New synthesis of KT5823 indolocarbazole aglycone

Article abstract of DOI:10.3987/com-00-8992

The indolo[2,3-α]pyrrolo[3,4-c]carbazole aglycone of the selective protein kinase G inhibitor KT5823 has been synthesised in four steps and 36percent overall yield from 2,3-dichloro-N-methylmaleimide by utilising an efficient new two step reduction sequen

Full text of DOI:10.3987/com-00-8992

HETEROCYCLES, Vol. 53, No10, 2000, pp. 2119 - 2122, Received, 30th June, 2000
NEW SYNTHESIS OF KT5823 INDOLOCARBAZOLE AGLYCONE
Guillaume E. Burtin,* David J. Madge* and David L. Selwood
The Wolfson Institute for Biomedical Research, Biological and Medicinal Chemistry,
University College London, Cruciform Building, Gower St., London WC1E 6AU, UK.
Abstract - The indolo[2,3-a]pyrrolo[3,4-c]carbazole aglycone of the selective protein
kinase G inhibitor KT5823 has been synthesised in four steps and 36% overall yield from
2,3-dichloro-N-methylmaleimide by utilising an efficient new two step reduction
sequence for the key imide to amide transformation.
In 1977 the first indolocarbazole alkaloid, staurosporine (1) was isolated from Streptomyces
staurosporeus.1 Since then, the number of isolated and synthesised indolocarbazole glycosides has grown
considerably due to their wide range of biological properties, from antifungal, antimicrobial, and
antitumor through to antihypertensive effects.2 Synthetic chemists have concentrated their efforts on the
synthesis of the naturally occurring indolocarbazole K252c (2) and several strategies have been reported.3
H
N
R
N
H
N
Me
N
O
O
O
O
O
N
N
H
N
H
N
N
N
H
N
H
N
O
O
Me
H
Me
H
MeO
MeO CO2Me
NHMe
(1) : staurosporine
(2) : K252c (R = H)
(3) : arcyriaflavin A
(4) : KT5823
(5) : KT5823 aglycone (R = Me)
For many years, the most widely used approaches to the aglycone (2) had involved the imide arcyriaflavin
4
A (3) as a key intermediate, although its reduction to the lactam (2) proved to be difficult. Several
groups have recently developed new synthetic pathways to give direct access to analogues of 2.5

As part of a program investigating the biological activity of (±)-KT5823 (4) towards the inhibition of
protein kinase G,6 we report here an efficient and high yielding synthesis of the KT5823 indolo[2,3-
a]pyrrolo[3,4-c]carbazole aglycone (5). Only one synthesis of lactam (5) has been described to date, with
no experimental detail and in poor yield.7 The strategy involving the bis-indolylmaleimide (7) was
considered as the most convenient route to prepare large quantities of aglycone (5) (Scheme 1).8
Treatment of 2,3-dichloro-N-methylmaleimide9 (6) with indole in presence of ethylmagnesium bromide
afforded intermediate (7).10 In our hands, the utilisation of 3 equivalents of indole-MgBr gave higher
yields. Oxidative cyclisation of the bis-indolylmaleimide (7) with a large excess of palladium chloride (5
eq.) in dimethylformamide provided product (8) in good yield.11 The use of a 2:1 mixture of toluene and
dimethylformamide allowed the reaction to go to completion with only 3.5 equivalents of expensive
PdCl2. Cyclisation using palladium acetate in acetic acid12 gave poor results and CuCl2 in methyl ethyl
ketone13 failed to provide the imide (8). The product obtained with DDQ/TsOH14 proved to be difficult to
purify.
Me
N
Me
N
O
O
O
O
Me
N
a
b
O
O
N
H
N
H
N
H
N
H
Cl
Cl
N
H
(6)
(7)
(8)
Scheme 1. (a) EtMgBr, toluene/THF, 90oC; 80%; (b) PdCl2, toluene/DMF,105oC, 90%.
Final reduction of imide (8) to lactam (5) required more attention. Although Clemmensen type reduction
of arcyriaflavin A (3) has been carried out by several groups to prepare K252c (2),4,15 treatment of (8)
with acid activated zinc16 in place of toxic zinc-amalgam gave the lactam (5) in poor yield. The use of
BH3.THF complex gave a mixture of reduced compounds.17 The two step reduction sequence developed
by Hill et al.12 and used by Somei et al.7 in the synthesis of aglycone (2) gave rise to the unexpected
ethoxyindolocarbazole (11)18 (Scheme 2). We assume that compound 11 results from the addition of
ethanol to the iminium species (10).
Formation of 10, which presumably occurs by elimination of the hydroxyl from intermediate (9) due to
the participation of indolic nitrogen, was also observed during attempts to purify crude 9 on silica gel

using a methanol-chloroform gradient. In this case partial elimination, followed by addition of methanol
to 10, led to a mixture of the alcohol (9) and the methoxy analogue of 11. Although hydrogenolysis of
compound 9 in ethyl acetate failed to give 5, the use of ethanol-acetic acid afforded a separable mixture
of 5 and 11.
Me
N
Me
N
Me
N
..
ROH
HO
O
O
EtO
O
a
b
(8)
N
N
N
N
H
N
N
+
H
H
H
H
H
(9)
(10)
(11)
Scheme 2. (a) NaBH4, DMF/MeOH; (b) 10% Pd/C, H2, EtOH, rt, 42% two steps
Efficient reduction of imide (8) was finally achieved using the two step sequence shown in Scheme 3.
Reduction of 8 with an excess of lithium aluminium hydride at room temperature yielded hydroxy
derivative (9). Treatment of crude hydroxy lactam (9) with a mixture of trifluoroacetic acid and
triethylsilane19,15b at room temperature led to the KT5823 aglycone (5)18 in 50% yield from 8. Utilisation
of toxic phenylselenol as a reducing agent of the hydroxy lactam has been avoided.3
Me
N
Me
N
Me
N
HO
O
O
O
O
a
b
N
N
N
N
N
N
H
H
H
H
H
H
(8)
(9)
(5)
Scheme 3. (a) LiAlH4, THF, rt; (b) Et3SiH, TFA, chloroform, rt, 50% two steps.
In summary, we have developed a new two step reduction sequence of indolopyrrolocarbazole giving
access to the KT5823 aglycone (5) in four steps and 36% overall yield from N-methyl dichloromaleimide.
ACKNOWLEDGMENTS
This work was supported in part by United Therapeutics Corp. (Silver Spring, USA).

REFERENCES
*
Corresponding author. Tel: (0044) 207 6796712; Fax: (0044) 207 6796799; e-mail: d.madge@ucl.ac.uk
1. (a) S. Omura, Y. Iwai, A. Hirano, A. Nakagawa, J. Awaya, H. Tsuchiya, Y. Takahashi, and R. Masuma, J.
Antibiot., 1977, 30, 275. (b) A. Furusaki, N. Hashiba, T. Matsumoto, A. Hirano, Y. Iwai, and S. Omura, J.
Chem. Soc., Chem. Commun., 1978, 800.
2. For a review see G. W. Gribble and S. J. Berthel, ‘Studies in Natural Products Chemistry’, Vol. 12, ed. by
Atta-ur-Rahman, Elsevier Science Publisher, New York, 1993, pp. 365-409.
3. For a short survey of the different strategies see: J. T. Link, S. Raghavan, M. Gallant, S. J. Danishefsky, T. C.
Chou, and L. M. Ballas, J. Am. Chem. Soc., 1996, 118, 2825.
4. G. Xie and J. W. Lown, Tetrahedron Lett., 1994, 35, 5555.
5. (a) S. Mahboobi, E. Eibler, M. Koller, S. Sumar, and A. Popp, J. Org Chem., 1999, 64, 4697; (b) Y.
Kobayashi, T. Fujimoto, and T. Fukuyama, J. Am. Chem. Soc., 1999, 121, 6501; (c) S. Eils and E.
Winterfeldt, Synthesis, 1999, 275; (d) J. L. Wood, D. T. Petsch, B. M. Stoltz, E. M. Hawkins, D. Elbaum, and
D. R. Stover, Synthesis, 1999, 1529.
6. H. Kase, K. Iwahashi, S. Nakanishi, Y. Matsuda, K. Yamada, M. Takahashi, C. Murakata, A. Sato, and M.
Kaneko, Biochem. Biophys. Res. Commun., 1987, 142, 436.
7. M. Somei, H. Hayashi, T. Izumi, and S. Ohmoto, Heterocycles, 1995, 41, 2161.
8. M. Brenner, H. Rexhausen, B. Steffan, and W. Steglich, Tetrahedron, 1988, 44, 2887.
9. 6 was prepared in 60% yield using an adaptation of H. M. Relles, J. Org Chem., 1972, 37, 3630.
10. M. M. Faul, K. A. Sullivan, and L. L. Winneroski, Synthesis, 1995, 1511.
11. M. Ohkubo, T. Nishimura, H. Jona, T. Honma, and H. Morishima, Tetrahedron, 1996, 52, 8099.
12. W. Harris, C. H. Hill, E. Keech, and P. Malsher, Tetrahedron Lett., 1993, 34, 8361.
13. (a) M. Ohkubo, T. Nishimura, H. Jona, T. Honma, S. Ito, and H. Morishima, Tetrahedron, 1997, 53, 5937; (b)
M. Ohkubo, T. Nishimura, T. Honma, I. Nishimura, S. Ito, T. Yoshinari, H. Arakawa, H. Suda, H. Morishima,
and S. Nishimura, Bioorg. Med. Chem. Lett., 1999, 9, 3307.
14. R. P. Joyce, J. A. Gainor, and S. M. Weinreb, J. Org Chem., 1987, 52, 1177.
15. (a) D. Toullec, P. Pianetti, H. Coste, P. Bellevergue, T. Grand-Perret, M. Ajakane, V. Baudet, P. Boissin, E.
Boursier, F. Loriolle, L. Duhamel, D. Charon, and J. Kirilovsky, J. Biol. Chem., 1991, 266, 15771; (b) I.
Hughes, W. P. Nolan, and R. A. Raphael, J. Chem. Soc., Perkin Trans I, 1990, 2475.
16. S. Yamamura, M. Toda ,and Y. Hirata, Org. Synth., Coll. Vol., 1988, 6, 289.
17. S. W. McCombie, R. W. Bishop, D. Carr, E. Dobek, M. P. Kirkup, P. Kirschmeier, S.-I. Lin, J. Petrin, K.
Rosinski, B. B. Shankar, and O. Wilson, Bioorg. Med. Chem. Lett., 1993, 3, 1537.
18. All new compounds gave satisfactory analytical and spectral data. Compound (11): 1H NMR (DMSO-d6, 400
MHz) δ 11.53 (NH), 11.39 (NH), 9.17 (d, J = 7.9 Hz, 1H), 8.31 (d, J = 7.3 Hz, 1H), 7.77 (d, J = 8.1 Hz, 1H),
7.74 (d, J = 8.1 Hz, 1H), 7.46 (q, J = 7.8 Hz, 2H), 7.30 (t, J = 7.5 Hz, 1H), 7.25 (t, J = 7.7 Hz, 1H), 6.62 (s,
1H, CHOEt), 3.19-3.11 (m, 1H, CH2CH3), 3.11 (s, 3H, Me), 2.86-2.82 (m, 1H, CH2CH3), 0.97 (t, J = 7.0 Hz,
3H, CH2CH3); 13C NMR (DMSO-d6, 100 MHz) δ 168.6 (C=O), 139.6, 139.4, 129.0, 127.9, 126.4, 125.4,
124.8, 122.4, 122.0, 119.9, 119.2, 118.9, 115.2, 114.8, 111.8, 111.5, 87.3 (CHOEt), 56.4 (CH2CH3), 26.4
(Me), 15.1 (CH2CH3); Anal. Calcd for C23H20N3O2: C, 74.56; H, 5.45; N, 11.35. Found: C, 74.37; H, 5.68; N,
1
11.07. Compound 5: H NMR (DMSO-d6, 400 MHz) δ 11.46 (NH), 11.30 (NH), 9.23 (d, J = 8.0 Hz, 1H),
8.02 (d, J = 7.8 Hz, 1H), 7.78 (d, J = 8.3 Hz, 1H), 7.72 (d, J = 8.1 Hz, 1H), 7.47 (t, J = 7.7 Hz, 1H), 7.42 (t, J
= 7.7 Hz, 1H), 7.30 (t, J = 7.5 Hz, 1H), 7.22 (t, J = 7.4 Hz, 1H), 5.02 (s, 2H, CH2), 3.25 (s, 3H, Me); 13C NMR
(DMSO-d6, 100 MHz) δ 169.5 (C=O), 139.3, 139.1, 130.0, 127.6, 125.4, 125.0, 122.8, 122.4, 120.9, 119.8,
118.9, 118.7, 115.4, 113.9, 111.9, 111.3, 51.6 (CH2), 29.2 (Me); MS (APCI+) m/z: 326 (M+H); Anal. Calcd
for C21H15N3O.0.7H2O: C, 74.62; H, 4.90; N, 12.43. Found: C, 74.82; H, 4.62; N, 12.22.
19. D. Dube and A. A. Scolte, Tetrahedron Lett., 1999, 40, 2295 and references therein.