Crystal structure of 6-chloro-5-iso­propyl­pyrimidine-2,4(1H,3H)-dione

Haress, N.G.; Ghabbour, H.A.; El-Emam, A.A.; Chidan Kumar, C.S.; Fun, H.-K.

doi:10.1107/S1600536814021382

organic compounds

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ISSN: 2056-9890

Volume 70| Part 11| November 2014| Pages o1144-o1145

doi:10.1107/S1600536814021382

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Crystal structure of 6-chloro-5-isopropylpyrimidine-2,4(1H,3H)-dione

Nadia G. Haress,^a Hazem A. Ghabbour,^a Ali A. El-Emam,^a,^b ‡ C. S. Chidan Kumar ^c § and Hoong-Kun Fun ^a,^c ^*¶

^aDepartment of Pharmaceutical Chemistry, College of Pharmacy, King Saud University, PO Box 2457, Riaydh 11451, Saudi Arabia, ^bKing Abdullah Institute for Nanotechnology (KAIN), King Saud University, Riyadh 11451, Saudi Arabia, and ^cX-ray Crystallography Unit, School of Physics, Universiti Sains Malaysia, 11800 USM, Penang, Malaysia
^*Correspondence e-mail: hfun.c@ksu.edu.sa

Edited by J. Simpson, University of Otago, New Zealand (Received 23 September 2014; accepted 26 September 2014; online 4 October 2014)

In the molecule of the title compound, C₇H₉ClN₂O₂, the conformation is determined by intramolecular C—H⋯O and C—H⋯Cl hydrogen bonds, which generate S(6) and S(5) ring motifs. The isopropyl group is almost perpendicular to the pyrimidine ring with torsion angles of −70.8 (3) and 56.0 (3)°. In the crystal, two inversion-related molecules are linked via a pair of N—H⋯O hydrogen bonds into R₂²(8) dimers; these dimers are connected into chains extending along the bc plane via an additional N—H⋯O hydrogen bond and weaker C—H⋯O hydrogen bonds. The crystal structure is further stabilized by a weak π–π interaction [3.6465 (10) Å] between adjacent pyrimidine-dione rings arranged in a head-to-tail fashion, producing a three-dimensional network.

Keywords: crystal structure; pyrimidine-2,4-dione; hydrogen bonds; π–π interaction.

CCDC reference: 1026350

1. Related literature

For the biological activity of pyrimidine-2,4(1H,3H)-diones, see: Miyasaka et al. (1989 ); Tanaka et al. (1995 ); Hopkins et al. (1996 ); El-Brollosy et al. (2009 ); Klein et al. (2001 ); Nencka et al. (2006 ); El-Emam et al. (2004 ). For the use of 5-alkyl-6-chloropyrimidine-2,4(1H,3H)-diones in synthesis, see: El-Emam et al. (2004). For related pyrimidine-2,4-dione structures, see: El-Brollosy et al. (2011 ); Al-Omary et al. (2014 ); Haress et al. (2014 ). For the synthesis of the title compound, see: Al-Turkistani et al. (2011 ); Koroniak et al. (1993 ). For hydrogen-bond motifs, see: Bernstein et al. (1995 ).

2. Experimental

2.1. Crystal data

C₇H₉ClN₂O₂
M_r = 188.61
Monoclinic, P 2₁ /c
a = 11.2244 (4) Å
b = 6.8288 (3) Å
c = 11.6641 (5) Å
β = 104.577 (2)°
V = 865.26 (6) Å³
Z = 4
Cu Kα radiation
μ = 3.62 mm⁻¹
T = 296 K
0.45 × 0.28 × 0.26 mm

2.2. Data collection

Bruker APEXII CCD diffractometer
Absorption correction: multi-scan (SADABS; Bruker, 2009 ) T_min = 0.292, T_max = 0.458
5647 measured reflections
1553 independent reflections
1444 reflections with I > 2σ(I)
R_int = 0.027

2.3. Refinement

R[F² > 2σ(F²)] = 0.040
wR(F²) = 0.114
S = 1.06
1553 reflections
122 parameters
H atoms treated by a mixture of independent and constrained refinement
Δρ_max = 0.27 e Å⁻³
Δρ_min = −0.32 e Å⁻³

Table 1
Hydrogen-bond geometry (Å, °)

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
N1—H1N1⋯O1ⁱ	0.83 (3)	2.01 (3)	2.833 (2)	169 (2)
N2—H1N2⋯O2ⁱⁱ	0.83 (3)	2.03 (3)	2.854 (2)	171 (2)
C5—H5⋯Cl1	0.94 (3)	2.57 (2)	3.132 (2)	118.7 (17)
C6—H6A⋯O1ⁱⁱⁱ	0.96	2.56	3.455 (3)	156
C6—H6B⋯O2	0.96	2.45	3.034 (3)	119
Symmetry codes: (i) $[-x+1, y+{\script{1\over 2}}, -z+{\script{1\over 2}}]$ ; (ii) -x+1, -y, -z+1; (iii) -x+1, -y+1, -z+1.

Data collection: APEX2 (Bruker, 2009); cell refinement: SAINT (Bruker, 2009); data reduction: SAINT; 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 and PLATON (Spek, 2009 ).

Supporting information

CCDC reference: 1026350

Crystal structure: contains datablocks global, I. DOI: 10.1107/S1600536814021382/sj5429sup1.cif

Structure factors: contains datablock I. DOI: 10.1107/S1600536814021382/sj5429Isup2.hkl

Supporting information file. DOI: 10.1107/S1600536814021382/sj5429Isup3.cml

checkCIF report

Comment top

Pyrimidine-2,4-diones and their related derivatives have long been known for their diverse chemotherapeutic activities including antiviral activity against HIV (Miyasaka et al., 1989; Tanaka et al., 1995; Hopkins et al., 1996; El-Emam et al., 2004). In addition, potent anticancer activity was observed for several pyrimidine-2,4-diones (Klein et al., 2001; Nencka et al., 2006). In a continuation of our interest in the chemical and pharmacological properties of pyrimidine and uracil derivatives (Al-Omary et al., 2014; Haress et al., 2014, El-Brollosy et al., 2009), we have synthesized the title compound (I) as a precursor to the synthesis of a potential chemotherapeutic agent (Al-Turkistani et al., 2011).

In the title compound (Fig. 1), the molecular conformation is stabilized by intramolecular C6–H6B···O2 and C5–H5···Cl1 hydrogen bonds incorporating S(6) and S(5) ring motifs respectively (Bernstein et al., 1995). The isopropyl group is almost perpendicular to the N1/N2/C1–C4 ring with the C3–C4–C5–C7 and C3–C4–C5–C6 torsion angles of -70.8 (3)° and 56.0 (3)° respectively. In the crystal structure, two adjacent molecules are linked via a pair of N2–H1N2···O2 intermolecular hydrogen bonds forming inversion related R₂²(8) dimers (Fig. 2).; these dimers are connected into chains via N1–H1N1···O1 and weak C6–H6A···O1 hydrogen bonds extending along the bc plane. The crystal structure is further stabilized by a weak π···π interaction [3.6465 (10) Å] producing a three-dimensional network.

Related literature top

For the biological activity of pyrimidine-2,4(1H,3H)-diones, see: Miyasaka et al. (1989); Tanaka et al. (1995); Hopkins et al. (1996); El-Brollosy et al. (2009); Klein et al. (2001); Nencka et al. (2006); El-Emam et al. (2004). For the use of 5-alkyl-6-chloropyrimidine-2,4(1H,3H)-diones in synthesis, see: El-Emam et al. (2004). For related pyrimidine-2,4-dione structures, see: El-Brollosy et al. (2011); Al-Omary et al. (2014); Haress et al. (2014), For the synthesis of the title compound, see: Al-Turkistani et al. (2011); Koroniak et al. (1993). For hydrogen-bond motifs, see: Bernstein et al. (1995).

Experimental top

5-Isopropylbarbituric acid (8.51 g, 0.05 mol) was added portionwise with stirring to a mixture of phosphorus oxychloride (19.2 ml) and N,N-dimethyl aniline (10.3 ml) over a period of 10 minutes. The mixture was then heated under reflux for one hour. On cooling, the mixture was poured onto crushed ice (200 g m), stirred for 30 minutes and extracted with diethyl ether (400 ml). The ethereal extract was dried over anhydrous sodium sulfate and evaporated under vacuum at room temperature to yield the intermediate 5-isopropyl-2,4,6-trichloropyrimidine as a white waxy solid. 10% Sodium hydroxide (20 ml) was then added to the intermediate and the mixture was heated under reflux for 30 minutes. On cooling, the mixture was acidified with hydrochloric acid to pH 1–2 and the separated precipitate was filtered, washed with cold water and crystallized from ethanol to yield 6.98 g (74%) of the title compound (C₇H₉ClN₂O₂) as colourless crystals. M.P.: 257–259 °C.

¹H NMR (DMSO-d₆, 500.13 MHz): δ 1.14 (d, 6H, CH₃, J = 7.2 Hz), 2.51–2.63 (m, 1H, CH), 11.22 (s, 1H, NH), 11.79 (s, 1H, NH). ¹³C NMR (DMSO-d₆, 125.76 MHz): δ 20.02 (CH₃), 26.52 (CH), 113.95 (C–5), 140.95 (C-6), 149.75 (C=O), 162.75 (C=O).

Computing details top

Data collection: APEX2 (Bruker, 2009); cell refinement: SAINT (Bruker, 2009); data reduction: SAINT (Bruker, 2009); 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) and PLATON (Spek, 2009).

Figures top

Fig. 1. The molecular structure of the title compound with atom labels and 30% probability displacement ellipsoids.

Fig. 2. Crystal packing of the title compound, showing the hydrogen bonding interactions as dashed lines. H-atoms not involved in the hydrogen bonding are omited for clarity.

6-Chloro-5-isopropylpyrimidine-2,4(1H,3H)-dione top

Crystal data top

C₇H₉ClN₂O₂	F(000) = 392
M_r = 188.61	D_x = 1.448 Mg m⁻³
Monoclinic, P2₁/c	Cu Kα radiation, λ = 1.54178 Å
Hall symbol: -P 2ybc	Cell parameters from 3341 reflections
a = 11.2244 (4) Å	θ = 3.9–69.4°
b = 6.8288 (3) Å	µ = 3.62 mm⁻¹
c = 11.6641 (5) Å	T = 296 K
β = 104.577 (2)°	Block, colourless
V = 865.26 (6) Å³	0.45 × 0.28 × 0.26 mm
Z = 4

Data collection top

Bruker APEXII CCD diffractometer	1553 independent reflections
Radiation source: fine-focus sealed tube	1444 reflections with I > 2σ(I)
Graphite monochromator	R_int = 0.027
ϕ and ω scans	θ_max = 69.7°, θ_min = 4.1°
Absorption correction: multi-scan (SADABS; Bruker, 2009)	h = −12→13
T_min = 0.292, T_max = 0.458	k = −8→7
5647 measured reflections	l = −13→10

Refinement top

Refinement on F²	Secondary atom site location: difference Fourier map
Least-squares matrix: full	Hydrogen site location: inferred from neighbouring sites
R[F² > 2σ(F²)] = 0.040	H atoms treated by a mixture of independent and constrained refinement
wR(F²) = 0.114	w = 1/[σ²(F_o²) + (0.0664P)² + 0.3263P] where P = (F_o² + 2F_c²)/3
S = 1.06	(Δ/σ)_max < 0.001
1553 reflections	Δρ_max = 0.27 e Å⁻³
122 parameters	Δρ_min = −0.32 e Å⁻³
0 restraints	Extinction correction: SHELXL, Fc^*=kFc[1+0.001xFc²λ³/sin(2θ)]^-1/4
Primary atom site location: structure-invariant direct methods	Extinction coefficient: 0.0181 (15)

Crystal data top

C₇H₉ClN₂O₂	V = 865.26 (6) Å³
M_r = 188.61	Z = 4
Monoclinic, P2₁/c	Cu Kα radiation
a = 11.2244 (4) Å	µ = 3.62 mm⁻¹
b = 6.8288 (3) Å	T = 296 K
c = 11.6641 (5) Å	0.45 × 0.28 × 0.26 mm
β = 104.577 (2)°

Data collection top

Bruker APEXII CCD diffractometer	1553 independent reflections
Absorption correction: multi-scan (SADABS; Bruker, 2009)	1444 reflections with I > 2σ(I)
T_min = 0.292, T_max = 0.458	R_int = 0.027
5647 measured reflections

Refinement top

R[F² > 2σ(F²)] = 0.040	0 restraints
wR(F²) = 0.114	H atoms treated by a mixture of independent and constrained refinement
S = 1.06	Δρ_max = 0.27 e Å⁻³
1553 reflections	Δρ_min = −0.32 e Å⁻³
122 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 F² against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F², conventional R-factors R are based on F, with F set to zero for negative F². The threshold expression of F² > σ(F²) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F² 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

	x	y	z	U_iso*/U_eq
Cl1	0.20279 (5)	0.71985 (9)	0.34179 (6)	0.0644 (3)
O1	0.55744 (12)	0.3674 (2)	0.28760 (12)	0.0465 (4)
O2	0.35879 (13)	0.0719 (2)	0.53323 (13)	0.0522 (4)
N1	0.39383 (14)	0.5227 (2)	0.32786 (14)	0.0386 (4)
N2	0.45727 (14)	0.2261 (2)	0.41291 (14)	0.0394 (4)
C1	0.29636 (16)	0.5181 (3)	0.37878 (16)	0.0371 (4)
C2	0.47498 (15)	0.3710 (3)	0.33892 (15)	0.0354 (4)
C3	0.36295 (16)	0.2134 (3)	0.46972 (15)	0.0371 (4)
C4	0.27390 (15)	0.3729 (3)	0.44806 (15)	0.0368 (4)
C5	0.16037 (17)	0.3635 (3)	0.49593 (17)	0.0437 (5)
C6	0.1871 (2)	0.3338 (5)	0.6280 (2)	0.0697 (7)
H6A	0.2438	0.4322	0.6675	0.105*
H6B	0.2226	0.2065	0.6480	0.105*
H6C	0.1119	0.3437	0.6527	0.105*
C7	0.0719 (2)	0.2127 (5)	0.4302 (3)	0.0815 (9)
H7A	0.0568	0.2355	0.3465	0.122*
H7B	−0.0041	0.2217	0.4533	0.122*
H7C	0.1065	0.0845	0.4486	0.122*
H1N1	0.403 (2)	0.617 (4)	0.2856 (19)	0.043 (6)*
H1N2	0.506 (2)	0.132 (4)	0.422 (2)	0.059 (7)*
H5	0.125 (2)	0.489 (4)	0.481 (2)	0.058 (7)*

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
Cl1	0.0662 (4)	0.0486 (4)	0.0892 (5)	0.0249 (2)	0.0395 (3)	0.0243 (3)
O1	0.0481 (7)	0.0421 (8)	0.0587 (9)	0.0001 (6)	0.0307 (6)	−0.0012 (6)
O2	0.0522 (8)	0.0506 (9)	0.0614 (9)	0.0157 (6)	0.0286 (6)	0.0227 (7)
N1	0.0458 (8)	0.0321 (8)	0.0424 (9)	0.0018 (6)	0.0197 (7)	0.0042 (6)
N2	0.0394 (8)	0.0381 (8)	0.0451 (9)	0.0095 (7)	0.0187 (7)	0.0061 (6)
C1	0.0392 (9)	0.0358 (9)	0.0379 (10)	0.0053 (7)	0.0124 (7)	−0.0011 (7)
C2	0.0373 (8)	0.0343 (9)	0.0365 (9)	−0.0012 (7)	0.0129 (7)	−0.0044 (7)
C3	0.0376 (9)	0.0394 (10)	0.0361 (10)	0.0033 (7)	0.0125 (7)	0.0025 (7)
C4	0.0380 (9)	0.0393 (10)	0.0346 (9)	0.0044 (7)	0.0120 (7)	0.0005 (7)
C5	0.0420 (9)	0.0460 (11)	0.0485 (11)	0.0095 (8)	0.0215 (8)	0.0060 (8)
C6	0.0585 (13)	0.108 (2)	0.0512 (14)	0.0016 (14)	0.0296 (11)	−0.0019 (13)
C7	0.0472 (13)	0.120 (3)	0.0849 (19)	−0.0208 (14)	0.0314 (13)	−0.0335 (17)

Geometric parameters (Å, º) top

Cl1—C1	1.7200 (18)	C4—C5	1.516 (2)
O1—C2	1.222 (2)	C5—C7	1.500 (3)
O2—C3	1.226 (2)	C5—C6	1.507 (3)
N1—C2	1.364 (2)	C5—H5	0.95 (3)
N1—C1	1.370 (2)	C6—H6A	0.9600
N1—H1N1	0.83 (2)	C6—H6B	0.9600
N2—C2	1.360 (2)	C6—H6C	0.9600
N2—C3	1.386 (2)	C7—H7A	0.9600
N2—H1N2	0.84 (3)	C7—H7B	0.9600
C1—C4	1.342 (3)	C7—H7C	0.9600
C3—C4	1.457 (2)

C2—N1—C1	121.92 (16)	C7—C5—C6	111.5 (2)
C2—N1—H1N1	117.6 (15)	C7—C5—C4	110.49 (17)
C1—N1—H1N1	120.4 (15)	C6—C5—C4	114.38 (17)
C2—N2—C3	126.87 (16)	C7—C5—H5	109.6 (16)
C2—N2—H1N2	116.1 (17)	C6—C5—H5	106.3 (15)
C3—N2—H1N2	116.9 (17)	C4—C5—H5	104.3 (15)
C4—C1—N1	124.69 (16)	C5—C6—H6A	109.5
C4—C1—Cl1	123.17 (14)	C5—C6—H6B	109.5
N1—C1—Cl1	112.13 (13)	H6A—C6—H6B	109.5
O1—C2—N2	123.07 (17)	C5—C6—H6C	109.5
O1—C2—N1	122.60 (17)	H6A—C6—H6C	109.5
N2—C2—N1	114.32 (15)	H6B—C6—H6C	109.5
O2—C3—N2	119.22 (16)	C5—C7—H7A	109.5
O2—C3—C4	124.43 (16)	C5—C7—H7B	109.5
N2—C3—C4	116.35 (16)	H7A—C7—H7B	109.5
C1—C4—C3	115.60 (16)	C5—C7—H7C	109.5
C1—C4—C5	123.76 (16)	H7A—C7—H7C	109.5
C3—C4—C5	120.54 (16)	H7B—C7—H7C	109.5

C2—N1—C1—C4	3.4 (3)	N1—C1—C4—C5	−175.22 (17)
C2—N1—C1—Cl1	−175.53 (14)	Cl1—C1—C4—C5	3.6 (3)
C3—N2—C2—O1	−176.48 (18)	O2—C3—C4—C1	177.94 (19)
C3—N2—C2—N1	4.4 (3)	N2—C3—C4—C1	−2.5 (2)
C1—N1—C2—O1	175.02 (17)	O2—C3—C4—C5	−5.7 (3)
C1—N1—C2—N2	−5.8 (2)	N2—C3—C4—C5	173.89 (16)
C2—N2—C3—O2	179.34 (18)	C1—C4—C5—C7	105.3 (2)
C2—N2—C3—C4	−0.3 (3)	C3—C4—C5—C7	−70.8 (3)
N1—C1—C4—C3	1.0 (3)	C1—C4—C5—C6	−128.0 (2)
Cl1—C1—C4—C3	179.85 (13)	C3—C4—C5—C6	56.0 (3)

Hydrogen-bond geometry (Å, º) top

D—H···A	D—H	H···A	D···A	D—H···A
N1—H1N1···O1ⁱ	0.83 (3)	2.01 (3)	2.833 (2)	169 (2)
N2—H1N2···O2ⁱⁱ	0.83 (3)	2.03 (3)	2.854 (2)	171 (2)
C5—H5···Cl1	0.94 (3)	2.57 (2)	3.132 (2)	118.7 (17)
C6—H6A···O1ⁱⁱⁱ	0.96	2.56	3.455 (3)	156
C6—H6B···O2	0.96	2.45	3.034 (3)	119

Symmetry codes: (i) −x+1, y+1/2, −z+1/2; (ii) −x+1, −y, −z+1; (iii) −x+1, −y+1, −z+1.

Hydrogen-bond geometry (Å, º) top

D—H···A	D—H	H···A	D···A	D—H···A
N1—H1N1···O1ⁱ	0.83 (3)	2.01 (3)	2.833 (2)	169 (2)
N2—H1N2···O2ⁱⁱ	0.83 (3)	2.03 (3)	2.854 (2)	171 (2)
C5—H5···Cl1	0.94 (3)	2.57 (2)	3.132 (2)	118.7 (17)
C6—H6A···O1ⁱⁱⁱ	0.9600	2.5600	3.455 (3)	156.00
C6—H6B···O2	0.9600	2.4500	3.034 (3)	119.00

Symmetry codes: (i) −x+1, y+1/2, −z+1/2; (ii) −x+1, −y, −z+1; (iii) −x+1, −y+1, −z+1.

Footnotes

‡Additonal correspondence author, e-mail: elemam5@hotmail.com.

§Thomson Reuters ResearcherID: C-3194-2011.

¶Thomson Reuters ResearcherID: A-3561-2009.

Acknowledgements

The financial support of the Deanship of Scientific Research and the Research Center for Female Scientific and Medical Colleges, King Saud University, is greatly appreciated. CSCK thanks Universiti Sains Malaysia (USM) for a postdoctoral research fellowship.

References

Al-Omary, F. A. M., Ghabbour, H. A., El-Emam, A. A., Chidan Kumar, C. S. & Fun, H.-K. (2014). Acta Cryst. E70, o179–o180. CSD CrossRef CAS IUCr Journals Google Scholar
Al-Turkistani, A. A., Al-Deeb, O. A., El-Brollosy, N. R. & El-Emam, A. A. (2011). Molecules, 16, 4764–4774. Web of Science CAS PubMed Google Scholar
Bernstein, J., Davis, R. E., Shimoni, L. & Chang, N.-L. (1995). Angew. Chem. Int. Ed. Engl. 34, 1555–1573. CrossRef CAS Web of Science Google Scholar
Bruker (2009). SADABS, APEX2 and SAINT. Bruker AXS Inc., Madison, Wisconsin, USA. Google Scholar
El-Brollosy, N. R., Al-Deeb, O. A., El-Emam, A. A., Pedersen, E. B., La Colla, P., Collu, G., Sanna, G. & Loddo, R. (2009). Arch. Pharm. 342, 663–670. CAS Google Scholar
El-Brollosy, N. R., El-Emam, A. A., Al-Deeb, O. A. & Ng, S. W. (2011). Acta Cryst. E67, o2839. Web of Science CSD CrossRef IUCr Journals Google Scholar
El-Emam, A. A., Massoud, M. A., El-Bendary, E. R. & El-Sayed, M. A. (2004). Bull. Korean Chem. Soc., 25, 991–996. CAS Google Scholar
Haress, N. G., Ghabbour, H. A., El-Emam, A. A., Chidan Kumar, C. S. & Fun, H.-K. (2014). Acta Cryst. E70, o768–o769. CSD CrossRef CAS IUCr Journals Google Scholar
Hopkins, A. L., Ren, J., Esnouf, R. M., Willcox, B. E., Jones, E. Y., Ross, C., Miyasaka, T., Walker, R. T., Tanaka, H., Stammers, D. K. & Stuart, D. I. (1996). J. Med. Chem. 39, 1589–1600. CrossRef CAS PubMed Web of Science Google Scholar
Klein, R. S., Lenzi, M., Lim, T. H., Hotchkiss, K. A., Wilson, P. & Schwartz, E. L. (2001). Biochem. Pharmacol. 62, 1257–1263. Web of Science CrossRef PubMed CAS Google Scholar
Koroniak, H., Jankowski, A. & Krasnowski, M. (1993). Org. Prep. Proced. Int. 25, 563–568. CrossRef CAS Google Scholar
Miyasaka, T., Tanaka, H., Baba, M., Hayakawa, H., Walker, R. T., Balzarini, J. & De Clercq, E. (1989). J. Med. Chem. 32, 2507–2509. CrossRef CAS PubMed Web of Science Google Scholar
Nencka, R., Votruba, I., Hrebabecký, H., Tloust'ová, E., Horská, K., Masojídková, M. & Holý, A. (2006). Bioorg. Med. Chem. Lett. 16, 1335–1337. Web of Science CrossRef PubMed CAS Google Scholar
Sheldrick, G. M. (2008). Acta Cryst. A64, 112–122. Web of Science CrossRef CAS IUCr Journals Google Scholar
Spek, A. L. (2009). Acta Cryst. D65, 148–155. Web of Science CrossRef CAS IUCr Journals Google Scholar
Tanaka, H., Takashima, H., Ubasawa, M., Sekiya, K., Inouye, N., Baba, M., Shigeta, S., Walker, R. T., De Clercq, E. & Miyasaka, T. (1995). J. Med. Chem. 38, 2860–2865. CrossRef CAS PubMed Web of Science Google Scholar