A short and straightforward approach towards 6-amino and 6-aminoalkyl thiazolo[4,5-c]pyridazines

Article abstract of DOI:10.1016/j.tetlet.2012.11.072

A facile and efficient synthesis of 6-amino and 6-aminoalkyl thiazolo[4,5-c]pyridazines is reported. The key step for the construction of this novel bicyclic scaffold was the reaction between 3-amino-4-bromopyridazine derivatives and alkylisothiocyanates.

Full text of DOI:10.1016/j.tetlet.2012.11.072

Accepted Manuscript
A short and straightforward approach towards 6-amino and 6-aminoalkyl thia‐
zolo[4,5-c]pyridazines
Alessandro Stella, Steven De Jonghe, Kenneth Segers, Piet Herdewijn
PII:
S0040-4039(12)02026-6
DOI:
http://dx.doi.org/10.1016/j.tetlet.2012.11.072
TETL 42168
Reference:
To appear in:
Tetrahedron Letters
Received Date:
Revised Date:
Accepted Date:
4 October 2012
13 November 2012
19 November 2012
Please cite this article as: Stella, A., De Jonghe, S., Segers, K., Herdewijn, P., A short and straightforward approach
towards 6-amino and 6-aminoalkyl thiazolo[4,5-c]pyridazines, Tetrahedron Letters (2012), doi: http://dx.doi.org/
10.1016/j.tetlet.2012.11.072
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A short and straightforward approach towards 6-amino
and 6-aminoalkyl thiazolo[4,5-c]pyridazines
Alessandro Stellaa,b, Steven De Jonghea,b, Kenneth Segersc, and Piet Herdewijn*a,b
a) Katholieke Universiteit Leuven, Interface Valorisation Platform, Kapucijnenvoer 33, 3000
Leuven, Belgium.
b) Katholieke Universiteit Leuven, Rega Institute for Medical Research, Laboratory of
Medicinal Chemistry, Minderbroedersstraat 10, 3000 Leuven, Belgium.
c) Katholieke Universiteit Leuven, Rega Institute for Medical Research, Laboratory of
Bacteriology, Minderbroedersstraat 10, 3000 Leuven, Belgium.
∗ Corresponding author. Tel. +32 (0)16/33.73.87 ; Fax : +32 (0)16/33.73.40 ; E-mail :
piet.herdewijn@rega.kuleuven.be
Abstract
A facile and efficient synthesis of 6-amino and 6-aminoalkyl thiazolo[4,5-c]pyridazines is reported.
The key step for the construction of this novel bicyclic scaffold was the reaction between 3-amino-
4-bromopyridazine derivatives and alkylisothiocyanates. The application of this methodology for
the synthesis of a small library of thiazolo[4,5-c]pyridazines is also described.
Keywords: Thiazolo[4,5-c]pyridazine, Isothiocyanates, Privileged structures, Drug-like structures
Heterocyclic structures are of fundamental importance in drug discovery and the development of
new strategies for the synthesis of novel heterocyclic scaffolds is an essential field of research in
organic and medicinal chemistry.1 In the last years, there has been a great interest in pyridazine-
based structures in medicinal chemistry, as pyridazines can be considered as privileged structures,
displaying a plethora of biological activities. The pyridazine moiety has been further fused to other
heterocyclic rings, affording fused bi- and tricyclic scaffolds.2 Several of these pyridazine-
containing bicyclic and tricyclic cores show interesting biological activities. For example, the
imidazo[1,2-b]pyridazine
1
displays activity against human picornaviruses,3 while
pyrrolopyridazine 2 is a novel acyl CoA:diacylglycerol acyltransferase (DGAT1) inhibitor with
potential applications in a variety of diseases including obesity, insulin resistance syndrome and
type II diabetes.4 Sulfur-containing pyridazine scaffolds5 also display interesting pharmacological

activities. For example, thienopyridazinone 3 is a melanine-concentrating hormone I antagonist,
which might be developed for the treatment of obesity,6 while fused thiazolo[4,5-d]pyridazine
derivative 4 displays antibacterial activity against S. aureus and E. coli.7
Figure 1
The medicinal chemistry community is continuously looking for new heterocyclic scaffolds that can
be used in a wide variety of screening campaigns. Based on the interest from our lab in the
synthesis and biological evaluation of novel, bicyclic heterocyclic structures, the thiazolo[4,5-
c]pyridazine scaffold was selected for synthesis. The chemistry of this heterocycle is still very much
unexplored as a search on SciFinder revealed only 14 known compounds based on this core
structure. Hence, very few reports dealing with the synthesis8 and biological activity9 of this novel
scaffold are known. To the best of our knowledge, there is no publication describing a general and
versatile method for the construction of the thiazolo[4,5-c]pyridazine core. Herein, we report a
convenient methodology for the synthesis of 6-amino- and 6-aminoalkyl thiazolo[4,5-c]pyridazines.
Our strategy for the construction of thiazolo[4,5-c]pyridazines is based on a methodology
elaborated by Liu and collaborators10 for the synthesis of 2-aminothiazolo[5,4-d]pyrimidines, in
which they start from a 4-chloro-5-amino-pyrimidine derivative. Similarly, we envisioned to
construct the thiazolo[4,5-c]pyridazine scaffold from an appropriate substituted 3-amino-4-bromo-
pyridazine analogue.
Starting from the commercially available 3-amino-6-chloropyridazine 5, a substituted phenyl ring
was introduced by a Suzuki cross-coupling reaction, followed by selective C-4 bromination of the
pyridazine ring,11 using bromine in MeOH, in presence of sodium bicarbonate (Scheme 1).
Scheme 1
In order to find the optimal reaction conditions for the construction of the thiazole moiety, the
reaction between 3-amino-4-bromo-6-(4-ethylphenyl)pyridazine 7a, as representative example, and
phenyl, as well as alkylisothiocyanates, has been studied, by varying different parameters such as
base, solvent, temperature and reaction time (Table 1).
Table 1

From Table 1 it is clear that the desired thiazolo[4,5-c]pyridazine was never obtained when
aromatic isothiocyanates were used for formation of the thiazole ring. Treatment of pyridazine
derivative 7a with phenylisothiocyanate at 80°C for 12h, in presence of Cs2CO3 and CH3CN as
solvent (Entry 1), led mainly to dehalogenation of the starting material, yielding compound 6a.
These results stand in sharp contrast with the results obtained by Liu and coworkers,10 wherein
reaction of 4,6-dichloro-5-amino-pyrimidine and a wide range of phenylisothiocyanates afforded
the desired 2-anilino-thiazolo[5,4-d]pyrimidines in excellent yields. Other combinations of solvents
and bases did not bring any improvement. Using DMF as solvent and Cs2CO3 as a base, at 50°C
showed only the presence of unreacted starting material (Entry 2), while sodium hydride in THF (at
50°C for 12h, Entry 3) led to debromination of the substrate 7a. When the reaction was carried out
without a base, only starting material 7a was recovered (Entry 4). In order to investigate if the
electronic properties of the aromatic isothiocyanate did have an influence on the reaction outcome,
4-nitrophenylisothiocyanate (as an example of an electron-withdrawing group) and 3,4,5-
trimethoxyphenylisothiocyanate (representative for an electron-donating moiety) were selected as
coupling partners. When the reaction was performed in toluene as solvent and using DABCO as a
base,12 both reactions failed. No desired product could be isolated, and only reductive removal of
the bromine was observed (Entries 6 and 7). Performing the reaction under microwave irradiation
(Entry 5), using DMSO as solvent at 150°C for 2 hours, likewise, led to dehalogenation of the
pyrazine moiety. All attempts to accomplish the ring closure with aromatic isothiocyanates were
unsuccessful and we were not able to prepare 6-anilino-thiazolo[4,5-c]pyridazines by this
methodology. However, applying this methodology to aliphatic isothiocyanates, the desired
thiazolo[4,5-c]pyridazines scaffold could be isolated. In particular, performing the reaction with n-
butylisothiocyanate at 80°C for 12 hours, using CH3CN as solvent and Cs2CO3 (2 eq) as base,
allowed the isolation of compound 8a in 29% yield (Entry 8). Encouraged by this result, further
optimization of the reaction parameters, by decreasing the reaction time to 6 hours, lowering the
reaction temperature to 60°C and reducing the amount of Cs2CO3 to 1.5 eq allowed the isolation of
compound 8a in 56% yield (Entry 9).
To demonstrate that this chemistry was useful for making series of analogues, a small library of
thiazolo[4,5-c]pyridazines was prepared (Table 2).
Table 2

Four different alkylisothiocyanates (n-butyl, cyclohexyl, n-propyl and allyl isothiocyanate) were
coupled with four different substituted pyridazines 7a-d bearing various substituents at C-6 position
(4-ethylphenyl, 4-fluorophenyl, 4-chlorophenyl and chlorine), leading to a small library of
thiazolo[4,5-c]pyridazines 8a-p.
The synthesis of 6-aminothiazolo[4,5-c]pyridazine derivatives 9a-d was also accomplished by
reaction of pyridazines 7a-d with potassium thiocyanate and bromine13 in acetic acid as solvent
(Scheme 2).
Scheme 2
Given the chemical novelty of the thiazolo[4,5-c]pyridazine scaffold, the library can be used for
screening campaigns in a wide variety of biological assays. Our lab has a main interest in the search
for novel antibacterial compounds,14a-d either via a phenotypic cell-based assay approach,14c or via a
target-based approach using SecA as drug target.14d Therefore, the thiazolo[4,5-c]pyridazine-based
compound library was first evaluated in these two assays, both relevant for antibacterial drug
discovery. Screening of the compounds for their ability to inhibit the growth of S. aureus at a
concentration of 64 µg/mL did not reveal any compounds with antibacterial activity. When the
thiazolo[4,5-c]pyridazines were investigated as potential inhibitors of S. aureus secA, none of the
analogues displayed any inhibitory activity when tested at a concentration of 200 µM.
In summary, we have developed a short and easy methodology for the synthesis of novel 6-amino
and 6-aminoalkyl thiazolo[4,5-c]pyridazine derivatives. The key step is the formation of a thiazole
moiety starting from a 3-amino-4-bromo-pyridazine derivative and an isothiocyanate. Whereas the
reaction proceeds smoothly with aliphatic isothiocyanates, using phenylisothiocyanates did not lead
to the expected 6-anilino-thiazolo[4,5-c]pyridazines. The methodology presented in this work
allows for the rapid synthesis of a wide variety of, hitherto unknown, 3,6-disubstituted thiazolo[4,5-
c]pyridazines.
Acknowledgments
Alessandro Stella is deeply indebted to the IWT (Agentschap voor Innovatie door Wetenschap en
Technologie) for providing a PhD-scholarship. Mass spectrometry was made possible by the
support of the Hercules Foundation of the Flemish Government (grant 20100225–7).
References
1) Dua, R.; Shrivastava, S.; Sonwane, S; K.; Srivastava S. K. Adv. Biol. Res.2011, 5, 120-144.

2) Wermuth, C. G. MedChemComm 2011, 2, 935-941.
3) Hamdouchi, C.; Sanchez-Martinez, C.; Gruber, J.; del Prado, M.; Lopez, J.; Rubio, A.;
Heinz, B. A. J. Med. Chem. 2003, 46, 4333-4341.
4) Fox, B. M.; Iio, K.; Li, R.; Choi, R.; Takashi, I.; Jackson, S.; Sagawa, S.; Shan, B.; Tanaka,
M.; Yoshida, A.; Kayser, F. Bioorg. Med. Chem. Lett. 2010, 20, 6030-6033.
5) a) Kaji, K.; Nagashima, H.; Nagao, S.; Tabashi, K.; Oda, H. Chem. Pharm. Bull. 1984, 32,
4437-4446. b) Matsuo, T.; Tsukamoto, Y.; Takagi, T.; Yaginuma, H. Chem. Pharm. Bull.
1982, 30, 1030-1032. c) Al-Mousawi, S. M.; Moustafa, M. S.; Elnagdi, M. H. Arkivoc
2010, 224-232. d) Bantick, J.; Cooper, M.; Thorne, P.; Perry, M. PCT WO 1999/29695.
6) Dyck, B.; Markison, S.; Zhao, L. R.; Tamiya, J.; Grey, J.; Rowbottom, M. W.; Zhang, M.
Z.; Vickers, T.; Sorensen, K.; Norton, C.; Wen, J.; Heise, C. E.; Saunders, J.; Conlon, P.;
Madan, A.; Schwarz, D.; Goodfellow, V. S. J. Med. Chem. 2006, 49, 3753-3756.
7) Makki, M. S. I.; Faidallah, H. M. J. Chin. Chem. Soc. 1996, 43, 433-438.
8) Fruit, C.; Turck, A.; Ple, N.; Queguiner, G. J. Heterocycl. Chem. 2002, 39, 1077-1082.
9) Bluth, R. Pharmazie 1982, 37, 208-212.
10)Liu, J.; Patch, R. J.; Schubert, C.; Player, M. R. J. Org. Chem. 2005, 70, 10194-10197.
11)Pastor-Fernandez, J.; Bartolome-Nebreda, J. M.; MacDonald, G. J.; Conde-Ceide, S.;
Delgado-Gonzalez, O.; Vanhoof, G. C. P.; Van Gool, M. L. M.; Martin-Martin, M. L.;
Alonso-De Diego, S. A. PCT WO 2011/051342.
12)Ding, Q. P.; He, X. D.; Wu, J. J. Comb. Chem. 2009, 11, 587-591.
13)Bernstein, J.; Stearns, B.; Shaw, E.; Lott, W. A. J. Am. Chem. Soc. 1947, 69, 1151-1158.
14) a) Kögler, M., Vanderhoydonck, B., De Jonghe, S., Rozenski, J., Van Belle, K., Herman, J.,
Louat, T., Parchina, N., Sibley, C., Lescrinier, E., Herdewijn, P. J. Med. Chem 2011, 13,
4847-4862. b) Dumbre, S.; Derouaux, A.; Lescrinier, E.; Piette, A.; Joris, B.; Terrak, M.;
Herdewijn, P. J. Am. Chem. Soc. 2012, 134, 9343-9351. c) Stella, A.; Segers, K.; De Jonghe,
S.; Vanderhoydonck, B.; Rozenski, J.; Anné; J.; Herdewijn, P. Chem. Biodiv. 2011, 8, 253-
265. d) Yang, M.-Y.; De Jonghe, S.; Segers, K.; Anné, J.; Herdewijn, P. Bioorg. Med.
Chem. 2011, 19, 702-714.
15)Representative example: Synthesis of N-Butyl-3-(4-ethylphenyl)-[1,3]thiazolo[4,5-
c]pyridazin-6-amine (8a) To a solution of 3-amino-4-bromo-6-(4-ethylphenyl)pyridazine 7a
(200 mg, 0.719 mmol) in CH3CN (6 mL), were added Cs2CO3 (1.5 eq, 1.08 mmol, 352 mg)
and butylisothiocyanate (1.5 eq, 1.08 mmol, 125 µL) and the mixture was stirred at 60°C for
6 hours. The solution was then diluited with AcOEt and brine, organic phase extracted, dried
over MgSO4 and evaporated under reduced pressure. The crude mixture was purified by
silica gel chromatography using Heptane/AcOEt : 1/1 as mobile phase, affording the title
1
compound in 56% yield. H-NMR (300 MHz, (D6)-DMSO): 8.88 (br s, 1H), 8.5 (s, 1H),
7.99 (d, J = 7.74 Hz, 2H), 7.35 (d, J = 7.74 Hz, 2H), 3.48 (br s, 2H), 2.66 (q, J = 7.41 Hz,
2H), 1.62 (m, 2H), 1.38 (m, J = 7.14, 2H), 1.22 (t, J = 7.35 Hz, 3H), 0.93 (t, J = 7.32 Hz,
3H). 13C-NMR (75 MHz, (D6)-DMSO): 168.0, 167.1, 151.2, 144.9, 134.4, 133.2, 128.4,
126.4, 117.0, 44.0, 30,7, 28.0, 19.6, 15.6, 13.8. HRMS: ESI+ calc. for (C17H21N4S)+
313.1481, found: 313.1484.

Table 1. Optimization of the reaction conditions
Br
NH2
S
N
RNCS
R
N
H
N
N
N
N
7a
Isothiocyanated
Entry
Base
Solvents
Temperature
– Reaction
time
Product
a
PhNCS
PhNCS
PhNCS
Cs2CO3 (2eq)
Cs2CO3 (2eq)
NaH (1.5 eq)
None
MeCN
DMF
THF
MeCN
DMSO
Toluene
Toluene
MeCN
MeCN
80°C - 12h
50°C - 4h
50°C - 12h
80°C - 12h
150°C - 2hc
80°C - 12h
80°C - 12h
80°C - 12h
60°C - 6h
-
-
1
2
3
4
5
6
7
8
9
b
a
-
b
PhNCS
p-NO2PhNCS
p-NO2PhNCS
-
a
None
-
a
DABCO (2eq)
DABCO (2eq)
Cs2CO3 (2eq)
Cs2CO3 (1.5eq)
-
a
3,4,5-trimethoxyPhNCS
ButylNCS
ButylNCS
-
8a (29%)
8a (56%)
b
aDebromination of the starting material was occurring and compound 6a was isolated. Starting material was isolated.
CReaction performed under microwave irradiation.d For all cases, 1.5 eq of isothiocyanates were used.
Table 2: Thiazolo[4,5-c]pyridazine library
X
Br
X
S
N
RNCS, Cs2CO3
CH3CN, 60°C 6h
R
N
H
N
N
N
NH2
N
Entry
X
R
Product
(Yield)
1
2
3
4
5
6
7
8
9
4-Ethylphenyl
4-Ethylphenyl
4-Ethylphenyl
4-Fluorophenyl
Cyclohexyl
Propyl
Allyl
8b (52%)
8c (71%)
8d (69%)
8e (58%)
8f (49%)
8g (76%)
8h (47%)
8i (53% )
8j (69%)
8k (71%)
8l (44%)
8m (48%)
8n (69%)
8o (73%)
8p (62%)
Butyl
4-Fluorophenyl Cyclohexyl
4-Fluorophenyl
4-Fluorophenyl
4-Chlorophenyl
Propyl
Allyl
Butyl
4-Chlorophenyl Cyclohexyl
10
11
12
13
14
15
4-Chlorophenyl
4-Chlorophenyl
Chlorine
Propyl
Allyl
Butyl
Cyclohexyl
Propyl
Allyl
Chlorine
Chlorine
Chlorine

Figure 1: Biologically active pyridazine-fused bi- and tricyclic scaffolds.
H
NH
N
O
N
S
N
F3C
H
N
N
N
N
3
NH2
N
N
N
S
N
N
H3C
N
O
HO
1
4
N
N
O
N
F
OEt
2
R
R
Cl
Br
a)
b)
N
N
N
N
NH2
N
NH2
N
NH2
5
6a R=Et
6b
R=F
7a R=Et
7b
R=F
6c
R=Cl
7c
R=Cl
Cl
Br
NH2
b)
N
N
7d
Scheme 1. Reagents and conditions: a) Arylboronic acid, Pd(PPh3)4, Na2CO3 2M, Toluene, 90°C, 12h. b) Br2, NaHCO3,
MeOH, rt.
R
Br
R
S
N
a)
NH2
N
N
N
NH2
N
7a-d
9a-d
7a R=4-Ethylphenyl
7b R=4-Fluorophenyl
7c R=4-Chlorophenyl
7d R =Chlorine
9a R=4-Ethylphenyl
9b R=4-Fluorophenyl
9c R=4-Chlorophenyl
9d R =Chlorine

Scheme 2. Reagents and conditions: a) KSCN, Br2, AcOH, 70°C.