New method for synthesis of EZH2 methyltransferase inhibitor GSK126

Article abstract of DOI:10.1080/00397911.2016.1193875

GSK126 is a potent small-molecule inhibitor of S-adenosyl-methionine-competitive EZH2 methyltransferase and has the potential to be used clinically for preventing unwanted histone methylation of tumor suppressor genes. In this article, we describe a new s

Full text of DOI:10.1080/00397911.2016.1193875

Synthetic Communications
An International Journal for Rapid Communication of Synthetic Organic
Chemistry
ISSN: 0039-7911 (Print) 1532-2432 (Online) Journal homepage: http://www.tandfonline.com/loi/lsyc20
New Method for Synthesis of EZH2
Methyltransferase Inhibitor GSK126
Chen Lu, Qiang Zhang & Xin Chen
To cite this article: Chen Lu, Qiang Zhang & Xin Chen (2016): New Method for Synthesis
of EZH2 Methyltransferase Inhibitor GSK126, Synthetic Communications, DOI:
10.1080/00397911.2016.1193875
To link to this article: http://dx.doi.org/10.1080/00397911.2016.1193875
View supplementary material
Accepted author version posted online: 02
Jun 2016.
Published online: 02 Jun 2016.
Submit your article to this journal
Article views: 5
View related articles
View Crossmark data
Full Terms & Conditions of access and use can be found at
http://www.tandfonline.com/action/journalInformation?journalCode=lsyc20
Download by: [University of Missouri-Columbia]
Date: 20 June 2016, At: 06:12

New Method for Synthesis of EZH2 Methyltransferase Inhibitor GSK126
Chen Lu1, Qiang Zhang1, Xin Chen1
1School of Pharmaceutical Engineering δ Life Sciences, Changzhou University,
Changzhou, Jiangsu, P. R. China
Corresponding Author: Xin Chen, School of Pharmaceutical Engineering δ Life Sciences,
Changzhou University, Changzhou, Jiangsu 213164, P. R. China. E-mail:
xinchen@cczu.edu.cn
Abstract
GSK126 is a potent small-molecule inhibitor of S-adenosyl-methionine-competitive
EZH2 methyltransferase, and has the potential to be used clinically for preventing
unwanted histone methylation of tumor suppressor genes. In this article, we describe a
new synthetic route that has been developed for synthesizing the title compound through
nine steps, starting from 2,5-dibromobenzoic acid. This synthetic method is economical
and suitable for multigram-scale preparation of GSK126 and related N-alkylated indole
derivatives.
Graphical Abstract:
1

KEYWORDS: GSK126, inhibitor, EZH2 methyltransferase, synthesis.
INTRODUCTION
EZH2 is a Polycomb group (PcG) protein homologous to Drosophila Enhancer of Zeste
and involved in gene silencing.[1] PcG proteins are key regulators of chromatin structure,
cell identity and development.[2] There are two main PcG complexes: Polycomb
repressive complex 1 (PRC1) and 2 (PRC2) in mammals. While both compact chromatin,
PRC1 catalyses the monoubiquitylation of histone H2A, whereas PRC2 is a highly
conserved histone methyltransferase that methylates lysine 27 of histone 3.[3] EZH2 is a
core catalytic component of PRC2, and its function is highly dependent upon its
recruitment by the PRC2 complex.[4] Abnormal EZH2 expression has been associated
with a wide range of cancers, including those of the skin [5], prostate [6], bladder [7] and
breast [8]. Therefore, EZH2 has become an attractive target for novel anti-cancer therapy.
The discovery of small molecule EZH2 inhibitors that prevent unwanted histone
methylation of tumor suppressor genes has raised the promising possibility of exploiting
the oncogenic addiction of tumor cells to EZH2.[9] So far several inhibitors of EZH2 have
been developed, including 3-deazaneplanocin A (DZNep)[10], GSK126 [11], EPZ005687
[12], EI1 [13], UNC1999 [14], and Sinefungin [15]. Among them, GSK126, developed by
GlaxoSmithKline, is a notable and promising drug candidate. GSK126 is a highly
selective, S-adenosyl-methionine-competitive, small-molecule inhibitor of EZH2
2

methyltransferase activity, and has 150-fold selectivity over EZH1 and a Ki of 0.5-3
nM.[11]
McCabe et al. reported the synthesis of GSK126 starting from 2-methyl-3-nitrobenzoic
acid.[11] The indole nucleus was constructed through the reductive amination/cyclization
procedure [16] by reacting a 2-methyl3-nitrobenzoate with dimethylformamide
dimethylacetal. They obtained a key chiral intermediate via chiral resolution, which is
prohibitively expensive and unsuitable for large-scale preparation of GSK126.
From the retrosynthesis analysis, GSK126 can be synthesized from halo-indole
carboxylic acid A through Suzuki-Miyaura cross-coupling with
1-(5-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-piperazine, and through amide
coupling with 3-(aminomethyl)-4,6-dimethyl-2(1H)-pyridinone (Scheme 1). The borate
ester is commercially available, and the pyridinone is readily obtained from reduction of
2-hydroxy-4,6-dimethylnicotinonitrile. The chirality in intermediate A might be
introduced by selective alkylation of indole B with (R)-isobutanol or the corresponding
active ester. 3-Methyl-1H-indole-4-carboxylic acid B can be achieved from the
Pd-catalyzed cyclization [17,18] of 1-allylamino-2-halobenzoate C, whereas the latter is
readily obtained via alkylation of the corresponding aniline with allyl bromide.
3

Our goal was to develop a general methodology for synthesizing GSK126 with high
chemical purity and high enantiomeric excess value in sufficient quantities to permit
laboratory bioassays and animal studies. Here we report convenient synthesis of the
target compound from commercially available and relatively inexpensive materials.
RESULTS AND DISCUSSION
By modifying the known procedure,[19] methyl 2,5-dibromo-3-nitrobenzoate[20] (3) was
obtained from 2,5-dibromobenzoic acid in two steps with about 40% overall yield
(Scheme 2). Then, in the presence of concentrated hydrochloric acid, the nitro group in 3
was reduced by iron powder, furnishing methyl 3-amino-2,5-dibromobenzoate (4) in 78%
yield. When glacial acetic acid was used to replace concentrated hydrochloric acid, the
yield was decreased to 38%. In the following step, alkylation of 4 with allyl bromide
gave methyl 3-(allylamino)-2,5-dibromobenzoate (5) in 75% yield.
By employing the known protocol for the palladium-catalyzed synthesis of indoles,[18] we
tried the intramolecular Heck reaction of 5 in the presence of PdCl2 and PCy3, but only a
trace amount of the desired product 6 was isolated. The existence of
electron-withdrawing carboxyl group likely affects the cyclization. Therefore, we chose
N-allyl-2,5-dibromoaniline (5a) as a substrate to optimize the intramolecular Heck
reaction conditions (catalysts and ligands), and the results are summarized in Table 1.
First, we tried the best condition reported in the literature (PdCl2(PCy3)2 as catalyst,
4

P(OPh)3 as ligand),[18] but 6-bromo-3-methyl-1H-indole (6a) was isolated in only 37%
yield (Table 1, entry 1). When the reaction was repeated without adding the ligand, the
yield was slightly increased to 43% (Table 1, entry 2), which indicates that a ligand is
probably unnecessary for the cyclization. Thus, other palladium catalysts have been tried
in absence of a ligand, such as PdCl2(PPh3)2 (Table 1, entry 3), PdCl2 (entry 4), Pd(OAc)2
(entry 5) and PdCO3 (entry 6). Among the screened palladium catalysts, Pd(OAc)2 gave
the highest yield (59%), whereas PdCO3 didn’t work for the cyclization at all. A
non-palladium catalyst, Cu(OAc)2 (entry 7), was also tested, but no desired product could
be isolated. As a result, we employed Pd(OAc)2 as the catalyst for the intramolecular
Heck reaction of 5, and smoothly obtained methyl
6-bromo-3-methyl-1H-indole-4-carboxylate (6) with 53% yield (Scheme 2).
With the optimized conditions for synthesizing the indole intermediate 6 in hand, we
turned our attention to the remaining steps in the synthetic sequence. The next key step is
introduction of a (S)-sec-butyl group at the N-1 position of 1H-indole 6. Our initial
attempts began with examination of the Mitsunobu reaction of 6 with (R)-isobutanol.
However, the standard Mitsunobu conditions (DIAD/PPh3)[21] failed to give any expected
condensation product. This result might be ascribed to the reduced basicity of 6 due to the
existence of two electron-withdrawing groups (carboxyl and bromo) in 6. More reactive
Mitsunobu-type reagents have been tried, such as
N,N,N’,N’-tetramethylazodicarboxamide/triphenylphosphine (TMAD/PPh3)[22] and
5

(cyanomethylene)trimethyl-phosphorane (CMMP)[23], but both gave only trace amounts
of the N-alkylation product. We converted (R)-isobutanol into the corresponding tosylate
and focused on the selective N-alkylation of 1H-indole 6 under basic conditions. After
testing different bases (such as NaH, KOH, NaOH and Cs2CO3) and solvents (such as
DMF, DMSO and THF), KOH/DMSO are found to be more effective for N-alkylation at
room temperature, resulting in (S)-methyl
6-bromo-1-(sec-butyl)-3-methyl-1H-indole-4-carboxylate (7) with 92% yield (Scheme 3).
To our delight, racemization didn’t occur during the alkylation under the basic condition,
as 96.5% ee was measured for 7 by a chiral HPLC analysis (Supplementary information).
In order to further confirm the selectivity of the N-alkylation, (R)-methyl
6-bromo-1-(sec-butyl)-3-methyl-1H-indole-4-carboxylate (7a), the R-enantiomer of 7,
has been prepared under identical reaction conditions, except using the (S)-tosylate
(Scheme 4), and 88% yield and 96.7% ee were obtained.
In the next phase of GSK126 synthesis, methyl 1H-indole-4-carboxylate 7 was
hydrolyzed by NaOH to give the corresponding carboxylic acid 8 in 93% yield, and then
the resulting acid was condensed with 3-(aminomethyl)-4,6-dimethyl-2(1H)-pyridinone
under the standard amide coupling conditions (EDC, HOAt, NMM) to generate
indole-4-carboxamide (9) with 82% yield. In the final step, the target molecule was
generated from Suzuki-Miyaura cross-coupling of 9 with
1-(5-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)- piperazine.[11] The identity of the final
6

product and the key intermediates was confirmed by the 1H NMR, 13C NMR spectra and
mass spectrometry. The synthetic route described is concise and has been adopted to
synthesize multi-gram quantities of GSK126 with good purity.
In summary, the conditions for the Pd-catalyzed intramolecular Heck reaction of
N-allyl-2,5-dibromoaniline have been optimized, and N-alkylation of the resulting
1H-indole with (R)-sec-butyl 4-methylbenzenesulfonate under basic conditions has
shown excellent selectivity. Based on these results, a new synthetic route has been
developed for synthesizing the EZH2 methyltransferase inhibitor GSK126. The described
synthetic method is economical and suitable for relatively large-scale preparation of the
title compound and the related N-alkylated indole derivatives.
EXPERIMENTAL
Methyl 6-Bromo-3-Methyl-1H-Indole-4-Carboxylate (6)
To a stirred solution of 5 (4.3g, 12.3 mmol) in DMF (50 mL) was added Pd(OAc)2
(276mg, 1.2 mmol, 0.1 equiv), KOAc (1.8g, 18.5 mmol, 1.5 equiv) and TBAB (596mg,
1.9 mmol, 0.15 equiv) under a stream of argon. After the mixture was heated at 100°C
with stirring for 1 h, the solvent was removed in vacuo, and the residue was then
extracted with DCM (3×50 mL), washed with H2O (2×30 mL) and brine (30 mL), and
dried (MgSO4). The crude product was purified by flash column chromatography (eluting
with hexanes/EtOAc, 20:1) to get indole 6 (1.72g, 53% yield) as a solid, m.p. 72-75゜C.
7

1H NMR (CDCl3, 300 MHz): δ2.36 (s，3H), 3.95 (s, 3H), 7.02 (s, 1H), 7.54 (d, J=1.1 Hz,
1H), 7.71 (d, J=1.1 Hz, 1H), 8.33 (s, 1H); 13C NMR (CDCl3, 75 MHz): δ13.2, 52.2, 112.6,
113.8, 117.8, 124.4, 125.8, 138.5, 167.8; HRMS (ESI): m/z calcd. for C11H10BrNO2Na
[M+Na]+ 289.9793, found 289.9785.
(S)-Methyl 6-Bromo-1-(Sec-Butyl)-3-Methyl-1H-Indole-4-Carboxylate (7)
TsCl (2.2g, 11.5 mmol) was added to a solution of (R)-isobutanol (775mg, 10.5 mmol),
Et3N (1.6g, 15.8 mmol) and DCM (30 mL) at 0°C, and the mixture was stirred at the
same temperature for 4h. The organic layer was separated, and the aqueous layer was
extracted with DCM (2×50mL). The combined organic layers were washed with H2O
(2×20 mL) and brine (30 mL), and dried (MgSO4). The solvent was removed, and the
residue was purified by flash column chromatography (eluting with hexane/EtOAc, 50:1)
to obtain (R)-sec-butyl 4-methylbenzenesulfonate (1.46g).
The tosylate obtained above (1.46g, 6.4 mmol) was dissolved in DMSO (11 mL), and
indole 6 (1.14g, 4.3 mmol) and KOH (2.17g, 38.7 mmol) was added, and the mixture was
stirred at rt for 6h. After completion of the reaction, 3N HCl (20 mL) was added to the
reaction mixture to adjust the pH to 3-4. The resulting mixture was extracted with EtOAc
(2×50 mL), and the combined organic layers were washed with H2O (2×20mL) and brine
(30 mL), and dried (MgSO4). The crude product was purified by flash column
chromatography (eluting with hexane/EtOAc, 20:1) to obtain 7 (1.28g, 92% yield) as
8

white powder; m.p. 36-37゜C; 96.5% ee, [δagr; ]25D +1.8° (c=0.1, CH2Cl2). 1H NMR
(CDCl3, 400 MHz): δ0.82 (t, J=7.4 Hz, 3H), 1.47 (d, J=6.4 Hz, 3H), 1.86 (m, 2H), 2.36 (s,
3H), 3.95 (s, 3H), 4.26-4.32 (m, 1H), 7.04 (s, 1H), 7.61 (d, J=1.2 Hz, 1H), 7.67 (d, J=2.0
Hz, 1H); 13C NMR (CDCl3, 100 MHz): δ10.9, 13.2, 21.0, 30.0, 52.1, 53.1, 111.4, 113.4,
115.8, 124.4, 124.5, 125.3, 138.6, 167.8; HRMS (ESI): m/z calcd. for C15H18BrNO2
[M-H]+ 324.0594, found 324.0601.
(S)-1-(Sec-Butyl)-N-((4,6-Dimethyl-2-Oxo-1,2-Dihydropyridin-3-Yl)Methyl)-3-Meth
yl-6-(6-(Piperazin-1-Yl)Pyridine-3-Yl)-1H-Indole-4-Carboxamine (GSK126)
To a 30 mL flask were added 9 (697mg, 1.57 mmol),
1-(5-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl), piperazine (590mg, 2.04 mmol, 1.3
equiv), 1,4-dioxane (29 mL), H2O (7mL) and K3PO4 (1g, 4.7 mmol, 3 equiv), and the
mixture was degassed with the aid of dry nitrogen three times. PdCl2(dpf)-CH2Cl2 adduct
(103mg, 0.13 mmol, 0.08 equiv) was then introduced into the reaction flask under a
stream of nitrogen, and the whole reaction mixture was heated at 90°C for 24h. The
cooled reaction mixture was concentrated in vacuo, and the residue was taken up into
MeOH (26 mL) and filtered off via celite. The filtrate was evaporated to dryness in vacuo,
and the crude product was purified by flash column chromatography (eluting with
DCM/MeOH, 10:1) to afford the final product (720mg, 87% yield) as gray powder. m.p.
186-188゜C (decomposed); [δagr; ]25D -11° (c=0.1, MeOH) (Lit.11 [δagr; ]25D -13.3°
(c=0.1, MeOH)). 1H NMR (500 MHz, DMSO-d6): δ0.75 (m, 3H), 1.37-1.44 (m, 4H),
9

1.75-1.87 (m, 2H), 2.11 (s, 3H), 2.25 (m, 3H), 2.77-2.85 (m, 4H), 3.45 (m, 4H), 4.35 (d,
J=5.3 Hz, 2H), 4.56-4.68 (m, 1H)，5.87 (s,1H), 6.88 (d, J=8.8 Hz, 1H), 7.17 (d, J=1.5 Hz,
1H), 7.26 (s, 1H), 7.73 (d, J=1.3 Hz, 1H), 7.91 (dd, J=8.8 Hz, 1H), 8.16 (t, J=5.1 Hz, 1H),
8.50 (d, J=2.5 Hz, 1H); 13C NMR (125 MHz, DMSO-d6): δ11.6, 12.6, 19.1, 19.9, 21.7,
30.4, 35.9, 46.3, 46.9, 52.4, 107.6, 108.2, 108.5, 110.6, 116.9, 122.6, 123.8, 125.2, 126.6,
130.6, 131.5, 136.7, 138.6, 143.5, 146.4, 150.2, 159.2, 164.0, 169.6; HRMS (ESI): m/z
calcd. for C31H39N6O2 [M+H]+ 527.3129, found 527.3129.
SUPPORTING INFORMATION
Full experimental detail and 1H and 13C NMR spectra can be found via the
Supplementary Content section of this article’s web page.
ACKNOWLEDGMENT
We are grateful for financial support from the National Science Foundation of China
(grant 21272029), the δldquo;333 High-level Talent Training Programδrdquo; of Jiangsu
Province of China, and the Priority Academic Program Development (PAPD) of Jiangsu
Higher Education Institutions.
10

REFERENCES
1. Vire, E.; Brenner, C.; Deplus, R.; Blanchon, L.; Fraga, M.; Didelot, C.; Morey, L.;
Eynde, A. V.; Bernard, D.; Vanderwinden, J.-M.; Bollen, M.; Esteller, M.; Croce, L. D.;
Launoit, Y.; Fuks, F. The Polycomb group protein EZH2 directly controls DNA
methylation. Nature, 2006, 439, 871-874.
2. Ding, X.; Wang. X.; Sontag, S.; Qin, J.; Wanek, P.; Lin, Q.; Zenke. M. The polycomb
protein Ezh2 impacts on induced pluripotent stem cell generation. Stem Cells Dev. 2014,
23, 931-40.
3. Margueron, R.; Reinberg, D. The Polycomb complex PRC2 and its mark in life.
Nature, 2011, 469, 343-349.
4. Tan, J. Z.; Yan, Y.; Wang, X. X.; Jiang, Y.; Xu, H. E. EZH2: biology, disease, and
structure-based drug discovery. Acta Pharmacol. Sin. 2014, 35, 161-74.
5. Zingg, D.; Debbache, J.; Schaefer, S. M.; Tuncer, E.; Frommel, S. C.; Cheng, P.;
Arenas-Ramirez, N.; Haeusel, J.; Zhang, Y.; Bonalli, M.; McCabe, M. T.; Creasy, C. L.;
Levesque, M. P.; Boyman, O.; Santoro, R.; Shakhova, O.; Dummer, R.; Sommer, L. The
epigenetic modifier EZH2 controls melanoma growth and metastasis through silencing of
distinct tumour suppressors. Nat. Commun. 2015, 6, 6051.
6. Varambally, S.; Dhanasekaran, S. M.; Zhou, M.; Barrette, T. R.; Kumar-Sinha, C.;
Sanda, M. G.; Ghosh, D.; Pienta, K. J.; Sewalt, R. G.; Otte, A. P.; Rubin, M. A.;
Chinnaiyan, A. M. The polycomb group protein EZH2 is involved in progression of
prostate cancer. Nature, 2002, 419, 624-629.
11

7. Arisan, S.; Buyuktuncer, E. D.; Palavan-Unsal, N.; Caδscedil;kurlu, T.; Cakir, O. O.;
Ergenekon, E. Increased expression of EZH2, a polycomb group protein, in bladder
carcinoma. Urol. Int. 2005, 75, 252-257.
8. Yoo, K. H.; Hennighausen, L. EZH2 methyltransferase and H3K27 methylation in
breast cancer. Int. J. Biol. Sci. 2012, 8, 59-65.
9. Lund, K.; Adams, P. D.; Copland, M. EZH2 in normal and malignant hematopoiesis.
Leukemia, 2014, 28, 44-49.
10. Fujiwara, T.; Saitoh, H.; Inoue, A.; Kobayashi, M.; Okitsu, Y.; Katsuoka, Y.; Fukuhara,
N.; Onishi, Y.; Ishizawa, K.; Ichinohasama, R.; Harigae, H. 3-Deazaneplanocin A
(DZNep), an inhibitor of S-adenosylmethionine-dependent methyltransferase, promotes
erythroid differentiation. J. Biol. Chem., 2014, 289, 8121δndash;8134.
11. McCabe, M. T.; Ott, H. M.; Ganji, G.; Korenchuk, S.; Thompson, C.; Van Aller, G. S.;
Liu, Y.; Graves, A. P.; Della Pietra, A. III, Diaz, E.; LaFrance, L. V.; Mellinger, M.;
Duquenne, C.; Tian, X.; Kruger, R. G.; McHugh, C. F.; Brandt, M.; Miller, W. H.; Dhanak,
D.; Verma, S. K.; Tummino, P. J.; Creasy, C. L. EZH2 inhibition as a therapeutic strategy
for lymphoma with EZH2-activating mutations. Nature, 2012, 492, 108δndash;112.
12. Knutson, S. K.; Wigle, T. J.; Warholic, N. M.; Sneeringer, C. J.; Allain, C. J.; Klaus, C.
R.; Sacks, J. d.; Raimondi, A.; Majer, C. R.; Song, J.; Scott, M. F.; Jin, L.; Smith, J. J.;
Olhava, E. J.; Chesworth, R.; Moyer, M. P.; Richon, V. M.; Copeland, R. A.; Keilhack, H.;
Pollock, R. M.; Kuntz, K. W. A selective inhibitor of EZH2 blocks H3K27 methylation
and kills mutant lymphoma cells. Nat. Chem. Biol. 2012, 8, 890δndash;896.
12

13. Qi, W.; Chan, H.; Teng, L.; Li, L.; Chuai, S.; Zhang, R.; Zeng, J.; Li, M.; Fan, H.; Lin,
Y.; Gu, J.; Ardayfio, O.; Zhang, J. H.; Yan, X.; Fang, J.; Mi, Y.; Zhang, M.; Zhou, T.;
Feng, G.; Chen, Z.; Li, G.; Yang, T.; Zhao, K.; Liu, X.; Yu, Z.; Lu, C. X.; Atadja, P.; Li, E.
Selective inhibition of Ezh2 by a small molecule inhibitor blocks tumor cells proliferation.
Proc. Natl. Acad. Sci. U.S.A. 2012, 109, 21360δndash;21365.
14. Konze, K. D.; Ma, A.; Li, F.; Barsyte-Lovejoy, D.; Parton, T.; Macnevin, C. J.; Liu, F.;
Gao, C.; Huang, X. P.; Kuznetsova, E.; Rougie, M.; Jiang, A.; Pattenden, S. G.; Norris, J.
L. James, L. I.; Roth, B. L.; Brown, P. J.; Frye, S. V.; Arrowsmith, C. H.; Hahn, K. M.;
Wang, G. G.; Vedadi, M.; Jin, J. An orally bioavailable chemical probe of the Lysine
Methyltransferases EZH2 and EZH1. ACS Chem. Biol., 2013, 8, 1324δndash;1334.
15. Couture, J. F.; Hauk, G.; Thompson, M. J.; Blackburn, G. M.; Trievel, R. C. Catalytic
roles for carbon-oxygen hydrogen bonding in SET domain lysine methyltransferases. J.
Biol. Chem. 2006, 281, 19280δndash;19287.
16. Faul, M. M.; Grutsch, J. L.; Kobierski, M. E.; Kopach, M. E.; Krumrich, C. A.;
Staszak, M. A.; Udodong, U.; Vicenzi, J. T.; Sullivan, K. A. Strategies for the synthesis
of N-(azacycloalkyl)bisindolylmaleimides: selective inhibitors of PKCδbgr;. Tetrahedron,
2003, 59, 7215-7229.
17. Hegedus, L. S.; Mulhern, T. A.; Mori, A. Palladium(0)-catalyzed synthesis of
indoloquinones. J. Org. Chem. 1985, 50, 4282-4288.
13

18. Yang, H.; Sun, P.; Zhu, Y.; Yan, H.; Lu, L.; Liu, D.; Rong, G.; Mao, J..
Palladium-catalyzed synthesis of indoles via intramolecular Heck reaction. Catalysis
Commun., 2013, 38, 21-25.
19. Baran, P. S.; Guerrero, C. A.; Corey, E. J. Short, enantioselective total synthesis of
Okaramine N. J. Am. Chem. Soc., 2003, 125, 5628-5629.
20. Wang, P.; Pan, Q.; Li, Y.; Gou, J. Intermediate of candesartan cilexetil and the
preparation method thereof. Faming Zhuanli Shengqing Gongkai Shuomingshu, CN
101648918, 2010; Chem. Abstr, 2010, 152, 358039.
21. Mitsunobu, O. Synthesis, 1981, 1-28.
22. Bombrun, A.; Casi, G. N-Alkylation of 1H-indoles and 9H-carbazoles with alcohols.
Tetrahedron Lett. 2002, 43, 2187-2190.
23. Sakamoto, I.; Kaku, H.; Tsunoda, T. Preparation of (cyanomethylene)-
trimethylphosphorane as new Mitsunobu-type reagent. Chem. Pharm. Bull. 2003, 51,
474-476.
14

Table 1. Cyclization of N-allyl-2,5-dibromoaniline (5a) under the intramolecular Heck
conditionsa
Entry
Catalystb
Ligand
Time
12h
12h
2h
Yield of indole 6a
1
2
3
4
5
6
7
PdCl2(PCy3)2
PdCl2(PCy3)2
PdCl2(PPh3)2
PdCl2
P(OPh)3
37%
43%
31%
51%
59%
NRc
NR
-
-
-
-
-
-
2h
Pd(OAc)2
PdCO3
2h
2h
Cu(OAc)2
2h
a Tetrabutyl ammonium bromide (TBAB) was used as promoter, KOAc was used as base,
DMF was used as solvent. All the reactions were performed at 100℃.
b Amount of catalyst used: 10 mol%.
c NR: no reaction.
15

Scheme 1. GSK126 and its retrosynthesis analysis
16

Scheme 2. Preparation of methyl 6-bromo-3-methyl-1H-indole-4-carboxylate (6).
Reagents and conditions: (a) HNO3, H2SO4, 100℃, 5h, 41%; (b) SOCl2, MeOH, reflux,
2h, 92%; (c) Fe, EtOH, conc. HCl, reflux, 8h, 78%; (d) Allyl bromide, K2CO3, DMF,
60℃, 24h, 75%; (e) TBAB, Pd(OAc)2, NaOAc, DMF, 100℃, 1h, 53%.
17

Scheme 3. New synthetic route to GSK126. Reagents and conditions: (a) KOH, DMSO,
rt, 6h, 92%; (b) NaOH, THF, MeOH, rt, 24h, 93%; (c)
3-(aminomethyl)-4,6-dimethyl-2(1H)-pyridinone hydrochloride, EDC, HOAt, NMM,
DMSO, rt, 32h, 82%; (d) 1-(5-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl) piperazine,
Pd(dppf), K3PO4, dioxane, H2O, 90℃, 24h, 87%.
18

Scheme 4. Preparation of (R)-methyl 6-bromo-1-(sec-butyl)-3-methyl-1H-
indole-4-carboxylate (7a)
19