A concise, metal-free approach to the synthesis of oxime ethers from cross-dehydrogenative-coupling of sp3 C-H bonds with oximes

Article abstract of DOI:10.1002/ejoc.200901321

A concise oxime ether synthesis by a metal-free dehydrogenative cross-coupling reaction between allylic sp³ C-H bonds and oximes promoted by 2,3-dichloro-5,6-dicyanoquinone (DDQ) is reported. The corresponding oxime ethers are obtained in good yields.

Full text of DOI:10.1002/ejoc.200901321

SHORT COMMUNICATION
DOI: 10.1002/ejoc.200901321
A Concise, Metal-Free Approach to the Synthesis of Oxime Ethers from
Cross-Dehydrogenative-Coupling of sp3 C–H Bonds with Oximes
Jun Jin,[a] Yan Li,[a] Zhi-jing Wang,[a] Wei-xing Qian,*[a] and Wei-liang Bao*[a]
Keywords: Oxime ethers / C-H activation / Allylic compounds / Cross-coupling / Dehydrogenation
A concise oxime ether synthesis by a metal-free dehydro-
genative cross-coupling reaction between allylic sp3 C–H
bonds and oximes promoted by 2,3-dichloro-5,6-dicyano-
quinone (DDQ) is reported. The corresponding oxime ethers
are obtained in good yields.
Introduction
The formation of carbon–heteroatom bonds from a com-
mon intermediate is of great significance to the drug discov-
ery process.[1] In particular, the oxime ether motif is a highly
effective pharmacophore and is widely applied in pesticide
and drug molecular design.[2] For example, oxime ether A
(Figure 1), which can be used to screen for possible antibac-
terial and antifungal activities, was found to be a potential
anticonvulsant compound.[2h] A series of novel benzoyl-
phenylureas containing oxime ether group B (Figure 1) ex-
hibits excellent larvicidal activities against oriental army-
worm and mosquito.[2d] Furthermore, oxime ethers have
also attracted a lot of attention in organic synthesis, as they
can be used as an efficient substrate in 1,2-addition reac-
tions with organometallic[3] or radical species.[4] The con-
ventional method for oxime ether synthesis is the condensa-
tion of O-alkylhydroxylamines with carbonyl com-
pounds.[5,6] The direct preparation of oxime ethers is com-
monly limited to base-catalyzed reactions of oximes with
alkyl or aryl halides.[7–9] Recently, several new methods have
been developed. Takemoto synthesized allylated oxime
ethers by using a transition-metal-catalyzed reaction.[10]
Copper iodide mediated cross-coupling of aryl halides with
oximes was achieved by Wailes.[11] Meyer recently found a
copper(II)-mediated cross-coupling of oximes with phen-
ylboronic acid.[12] Direct synthesis of oxime ethers from un-
activated alkenes catalyzed by cobalt was reported by Car-
reira.[13] Although these methods are efficient, they have the
drawback of using a metal catalyst, which generates metal-
ion-containing waste.[14]
Figure 1. Valuable structures containing an oxime ether motif.
Recently, groups have focused on constructing C–O
bonds by the direct utilization of C–H bonds, because the
substrates do not need to be prefunctionalized and the reac-
tion is atom economic.[15] Allylic sp3 C–H bonds were em-
ployed for the construction of C–X (X = C/O) bonds.[16]
We have developed an oxidative coupling reaction between
diarylallylic sp3 C–H and activated methylenic sp3 C–H
bonds mediated by DDQ.[16b] A concise metal-free oxidat-
ive cross-coupling reaction between 1,3-diarylallylic sp3 C–
H bonds and alcohols/thiols to form C–O/C–S bonds has
also been uncovered by our group.[16c] It is worthy to note
that generation of oxime ethers directly from allylic sp3 C–
H bonds and oxime without a metal catalyst has been
studied rarely.
Encouraged by the progress in the activation of diarylal-
lylic sp3 C–H bonds, we hope to form oxime ethers by sim-
ilar methodology. Herein, we report a concise metal-free
oxidative coupling reaction promoted by 2,3-dichloro-5,6-
dicyanoquinone (DDQ) between allylic sp3 C–H bonds and
oximes.
Results and Discussion
[a] Department of Chemistry, Xixi Campus, Zhejiang University,
Hangzhou 310028, P. R. China
Fax: +86-571-8827-3814
We chose 1,3-diphenylpropene (1a) and benzaldehyde ox-
ime (2a) as the standard substrates to search for the opti-
mized reaction conditions (Table 1). Initially, we examined
E-mail: wlbao@css.zju.edu.cn
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.200901321.
Eur. J. Org. Chem. 2010, 1235–1238
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J. Jin, Y. Li, Z.-j. Wang, W.-x. Qian, W.-l. Bao
SHORT COMMUNICATION
the cross-coupling reaction in CH2Cl2. To our delight, de- Table 1. Screening of reaction conditions.[a]
sired product 3a was obtained in 65% yield at room tem-
perature in the presence of DDQ as the oxidant (Table 1,
Entry 2). No product was observed in the presence of
PhI(OAc)2 and tert-butyl peroxide oxidants (Table 1, En-
tries 12 and 13). To improve the reaction yield, we examined
the ratios of 1,3-diphenylpropene to DDQ. It was found
that when 1,3-diphenylpropene/DDQ = 1:1.2, the result was
better (Table 1, Entry 1). Decreasing the amount of DDQ
Entry
Oxidant
Solvent
T [°C] Yield[b] [%]
resulted in reduced yields. If the amount of DDQ was in-
creased to more than 1.5 equivalents, the yield was not im-
proved (Table 1, Entries 10 and 11). No product was de-
tected in the absence of DDQ (Table 1, Entry 14). Various
solvents were tested to find the best one. When CH3NO2
was used as the solvent, the reaction was slower and the
yield was lower (Table 1, Entry 5). The reaction could pro-
ceed in 1,4-dioxane, CH3CN, THF, and CHCl3, but the re-
sults were not so good (Table 1, Entries 3, 4, 6, and 7). A
drastic decrease in yield occurred when the reaction was
warmed (Table 1, Entry 9). Setting the reaction temperature
to 0 °C, the oxime ether formed sluggishly and led to 63%
product yield after 5 h (Table 1, Entry 8). Therefore, the op-
timized reaction conditions are: 1,3-diphenylpropene
(0.5 mmol)/oxime (0.6 mmol)/DDQ (0.6 mmol)/CH2Cl2
(2 mL) at room temperature.
1
2
3
4
5
6
7
8
DDQ
DDQ
DDQ
DDQ
DDQ
DDQ
DDQ
DDQ
DDQ
CH2Cl2
CH2Cl2
1,4-dioxane
CH3CN
CH3NO2
THF
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
0
35
r.t.
r.t.
r.t.
71[a]
65[c]
45
48
40
55
63
63
57
CHCl3
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
9
10
11
12
13
DDQ
DDQ
65[d]
74[e]
0
PhI(OAc)2
2-(tert-butylperoxy)-2-
methypropane
–
CH2Cl2
CH2Cl2
r.t.
r.t.
0
0
14
[a] 1a (0.5 mmol), 2a (0.6 mmol), solvent (2 mL), DDQ (0.6 mmol),
r.t. (22–27 °C), 1 h. [b] Isolated yield. [c] 1a (0.6 mmol), DDQ
(0.6 mmol). [d] DDQ (0.45 mmol). [e] DDQ (0.75 mmol).
With the optimized conditions in hand, various sub-
strates were subjected to the reaction, and the correspond-
ing results are listed in Table 2. Both aliphatic and aromatic
oximes were compatible in the current cross-coupling reac- an electron-donating group on the aromatic ring (Table 2,
tion. In addition, the electronic effect of the substituent on Entries 2, 3 and 8) reacted rapidly, whereas those with elec-
the aromatic oximes was also investigated. Substrates with tron-withdrawing groups (Table 2, Entries 4–7) reacted
Table 2. Formation of oxime ether with 1,3-diphenylpropene (1) and oxime 2.[a]
Entry
R1, R2
R3, R4
Time [min]
Product
Yield[b] [%]
1
2
3
4
5
6
7
8
H, H: 1a
C6H5, H: 2a
60
50
40
65
90
80
65
50
65
70
70
60
70
75
55
50
70
80
3a
3b
3c
3d
3e
3f
3g
3h
3i
3j
3k
3l
3m
3n
3o
3p
3q
3r
71
77
85
67
52
61
57
82
68
63
55
68
65
61
65
83
77[c]
67[c]
1a
4-CH3C6H4, H: 2b
4-CH3OC6H4, H: 2c
4-ClC6H4, H: 2d
3-NO2C6H4, H: 2e
3-ClC6H4, H: 2f
2-BrC6H4, H: 2g
2-CH3OC6H4, H: 2h
C6H5, CH3: 2i
1a
1a
1a
1a
1a
1a
9
1a
10
11
12
13
14
15
16
17
18
1a
C6H5, C6H5: 2j
1a
2-furanyl, H: 2k
1-naphthyl, H: 2l
styryl, H: 2m
1a
1a
1a
CH2(CH2)3CH2: 2n
4-CH3C6H4, H: 2b
4-CH3C6H4, H: 2b
4-CH3C6H4, H: 2b
4-CH3C6H4, H: 2b
Cl, Cl: 1b
CH3O, CH3O: 1c
H, CH3: 1d
H, Cl: 1e
[a] 1 (0.5 mmol), 2 (0.6 mmol), CH2Cl2 (2 mL), DDQ (0.6 mmol, unless noted), at r.t. [b] Isolated yield. [c] For these two substrates both
α- and γ-ethers were formed. For 3q and 3r the ratio of the α- and γ-oxime ethers was 2:1 and 3:5, respectively.
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Eur. J. Org. Chem. 2010, 1235–1238

Synthesis of Oxime Ethers from Cross-Dehydrogenative-Coupling
slowly and usually resulted in a lower yield. Interestingly,
the keto oxime also gave the desired products in good yield
(Table 2, Entries 9 and 10). To expand the scope of sub-
strates, we further examined substituted 1,3-diphenylprope-
nes. The corresponding products were obtained in good
yield (Table 2, Entries 15–18). 1,3-Diphenylpropene bearing
a methyl group on the aromatic ring could react better than
the one with an electron-withdrawing group. However, as
for the asymmetrically substituted substrates 1d and 1e,
both α- and γ-oxime ethers were formed. According to our
previous research,[16c] the ratio of α- and γ-oxime ethers was
2:1 and 3:5, respectively (Table 2, Entries 17 and 18).
Encouraged by the above results, we further investigated
the reactions between 3-methyl-1-phenylbut-1-ene and ox-
ime. To our delight, the corresponding oxime ether could
be successfully formed in moderate to good yield. As indi-
cated in Table 3, the reaction was highly selective, as the E-
oxime ether was dominant. Oximes containing an electron-
rich group on the aromatic ring (Table 3, Entries 1, 4, and
6) reacted rapidly with better stereoselectivity. As for cyclo-
hexanone oxime, no Z-oxime ether was produced.
Scheme 1.
Conclusions
In summary, a novel direct cross-dehydrogenative-cou-
pling (CDC) between allylic compounds and oximes was
developed by using DDQ as a promoter. This method pro-
vides a highly efficient, fast, and convenient approach to
synthesize oxime ethers.
Table 3. Formation of oxime ether with 3-methyl-1-phenylbut-1-ene
(1f) and oxime 2.[a]
Experimental Section
General Procedure: To a 25-mL round-bottom flask charged with
1,3-diarylpropene (0.5 mmol) and benzaldehyde oxime (0.6 mmol)
in CH2Cl2 (2 mL) was added DDQ (0.6 mmol). The mixture was
stirred at room temperature and monitored by TLC. After the
starting material was completely consumed, the reaction mixture
was directly passed through Celite and rinsed with ethyl ether
(3ϫ10 mL). Then, the combined extracts were washed with brine,
dried with MgSO4, and concentrated. Purification was done by col-
umn chromatography on silica gel (200–300 mesh; petroleum ether/
ethyl acetate, 20:1) to give the pure product.
Entry R3, R4
Time [min] Product (E/Z) Yield[b] [%]
1
2
3
4
5
6
4-CH3C6H4, H: 2b
60
140
130
50
4a/4aЈ (10:1)
4b/4bЈ (4:1)
4c/4cЈ (4:1)
4d/4dЈ (50:3)
4e/4eЈ (3:1)
4f/4fЈ (Ͼ99:1)
75
55
59
80
65
67
3-NO2C6H4, H: 2e
2-BrC6H4, H: 2g
2-CH3OC6H4, H: 2h
C6H5, CH3: 2i
120
70
Supporting Information (see footnote on the first page of this arti-
cle): Full characterization details and copies of the 1H and 13C
NMR spectra of the prepared compounds.
CH2(CH2)3CH2: 2n
[a] 1 (0.5 mmol), 2 (0.6 mmol), CH2Cl2 (2 mL), DDQ (0.6 mmol,
unless noted). [b] Isolated yield.
Acknowledgments
On the basis of the experimental observations and the
literature,[17] a tentative mechanism was thus proposed
This work was financially supported by the Specialized Research
Fund for the Doctoral Program of Higher Education of China
(Scheme 1). The formation of oxime ethers may follow two (20060335036).
pathways: hydride transferred directly from the allylic posi-
tion to DDQ and/or proton abstraction after an electron
was abstracted from the allylic double bond by DDQ. In
our experiments, when 2-bromobenzaldehyde oxime was
treated with 1,3-diphenylpropene, the self-coupling product
of 1,3-diphenylpropene was observed, which implied the
coupling reaction may proceed through a single-electron-
transfer process. A rearranged product from 1d or 1e was
also obtained in the experiments. Therefore, an allylic cat-
ion may also be involved in the reaction. The incoming nu-
cleophile can attack at the original allylic or γ-position to
form the isomerized products.
[1] a) N. Lazarova, S. S. Zoghbi, J. Hong, N. Seneca, E. Tuan,
R. L. Gladding, J. S. Liow, A. Taku, R. B. Innis, V. W. Pike, J.
Med. Chem. 2008, 51, 6034; b) R. Pratap, P. G. Parrish, D.
Venkataraman, M. K. Lakshman, J. Am. Chem. Soc. 2009,
131, 12240; c) P. Lagisetty, L. M. Russon, M. K. Lakshman,
Angew. Chem. Int. Ed. 2006, 45, 3660; d) M. K. Lakshman,
Curr. Org. Synth. 2005, 2, 83; e) P. Cristau, J. P. Vors, J. Zhu,
Tetrahedron 2003, 59, 7859; f) S. B. Singh, G. R. Pettit, J. Org.
Chem. 1990, 55, 2797; g) M. E. Jung, J. C. Rohloff, J. Org.
Chem. 1985, 50, 4909–4913; h) D. Sames, R. Polt, J. Org. Chem.
1994, 59, 4596–4601.
[2] a) P. Cozzi, G. Carganico, D. Fusar, M. Grossoni, M. Menich-
incheri, V. Pinciroli, R. Tonani, F. Vaghi, P. Salvati, J. Med.
Eur. J. Org. Chem. 2010, 1235–1238
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
1237

J. Jin, Y. Li, Z.-j. Wang, W.-x. Qian, W.-l. Bao
SHORT COMMUNICATION
Chem. 1993, 36, 2964; b) J. X. Huang, Y. M. Jia, X. M. Liang,
W. J. Zhu, J. J. Zhang, Y. H. Dong, H. Z. Yuan, S. H. Qi, J. P.
Wu, F. H. Chen, D. Q. Wang, J. Agric. Food Chem. 2007, 55,
10857; c) S. Tu, L. H. Xu, L. Y. Ye, X. Wang, Y. Sha, Z. Y.
Xiao, J. Agric. Food Chem. 2008, 56, 5247; d) R. Sun, M. Lv,
L. Chen, Q. Li, H. Song, F. Bi, R. Huang, Q. Wang, J. Agric.
Food Chem. 2008, 56,11376; e) K. Bhandari, N. Srinivas,
G. B. S. Keshava, P. K. Shukla, Eur. J. Med. Chem. 2009, 44,
437; f) S. M. Johnson, H. M. Petrassi, S. K. Palaninathan,
N. N. Mohamedmohaideen, H. E. Purkey, C. Nichols, K. P.
Chiang, T. Walkup, J. C. Sacchettini, K. B. Sharpless, J. W.
Kelly, J. Med. Chem. 2005, 48, 1576; g) Y. Gai, Y. S. Or, Z.
Wang, WO 076166, 2009; h) A. Karakurt, S. Dalkara, M. Oz-
alp, S. Ozbey, E. Kendi, J. P. Stables, Eur. J. Med. Chem. 2001,
36, 421.
J. W. Kelly, J. Med. Chem. 2005, 48, 1576; d) H. Goda, H.
Ihara, C. Hirayama, M. Sato, Tetrahedron Lett. 1994, 35, 1565.
M. N. S. Rad, S. Behrouz, M. Dianat, Synthesis 2008, 2055.
a) E. Abele, R. Abele, K. Rubina, J. Popelis, I. Sleiksa, E. Lu-
kevics, Synth. Commun. 1998, 28, 2621; b) T. Yamada, K.
Goto, Y. Mitsuda, J. Tsuji, Tetrahedron Lett. 1987, 28, 4557; c)
A. Karakurt, S. Dalkara, M. Özalp, S. Özbey, E. Kendi, J. P.
[6]
[7]
Stables, Eur. J. Med. Chem. 2001, 36, 421; d) S. Merkas, M.
ˇ
Litvic´, I. Cepanec, V. Vinkovic´, Molecules 2005, 10, 1429; e) E.
Abele, E. Lukevics, Org. Prep. Proced. Int. 2000, 32, 237; f) T.
Banerjee, P. Dureja, Molecules 2005, 10, 990; g) C. B. Li, Y.
Cui, W. Q. Zhang, J. L. Li, S. M. Zhang, M. C. K. Choi, A. S.
Chan, Chin. Chem. Lett. 2002, 13, 95.
[8]
a) P. D. Nanoppa, K. Pandurangan, U. Maitra, S. Wailes, Org.
Lett. 2007, 9, 2767; b) A. Mooradian, P. E. Dupont, J. Hetero-
cycl. Chem. 1967, 4, 441.
X. Jia, X. Wang, C. Yang, Y. Da, L. Yang, Z. Liu, Tetrahedron
2009, 65, 2334.
a) H. Miyabe, K. Yoshida, V. K. Reddy, A. Matsumura, Y. Tak-
emoto, J. Org. Chem. 2005, 70, 5630; b) H. Miyabe, A. Matsu-
mura, K. Moriyama, Y. Takemoto, Org. Lett. 2004, 6, 4631;
c) H. Miyabe, A. Matsumura, K. Yoshida, M. Yamauchi, Y.
Takemoto, Synlett 2004, 2123.
P. De, Nonappa, K. Pandurangan, U. Maitra, S. Wailes, Org.
Lett. 2007, 9, 2767.
[3] For some examples of the addition of organometallic nucleo-
philes to oxime ethers, see: a) D. Seomoon, J. A, P. H. Lee, Org.
Lett. 2009, 11, 2401; b) H. Miyabe, Y. Yamaoka, T. Naito, Y.
Takemoto, J. Org. Chem. 2004, 69, 1415; c) T. S. Cooper, P.
Laurent, C. J. Moody, A. K. Takle, Org. Biomol. Chem. 2004,
2, 265; d) C. J. Moody, A. P. Lightfoot, P. T. Gallagher, Synlett
1997, 659; e) J. A. Marco, M. Carda, J. Murga, F. González,
E. Falomir, Tetrahedron Lett. 1997, 38, 1841; f) S. Hanessian,
P.-P. Lu, J. Y. Sanceau, P. Chemla, K. Gohda, R. Fonne-Pfister,
L. Prade, S. W. Cowan-Jacob, Angew. Chem. Int. Ed. 1999, 38,
3160; g) S. Hanessian, R. Y. Yang, Tetrahedron Lett. 1996, 37,
5273; h) H. Miyabe, Y. Yamaoka, T. Naito, Y. Takemoto, J.
[9]
[10]
[11]
[12] A. Ali, A. G. Meyer, K. L. Tuck, Synlett 2009, 955.
Org. Chem. 2003, 68, 6745; i) R. K. Dieter, R. Datar, Can. J. [13] B. Gaspar, E. M. Carreira, J. Am. Chem. Soc. 2009, 131, 13214.
Chem. 1993, 71, 814.
[14] H. M. Meshram, B. Eeshwaraiah, M. Sreenivas, D. Aravind,
B. S. Sundar, J. S. Yadav, Synth. Commun. 2009, 39, 1857.
[15] a) B. M. Trost, Acc. Chem. Res. 2002, 35, 695; b) B. M. Trost,
Angew. Chem. 1995, 107, 285; Angew. Chem. Int. Ed. Engl.
1995, 34, 259; c) B. M. Trost, Science 1991, 254, 1471.
[16] a) Z. P. Li, C.-J. Li, J. Am. Chem. Soc. 2006, 128, 56; b) D. P.
Cheng, W. L. Bao, Adv. Synth. Catal. 2008, 350, 1263; c) Y. Li,
W. B a o, Adv. Synth. Catal. 2009, 351, 865; d) S. A. Reed, M. C.
White, J. Am. Chem. Soc. 2008, 130, 3316; e) B. T. Guan, S. K.
Xiang, B. Q. Wang, Z. P. Sun, Y. Wang, K. Q. Zhao, Z. J. Shi,
J. Am. Chem. Soc. 2008, 130, 3268; f) G. S. Liu, G. Y. Yin, L.
Wu, Angew. Chem. Int. Ed. 2008, 47, 4733; g) Y. Z. Li, B. J. Li,
X. Y. Lu, S. Lin, Z. J. Shi, Angew. Chem. Int. Ed. 2009, 48,
3817.
[4] For some examples of radical additions to oxime ethers, see: a)
H. Rahaman, M. Ueda, O. Miyata, T. Naito, Org. Lett. 2009,
11, 2651; b) O. Miyata, S. Takahashi, A. Tamura, M. Ueda, T.
Naito, Tetrahedron 2008, 64, 1270; c) H. Rahaman, A. Shirai,
O. Miyata, T. Naito, Tetrahedron Lett. 2008, 49, 5789; d) H.
Miyabe, M. Ueda, K. Fujii, A. Nishimura, T. Naito, J. Org.
Chem. 2003, 68, 5618; e) H. Miyabe, M. Ueda, A. Nishimura,
T. Naito, Org. Lett. 2002, 4, 131; f) H. Miyabe, C. Ushiro, M.
Ueda, K. Yamakawa, T. Naito, J. Org. Chem. 2000, 65, 176; g)
H. Miyabe, K. Fujii, T. Naito, Org. Lett. 1999, 1, 569; h) H.
Miyabe, R. Shibata, C. Ushiro, T. Naito, Tetrahedron Lett.
1998, 39, 631; i) D. J. Hart, F. L. Seely, J. Am. Chem. Soc. 1988,
110, 1631.
[5] a) H. Goda, M. Sato, H. Ihara, C. Hirayama, Synthesis 1992,
849; b) K. G. Watson, R. N. Brown, R. Cameron, D. K. Chal-
mers, S. Hamilton, B. Jin, G. Y. Krippner, A. Luttick, D. B.
McConnell, P. A. Reece, J. Ryan, P. C. Stanislawski, S. P.
Tucker, W.-Y. Wu, D. L. Barnard, R. W. Sidwell, J. Med. Chem.
2003, 46, 3181; c) S. M. Jahnson, H. M. Petrassi, S. K. Palanin-
athan, N. N. Mohamedmohaideen, H. E. Purkey, C. Nichols,
K. P. Chiang, T. Walkup, J. C. Sacchettini, K. B. Sharpless,
[17] a) P. P. Fu, R. G. Harvey, Chem. Rev. 1978, 78, 317; b) Y.
Zhang, C.-J. Li, J. Am. Chem. Soc. 2006, 128, 4242; c) B.-P.
Ying, B. G. Trogden, D. T. Kohlman, S. X. Liang, Y. C. Xu,
Org. Lett. 2004, 6, 1523; d) P. C. Montevecchi, M. L. Navac-
chia, J. Org. Chem. 1998, 63, 8035; e) B. A. Snider, Chem. Rev.
1996, 96, 339.
Received: November 18, 2009
Published Online: January 20, 2010
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