A facile synthesis of 2-aroylindoles by the oxidation of 2-arylmethylindoles using Sarett reagent

Article abstract of DOI:10.1081/SCC-120021513

A facile synthesis of 2-aroylindoles by the oxidation of 3-substituted 2-arylmethylindoles using Sarett reagent has been reported. The phenylthio group at 3-position has been cleaved by Raney nickel.

Full text of DOI:10.1081/SCC-120021513

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A Facile Synthesis of 2-Aroylindoles by the Oxidation of
2-Arylmethylindoles Using Sarett Reagent
Nagappan Arumugam a & Panayencheri C. Srinivasan b
a R & D Department , Amrutanjan Ltd. , Chennai, India
b Department of Organic Chemistry , University of Madras , Guindy Campus, Chennai, India
Published online: 17 Aug 2006.
To cite this article: Nagappan Arumugam & Panayencheri C. Srinivasan (2003) A Facile Synthesis of 2-Aroylindoles by the
Oxidation of 2-Arylmethylindoles Using Sarett Reagent, Synthetic Communications: An International Journal for Rapid
Communication of Synthetic Organic Chemistry, 33:13, 2313-2320, DOI: 10.1081/SCC-120021513
To link to this article: http://dx.doi.org/10.1081/SCC-120021513
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SYNTHETIC COMMUNICATIONSÕ
Vol. 33, No. 13, pp. 2313–2320, 2003
A Facile Synthesis of 2-Aroylindoles by the
Oxidation of 2-Arylmethylindoles Using
Sarett Reagent
Nagappan Arumugam1 and Panayencheri C. Srinivasan2,
*
1R & D Department, Amrutanjan Ltd.,
Chennai, India
2Department of Organic Chemistry, University of
Madras, Guindy Campus, Chennai, India
ABSTRACT
A facile synthesis of 2-aroylindoles by the oxidation of 3-substituted
2-arylmethylindoles using Sarett reagent has been reported. The
phenylthio group at 3-position has been cleaved by Raney nickel.
Key Words: 2-Arylmethylindoles; 2-Aroylindoles; Sarett reagent;
Raney nickel; Chromium pyridine complex.
*Correspondence: C. Srinivasan, Department of Organic Chemistry, University
of Madras, Guindy Campus, Chennai, 600 025, India; Fax: 91-44-2352494;
E-mail: pancol2000@yahoo.co.in.
2313
DOI: 10.1081/SCC-120021513
Copyright & 2003 by Marcel Dekker, Inc.
0039-7911 (Print); 1532-2432 (Online)
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2314
Arumugam and Srinivasan
Recently antimitotic activity of some 2-aroylindoles has been
reported.[1] 3-Iodo-2-aroylindole has been used as a precursor for the
synthesis b- carbolines.[2] The synthesis of 2-aroylindoles from2-Lithio-
have been
[6]
N-blocked indoles[3–5] and from2-nitrobenzaldehydes
reported. In continuation of our studies towards the synthesis of fused
heterocycles,[7] we were interested in developing a new synthesis of
2-aroylindoles. The oxidation of an active methylene group to the corre-
sponding keto group by chromium trioxide has been reported.[8–10] So
far there is no report on the oxidation of a 2-benzylindole to the
corresponding ketone. So we report here the use of chromium trioxide-
pyridine complex in pyridine [Sarett reagent][11] for the oxidation of
2-arylmethylindoles to 2-aroylindoles.
Treatment of 1-phenylsulfonyl-3-phenylthioindol-2-ylmethanol[12–13]
1a–c, 3-chloro-1-phenylsulfonylindol-2-ylmethanol 1d or 3-bromo-1-phe-
nylsulfonylindol-2-ylmethanol 1e with different arenes in chloroformor
1,2-dichloroethane in the presence of BF3ꢀOEt2 afforded the correspond-
ing 2-arylmethylindoles 2a–e. Compounds 2a–e were oxidized to the cor-
responding 2-aroylindoles 3a–e by chromium trioxide in pyridine(sarett
reagent). Compounds 2a, 2b, 2d, and 2e required 5 equiv. of the oxidant.
Compound 2c required 10 equiv. for oxidation. The phenylthio group
was removed by Raney nickel in boiling ethanol to afford compound
4a–c; R1 ¼ H. Treatment of 4a–c; R1 ¼ H and 3d–e with 10% NaOH in
boiling alcohol afforded 5a–c; R1 ¼ H and 5d–e. In conclusion, a facile
oxidation of 2-arylmethylindoles by chromium trioxide to
2-aroylindoles with various substituents has been accomplished.
All melting points are uncorrected. Merck silica gel 60 GF 254
thin-layer plates were employed for TLC. Merck silica gel (finer than
350 mesh) was used for flash column chromatography. IR spectra were
recorded on Perkin-Elmer FT-paragon 1000 and Shimadzu-8300 FT
instruments. 1H-NMR spectra were recorded on a Brucker 304 MHz
instrument. Mass spectra were recorded on Finningan MAT 8230 instru-
ment. Chromium trioxide was purchased from Merck-India.
Compound 1d. 3-Chloro-2-methylindole was prepared according to
the Lit. procedure[14] from2-methylindole.
3-Chloro-2-methyl-1-phenylsulfonylindole. To a solution of 3-chloro-
2-methylindole (1.7 g; 10.3 mmol) in benzene (30 mL) was added 50%
sodium hydroxide solution (3 mL) followed by cetyltrimethylammonium-
bromide (5 mg). phenylsulfonylchloride (1.5 mL) was added in one lot at
RT and stirred for 24 h. The completion of the reaction was monitored by
TLC. Water (50 mL) was added and the benzene layer was separated and
distilled off. The residue was digested with chloroformand filtered. The

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2-Aroylindoles
2315
Scheme 1.
filtrate was concentrated and the pasty mass was crystallized from
methanol. (82%) m.p. 110–112ꢁC. IR (KBr): 1446, 1374, 1236, 1183,
1
1097, 751, 793, 587, 565 cmÀ1. H NMR (CDCl3/TMS): 2.91 (s, 3H),
6.66–7.85 (m, 9H). (Calcd. For C15H12ClNO2S: C, 58.92; H, 3.96; N,
4.58. Found: C, 58.82; H, 3.70; N, 4.86%).
3-Chloro-2-hydroxymethyl-1-phenylsulfonylindole. To a solution of 3-
Chloro-1-phenylsulfonyl-2-methylindole (1 g, 3.2 mmol) in Carbontetra-
chloride (25 mL), N-bromosuccinimide (0.7 g), and dibenzoylperoxide
(5 mg) were added. The solution was refluxed for 4 h. The succinimide
was filtered and carbontetrachloride (20 mL) was distilled out. The residue
was dissolved in acetonitrile (20 mL). To this a solution of potassium
carbonate (1 g) in water (2 mL) was added. It was refluxed for 3 h. The
lower layer was discarded. Acetonitrile was distilled out completely. The
residue was dissolved in chloroform(25 mL) and filtered. Chloroformwas
distilled out. The pasty mass was taken for next step after IR and 1H NMR
analysis. (80%), IR (KBr): 3451, 1591, 1480, 1183, 793, 587 cmÀ1
1H NMR (CDCl3/TMS): 2.31 (s, 1H), 4.82 (s, 2H), 6.91–7.98 (m, 9H).
.
Compound 1e. 3-Bromo-2-bromomethyl-1-phenylsulfonylindole was
prepared according to the Lit. procedure.[15] To a solution of 3-bromo-
2-bromomethyl-l-phenylsulfonylindole (27.9 mmol) in acetonitrile
(200 mL), a solution of potassium carbonate (10 g) in water (20 mL) was
added. It was refluxed for 5 h. The bottomlayer was discarded.

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2316
Arumugam and Srinivasan
Acetonitrile was distilled out completely. The residue was dissolved in
chloroform(100 mL) and filtered. The chloroformwas distilled off com-
pletely. The residue was crystallized from methanol. (78.4%), m.p.:
128–130ꢁC (MeOH). IR (KBr): 3557, 3435, 1448, 1315, 896, 764,
1
471 cmÀ1. H NMR (CDCl3): 2.31 (s, 1H), 4.88 (s, 3H), 6.8–7.99 (m,
9H). (Calcd. for C15H12BrNO3S: C, 49.19; H, 3.30; N, 3.82. Found: C,
49.29; H, 2.88; N, 3.44%).
Compound 2: General Procedure
1-Phenylsulfonyl-2-bromomethyl-3-phenylthioindole[11] was con-
verted by partial hydrolysis (CH3CN-H2O, K2CO3, Á, 3h) into 1-phenyl-
sulfonyl-3-phenylthioindol-2-ylmethanol by the published procedure.[12]
To a solution of 1-phenylsulfonyl-3-phenylthioindol-2-ylmethanol or
3-chloro-l-phenylsulfonylindol-2-ylmethanol 1d or 3-bromo-1-phenylsul-
fonylindol-2-ylmethanol 1e (25 mmol) in chloroform (400 mL), a solution
of arene (25 mmol) in the same solvent (25 mL) was added followed by
anhydrous magnesium sulfate (10 g) and borontrifluoride etherate
(2.0 mL). [For 2c, 1,2-dichloroethane was used as solvent]. The resulting
solution was refluxed for 3 h. Then water (100 mL) was added and the
organic layer was separated. The organic layer was washed with 20%
hydrochloric acid (1 Â 50 mL) followed by water and saturated bicarbo-
nate solution. The solvent was removed by distillation after drying over
anhydrous sodiumsulfate. The residue was chromatographed on a silica
gel column (350 mesh) and eluted successively with 20% ethylacetate in
hexane followed by 25% and then finally with 30%. Thirty percent ethyl-
acetate in hexane eluent gave 2-arylmethylindole which was then crystal-
lized.
Compound 2a. (76%), m.p.: 154–156ꢁC. (benzene–hexane 1:1). IR
.
(KBr): 3425, 1727, 1666, 1373, 1172 cmÀ1 1H NMR (CDCl3/TMS):
2.25 (s, 3H), 3.67 (s, 3H), 3.84 (s, 3H), 4.52 (s, 2H), 6.86–8.25 (m, 17H,
NHþarom). MS: m/z (%) 481(100), 480 (30), 479 (50), 328 (38), 77 (18).
(Calcd. for C32H28N2O6S2: C, 64.04; H, 4.70; N, 4.66. Found: C, 64.02;
H, 4.45; N, 4.35%).
Compound 2b. (85%), m.p.: 182–184ꢁC. (benzene). IR(KBr): 3409,
1
1689, 1365, 1172 cmÀ1. H NMR (CDCl3/TMS): 2.22 (s, 3H), 3.81 (s,
3H), 4.50 (s, 2H), 6.81–8.19 (m, 17H, NHþarom). MS: m/z (%) [Br81 and
Br79] 481 (38), 479 (40), 372 (2.5), 370 (2.5), 329 (54), 327 (52), 248 (28),
236 (40), 205 (30), 77 (100). (Calcd. for C30H25N2O4S2Br: C, 58.11; H,
4.06; N, 4.51. Found: C, 55.08; H, 3.75; N, 3.18%).
Compound 2c. (78%), m.p.: 178–180ꢁC. (benzene). IR (KBr): 3394,
1662, 1371, 1172 cmÀ1. 1H NMR (CDCl3/TMS): 1.93 (s, 3H), 2.24 (s, 3H),

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2-Aroylindoles
2317
4.59 (s, 2H), 6.47–8.18 (m, 18H, NHþarom). MS: m/z (%) 526 (Mþ, 2), 385
(38), 233 (100), 77 (48). (Calcd. for C30H26N2O3S2: C, 68.48; N, 4.98, H,
5.32. Found: C, 68.64, H, 4.73, N 5.01%).
Compound 2d. (85%) m.p.: 116–118ꢁC. (benzene–hexane 1:1). IR
(KBr): 1183, 1371 cmÀ1 1H NMR (CDCl3/TMS): 3.75 (s, 3H), 4.46
.
(s, 2H), 6.7–8.1 (m, 13H). MS: m/z (%) 380 (1), 378 (2), 287 (2), 285
(6), 272 (48), 270 (52), 237 (36), 235 (100). (Calcd. for C22H18ClNO3S: C,
64.15; N, 3.40; H, 4.40. Found: C, 64.38; H, 4.19; N, 3.13%).
Compound 2e. (76%), m.p.: 202–204ꢁC. (benzene). IR(KBr): 3217,
1
1660 cmÀ1. H NMR (CDCl3/TMS): 2.27 (s, 3H), 3.88 (s, 3H), 4.38 (s,
2H), 7.21–8.15 (m, 12H, NHþarom). MS: m/z (%) [Br81 and Br79] 594
(Mþ,3.3), 592 (4.7), 590 (3.5), 454 (12.3), 453 (50.2), 452 (22.6), 451
(100), 450 (13.4), 208 (3.4), 77 (37.2). (Calcd. for C24H20 Br2N2O4S: C;
48.67; H, 3.4; N, 4.73. Found: C, 48.85; H, 3.18; N, 4.98%).
Oxidation of 2a–e: General Procedure
Chromium trioxide (60 g) was slowly added to dry pyridine (600 mL)
at 0–5ꢁC over a period of 1 h. After the addition of 50% of chromium
(VI) oxide, a color change fromdark brown to an orange red indicated
the formation dipyridine chromium (VI) oxide complex. Then the tem-
perature was allowed to come to RT. A solution of 2-arylmethylindole
(12 g, 20 mmol) in pyridine (30 mL) was added in one portion to the
dipyridine chromium (VI) oxide complex and the temperature of the
reaction mixture was maintained at 50–60ꢁC with vigorous stirring for
48 h. [For compound 2C, 105 g of CrO3 and 1000 mL of pyridine was
used and the temperature was maintained for 60–70ꢁC]. Complete dis-
appearance of the starting material was ascertained by periodic TLC
analysis. Then the excess pyridine was distilled off under reduced pressure
and the residue was poured over crushed ice (500 g). The pH was adjusted
to 2 by the addition of concentrated hydrochloric acid. The precipitated
solid was filtered, washed with ice-cold water, pressed dry and desiccated
over anhydrous calciumchloride. Then it was extracted on a soxhlet
apparatus with chloroform. Removal of chloroform by distillation gave
the crude product which was then purified by crystallization.
Note: The chromium trioxide was made into granules in such a way that
it should sink into the cold pyridine solution immediately after the
addition. If it floats on the pyridine surface it becomes pyrophoric.
Compound 3a. (60%), m.p.: 190–191ꢁC. (DME). IR (KBr): 3249,
1
1730, 1698, 1631 cmÀ1. H NMR (CDCl3/TMS): 2.33 (s, 3H), 3.71 (s,
3H), 4.00 (s, 3H), 7.03–8.63 (m, 16Harom) 11.85 (s, 1H). MS: m/z (%)

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2318
Arumugam and Srinivasan
614 (Mþ, 20), 613 (16), 431 (14), 323 (22), 322 (100). The structure of acid
obtained by hydrolysis of ester moiety in 3a has been determined by
single crystal X-ray analysis. (Calcd. for C32H26N2O7S2: C, 62.52; H,
4.26; N, 4.56. Found: C, 62.80; H, 4.18; N, 4.39%).
Compound 3b. (50%), m.p.: 226–228ꢁC. (Benzene–hexane 1:1). IR
(KBr): 3259, 1688, 1622 cmÀ1 1H NMR (CDCl3/TMS): 2.16 (s, 3H),
.
3.89 (s, 3H), 6.95 À 8.08 (m, 16Harom), 11.82 (s, 1H). MS: m/z (%) 626
(6), 624 (6). 481 (6), 479 (3), 453 (2), 451 (2), 344 (28), 342 (25), 78 (100).
(Calcd. for C30H23N2O4S2Br: C, 56.78; H, 3.65; N, 4.41. Found: C 56.68;
H, 3.53; N, 4.56%)
Compound 3c. (65%), m.p.: 180-–182ꢁC. (Benzene). IR (KBr): 3301,
1698, 1641 cmÀ1 1H NMR (CDCl3/TMS): 1.98 (s, 3H), 2.22 (s, 3H),
.
7.01–8.70 (m, I7Harom), 11.26 (s, 1H). MS: m/z (%) 540 (Mþ, 8), 385
(2), 357 (6), 248 (100), 247 (16), 77 (54). (Calcd. for C30H24N2O4S2: C,
66.65; H, 4.47; N, 5.18. Found: C, 66.67; H, 4.29: N; 4.97%).
Compound 3d. (56%), m.p.: 140–141ꢁC (Benzene–hexane 1:1). IR
(KBr): 1688, cmÀ1 1H NMR. (CDCl3/TMS): 3.82 (s, 3H), 6.95–8.30
.
(m, 13Harom), MS: m/z (%) 362 (2), 360 (6), 286 (36), 256 (22), 179
(16), 177 (16), 177 (38), 77 (100), (Calcd. for C22H16CINO4S: C, 62.05;
H, 3.79; N, 3.29. Found: C, 62.18; H, 3.83; N, 3.91%).
Compound 3e. (61%), m.p.: 244–246 ꢁC (DME). IR (KBr): 3257,
1708, 1623, 728 cmÀ1 1H NMR (CDCl3/TMS): 2.35 (s, 3H), 4.03 (s,
.
3H), 7.26–8.67 (m, 11Harom), 11.76 (s, 1H). MS: m/z (%) 608 (Mþ,4.8),
606 (9.9), 485 (16.8), 483 (4), 344 (34), 342 (32), 77 (100). (Calcd. for
C24H18 Br2 N2O5S: C, 47.55; H, 2.99; N, 4.62. Found: C, 47.88; H, 2.89;
N, 4.91%).
Desulfurization of 3a–c: General Procedure
A solution of compound 3 (15 mmol) in ethanol (300 mL) containing
Raney nickel (50 g) was refluxed for 1 h. The catalyst was filtered off and
washed with hot ethanol (3 Â 20 mL). The combined filtrates were con-
centrated and the residue was crystallized . The products were directly
1
used for next step after IR and H NMR analysis.
Compound 4a. (87%), m.p.: 212–214ꢁC. (Acetone—hexane). IR(KBr):
3426, 1727, 1700, 1626, 1371, 1172 cmÀ1. 1H NMR (CDCl3/TMS): 2.32 (s,
311), 3.78 (s, 3H), 4.04 (s, 3H), 6.84 (s, 1H, indole-3H), 7.25–8.63 (m,
11Harom) 11.81 (s, 1H).
Compound 4b. (85%), m.p.: 144–146ꢁC. (Benzene–hexane). IR (KBr):
1688, 1622, 1371, 1172 cmÀ1. 1H NMR (CDCl3/TMS): 2.21 (s, 3H), 3.85 (s,
3H), 6.15 (s, 1Hindole-3H), 6.82–8.52 (m, 11Harom), 11.81 (s, 1H).

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2-Aroylindoles
2319
Compound 4c. (88%), m.p.: 158–160ꢁC. (Methanol). IR (KBr): 3347,
1686, 1639 cmÀ1. 1H NMR (CDCl3/TMS): 1.93 (s, 3H), 2.24 (s, 3H), 6.47
(s, lHindole-3H), 6.98–8.18 (m, 12Harom), 11.00 (s, 1H).
Hydrolysis of 4a–c; R1 ¼ H and 3d–e: General Procedure
A solution of compound (15 mmol) in alcohol (300 mL) was refluxed
with 10% sodium hydroxide solution (50 mL) for 30 min. The solution
was poured over ice and the precipitated solid was filtered, washed
with water, dried over anhydrous calciumchloride and purified by
crystallization .
Compound 5a. (80%), m.p.: 179–181ꢁC (MeOH). IR (KBr): 3477,
3336, 1693, 1625, 1612 cmÀ1 1H NMR (CDCl3/TMS): 2.29 (s, 3H),
.
3.76 (s, 3H); 4.02 (s, 3H), 6.84 (s, lHindole-3H), 7.23–8.61 (m, 7H,
NHþarom), 11.79 (s, 1H). MS: m/z (%) 324 (30), 323 (100), 250 (12), 236
(12), 205 (10), 77 (18). (Calcd. for C20H18N2O5: C, 65.57: H, 4.91; N,
7.65. Found: C, 65.68; H, 4.62; N, 7.32%).
Compound 5b. (85%), m.p.: 190–92ꢁC (MeOH). IR (KBr): 3365,
1
3305, 1607, 1586 cmÀ1. H NMR (CDCl3/TMS): 2.16 (s, 3H). 3.89 (s,
3H), 6.95 (s, lHindole-3H), 6.98–8.08 (m, 7H. NHþarom). 11.81 (s, 1H). MS:
m/z (%) 266 (100), 265 (90). 250 (8), 249 (20), 238 (9), 237 (28), 206 (8),
150 (22), 89 (54). (Calcd. for C18H15BrN2O3: C, 55.81; H, 3.87; N, 7.23.
Found: C, 55.82; H; 3.66; N, 6.92%).
Compound 5c. (80%), m.p.: 290–292ꢁC (MeOH). IR (KBr): 3224,
1685, 1621 cmÀ1 1H NMR (CDCl3/TMS): 1.90 (s, 3H), 2.22 (s, 3H),
.
6.45 (s, 1Hindole-3H), 6.97–8.25 (m, 8H, NHþarom), 11.81 (s, 1H ). MS:
m/z (%) 286 (16), 285 (24), 284 (43), 256 (12), 178 (16), 177 (36), 135
(48), 392 (26), 77 (100). (Calcd. for C18H16N2O2: C, 73.97; H, 5.47; N,
9.58. Found : C, 74.10: H, 5.26; N, 9.35%).
Compound 5d. (82%), m.p.: 168–170ꢁ. (MeOH); IR (KBr): 3293,
1619, 1594 cmÀ1
.
1H NMR (CDCl3/TMS): 3.87 (s, 3H), 6.95–7.91
(m, 9H, NHþarom). MS: m/z (%) 236 (12), 234 (24), 223 (100), 222 (34),
207 (20), 205 (12), 134 (10), 133 (10), 117 (22). (Calcd. for C16H12ClN2O2:
C, 67.26; H, 4.23; N, 4.91. Found: C, 67. 18; H, 4.61; N, 4.81%).
Compound 5e. (68%), m.p.: 292–294ꢁC Dec. (MeOH). IR (KBr):
3614, 3543, 3428, 1669, 1615 cmÀ1 1H NMR (CDCl3/TMS): 2.38 (s,
.
3H), 3.89 (s, 3H), 7.16–8.92 (m, 7H, NHþarom), 11.81 (s, 1H). MS: m/z
(%) 468 (Mþ, 4.8), 466 (10.16), 485 (16.7), 483 (3), 343 (34), 341 (35), 77
(100). (Calcd. for C18H14Br2N2O3: C, 46.38: H, 3.03; N, 6.01. Found: C
46.19; H, 2.91; N, 5.90%).

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2320
Arumugam and Srinivasan
ACKNOWLEDGMENTS
The authors thank UGC-SAP for instrument grants to the
Department of Organic chemistry, University of Madras, Amrutanjan
Ltd. for providing lab facilities to NA, Prof. D. Velmurugan, Dept. of
Biophysics and Crystallography, University of Madras for x-ray analysis
of carboxilic acid derivative of 3a and Dr. R. Murugesan, RSIC, IIT,
Chennai for mass spectra.
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Received in the Netherlands October 15, 2002