organic compounds\(\def\hfill{\hskip 5em}\def\hfil{\hskip 3em}\def\eqno#1{\hfil {#1}}\)

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ISSN: 2056-9890

3-Amino-5-methyl-5-(4-pyrid­yl)hydantoin

aInstitute of Inorganic Chemistry, University of Vienna, Waehringerstrasse 42, A-1090 Vienna, Austria, bDepartment of Organic Chemistry, Faculty of Pharmacy, Medical University – Sofia, 2 Dunav Str., 1000 Sofia, Bulgaria, and cDepartment of Chemistry, Faculty of Pharmacy, Medical University – Sofia, 2 Dunav Str., 1000 Sofia, Bulgaria
*Correspondence e-mail: hristo.varbanov@univie.ac.at

(Received 23 March 2009; accepted 27 March 2009; online 2 April 2009)

The title compound, 3-amino-5-methyl-5-(4-pyrid­yl)imid­azol­idine-2,4-dione, C9H10N4O2, was obtained by reaction of 5-methyl-5-(4-pyrid­yl)hydantoin with hydrazine. It crystallizes as a racemate in the tetra­gonal space group I41/a with one mol­ecule in the asymmetric unit. The dihedral angle between the pyridine ring and the five-membered hydantoin ring is 47.99 (3)° In the crystal structure, mol­ecules are joined in a three-dimensional hydrogen-bonded network by N—H⋯N and N—H⋯O links.

Related literature

For the biological activity of hydantoin derivatives and their metal complexes, see: Rajic et al. (2006[Rajic, Z., Zorc, B., Raic-Malic, S., Ester, K., Kralij, M., Pavelic, K., Balzarini, J., De Clercq, E. & Mintas, M. (2006). Molecules, 11, 837-848.]); Bazil et al. (1998[Bazil, C. & Pedley, T. (1998). Annu. Rev. Med. 49, 135-162.]); Bakalova et al. (2005[Bakalova, A., Buyukliev, R., Momekov, G., Ivanov, D., Todorov, D., Konstantinov, S. & Karaivanova, M. (2005). Eur. J. Med. Chem. 40, 590-596.], 2008[Bakalova, A., Varbanov, H., Buyukliev, R., Momekov, G., Ferdinandov, D., Konstantinov, S. & Ivanov, D. (2008). Eur. J. Med. Chem. 43, 958-965.], 2009[Bakalova, A., Varbanov, H., Buyukliev, R., Momekov, G. & Ivanov, D. (2009). J. Therm. Anal. Calorim. 95, 241-246.]). For crystal structures of other 3-amino substituted hydantoins and their metal complexes, see: Shivachev et al. (2005[Shivachev, B., Petrova, R. & Naydenova, E. (2005). Acta Cryst. C61, o524-o526.]); Bakalova et al. (2007[Bakalova, A., Petrova, R., Shivatchev, B. & Varbanov, H. (2007). J. Coord. Chem. 60, 1701-1707.]). For the synthesis of 5-methyl-5-(4-pyridyl)-hydantoin, see: Chu & Teague (1958[Chu, C.-C. & Teague, P. C. (1958). J. Org. Chem. 23, 1578.]). For the preparation of 3-amino­hydantoins, see: Davidson (1964[Davidson, J. (1964). J. Chem. Soc. pp. 4646-4656.]).

[Scheme 1]

Experimental

Crystal data
  • C9H10N4O2

  • Mr = 206.21

  • Tetragonal, I 41 /a

  • a = 12.8282 (5) Å

  • c = 22.9016 (17) Å

  • V = 3768.8 (3) Å3

  • Z = 16

  • Mo Kα radiation

  • μ = 0.11 mm−1

  • T = 100 K

  • 0.50 × 0.50 × 0.50 mm

Data collection
  • Bruker X8 APEXII CCD diffractometer

  • Absorption correction: multi-scan (SADABS; Bruker, 2005[Bruker (2005). APEX2, SAINT and SADABS. Bruker AXS Inc., Madison, Wisconsin, USA.]) Tmin = 0.948, Tmax = 0.948

  • 51879 measured reflections

  • 2765 independent reflections

  • 2179 reflections with I > 2σ(I)

  • Rint = 0.076

Refinement
  • R[F2 > 2σ(F2)] = 0.046

  • wR(F2) = 0.127

  • S = 1.03

  • 2765 reflections

  • 144 parameters

  • H atoms treated by a mixture of independent and constrained refinement

  • Δρmax = 0.56 e Å−3

  • Δρmin = −0.44 e Å−3

Table 1
Hydrogen-bond geometry (Å, °)

D—H⋯A D—H H⋯A DA D—H⋯A
N2—H2⋯N4i 0.88 2.69 3.526 159
N2—H2⋯O2i 0.88 2.26 2.9184 (15) 131
N4—H4A⋯O1ii 0.977 (19) 2.139 (19) 3.0676 (16) 158.07
N4—H4B⋯O1iii 0.96 (2) 2.17 (2) 3.1003 (16) 162.23
Symmetry codes: (i) [-y+{\script{5\over 4}}, x-{\script{1\over 4}}, z-{\script{1\over 4}}]; (ii) [y-{\script{1\over 4}}, -x+{\script{5\over 4}}, -z+{\script{1\over 4}}]; (iii) [-y+{\script{5\over 4}}, x+{\script{1\over 4}}, -z+{\script{1\over 4}}].

Data collection: APEX2 (Bruker, 2005[Bruker (2005). APEX2, SAINT and SADABS. Bruker AXS Inc., Madison, Wisconsin, USA.]); cell refinement: SAINT (Bruker, 2005[Bruker (2005). APEX2, SAINT and SADABS. Bruker AXS Inc., Madison, Wisconsin, USA.]); data reduction: SAINT; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008[Sheldrick, G. M. (2008). Acta Cryst. A64, 112-122.]); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008[Sheldrick, G. M. (2008). Acta Cryst. A64, 112-122.]); molecular graphics: SHELXTL (Sheldrick, 2008[Sheldrick, G. M. (2008). Acta Cryst. A64, 112-122.]); software used to prepare material for publication: SHELXTL.

Supporting information


Comment top

Some hydantoin derivatives are biologically active molecules with anticonvulsive, antiarythmic, antimicrobial, antiviral or cytostatic activitiy (Rajic et al., 2006; Bazil et al., 1998). 5-Methyl-5-(4-pyridyl)hydantoin was synthesized by Chu (Chu et al. 1958). This compound was used as starting material for preparation of new 3-amino-5-methyl-5-(4-pyridyl)hydantoin (AMPH). These hydantoin derivatives were utilized as carrier ligands for synthesis of new platinum and palladium complexes with potential cytotoxic activity. (Bakalova et al., 2008, 2009). In the recent work we synthesized AMPH (I) by the method of Davidson (Davidson, 1964) with some modifications. The new compound was characterized by elemental analysis, IR, 1H and 13C NMR spectroscopy and molar conductivity. Suitable crystals of AMPH for X-ray diffraction analysis have been isolated and its structure was determined. The result of X-ray diffraction study of 3-amino-5-methyl-5-(4-pyridyl)hydantoin is shown in Fig. 1. The racemic compound crystallizes in the tetragonal space group I41/a with one molecule in the asymmetric unit. The presence of the sp3-hybridized chiral carbon atom C1 is responsible for the dihedral angle between the pyridine ring and five-membered C3N2 ring of ca 48°. The sum of the angles around N4 is clearly smaller than 360° (327.0°), indicating its trigonal-pyramidal configuration. The lone-pair region at N4 is directed towards adjacent atom O1. The secondary amine nitrogen N2 acts as a proton donor in an intermolecular bifurcated hydrogen bonding interactions with the nitrogen atom N4 and oxygen atom O2 of the neighbouring molecule of AMPH (Fig. 2) The hydrazinic atom N4 is involved in two intermolecular H bonds with the atoms O1ii and O1iii of the two different neighbouring molecules (Table 1).

Related literature top

For the biological activity of hydantoin derivatives and their metal complexes, see: Rajic et al. (2006); Bazil et al. (1998); Bakalova et al. (2005, 2008, 2009). For crystal structures of other 3-amino substituted hydantoins and their metal complexes, see: Shivachev et al. (2005); Bakalova et al. (2007). For the preparation of 3-amino-5-methyl-5-(4-pyridyl)hydantoin, see: Chu & Teague (1958). For the synthesis, see: Davidson (1964).

Experimental top

3-Amino-5-methyl-5-(4-pyridyl)hydantoin was synthesized by dissolving 5-methyl-5(4-pyridyl)hydantoin (1.91 g, 10 mmol) in 98% N2H4.H2O (5 cm3) and refluxing the solution for 2 h. The reaction mixture was cooled to room temperature and water (15 cm3) was added. The solution was placed in refrigerator for 24 h. The white product was filtered off, recrystallized from ethanol and dried at 373 K for 5 h. The purity was checked by TLC. The substance is soluble in DMSO and weakly soluble in water and ethanol. Yield: 1.26 g, 61%, m.p. = 523.2–524.7 K. Crystals, suitable for X-ray data collection were grown by slow evaporation from ethanol solution at 277 K. Analysis calculated for C9H10N4O2: C 52.42, H 4.89, N 27.17%. Found: C 52.23, H 4.46, N 26.83%. λM = 0.979 S.cm2.mol-1; IR(pellets KBr)/cm-1: 3314.0, 3276.0, 1766.9, 1713.0, 1597.2 and 1411.1. 1H NMR (250 MHz; DMSO-d6): 8.95 (1H, s, N(1)—H), 8.59 (2H, d, J=7 Hz, H-2, H-6), 7.50 (2H, d, J=7 Hz, H-3, H-5), 4.80 (2H, s, NH2), 1.66 (3H, s, CH3). 13C NMR (62.5 MHz; DMSO-d6): 172.7 (C=O-4`), 155.5 (C=O-2`), 150.0 (C-2,C-6), 148.4 (C-4), 120.6 (C-3, C-5), 60.8 (C-5`), 24.7 (CH3).

Refinement top

H atoms were placed at calculated positions [N—H = 0.88 Å, C—H = 0.95 and 0.98 Å] and refined as riding atoms in the subsequent least squares model refinements, except two hydrogen atoms at N4 which were localized from difference map. The isotropic thermal parameters of hydrogen atoms in the positions of which were calculated were estimated to be 1.2 or 1.5 times the values of the equivalent isotropic thermal parameters of the atoms to which H atoms were bonded.

Computing details top

Data collection: APEX2 (Bruker, 2005); cell refinement: SAINT (Bruker, 2005); data reduction: SAINT (Bruker, 2005); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: SHELXTL (Sheldrick, 2008); software used to prepare material for publication: SHELXTL (Sheldrick, 2008).

Figures top
[Figure 1] Fig. 1. View of the molecule of AMPH with atom labeling scheme; the thermal ellipsoids are drawn at 50% probability level.
[Figure 2] Fig. 2. Fragment of the crystal structure of AMPH showing the intermolecular hydrogen bonding interactions. [Symmetry codes: (i) –y +1.25, x – 0.25, z – 0.25; (ii) y – 0.25, –x + 1.25, –z + 0.25; (iii) –y +1.25, x + 0.25, –z + 0.25].
3-amino-5-methyl-5-(4-pyridyl)imidazolidene-2,4-dione top
Crystal data top
C9H10N4O2Dx = 1.454 Mg m3
Mr = 206.21Mo Kα radiation, λ = 0.71073 Å
Tetragonal, I41/aCell parameters from 847 reflections
Hall symbol: -I 4adθ = 2.9–29.5°
a = 12.8282 (5) ŵ = 0.11 mm1
c = 22.9016 (17) ÅT = 100 K
V = 3768.8 (3) Å3Block, colourless
Z = 160.50 × 0.50 × 0.50 mm
F(000) = 1728
Data collection top
Bruker X8 APEXII CCD
diffractometer
2765 independent reflections
Radiation source: fine-focus sealed tube2179 reflections with I > 2σ(I)
Graphite monochromatorRint = 0.076
ω scansθmax = 30.1°, θmin = 2.9°
Absorption correction: multi-scan
(SADABS; Bruker, 2005)
h = 1818
Tmin = 0.948, Tmax = 0.948k = 1818
51879 measured reflectionsl = 3232
Refinement top
Refinement on F2Primary atom site location: structure-invariant direct methods
Least-squares matrix: fullSecondary atom site location: difference Fourier map
R[F2 > 2σ(F2)] = 0.046Hydrogen site location: inferred from neighbouring sites
wR(F2) = 0.127H atoms treated by a mixture of independent and constrained refinement
S = 1.03 w = 1/[σ2(Fo2) + (0.0631P)2 + 3.9566P]
where P = (Fo2 + 2Fc2)/3
2765 reflections(Δ/σ)max < 0.001
144 parametersΔρmax = 0.56 e Å3
0 restraintsΔρmin = 0.44 e Å3
Crystal data top
C9H10N4O2Z = 16
Mr = 206.21Mo Kα radiation
Tetragonal, I41/aµ = 0.11 mm1
a = 12.8282 (5) ÅT = 100 K
c = 22.9016 (17) Å0.50 × 0.50 × 0.50 mm
V = 3768.8 (3) Å3
Data collection top
Bruker X8 APEXII CCD
diffractometer
2765 independent reflections
Absorption correction: multi-scan
(SADABS; Bruker, 2005)
2179 reflections with I > 2σ(I)
Tmin = 0.948, Tmax = 0.948Rint = 0.076
51879 measured reflections
Refinement top
R[F2 > 2σ(F2)] = 0.0460 restraints
wR(F2) = 0.127H atoms treated by a mixture of independent and constrained refinement
S = 1.03Δρmax = 0.56 e Å3
2765 reflectionsΔρmin = 0.44 e Å3
144 parameters
Special details top

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Refinement. Refinement of F^2^ against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F^2^, conventional R-factors R are based on F, with F set to zero for negative F^2^. The threshold expression of F^2^ > σ(F^2^) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F^2^ are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top
xyzUiso*/Ueq
O10.59104 (8)0.59567 (7)0.06315 (4)0.0171 (2)
O20.68145 (8)0.42948 (8)0.23252 (4)0.0203 (2)
N10.58287 (9)0.06249 (9)0.13055 (5)0.0176 (2)
N20.67304 (9)0.43740 (9)0.07949 (5)0.0150 (2)
H20.68200.41720.04310.018*
N30.62479 (8)0.53105 (8)0.15592 (5)0.0128 (2)
N40.57993 (10)0.61113 (9)0.18907 (5)0.0181 (2)
H4A0.5062 (15)0.6163 (15)0.1787 (8)0.027 (5)*
H4B0.6100 (16)0.6781 (17)0.1806 (9)0.036 (5)*
C10.70691 (10)0.37686 (10)0.12997 (5)0.0136 (3)
C20.65758 (10)0.26869 (10)0.13127 (5)0.0131 (2)
C30.63039 (11)0.21921 (10)0.07925 (6)0.0165 (3)
H30.63650.25470.04300.020*
C40.59419 (11)0.11717 (11)0.08121 (6)0.0178 (3)
H40.57640.08430.04540.021*
C50.60863 (11)0.11133 (11)0.18034 (6)0.0179 (3)
H50.60080.07420.21600.021*
C60.64606 (11)0.21278 (10)0.18292 (6)0.0165 (3)
H60.66350.24350.21940.020*
C70.82617 (11)0.36654 (11)0.13212 (7)0.0201 (3)
H7A0.85780.43600.13120.030*
H7B0.85020.32620.09840.030*
H7C0.84660.33080.16820.030*
C80.67014 (10)0.44623 (10)0.18059 (6)0.0139 (3)
C90.62667 (10)0.52740 (10)0.09462 (5)0.0132 (3)
Atomic displacement parameters (Å2) top
U11U22U33U12U13U23
O10.0197 (5)0.0162 (5)0.0154 (5)0.0016 (4)0.0004 (3)0.0034 (3)
O20.0299 (6)0.0178 (5)0.0131 (5)0.0007 (4)0.0040 (4)0.0002 (4)
N10.0152 (5)0.0153 (5)0.0223 (6)0.0003 (4)0.0004 (4)0.0004 (4)
N20.0195 (5)0.0145 (5)0.0109 (5)0.0020 (4)0.0019 (4)0.0013 (4)
N30.0136 (5)0.0132 (5)0.0117 (5)0.0002 (4)0.0011 (4)0.0005 (4)
N40.0194 (6)0.0169 (6)0.0180 (6)0.0016 (4)0.0023 (4)0.0030 (4)
C10.0153 (6)0.0130 (6)0.0126 (6)0.0006 (4)0.0000 (4)0.0010 (4)
C20.0114 (5)0.0127 (6)0.0151 (6)0.0014 (4)0.0006 (4)0.0005 (4)
C30.0177 (6)0.0172 (6)0.0146 (6)0.0010 (5)0.0012 (5)0.0008 (5)
C40.0178 (6)0.0172 (6)0.0183 (6)0.0000 (5)0.0019 (5)0.0027 (5)
C50.0193 (6)0.0162 (6)0.0182 (6)0.0008 (5)0.0013 (5)0.0031 (5)
C60.0189 (6)0.0160 (6)0.0145 (6)0.0008 (5)0.0005 (5)0.0006 (5)
C70.0140 (6)0.0175 (6)0.0286 (7)0.0002 (5)0.0006 (5)0.0027 (5)
C80.0140 (6)0.0134 (6)0.0143 (6)0.0025 (4)0.0013 (4)0.0001 (4)
C90.0120 (6)0.0151 (6)0.0126 (6)0.0019 (5)0.0011 (4)0.0004 (4)
Geometric parameters (Å, º) top
O1—C91.2229 (16)C1—C81.5355 (18)
O2—C81.2174 (16)C1—C71.5364 (19)
N1—C41.3378 (18)C2—C61.3913 (18)
N1—C51.3425 (18)C2—C31.3942 (18)
N2—C91.3442 (17)C3—C41.3897 (19)
N2—C11.4589 (16)C3—H30.9500
N2—H20.8800C4—H40.9500
N3—C81.3571 (17)C5—C61.3884 (19)
N3—N41.4010 (16)C5—H50.9500
N3—C91.4047 (17)C6—H60.9500
N4—H4A0.978 (19)C7—H7A0.9800
N4—H4B0.96 (2)C7—H7B0.9800
C1—C21.5254 (18)C7—H7C0.9800
C4—N1—C5116.49 (12)N1—C4—C3123.89 (13)
C9—N2—C1112.63 (11)N1—C4—H4118.1
C9—N2—H2123.7C3—C4—H4118.1
C1—N2—H2123.7N1—C5—C6123.97 (13)
C8—N3—N4122.57 (11)N1—C5—H5118.0
C8—N3—C9112.46 (10)C6—C5—H5118.0
N4—N3—C9124.96 (11)C5—C6—C2118.94 (12)
N3—N4—H4A108.4 (11)C5—C6—H6120.5
N3—N4—H4B112.4 (12)C2—C6—H6120.5
H4A—N4—H4B106.2 (17)C1—C7—H7A109.5
N2—C1—C2112.09 (10)C1—C7—H7B109.5
N2—C1—C8101.43 (10)H7A—C7—H7B109.5
C2—C1—C8112.65 (10)C1—C7—H7C109.5
N2—C1—C7111.58 (11)H7A—C7—H7C109.5
C2—C1—C7109.52 (11)H7B—C7—H7C109.5
C8—C1—C7109.37 (11)O2—C8—N3126.83 (12)
C6—C2—C3117.73 (12)O2—C8—C1126.77 (12)
C6—C2—C1121.97 (11)N3—C8—C1106.38 (11)
C3—C2—C1120.09 (11)O1—C9—N2128.95 (12)
C4—C3—C2119.00 (12)O1—C9—N3123.97 (12)
C4—C3—H3120.5N2—C9—N3107.08 (11)
C2—C3—H3120.5
C9—N2—C1—C2121.59 (12)N4—N3—C8—O22.7 (2)
C9—N2—C1—C81.19 (14)C9—N3—C8—O2178.47 (13)
C9—N2—C1—C7115.16 (12)N4—N3—C8—C1178.74 (11)
N2—C1—C2—C6155.89 (12)C9—N3—C8—C10.11 (14)
C8—C1—C2—C642.24 (17)N2—C1—C8—O2179.19 (13)
C7—C1—C2—C679.71 (15)C2—C1—C8—O260.81 (17)
N2—C1—C2—C329.50 (16)C7—C1—C8—O261.23 (17)
C8—C1—C2—C3143.15 (12)N2—C1—C8—N30.61 (13)
C7—C1—C2—C394.90 (14)C2—C1—C8—N3120.62 (11)
C6—C2—C3—C40.49 (19)C7—C1—C8—N3117.35 (12)
C1—C2—C3—C4174.35 (12)C1—N2—C9—O1179.01 (13)
C5—N1—C4—C30.0 (2)C1—N2—C9—N31.30 (14)
C2—C3—C4—N10.4 (2)C8—N3—C9—O1179.43 (12)
C4—N1—C5—C60.4 (2)N4—N3—C9—O11.8 (2)
N1—C5—C6—C20.4 (2)C8—N3—C9—N20.86 (14)
C3—C2—C6—C50.1 (2)N4—N3—C9—N2177.95 (11)
C1—C2—C6—C5174.61 (12)
Hydrogen-bond geometry (Å, º) top
D—H···AD—HH···AD···AD—H···A
N2—H2···O2i0.882.262.9184 (15)131
N2—H2···N4i0.882.693.526159
N4—H4A···O1ii0.977 (19)2.139 (19)3.0676 (16)158.07
N4—H4B···O1iii0.96 (2)2.17 (2)3.1003 (16)162.23
Symmetry codes: (i) y+5/4, x1/4, z1/4; (ii) y1/4, x+5/4, z+1/4; (iii) y+5/4, x+1/4, z+1/4.

Experimental details

Crystal data
Chemical formulaC9H10N4O2
Mr206.21
Crystal system, space groupTetragonal, I41/a
Temperature (K)100
a, c (Å)12.8282 (5), 22.9016 (17)
V3)3768.8 (3)
Z16
Radiation typeMo Kα
µ (mm1)0.11
Crystal size (mm)0.50 × 0.50 × 0.50
Data collection
DiffractometerBruker X8 APEXII CCD
diffractometer
Absorption correctionMulti-scan
(SADABS; Bruker, 2005)
Tmin, Tmax0.948, 0.948
No. of measured, independent and
observed [I > 2σ(I)] reflections
51879, 2765, 2179
Rint0.076
(sin θ/λ)max1)0.705
Refinement
R[F2 > 2σ(F2)], wR(F2), S 0.046, 0.127, 1.03
No. of reflections2765
No. of parameters144
H-atom treatmentH atoms treated by a mixture of independent and constrained refinement
Δρmax, Δρmin (e Å3)0.56, 0.44

Computer programs: APEX2 (Bruker, 2005), SAINT (Bruker, 2005), SHELXS97 (Sheldrick, 2008), SHELXL97 (Sheldrick, 2008), SHELXTL (Sheldrick, 2008).

Hydrogen-bond geometry (Å, º) top
D—H···AD—HH···AD···AD—H···A
N2—H2···O2i0.8802.2622.9184 (15)131.26
N2—H2···N4i0.8802.6903.526159.11
N4—H4A···O1ii0.977 (19)2.139 (19)3.0676 (16)158.07
N4—H4B···O1iii0.96 (2)2.17 (2)3.1003 (16)162.23
Symmetry codes: (i) y+5/4, x1/4, z1/4; (ii) y1/4, x+5/4, z+1/4; (iii) y+5/4, x+1/4, z+1/4.
 

Acknowledgements

The authors are indebted to Professor V. Arion of the Institute of Inorganic Chemistry of the University of Vienna for discussions about the X-ray data.

References

First citationBakalova, A., Buyukliev, R., Momekov, G., Ivanov, D., Todorov, D., Konstantinov, S. & Karaivanova, M. (2005). Eur. J. Med. Chem. 40, 590–596.  Web of Science CrossRef PubMed CAS Google Scholar
First citationBakalova, A., Petrova, R., Shivatchev, B. & Varbanov, H. (2007). J. Coord. Chem. 60, 1701–1707.  Web of Science CSD CrossRef CAS Google Scholar
First citationBakalova, A., Varbanov, H., Buyukliev, R., Momekov, G., Ferdinandov, D., Konstantinov, S. & Ivanov, D. (2008). Eur. J. Med. Chem. 43, 958–965.  Web of Science CrossRef PubMed CAS Google Scholar
First citationBakalova, A., Varbanov, H., Buyukliev, R., Momekov, G. & Ivanov, D. (2009). J. Therm. Anal. Calorim. 95, 241–246.  Web of Science CrossRef CAS Google Scholar
First citationBazil, C. & Pedley, T. (1998). Annu. Rev. Med. 49, 135–162.  Web of Science CAS PubMed Google Scholar
First citationBruker (2005). APEX2, SAINT and SADABS. Bruker AXS Inc., Madison, Wisconsin, USA.  Google Scholar
First citationChu, C.-C. & Teague, P. C. (1958). J. Org. Chem. 23, 1578.  CrossRef Web of Science Google Scholar
First citationDavidson, J. (1964). J. Chem. Soc. pp. 4646–4656.  Google Scholar
First citationRajic, Z., Zorc, B., Raic-Malic, S., Ester, K., Kralij, M., Pavelic, K., Balzarini, J., De Clercq, E. & Mintas, M. (2006). Molecules, 11, 837–848.  Web of Science CrossRef PubMed CAS Google Scholar
First citationSheldrick, G. M. (2008). Acta Cryst. A64, 112–122.  Web of Science CrossRef CAS IUCr Journals Google Scholar
First citationShivachev, B., Petrova, R. & Naydenova, E. (2005). Acta Cryst. C61, o524–o526.  Web of Science CSD CrossRef CAS IUCr Journals Google Scholar

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