Synthesis of 7-keto-Goe6976 (ICP-103)

Article abstract of DOI:10.1081/SCC-200049803

An efficient synthesis of 7-keto-Goe6976 (ICP-103) (3) is described that employs palladium(II) inflate in the final oxidative cyclization step.

Full text of DOI:10.1081/SCC-200049803

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Synthesis of 7‐Keto‐Gö6976 (ICP‐103)
Sudipta Roy a , Alan Eastman b & Gordon W. Gribble a
a Department of Chemistry, Dartmouth College, Hanover, USA
b Department of Pharmacology and Toxicology, Dartmouth Medical School,
Hanover, USA
Published online: 12 Apr 2011.
To cite this article: Sudipta Roy , Alan Eastman & Gordon W. Gribble (2005): Synthesis of 7‐Keto‐Gö6976
(ICP‐103), Synthetic Communications: An International Journal for Rapid Communication of Synthetic Organic
Chemistry, 35:4, 595-601
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Synthetic Communicationsw, 35: 595–601, 2005
Copyright # Taylor & Francis, Inc.
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1081/SCC-200049803
¨
Synthesis of 7-Keto-Go6976 (ICP-103)
Sudipta Roy
Department of Chemistry, Dartmouth College, Hanover, USA
Alan Eastman
Department of Pharmacology and Toxicology, Dartmouth Medical
School, Hanover, USA
Gordon W. Gribble
Department of Chemistry, Dartmouth College, Hanover, USA
¨
Abstract: An efficient synthesis of 7-keto-Go6976 (ICP-103) (3) is described that
employs palladium(II) triflate in the final oxidative cyclization step.
¨
Keywords: Indolo[2,3-a]carbazole, Go6976, palladium(II) triflate
Indolo[2,3-a]carbazole and indolo[2,3-a]pyrrolo[3,4-c]carbazole alkaloids are
an interesting class of natural products because of their high degree of biologi-
cal activity, including antitumor, antifungal, and antimicrobial properties.[1]
One member of this class, UCN-01 (1), generated considerable interest in
our laboratory when it was found to be a potent inhibitor of DNA damage-
induced S and G2 cell cycle checkpoints, which led to increased killing of
tumor cells.[2] Although UCN-01 is well recognized as a protein kinase C
inhibitor,[3] this checkpoint inhibition was attributed to its ability to inhibit
Chk1.[4] In a clinical trial, UCN-01 was found to bind avidly to human
serum proteins thereby compromising its potential therapeutic activity.[5]
Therefore, we have initiated an Indolocarbazole Project (ICP) to identify
Received in the USA 4 November 2004
Address correspondence to Gordon W. Gribble, Department of Chemistry,
Dartmouth College, Hanover, NH 03755, USA. E-mail: ggribble@dartmouth.edu

596
S. Roy, A. Eastman, and G. W. Gribble
other potent and selective checkpoint inhibitors that would not be inactivated
by human serum proteins. We initially synthesized and tested a K252a
analogue, ICP-1, which overcame the problems of protein binding but had
considerably reduced potency.[6] More recently, we established that Go6976
¨
(2) is a very potent checkpoint inhibitor even in the presence of human
serum,[7] and 2 has subsequently been shown to potently inhibit Chk1.[8] As
a consequence, we have begun structure/activity analysis of analogs of
¨
¨
Go6976. We now report an efficient synthesis of 7-keto-Go6976 (3), which
we name ICP-103.[9]
The requisite N-methylindole and N-cyanoethylindole fragments were
synthesized as shown in Scheme 1. Indole-1-propionitrile (4) was prepared
in 82% yield from the reaction of indole with acrylonitrile in presence of
Triton-B.[10] Alkylation of indole-3-acetic acid (5) afforded 1-methylindole-
3-acetic acid (6)[11] in excellent yield (Scheme 1).
Indole-1-propionitrile (4) was treated with oxalyl chloride to give [1-(2-
cyanoethyl)indolyl]-3-glyoxylyl chloride (7).[12] The crude reaction mixture
was allowed to react with 1-methylindole-3-acetic acid (6) in presence of
triethylamine. Anhydride 8 was obtained in 61% yield over the two steps
Scheme 1.

¨
Synthesis of 7-Keto-Go6976
597
Scheme 2.
(Scheme 2), and was converted to imide 9 under mild amination conditions.
Due to the potentially labile nitrile functionality, ammonia was generated
in situ from 1,1,1,3,3,3-hexamethyldisilazane (HMDS) and methanol and
these conditions were used to convert anhydride 8 to imide 9 at room
temperature.[13]
The final cyclization step to afford the target molecule 3 was initially
attempted by using 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ) in
presence of catalytic p-toluenesulfonic acid.[14] This particular oxidative
cyclization can be difficult in the indolocarbazole system and, therefore,
we delayed this reaction to the final step of our synthesis. Indeed, poor
yields were obtained using DDQ as the oxidant. Different solvent systems
(such as benzene,[15] chlorobenzene[16]) and temperatures did not improve
the yield, and inseparable mixtures were obtained. The use of PdCl2 did
not increase the yield of this cyclization reaction.[17] Finally, we found
Pd(OTf)2 to be a superior reagent for this cyclization, especially with
bulky substituents on N-12 and N-13 of the bisindolemaleimide.[18] Thus,
using Pd(OTf)2 in DMF, we were able to improve the yield of this
oxidative cyclization to 45%. Overall, ICP-103 (3) was obtained in 26%
yield from 4.
In work to be reported separately, we find that ICP-103 (3) is almost as
¨
potent as Go6976 (2) at abrogating DNA damage-induced cell cycle arrest.
¨
Using an assay previously described,[7] Go6976 abrogated S and G2 arrest
at 10 nM, whereas ICP-103 was effective at 30 nM.

598
S. Roy, A. Eastman, and G. W. Gribble
EXPERIMENTAL
Melting points were determined with a Mel-Temp Laboratory Device
1
apparatus and are uncorrected. H and 13C NMR spectra were recorded on
either a Varian XL-300 or 500 Fourier-transform NMR spectrometer. Both
low- and high-resolution mass spectra were carried out at the Mass Spectro-
metry Laboratory, School of Chemical Sciences, University of Illinois at
Urbana Champaign. Elemental analyses were done by Atlantic Microlab
Inc. THF, methylene chloride, and triethylamine were distilled before use.
Indole-1-propionitrile (4)
To a magnetically stirred solution of indole (2.93g, 25mmol) in dioxane (25 mL)
at 08C was added acrylonitrile (2.5 mL, 37.5 mmol) followed by the dropwise
addition of Triton-B (0.8 mL). The resulting mixture was slowly allowed to
warm to room temperature (rt) and stirred for 20h. Then 5% acetic acid was
added dropwise to neutralize the base catalyst. The solution was washed with
brine and dried over sodium sulfate. The solvent was removed in vacuo
and the residue was purified by column chromatography (6:4 hexanes:ethyl
acetate) to give 3.49g (82%) of 4 as a colorless oil that solidified on prolonged
drying to furnish a white solid; m.p. 44–468C (Lit.[19] 47–488C); 1H (CDCl3)
d 7.73 (d, 1H, J ¼ 7.8 Hz), 7.37–7.18 (m, 4H), 6.62 (dd, 1H, J ¼ 3.2 Hz,
0.5 Hz), 4.44 (t, 2H, J ¼ 6.7 Hz), 2.80 (t, 2H, J ¼ 6.8 Hz); 13C (CDCl3) d
135.5, 129.1, 127.6, 122.4, 121.6, 120.3, 117.4, 108.8, 103.0, 42.2, 19.2.
1-Methylindole-3-acetic Acid (6)
To a magnetically stirred suspension of NaH (6 g, 150 mmol, 60% mineral oil
dispersion) in THF (125 mL) at 08C was added a solution of indole-3-acetic
acid (5) (5.25 g, 30 mmol) in THF (50 mL). After stirring the reaction
mixture at the same temperature for 30 min, a solution of methyl iodide
(14.2 g, 100 mmol) in THF (50 mL) was added and stirred for 16 h at rt.
Excess hydride was carefully destroyed by slow addition of MeOH
followed by water. The mixture was extracted with Et2O. The aqueous layer
was acidified with 6N HCl and extracted with CH2Cl2. The combined
organic extracts were dried over sodium sulfate and concentrated to about
40–50 mL. Petroleum ether (b.p. 38.5–558C) was then added slowly until a
cream-colored solid precipitated. The solid was recrystallized from ethanol
to give 5.33 g (94%) of 6; m.p. 127–1288C (Lit.[20] 127–128.58C);
1H (CDCl3) d 7.64–7.62 (m, 1H), 7.34–7.26 (m, 2H), 7.19–7.15 (m, 1H),
7.07 (s, 1H), 3.83 (s, 2H), 3.78 (s, 3H); 13C (CDCl3) d 178.8, 137.0, 128.1,
127.7, 122.0, 119.5, 119.1, 109.5, 106.2, 32.9, 31.2.

¨
Synthesis of 7-Keto-Go6976
599
3-(1-Cyanoethyl-3-indolyl)-4-(1-methyl-3-indolyl)maleic
Anhydride (8)
To a magnetically stirred solution of indole-1-propionitrile (4) (0.85 g,
5 mmol) in dichloromethane (50 mL) at 08C was added dropwise oxalyl
chloride (0.7 g, 5.5 mmol). The mixture was stirred at 0–58C for 30 min.
The solvent was removed in vacuo and the organic residue was redissolved
in dichloromethane (50 mL). This solution was then added dropwise to a
stirred mixture of 1-methylindole-3-acetic acid (6) (0.95 g, 5 mmol) and tri-
ethylamine (1.0 g, 10 mmol) in dichloromethane (25 mL). After 8 h of
stirring at rt, the solvent was evaporated in vacuo. The residue was purified
by column chromatography (99:1 dichloromethane:methanol) to give 1.2 g
1
(61%) of 8 as a red solid; m.p. 203–2058C; H (acetone-d6) d 7.94 (d, 2H,
J ¼ 13.2 Hz), 7.62 (d, 1H, J ¼ 8.4 Hz), 7.43 (d, 1H, J ¼ 8.4 Hz), 7.15–7.04
(m, 3H), 6.82–6.70 (m, 3H), 4.71 (t, 2H, J ¼ 6.6 Hz), 3.95 (s, 3H), 3.08
(t, 2H, J ¼ 6.6 Hz); LRMS (EI): m/z 395 (Mþ), 365, 340, 323, 259, 205,
183.1, 171, 155, 143, 131 (100%), 103, 77; HRMS (EI): Calcd. for
C24H17N3O3: 395.1270. Found: 395.1268; Anal. Calcd. for C24H17N3O3: C,
72.90; H, 4.33; N, 10.63. Found: C, 72.77; H, 4.25; N, 10.62.
2-(1-Cyanoethyl-3-indolyl)-3-(1-methyl-3-indolyl)maleimide (9)
A magnetically stirred solution of anhydride 8 (0.4 g, 1 mmol) in DMF (5 mL)
was added a 1,1,1,3,3,3-hexamethyldisilazane (1.61 g, 10 mmol) followed by
methanol (0.16 g, 5 mmol). The completely closed solution was stirred at rt
for 18 h. Then the mixture was poured into water and extracted thrice with
ethyl acetate. The combined organic extracts were washed with water. The
separated organic layer was dried over magnesium sulfate and the solvent
was removed in vacuo to give 0.37 g (95%) of 9 as a red solid: m.p. 249–
2518C; 1H (DMSO-d6) 10.96 (brs, 1H), 7.86 (d, 2H, J ¼ 17.6 Hz), 7.57
(d, 1H, J ¼ 8.1 Hz), 7.40 (d, 1H, J ¼ 8.1 Hz), 7.07–6.99 (m, 2H), 6.88
(d, 1H, J ¼ 8.1 Hz), 6.72–6.58 (m, 3H), 4.57 (t, 2H, J ¼ 6.4 Hz), 3.37
(s, 3H), 3.00 (t, 2H, J ¼ 6.4 Hz); LRMS (EI): m/z 394 (Mþ, 100%), 354,
339, 311, 283, 268, 177, 142, 121; HRMS (EI): Calcd. for C24H18N4O2:
394.1430. Found: 394.1429; Anal. Calcd. for C24H18N4O2: C, 73.08; H,
4.60; N, 14.20. Found: C, 72.99; H, 4.67; N, 14.16.
ICP-103 (3)
A mixture of maleimide 9 (39.4 mg, 0.1 mmol) and Pd(OTf)2 (166 mg,
0.5 mmol) in DMF (15 mL) was heated at 908C for 2 h. The reaction
mixture was then cooled, diluted with ethyl acetate and washed with 0.5N

600
S. Roy, A. Eastman, and G. W. Gribble
HCl. The organic phase was dried over sodium sulfate and filtered through
Hyflo. The solvent was removed in vacuo and the residue was purified by
column chromatography (97:3 dichloromethane:methanol) to give 17.7 mg
1
(45%) of 3 as a fluorescent yellow solid; m.p. 291–2938C; H (DMSO-d6)
d 11.17 (brs, 1H), 9.17–9.10 (m, 2H), 7.99–7.96 (m, 1H), 7.83–7.81
(m, 1H), 7.71–7.67 (m, 2H), 7.49–7.44 (m, 2H), 5.16–5.12 (t, 2H,
J ¼ 6.5 Hz), 4.23 (s, 3H), 2.83–2.79 (t, 2H, J ¼ 6.6 Hz); LRMS (EI): m/z
392 (Mþ), 352, 339, 53 (100%); HRMS (EI): Calcd. for C24H16N4O2:
392.1273. Found: 392.1274. Anal. Calcd. for C24H16N4O2.1/3H2O: C,
72.35; H, 4.22; N 14.06. Found: C, 72.49; H, 4.13; N, 14.21.
ACKNOWLEDGMENTS
This investigation was supported in part by the donors of the Petroleum
Research Fund (PRF), administered by the American Chemical Society, the
National Institutes of Health (CA 82220), and Wyeth.
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Synthesis of 7-Keto-Go6976
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