Chinese Journal of Chemistry, 2006, 24, 1402—1405 Full Paper

* E-mail: zhaog@mail.sioc.ac.cn; rewcao2002@yahoo.com.cn Received February 28, 2006; revised May 18, 2006; accepted June 12, 2006. Project supported by the National Natural Science Foundation of China (Nos. 20525208, 20532040, 20390050).

© 2006 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

An Efficient Method for Catalytic Asymmetric Reduction of Diketones and Application of Synthesis to Chiral 2,5-

Diphenylpyrrolidine and 2,5-Diphenylthiolane

LI, Xianga,b(李响) ZHAO, Gang*,a(赵刚) CAO, Wei-Guo*,b(曹卫国) a Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, Shanghai 200032, China

b Department of Chemistry, Shanghai University, Shanghai 200436, China

Asymmetric reduction of diketones with borane reagents generated in situ using cheap and available NaBH4 and SnCl2 in the presence of (S)-(－)-α,α-diphenyl-2-pyrrolidinemethanol was successfully achieved to yield the corre-sponding chiral diols with excellent stereoselectivity and enantioselectivity. And the chiral diol was transformed into optically pure C2-symmetricl chiral amine or thioether.

Keywords asymmetric reduction, chiral diol, chiral 2,5-diphenylpyrrolidine, chiral 2,5-diphenylthiolane

Introduction

Chiral dialcohols are structural motifs often found in various important natural products and have also been proven valuable as chiral ligands and auxiliaries in stereoselective organic syntheses. The enantioselective synthesis of chiral dialcohols is still a stimulating sub-ject. One of the most successful methods is the enanti-oselective reduction of prochiral diketones by chiral catalysts and reagents. During the past few years, a large number of reducing agents and chiral catalysts have been developed that facilitate the reduction reaction un-der a wide range of conditions.1 The asymmetric reduc-tion of the diketones to obtain the corresponding dial-cohols has been reported using the CBS oxazaborolidine catalyst.2 In this reduction, the borane reagents have been used, but the borane reagents such as BH3•THF, BH3•SMe2, diborane and catecholborane suffer from drawbacks such as thermal decomposition, low concen-tration, noxious odor or great expense. During our on-going research in asymmetric reduction of ketones,3 accordingly, in this paper, we would like to describe the asymmetric reduction of diketones with borane reagents generated in situ using cheap and readily available NaBH4 and SnCl2 in combination with (S)-(－)-α,α-di-phenyl-2-pyrrolidinemethanol. And then, we try to trans- form the diol into the optically pure C2-symmetrical chiral amine and thioether which are important chiral auxiliaries in asymmetric synthesis such as the asy- mmetric α-chlorination of aldehyde,4 asymmetric sul- fonium ylide epoxidation,5 the electrophilic sulfeny- lation of unsaturated carbon-carbon bonds6 and so on.

Results and discussion

We have reported that the SnCl2 or SnCl4•2Et2O promoted a catalytic asymmetric reduction of simple ketones by sodium borohydride.7 Herein we wish to report the use of this system on the reduction of a series of diketones. The required diphenylbutanediones 1a—1g can be easily prepared using the efficient methodol-ogy reported by Ceylan et al.,8 the diketones 1h—1j were prepared through Friedel-Crafts acylation of ben-zene with acyl chloride, and the chiral (S)-( － )-α,α-diphenyl-2-pyrrolidinemethanol (3) was prepared from (S)-proline in several steps.9 The results are summarized in Table 1.

When the 1,4-diones were subjected to the present system, 1,4-diols were obtained in excellent yield with good diastereoselectivity and excellent enantioselectiv-ity irrespective of the substituent groups (electron- withdrawing or electron-donating) (Entries 1—8), and 1,5-dione was also smoothly reduced to the required products with 92% dl selectivity and ＞99% ee (Entry 9). Whereas 1,2-diols were obtained with low di-astereoselectivity though the enantioselectivity was high (Entries 11—13). 1H NMR shows that 1,3-dione mainly existed in the form of enol which the system is not suit-able for, as a result, the 1,3-dione led to the desired product in both low diastereoselectivity and enantiose-lectivity (Entry 10).

1,4-Diols could be transformed into C2-symmetrical chiral amines and thioethers which were important chiral auxiliaries in asymmetric synthesis.4-6 After 1,4- diol was mesylated with methanesulfonyl chloride, the development of (1R,4R)-1,4-bis(methanesulfonyloxy)-

Chiral dialcohol Chin. J. Chem., 2006 Vol. 24 No. 10 1403

© 2006 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Table 1 Asymmetric reduction of diones to diols using NaBH4 and SnCl2 in combination with (S)-(－ )-α,α-diphenyl-2-pyr-rolidinemethanol

Entry n R1 and R2 dl/mesoa eea %

1 2 R1＝R2

＝Ph (1a) 92∶8b ＞99

2 2 R1＝R2

＝p-F-Ph (1b) 91∶9b ＞99

3 2 R1＝R2

＝p-Cl-Ph (1c) 89∶11b 97

4 2 R1＝R2

＝p-Br-Ph (1d) 93∶7b 99

5 2 R1＝R2

＝m-Br-Ph (1e) 88∶12b ＞99

6 2 R1＝R2

＝p-CH3O-Ph (1f) —b,c 92

7 2 R1＝R2

＝p-Ph-Ph (1g) —b,c 97

8 2 R1＝Ph, R2

＝OEt (1h) — 91

9 3 R1＝R2

＝Ph (1i) 92∶8b ＞99

10 1 R1＝R2

＝Ph (1j) 52∶48 33

11 0 R1＝R2

＝Ph (1k) 63∶37 97

12 0 R1＝R2

＝p-CH3Ph (1l) 72∶28 ＞99

13 0 R1＝R2

＝p-C2H5OPh (1m) 73∶27 ＞99 a Determined by HPLC using a chiral stationary phase. b Analyti-cal samples were converted to acetylates. c No mesomer was ob-served by 1H NMR or HPLC even though the analytical samples were converted to acetylates, benzoylates or benzylates.

1,4-diphenylbutane into 2,5-diphenylpyrrolidine was successfully obtained in only two steps, and the 2,5-di- phenylthiolane was easily synthesized by the reaction of (1R,4R)-1,4-bis(methanesulfonyloxy)-1,4-diphenylbutane with sodium sulfide nonahydrate (Scheme 1).

Scheme 1

In conclusion, we have developed an efficient and practical method for catalytic asymmetric reduction of diketones with high diastereoselectivity and enantiose-lectivity and successfully transformed part of them into some optically pure C2-symmetricl chiral amines and thioethers, especially the (2S,5S)-2,5-diphenylthiolanes

was first reported.

Experimental

All the reactions were carried out under a dry Ar atmosphere with freshly distilled THF from sodium. 1H NMR and 13C NMR spectra were recorded respectively at 300 and 75 MHz with Varian Mercury 300 spec-trometer. Chemical shifts were reported by using tetramethylsilane as an internal standard. IR spectra were recorded on a Shimadzu IR-440. Optical rotations were taken with a Perkin Elmer 241 Autopol polarime-ter. Mass spectra were measured with a Finnigan MAT-95 spectrometer. Elemental analyses were per-formed on Carlo- ERBA 1106 instrument.

General procedure for the asymmetric reduction of prochiral diketones

To a suspension of sodium borohydride (92 mg, 2.4 mmol) in THF (10 mL) was added SnCl2 (230 mg, 1.2 mmol) at room temperature. Black precipitates appeared along with generation of gas. After the resulting mixture was stirred at room temperature for 1 h, catalyst 3 (50 mg, 0.2 mmol) was added and the reaction mixture was refluxed and stirred for another 0.5 h. Then a solution of diketone (1 mmol) in THF (8 mL) was added over 2 h. After the addition was completed, the mixture was treated with methanol, filtered and washed by EtOAc. The solution was evaporated and purified by silica gel chromatography to give the corresponding pure prod-ucts.

(1R,4R)-1,4-Diphenylbutane-1,4-diol (2a): White solid, yield 92.0%; 23.5

D[ ]α ＋58.4 (c 1.05, CHCl3) [lit.10

20 (c 1.02, CHCl3)]; 1H NMR (CDCl3, 300 MHz) δ:

7.25—7.33 (m, 10H), 4.58—4.60 (m, 2H), 3.73 (s, 2H), 1.73—2.03 (m, 4H); IR (thin film) v: 3328, 3028, 1603, 1493, 1451, 1201 cm－1. Enantiomeric excess was de-termined by HPLC with a Chiralcel AD column (hex-ane∶2-propanol＝9∶1, V∶V, flow rate＝1.0 mL/min), tr(R,R)＝7.0 min, tr(S,S)＝10.5 min, tr(meso)＝7.6 min.

(1R,4R)-1,4-Bis(4-fluorophenyl)butane-1,4-diol (2b): White solid, yield 93.0%; 26.0

D[ ]α ＋53.0 (c 1.05, CHCl3);

1H NMR (CDCl3, 300 MHz) δ: 7.22—7.28 (m, 4H), 6.97—7.03 (m, 4H), 4.62—4.65 (m, 2H), 3.13 (s, 2H), 1.72—1.88 (m, 4H); 13C NMR (CDCl3, 75 MHz) δ: 163.1, 161.2, 140.3, 140.3, 127.4, 127.4, 115.3, 115.2, 73.9, 35.60; IR (thin film) v: 3326, 3048, 2923, 2825, 1606, 1509, 1447, 965, 909 cm－1. Anal. calcd for C16H16F2O2: C 69.05, H 5.79; found C 68.86, H 5.79. Enantiomeric excess was determined by HPLC with a Chiralcel AD column (hexane∶2-propanol＝9∶1, V∶V, flow rate＝1.0 mL/min.), tr(R,R)＝7.9 min, tr(S,S)＝12.2 min, tr(meso)＝9.1 min.

(1R,4R)-1,4-Bis(4-chlorophenyl)butane-1,4-diol (2c): White solid, yield 91.8%; 21.5

D[ ]α ＋24.4 (c 1.05, CHCl3);

1H NMR (CDCl3, 300 MHz) δ: 7.23—7.32 (m, 8H), 4.68—4.70 (m, 2H), 2.84 (s, 1H), 2.61 (s, 1H), 1.75—1.85 (m, 4H); 13C NMR (CDCl3, 75 MHz) δ: 142.9, 133.1, 128.5, 127.1, 73.7, 35.8; IR (thin film) v:

1404 Chin. J. Chem., 2006, Vol. 24, No. 10 LI, ZHAO & CAO

© 2006 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

3053, 2923, 2854, 1597, 1491, 908 cm－1. Anal. calcd for C16H16Cl2O2: C 61.75, H 5.18; found C 61.56, H 5.28. Enantiomeric excess was determined by HPLC with a Chiralcel AD column (hexane∶2-propanol＝9∶1, V∶V, flow rate＝1.0 mL/min), tr(R,R)＝9.4 min, tr(S,S)＝17.3 min, tr(meso)＝11.6 min.

(1R,4R)-1,4-Bis(4-bromophenyl)butane-1,4-diol (2d): White solid, yield 92.0%; 26.4

D[ ]α ＋23.1 (c 1.04, CHCl3);

1H NMR (CDCl3, 300 MHz) δ: 7.42—7.45 (m, 4H), 7.15—7.26 (m, 4H), 4.60—4.62 (m, 2H), 3.23(s, 2H), 1.73—1.83 (m, 4H); 13C NMR (CDCl3, 75 MHz) δ: 143.5, 131.6, 127.5, 121.3, 73.8, 35.8; IR (thin film) v: 3348, 2924, 2854, 1592, 1487, 903 cm－1. Anal. calcd for C16H16Br2O2: C 48.03, H 4.03; found C 48.01 H 4.24. Enantiomeric excess was determined by HPLC with a Chiralcel AD column (hexane∶2-propanol＝9∶1, V∶V, flow rate＝1.0 mL/min), tr(R,R)＝9.3 min, tr(S,S)＝17.1 min, tr(meso)＝12.0 min.

(1R,4R)-1,4-Bis(3-bromophenyl)butane-1,4-diol (2e): White solid, yield 87.0%; 26.4

D[ ]α ＋23.1 (c 0.85, CHCl3);

1H NMR (CDCl3, 300 MHz) δ: 7.15—7.47 (m, 8H), 4.61—4.63 (m, 2H), 3.29 (s, 2H), 1.80—1.84 (m, 4H); 13C NMR (CDCl3, 75 MHz) δ: 146.841, 130.592, 130.1, 128.9, 124.4, 122.6, 73.8, 36.0; IR (thin film) v: 3354, 3062, 2923, 2859, 1594, 1475, 997, 907 cm－1. Anal. calcd for C16H16Br2O2: C 48.03, H 4.03; found C 48.36, H 4.40. Enantiomeric excess was determined by HPLC with a Chiralcel AD column (hexane∶2-pro- panol＝9∶1, V∶V, flow rate＝1.0 mL/min), tr(R,R)＝30.6 min, tr(S,S)＝19.7 min, tr(meso)＝27.8 min.

(1R,4R)-1,4-Bis(4-methoxylphenyl)butane-1,4-di- ol (2f): Yellow liquid, yield 85.0%; 22.0

D[ ]α ＋41.6 (c 1.00, CHCl3);

1H NMR (CDCl3, 300 MHz) δ: 7.23—7.26 (m, 4H), 6.83—6.86 (m, 4H), 4.74—4.80 (m, 2H), 3.76 (s, 6H), 2.83 (s, 2H), 1.41—1.43 (m, 4H); 13C NMR (CDCl3, 75 MHz) δ: 159.0, 138.2, 126.8, 113.8, 69.7, 55.3, 25.1; IR (thin film) v: 3418, 2970, 2928, 1612, 1586, 1513, 897, 832 cm－1. Enantiomeric excess was determined by HPLC with a Chiralcel AD column (hexanes∶2-propanol＝9∶1, flow rate＝1.0 mL/min), tr(R,R)＝5.7 min, tr(S,S)＝6.3 min.

(1R,4R)-1,4-Bis(biphenyl-4-yl)butane-1,4-diol (2g): White solid, yield 92.0%; 26.6

D[ ]α ＋ 43.6 (c 1.04, CHCl3);

1H NMR (CDCl3, 300 MHz) δ: 7.26—7.60 (m, 18H), 4.96—4.98 (m, 2H), 1.83 (s, 2H), 1.54—1.64 (m, 4H); 13C NMR (CDCl3, 75 MHz) δ: 144.8, 140.9, 140.5, 128.8, 127.3, 127.1, 125.9, 70.2, 25.2; IR (thin film) v: 3327, 3055, 2927, 2924, 1565, 1486, 1451, 896 cm－1. Anal. calcd for C28H26O2: C 85.25, H 6.64; found C 84.88, H 6.87. Enantiomeric excess was determined by HPLC with a Chiralcel AD column (hexane∶2-pro- panol＝9∶1, V∶V, flow rate＝1.0 mL/min), tr(R,R)＝5.7 min, tr(S,S)＝6.3 min.

1-Phenyl-(1R)-butane-1,4-diol (2h): White solid, yield 84.7%; 26.4

D[ ]α ＋27.5 (c 0.75, CH3OH) [lit.12 －28.0 (c 1.27, CH3OH) for S isomer]; 1H NMR (CDCl3, 300 MHz) δ: 7.26—733 (m, 5H), 4.66—4.68 (m, 2H), 3.61—3.63 (m, 2H), 3.39 (s, 1H), 2.99 (s, 1H), 1.78—

1.83 (m, 2H), 1.62—1.64 (m, 2H); IR (thin film) v: 3343, 3064, 3033, 1496, 1447, 948 cm－1. Enantiomeric excess was determined by HPLC with a Chiralcel OD column (hexane∶2-propanol＝9∶1, V∶V, flow rate＝1.0 mL/min), tr(R,R)＝18.6 min, tr(S,S)＝20.2 min.

(1R,5R)-( ＋ )-1,5-Diphenyl-pentane-1,5-diol (2i): White solid, yield 94.8%; 27.5

D[ ]α ＋17.9 (c 1.05, CH3- OH) [lit.10 20.0 (c 2.00, CH3OH)]; 1H NMR (CDCl3, 300 MHz) δ: 7.25—7.33 (m, 10H), 4.62—4.69 (m, 2H), 2.05 (s, 2H), 1.72—1.79 (m, 4H), 1.46—1.49 (m, 2H); 13C NMR (CDCl3, 75 MHz) δ: 144.8, 128.5, 127.6, 125.8, 74.4, 38.8, 22.3; IR (film) v: 3566, 3412, 3061, 3028, 2937, 2863, 1493, 1453, 910, 762 cm－1. Enanti-omeric excess was determined by HPLC with a Chiral-cel AD column (hexane∶2-propanol＝9∶1, V∶V, flow rate＝1.0 mL/min), tr(R,R)＝7.3 min, tr(S, S)＝9.3 min, tr(meso)＝8.5 min.

(1R,3R)-(＋)-1,3-Diphenyl-propane-1,3-diol (2j): White solid, yield 83.8%; 23.4

D[ ]α ＋11.65 (c 1.00, EtOH) [lit.11 67.5 (c 0.3, EtOH)]; 1H NMR (CDCl3, 300 MHz) δ: 7.16—7.24 (m, 10H), 4.81—4.86 (m, 2H), 3.68 (s, 1H), 3.23 (s, 1H), 1.99—2.01 (m, 2H); IR (film) v: 3546, 3421, 3029, 2946, 2914, 1602, 1493, 913 cm－1. Enan-tiomeric excess was determined by HPLC with a Chiralcel OD column (hexane∶2-propanol＝9∶1, V∶V, flow rate＝1.0 mL/min), tr(R,R)＝7.3 min, tr(S,S)＝9.3 min, tr(meso)＝8.5 min.

(S,S)-(－)-1,2-Diphenyl-ethane-1,2-diol (2k): White solid, yield 92.1%; 23.5

D[ ]α －55.6 (c 0.96, EtOH) [lit.13 －94.5 (c 1.0, EtOH)]. Enantiomeric excess was determined by HPLC with a Chiralcel AD column (hexane∶2-propanol＝9∶1, V∶V, flow rate＝1.0 mL/min), tr(S,S)＝15.2 min, tr(R,R)＝16.2 min, tr(meso)＝17.5 min.

(S,S)-(－)-1,2-Di-p-tolyl-ethane-1,2-diol (2l): White solid, yield 93.1%; 23.5

D[ ]α －94.9 (c 0.88, EtOH) [lit.13

－102.5 (c 1.0, EtOH)]. Enantiomeric excess was determined by HPLC with a Chiralcel AD column (hex-ane∶2-propanol＝9∶1, V∶V, flow rate＝1.0 mL/min), tr(S,S)＝15.6 min, tr(R,R)＝16.2 min, tr(meso)＝19.0 min.

(S,S)-(－ )-1,2-Bis-(4-ethoxy-phenyl)-ethane-1,2-di-ol (2m): White solid, yield 94.0%; 27.9

D[ ]α －92.2 (c 0.82, EtOH) [lit.14

－100.5 (c 1.0, EtOH)]. Enanti-omeric excess was determined by HPLC with a Chiral-cel OJ column (hexane∶2-propanol＝4∶1, V∶V, flow rate＝0.75 mL/min), tr(S,S)＝17.7 min, tr(R,R)＝19.3 min, tr(meso)＝21.7 min.

Procedure for synthesis of (1R,4R)-1,4-bis(methan- esulfonyloxy)-1,4-diphenylbutane (4)10

To a solution of methanesulfonyl chloride (400 µL, 5.3 mmol) in DCM (20 mL) at －20 was added a ℃

solution of (1R,4R)-1,4-diphenylbutane-1,4-diol (Entry 1, 500 mg, 2.1 mmol) and triethylamine (870 µL, 6.2 mmol) in DCM (20 mL). The mixture was stirred for 120 min at －20 and then quenched with sat℃ urated NH4Cl. The mixture was warmed to room temperature and the solvent was removed in vacuo to approximately

Chiral dialcohol Chin. J. Chem., 2006 Vol. 24 No. 10 1405

© 2006 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

10 mL. The solution was then diluted with ethyl acetate and washed with water∶brine∶ saturated NaHCO3 (1∶2∶1, V∶V∶V), dried over MgSO4, filtered through celite and concentrated in vacuo to 8 mL. The solution was then cooled to 0

℃, stirred and the crude dimesylate was precipitated out by dropwise addition of hexane, the desired (1R,4R)-1,4-bis(methanesulfonyl- oxy)-1,4-diphenylbutane (4) (680 mg, 82.9%) was ob-tained as a moderately unstable compound which was used directly in the next step.

Procedure for Synthesis of (2S,5S)-2,5-diphenylthio- lanes (7)

A solution of the known mesylate 4 (458 mg, 1.15 mmol) and sodium sulfide nonahydrate (freshly recrys-tallized in EtOH, 2.54 g, 10.58 mmol) in dimethylsul-foxide (2.5 mL) was stirred at 0 for 21 h. The rea℃ c-tion was monitored by thin layer chromatography (TLC). Water was then added and the aqueous layer was ex-tracted with diethyl ether. The combined organic layers were washed with water and dried over MgSO4. After filtration, the solution was concentrated in vacuo. Puri-fication by flash column chromatography (pentane/di- ethyl ether, 50∶1, V∶V) afforded 7 (220 mg, 80.0%) as a white solid. 23.0

D[ ]α ＋15.76 (c 1.0, CHCl3);

1H NMR (CDCl3, 300 MHz) δ: 7.25—7.49 (m, 10H), 4.83—4.88 (m, 2H), 2.60—2.64 (m, 2H), 2.11—2.18 (m, 2H); 13C NMR (CDCl3, 75 MHz) δ: 142.5, 128.4, 127.2, 54.3, 41.0; EIMS (70 ev) m/z (%): 240 (M＋). HRMS calcd for C16H16S (M ＋ ) 240.0973, found 240.0979.

Procedure for synthesis of (2S,5S)-2,5-diphenylpyr- rolidine (6)

Allyl amine (25 mL, 0.33 mol) was added to a cooled flask (0 ) co℃ ntaining the dimesylate (670 mg, 1.68 mmol) and the resultant solution stirred at this temperature for 20 h. After warming to room tempera-ture, the excess allyl amine was removed in vacuo and the residue dissolved in ether (70 mL) and washed with saturated NaHCO3 and brine, dried over MgSO4 and concentrated to afford the crude product as yellow oil. Flash chromatography (eluting with 3% ether/petrol) yielded the amine 5 as colourless oil (330 mg, 74.8% yield). 20

D[ ]α －114.7 (c 1.57, CHCl3) [lit.10 －115 (c 0.56, CHCl3)];

1H NMR (CDCl3, 300 MHz) δ: 7.25—7.36 (m, 10H), 5.59—5.67 (m, 1H), 4.87—4.95 (m, 2H), 4.31— 4.34 (m, 2H), 2.67—2.96 (m, 2H), 2.50—2.54 (m, 2H), 1.89—1.94 (m, 2H); IR (film) v: 3081, 3027, 2968, 2815, 1601, 1491, 1454, 1227, 997, 914 cm－1. (2S,5S)-N-allyl-trans-2,5-diphenylpyrrolidine 5 (1.28 g, 4.86 mmol) and (Ph3P)3RhCl (Wilkinson’s catalyst, 22 mg, 0.047 mmol) were dissolved in 13 mL

of 84∶16 (w/w) CH3CN∶H2O mixture and placed in a 50 mL three-necked flask fitted with distillation head. The mixture was purged with nitrogen gas and heated to boiling for 5 h. The reaction was then cooled to room temperature and diluted with ether. The layers were separated and the organic layer washed with brine, and the combined aqueous washes were back extracted with ether. The combined organic extracts were dried (MgSO4) and concentrated in vacuo. The resulting oil was purified by flash chromatography (eluting with 33% ethyl acetate in petrol) to yield the desired amine 6 (960 mg, 88.6%) as yellow oil. 24.1

D[ ]α －99.9 (c 1.00, CHCl3) [it.

10－108.2 (c 0.45, CHCl3)];

1H NMR (CDCl3, 300 MHz) δ: 7.26—7.50 (m, 10H), 4.56—4.61 (t, J＝7.30 Hz, 2H), 2.39—2.49 (m, 2H), 2.14 (s, 1H), 1.91—1.99 (m, 2H); IR (film) v: 3367, 3061, 3026, 2956, 2925, 2853, 1602, 1492, 1451, 751 cm－1.

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(E0602283 SONG, J. P.; DONG, H. Z.)