(1E,4E)-1-(Thio­phen-2-yl)-5-(2,6,6-trimethyl­cyclo­hex-1-en-1-yl)penta-1,4-dien-3-one

Zhang, Y.-L.; Xiang, L.-F.; Zou, P.; Jin, Y.-J.; Yang, S.-L.

doi:10.1107/S1600536812022465

organic compounds

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ISSN: 2056-9890

Volume 68| Part 6| June 2012| Page o1859

doi:10.1107/S1600536812022465

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(1E,4E)-1-(Thiophen-2-yl)-5-(2,6,6-trimethylcyclohex-1-en-1-yl)penta-1,4-dien-3-one

Ya-Li Zhang,^a Liu-Fang Xiang,^a Peng Zou,^a Yi-Jun Jin ^a and Shu-Lin Yang ^a ^*

^aInstitute of Biotechnology, Nanjing University of Science and Technology, Nanjing, Jiangsu Province 210094, People's Republic of China
^*Correspondence e-mail: shulin_yang@126.com

(Received 6 May 2012; accepted 17 May 2012; online 23 May 2012)

In the title curcumin–ionone derivative, C₁₈H₂₂OS, the dihedral angle between the thiazole ring and the mean plane through the cyclohexene ring is 5.16 (10)°. The molecule has an E conformation for each of the olefinic bonds.

Related literature

For related structures, see: Liang et al. (2007 ); Zou et al. (2012 ). For background to the biological properties of curcumin–ionone derivatives, see: Anand et al. (2008 ); Zhao et al. (2010a ,b ); Zhou et al. (2009a ,b ).

Experimental

Crystal data

C₁₈H₂₂OS
M_r = 286.42
Monoclinic, P 2₁ /m
a = 8.3401 (17) Å
b = 6.9084 (14) Å
c = 13.994 (3) Å
β = 96.082 (4)°
V = 801.7 (3) Å³
Z = 2
Mo Kα radiation
μ = 0.20 mm⁻¹
T = 293 K
0.36 × 0.30 × 0.15 mm

Data collection

Bruker SMART CCD area-detector diffractometer
Absorption correction: multi-scan (SADABS; Bruker, 2002 ) T_min = 0.674, T_max = 1.000
4888 measured reflections
1711 independent reflections
1555 reflections with I > 2σ(I)
R_int = 0.020

Refinement

R[F² > 2σ(F²)] = 0.046
wR(F²) = 0.137
S = 1.05
1711 reflections
145 parameters
4 restraints
H atoms treated by a mixture of independent and constrained refinement
Δρ_max = 0.32 e Å⁻³
Δρ_min = −0.17 e Å⁻³

Data collection: SMART (Bruker, 2002); cell refinement: SAINT (Bruker, 2002); 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.

Supporting information

Crystal structure: contains datablocks I, global. DOI: 10.1107/S1600536812022465/ng5270sup1.cif

Structure factors: contains datablock I. DOI: 10.1107/S1600536812022465/ng5270Isup2.hkl

Supporting information file. DOI: 10.1107/S1600536812022465/ng5270Isup3.cml

checkCIF report

Comment top

The β-ionone unit is a phytochemical present in many fruit, vegetable and grain. It is found to exert in vitro anticarcinogenic and antitumor activities; ionone derivatives are valuable intermediates for the chemo-enzymatic synthesis of carotenoids, astaxanthin and zeaxanthin (Zhou et al., 2009a,b). On the other hand, curcumin (diferuloylmethane) is a polyphenolic phytochemical found in turmeric (Curcuma longa) that has useful medicinal properties (Anand et al., 2008). Previously, we have evaluated mono-carbonylanalogues of curcumin for anti-inflammatory properties and discussed structure-activity relationships (Liang et al., 2007; Zhao et al.,2010a,b).

In the ionone-based curcumin title compound (Scheme I), all bond dimensions are normal. The dihedral angles between the thiazole ring and the cyclohexene ring is 5.16 (10)°.

Related literature top

For related structures, see: Liang et al. (2007); Zou et al. (2012). For background to the biological properties of curcumin–ionone derivatives, see: Anand et al. (2008); Zhao et al. (2010a,b); Zhou et al. (2009a,b).

Experimental top

To the mixture of β-ionone (2.5 mmol, 0.481 g) and thiophene-2-carbaldehyde (2.5mmol) in 10 ml EtOH, 1 ml of 10% NaOH was added and the mixture was stirred for 12 h at room temperature. After addition of 10 ml H₂O, the solution was extracted by 3×10 ml CH₂Cl₂. The crude product obtained from the combined organic layers was purified by silica gel column chromatography (elutant: EtOAc/hexane). Crystals were obtained from an ethanol/EtOAc solution (1:2, v/v) at 293 K.

Computing details top

Data collection: SMART (Bruker, 2002); cell refinement: SAINT (Bruker, 2002); data reduction: SAINT (Bruker, 2002); 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

Fig. 1. The molecular structure of the title compound, showing 30% displacement ellipsoids for the non-hydrogen atoms. Hydrogen atoms are drawn as spheres of arbitrary radius.

(1E,4E)-1-(Thiophen-2-yl)-5-(2,6,6-trimethylcyclohex-1-en-1- yl)penta-1,4-dien-3-one top

Crystal data top

C₁₈H₂₂OS	F(000) = 308
M_r = 286.42	D_x = 1.186 Mg m⁻³
Monoclinic, P2₁/m	Mo Kα radiation, λ = 0.71073 Å
a = 8.3401 (17) Å	Cell parameters from 2746 reflections
b = 6.9084 (14) Å	θ = 4.9–56.5°
c = 13.994 (3) Å	µ = 0.20 mm⁻¹
β = 96.082 (4)°	T = 293 K
V = 801.7 (3) Å³	Prismatic, colorless
Z = 2	0.36 × 0.30 × 0.15 mm

Data collection top

Bruker SMART CCD area-detector diffractometer	1711 independent reflections
Radiation source: fine-focus sealed tube	1555 reflections with I > 2σ(I)
Graphite monochromator	R_int = 0.020
phi and ω scans	θ_max = 26.0°, θ_min = 1.5°
Absorption correction: multi-scan (SADABS; Bruker, 2002)	h = −10→8
T_min = 0.674, T_max = 1.000	k = −8→8
4888 measured reflections	l = −17→16

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)] = 0.046	H atoms treated by a mixture of independent and constrained refinement
wR(F²) = 0.137	w = 1/[σ²(F_o²) + (0.091P)² + 0.1463P] where P = (F_o² + 2F_c²)/3
S = 1.05	(Δ/σ)_max = 0.002
1711 reflections	Δρ_max = 0.32 e Å⁻³
145 parameters	Δρ_min = −0.17 e Å⁻³
4 restraints	Extinction correction: SHELXL97 (Sheldrick, 2008), Fc^*=kFc[1+0.001xFc²λ³/sin(2θ)]^-1/4
Primary atom site location: structure-invariant direct methods	Extinction coefficient: 0.013 (5)

Crystal data top

C₁₈H₂₂OS	V = 801.7 (3) Å³
M_r = 286.42	Z = 2
Monoclinic, P2₁/m	Mo Kα radiation
a = 8.3401 (17) Å	µ = 0.20 mm⁻¹
b = 6.9084 (14) Å	T = 293 K
c = 13.994 (3) Å	0.36 × 0.30 × 0.15 mm
β = 96.082 (4)°

Data collection top

Bruker SMART CCD area-detector diffractometer	1711 independent reflections
Absorption correction: multi-scan (SADABS; Bruker, 2002)	1555 reflections with I > 2σ(I)
T_min = 0.674, T_max = 1.000	R_int = 0.020
4888 measured reflections

Refinement top

R[F² > 2σ(F²)] = 0.046	4 restraints
wR(F²) = 0.137	H atoms treated by a mixture of independent and constrained refinement
S = 1.05	Δρ_max = 0.32 e Å⁻³
1711 reflections	Δρ_min = −0.17 e Å⁻³
145 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	Occ. (<1)
S1	1.08493 (6)	0.2500	0.58526 (4)	0.0487 (3)
O1	0.4327 (2)	0.2500	0.53246 (12)	0.0817 (7)
C1	1.2154 (3)	0.2500	0.68697 (16)	0.0521 (6)
H1	1.3270	0.2500	0.6874	0.063*
C2	1.1384 (3)	0.2500	0.76737 (16)	0.0533 (6)
H2	1.1919	0.2500	0.8292	0.064*
C3	0.9688 (3)	0.2500	0.74818 (15)	0.0464 (5)
H3	0.8982	0.2500	0.7954	0.056*
C4	0.9204 (3)	0.2500	0.65023 (14)	0.0416 (5)
C5	0.7575 (3)	0.2500	0.60586 (16)	0.0463 (5)
H5	0.6772	0.2500	0.6472	0.056*
C6	0.7078 (3)	0.2500	0.51249 (15)	0.0504 (6)
H6	0.7839	0.2500	0.4685	0.060*
C7	0.5361 (3)	0.2500	0.47694 (16)	0.0525 (6)
C8	0.4939 (3)	0.2500	0.37222 (15)	0.0537 (6)
H8	0.5754	0.2500	0.3317	0.064*
C9	0.3412 (3)	0.2500	0.33499 (15)	0.0478 (5)
H9	0.2683	0.2500	0.3809	0.057*
C10	0.2646 (3)	0.2500	0.23645 (14)	0.0442 (5)
C11	0.1024 (3)	0.2500	0.22224 (15)	0.0439 (5)
C12	0.0090 (3)	0.2500	0.12463 (18)	0.0631 (7)
C13	0.1137 (5)	0.3120 (7)	0.0454 (2)	0.0749 (18)	0.50
C14	0.2610 (5)	0.1876 (8)	0.0579 (2)	0.0745 (19)	0.50
C15	0.3698 (3)	0.2500	0.15222 (17)	0.0624 (7)
C16	−0.0078 (3)	0.2500	0.30125 (17)	0.0516 (6)
H16A	−0.0320	0.3810	0.3175	0.077*	0.50
H16B	0.0445	0.1857	0.3569	0.077*	0.50
H16C	−0.1060	0.1833	0.2796	0.077*	0.50
C17	0.4761 (3)	0.0695 (4)	0.15515 (17)	0.0931 (8)
H17A	0.5515	0.0723	0.2119	0.140*
H17B	0.5338	0.0669	0.0993	0.140*
H17C	0.4099	−0.0440	0.1560	0.140*
H14A	0.317 (4)	0.2500	0.005 (2)	0.091 (11)*
H12A	−0.061 (3)	0.137 (3)	0.1184 (16)	0.077 (6)*
H14B	0.253 (8)	0.046 (3)	0.061 (5)	0.13 (3)*	0.50
H13A	0.060 (4)	0.2500	−0.0122 (18)	0.096 (11)*
H13B	0.159 (5)	0.445 (3)	0.048 (3)	0.052 (11)*	0.50

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
S1	0.0413 (4)	0.0712 (4)	0.0340 (3)	0.000	0.0062 (2)	0.000
O1	0.0398 (9)	0.168 (2)	0.0387 (9)	0.000	0.0092 (7)	0.000
C1	0.0394 (11)	0.0740 (16)	0.0423 (11)	0.000	0.0013 (9)	0.000
C2	0.0513 (13)	0.0721 (15)	0.0351 (11)	0.000	−0.0017 (9)	0.000
C3	0.0454 (12)	0.0565 (13)	0.0375 (10)	0.000	0.0053 (9)	0.000
C4	0.0409 (11)	0.0488 (11)	0.0356 (10)	0.000	0.0065 (8)	0.000
C5	0.0399 (11)	0.0601 (13)	0.0398 (11)	0.000	0.0088 (8)	0.000
C6	0.0403 (12)	0.0745 (16)	0.0372 (11)	0.000	0.0085 (9)	0.000
C7	0.0407 (12)	0.0787 (16)	0.0388 (11)	0.000	0.0069 (9)	0.000
C8	0.0451 (12)	0.0802 (17)	0.0364 (11)	0.000	0.0075 (9)	0.000
C9	0.0437 (11)	0.0646 (14)	0.0357 (10)	0.000	0.0066 (8)	0.000
C10	0.0453 (11)	0.0540 (12)	0.0333 (10)	0.000	0.0043 (8)	0.000
C11	0.0469 (11)	0.0470 (11)	0.0374 (10)	0.000	0.0026 (9)	0.000
C12	0.0506 (14)	0.093 (2)	0.0437 (13)	0.000	−0.0045 (11)	0.000
C13	0.071 (2)	0.115 (6)	0.0359 (15)	0.016 (2)	−0.0032 (14)	0.0065 (18)
C14	0.069 (2)	0.121 (6)	0.0340 (15)	0.018 (2)	0.0062 (14)	−0.0074 (17)
C15	0.0488 (13)	0.103 (2)	0.0362 (11)	0.000	0.0087 (10)	0.000
C16	0.0444 (12)	0.0626 (14)	0.0486 (12)	0.000	0.0089 (9)	0.000
C17	0.0879 (16)	0.113 (2)	0.0838 (15)	0.0131 (14)	0.0331 (12)	−0.0305 (14)

Geometric parameters (Å, º) top

S1—C1	1.698 (2)	C12—C13	1.543 (5)
S1—C4	1.724 (2)	C12—C13ⁱ	1.543 (5)
O1—C7	1.220 (3)	C12—H12A	0.97 (2)
C1—C2	1.353 (3)	C13—C13ⁱ	0.856 (10)
C1—H1	0.9300	C13—C14ⁱ	1.222 (6)
C2—C3	1.411 (3)	C13—C14	1.494 (6)
C2—H2	0.9300	C13—H13A	0.979 (18)
C3—C4	1.387 (3)	C13—H13B	0.994 (19)
C3—H3	0.9300	C14—C14ⁱ	0.862 (11)
C4—C5	1.433 (3)	C14—C13ⁱ	1.222 (6)
C5—C6	1.328 (3)	C14—C15	1.580 (4)
C5—H5	0.9300	C14—H14A	1.015 (18)
C6—C7	1.465 (3)	C14—H14B	0.99 (2)
C6—H6	0.9300	C15—C17	1.528 (3)
C7—C8	1.470 (3)	C15—C17ⁱ	1.528 (3)
C8—C9	1.324 (3)	C15—C14ⁱ	1.580 (4)
C8—H8	0.9300	C16—H16A	0.9600
C9—C10	1.457 (3)	C16—H16B	0.9600
C9—H9	0.9300	C16—H16C	0.9600
C10—C11	1.346 (3)	C17—H17A	0.9600
C10—C15	1.542 (3)	C17—H17B	0.9600
C11—C12	1.499 (3)	C17—H17C	0.9600
C11—C16	1.511 (3)

C1—S1—C4	91.89 (10)	C13ⁱ—C13—H13A	64.1 (7)
C2—C1—S1	112.24 (18)	C14ⁱ—C13—H13A	119 (2)
C2—C1—H1	123.9	C14—C13—H13A	98.3 (16)
S1—C1—H1	123.9	C12—C13—H13A	103 (2)
C1—C2—C3	113.3 (2)	C13ⁱ—C13—H13B	158 (3)
C1—C2—H2	123.3	C14ⁱ—C13—H13B	68 (3)
C3—C2—H2	123.3	C14—C13—H13B	103 (3)
C4—C3—C2	111.63 (19)	C12—C13—H13B	118 (2)
C4—C3—H3	124.2	H13A—C13—H13B	125 (3)
C2—C3—H3	124.2	C14ⁱ—C14—C13ⁱ	89.9 (3)
C3—C4—C5	126.24 (19)	C14ⁱ—C14—C13	54.9 (3)
C3—C4—S1	110.90 (16)	C13ⁱ—C14—C13	35.0 (4)
C5—C4—S1	122.86 (16)	C14ⁱ—C14—C15	74.18 (19)
C6—C5—C4	127.5 (2)	C13ⁱ—C14—C15	126.7 (3)
C6—C5—H5	116.2	C13—C14—C15	109.5 (3)
C4—C5—H5	116.2	C14ⁱ—C14—H14A	64.9 (6)
C5—C6—C7	121.74 (19)	C13ⁱ—C14—H14A	115 (2)
C5—C6—H6	119.1	C13—C14—H14A	96.0 (16)
C7—C6—H6	119.1	C15—C14—H14A	103 (2)
O1—C7—C6	121.0 (2)	C14ⁱ—C14—H14B	175 (4)
O1—C7—C8	121.6 (2)	C13ⁱ—C14—H14B	86 (4)
C6—C7—C8	117.42 (19)	C13—C14—H14B	121 (4)
C9—C8—C7	120.7 (2)	C15—C14—H14B	106 (4)
C9—C8—H8	119.6	H14A—C14—H14B	119 (4)
C7—C8—H8	119.6	C17—C15—C17ⁱ	109.3 (3)
C8—C9—C10	132.8 (2)	C17—C15—C10	110.80 (14)
C8—C9—H9	113.6	C17ⁱ—C15—C10	110.80 (14)
C10—C9—H9	113.6	C17—C15—C14ⁱ	121.7 (2)
C11—C10—C9	118.22 (19)	C17ⁱ—C15—C14ⁱ	94.6 (2)
C11—C10—C15	122.08 (19)	C10—C15—C14ⁱ	108.4 (2)
C9—C10—C15	119.7 (2)	C17—C15—C14	94.6 (2)
C10—C11—C12	123.5 (2)	C17ⁱ—C15—C14	121.7 (2)
C10—C11—C16	124.87 (19)	C10—C15—C14	108.4 (2)
C12—C11—C16	111.65 (19)	C14ⁱ—C15—C14	31.6 (4)
C11—C12—C13	112.1 (2)	C11—C16—H16A	109.5
C11—C12—C13ⁱ	112.1 (2)	C11—C16—H16B	109.5
C13—C12—C13ⁱ	32.2 (4)	H16A—C16—H16B	109.5
C11—C12—H12A	109.2 (14)	C11—C16—H16C	109.5
C13—C12—H12A	122.5 (13)	H16A—C16—H16C	109.5
C13ⁱ—C12—H12A	95.3 (13)	H16B—C16—H16C	109.5
C13ⁱ—C13—C14ⁱ	90.1 (3)	C15—C17—H17A	109.5
C13ⁱ—C13—C14	54.9 (3)	C15—C17—H17B	109.5
C14ⁱ—C13—C14	35.2 (5)	H17A—C17—H17B	109.5
C13ⁱ—C13—C12	73.90 (18)	C15—C17—H17C	109.5
C14ⁱ—C13—C12	122.3 (3)	H17A—C17—H17C	109.5
C14—C13—C12	106.1 (3)	H17B—C17—H17C	109.5

C4—S1—C1—C2	0.0	C11—C12—C13—C14	51.6 (4)
S1—C1—C2—C3	0.0	C13ⁱ—C12—C13—C14	−45.1 (3)
C1—C2—C3—C4	0.0	C13ⁱ—C13—C14—C14ⁱ	180.0
C2—C3—C4—C5	180.0	C12—C13—C14—C14ⁱ	−123.8 (3)
C2—C3—C4—S1	0.0	C14ⁱ—C13—C14—C13ⁱ	180.0
C1—S1—C4—C3	0.0	C12—C13—C14—C13ⁱ	56.2 (3)
C1—S1—C4—C5	180.0	C13ⁱ—C13—C14—C15	−127.0 (4)
C3—C4—C5—C6	180.0	C14ⁱ—C13—C14—C15	53.0 (4)
S1—C4—C5—C6	0.0	C12—C13—C14—C15	−70.7 (4)
C4—C5—C6—C7	180.0	C11—C10—C15—C17	−119.22 (17)
C5—C6—C7—O1	0.0	C9—C10—C15—C17	60.78 (17)
C5—C6—C7—C8	180.0	C11—C10—C15—C17ⁱ	119.22 (17)
O1—C7—C8—C9	0.0	C9—C10—C15—C17ⁱ	−60.78 (17)
C6—C7—C8—C9	180.0	C11—C10—C15—C14ⁱ	16.7 (2)
C7—C8—C9—C10	180.0	C9—C10—C15—C14ⁱ	−163.3 (2)
C8—C9—C10—C11	180.0	C11—C10—C15—C14	−16.7 (2)
C8—C9—C10—C15	0.0	C9—C10—C15—C14	163.3 (2)
C9—C10—C11—C12	180.0	C14ⁱ—C14—C15—C17	−150.87 (16)
C15—C10—C11—C12	0.0	C13ⁱ—C14—C15—C17	131.5 (6)
C9—C10—C11—C16	0.0	C13—C14—C15—C17	166.3 (3)
C15—C10—C11—C16	180.0	C14ⁱ—C14—C15—C17ⁱ	−34.77 (18)
C10—C11—C12—C13	−17.4 (2)	C13ⁱ—C14—C15—C17ⁱ	−112.4 (5)
C16—C11—C12—C13	162.6 (2)	C13—C14—C15—C17ⁱ	−77.6 (4)
C10—C11—C12—C13ⁱ	17.4 (2)	C14ⁱ—C14—C15—C10	95.42 (9)
C16—C11—C12—C13ⁱ	−162.6 (2)	C13ⁱ—C14—C15—C10	17.8 (6)
C11—C12—C13—C13ⁱ	96.72 (10)	C13—C14—C15—C10	52.6 (4)
C11—C12—C13—C14ⁱ	17.1 (6)	C13ⁱ—C14—C15—C14ⁱ	−77.6 (6)
C13ⁱ—C12—C13—C14ⁱ	−79.6 (5)	C13—C14—C15—C14ⁱ	−42.8 (4)

Symmetry code: (i) x, −y+1/2, z.

Experimental details

Crystal data
Chemical formula	C₁₈H₂₂OS
M_r	286.42
Crystal system, space group	Monoclinic, P2₁/m
Temperature (K)	293
a, b, c (Å)	8.3401 (17), 6.9084 (14), 13.994 (3)
β (°)	96.082 (4)
V (Å³)	801.7 (3)
Z	2
Radiation type	Mo Kα
µ (mm⁻¹)	0.20
Crystal size (mm)	0.36 × 0.30 × 0.15

Data collection
Diffractometer	Bruker SMART CCD area-detector diffractometer
Absorption correction	Multi-scan (SADABS; Bruker, 2002)
T_min, T_max	0.674, 1.000
No. of measured, independent and observed [I > 2σ(I)] reflections	4888, 1711, 1555
R_int	0.020
(sin θ/λ)_max (Å⁻¹)	0.617

Refinement
R[F² > 2σ(F²)], wR(F²), S	0.046, 0.137, 1.05
No. of reflections	1711
No. of parameters	145
No. of restraints	4
H-atom treatment	H atoms treated by a mixture of independent and constrained refinement
Δρ_max, Δρ_min (e Å⁻³)	0.32, −0.17

Computer programs: SMART (Bruker, 2002), SAINT (Bruker, 2002), SHELXS97 (Sheldrick, 2008), SHELXL97 (Sheldrick, 2008), SHELXTL (Sheldrick, 2008).

Acknowledgements

The authors are grateful for the Program Foundation of the Ministry of Education of China (grant No. 20093219110013) and the Nanjing University of Science and Technology Training Grant (to S-LY and Y-JJ). The crystallographic services at the Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, are gratefully acknowledged.

References

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