(S)-2-[(S,E)-4-(4-Chloro­phen­yl)-1-nitro­but-3-en-2-yl]cyclo­hexa­none

Li, Z.; Guo, Y.; Li, B.; Luo, S.

doi:10.1107/S1600536809029213

organic compounds

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ISSN: 2056-9890

Volume 65| Part 8| August 2009| Page o2023

doi:10.1107/S1600536809029213

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(S)-2-[(S,E)-4-(4-Chlorophenyl)-1-nitrobut-3-en-2-yl]cyclohexanone

Zhaobo Li,^a Yi Guo,^a Bailin Li ^b and Shuping Luo ^a ^*

^aState Key Laboratory Breeding Base of Green Chemistry-Synthesis Technology, Zhejiang University of Technology, Hangzhou 310014, People's Republic of China, and ^bDepartment of Pharmaceutical and Chemical Engineering, Taizhou College, Linhai, Zhejiang 317000, People's Republic of China
^*Correspondence e-mail: boyzb@163.com

(Received 11 July 2009; accepted 23 July 2009; online 29 July 2009)

The title compound, C₁₆H₁₈ClNO₃, was obtained by the organocatalytic asymmetric Michael addition of cyclohexanone to 1-chloro-4-[(1E,3E)-4-nitrobuta-1,3-dienyl]benzene. The double bond has an E configuration. The cyclohexanone ring adopts a chair conformation. The conformation of the molecule is stabilized by a weak intramolecular C—H⋯O hydrogen bond.

Related literature

For asymmetric Michael addition reactions employing chiral organocatalysts, see: Belot et al. (2008 ); Dalko & Moisan (2004 ); Yu et al. (2009 ). For details of the synthesis, see: Xu et al. (2008 ); For puckering parameters, see: Cremer & Pople (1975 ).

Experimental

Crystal data

C₁₆H₁₈ClNO₃
M_r = 307.78
Orthorhombic, P 2₁ 2₁ 2₁
a = 5.5300 (3) Å
b = 8.5175 (6) Å
c = 34.0903 (18) Å
V = 1605.71 (17) Å³
Z = 4
Mo Kα radiation
μ = 0.25 mm⁻¹
T = 296 K
0.48 × 0.32 × 0.28 mm

Data collection

Rigaku R-AXIS RAPID diffractometer
Absorption correction: multi-scan (ABSCOR; Higashi, 1995 ) T_min = 0.879, T_max = 0.933
15644 measured reflections
2148 independent reflections
1391 reflections with F² > 2σ(F²)
R_int = 0.034

Refinement

R[F² > 2σ(F²)] = 0.044
wR(F²) = 0.124
S = 1.00
2148 reflections
192 parameters
H-atom parameters constrained
Δρ_max = 0.22 e Å⁻³
Δρ_min = −0.27 e Å⁻³
Absolute structure: Flack (1983 ), 1486 Friedel pairs
Flack parameter: 0.27 (18)

Table 1
Hydrogen-bond geometry (Å, °)

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
C8—H81⋯O1	0.97	2.37	3.020 (4)	124

Data collection: PROCESS-AUTO (Rigaku, 2006 ); cell refinement: PROCESS-AUTO; data reduction: CrystalStructure (Rigaku, 2007 ); program(s) used to solve structure: SIR97 (Altomare et al., 1999 ); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008 ); molecular graphics: ORTEP-3 for Windows (Farrugia, 1997 ); software used to prepare material for publication: CrystalStructure.

Supporting information

Crystal structure: contains datablocks global, I. DOI: 10.1107/S1600536809029213/bx2224sup1.cif

Structure factors: contains datablock I. DOI: 10.1107/S1600536809029213/bx2224Isup2.hkl

checkCIF report

Comment top

As one of the most important chiral carbon-carbon bond-forming processes in modern organic chemistry, the field of asymmetric Michael addition employing chiral organocatalysts has gained more and more attention and become the focus of intense research efforts (Dalko & Moisan, 2004; Belot et al., 2008; Yu et al., 2009). Consequently, we have synthesized a series of Michael adducts by employing cyclo-ketones to nitrodienes in our laboratory. We report here the crystal structure and the absolute configuration of the title compound, (I). The cyclohexanone ring adopts a chair conformation as shown by the Cremer & Pople (1975) puckering parameters [Q_T= 0.568 (4) Å , θ=3.9 (4)° , ϕ=355 (6) °]. The C1—C9—C10—C11 torsion angle of 175.2 (3) confirms the E configuration of the molecule with respect to the C9═C10 bond. The conformation of (I) is stabilized by one weak intramolecular C—H···O hydrogen bond, Table 1, Fig 1.

Related literature top

For asymmetric Michael addition employing

chiral organocatalysts, see: Belot et al. (2008); Dalko & Moisan (2004); Yu et al. (2009). For details of the synthesis, see: Xu et al. (2008); For puckering parameters, see: Cremer & Pople (1975).

Experimental top

A 1,2-dichloroethane (0.5 ml) solution of cyclohexanone (0.25 mmol) and 1-chloro-4-((1E,3E)-4-nitrobuta-1,3-dienyl)benzene (0.25 mmol) in the presence of (S)-1-methyl-2-(pyrrolidin-2-ylmethylthio)-1H-imidazole (0.025 mmol) as amine catalyst and (R)-2-(3-(3,5-bis(trifluoromethyl)phenyl)thioureido)-2-phenylacetic acid (0.025 mmol) as acid module at room tempreture was stirred vigorously (Xu et al., 2008). After completion of the reaction, the resulted reaction mixture was purified directly by silica gel column chromatography (eluent: petroleum ether-EtOAc). Single crystals were obtained by slow evaporation of an ethanol-dichloromethane solution.

Computing details top

Data collection: PROCESS-AUTO (Rigaku, 2006); cell refinement: PROCESS-AUTO (Rigaku, 2006); data reduction: CrystalStructure (Rigaku Americas, 2007); program(s) used to solve structure: SIR97 (Altomare et al., 1999); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ORTEP-3 for Windows (Farrugia, 1997); software used to prepare material for publication: CrystalStructure (Rigaku Americas, 2007).

Figures top

Fig. 1. The asymmetric unit of the title compound with the atomic labeling scheme; displacement ellipsoids are drawn at the 30% probability level. Dotted lines show hydrogen bonding.

(I) top

Crystal data top

C₁₆H₁₈ClNO₃	F(000) = 648.00
M_r = 307.78	D_x = 1.273 Mg m⁻³
Orthorhombic, P2₁2₁2₁	Mo Kα radiation, λ = 0.71075 Å
Hall symbol: P 2ac 2ab	Cell parameters from 9533 reflections
a = 5.5300 (3) Å	θ = 3.0–27.4°
b = 8.5175 (6) Å	µ = 0.25 mm⁻¹
c = 34.0903 (18) Å	T = 296 K
V = 1605.71 (17) Å³	Chunk, colourless
Z = 4	0.48 × 0.32 × 0.28 mm

Data collection top

Rigaku R-AXIS RAPID diffractometer	1391 reflections with F² > 2σ(F²)
Detector resolution: 10.00 pixels mm^-1	R_int = 0.034
ω scans	θ_max = 27.4°
Absorption correction: multi-scan (ABSCOR; Higashi, 1995)	h = −7→6
T_min = 0.879, T_max = 0.933	k = −11→11
15644 measured reflections	l = −43→44
2148 independent reflections

Refinement top

Refinement on F²	(Δ/σ)_max = 0.001
R[F² > 2σ(F²)] = 0.044	Δρ_max = 0.22 e Å⁻³
wR(F²) = 0.124	Δρ_min = −0.27 e Å⁻³
S = 1.00	Extinction correction: SHELXL97 (Sheldrick, 2008)
2148 reflections	Extinction coefficient: 0.027 (2)
192 parameters	Absolute structure: Flack (1983), 1486 Friedel pairs
H-atom parameters constrained	Absolute structure parameter: 0.27 (18)
w = 1/[σ²(F_o²) + (0.032P)² + 1.2P] where P = (F_o² + 2F_c²)/3

Crystal data top

C₁₆H₁₈ClNO₃	V = 1605.71 (17) Å³
M_r = 307.78	Z = 4
Orthorhombic, P2₁2₁2₁	Mo Kα radiation
a = 5.5300 (3) Å	µ = 0.25 mm⁻¹
b = 8.5175 (6) Å	T = 296 K
c = 34.0903 (18) Å	0.48 × 0.32 × 0.28 mm

Data collection top

Rigaku R-AXIS RAPID diffractometer	2148 independent reflections
Absorption correction: multi-scan (ABSCOR; Higashi, 1995)	1391 reflections with F² > 2σ(F²)
T_min = 0.879, T_max = 0.933	R_int = 0.034
15644 measured reflections

Refinement top

R[F² > 2σ(F²)] = 0.044	H-atom parameters constrained
wR(F²) = 0.124	Δρ_max = 0.22 e Å⁻³
S = 1.00	Δρ_min = −0.27 e Å⁻³
2148 reflections	Absolute structure: Flack (1983), 1486 Friedel pairs
192 parameters	Absolute structure parameter: 0.27 (18)

Special details top

Geometry. ENTER SPECIAL DETAILS OF THE MOLECULAR GEOMETRY

Refinement. Refinement using all reflections. The weighted R-factor (wR) and goodness of fit (S) are based on F². R-factor (gt) are based on F. The threshold expression of F² > 2.0 σ(F²) is used only for calculating R-factor (gt).

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å²) top

	x	y	z	U_iso*/U_eq
Cl1	0.6163 (3)	0.6280 (2)	1.07164 (3)	0.1211 (6)
O1	0.9458 (5)	0.1436 (3)	0.77121 (8)	0.0735 (8)
O2	1.2699 (5)	0.5169 (4)	0.83637 (10)	0.0841 (9)
O3	0.9882 (6)	0.6855 (3)	0.84395 (12)	0.1019 (12)
N1	1.0607 (6)	0.5581 (4)	0.83299 (10)	0.0633 (8)
C1	0.8554 (6)	0.3047 (4)	0.84336 (9)	0.0497 (8)
C2	0.6749 (6)	0.1873 (4)	0.82559 (9)	0.0486 (7)
C3	0.6291 (9)	0.0456 (4)	0.85244 (10)	0.0705 (12)
C4	0.4412 (9)	−0.0647 (5)	0.83580 (12)	0.0801 (13)
C5	0.5077 (9)	−0.1218 (5)	0.79532 (12)	0.0764 (12)
C6	0.5632 (7)	0.0143 (4)	0.76796 (10)	0.0643 (10)
C7	0.7474 (7)	0.1200 (4)	0.78612 (10)	0.0532 (8)
C8	0.8838 (7)	0.4463 (4)	0.81622 (10)	0.0562 (9)
C9	0.7707 (7)	0.3624 (4)	0.88279 (9)	0.0552 (8)
C10	0.8919 (7)	0.3590 (4)	0.91561 (10)	0.0601 (9)
C11	0.8194 (7)	0.4250 (4)	0.95375 (10)	0.0596 (9)
C12	0.6144 (9)	0.5173 (5)	0.95838 (12)	0.0754 (12)
C13	0.5508 (9)	0.5783 (5)	0.99460 (12)	0.0809 (13)
C14	0.6932 (9)	0.5491 (5)	1.02636 (12)	0.0754 (12)
C15	0.8973 (10)	0.4591 (6)	1.02286 (12)	0.0837 (14)
C16	0.9577 (8)	0.3972 (5)	0.98675 (11)	0.0726 (12)
H1	1.0127	0.2531	0.8464	0.060*
H2	0.5206	0.2421	0.8221	0.058*
H9	0.6159	0.4048	0.8839	0.066*
H10	1.0415	0.3092	0.9148	0.072*
H12	0.5177	0.5386	0.9367	0.090*
H13	0.4118	0.6389	0.9972	0.097*
H15	0.9943	0.4398	1.0446	0.100*
H16	1.0952	0.3350	0.9846	0.087*
H31	0.7795	−0.0114	0.8558	0.085*
H32	0.5737	0.0833	0.8777	0.085*
H41	0.4259	−0.1546	0.8531	0.096*
H42	0.2875	−0.0101	0.8344	0.096*
H51	0.3737	−0.1816	0.7847	0.092*
H52	0.6493	−0.1886	0.7972	0.092*
H61	0.6255	−0.0263	0.7434	0.077*
H62	0.4162	0.0732	0.7631	0.077*
H81	0.9403	0.4111	0.7908	0.067*
H82	0.7287	0.4981	0.8133	0.067*

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
Cl1	0.1546 (14)	0.1453 (13)	0.0635 (6)	−0.0040 (13)	0.0179 (8)	−0.0299 (7)
O1	0.0658 (17)	0.0833 (19)	0.0716 (16)	−0.0074 (17)	0.0162 (14)	−0.0129 (15)
O2	0.0494 (16)	0.086 (2)	0.117 (2)	−0.0064 (16)	−0.0101 (17)	0.006 (2)
O3	0.087 (2)	0.0605 (17)	0.158 (3)	−0.005 (2)	−0.018 (2)	−0.023 (2)
N1	0.0585 (19)	0.0535 (18)	0.078 (2)	−0.0089 (18)	−0.0045 (18)	0.0029 (17)
C1	0.0521 (19)	0.0498 (18)	0.0472 (16)	−0.0037 (18)	−0.0020 (16)	−0.0009 (15)
C2	0.0516 (18)	0.0480 (18)	0.0462 (15)	−0.0035 (17)	0.0013 (15)	−0.0015 (14)
C3	0.101 (3)	0.061 (2)	0.0486 (18)	−0.032 (2)	0.001 (2)	0.0005 (17)
C4	0.109 (3)	0.067 (2)	0.064 (2)	−0.037 (2)	−0.000 (2)	−0.003 (2)
C5	0.095 (3)	0.058 (2)	0.076 (2)	−0.010 (2)	−0.009 (2)	−0.009 (2)
C6	0.072 (2)	0.068 (2)	0.0534 (19)	−0.009 (2)	−0.0023 (19)	−0.0126 (18)
C7	0.057 (2)	0.053 (2)	0.0501 (17)	0.0003 (19)	−0.0022 (17)	−0.0014 (16)
C8	0.060 (2)	0.050 (2)	0.0583 (19)	−0.014 (2)	−0.0097 (18)	0.0038 (16)
C9	0.058 (2)	0.055 (2)	0.0526 (17)	−0.0046 (19)	−0.0029 (17)	−0.0023 (17)
C10	0.063 (2)	0.064 (2)	0.0537 (18)	0.005 (2)	−0.0053 (18)	−0.0025 (17)
C11	0.064 (2)	0.060 (2)	0.0547 (19)	−0.005 (2)	−0.0032 (18)	−0.0040 (17)
C12	0.079 (2)	0.089 (3)	0.058 (2)	0.010 (2)	−0.006 (2)	−0.006 (2)
C13	0.085 (3)	0.089 (3)	0.069 (2)	0.014 (3)	0.002 (2)	−0.012 (2)
C14	0.090 (3)	0.079 (2)	0.058 (2)	−0.012 (3)	0.006 (2)	−0.007 (2)
C15	0.097 (3)	0.101 (3)	0.053 (2)	−0.006 (3)	−0.011 (2)	−0.005 (2)
C16	0.075 (2)	0.085 (3)	0.059 (2)	0.004 (2)	−0.011 (2)	−0.003 (2)

Geometric parameters (Å, º) top

Cl1—C14	1.736 (4)	C14—C15	1.370 (7)
O1—C7	1.226 (4)	C15—C16	1.380 (5)
O2—N1	1.214 (4)	C1—H1	0.980
O3—N1	1.216 (4)	C2—H2	0.980
N1—C8	1.480 (5)	C3—H31	0.970
C1—C2	1.537 (4)	C3—H32	0.970
C1—C8	1.528 (4)	C4—H41	0.970
C1—C9	1.506 (4)	C4—H42	0.970
C2—C3	1.536 (5)	C5—H51	0.970
C2—C7	1.516 (4)	C5—H52	0.970
C3—C4	1.512 (6)	C6—H61	0.970
C4—C5	1.509 (5)	C6—H62	0.970
C5—C6	1.519 (5)	C8—H81	0.970
C6—C7	1.494 (5)	C8—H82	0.970
C9—C10	1.305 (4)	C9—H9	0.930
C10—C11	1.472 (5)	C10—H10	0.930
C11—C12	1.389 (6)	C12—H12	0.930
C11—C16	1.381 (5)	C13—H13	0.930
C12—C13	1.385 (6)	C15—H15	0.930
C13—C14	1.362 (6)	C16—H16	0.930

O2—N1—O3	122.9 (3)	C2—C3—H32	108.8
O2—N1—C8	118.7 (3)	C4—C3—H31	108.8
O3—N1—C8	118.4 (3)	C4—C3—H32	108.8
C2—C1—C8	110.0 (2)	H31—C3—H32	109.5
C2—C1—C9	111.2 (2)	C3—C4—H41	108.8
C8—C1—C9	108.3 (2)	C3—C4—H42	108.8
C1—C2—C3	112.5 (2)	C5—C4—H41	108.8
C1—C2—C7	115.1 (2)	C5—C4—H42	108.8
C3—C2—C7	106.0 (2)	H41—C4—H42	109.5
C2—C3—C4	112.2 (3)	C4—C5—H51	109.0
C3—C4—C5	112.1 (3)	C4—C5—H52	109.0
C4—C5—C6	111.4 (3)	C6—C5—H51	109.0
C5—C6—C7	110.1 (3)	C6—C5—H52	109.0
O1—C7—C2	122.9 (3)	H51—C5—H52	109.5
O1—C7—C6	122.5 (3)	C5—C6—H61	109.3
C2—C7—C6	114.5 (3)	C5—C6—H62	109.3
N1—C8—C1	110.0 (2)	C7—C6—H61	109.3
C1—C9—C10	126.8 (3)	C7—C6—H62	109.3
C9—C10—C11	127.5 (3)	H61—C6—H62	109.5
C10—C11—C12	122.6 (3)	N1—C8—H81	109.3
C10—C11—C16	120.2 (3)	N1—C8—H82	109.3
C12—C11—C16	117.2 (3)	C1—C8—H81	109.3
C11—C12—C13	121.4 (4)	C1—C8—H82	109.3
C12—C13—C14	119.6 (4)	H81—C8—H82	109.5
Cl1—C14—C13	119.6 (3)	C1—C9—H9	116.6
Cl1—C14—C15	119.7 (3)	C10—C9—H9	116.6
C13—C14—C15	120.6 (4)	C9—C10—H10	116.2
C14—C15—C16	119.4 (4)	C11—C10—H10	116.2
C11—C16—C15	121.8 (4)	C11—C12—H12	119.3
C2—C1—H1	109.1	C13—C12—H12	119.3
C8—C1—H1	109.1	C12—C13—H13	120.2
C9—C1—H1	109.1	C14—C13—H13	120.2
C1—C2—H2	107.6	C14—C15—H15	120.3
C3—C2—H2	107.6	C16—C15—H15	120.3
C7—C2—H2	107.6	C11—C16—H16	119.1
C2—C3—H31	108.8	C15—C16—H16	119.1

O2—N1—C8—C1	64.7 (4)	C3—C4—C5—C6	−52.9 (5)
O3—N1—C8—C1	−112.9 (3)	C4—C5—C6—C7	52.5 (5)
C2—C1—C8—N1	−179.0 (2)	C5—C6—C7—O1	118.3 (4)
C8—C1—C2—C3	−177.3 (3)	C5—C6—C7—C2	−58.0 (4)
C8—C1—C2—C7	61.1 (3)	C1—C9—C10—C11	175.2 (3)
C2—C1—C9—C10	126.2 (4)	C9—C10—C11—C12	−7.3 (6)
C9—C1—C2—C3	−57.3 (4)	C9—C10—C11—C16	173.2 (4)
C9—C1—C2—C7	−178.8 (2)	C10—C11—C12—C13	−179.8 (4)
C8—C1—C9—C10	−112.8 (4)	C10—C11—C16—C15	179.0 (4)
C9—C1—C8—N1	59.2 (3)	C12—C11—C16—C15	−0.6 (6)
C1—C2—C3—C4	176.8 (3)	C16—C11—C12—C13	−0.2 (6)
C1—C2—C7—O1	7.5 (5)	C11—C12—C13—C14	0.8 (7)
C1—C2—C7—C6	−176.2 (3)	C12—C13—C14—Cl1	179.0 (3)
C3—C2—C7—O1	−117.6 (4)	C12—C13—C14—C15	−0.5 (7)
C3—C2—C7—C6	58.7 (4)	Cl1—C14—C15—C16	−179.8 (3)
C7—C2—C3—C4	−56.6 (4)	C13—C14—C15—C16	−0.3 (6)
C2—C3—C4—C5	56.4 (4)	C14—C15—C16—C11	0.8 (7)

Hydrogen-bond geometry (Å, º) top

D—H···A	D—H	H···A	D···A	D—H···A
C8—H81···O1	0.97	2.37	3.020 (4)	124

Experimental details

Crystal data
Chemical formula	C₁₆H₁₈ClNO₃
M_r	307.78
Crystal system, space group	Orthorhombic, P2₁2₁2₁
Temperature (K)	296
a, b, c (Å)	5.5300 (3), 8.5175 (6), 34.0903 (18)
V (Å³)	1605.71 (17)
Z	4
Radiation type	Mo Kα
µ (mm⁻¹)	0.25
Crystal size (mm)	0.48 × 0.32 × 0.28

Data collection
Diffractometer	Rigaku R-AXIS RAPID diffractometer
Absorption correction	Multi-scan (ABSCOR; Higashi, 1995)
T_min, T_max	0.879, 0.933
No. of measured, independent and observed [F² > 2σ(F²)] reflections	15644, 2148, 1391
R_int	0.034
(sin θ/λ)_max (Å⁻¹)	0.648

Refinement
R[F² > 2σ(F²)], wR(F²), S	0.044, 0.124, 1.00
No. of reflections	2148
No. of parameters	192
No. of restraints	?
H-atom treatment	H-atom parameters constrained
Δρ_max, Δρ_min (e Å⁻³)	0.22, −0.27
Absolute structure	Flack (1983), 1486 Friedel pairs
Absolute structure parameter	0.27 (18)

Computer programs: PROCESS-AUTO (Rigaku, 2006), CrystalStructure (Rigaku Americas, 2007), SIR97 (Altomare et al., 1999), SHELXL97 (Sheldrick, 2008), ORTEP-3 for Windows (Farrugia, 1997).

Hydrogen-bond geometry (Å, º) top

D—H···A	D—H	H···A	D···A	D—H···A
C8—H81···O1	0.97	2.37	3.020 (4)	124

Acknowledgements

We acknowledge the help of Professor Jianming Gu of Zhejiang University. We are also grateful for financial support by the Catalytic Hydrogenation Research Center of Zhejiang University of Technology.

References

Altomare, A., Burla, M. C., Camalli, M., Cascarano, G. L., Giacovazzo, C., Guagliardi, A., Moliterni, A. G. G., Polidori, G. & Spagna, R. (1999). J. Appl. Cryst. 32, 115–119. Web of Science CrossRef CAS IUCr Journals Google Scholar
Belot, S., Massaro, A., Tenti, A., Mordini, A. & Alexakis, A. (2008). Org. Lett. 10, 4557–4560. Web of Science CrossRef PubMed CAS Google Scholar
Cremer, D. & Pople, J. A. (1975). J. Am. Chem. Soc. 97, 1354–1358. CrossRef CAS Web of Science Google Scholar
Dalko, P. I. & Moisan, L. (2004). Angew. Chem. Int. Ed. 43, 5138–5175. Web of Science CrossRef CAS Google Scholar
Farrugia, L. J. (1997). J. Appl. Cryst. 30, 565. CrossRef IUCr Journals Google Scholar
Flack, H. D. (1983). Acta Cryst. A39, 876–881. CrossRef CAS Web of Science IUCr Journals Google Scholar
Higashi, T. (1995). ABSCOR. Rigaku Corporation, Tokyo, Japan. Google Scholar
Rigaku (2006). PROCESS-AUTO. Rigaku Corporation, Tokyo, Japan. Google Scholar
Rigaku (2007). CrystalStructure. Rigaku Americas, The Woodlands, Texas, USA. Google Scholar
Sheldrick, G. M. (2008). Acta Cryst. A64, 112–122. Web of Science CrossRef CAS IUCr Journals Google Scholar
Xu, D., Yue, H., Luo, S., Xia, A., Zhang, S. & Xu, Z. (2008). Org. Biomol. Chem. 6, 2054–2057. Web of Science CrossRef PubMed CAS Google Scholar
Yu, Z., Liu, X., Zhou, L., Lin, L. & Feng, X. (2009). Angew. Chem. Int. Ed. 48, 5195–5198. Web of Science CSD CrossRef CAS Google Scholar