 © Thieme Stuttgart · New York

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J. S. Yadav et al.
PAPER
EIMS: m/z (%) = 251 (M+, 40), 186 (50), 154 (30),121 (40), 77
2-Methyl-3-(2-methyl-1H-indol-3-yl)-1,4-dihydro-1,4-naphthal-
(100), 41 (40).
enedione (5o)
Solid, mp 130 °C.
2-Methoxy-5-(2-methyl-1H-indol-3-yl)benzo-1,4-quinone (4k)
Solid, mp 198–199 °C.
IR (KBr): 3389, 2927, 2855, 2360, 1656, 1561, 1245, 1010, 769,
595 cm–1.
IR (KBr): 3270, 2362, 1738, 1667, 1635, 1591, 1565, 1458, 1421,
1294, 1118, 715 cm–1.
1H NMR (200 MHz, CDCl3): = 2.40 (s, 3 H), 3.90 (s, 3 H), 5.98–
6.05 (m, 1 H), 6.78–6.81 (m, 1 H), 7.10–7.20 (m, 2 H), 7.25–7.30
(m, 1 H), 7.45–7.50 (m, 1 H), 8.20 (br s, 1 H, NH).
13C NMR (50 MHz, CDCl3): = 14.1, 87.9, 106.7, 108.6, 111.8,
119.7, 120.8, 122.2, 128.1, 129.8, 136.3, 139.4, 143.4, 159.3, 182.4,
187.6.
1H NMR (200 MHz, CDCl3): = 2.10 (s, 3 H), 2.30 (s, 3 H), 7.05–
7.20 (m, 2 H), 7.25–7.30 (m, 2 H), 7.70–7.80 (m, 2 H), 8.10–8.25
(m, 2 H), 8.30 (br s, 1 H, NH).
13C NMR (50 MHz, CDCl3): = 14.1, 16.3, 106.6, 112.1, 120.1,
120.4, 121.7, 127.0, 127.4, 128.9, 133.2, 133.3, 135.0, 136.6, 136.9,
141.8, 145.6, 184.6, 186.3.
EIMS: m/z (%) = 301 (M+, 15), 286 (15), 149 (60), 69 (70), 42
(100).
EIMS: m/z (%) = 267 (M+, 35), 156 (40), 142 (100), 69 (50), 43
(70).
Acknowledgment
3,5-Dimethyl-2-(2-methyl-1H-indol-3-yl)benzo-1,4-quinone (4l)
Solid, mp 72–74 °C.
IR (KBr): 2361, 1521, 1376, 1263, 772 cm–1.
B. V. S. R. and T. S. thank CSIR, New Delhi for the award of fel-
lowships.
1H NMR (200 MHz, CDCl3): = 2.0 (s, 3 H), 2.20 (s, 3 H), 2.30 (s,
3 H), 6.70 (s, 1 H), 7.05–7.20 (m, 2 H), 7.25–7.30 (m, 2 H), 8.10 (br
s, 1 H, NH).
EIMS: m/z (%) = 265 (M+, 100), 250 (50), 222 (20), 194 (20), 168
(30), 118 (15), 69 (25), 43 (50).
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2-(1H-Indol-3-yl)-1,4-naphthoquinone (5m)
Solid, mp 176 °C.
IR (KBr): 3398, 2362, 1617, 1457, 1219, 772 cm–1.
1H NMR (200 MHz, CDCl3): = 7.27–7.35 (m, 2 H), 7.40–7.48 (m,
2 H), 7.71–7.80 (m, 2 H), 7.95–8.05 (m, 1 H), 8.10–8.19 (m, 2 H),
8.55–8.60 (m, 1 H), 8.65 (br s, 1 H, NH).
13C NMR (50 MHz, CDCl3): = 107.7, 112.9, 120.3, 121.7, 123.0,
125.1, 125.5, 126.9, 128.0, 131.9, 132.3, 132.8, 134.0, 134.4, 137.0,
142.5, 184.6, 185.3.
EIMS: m/z (%) = 273 (M+, 20), 158 (100), 142 (90), 104 (75), 76
(70), 43 (90).
2-(2-Methyl-1H-indol-3-yl)-1,4-naphthoquinone (5n)
Solid, mp 180 °C.
IR (KBr): 3274, 2922, 1739, 1634, 1457, 1254, 714 cm–1.
1H NMR (200 MHz, CDCl3): = 2.50 (s, 3 H), 7.10 (s, 1 H), 7.15–
7.20 (m, 2 H), 7.25–7.30 (m, 1 H), 7.50–7.60 (m, 1 H), 7.70–7.80
(m, 2 H), 8.10–8.20 (m, 2 H), 8.25 (br s, 1 H, NH).
13C NMR (50 MHz, CDCl3): = 14.2, 107.6, 111.1, 119.7, 121.2,
122.6, 126.3, 127.4, 128.0, 132.6, 133.2, 133.9, 134.1, 135.1, 135.9,
137.5, 144.8, 185.2, 185.7.
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EIMS: m/z (%) = 287 (M+, 20), 270 (15), 230 (100).
Synthesis 2004, No. 1, 106–110 © Thieme Stuttgart · New York