An improved synthesis of 5-(2,6-dichlorophenyl)-2-(phenylthio)-6H- pyrimido[1,6-b]pyridazin-6-one (a VX-745 analog)

Article abstract of DOI:10.3390/61200959

An improved procedure for the synthesis an analog of the p38 inhibitor compound VX-745 is reported.

Full text of DOI:10.3390/61200959

Molecules 2001, 6, 959-963
molecules
ISSN 1420-3049
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An Improved Synthesis of 5-(2,6-Dichlorophenyl)-2-(phenyl-
thio)-6H-pyrimido[1,6-b]pyridazin-6-one (a VX-745 analog)†
1
Matthias Treu , Ulrich Jordis*,1 and Ving J. Lee2
1 Institute of Organic Chemistry, VUT, A-1060 Vienna, Getreidemarkt 9, Austria
2 Iconix Pharmaceuticals, Inc., 830 Maude Avenue, Mountain View, CA 94043
†
This paper was presented at the Fifth Electronic Conference on Synthetic Organic Chemistry (ECSOC-5),
September 1-30, 2001 (Paper A0039).
* Author to whom correspondence should be addressed; email ujordis@pop.tuwien.ac.at; Tel. +43 (1)
58801.15460, Fax +43(1)58801.15499
Received: 22 October 2001; in revised form 30 October 2001 / Accepted: 30 October 2001 / Published:
30 November 2001
Abstract: An improved procedure for the synthesis an analog of the p38 inhibitor compound VX-
745 is reported.
Keywords: nucleophilic substitution, DMF-acetal heterocyclization.
Introduction
Mitogen-activated protein (MAP) kinases are key enzymes involved in signal transduction and the
amplification of cellular responses to stumuli. P38 MAP kinase is a specific member of the MAP kinase family
associated with the onset and progression of inflammation, and several groups are working to develop p38
MAP kinase inhibitors as potential treatments for inflammatory and neurological disease. Vertex discovered
the 3D structure of p38MAP kinase in 1996 and computer modeling and testing suggested the design of VX-
475 [1].

Molecules 2001, 6
960
For comparison purposes we needed a sample of a VX-745 analog and followed the synthesis as
described in a general procedure for the preparation of p38 inhibitor compounds by Bemis and coworkers
[2,3]. As we had difficulties in duplicating some of the experiments and only limited physical data were
mentioned by the authors, we herein report our experience with and improvements of the synthesis of this VX-
745 analog.
Results and Discussion
The use of NaH, instead of NaNH2 [2,3], for the deprotonation of dichlorophenylacetonitrile 1 and
aqueous workup using satd. NH4Cl (instead of satd. Na2CO3) afforded 2 as a shiny red crystalline product
without chromatographic purification. The yield could be improved to 43% on a 9 g scale, whereas Bemis
[2,3] reported a yield of 28% without comments on the physical state. HPLC and NMR analysis (no benzylic
protons could be detected) revealed the formation of a mixture of tautomers of 2. Since 2 obtained from the
modified procedure was quite insoluble in the solvents given by Bemis, DMF was used as solvent for this step.
A modified aqueous workup and direct recrystallization of the crude product from EtOH allowed us to avoid
column chromatography and gave the desired product 3 as orange crystals in a yield of 72%. The absence of
signals of the benzylic proton and the complex peak pattern proved again the formation of at least two
tautomeric species. Bemis provides no information on the physical state of 4; while in the general procedure
the compounds obtained are described as oils.
Scheme 1.
CN
1. NaH
2. dichloropyridazine
CN
Cl
CN
Cl
H
N
Cl
Cl
N
N
N
43%
Cl
Cl
Cl
Cl
1. NaH
1
2
2. PhSH
72%
S
N
N
S
N
N
N
X
O
Cl
Cl
Cl
Cl
5
dimethyl formamide
dimethyl acetal
41%
3 X = CN
H2SO4
64%
4 X = CONH2
Partial hydrolysis of the nitrile moity of 3 was performed using shorter reaction times than described in the
literature and a modified workup, (no neutralization of the H SO4) to afford 4 as bright orange crystals. Bemis
2

Molecules 2001, 6
961
and coworkers again report no details for the final conversion of 4 to 5. When pure 4 was reacted with
dimethylformamide dimethyl acetal the VX-745 analog 5 was formed as a yellow precipitate, which
crystallized from AcOH/water with a yield of 41% (see Scheme 1).
Conclusions
We have prepared the VX-745 analog 5 in a 4-step sequence and increased the overall yield from 3.5%
to 16%. Spectral data, HPLC- and CHN-analysis proved identity as well as high purity (> 99%).
Experimental
General
1
Melting points were measured on a Kofler hot stage apparatus. H-NMR-spectra were recorded on a
Bruker AC-200 (200 MHz) pulse Fourier-transform NMR spectrometer in CDCl , DMSO-d6 or TFA-d1
3
using tetramethylsilane as an internal standard. Thin layer chromatography (TLC) was performed on precoated
plates (Merck TLC aluminum sheets silica 60 F ) with detection by UV light or with phosphomolybdic acid
254
in aqueous EtOH by heating. All reactions were magnetically stirred under an argon atmosphere.
6-Chloro-a-(2,6-dichlorophenyl)pyridazine-3-acetonitrile (2).
Sodium hydride (4.06 g, 84.5 mmol, 55% dispersion in mineral oil) was washed with anhydrous light
petroleum (3 x 35 mL) and then suspended in anhydrous THF (30 mL). 2,6-Dichlorophenyl-acetonitrile (1)
(13.1 g, 70.4 mmol) in anhydrous THF (50 mL) was added at RT and the resulting mixture stirred for 30 min.
at this temperature. 3,6-Dichloropyridazine in anhydrous THF (50 mL) was then added at RT and stirring
continued for another 30 min. Saturated NH4Cl solution (350 mL) was added and the layers were separated.
The aqueous layer was extracted with EtOAc (3 x 75 mL), the combined organic layers were washed with
water (2 x 400 mL) and brine (1 x 400 mL), dried (Na2SO4), filtered and concentrated in vacuo. The residue
was dissolved in anhydrous EtOH (150 mL) and crystallized at -20 °C for 48 h. Yield: red crystals (8.70 g,
43%, lit. [2,3] 28%); mp. 124-31 °C.
a-(2,6-Dichlorophenyl)-6-phenylthiopyridazine-3-acetonitrile (3).
Sodium hydride (1.23 g, 28.1 mmol, 55% dispersion in mineral oil) was washed with anhydrous light
petroleum (3 x 25 mL) and suspended in anhydrous DMF (10 mL). Thiophenol (3.10 g, 28.1 mmol) in
anhydrous DMF (10 mL) was added at RT and stirred for 10 min. 6-Chloro-a -(2,6-dichloro-
phenyl)pyridazine-3-acetonitrile (2) (8.00 g, 26.8 mmol) in anhydrous DMF (20 mL) was added at RT and
the reaction mixture stirred for 45 min. at 100 °C. The solution was cooled and partitioned between water (1
L) and EtOAc (200 mL). The aqueous layer was extracted with EtOAc (5 x 75 mL), the combined organic

Molecules 2001, 6
962
layers were washed with water (5 x 400 mL), 1 N NaOH (2 x 250 mL) and brine (1 x 500 mL), dried
(Na2SO4), filtered and concentrated in vacuo. The residue was crystallized from anhydrous EtOH (50 mL).
Yield: orange crystals (7.15 g, 72%); mp. 188-91 °C (lit. [2,3] oil).
a-(2,6-Dichlorophenyl)-6-phenylthiopyridazine-3-acetamide (4).
a -(2,6-Dichlorophenyl)-6-phenylthiopyridazine-3-acetonitrile (3) (6.50 g, 14.8 mmol) was stirred in
concentrated H SO4 (200 mL) at 100 °C for 75 min. The solution was cooled to RT, poured into cold water
2
(4 L) and stirred for 10 min. The precipitate was filtered, washed with water (6 x 100 mL), dissolved in
MeOH (25 mL)/EtOAc (100 mL), dried (Na2SO4), filtered, concentrated in vacuo and triturated with iPr2O
(2 x 10 mL). Yield: orange crystals (3.70 g, 64%); mp. 120-32 °C (lit. [2,3] oil).
5-(2,6-Dichlorophenyl)-2-(phenylthio)-6H-pyrimido[1,6-b]pyridazin-6-one (5).
a -(2,6-Dichlorophenyl)-6-phenylthiopyridazine-3-acetamide (4) (3.40 g, 8.71 mmol) and N,N-dimethyl-
formamide dimethyl acetal (2.51 g, 18.9 mmol) in anhydrous toluene (60 mL) were stirred at 100 °C for 2 h.
The solution was stirred at RT for an additional 2 h, and the precipitate thus formed was collected by suction
filtration and washed successively with toluene (2 x 50 mL) and light petroleum (4 x 50 mL). The crude
product was dissolved in hot AcOH (10 mL), then water (30 mL) was added dropwise, and the resulting
suspension was stirred at RT for 1 h. The precipitate was filtered off and washed with water (5 x 20 mL),
iPrOH (3 x 20 mL) and iPr2O (3 x 20 mL) and dried (50 °C/50 mbar) to afford the title compound as yellow
crystals. Yield: (1.45 g, 41%, lit. [2,3] ~ 10%); mp.: 262 - 265 °C; Anal. Calcd for C H11Cl N3OS: C,
19
2
1
57.01; H, 2.77; N, 10.50. Found: C, 56.73; H, 2.97; N, 10.22; H-NMR (CF3COOD) d 10.00 (s, 1H),
8.33 – 8.09 (m, 8H), 8.04 (s, 2H). The purity of the product was determined to be > 99% by HPLC; HPLC
conditions: Injection volume: 10 µL, run time: 20 min, column: Phenomenex Synergi Polar-RP 4,
temperature: ambient, solvent A: MeCN : H O = 97: 3, solvent B: MeCN : 0.1% TFA = 97 : 3, gradient
2
used: see following table.
Table 1. HPLC-gradient
Time Flow %A
%B
70.0
70.0
10.0
70.0
70.0
0.00
3.00
1.00 30.0
1.00 30.0
15.00 1.00 90.0
20.00 1.00 30.0
50.00 0.20 30.0

Molecules 2001, 6
963
References
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Sample Availability: Available from MDPI.
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