(4S,5S)-2-(2-Thien­yl)-1,3-dioxolane-4,5-dicarboxamide

Xu, W.; Yang, Z.; Li, X.-H.; Liu, B.-N.; Wang, D.-C.

doi:10.1107/S1600536809008368

organic compounds

CRYSTALLOGRAPHIC
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ISSN: 2056-9890

Volume 65| Part 4| April 2009| Page o764

doi:10.1107/S1600536809008368

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(4S,5S)-2-(2-Thienyl)-1,3-dioxolane-4,5-dicarboxamide

Wei Xu,^a Zheng Yang,^a Xin-Hua Li,^a Bo-Nian Liu ^b and De-Cai Wang ^a ^*

^aState Key Laboratory of Materials-Oriented Chemical Engineering, College of Life Science and Pharmaceutical Engineering, Nanjing University of Technology, Xinmofan Road No. 5 Nanjing, Nanjing 210009, People's Republic of China, and ^bCollege of Science, Nanjing University of Technoolgy, Xinmofan Road No. 5 Nanjing, Nanjing 210009, People's Republic of China
^*Correspondence e-mail: dcwang@njut.edu.cn

(Received 18 February 2009; accepted 7 March 2009; online 14 March 2009)

In the title compound, C₉H₁₀N₂O₄S, which is an important intermediate for the preparation of antitumor platinum drugs, the dioxolane ring adopts an envelope conformation with the C atom bonded to the thienyl ring at the flap position. Intramolecular N—H⋯O and C—H⋯O hydrogen bonds result in the formation of two five-membered rings having envelope conformations. In the crystal structure, intermolecular N—H⋯O and C—H⋯O hydrogen bonds link the molecules into a three-dimensional network.

Related literature

For general background, see: Kim et al. (1994 ); Pandey et al. (1997 ). For bond-length data, see: Allen et al. (1987 ).

Experimental

Crystal data

C₉H₁₀N₂O₄S
M_r = 242.25
Monoclinic, P 2₁
a = 8.9250 (18) Å
b = 4.796 (1) Å
c = 12.109 (2) Å
β = 90.60 (3)°
V = 518.29 (18) Å³
Z = 2
Mo Kα radiation
μ = 0.31 mm⁻¹
T = 294 K
0.30 × 0.20 × 0.10 mm

Data collection

Enraf–Nonius CAD-4 diffractometer
Absorption correction: ψ scan (North et al., 1968 ) T_min = 0.912, T_max = 0.969
2135 measured reflections
2007 independent reflections
1820 reflections with I > 2σ(I)
R_int = 0.015
3 standard reflections frequency: 120 min intensity decay: 1%

Refinement

R[F² > 2σ(F²)] = 0.040
wR(F²) = 0.124
S = 1.01
2007 reflections
146 parameters
1 restraint
H-atom parameters constrained
Δρ_max = 0.25 e Å⁻³
Δρ_min = −0.34 e Å⁻³
Absolute structure: Flack (1983 ), 876 Friedel pairs
Flack parameter: 0.04 (12)

Table 1
Hydrogen-bond geometry (Å, °)

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
N1—H1A⋯O3ⁱ	0.86	2.06	2.909 (4)	167
N1—H1B⋯O1	0.86	2.30	2.673 (4)	106
N2—H2A⋯O3ⁱⁱ	0.86	2.29	3.059 (3)	149
N2—H2B⋯O4ⁱⁱⁱ	0.86	2.28	3.077 (3)	154
C6—H6A⋯O4ⁱⁱⁱ	0.98	2.27	3.049 (3)	136
C7—H7A⋯O4	0.98	2.45	2.866 (3)	105
Symmetry codes: (i) $[-x, y+{\script{1\over 2}}, -z+1]$ ; (ii) $[-x+1, y-{\script{1\over 2}}, -z+1]$ ; (iii) x, y+1, z.

Data collection: CAD-4 Software (Enraf–Nonius, 1989 ); cell refinement: CAD-4 Software; data reduction: XCAD4 (Harms & Wocadlo, 1995 ); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008 ); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ORTEP-3 for Windows (Farrugia, 1997 ); software used to prepare material for publication: WinGX (Farrugia, 1999 ).

Supporting information

Crystal structure: contains datablocks global, I. DOI: 10.1107/S1600536809008368/hk2628sup1.cif

Structure factors: contains datablock I. DOI: 10.1107/S1600536809008368/hk2628Isup2.hkl

checkCIF report

Comment top

Antitumor platinum drug is one kind of the most effective anticancer agents currently available. (2S,3S)-Diethyl 2,3-O-alkyltartrate analogues are starting materials for the syntheses of platinum complexes with antitumor activity (Kim et al., 1994), and are also important intermediates in organic syntheses (Pandey et al., 1997). As part of our studies on the syntheses and characterizations of these compounds, we have synthesized the title compound and reported herein its crystal structure.

In the molecule of the title compound (Fig 1), the bond lengths (Allen et al., 1987) and angles are within normal ranges. Ring A (S/C1-C4) is, of course, planar, while ring B (O1/O2/C5-C7) adopts an envelope conformation with C5 atom displaced by 0.529 (3) Å from the plane of the other ring atoms. The intramolecular N-H···O and C-H···O hydrogen bonds (Table 1) result in the formations of two five-membered rings C (O1/N1/C7/C8/H1B) and D (O4/C6/C7/C9/H7A), having envelope conformations with atoms O1 and O4 displaced by -0.424 (3) Å and 0.461 (3) Å, respectively, from the planes of the other ring atoms.

In the crystal structure, intermolecular N-H···O and C-H···O hydrogen bonds (Table 1) link the molecules (Fig. 2), in which they may be effective in the stabilization of the structure.

Related literature top

For general background, see: Kim et al. (1994); Pandey et al. (1997). For bond-length data, see: Allen et al. (1987).

Experimental top

For the preparation of the title compound, a mixture of thiophene-2-carbaldehyde (272 mg, 2.43 mmol), (2S,3S)-diethyltartrate (500 mg, 2.43 mmol), anhydrous copper sulfate (776 mg, 2.86 mmol) and methanesulfonic acid (1 drop) in anhydrous toluene (8 ml) was stirred at room temperature for 12 h. Anhydrous potassium carbonate (40 mg) was added, and then stirred for a further 20 min. The resulting colorless precipitate was obtained by evaporation, and dried in the vacuo. This product (10 mmol) was dissolved in anhydrous ethanol (50 ml), then a current of dry ammonia, dried with calcium chloride, was passed over the reaction mixture at room temperature for about 4 h. The reaction mixture was evaporated to dryness. Crystals suitable for X-ray analysis were obtained by slow evaporation of an ethanol solution after three weeks.

Computing details top

Data collection: CAD-4 Software (Enraf–Nonius, 1989); cell refinement: CAD-4 Software (Enraf–Nonius, 1989); data reduction: XCAD4 (Harms & Wocadlo, 1995); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ORTEP-3 for Windows (Farrugia, 1997); software used to prepare material for publication: WinGX (Farrugia, 1999).

Figures top

	Fig. 1. The molecular structure of the title molecule, with the atom-numbering scheme. Hydrogen bonds are shown as dashed lines.
	Fig. 2. A partial packing diagram of the title compound. Hydrogen bonds are shown as dashed lines.

(4S,5S)-2-(2-Thienyl)-1,3-dioxolane-4,5-dicarboxamide top

Crystal data top

C₉H₁₀N₂O₄S	F(000) = 252
M_r = 242.25	D_x = 1.552 Mg m⁻³
Monoclinic, P2₁	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: P 2yb	Cell parameters from 25 reflections
a = 8.9250 (18) Å	θ = 9–13°
b = 4.796 (1) Å	µ = 0.31 mm⁻¹
c = 12.109 (2) Å	T = 294 K
β = 90.60 (3)°	Block, colorless
V = 518.29 (18) Å³	0.30 × 0.20 × 0.10 mm
Z = 2

Data collection top

Enraf–Nonius CAD-4 diffractometer	1820 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tube	R_int = 0.015
Graphite monochromator	θ_max = 26.0°, θ_min = 1.7°
ω/2θ scans	h = 0→10
Absorption correction: ψ scan (North et al., 1968)	k = −5→5
T_min = 0.912, T_max = 0.969	l = −14→14
2135 measured reflections	3 standard reflections every 120 min
2007 independent reflections	intensity decay: 1%

Refinement top

Refinement on F²	Hydrogen site location: inferred from neighbouring sites
Least-squares matrix: full	H-atom parameters constrained
R[F² > 2σ(F²)] = 0.040	w = 1/[σ²(F_o²) + (0.09P)²] where P = (F_o² + 2F_c²)/3
wR(F²) = 0.124	(Δ/σ)_max < 0.001
S = 1.01	Δρ_max = 0.25 e Å⁻³
2007 reflections	Δρ_min = −0.34 e Å⁻³
146 parameters	Extinction correction: SHELXL97 (Sheldrick, 2008), Fc^*=kFc[1+0.001xFc²λ³/sin(2θ)]^-1/4
1 restraint	Extinction coefficient: 0.134 (16)
Primary atom site location: structure-invariant direct methods	Absolute structure: Flack (1983), 876 Friedel pairs
Secondary atom site location: difference Fourier map	Absolute structure parameter: 0.04 (12)

Crystal data top

C₉H₁₀N₂O₄S	V = 518.29 (18) Å³
M_r = 242.25	Z = 2
Monoclinic, P2₁	Mo Kα radiation
a = 8.9250 (18) Å	µ = 0.31 mm⁻¹
b = 4.796 (1) Å	T = 294 K
c = 12.109 (2) Å	0.30 × 0.20 × 0.10 mm
β = 90.60 (3)°

Data collection top

Enraf–Nonius CAD-4 diffractometer	1820 reflections with I > 2σ(I)
Absorption correction: ψ scan (North et al., 1968)	R_int = 0.015
T_min = 0.912, T_max = 0.969	3 standard reflections every 120 min
2135 measured reflections	intensity decay: 1%
2007 independent reflections

Refinement top

R[F² > 2σ(F²)] = 0.040	H-atom parameters constrained
wR(F²) = 0.124	Δρ_max = 0.25 e Å⁻³
S = 1.01	Δρ_min = −0.34 e Å⁻³
2007 reflections	Absolute structure: Flack (1983), 876 Friedel pairs
146 parameters	Absolute structure parameter: 0.04 (12)
1 restraint

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
S	0.14355 (10)	0.3977 (2)	0.03359 (7)	0.0579 (3)
O1	0.1962 (2)	0.0273 (4)	0.23675 (15)	0.0388 (5)
O2	0.4057 (2)	0.2850 (4)	0.21945 (16)	0.0409 (5)
O3	0.1404 (2)	0.2632 (5)	0.51056 (16)	0.0475 (6)
O4	0.5298 (2)	−0.1909 (4)	0.3765 (2)	0.0496 (6)
N1	0.0403 (3)	0.4070 (7)	0.3492 (2)	0.0504 (6)
H1A	−0.0214	0.5180	0.3811	0.060*
H1B	0.0405	0.3944	0.2784	0.060*
N2	0.6553 (2)	0.2124 (5)	0.39050 (18)	0.0383 (5)
H2A	0.7383	0.1315	0.4079	0.046*
H2B	0.6516	0.3912	0.3856	0.046*
C1	0.1625 (4)	0.3760 (10)	−0.1056 (3)	0.0639 (10)
H1C	0.1063	0.4803	−0.1558	0.077*
C2	0.2684 (4)	0.1899 (10)	−0.1349 (3)	0.0622 (10)
H2C	0.2952	0.1552	−0.2076	0.075*
C3	0.3340 (4)	0.0537 (9)	−0.0427 (2)	0.0548 (8)
H3A	0.4071	−0.0837	−0.0480	0.066*
C4	0.2777 (3)	0.1472 (6)	0.0543 (2)	0.0397 (7)
C5	0.3239 (3)	0.0656 (6)	0.1672 (2)	0.0381 (6)
H5A	0.3843	−0.1049	0.1653	0.046*
C6	0.3965 (3)	0.2327 (6)	0.3351 (2)	0.0323 (6)
H6A	0.3940	0.4094	0.3757	0.039*
C7	0.2451 (3)	0.0773 (6)	0.3475 (2)	0.0329 (6)
H7A	0.2605	−0.0999	0.3863	0.039*
C8	0.1346 (3)	0.2558 (6)	0.4091 (2)	0.0347 (6)
C9	0.5335 (3)	0.0618 (6)	0.3713 (2)	0.0319 (6)

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
S	0.0688 (6)	0.0563 (5)	0.0484 (4)	0.0176 (4)	0.0005 (4)	0.0024 (4)
O1	0.0370 (10)	0.0429 (11)	0.0365 (10)	−0.0130 (8)	0.0048 (8)	−0.0072 (8)
O2	0.0340 (9)	0.0457 (11)	0.0428 (10)	−0.0131 (8)	−0.0024 (7)	0.0140 (9)
O3	0.0439 (11)	0.0656 (15)	0.0331 (10)	−0.0154 (10)	0.0051 (8)	−0.0026 (10)
O4	0.0446 (11)	0.0252 (11)	0.0788 (16)	−0.0001 (8)	−0.0073 (10)	0.0023 (9)
N1	0.0458 (13)	0.0636 (17)	0.0418 (12)	0.0139 (14)	0.0058 (10)	−0.0047 (15)
N2	0.0315 (11)	0.0310 (12)	0.0523 (14)	−0.0012 (9)	−0.0047 (9)	0.0023 (10)
C1	0.082 (2)	0.066 (2)	0.0434 (17)	−0.002 (2)	−0.0114 (16)	0.0095 (18)
C2	0.065 (2)	0.083 (3)	0.0388 (17)	−0.011 (2)	0.0067 (14)	−0.0020 (17)
C3	0.0546 (18)	0.068 (2)	0.0412 (16)	0.0068 (17)	0.0039 (13)	−0.0059 (16)
C4	0.0343 (13)	0.0417 (16)	0.0432 (15)	−0.0043 (11)	0.0030 (11)	0.0010 (13)
C5	0.0369 (13)	0.0371 (15)	0.0405 (14)	−0.0013 (12)	0.0075 (11)	−0.0011 (12)
C6	0.0313 (12)	0.0255 (12)	0.0402 (14)	−0.0037 (10)	0.0006 (10)	0.0013 (10)
C7	0.0343 (13)	0.0280 (13)	0.0364 (13)	−0.0069 (11)	0.0005 (10)	0.0015 (11)
C8	0.0271 (12)	0.0412 (16)	0.0360 (14)	−0.0115 (11)	0.0017 (10)	−0.0035 (11)
C9	0.0314 (12)	0.0339 (15)	0.0304 (12)	0.0005 (10)	0.0005 (10)	−0.0003 (10)

Geometric parameters (Å, º) top

S—C1	1.698 (3)	C1—C2	1.350 (6)
S—C4	1.713 (3)	C1—H1C	0.9300
O1—C5	1.436 (3)	C2—C3	1.415 (5)
O1—C7	1.426 (3)	C2—H2C	0.9300
O2—C5	1.425 (3)	C3—C4	1.359 (4)
O2—C6	1.426 (3)	C3—H3A	0.9300
O3—C8	1.230 (3)	C4—C5	1.477 (4)
O4—C9	1.214 (4)	C5—H5A	0.9800
N1—C8	1.322 (4)	C6—C7	1.552 (3)
N1—H1A	0.8600	C6—C9	1.532 (4)
N1—H1B	0.8600	C6—H6A	0.9800
N2—C9	1.324 (3)	C7—C8	1.509 (4)
N2—H2A	0.8600	C7—H7A	0.9800
N2—H2B	0.8600

C1—S—C4	91.47 (18)	O2—C5—O1	103.9 (2)
C7—O1—C5	107.04 (19)	O2—C5—C4	110.6 (2)
C5—O2—C6	105.76 (18)	O2—C5—H5A	110.3
C8—N1—H1A	120.0	C4—C5—H5A	110.3
C8—N1—H1B	120.0	O2—C6—C7	103.76 (19)
H1A—N1—H1B	120.0	O2—C6—C9	108.7 (2)
C9—N2—H2A	120.0	O2—C6—H6A	110.0
C9—N2—H2B	120.0	C7—C6—H6A	110.0
H2A—N2—H2B	120.0	C9—C6—C7	114.1 (2)
S—C1—H1C	123.9	C9—C6—H6A	110.0
C2—C1—S	112.3 (3)	O1—C7—C6	104.4 (2)
C2—C1—H1C	123.9	O1—C7—C8	111.4 (2)
C1—C2—C3	112.5 (3)	O1—C7—H7A	110.2
C1—C2—H2C	123.7	C6—C7—H7A	110.2
C3—C2—H2C	123.7	C8—C7—C6	110.5 (2)
C2—C3—H3A	124.0	C8—C7—H7A	110.2
C4—C3—C2	112.1 (3)	O3—C8—N1	123.5 (3)
C4—C3—H3A	124.0	O3—C8—C7	119.3 (2)
C3—C4—S	111.7 (2)	N1—C8—C7	117.1 (2)
C3—C4—C5	127.7 (3)	O4—C9—N2	123.9 (3)
C5—C4—S	120.6 (2)	O4—C9—C6	121.8 (2)
O1—C5—C4	111.2 (2)	N2—C9—C6	114.2 (2)
O1—C5—H5A	110.3

C4—S—C1—C2	−1.2 (3)	S—C4—C5—O2	−69.8 (3)
C1—S—C4—C3	0.2 (3)	C3—C4—C5—O1	−137.8 (3)
C1—S—C4—C5	177.7 (2)	C3—C4—C5—O2	107.3 (4)
C7—O1—C5—O2	−34.8 (2)	O2—C6—C7—O1	7.5 (3)
C7—O1—C5—C4	−153.8 (2)	O2—C6—C7—C8	−112.4 (2)
C5—O1—C7—C8	135.7 (2)	C9—C6—C7—O1	−110.7 (2)
C5—O1—C7—C6	16.5 (3)	C9—C6—C7—C8	129.4 (2)
C6—O2—C5—O1	39.8 (2)	O1—C7—C8—O3	162.6 (2)
C6—O2—C5—C4	159.2 (2)	O1—C7—C8—N1	−20.4 (3)
C5—O2—C6—C7	−28.8 (3)	C6—C7—C8—O3	−81.8 (3)
C5—O2—C6—C9	93.0 (2)	C6—C7—C8—N1	95.1 (3)
S—C1—C2—C3	1.7 (5)	O2—C6—C9—O4	−94.2 (3)
C1—C2—C3—C4	−1.6 (5)	O2—C6—C9—N2	82.7 (3)
C2—C3—C4—S	0.7 (4)	C7—C6—C9—O4	21.0 (4)
C2—C3—C4—C5	−176.6 (3)	C7—C6—C9—N2	−162.1 (2)
S—C4—C5—O1	45.1 (3)

Hydrogen-bond geometry (Å, º) top

D—H···A	D—H	H···A	D···A	D—H···A
N1—H1A···O3ⁱ	0.86	2.06	2.909 (4)	167
N1—H1B···O1	0.86	2.30	2.673 (4)	106
N2—H2A···O3ⁱⁱ	0.86	2.29	3.059 (3)	149
N2—H2B···O4ⁱⁱⁱ	0.86	2.28	3.077 (3)	154
C6—H6A···O4ⁱⁱⁱ	0.98	2.27	3.049 (3)	136
C7—H7A···O4	0.98	2.45	2.866 (3)	105

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

Experimental details

Crystal data
Chemical formula	C₉H₁₀N₂O₄S
M_r	242.25
Crystal system, space group	Monoclinic, P2₁
Temperature (K)	294
a, b, c (Å)	8.9250 (18), 4.796 (1), 12.109 (2)
β (°)	90.60 (3)
V (Å³)	518.29 (18)
Z	2
Radiation type	Mo Kα
µ (mm⁻¹)	0.31
Crystal size (mm)	0.30 × 0.20 × 0.10

Data collection
Diffractometer	Enraf–Nonius CAD-4 diffractometer
Absorption correction	ψ scan (North et al., 1968)
T_min, T_max	0.912, 0.969
No. of measured, independent and observed [I > 2σ(I)] reflections	2135, 2007, 1820
R_int	0.015
(sin θ/λ)_max (Å⁻¹)	0.616

Refinement
R[F² > 2σ(F²)], wR(F²), S	0.040, 0.124, 1.01
No. of reflections	2007
No. of parameters	146
No. of restraints	1
H-atom treatment	H-atom parameters constrained
Δρ_max, Δρ_min (e Å⁻³)	0.25, −0.34
Absolute structure	Flack (1983), 876 Friedel pairs
Absolute structure parameter	0.04 (12)

Computer programs: CAD-4 Software (Enraf–Nonius, 1989), XCAD4 (Harms & Wocadlo, 1995), SHELXS97 (Sheldrick, 2008), SHELXL97 (Sheldrick, 2008), ORTEP-3 for Windows (Farrugia, 1997), WinGX (Farrugia, 1999).

Hydrogen-bond geometry (Å, º) top

D—H···A	D—H	H···A	D···A	D—H···A
N1—H1A···O3ⁱ	0.86	2.06	2.909 (4)	167.00
N1—H1B···O1	0.86	2.30	2.673 (4)	106.00
N2—H2A···O3ⁱⁱ	0.86	2.29	3.059 (3)	149.00
N2—H2B···O4ⁱⁱⁱ	0.86	2.28	3.077 (3)	154.00
C6—H6A···O4ⁱⁱⁱ	0.98	2.27	3.049 (3)	136.00
C7—H7A···O4	0.98	2.45	2.866 (3)	105.00

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

Acknowledgements

The authors thank the Center of Testing and Analysis, Nanjing University, for support.

References

Allen, F. H., Kennard, O., Watson, D. G., Brammer, L., Orpen, A. G. & Taylor, R. (1987). J. Chem. Soc. Perkin Trans. 2, pp. S1–19. CrossRef Web of Science Google Scholar
Enraf–Nonius (1989). CAD-4 Software. Enraf–Nonius, Delft, The Netherlands. Google Scholar
Farrugia, L. J. (1997). J. Appl. Cryst. 30, 565. CrossRef IUCr Journals Google Scholar
Farrugia, L. J. (1999). J. Appl. Cryst. 32, 837–838. CrossRef CAS IUCr Journals Google Scholar
Flack, H. D. (1983). Acta Cryst. A39, 876–881. CrossRef CAS Web of Science IUCr Journals Google Scholar
Harms, K. & Wocadlo, S. (1995). XCAD4. University of Marburg, Germany. Google Scholar
Kim, D. K., Kim, G., Gam, J. S., Cho, Y. B., Kim, H. T., Tai, J. H., Kim, K. H., Hong, W. S. & Park, J. G. (1994). J. Med. Chem. 37, 1471–1485. CrossRef CAS PubMed Web of Science Google Scholar
North, A. C. T., Phillips, D. C. & Mathews, F. S. (1968). Acta Cryst. A24, 351–359. CrossRef IUCr Journals Web of Science Google Scholar
Pandey, G., Hajra, S., Ghorai, M. K. & Kumar, K. R. (1997). J. Org. Chem. 62, 5966–5973. CSD CrossRef CAS Web of Science Google Scholar
Sheldrick, G. M. (2008). Acta Cryst. A64, 112–122. Web of Science CrossRef CAS IUCr Journals Google Scholar