Reactions of 3,6-bis(3,5-dimethyl-4-r-pyrazol-1-yl)-1,2,4,5-tetrazines with indole and 1,3,3-trimethyl-2-methyleneindoline

Article abstract of DOI:10.3987/COM-11-12165

It has been found that 3,6-bis(3,5-dimethyl-4-R-pyrazol-1-yl)-1,2,4,5- tetrazines react with indole and 1,3,3-trimethyl-2-methyleneindoline to give pyridazines as [4+2]cycloaddition products. 1,3,3-Trimethyl-2-methyleneindoline has been shown to act as C-nucleophile in the substitution of pyrazolyl group as well as in the reactions of tetrazine ring expansion in [1,2,4]triazolo[4,3-b] [1,2,4,5]tetrazine derivatives.

Full text of DOI:10.3987/COM-11-12165

HETEROCYCLES, Vol. 83, No. 6, 2011
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HETEROCYCLES, Vol. 83, No. 6, 2011, pp. 1363 - 1370. © The Japan Institute of Heterocyclic Chemistry
Received, 7th February, 2011, Accepted, 24th March, 2011, Published online, 12th April, 2011
DOI: 10.3987/COM-11-12165
REACTIONS OF 3,6-BIS(3,5-DIMETHYL-4-R-PYRAZOL-1-YL)-1,2,4,5-
TETRAZINES
WITH
INDOLE
AND
1,3,3-TRIMETHYL-2-
METHYLENEINDOLINE
Rashida I. Ishmetova, Nina K. Ignatenko, Ilya N. Ganebnykh, Svetlana G.
Tolshchina, Pavel A. Slepukhin, and Gennady L. Rusinov*
Dedicated to Professor Dr. V. N. Charushin on the occasion of his 60th
birthday
I. Ya. Postovsky Institute of Organic Synthesis of RAS, 620990, S. Kovalevskoy
st, 22, Yekaterinburg, Russia. Tel.(fax): +7-(343)-374-11-89; E-mail:
rusinov@ios.uran.ru
Abstract – It has been found that 3,6-bis(3,5-dimethyl-4-R-pyrazol-1-yl)-1,2,4,5-
tetrazines react with indole and 1,3,3-trimethyl-2-methyleneindoline to give
pyridazines as [4+2]cycloaddition products. 1,3,3-Trimethyl-2-methyleneindoline
has been shown to act as C-nucleophile in the substitution of pyrazolyl group as
well
as
in
the
reactions
of
tetrazine
ring
expansion
in
[1,2,4]triazolo[4,3-b][1,2,4,5]tetrazine derivatives.
Substituted indoles are the structural elements of a large number of natural alkaloids demonstrating
various biological activities. Indole as would be expected is widely used as a building block in the design
of potential biologically active compounds.1 The ability of indole to act as nucleophile in the substitution
reactions or as dienophile in [4+2]cycloaddition processes can be used for an attachment of the indole
moiety to heterocyclic compounds. The reaction of 3,6-disubstituted s-tetrazines bearing
electron-withdrawing substituents (2-Py, CO2CH3) with indole and its N-derivatives has been observed to
give rise to 1,4-disubstituted 5H-pyridazino[4,5-b]indoles.2,3 It has been established earlier that the
reaction of indole or 5-bromoindole with 3,6-bis(3,5-dimethylpyrazol-1-yl)-1,2,4,5-tetrazine (1a) in
acetonitrile in the presence of triethylamine results in the formation of mono- and disubstitution
products.4 3,6-Bis(3,5-dimethyl-4-R-pyrazol-1-yl)-1,2,4,5-tetrazines (1a-c) (R=H, Br, Cl, respectively)

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HETEROCYCLES, Vol. 83, No. 6, 2011
appear to undergo [4+2]cycloaddition reaction with enamines yielding 1,4-dihydropyridazines.5 On the
other hand the reactions of 1a-c with anhydrobases formed in situ from N-methylquinaldinium iodide give
rise to the products of nucleophilic substitution of pyrazolyl group along with the products of
cycloaddition reaction.6
In the course of this study it has been established that 3,6-bis(3,5-dimethyl-4-R-pyrazol-1-yl)-
1,2,4,5-tetrazines (1a-c) are able to react with indole and 1,3,3-trimethyl-2-methyleneindoline to give
[4+2]cycloaddition products.
1
The interaction of tetrazines (1a-c) with indole proceeded in toluene at reflux (method A). The H NMR
spectra of the reaction mixtures exhibit the signals of the protons of pyridazines (3a-c) along with the
chemical shifts of the protons of dihydrotetrazines (4a-c). Compounds (4a-c) derive from the
reduction-oxidation reactions of the intermediates (2a-c) and tetrazines (1a-c) due to the high
electrophilicity of latter.6 [4+2]Cycloaddition reactions of indole with 3,6-bis(3,5-dimethyl-4-R-
pyrazol-1-yl)-1,2,4,5-tetrazines also proceed upon melting (method B) giving rise to pyridazines (3) as
exemplified by (3a) (Scheme 1). The structure of (3a) was established by X-ray analysis (Figure 1).
Me
N
Me
N
N
H
Me
N
Me
N
R
R
N N
R
R
N N
toluene,
N
N
Me
Me
-N2
N
N
N N
1a-c
Me
Me
HN
2a-c
Me
Me
Me
Me
R
R
R
R
HN N
N NH
N
N N
N
N
N
N
+
N
N
N
Me
Me
Me
Me
HN
4a-c
3a-c
1-4: R=H(a), Br (b), Cl (c)
Scheme 1
It has been found that the interaction of tetrazines (1a-c) with 1,3,3-trimethyl-2-methyleneindoline in
acetonitrile proceeds in mild conditions resulting in formation of unstable spiro intermediates (5a-c)
followed by the pyrrole ring opening to yield pyridazines (6a-c) (Scheme 2).

HETEROCYCLES, Vol. 83, No. 6, 2011
1365
Me
N
Me
Me
N
Me
N
R
R
R
R
N N
N N
N
N
N
N
N
Me
Me
Me
Me
N
Me
Me
Me
Me
Me
NH
Me
1a-c
5a-c
6a-c
1,5,6: R=H (a), Br (b), Cl (c)
MeCN
-N2
Me
Me
1a
Me
N N
N
TEA, MeCN (DMF)
Me
Me
N
N
6a
+
N N
Me
Me
Me
N
8
Me
7
MeCN, TEA
N
Me
N
N
Me
N
Me
HN
N N
N N
N
N
8 =
N
N
N
N
Me
OH
Me
N
Me
N
9
Scheme 2
1
The structural data for compounds (6a-c) were obtained by NMR H spectroscopy and X-ray analysis
(Figure 2).
Figure 2. The X-ray structure of compound 6a
Figure 1. The X-ray structure of compound 3a
(50% probability thermal ellipsoids)
(50% probability thermal ellipsoids)

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HETEROCYCLES, Vol. 83, No. 6, 2011
In the 1H NMR spectra of (6a-c) the signals of aromatic protons of pyridazine ring appear as singlets at δ
8.37-8.25 ppm. The signals of methyl protons of the amino substitutents are observed as doublets at δ
2.64-2.72 ppm.
The substitution of the pyrazolyl group in tetrazine (1a) by 1,3,4-trimethyl-2-methyleneindoline proceeds
upon the addition of triethylamine (Scheme 2), although the major compound is (6a). Evidence for the
formation of (7) in reaction mixture was obtained by LC/MS data. Yield of (7) increased to 11% on
transfer from acetonitrile to DMF. The structure of (7) was established by X-ray analysis (Figure 3).
Substituted [1,2,4]triazolo[4,3-b][1,2,4,5]tetrazines being the structural derivatives of aromatic tetrazines
(1a-c) keep the lability of heterocyclic moiety in the position 6 and react with nucleophiles of different
types.6,10,11 Specific allocation of the electron density prevents triazolotetrazines from [4+2]cycloaddition
processes. In our previous studies we have found that cyclic enamines are prone to demonstrate both
dienophilic and C-nucleophilic properties reacting in extraordinary manner with 3,6-disubstituted
s-tetrazines12 and [1,2,4]triazolo[4,3-b][1,2,4,5]tetrazines.6,13
In the course of the study of the interaction of C-nucleophiles with substituted triazolo[4,3-b]-s-tetrazines
we have investigated the reactions of 6-(3,5-dimethylpyrazol-1-yl)[1,2,4]triazolo[4,3-b]-s-tetrazine (8)
with indole and 1,3,3-trimethyl-2-indoline. It has been found that indole doesn’t react with (8) at room
temperature in acetonitrile, whereas hardly separating complex mixture was obtained in the presence of
triethylamine. The interaction of triazolotetrazine (8) with 1,3,3-trimethyl-2-methyleneindoline resulted in
the formation of tetrazine ring expansion product
–
2-(6-(3,5-dimethylpyrazol-1-yl)-7H-
[1,2,4]triazolo[4,3-b][1,2,4,6]tetrazepine-8-yl)-1,3,3-trimethylindoline-2-ol (9) in 19% yield (Scheme 2).
Expansion of the tetrazine ring has first been found to occur when [1,2,4]triazolo[4,3-b][1,2,4,5]tetrazines
were allowed to react with CH-active compounds in the presence of triethylamine.13 The structure of 9
was strictly proved by 1H NMR spectroscopy and X-ray analysis (Figure 4).
Figure 3. The X-ray structure of compound 7
Figure 4. The X-ray structure of compound 9 (50%
(50% probability thermal ellipsoids)
probability thermal ellipsoids)

HETEROCYCLES, Vol. 83, No. 6, 2011
1367
In summary, it has been shown that 3,6-bis(3,5-dimethyl-4-R-pyrazol-1-yl)-1,2,4,5-tetrazines (1a-c) are
able to react with indole and 1,3,3-trimethyl-2-methyleneindoline to give [4+2]cycloaddition products.
Reaction of 6-(3,5-dimethylpyrazol-1-yl)[1,2,4]triazolo[4,3-b][1,2,4,5]tetrazine with 2-methyleneindoline
results in the formation of novel (7H-[1,2,4]triazolo[4,3-b][1,2,4,6]tetrazepine-8-yl)-1,3,3-
trimethylindoline-2-ol (9).
EXPERIMENTAL
General: Starting 3,6-bis(3,5-dimethyl-4-R-pyrazol-1-yl)-1,2,4,5-tetrazines (1a-c) were synthesized
according to known procedures.5,7 6-(3,5-Dimethylpyrazol-1-yl)[1,2,4]triazolo[4,3-b][1,2,4,5]tetrazine (8)
was obtained as described in the literature.8 1H NMR spectra were recorded in CDCl3 using Bruker
Avance DRX-400 spectrometer operating at 400 MHz frequency with TMS as internal standard. Mass
spectral data were obtained on a Shimadzu LCMS-2010 instrument operating in APCI mode. Melting
points were determined on Boëtius apparatus. Elemental analyses were performed with an automatic
analyzer Carlo Erba 1108. The Sorbfil ® TLC plates were used to control the reaction progress and the
purity of the obtained compounds with 1:1 benzene/acetonitile mixture as eluent. X-Ray crystallography
analyses were performed on a diffractometer “Xcalibur-3” (Oxford-Diffraction) equipped with CCD
detector at 295(2)o К, (λMoKα, graphite monochromator, ω-scan). Corrections on absorbance were not
used. The structures for all compounds were solved by direct method and refined using SHELX program
package with anisotropic approximation (isotropic for hydrogen atoms). Hydrogen atoms of OH- and
NH- groups were refined independently; other hydrogen atoms were included into the adjustment in the
rider model with dependent heat parameters.9
CCDC 816604 (3a), 816605 (6a), 816606 (7), 816607 (9), contain the supplementary crystallographic
data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data
Centre via http://www.ccdc.cam.ac.uk/data_request/cif.
1,4-Di(3,5-dimethyl-4-R-pyrazol-1-yl)-5H-pyridazino[4,5-b]indoles (3a-c). Method A: A mixture of
0.001 mol of the starting tetrazine (1) and 0.001 mol of indole in 15 mL of toluene was refluxed for 3 h.
During the reaction the color of the reaction mixture changed from bright red to light yellow. Toluene
was removed, and the residue was triturated with MeOH and filtered off. The formed precipitate was
suspended in a mixture of 10 mL of MeCN and 0.5 mL of AcOH. A solution of 50 mg of sodium nitrite
in 2 mL of water was added to the precipitate. Partial dissolvation of the precipitate was observed, and the
solution became red. A colorless residue was filtered off, recrystallized from MeOH, and then dried in
vacuum for 6 h.

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HETEROCYCLES, Vol. 83, No. 6, 2011
Method B: 0.001 mol of the starting tetrazine (1) and 0.001 mol of indole were mixed together. This
mixture in a round-bottom flask was held at 140-150 oC until no nitrogen liberation observed (15-20 min).
After cooling the mixture was crystallized from MeOH, filtered off and then isolated as described in the
method A.
1,4-Di-(3,5-dimethylpyrazol-1-yl)-5H-pyridazino[4,5-b]indole (3a). Yield 56%. Mp. 158-159 ºC (from
MeOH). Anal. Calcd for C20H19N7: C 67.21; H 5.36; N 27.43. Found: C 67.38; H 5.39; N 27.75. 1H NMR
(δ, ppm): 2.43, 2.45, 2.48, 2.92 (all s, 4x3H, 4CH3); 6.15, 6.18 (both s, H(C4) in pyrazole ring);
7.88-7.90; 7.68-7.70; 7.59-7.64; 7.30-7.34 (all m, 4H in benzene ring); 11.03 (br.s., 1H, NH in pyrrole
ring).
1,4-Di-(4-bromo-3,5-dimethylpyrazol-1-yl)-5H-pyridazino[4,5-b]indole (3b). Yield 74%. Mp 256-257
ºC (from MeOH). Anal. Calcd for C20H17Br2N7: C 46.62; H 3.33; N 19.03. Found: C 46.36; H 3.60; N
19.10. 1H NMR, δ, ppm: 2.94, 2.51, 2.48, 2.44 (all s, 3H x 4CH3); 7.71-7.62; 7.64-7.59; 7.37-7.33 (all m,
4H in benzene ring); 10.88 (br. s., 1H, NH in pyrrole ring).
1,4-Di-(4-chloro-3,5-dimethylpyrazol-1-yl)-5H-pyridazino[4,5-b]indole (3c). Yield 71%. Mp 240-242
ºC (from MeOH). Anal. Calcd for C20H17Cl2N7: C 56.34; H 4.02; N 23.00. Found: C 56.38; H 3.97; N
1
23.03. H NMR, δ, ppm: 2.93, 2.51, 2.47, 2.44 (all s, 4x3H, 4CH3); 7.96-7.94; 7.72-7.66; 7.65-7.62;
7.37-7.33 (all m, 4H in benzene ring); 10.89 (br. s., 1H, NH in pyrrole ring).
Typical procedure for the synthesis of (2-{1[3,6-Bis-(3,5-dimethyl-4-R-pyrazol1-yl)pyridazin-4-yl]-1-
ethylmethyl}phenyl)methylamines (6a-c). To a suspension of 0.001 mol of 3,6-bis(3,5-dimethyl-
4-R-pyrazol-1-yl)-1,2,4,5-tetrazine (1a-c) in 15 mL of MeCN 0.25 mL (0.002 mol) of 1,3,3-trimethyl-
2-methyleneindoline was added. Reaction mixture was heated for 3-5 min until nitrogen liberation started.
During this process the reaction color changed from bright red to brown-red. TLC was used to check the
reaction completion. After 5-10 min the reaction mixture was filtered, and the filtrate was chilled using
ice bath. The precipitated in few minutes colorless crystals were filtered off, washed by MeOH and
recrystallized from MeCN. Yield 63-85%.
(2-{1[3,6-Bis-(3,5-dimethylpyrazol-1-yl)pyridazin-4-yl]-1-ethylmethyl}phenyl)methylamine
(6a).
Yield 68%. Mp 133-135 ºC (from MeOH). Anal. Calcd for C24H29N7: C 69.36; H 7.03; N 23.60. Found:
1
C 69.20; H 7.09; N 23.98. H NMR, δ, ppm: 2.00, 2.21, 2.30, 2.21, (all s,4x3H, 4CH3); 2.72 (d, 3H,
-NHCH3); 1.74 (s, 6H, 2 CH3 in -C(CH3)2); 3.70 (m,1H, -NHCH3); 5.68, 6.05 (both s, 2H, in pyrazole
rings); 6.52-6.59, 7.10-7.20 (both m, 4H, in benzene ring); 8.25 (s, 1H, in pyridazine ring).
(2-{1[3,6-Bis-(4-bromo-3,5-dimethylpyrazol-1-yl)pyridazine-4-yl]-1-ethylmethyl}phenyl)methyl-
amine (6b). Yield 63%. Mp 115-117 ºC (from MeOH). Anal. Calcd for C24H27Br2N7: C 49.84; H 5.02; N
1
18.60. Found: C 50.09; H 5.00; N 18.57. H NMR, δ, ppm: 2.00, 2.18, 2.35, 2.60, 2.78 (all s, 5x3H, 5
CH3, in pyrazole rings, HN-CH3 ); (all s, 4x3H, 4CH3); 1.87 (s, 6H, 2 CH3 in -C(CH3)2); 3.25 (m,1H,

HETEROCYCLES, Vol. 83, No. 6, 2011
1369
-NHCH3); 6.42-6.44, 6.55-6.59, 7.02-7.04, 7.07-7.11 (all m, 4H, in benzene ring); 8.37 (s, 1H, in
pyridazine ring).
(2-{1[3,6-Bis-(4-chloro-3,5-dimethylpyrazol-1-yl)pyridazine-4-yl]-1-ethylmethyl}phenyl)methyl-
amine (6c). Yield 85%. Mp 105-108 ºC (from MeOH). Anal. Calcd for C24H27Cl2N7: C 59.50; H 5.62; N
1
20.25. Found: C 59.37; H 5.70; N 20.06. H NMR, δ, ppm: 2.00, 2.17, 2.34 (all s, of 3H, 3 CH3, in
pyrazole ring), 2.72-2.76 (m, 6H, CH3 in pyrazole ring, -NH-CH3), (all s, of 3H, 4CH3); 1.84, 1.89 (both s,
6H, 2 CH3 in -C(CH3)2); 3.20-3.25 (m,1H, -NHCH3); 7.30-7.34, 7.59-7.64, 7.68-7.70, 7.88-7.90 (all m,
4H in benzene ring); 8.36 (s, 1H in pyridazine ring).
2-((6-(3,5-Dimethylpyrazol-1-yl)-1,2,4,5-tetrazin-3-yl)methylene)-1,3,3-trimethylindoline (7). Yield
11% Mp 198-201 ºC (from MeOH). MS (APCI): m/z (%) = 348 (100) [M+1]+. Anal. Calcd for C19H21N7:
1
C 65.68; H 6.09; N 28.23. Found: C 65.46; H 6.32; N 27.95. H NMR, δ, ppm: 1.60 (s, 6H, 2 CH3 in
indoline), 2.37, 2.67 (all s, 2x3H, 2CH3 in pyrazole ring); 3.36 (s, 3H, in -NCH3); 5.92 (s, 1H, methylene);
6.14 (s, 1H, in pyrazole ring); 6.81, 7.01, 7.21 (all m, 4H, in benzene ring).
2-(6-(3,5-Dimethylpyrazol-1-yl)-7H-[1,2,4]triazolo[4,3-b][1,2,4,6]tetrazepin-8-yl)-1,3,3-trimethyl-
indolin-2-ol (9). To a suspension of 216 mg (1 mmol) of 6-(3,5-dimethylpyrazol-1-yl)-1,2,4-
triazolo[4,3-b]-1,2,4,5-tetrazine (7) in 10 mL of MeCN 0.14 mL (1 mmol) of triethylamine was added
with 200 mg (1.15 mmol) of indoline. The mixture was stirred at room temperature at the magnetic stirrer
for 2 h. After cooling of a mixture by ice a precipitated product was filtered off and washed by MeCN.
Yield 75 mg (19%). Mp 182 ºC (from EtOH). Anal. Calcd for C20H23N9O: C 59.24; H 5.71; N 31.09.
Found: C 59.28; H 5.89; N 30.54. 1H NMR, δ, ppm: 1.30, 1.40, 2.15, 2.50, 2.76 (all s, 5x3H, 5CH3); 6.27
(s, 1H, in pyrazole ring); 6.56 (s, 1H, OH), 6.70-6.74, 7.03-7.10 (all m, 4H, in benzene ring); 8.32 (s, 1H
in triazole ring), 9.92 (s, 1H, NH in tetrazepine ring).
ACKNOWLEDGEMENTS
The work was performed in the context and under financial support of the Russian Foundation for basic
Research (Project N11-03-00545a), the Integration Project of RAS 09-I-3-2004, Program of Presidium of
RAS 09-P-3-2001, State Contract 02 74011 0620 and State Program for Supporting of Leading Scientific
Schools of Russian Federation (grant NSh-65261.2010.3).
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