Tautomeric variety and methylation of 3,6-dihydroxy-4-methylpyridazine

Article abstract of DOI:10.1016/j.molstruc.2004.02.020

The method of exhaustive methylation has been applied for investigating tautomeric stability and its temperature variation of 6-hydroxy-5-methyl-3- pyridazinone (1) and its N-phenyl derivative (2) - two model compounds chosen for studying tautomeric trans

Full text of DOI:10.1016/j.molstruc.2004.02.020

Journal of Molecular Structure 694 (2004) 85–90
www.elsevier.com/locate/molstruc
Tautomeric variety and methylation of 3,6-dihydroxy-4-methylpyridazine
a,
Anna Katrusiak , Andrzej Katrusiak
b
*
a
´ ´
Department of Organic Chemistry, Poznan University of Medical Sciences, Grunwaldzka 6, Poznan 60-780, Poland
b
´
Faculty of Chemistry, University of Adam Mickiewicz, Grunwaldzka 6, Poznan 60-780, Poland
Received 10 December 2003; revised 28 January 2004; accepted 2 February 2004
Abstract
The method of exhaustive methylation has been applied for investigating tautomeric stability and its temperature variation of 6-hydroxy-5-
methyl-3-pyridazinone (1) and its N-phenyl derivative (2) — two model compounds chosen for studying tautomeric transformations of
pyridazine medicinal drugs, either in the process of their production, or then their activity in the living tissue. The resulting N-methylated and
1
O-methylated products were separated, and characterized by H NMR and X-ray diffraction. The methylation of 1 and 2 in the dimethyl
sulfate as well as in its aqueous solution leads predominantly to the O-methylated forms, but the rising temperature favours the N-
methylation.
q 2004 Elsevier B.V. All rights reserved.
Keywords: Methylation; Methylpyridazine; H-tautomer; Column chromatography; X-ray diffraction
1. Introduction
of certain drugs. A well-known example is that of morphine
derivatives, constituting an important group of pain-killers.
The replacement of the hydrogen atom of the hydroxyl
group at C-6 in morphine by a methyl group results in a new
pharmacological activity. O-Methyl morphine (CODEINE)
is an anti-coughing drug. The N-methyl group conditions
anti-pain and anti-coughing activity, while the elimination
of this group reduces not only the drug activity but also
toxicity. It is also well known that polar amines in some
drugs can be N-methylated to reduce their polarity
and improve membrane permeability. Many of antiepilep-
tics and hypnotics take advantage of this reaction, for
example hexobarbitone [2]. In certain groups of
substances, the methylation may stabilize one of several
possible H-tautomers and can be applied for increasing
stability of drugs or for inducing their required physical
properties, such as solubility. Furthermore, the methylation
may be applied as a method of investigating the tautomeric
stability of substrates [3,4]. The same method can be used
for investigating molecular transformations, for example at
varied thermodynamical conditions of temperature or pH,
mimicking the physiological conditions.
H-Tautomers of nucleic-acid bases and their analogues
are of primary significance for their biological functions.
Although a considerable number of antiviral drugs is known
to undergo tautomeric transformations the exact ratio and
pathways of these transformations are strongly dependent
on the macroscopic conditions (temperature, solvent, pH,
etc.) and complex for general description. Most recently, we
described the tautomeric transformations of 4,6-dihydroxy-
pirymidine [1]. Presently, we have investigated the
tautomeric transformations of another dihydroxydiazine:
3,6-dihydroxypyridazine. In this compound, the molecular
symmetry has been broken by introducing a methyl
substituent at the C-5 site inactive for tautomeric trans-
formations (Scheme 1); to extend this study also one of the
nitrogen atoms in this compound has been substituted by the
phenyl substituent to block its tautomeric transformations. It
is well established, that the pharmaceutical activity of drugs
can be considerably modified by methyl substituents.
Therefore, the reactions of methylation are of particular
interest. The methyl group can either increase or decrease
activity, or can considerably modify efficacity and toxicity
In the present study, we have revealed the equilibra of the
H-tautomers of 6-hydroxy-5-methyl-3-pyridazinone (1) and
its N-phenyl derivative (2). Factors influencing the direction
of methylation have been described, and a series of new
*
Corresponding author. Tel.: þ48-61-86-99-181; fax: þ48-61-86-
58-008.
E-mail address: akatrus@amp.edu.pl (A. Katrusiak).
0022-2860/$ - see front matter q 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2004.02.020

86
A. Katrusiak, A. Katrusiak / Journal of Molecular Structure 694 (2004) 85–90
Scheme 1. Tautomeric forms of compound 1: 3,6-dihydroxy-4-methylpyridazine (1a), 6-hydroxy-5-methyl-3-pyridazinone (1b), 4-methyl-3,6-
pyridazinedione (1c), and 6-hydroxy-4-methyl-3-pyridazinone (1d).
methylated derivatives have been characterized by spec-
troscopy and X-ray diffraction. The systematic study of a
series of methylated derivatives was also aimed at under-
standing the atomic-scale mechanisms leading to the
changes of pharmaceutical properties of these compounds.
6-methoxy-5-methyl-3-pyridazinone (5) and 1,2,4-tri-
1
methyl-3,6-pyridazinedione (6) were obtained. In the H
NMR spectrum of compound 3 three signals of the methyl
groups were observed: at 2.19, 3.67 and 3.79 ppm the proton
signals of C-, N-, and O-methyl group, respectively. The 1H
NMR spectrum of compound 4 consists of C-methyl protons
signal at 2.22 ppm, and O-methyl protons signal at
1
3.81 ppm. In the H NMR spectrum of compound 5 the
2. Results and discussion
signal of the C-methyl group is at 2.15 ppm and the signal of
the O-methyl group is observed at 3.85 ppm. Owing to their
‘acidic’ hydrogen atoms compounds 4 and 5 predominantly
exist in two H-tautomeric forms. Compound 4 can transform
between the 6-methoxy-4-methyl-3-pyridazinone and 3-
hydroxy-6-methoxy-4-methylpyridazine. The X-ray diffrac-
tion showed that in the crystalline state compound 4 exists
exclusively in the lactam form. However, a broad signal at
about 11.56 ppm in the 1H NMR spectrum, characteristic of
hydroxyl group protons, indicates that a lactam–lactim
equilibrium of H-tautomers of 4 exists in the solution.
Similarly, both lactam and lactim forms have been observed
in the solution of compound 5. The wide signal at about
11.52 ppm in the 1H NMR spectrum of solution indicates a
2.1. Synthesis
The exhaustive methylations of tautomeric 6-hydroxy-5-
methyl-3-pyridazinone (1) and its N-phenyl derivative (2)
were carried out either (i) with excessive dimethyl sulfate in
20% sodium carbonate solution at room temperature, or (ii)
with neat excessive dimethyl sulfate at 140–150 8C. The
methylation of tautomeric compound 1 with dimethyl
sulfate at room temperature in 20% Na2CO3 yielded mixture
of four methylated products which were separated by
column chromatography (Scheme 2).
The isolated products were identified by 1H NMR
spectra and X-ray diffraction: 6-methoxy-2,4-dimethyl-3-
pyridazinone (3), 6-methoxy-4-methyl-3-pyridazinone (4),
1
lactam–lactim equilibrium. The broad signals in the H
Scheme 2. Methylation products of compound 1 with (CH3)2SO4: (i) at room temperature in 20% Na2CO3, and (ii) at 140–150 8C.

A. Katrusiak, A. Katrusiak / Journal of Molecular Structure 694 (2004) 85–90
87
Scheme 3. Methylation products of compound 2 with (CH3)2SO4: (i) at room temperature in 20% Na2CO3, and (ii) at 140–150 8C.
NMR spectrum disappeared after adding a drop of
deuterated water.
The methylation of compounds 1 and 2 with dimethyl
sulfate at room temperature in the aqueous solution lead
predominantly to the O-derivatives, whereas the methyl-
ation conducted on the crystal surface of 1 or 2 at higher
temperature (140–150 8C) and at the absence of water
favoured the methylation at the nitrogen atom. These
products are consistent with the presence of lactam groups
in the crystal structures of 1 and 2. It is thus plausible, that
these compounds in the solution at room temperature
transform into the lactim form. The lactim H-tautomer of
the substrates leads to the O-methyl derivatives, as main
products of the methylation reactions.
The methylation of 1 with neat dimethyl sulfate at 140–
150 8C yielded two dimethylated products: compounds 3
1
and 6 as the main products (Scheme 2). In the H NMR
spectrum of compound 6 signals of three methyl groups
appeared: at 2.19 ppm (C–CH3), 3.67 ppm (N–CH3), and
3.79 ppm (O–CH3), and a proton signal at 6.79 ppm.
Compounds 4 and 5 have been formed when only one
methyl group have been substituted at one of the oxygen
atoms. No products with one methyl group at the nitrogen
atom could be found. N-Methylated derivative of compound
1, 6-hydroxy-2,5-dimethyl-3-pyridazinone (9), has been
obtained from citraconic anhydride and methylhydrazine
and its structure confirmed by X-ray diffraction.
2.2. Crystal structure
The methylation of compound 2 with dimethyl sulfate
at 140–150 8C gave mainly one product, 1,5-dimethyl-2-
phenyl-3,6-pyridazinedione (7), and a trace amount of
The molecular structures of compounds 3, 4 and 9 have
been confirmed by X-ray diffraction — their ORTEP drawing
are shown in Figs. 1–3. These structures illustrate systematic
changes induced by methylation in the association of the
pyridazinederivatives. Namely, themethylsubstituentsblock
the H-donor groups, and in this way modify the associating
properties and interactions of the molecules. Thus in
compound 3, at the absence of the hydroxyl and N hydrogens,
the shortest of intermolecular contacts are comparable to the
sums of appropriate van der Waals interactions (Fig. 1).
The presence of both the H-donors in structure 4 leads to the
formations of NH· · ·O bonded dimers — the dimers in turn
interact by van der Waals interactions in the crystal lattice
(Fig. 2). At the presence of the hydroxyl H-donor only, the
molecules of 9 form linear OH· · ·O hydrogen-bonded chains;
and the N-atom substituted by the methyl prevents formation
of any other H-bonds. Finally, at the absence of the blocking
methyls in maleic hydrazide (MH) the OH· · ·O bonded linear
1
6-methoxy-5-methyl-2-phenyl-3-pyridazinone (8). The H
NMR spectrum of compound 7 consists of two signals of
methyl groups: at 2.23 ppm (C–CH3) and 3.21 ppm
(N–CH3), and of a signal of the phenyl protons at
7.32–7.54 ppm. When 2 was methylated with dimethyl
sulfate at room temperature in aqueous sodium-carbonate
solution, a mixture of compound 8, as the main reactions
product, and of compound 7 were observed. The 1H
NMR spectrum of compound 8 consists of the C-
methyl protons signal at 2.17 ppm, O-methyl protons
signal at 3.88 ppm and phenyl protons at 7.35–7.72 ppm
(Scheme 3).
In each 1H NMR spectrum a long-range coupling
between the C-methyl group and hydrogen atom at C-4(5)
has been observed.

88
A. Katrusiak, A. Katrusiak / Journal of Molecular Structure 694 (2004) 85–90
Fig. 1. A pair of symmetry-independent molecules of 3: the thermal ellipsoids have been drawn at 50% probability level. All potential hydrogen-bond H-donor
and H-acceptor groups are blocked by the methyl substituents, and no intermolecular contacts shorter than van der Waals interactions are present in this crystal
structure.
chains are linked by NH· · ·O hydrogen bonds into ribbons.
The crystal structures of MH, 3, 4, and 9 illustrate one of
possible mechanisms induced by the methyl substitution to
the properties of these compounds, and also other medicinal
drugs.
3. Experimental
3.1. X-Ray diffraction experiments
The crystal structures were measured using a KUMA-
˚
At the same time the methylation at N and O did not
transform significantly the molecular dimensions of the
pyridazine ring. The crystal structures of the methylated
derivatives of 1 revealed the interactions and angular
distortions of the pyridazine ring (Table 1) clearly
correspond to those in unsubstituted MH [5,6]. The
sequence of –N(1)–N(2)yC(3)–C(4)yC(5)–C(6)– endo-
cyclic bonds results in the endocyclic angles sharpened at
unsubstituted N(1), opened at substituted N(2), sharpened at
C(3), and opened at C(6).
CCD diffractometer, l ¼ 0:71073 A, solved with
SHELXS-93 [7] and refined with SHELXL-93 [8]. The
H-atoms were located from difference Fourier maps and
with isotropic thermal parameter. The final coordinates
have been deposited in the Cambridge Crystallo-
graphic Database Centre as supplementary publications
Nos. CCDC225328 (3), CCDC225327 (4), and
CCDC225326 (9). Copies can be obtained on request
from deposit.request@ccdc.cam.ac.uk. The experimental
details are listed below.
Fig. 2. A centrosymmetric dimer of hydrogen bonded molecules of 4. The thermal-vibrations ellipsoids have been drawn at 50% probability level, and the
˚
hydrogen bonds shown as the dashed lines. The hydrogen-bond dimensions are: O· · ·N 2.805(2) A, H· · ·O 1.87(1) A, and angle N–H· · ·O 176.7(7)8.
˚

A. Katrusiak, A. Katrusiak / Journal of Molecular Structure 694 (2004) 85–90
89
0
˚
Fig. 3. A three-member fragment of the OH· · ·O hydrogen-bonded linear chain of molecules of 9. The hydrogen-bond dimensions are O(6)· · ·O(3 ) 2.605(4) A,
˚
H(6)· · ·O(3 ) 1.64(4) A, and O(6)–H(6)· · ·O(3 ) 161(3)8.
0
0
Table 1
data R1 0.1693, wR2 0.1121, isotropic extinction coefficient
23
˚
0.0068(8); max/min DF:0:163=20:184 e A
Endocyclic valency angels describing deformations in the pyridazine rings
in MH, two symmetry-independent molecules of compounds 3 (denoted 3a
and 3b), 4, and 9
.
6-Methoxy-4-methyl-3-pyridazinone (4), C6H8N2O2,
ꢀ
Mr ¼ 140:14; space group P1; a ¼ 4:0730ð10Þ; b ¼
˚
MH
3a
3b
4
9
8:168ð2Þ; c ¼ 11:029ð2Þ A, a ¼ 107:98ð3Þ; b ¼ 100:11ð3Þ;
3
˚
g ¼ 96:44ð3Þ8; V ¼ 338:11ð13Þ A , Z ¼ 2; T ¼ 293ð2Þ K;
Dx ¼ 1:377; m ¼ 0:105 mm21, l ¼ 0:71073 A, numbers of
C(6)–N(1)–N(2) 115.4 115.2(2) 115.4(2) 115.05(11) 116.1(2)
C(3)–N(2)–N(1) 127.3 126.0(2) 126.2(2) 127.43(11) 125.6(2)
N(2)–C(3)–C(4) 115.0 115.6(2) 115.2(3) 115.48(11) 115.0(2)
C(5)–C(4)–C(3) 118.6 119.1(3) 119.2(3) 117.88(12) 122.0(3)
C(4)–C(5)–C(6) 118.5 119.4(3) 119.0(3) 120.24(12) 116.6(3)
N(1)–C(6)–C(5) 123.7 124.7(3) 124.9(2) 123.90(11) 124.7(2)
˚
reflections: total 3296, ðRint ¼ 0:0208Þ; independent 1707,
with I . 2sI 1371; ranges of Miller indices h; k; l : 23/5,
211/11, 215/14; isotropic extinction coefficient 0.053(14),
R factors: all data 0.0580, R factor for I . 2sI 0.0483, wR2
all 0.1085, wR2 for I . 2sI 0.1071, GOF 1.925; max/min
23
˚
DF 0.278/20.198 e A
6-Hydroxy-2,5-dimethyl-3-pyridazinone(9), C6H8N2O2,
.
6-Methoxy-2,4-dimethyl-3-pyridazinone (3), C7H10N2O2,
Mr ¼ 154:17; T ¼ 293ð2Þ K, space group P21/c, a ¼
Mr ¼ 140:14; P21=c; a ¼ 5:7430ð9Þ; b ¼ 16:082ð3Þ; c ¼
˚
3
17:400ð3Þ; b ¼ 12:230ð2Þ; c ¼ 7:2800ð10Þ A, b ¼ 92:50ð3Þ8;
˚
˚
7:0444ð11Þ A, b ¼ 103:411ð14Þ8; V ¼ 632:90ð10Þ A , Z ¼
3
3
21
,
4; T ¼ 100ð2Þ K, Dx ¼ 1:471 g cm23, m ¼ 0:113 mm21, l
˚
V¼1547:7ð4Þ A , Z¼8; Dx ¼1:323 g/cm , m¼0:099 mm
˚
ranges of Miller indices 223/21, 216/15, 24/9; numbers
of reflections: all 5685, unique 3323 ðRint ¼0:0516Þ; GOF
for F2 1.467, R indices ½I.2sIŠ R1 0.0757, wR2 0.0981, all
(Mo Ka) ¼ 0.71073 A, numbers of reflections: total 4072,
independent 1589 ðRint ¼ 0:0587Þ; I . 2sI ¼ 1003; Miller
indices 27/7, 221/21, 27/9; wR2 factor for all data
Table 2
Characterisation of compounds 3–9
Comp.
1H NMR (ppm, CDCl3)
m.p. (8C)
Yield (%)
3
4
5
6
7
8
9
2.19 (3H, C–CH3, d, J ¼ 1:373 Hz), 3.67 (3H, N–CH3, s), 3.79 (3H, O–
CH3, s), 6.79 (1H, C–5, q, J ¼ 1:373 Hz)
110
168
(i) 27, (ii) 23
(i) 37, (ii) –
(i) 34, (ii) –
(i) Traces, (ii) 68
(i) 37, (ii) 91
(i) 60, (ii) Traces
87
2.22 (3H, C–CH3, d, J ¼ 1:374 Hz), 3.81 (3H, O–CH3, s), 6.86 (1H, C–5,
q, J ¼ 1:374 Hz), 1.72, 11.56 (1H, NH, OH)
2.15 (3H, C–CH3, d, J ¼ 1:373 Hz), 3.85 (3H, O–CH3, s), 6.74 (1H, C–4,
q, J ¼ 1:373 Hz) 1.68, 11.52 (1H, NH, OH)
192
2.16 (3H, C–CH3, d, J ¼ 1:374 Hz), 3.62 (3H, N–CH3, s), 3.65 (3H, N–
CH3, s), 6.77 (1H, C–5, q, J ¼ 1:374 Hz)
84–85
90
2.23 (3H, C–CH3, d, J ¼ 1:373 Hz), 3.21 (3H, N–CH3, s), 6.85 (1H, C–4,
q, J ¼ 1:373 Hz), 7.40–7.54 (5H, H–C6H5, m)
2.17 (3H, C–CH3, d, J ¼ 1:374 Hz), 3.88 (3H, O–CH3, s), 6.82 (1H, C–4,
q, J ¼ 1:374 Hz), 7.35–7.72 (5H, H–C6H5, m)
65
2.03 (3H, C–CH3, d, J ¼ 1:373 Hz), 3.44 (3H, N–CH3, s), 6.74 (1H, C–4,
q, J ¼ 1:373 Hz), 11.22 (1H, OH)
265

90
A. Katrusiak, A. Katrusiak / Journal of Molecular Structure 694 (2004) 85–90
0.1568, wR2 for I . 2si 0.0840, wR1 for all data 0.0473,
wR1 for I . 2si 0.0348, GOF 1.275, max/min DF
The residue was separated by column chromatography.
(ii) with neat excessive dimethyl sulfate at 140–150 8C.
0.05 mole of a given 3-pyridazinone and 13 cm3 of
dimethyl sulfate heated at 140–150 8C on a silica bath
for 3 h. Tha reaction mixture was neutralized to pH ¼ 8
by 20% aq. Na2CO3. The residue was filtered, and the
filtrate extracted with chloroform. The extracts were
washed with water, dried (MgSO4) and evaporated. The
residue was separated by the column chromatography.
23
˚
0.247/20.208 e A
.
3.2. Chemical synthesis
Melting points were determined on a Boetius apparatus
and were uncorrected. 1H NMR spectra were recorded on a
Varian spectrometer at 300 MHz with TMS as the internal
standard. The reaction products were separated by a column
chromatography using as a mobile phase 3:2 acetone:
hexane mixture. The column (B2, length 50 cm) was filled
by Silica Gel (0.040–0.063 mm, 230–400 mesh ASTM,
Merck).
1
Chemical H NMR shifts, the melting points and the
yieldsof compounds 3–9 in the discussedreactions have
been listed in Table 2.
6-Hydroxy-5-methyl-3-pyridazinone (1), and 6-hydroxy-
5-methyl-2-phenyl-3-pyridazinone (2) had been prepared by
standard techniques from citraconic anhydride and hydra-
zine hydrate or phenylhydrazine respectively [9,10].
Similarly, 6-hydroxy-2,5-dimethyl-3-pyridazinone (9) had
been synthesized from citraconic anhydride and methyl
hydrazine.
References
[1] A. Katrusiak, A. Katrusiak, Org. Lett. 5 (2003) 1903–1905.
[2] L.P. Graham, An Introduction to Medicinal Chemistry, Oxford
University Press, Oxford, 2001, p. 241.
[3] L.N. Short, H.W. Thompson, J. Chem. Soc. (1952) 168–187.
[4] A.R. Katritzky, F.D. Popp, A.J. Waring, J. Chem. Soc. B (1966)
565–568.
General methods of methylation:
[5] A. Katrusiak, Acta Crystallogr. C 49 (1993) 36–39.
[6] A. Katrusiak, Acta Crystallogr. B 57 (2001) 697–704.
[7] G. Sheldrick, SHELXS-93, Programme for Crystal Structure Determi-
nation, University of Goettingen, Goettingen, 1993.
[8] G. Sheldrick, SHELXS-93, Programme for Crystal Structure Refine-
ment, University of Goettingen, Goettingen, 1993.
(i) with dimethyl sulfate in 20% aqueous sodium carbonate
solution 0.05 mole of a given 3-pyridazinone, 30,0 cm3
of 20% aq. Na2CO3 and 13 cm3 of dimethyl sulfate were
stirred at room temperature for 2 h. The reaction mixture
was extracted with chloroform. The extracts were
washed with water, dried (MgSO4) and evaporated.
[9] R.H. Mizzoni, P.E. Spoerri, J. Am. Chem. Soc. 76 (1954) 2201.
[10] J. Druey, A. Hu¨ni, Meier Kd, Ringier B.H., Stachelin A., Helv. Chim.
Acta 37 (1954) 510.