Palladium-Catalyzed C(sp3)-H Arylation of N-Boc Benzylalkylamines via a Deprotonative Cross-Coupling Process

Article abstract of DOI:10.1002/chem.201502017

Diarylmethylamines are key intermediates and products in the pharmaceutical industry. Herein we disclose a novel method toward the synthesis of these important compounds via C-H functionalization. Presented is a reversible deprotonation of N-Boc benzylalkylamines at the benzylic C-H with in situ arylation by a NiXantPhos-based palladium catalyst (50-93 % yield, 29 examples). The method is also successful with N-Boc-tetrahydroisoquinolines. The advantages of this method are it avoids strong bases, low temperatures, and the need to transmetallate to main group metals for the coupling. Skipping steps! Diarylmethylamines are key intermediates and products in the pharmaceutical industry. A novel method toward the synthesis of these important compounds via C-H functionalization is reported. A reversible deprotonation of N-Boc benzylalkylamines at the benzylic C-H is coupled with in situ arylation with a (NiXantPhos)Pd-based catalyst (50-93 % yield, 29 examples). The advantages of this method are that it avoids strong bases, low temperatures, and the need to transmetallate to main group metals for the coupling.

Full text of DOI:10.1002/chem.201502017

DOI: 10.1002/chem.201502017
Communication
&
Diarylmethylamines
Palladium-Catalyzed C(sp3)ÀH Arylation of N-Boc
Benzylalkylamines via a Deprotonative Cross-Coupling Process
Nusrah Hussain, Byeong-Seon Kim, and Patrick J. Walsh*[a]
by the use of strong bases, low temperatures (À788C), and the
Abstract: Diarylmethylamines are key intermediates and
need to transmetallate to main group metals.
products in the pharmaceutical industry. Herein we dis-
Our approach to arylation adjacent to amino groups focuses
close a novel method toward the synthesis of these im-
on reversible in situ CÀH deprotonations of benzylic amines
portant compounds via CÀH functionalization. Presented
under catalytic conditions. To circumvent the low acidity of
is a reversible deprotonation of N-Boc benzylalkylamines
benzylic amine CÀH groups, we initially employed (h6-C6H5-
at the benzylic CÀH with in situ arylation by a NiXant-
CH2NR2)Cr(CO)3 activation to decrease the pKa of benzylic CÀH
Phos-based palladium catalyst (50–93% yield, 29 exam-
bonds (Scheme 1a).[14] The corresponding diarylmethylamines
ples). The method is also successful with N-Boc-tetrahy-
were obtained in excellent yields and enantioselectivities. To
droisoquinolines. The advantages of this method are it
avoid the stoichiometric use of chromium, we employed
avoids strong bases, low temperatures, and the need to
a direct arylation of benzylic C(sp3)ÀH bonds using 2-azaallyl
transmetallate to main group metals for the coupling.
anions.[15] This strategy, introduced by Oshima and co-work-
ers[16] and rendered synthetically useful by Buchwald’s group[17]
and by us,[18] takes advantage of the stability of 2-azaallyl
Diarylmethylamines are an important class of compounds that
have had a significant impact in pharmaceutical sciences. For
example, diarylmethylamines are core structures of Zyrtec,[1]
Levocetirizine,[2] Meclizine,[3] and Solifenacin.[4] As such, their
synthesis has attracted much attention. Generally, diarylmethyl-
amines are prepared by nucleophilic addition of organometal-
lic reagents to imines.[5] More recently, functionalization of sp3-
hybridized CÀH bonds adjacent to nitrogen has emerged as
a powerful method for the formation of CÀC bonds.[6]
anions (Scheme 1b).
Direct deprotonation and functionalization of the benzylic
CÀH bonds in secondary benzylamine derivatives under cata-
lytic conditions is an attractive approach to elaborate amines,
but remains challenging. The difficulty arises from the low
acidity of sp3-hybridized benzylic CÀH bonds adjacent to
amino groups,[7] which usually require alkyl lithiums for depro-
tonation.[8] The resulting lithiated benzylic amines are quite
versatile and can be captured with a variety of electrophiles.[8,9]
The strong bases used in these deprotonations, however, are
incompatible with the vast majority of cross-coupling catalysts.
To circumvent this incompatibility, Baudoin,[10] Knochel,[11]
Campos,[12] and Dieter[13] have established two-step methods
that commence with direct lithiation of secondary amines with
strong bases, such as sBuLi, followed by in situ transmetalla-
tion to zinc, boron, or copper and subsequent coupling with
aryl halides. The practicality of these approaches is diminished
Scheme 1. a) Synthesis of diarylmethylamines based on an h6-arene-activa-
tion strategy. b) Cross-coupling of N-benzyl benzophenone ketimines.
Considering the importance of diarylmethylamines in medic-
inal chemistry,[19] we envisioned a reversible deprotonation/
functionalization of N-Boc benzylalkylamines at the benzylic
CÀH bonds under catalytic conditions. The Boc group is
known to be useful in synthesis and was chosen for its ability
to increase the acidity of the benzylic CÀH bonds and to direct
the base to facilitate deprotonation. Herein we report the first
direct arylation of N-Boc-protected benzylalkylamines with aryl
halides to provide N-Boc-diarylmethylamines (Scheme 2).
Our general approach to deprotonative cross-coupling pro-
cesses (DCCP) involves initial identification of a base for rever-
sible deprotonation and an arylation catalyst that is compatible
with the basic reaction conditions.[20] From our experience with
DCCP of weakly acidic substrates (pKa 25–35), we chose van
Leeuwen’s NiXantPhos ligand (Scheme 2) as a starting point.[21]
We have recently demonstrated that NiXantPhos is deproton-
ated under basic reaction conditions, leading significantly en-
hanced catalyst reactivity.[21c] To identify a suitable base for the
[a] Dr. N. Hussain, B.-S. Kim, Prof. P. J. Walsh
Department of Chemistry, University of Pennsylvania
231 S, 34th St. Philadelphia, PA 19104 (USA)
E-mail: pwalsh@sas.upenn.edu
Homepage: http://titanium.chem.upenn.edu/walsh/index.html
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201502017.
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the reaction temperature from 85 to 508C had a detri-
mental effect on the yield (20%, entry 12).
Although the combination of Pd(OAc)2 and NiXant-
Phos as precatalyst afforded the diarylmethylamine
product 4a in excellent yield, the use of 3 equiv of N-
Boc benzylmethylamine (entries 9 and 10) required
further attention. Reducing the equivalents of the
Scheme 2. Palladium-catalyzed DCCP of N-Boc benzylalkylamines followed by deprotec-
benzylmethylamine pro-nucleophile from
3
to
tion.
1 equiv in THF at 858C resulted in a drop in yield of
4a from >95 to 61% yield, along with 30% unreact-
reversible deprotonation of the weakly acidic sp3-hybridized
CÀH bond adjacent to nitrogen, we screened six bases [LiN(-
SiMe3)2, NaN(SiMe3)2, KN(SiMe3)2, LiOtBu, NaOtBu, and KOtBu]
with the Pd(OAc)2/NiXantPhos system at 858C in cyclopentyl-
methyl ether (CPME) for 24 h. As illustrated in Table 1, the
bases leading to arylation products were MN(SiMe3)2 (M=Li,
Na, K), affording 10–40% assay yields (AY, determined by
1H NMR spectroscopy of the crude products) of the diarylme-
thylamines in CPME (entries 1–3). None of the MOtBu (M=Li,
Na, K) bases generated detectable amounts of arylated prod-
ucts (entries 4–6). Examination of four ethereal solvents [THF,
DME, dioxane, and CPME] indicated that THF was an excellent
choice (entry 9; 99% assay yield). To optimize the reaction con-
ditions with the NiXantPhos/Pd(OAc)2 system, we examined
different ratios of the benzylmethylamine pro-nucleophile, 4-
bromotoluene, and LiN(SiMe3)2 at 50 and 858C. When 3 equiv
of N-Boc benzylmethylamine (1a), 3 equiv of LiN(SiMe3)2 or
NaN(SiMe3)2, and 1 equiv of 4-Tol-Br were used in THF, the de-
sired arylated product was obtained in quantitative yield (en-
tries 9 and 10). KN(SiMe3)2, on the other hand, gave the prod-
uct 4a in only 21% yield (entry 11). Furthermore, decreasing
ed N-Boc benzylmethylamine 1a (entry 13). Changing base
from LiN(SiMe3)2 to NaN(SiMe3)2 also resulted in a drop in yield
of 4a to 57% (entry 15). The best result was obtained when
1.1 equiv of N-Boc benzylmethylamine, 1 equiv of 4-bromoto-
luene 3a, and 4 equiv of LiN(SiMe3)2 at 858C were used in THF
for 24 h, producing the product 4a in 99% AY and 88% isolat-
ed yield (entry 16).
With the optimized conditions (entry 16, Table 1), we sought
to evaluate the substrate scope of the arylation of N-Boc ben-
zylmethylamine (Table 2). Overall, the DCCP showed good to
excellent reactivity with a variety of aryl electrophiles. For ex-
ample, the arylated products were obtained in 50–91% yields
for para, meta, and ortho alkyl-substituted aryl bromides, chlor-
ides, an aryl iodide, and an aryl triflate (4a, 4c–4e, Table 2).
Sterically hindered 2-bromotoluene proved to be a more chal-
lenging substrate, furnishing the product 4e in 50% yield. On
the other hand, 1-bromonaphthalene resulted in 4 f in 78%
yield. Aryl halides bearing 3-methoxy and 3-N,N-dimethylamino
groups led to coupling products 4g and 4h in 64 and 93%
yields, respectively. Electron-rich 4-bromo- and 4-chloroanisole
yielded the coupling product 4i in 87 and 85% yield, respec-
tively. The reactivity with 1-bromo- and 1-chloro-4-fluoroben-
zene was lower, giving 4j in 70 and 60% yield, respectively.
Acetals are known to undergo CÀO bond cleavage with reac-
tive organometallic reagents,[22] however, the CÀH functionali-
zation product 4k was isolated in 68% yield. With heterocyclic
4-chlorophenyl pyrrole, the arylated product 4l was obtained
in 93% yield. N-TBS 5-bromoindole furnished the heterocyclic
product 4m in 50% yield.
Table 1. Optimization of the DCCP of N-Boc benzylmethylamine with 4-
bromotoluene.
We next turned our attention to the scope of N-Boc benzyl-
methylamine derivatives (Table 3). Heterocyclic N-Boc methyl(-
pyridin-3-methyl)amine exhibited good reactivity with alkyl-
substituted, electron-donating and electron-deficient aryl bro-
mides (4n–4p, 72–98% yield). The benzylmethylamine bearing
a 3-OMe substituent furnished the desired products in 57–90%
yield (4g, 4q, and 4r). The tert-butyl methyl(naphthalen-1-yl-
methyl)carbamate also afforded products in 57–80% yield (4s,
4t, and 4u). N-Boc 4-fluorobenzylmethylamine coupled with
bromobenzene and 4-bromoanisole in 76 and 73% yields (4j
and 4v), respectively. When the methyl group on N-Boc ben-
zylmethylamine was changed to N-ethyl group, the arylated
products with 4-bromotoluene and 4-bromoanisole were ob-
tained in 75 and 60% yield, respectively (4w and 4x).
Entry
Base
1a:2:3b
Solvent
Yield[a] [%]
1
2
3
4
5
6
7
8
LiN(SiMe3)2
NaN(SiMe3)2
KN(SiMe3)2
LiOtBu
NaOtBu
KOtBu
LiN(SiMe3)2
LiN(SiMe3)2
LiN(SiMe3)2
NaN(SiMe3)2
KN(SiMe3)2
LiN(SiMe3)2
LiN(SiMe3)2
LiN(SiMe3)2
NaN(SiMe3)2
LiN(SiMe3)2
3:3:1
3:3:1
3:3:1
3:3:1
3:3:1
3:3:1
3:3:1
3:3:1
3:3:1
3:3:1
3:3:1
3:3:1
1:3:3
1:3:1
1:3:3
1.1:4:1
CPME
CPME
CPME
CPME
CPME
CPME
dioxane
DME
THF
THF
THF
THF
THF
40
30
10
–
–
–
15
10
99
99
21
20[b]
61[c]
74[d]
57[e]
99 (88)[f]
9
10
11
12
13
14
15
16
THF
THF
THF
Unfortunately, reactions with 2-thiophenyl, 2-furyl, 2- and 4-
pyridyl-derived benzylmethylamines did not yield any detecta-
ble amounts of coupling products under these conditions.
These substrates are significantly more acidic than N-Boc ben-
1
[a] Assay yield determined by H NMR analysis of unpurified reaction mix-
ture with internal standard CH2Br2. [b] Reaction at 508C. [c] 30% unreact-
ed 1a. [d] 20% unreacted 1a. [e] 18%, unreacted 1a. [f] Isolated yield.
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Table 2. Cross-coupling of N-Boc benzylmethylamine with aryl electrophi-
les.[a]
Table 3. Cross-coupling of N-Boc benzylalkylamines with aryl bromides.[a]
4n
4o
4p
98%
88%
72%
4a
4b
4c
88% (X=Br)
78% (X=I)
90% (X=Br)
87% (X=Cl)
90% (X=Br)
91% (X=OTf)
4g
4q
4r
90%
87%
57%
4d
4e
4 f
86% (X=Br)
50% (X=Br)
78% (X=Br)
4s
70%
4t
57%
4u
80%[b]
4g
4h
4i
64% (X=Br)
93% (X=Br)
87% (X=Br)
85% (X=Cl)
4j
4v
76%
73%
4w
4x
4j
4k
4l
75%
60%
70% (X=Br)
60% (X=Cl)
68% (X=Br)
93% (X=Cl)
[a] See the Supporting Information for experimental details. [b] 10/
20 mol% of Pd(OAc)2 and NiXantPhos used.
We next desired to expand our method to the synthesis of
1-phenyl-1,2,3,4-tetrahydroisoquinoline (5aa, Scheme 4), a key
intermediate in the antimuscarinic agent Solifenacin (Vesi-
care).[23] 1-Arylated tetrahydroisoquinolines have also been
studied for their anti-HIV activities.[24]
4m
50% (X=Br)
[a] See the Supporting Information for experimental details.
zylmethylamine and will require different conditions to form
the desired products.
Under our DCCP conditions, the cross-coupling of 1aa took
place smoothly to provide 4aa in 81% isolated yield
(Scheme 4). The arylated tetrahydroisoquinoline products de-
rived from 4-bromoanisole and 1-bromo-4-fluorobenzene were
obtained in 76 and 65% yields, respectively (4ab and 4ac,
Scheme 4). Toward the synthesis of the key intermediate (5aa),
the arylated product (4aa) was deprotected with HCl followed
by basic workup to provide 1-phenyl-1,2,3,4-tetrahydroisoqui-
noline 5aa in 95% yield. Racemic 5aa has been used in the
synthesis of Solifenacin (6, Scheme 4).[25]
The Boc-protected 4c was treated with hydrochloric acid fol-
lowed by basic work up to provide the secondary amine 5c in
75% yield (Scheme 3).
In summary, we have developed the first direct a-arylation
of N-Boc benzylalkylamines with aryl electrophiles by using de-
protonative cross-coupling processes. This method avoids two-
step procedures involving low temperature deprotonation
Scheme 3. Deprotection of the Boc group to provide 5c from 4c.
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[8] a) M. Schlosser, D. Limat, J. Am. Chem. Soc. 1995, 117, 12342–12343;
b) J. E. Resek, P. Beak, J. Am. Chem. Soc. 1994, 116, 405–406.
[9] a) J. Clayden, J. Dufour, D. M. Grainger, M. Helliwell, J. Am. Chem. Soc.
2007, 129, 7488–7489; b) I. Fernµndez, J. Gonzµlez, F. López-Ortiz, J.
Am. Chem. Soc. 2004, 126, 12551–12564; c) N. C. Faibish, Y. S. Park, S.
Lee, P. Beak, J. Am. Chem. Soc. 1997, 119, 11561–11570.
[10] A. Millet, D. Dailler, P. Larini, O. Baudoin, Angew. Chem. Int. Ed. 2014, 53,
2678–2682; Angew. Chem. 2014, 126, 2716–2720.
[11] S. Seel, T. Thaler, K. Takatsu, C. Zhang, H. Zipse, B. F. Straub, P. Mayer, P.
Knochel, J. Am. Chem. Soc. 2011, 133, 4774–4777.
[12] K. R. Campos, A. Klapars, J. H. Waldman, P. G. Dormer, C.-y. Chen, J. Am.
Chem. Soc. 2006, 128, 3538–3539.
[13] R. K. Dieter, S. J. Li, J. Org. Chem. 1997, 62, 7726–7735.
[14] a) G. I. McGrew, C. Stanciu, J. Zhang, P. J. Carroll, S. D. Dreher, P. J. Walsh,
Angew. Chem. Int. Ed. 2012, 51, 11510–11513; Angew. Chem. 2012, 124,
11678–11681; b) G. I. McGrew, J. Temaismithi, P. J. Carroll, P. J. Walsh,
Angew. Chem. Int. Ed. 2010, 49, 5541–5544; Angew. Chem. 2010, 122,
5673–5676.
[15] a) Z. Li, Y.-Y. Jiang, A. A. Yeagley, J. P. Bour, L. Liu, J. J. Chruma, Y. Fu,
Chem. Eur. J. 2012, 18, 14527–14538; b) J. D. Weaver, I. I. I. A. Recio, A. J.
Grenning, J. A. Tunge, Chem. Rev. 2011, 111, 1846–1913; c) A. J. Gren-
ning, J. A. Tunge, J. Am. Chem. Soc. 2011, 133, 14785–14794; d) W. H.
Fields, J. J. Chruma, Org. Lett. 2010, 12, 316–319; e) A. A. Yeagley, M. A.
Lowder, J. J. Chruma, Org. Lett. 2009, 11, 4022–4025; f) W. H. Fields,
A. K. Khan, M. Sabat, J. J. Chruma, Org. Lett. 2008, 10, 5131–5134;
g) A. A. Yeagley, J. J. Chruma, Org. Lett. 2007, 9, 2879–2882; h) E. C.
Burger, J. A. Tunge, J. Am. Chem. Soc. 2006, 128, 10002–10003; i) J. A.
Tunge, E. C. Burger, Eur. J. Org. Chem. 2005, 2005, 1715–1726.
[16] a) T. Niwa, T. Suehiro, H. Yorimitsu, K. Oshima, Tetrahedron 2009, 65,
5125–5131; b) T. Niwa, H. Yorimitsu, K. Oshima, Org. Lett. 2008, 10,
4689–4691.
Scheme 4. Cross-coupling of N-Boc tetrahydroisoquinoline 1aa, and depro-
tection to give 1-phenyl tetrahydroisoquinoline 5aa, a key intermediate for
Solifenacin (6).
with very strong bases and transmetallation to main group in-
termediates previously used. Key to success of this approach
are the development of conditions for the reversible in situ CÀ
H deprotonation of the substrates and the application of
a Pd(NiXantPhos)-based catalyst that enables the arylation to
be conducted under mild conditions.
[17] Y. Zhu, S. L. Buchwald, J. Am. Chem. Soc. 2014, 136, 4500–4503.
[18] a) M. Li, B. Yücel, J. Adrio, A. Bellomo, P. J. Walsh, Chem. Sci. 2014, 5,
2383–2391; b) M. Li, S. Berritt, P. J. Walsh, Org. Lett. 2014, 16, 4312–
4315.
Acknowledgements
[19] D. Ameen, T. J. Snape, Med. Chem. Commun. 2013, 4, 893–907.
[20] a) F. Gao, B.-S. Kim, P. J. Walsh, Chem. Commun. 2014, 50, 10661–10664;
b) S. Montel, T. Jia, P. J. Walsh, Org. Lett. 2014, 16, 130–133; c) G.
Frensch, N. Hussain, F. A. Marques, P. J. Walsh, Adv. Synth. Catal. 2014,
356, 2517–2524; d) N. Hussain, G. Frensch, J. Zhang, P. J. Walsh, Angew.
Chem. Int. Ed. 2014, 53, 3693–3697; Angew. Chem. 2014, 126, 3767–
3771; e) B. Zheng, T. Jia, P. J. Walsh, Org. Lett. 2013, 15, 1690–1693; f) J.
Zhang, A. Bellomo, A. D. Creamer, S. D. Dreher, P. J. Walsh, J. Am. Chem.
Soc. 2012, 134, 13765–13772.
P.J.W acknowledges the NIH (National Institute of General Med-
ical Sciences GM 104349) and the NSF (CHE-1152488).
Keywords: CÀH
functionalization
·
cross-coupling
·
diarylmethylamines · NiXantPhos ligand · pallladium
[21] a) L. A. van der Veen, P. H. Keeven, G. C. Schoemaker, J. N. H. Reek, P. C. J.
Kamer, P. W. N. M. van Leeuwen, M. Lutz, A. L. Spek, Organometallics
2000, 19, 872–883; b) P. C. J. Kamer, P. W. N. M. van Leeuwen, J. N. H.
Reek, Acc. Chem. Res. 2001, 34, 895–904; c) J. Zhang, A. Bellomo, N.
Trongsiriwat, T. Jia, P. J. Carroll, S. D. Dreher, M. T. Tudge, H. Yin, J. R.
Robinson, E. J. Schelter, P. J. Walsh, J. Am. Chem. Soc. 2014, 136, 6276–
6287.
[22] P. Müller, P. Nury, G. Bernardinelli, Eur. J. Org. Chem. 2001, 4137–4147.
[23] C. Bolchi, M. Pallavicini, L. Fumagalli, V. Straniero, E. Valoti, Org. Process
Res. Dev. 2013, 17, 432–437.
[1] J. J. Li, D. S. Johnson, D. R. Sliskovic, B. Roth, Contemporary Drug Synthe-
sis, Wiley-Interscience, 2004.
[2] J. A. Grant, J.-M. Riethuisen, B. Moulaert, C. DeVos, Ann. Allergy Asthma
Immunol. 2002, 88, 190–197.
[3] J.-H. Fuhrhop, G. Li, Organic Synthesis: Concepts and Methods, 3rd ed.,
Wiley, 2003, p. 237.
[4] Y. Ko, D. C. Malone, E. P. Armstrong, Pharmacotherapy 2006, 26, 1694–
1702.
[5] a) D. L. Silverio, S. Torker, T. Pilyugina, E. M. Vieira, M. L. Snapper, F. Haeff-
ner, A. H. Hoveyda, Nature 2013, 494, 216–221; b) H.-C. Xu, S. Chowd-
hury, J. A. Ellman, Nat. Protoc. 2013, 8, 2271–2280; c) S. Kobayashi, Y.
Mori, J. S. Fossey, M. M. Salter, Chem. Rev. 2011, 111, 2626–2704; d) S. J.
Zuend, M. P. Coughlin, M. P. Lalonde, E. N. Jacobsen, Nature 2009, 461,
968–970; e) K.-i. Yamada, K. Tomioka, Chem. Rev. 2008, 108, 2874–2886.
[6] a) Z. Li, C.-J. Li, J. Am. Chem. Soc. 2005, 127, 6968–6969; b) Z. Li, C.-J. Li,
Org. Lett. 2004, 6, 4997–4999; c) Z. Li, C.-J. Li, J. Am. Chem. Soc. 2004,
126, 11810–11811.
[24] K.-x. Chen, H.-y. Xie, Z.-g. Li, J.-r. Gao, Bioorg. Med. Chem. Lett. 2008, 18,
5381–5386.
[25] a) Z.-S. Ye, R.-N. Guo, X.-F. Cai, M.-W. Chen, L. Shi, Y.-G. Zhou, Angew.
Chem. Int. Ed. 2013, 52, 3685–3689; Angew. Chem. 2013, 125, 3773–
3777; b) N. C. Niphade, K. M. Jagtap, A. C. Mali, P. V. Solanki, M. N.
Jachak, V. T. Mathad, Monatsh. Chem. 2011, 142, 1181–1186; c) R. Naito,
Y. Yonetoku, Y. Okamoto, A. Toyoshima, K. Ikeda, M. Takeuchi, J. Med.
Chem. 2005, 48, 6597–6606.
[7] a) P. Beak, A. Basu, D. J. Gallagher, Y. S. Park, S. Thayumanavan, Acc.
Chem. Res. 1996, 29, 552–560; b) A. I. Meyers, Aldrichimica Acta 1985,
18, 59–68.
Received: May 22, 2015
Published online on June 30, 2015
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