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ISSN: 2056-9890

1,3,5-Tris(6-chloro­pyrazin-2-yl­­oxy)benzene

aSchool of Chemistry and Chemical Engineering, Shandong University, Jinan 250100, People's Republic of China
*Correspondence e-mail: yugp2005@yahoo.com.cn

(Received 19 November 2007; accepted 26 November 2007; online 6 December 2007)

In the title compound, C18H9Cl3N6O3, all bond lengths and angles are normal. The dihedral angles between the benzene ring and the three pyrazine rings are 72.67 (2), 60.73 (3) and 77.74 (2)°. The crystal packing is stabilized by van der Waals forces and by a weak ππ stacking inter­action between pyrazine rings, with a centroid–centroid distance of 3.487 (2) Å.

Related literature

For related literatures see: Carter & Boer (1974[Carter, D. R. & Boer, F. P. (1974). J. Chem. Soc. Perkin Trans. 2, pp. 1841-1844.]); Seitz et al. (2002[Seitz, L. E., Suling, W. J. & Reynolds, R. C. (2002). J. Med. Chem. 45, 5604-5606.]); Temple et al. (1970[Temple, C. Jr, Rose, J. D. & Montgomery, J. A. (1970). J. Med. Chem. 13, 1234-1235.]).

[Scheme 1]

Experimental

Crystal data
  • C18H9Cl3N6O3

  • Mr = 463.66

  • Triclinic, [P \overline 1]

  • a = 9.680 (2) Å

  • b = 10.658 (2) Å

  • c = 11.039 (3) Å

  • α = 72.768 (3)°

  • β = 68.308 (3)°

  • γ = 69.342 (3)°

  • V = 972.0 (4) Å3

  • Z = 2

  • Mo Kα radiation

  • μ = 0.51 mm−1

  • T = 298 (2) K

  • 0.58 × 0.31 × 0.30 mm

Data collection
  • Bruker SMART CCD area-detector diffractometer

  • Absorption correction: multi-scan (SADABS; Sheldrick, 2004[Sheldrick, G. M. (2004). SADABS. University of Göttingen, Germany.]) Tmin = 0.758, Tmax = 0.863

  • 4925 measured reflections

  • 3375 independent reflections

  • 2797 reflections with I > 2σ(I)

  • Rint = 0.017

Refinement
  • R[F2 > 2σ(F2)] = 0.041

  • wR(F2) = 0.108

  • S = 1.04

  • 3375 reflections

  • 271 parameters

  • H-atom parameters constrained

  • Δρmax = 0.22 e Å−3

  • Δρmin = −0.39 e Å−3

Data collection: SMART (Bruker, 2001[Bruker (2001). SMART and SAINT. Bruker AXS Inc., Madison, Wisconsin, USA.]); cell refinement: SAINT (Bruker, 2001[Bruker (2001). SMART and SAINT. Bruker AXS Inc., Madison, Wisconsin, USA.]); data reduction: SAINT; program(s) used to solve structure: SHELXTL (Sheldrick, 2001[Sheldrick, G. M. (2001). SHELXTL. Version 5.0. Bruker AXS Inc., Madison, Wisconsin, USA.]); program(s) used to refine structure: SHELXTL; molecular graphics: SHELXTL; software used to prepare material for publication: SHELXTL and local programs.

Supporting information


Comment top

Pyrazine derivatives were shown to display antimycobacterial (Seitz et al., 2002) and potential antimalarial (Temple et al., 1970) activities. The title compound was prepared for the screening of these bioactivities. We report here the crystal structure of (I).

In (I) (Fig. 1), all bond lengths and angles are normal and in a good agreement with those reported previously (Carter & Boer, 1974). The dihedral angles between benzene ring (C1—C6) and three Pyrazine rings (C7—C10/N1/N2; C11—C14/N3/N4; C15—C18/N5/N6) are 72.67 (2), 60.73 (3) and 77.74 (2)°, respectively. The crystal packing is stabilized by weak ππ stacking interactions and van der Waals forces, proved by the shorter distance Cg1···Cg1ii of 3.487 (2) Å, where Cg1 is a centroid of Pyrazine ring (C11—C14/N3/N4) [symmetry code:(ii) –X,1-Y,2-Z].

Related literature top

For related literatures see: Carter & Boer (1974); Seitz et al. (2002); Temple et al. (1970).

Experimental top

A flask was charged with 1.26 g (10 mmol) of 1,3,5-trihydroxybenzene, 4.47 g (30 mmol) of 2,6-dichloropyrazine and 3.18 g (30 mmol) of sodium carbonate, followed by addition of 50 ml of dried MeCN. The resultant mixture was refluxed over night. On cooling, the reaction mixture was filtered and the filtrate was portioned between 150 ml of water and 200 ml of dichloromethane. The organic phase was washed with brine, dried over sodium sulfate and evaporated on a rotary evaporator to furnish the crude product as a residue, which was chromatographed on silica gel to afford the pure product as colorless prisms. Crystals suitable for X-ray diffraction were obtained via slow evaporation of a solution of the title compound in ethyl acetate.

Refinement top

All H atoms were placed in calculated positions, with C—H = 0.93 Å, and included in the final cycles of refinement using a riding model, with Uiso(H) = 1.2 times Ueq(C).

Structure description top

Pyrazine derivatives were shown to display antimycobacterial (Seitz et al., 2002) and potential antimalarial (Temple et al., 1970) activities. The title compound was prepared for the screening of these bioactivities. We report here the crystal structure of (I).

In (I) (Fig. 1), all bond lengths and angles are normal and in a good agreement with those reported previously (Carter & Boer, 1974). The dihedral angles between benzene ring (C1—C6) and three Pyrazine rings (C7—C10/N1/N2; C11—C14/N3/N4; C15—C18/N5/N6) are 72.67 (2), 60.73 (3) and 77.74 (2)°, respectively. The crystal packing is stabilized by weak ππ stacking interactions and van der Waals forces, proved by the shorter distance Cg1···Cg1ii of 3.487 (2) Å, where Cg1 is a centroid of Pyrazine ring (C11—C14/N3/N4) [symmetry code:(ii) –X,1-Y,2-Z].

For related literatures see: Carter & Boer (1974); Seitz et al. (2002); Temple et al. (1970).

Computing details top

Data collection: SMART (Bruker, 2001); cell refinement: SAINT (Bruker, 2001); data reduction: SAINT (Bruker, 2001); program(s) used to solve structure: SHELXTL (Sheldrick, 2001); program(s) used to refine structure: SHELXTL (Sheldrick, 2001); molecular graphics: SHELXTL (Sheldrick, 2001); software used to prepare material for publication: SHELXTL and local programs.

Figures top
[Figure 1] Fig. 1. View of the title compound (I), with displacement ellipsoids drawn at the 40% probability level.
1,3,5-Tris(6-chloropyrazin-2-yloxy)benzene top
Crystal data top
C18H9Cl3N6O3Z = 2
Mr = 463.66F(000) = 468
Triclinic, P1Dx = 1.584 Mg m3
Hall symbol: -P 1Mo Kα radiation, λ = 0.71073 Å
a = 9.680 (2) ÅCell parameters from 1699 reflections
b = 10.658 (2) Åθ = 2.8–23.1°
c = 11.039 (3) ŵ = 0.51 mm1
α = 72.768 (3)°T = 298 K
β = 68.308 (3)°Prism, colorless
γ = 69.342 (3)°0.58 × 0.31 × 0.30 mm
V = 972.0 (4) Å3
Data collection top
Bruker SMART CCD area-detector
diffractometer
3375 independent reflections
Radiation source: fine-focus sealed tube2797 reflections with I > 2σ(I)
Graphite monochromatorRint = 0.017
φ and ω scansθmax = 25.0°, θmin = 2.0°
Absorption correction: multi-scan
(SADABS; Sheldrick, 2004)
h = 911
Tmin = 0.758, Tmax = 0.863k = 1212
4925 measured reflectionsl = 1312
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)] = 0.041Hydrogen site location: inferred from neighbouring sites
wR(F2) = 0.108H-atom parameters constrained
S = 1.04 w = 1/[σ2(Fo2) + (0.0537P)2 + 0.2339P]
where P = (Fo2 + 2Fc2)/3
3375 reflections(Δ/σ)max = 0.001
271 parametersΔρmax = 0.22 e Å3
0 restraintsΔρmin = 0.39 e Å3
Crystal data top
C18H9Cl3N6O3γ = 69.342 (3)°
Mr = 463.66V = 972.0 (4) Å3
Triclinic, P1Z = 2
a = 9.680 (2) ÅMo Kα radiation
b = 10.658 (2) ŵ = 0.51 mm1
c = 11.039 (3) ÅT = 298 K
α = 72.768 (3)°0.58 × 0.31 × 0.30 mm
β = 68.308 (3)°
Data collection top
Bruker SMART CCD area-detector
diffractometer
3375 independent reflections
Absorption correction: multi-scan
(SADABS; Sheldrick, 2004)
2797 reflections with I > 2σ(I)
Tmin = 0.758, Tmax = 0.863Rint = 0.017
4925 measured reflections
Refinement top
R[F2 > 2σ(F2)] = 0.0410 restraints
wR(F2) = 0.108H-atom parameters constrained
S = 1.04Δρmax = 0.22 e Å3
3375 reflectionsΔρmin = 0.39 e Å3
271 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
xyzUiso*/Ueq
Cl10.04661 (7)0.67966 (7)0.44208 (6)0.0642 (2)
Cl20.43548 (9)1.27994 (7)0.59901 (7)0.0772 (2)
Cl30.32169 (10)1.47146 (8)0.07383 (9)0.0893 (3)
O10.43622 (16)0.85408 (15)0.05573 (15)0.0530 (4)
O20.1867 (2)0.93858 (16)0.27062 (15)0.0616 (4)
O30.01216 (16)1.23869 (14)0.03280 (16)0.0513 (4)
N10.1102 (3)1.4653 (2)0.2505 (2)0.0743 (6)
N20.4938 (2)0.6220 (2)0.3517 (2)0.0701 (6)
N30.2331 (3)1.0283 (2)0.6181 (2)0.0672 (6)
N40.1437 (2)1.34503 (17)0.05891 (17)0.0469 (4)
N50.3057 (2)1.09657 (17)0.42807 (17)0.0459 (4)
N60.25692 (18)0.76867 (16)0.23874 (16)0.0409 (4)
C10.0239 (4)1.4929 (3)0.2147 (3)0.0718 (8)
H1B0.03381.55410.25390.086*
C20.3070 (3)1.1226 (3)0.6499 (2)0.0600 (6)
H2B0.33581.16740.73770.072*
C30.1181 (3)1.3782 (3)0.1916 (2)0.0610 (6)
H3B0.21041.35600.21470.073*
C40.5191 (3)0.7021 (3)0.2337 (2)0.0626 (7)
H4B0.61880.70940.18670.075*
C50.1484 (3)1.4320 (2)0.1203 (2)0.0542 (6)
C60.1940 (3)0.9688 (3)0.4911 (2)0.0602 (6)
H6B0.14150.90250.46430.072*
C70.3414 (2)1.1549 (2)0.5553 (2)0.0488 (5)
C80.2308 (3)1.0047 (2)0.3975 (2)0.0470 (5)
C90.3493 (3)0.6156 (2)0.4159 (2)0.0540 (6)
H9A0.32640.56140.50000.065*
C100.2025 (3)0.9832 (2)0.1693 (2)0.0467 (5)
C110.3125 (2)0.8988 (2)0.1081 (2)0.0467 (5)
H11A0.38120.82020.13860.056*
C120.2348 (2)0.68880 (19)0.3581 (2)0.0407 (5)
C130.3169 (2)0.9348 (2)0.0002 (2)0.0421 (5)
C140.0097 (2)1.3191 (2)0.0952 (2)0.0440 (5)
C150.1076 (2)1.12977 (19)0.0168 (2)0.0403 (5)
C160.2156 (2)1.0492 (2)0.0488 (2)0.0409 (4)
H16A0.21961.07130.12260.049*
C170.0996 (2)1.0992 (2)0.1265 (2)0.0444 (5)
H17A0.02681.15530.16990.053*
C180.3995 (2)0.77556 (19)0.1787 (2)0.0419 (5)
Atomic displacement parameters (Å2) top
U11U22U33U12U13U23
Cl10.0539 (3)0.0787 (4)0.0503 (4)0.0223 (3)0.0150 (3)0.0057 (3)
Cl20.1024 (5)0.0756 (4)0.0561 (4)0.0460 (4)0.0100 (4)0.0072 (3)
Cl30.0920 (5)0.0802 (5)0.1186 (7)0.0405 (4)0.0433 (5)0.0155 (4)
O10.0408 (8)0.0582 (9)0.0452 (9)0.0084 (7)0.0133 (7)0.0055 (7)
O20.0966 (13)0.0628 (10)0.0390 (9)0.0401 (9)0.0227 (8)0.0036 (7)
O30.0463 (8)0.0467 (8)0.0619 (10)0.0073 (6)0.0212 (7)0.0118 (7)
N10.0842 (17)0.0791 (15)0.0478 (13)0.0113 (13)0.0091 (11)0.0225 (11)
N20.0501 (12)0.0856 (15)0.0539 (13)0.0002 (10)0.0244 (10)0.0053 (11)
N30.0846 (15)0.0855 (15)0.0436 (12)0.0288 (12)0.0243 (11)0.0158 (11)
N40.0521 (10)0.0416 (9)0.0439 (10)0.0120 (8)0.0150 (8)0.0040 (8)
N50.0531 (10)0.0477 (10)0.0367 (10)0.0103 (8)0.0156 (8)0.0090 (8)
N60.0451 (10)0.0397 (9)0.0361 (9)0.0061 (7)0.0174 (8)0.0044 (7)
C10.105 (2)0.0596 (15)0.0553 (16)0.0137 (15)0.0331 (16)0.0168 (13)
C20.0684 (15)0.0727 (16)0.0363 (12)0.0164 (13)0.0167 (11)0.0086 (11)
C30.0580 (14)0.0666 (15)0.0427 (13)0.0117 (12)0.0069 (11)0.0049 (12)
C40.0421 (12)0.0782 (16)0.0521 (15)0.0057 (11)0.0173 (11)0.0003 (12)
C50.0693 (15)0.0441 (12)0.0515 (14)0.0153 (11)0.0271 (12)0.0008 (10)
C60.0758 (16)0.0650 (14)0.0508 (15)0.0256 (13)0.0228 (12)0.0137 (12)
C70.0502 (12)0.0509 (12)0.0394 (12)0.0091 (10)0.0103 (9)0.0097 (10)
C80.0562 (12)0.0474 (12)0.0385 (12)0.0122 (10)0.0172 (10)0.0080 (9)
C90.0569 (14)0.0536 (12)0.0395 (12)0.0016 (10)0.0201 (10)0.0010 (10)
C100.0627 (13)0.0500 (12)0.0312 (11)0.0271 (10)0.0145 (10)0.0016 (9)
C110.0499 (12)0.0434 (11)0.0393 (12)0.0156 (9)0.0046 (9)0.0049 (9)
C120.0454 (11)0.0378 (10)0.0363 (11)0.0049 (8)0.0156 (9)0.0067 (8)
C130.0395 (10)0.0442 (11)0.0357 (11)0.0132 (8)0.0114 (9)0.0045 (9)
C140.0487 (12)0.0385 (10)0.0361 (11)0.0070 (9)0.0142 (9)0.0012 (8)
C150.0423 (11)0.0369 (10)0.0390 (11)0.0141 (8)0.0114 (9)0.0004 (8)
C160.0450 (11)0.0460 (11)0.0325 (10)0.0157 (9)0.0140 (9)0.0017 (8)
C170.0537 (12)0.0446 (11)0.0379 (11)0.0208 (10)0.0214 (10)0.0070 (9)
C180.0429 (11)0.0400 (10)0.0395 (11)0.0049 (8)0.0158 (9)0.0060 (9)
Geometric parameters (Å, º) top
Cl1—C121.735 (2)C1—C51.371 (4)
Cl2—C71.728 (2)C1—H1B0.9300
Cl3—C51.729 (3)C2—C71.367 (3)
O1—C181.360 (2)C2—H2B0.9300
O1—C131.402 (2)C3—C141.390 (3)
O2—C81.351 (3)C3—H3B0.9300
O2—C101.411 (3)C4—C181.386 (3)
O3—C141.357 (3)C4—H4B0.9300
O3—C151.396 (2)C6—C81.394 (3)
N1—C31.317 (3)C6—H6B0.9300
N1—C11.323 (4)C9—C121.364 (3)
N2—C41.319 (3)C9—H9A0.9300
N2—C91.330 (3)C10—C171.369 (3)
N3—C21.326 (3)C10—C111.377 (3)
N3—C61.326 (3)C11—C131.374 (3)
N4—C141.312 (3)C11—H11A0.9300
N4—C51.320 (3)C13—C161.377 (3)
N5—C81.312 (3)C15—C171.379 (3)
N5—C71.325 (3)C15—C161.382 (3)
N6—C181.310 (3)C16—H16A0.9300
N6—C121.328 (2)C17—H17A0.9300
C18—O1—C13118.64 (15)O2—C8—C6116.6 (2)
C8—O2—C10118.93 (16)N2—C9—C12119.8 (2)
C14—O3—C15121.38 (16)N2—C9—H9A120.1
C3—N1—C1117.2 (2)C12—C9—H9A120.1
C4—N2—C9117.46 (19)C17—C10—C11122.6 (2)
C2—N3—C6116.8 (2)C17—C10—O2119.58 (19)
C14—N4—C5115.09 (19)C11—C10—O2117.48 (19)
C8—N5—C7114.59 (18)C13—C11—C10117.54 (19)
C18—N6—C12114.85 (16)C13—C11—H11A121.2
N1—C1—C5120.6 (2)C10—C11—H11A121.2
N1—C1—H1B119.7N6—C12—C9124.2 (2)
C5—C1—H1B119.7N6—C12—Cl1116.16 (14)
N3—C2—C7120.9 (2)C9—C12—Cl1119.64 (17)
N3—C2—H2B119.6C11—C13—C16122.78 (18)
C7—C2—H2B119.6C11—C13—O1117.27 (18)
N1—C3—C14120.9 (2)C16—C13—O1119.85 (19)
N1—C3—H3B119.6N4—C14—O3120.43 (19)
C14—C3—H3B119.6N4—C14—C3122.7 (2)
N2—C4—C18121.0 (2)O3—C14—C3116.8 (2)
N2—C4—H4B119.5C17—C15—C16122.55 (19)
C18—C4—H4B119.5C17—C15—O3115.06 (17)
N4—C5—C1123.6 (2)C16—C15—O3121.97 (19)
N4—C5—Cl3116.79 (18)C13—C16—C15116.97 (19)
C1—C5—Cl3119.6 (2)C13—C16—H16A121.5
N3—C6—C8120.7 (2)C15—C16—H16A121.5
N3—C6—H6B119.6C10—C17—C15117.57 (19)
C8—C6—H6B119.6C10—C17—H17A121.2
N5—C7—C2124.0 (2)C15—C17—H17A121.2
N5—C7—Cl2116.06 (17)N6—C18—O1120.19 (17)
C2—C7—Cl2119.97 (18)N6—C18—C4122.7 (2)
N5—C8—O2120.29 (18)O1—C18—C4117.13 (19)
N5—C8—C6123.1 (2)
C3—N1—C1—C50.2 (4)N2—C9—C12—Cl1179.78 (19)
C6—N3—C2—C70.5 (4)C10—C11—C13—C161.0 (3)
C1—N1—C3—C140.4 (4)C10—C11—C13—O1175.44 (17)
C9—N2—C4—C180.6 (4)C18—O1—C13—C11111.4 (2)
C14—N4—C5—C10.4 (3)C18—O1—C13—C1672.1 (2)
C14—N4—C5—Cl3178.98 (14)C5—N4—C14—O3175.50 (17)
N1—C1—C5—N40.7 (4)C5—N4—C14—C30.2 (3)
N1—C1—C5—Cl3179.2 (2)C15—O3—C14—N433.3 (3)
C2—N3—C6—C80.4 (4)C15—O3—C14—C3150.69 (19)
C8—N5—C7—C21.0 (3)N1—C3—C14—N40.7 (3)
C8—N5—C7—Cl2178.31 (15)N1—C3—C14—O3175.2 (2)
N3—C2—C7—N50.2 (4)C14—O3—C15—C17150.15 (18)
N3—C2—C7—Cl2179.10 (19)C14—O3—C15—C1637.0 (3)
C7—N5—C8—O2179.47 (19)C11—C13—C16—C150.8 (3)
C7—N5—C8—C61.1 (3)O1—C13—C16—C15175.49 (16)
C10—O2—C8—N58.9 (3)C17—C15—C16—C130.1 (3)
C10—O2—C8—C6171.7 (2)O3—C15—C16—C13172.23 (17)
N3—C6—C8—N50.5 (4)C11—C10—C17—C150.6 (3)
N3—C6—C8—O2179.9 (2)O2—C10—C17—C15172.20 (17)
C4—N2—C9—C121.0 (4)C16—C15—C17—C100.8 (3)
C8—O2—C10—C1776.1 (3)O3—C15—C17—C10172.01 (17)
C8—O2—C10—C11110.8 (2)C12—N6—C18—O1179.91 (17)
C17—C10—C11—C130.2 (3)C12—N6—C18—C41.8 (3)
O2—C10—C11—C13173.20 (17)C13—O1—C18—N64.8 (3)
C18—N6—C12—C91.4 (3)C13—O1—C18—C4177.0 (2)
C18—N6—C12—Cl1178.83 (14)N2—C4—C18—N60.9 (4)
N2—C9—C12—N60.0 (3)N2—C4—C18—O1179.0 (2)

Experimental details

Crystal data
Chemical formulaC18H9Cl3N6O3
Mr463.66
Crystal system, space groupTriclinic, P1
Temperature (K)298
a, b, c (Å)9.680 (2), 10.658 (2), 11.039 (3)
α, β, γ (°)72.768 (3), 68.308 (3), 69.342 (3)
V3)972.0 (4)
Z2
Radiation typeMo Kα
µ (mm1)0.51
Crystal size (mm)0.58 × 0.31 × 0.30
Data collection
DiffractometerBruker SMART CCD area-detector
Absorption correctionMulti-scan
(SADABS; Sheldrick, 2004)
Tmin, Tmax0.758, 0.863
No. of measured, independent and
observed [I > 2σ(I)] reflections
4925, 3375, 2797
Rint0.017
(sin θ/λ)max1)0.595
Refinement
R[F2 > 2σ(F2)], wR(F2), S 0.041, 0.108, 1.04
No. of reflections3375
No. of parameters271
H-atom treatmentH-atom parameters constrained
Δρmax, Δρmin (e Å3)0.22, 0.39

Computer programs: SMART (Bruker, 2001), SAINT (Bruker, 2001), SHELXTL (Sheldrick, 2001), SHELXTL and local programs.

 

References

First citationBruker (2001). SMART and SAINT. Bruker AXS Inc., Madison, Wisconsin, USA.  Google Scholar
First citationCarter, D. R. & Boer, F. P. (1974). J. Chem. Soc. Perkin Trans. 2, pp. 1841–1844.  CrossRef Google Scholar
First citationSeitz, L. E., Suling, W. J. & Reynolds, R. C. (2002). J. Med. Chem. 45, 5604–5606.  Web of Science CrossRef PubMed CAS Google Scholar
First citationSheldrick, G. M. (2001). SHELXTL. Version 5.0. Bruker AXS Inc., Madison, Wisconsin, USA.  Google Scholar
First citationSheldrick, G. M. (2004). SADABS. University of Göttingen, Germany.  Google Scholar
First citationTemple, C. Jr, Rose, J. D. & Montgomery, J. A. (1970). J. Med. Chem. 13, 1234–1235.  CrossRef CAS PubMed Web of Science Google Scholar

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