DOI: 10.1002/chem.200(( will be filled in by the editorial staff ))
[a]
[b]
[c]
1

Abstract: The allene moiety represents an excellent building block for allene cyclizative cross-coupling reactions, affording heterocyclic skeletons in a single step.
This strategy is of particular interest when two different allene derivatives are involved in a series of metal-catalyzed cross-coupling heterocyclization processes. This Concept article is focussed on the Pd-catalyzed union of two different allenic moieties, with cyclization of at least one of them by intramolecular cyclometallation. These new, versatile, and highly effective transformations are complex multistep processes leading to potential privileged structures that could find wide applications in related medicinal chemistry.
Keywords: allenes • coupling reactions • cyclization • palladium • selectivity
Allenes are a class of compounds with a 1,2-diene functionality possessing two perpendicular

-orbitals, showing unique properties and reactivity as well as interesting stereoselectivity due to the presence of the axial chirality.
[1] However, for a long period of time allenes were considered as highly unstable, which hindered development of the chemistry of allenes. During the past decade, the allene moiety has developed from almost a rarity to an established member of the weaponry utilized in modern organic synthetic chemistry.
[2] The literature contains an impressive number of transition-metal-catalyzed reactions involving cycloisomerization as well as the cyclization-coupling sequences of functionalized allenes for the synthesis of carbo- and heterocyclic compounds.
[3] However, cross-coupling reactions of two different allenes are almost unexplored. The present overview is devoted to cross-coupling reactions between two different classes of functionalized allenes to give interesting cyclic compounds in a single step based on metal catalysis. In 2002, Ma and coworkers reported the first oxidative cyclization-coupling reaction between two different allenes, namely, allenoic acids 1 with 1,2-allenyl ketones 2 , which afforded polysubstituted 4-(3’furanyl)-2(5H)-furanones 3 , which are not readily available from the known methods, in a single step (Scheme 1).
[4] Under the catalysis of 5 mol% [PdCl
2
(MeCN)
2
] the reaction takes place using 5 equivalents of the 1,2-allenyl ketone 2 in CH
3
CN as solvent. When the reaction was carried out in CH
2
Cl
2 or toluene,
[a] Prof. Dr. Benito Alcaide
Grupo de Lactamas y Heterociclos Bioactivos, Departamento de
Química Orgánica I, Facultad de Química, Unidad Asociada al CSIC
Universidad Complutense de Madrid
Facultad de Química, E-28040-Madrid (Spain)
Fax: +34-91-3944103
E-mail: alcaideb@quim.ucm.es
[b] Dr. Pedro Almendros
Instituto de Química Orgánica General
Consejo Superior de Investigaciones Científicas (CSIC)
Juan de la Cierva 3, 28006 Madrid (Spain)
Fax: +34-91-5644853
E-mail: Palmendros@iqog.csic.es
[c] Dr. Teresa Martínez del Campo
Department of Chemistry, Chemistry Research Laboratory
University of Oxford
12 Mansfield Road, OX1 3TA (United Kingdom) or with equimolecular ratio of ketone 2 , the major by-product was the product derived from the cycloisomerization reaction of the allenoic acid without cross-coupling.
[5] Differently substituted 2,3allenoic acids 1 could be used: monosubstituted, disubstituted as well as fully substituted. The substituents could be aryl, alkyl, benzyl and allyl; while the substituent of the 1,2-allenyl ketone moiety could be alkyl or benzyl groups. Polysubstituted 1,2allenyl ketones, however, could not be used to yield the corresponding cyclization–coupling products, probably due to the low reactivity towards oxypalladation. On the other hand, it was possible to perform the synthesis of an optically active

-(3’furanyl)butenolide of type 3 from an optically active 2,3-allenoic acid by a highly efficient chirality transfer under the standard reaction conditions. A major limitation of the above heterocoupling is the requirement of 5 equivalents of the allenone component, because efficient synthetic methods are desirable to be economical in atom count (maximum number of atoms of reactants appearing in the products).
R
4
O
R
1
R
2
1
R
3
COOH
+
O
2
R
4
5 mol% [PdCl
2
(MeCN)
2
]
CH
3
CN, RT, 4h
R 3
R
1
R 2
O O
3 (61

92%)
Br H
3
C
O
H
C
3
H
7
COOH
+
99% ee
O
CH
3
5 mol% [PdCl
2
(MeCN)
2
]
CH
3
CN, RT, 4h
Br
C
3
H
7
O O
76%, 99% ee
Scheme 1. Cyclization–coupling reaction of allenoic acids and 1,2-allenyl ketones.
A plausible mechanism was proposed for this cyclization– coupling reaction. Intermediate 4 is formed from the 1,2-allenyl ketone and the 2,3-allenoic acid via double oxypalladation, which is further supported by the highly efficient chirality transfer. After the reductive elimination of 4 , the final product is formed along with the Pd 0 species. Then, the Pd 0 species cyclometalate with 2 equiv of penta-3,4-dien-2-one to form the cyclic palladaintermediate 5 , which may be protonated with the in situ generated
HCl (2 equiv) to regenerate L n
PdCl
2
to enable the catalytic cycle
(Scheme 2). The formation of intermediate 5 is necessary in order to reoxidize the Pd 0 to Pd II species.
R
4 O
R
1 R
3
R
2
1
HO
+
O
R
1
Pd
R
2
O
4
R
3
O
R
4 O
R
3
2
O
R
4
R
1
R
2
O
3
O
L n
PdCl
2
2 HCl L
X
Pd
0
O
R
4
O
PdL m
R
4
2
O
R
4
5
Scheme 2. Proposed catalytic cycle for the oxidative cyclization–coupling reaction between allenoic acids and allenones.
2

Later, Ma and coworkers extended this type of oxidative cyclodimerization reaction to 2,3-allenamides 6 and 1,2-allenyl ketones 2 to form 4-(furan-3’-yl)furanimines 7 , the O -attack product, as Z -isomers (Scheme 3).
[6] The substituent on the nitrogen atom could be benzyl, alkyl or hydrogen. The 1,2-allenyl ketones that bear alkyl, aryl and benzyl groups were also successfully employed to carry out the cyclizative cross-coupling.
It was necessary to use acetic acid as cosolvent in order to regenerate the catalytically active Pd II species via protonation. It should be noted that when 1.0 equiv of benzoquinone was introduced to the reaction system, both the loading of 1,2-allenyl ketones 2 and catalyst could be reduced without decreasing the yield. Benzoquinone together with acetic acid, may be responsible for the facile regeneration of the catalytically active Pd II from the in situ generated Pd 0 . The mechanism shown in Scheme 4 was proposed to explain the cyclization reaction. Initial coordination of
Pd II species with allenes 2 and 6 was followed by double cyclic oxypalladation to form intermediates 8 . Subsequent reductive elimination of 8 yielded 4-(furan-3’-yl)furanimines 7 and the Pd 0 species. Then, the in situ generated Pd 0 species was reoxidized by benzoquinone in the presence of two protons to the catalytically active Pd II species to complete the catalytic cycle.
R
R
1
2
6
H
CONHR
3
+
O
2
R
4
1 mol% [PdCl
2
(MeCN)
2
]
1 equiv benzoquinone
HOAc, 30 o
C
R
4 O
H
R
1
N
R
2
O
Z 7 (55

86%)
R
3
R
1
= Me, (CH
2
)
5
; R
2
= Me, Et; R
3
= H, Bn; R
4
= alkyl, Ph
O
Me
Me
Me
H
CONHBn
+
O
Me
1 mol% [PdCl
2
(MeCN)
2
]
1 equiv benzoquinone
HOAc, 30 o
C, 2h
H
Me
Me
O
(86%)
N
Bn
Scheme 3. Palladium-catalyzed oxidative cyclization-coupling reaction between 2,3allenamides 6 and 1,2-allenyl ketones 2 .
R
1
H
Hydroquinone
BQ + 2H
+
Pd
0
L n
PdCl
2
L n
R
4 O
R
2
6
O
+
NHR
3
R
4
O
2
R
4 O
Pd
R
1
R
2 O
H
N
R
3
H
R
1
R
2
O
N
R
3
8
Z 7
Scheme 4. Proposed mechanism for the oxidative cyclization-coupling reaction of allenamides with allenones.
It should be noted that in the above two cyclization-coupling sequences (Scheme 1 and Scheme 3), the Pd II species was regenerated by consuming a large amount of 1,2-allenyl ketone via cyclometalation/protonation or by a additional oxidant
(benzoquinone; see Scheme 4) via direct oxidation. An interesting heterocyclization protocol was reported wherein two different allenes reacted, regenerating a catalytically active Pd II species directly on completion of reaction.
[7] This cyclization-coupling was performed using 2,3-allenoic acids 1 and 2,3-allenols 9 under
PdCl
2
catalysis and dimethylacetamide as solvent to afford 4-
(1’,3’-dien-2’-yl)butenolides
10 (Scheme 5). Thus, the two allenes function differently, that is, the 2,3-allenoic acid forms the butenolide skeleton while the 2,3-allenol introduces the 1,3-diene substituent to the

-position of the butenolide formed. Various differently substituted 2,3-allenoic acids that bear an alkyl, a benzyl and an aryl group were successfully cross-coupled with
2,3-allenols to afford butenolides in moderate yields. It is important to note that with R 4 ≠ H and R 5 ≠ H, a very high stereoselectivity was observed affording the products ( E )10 exclusively. When optically active allenoic acid ( R )1a was used as a mechanistic probe to react with 9a , product 10a was formed with partial racemization (Scheme 6). Thus, to neutralize any in situ generated basic species, trifluoroacetic acid (TFA) was added to the reaction mixture, and as a result, the racemization was indeed mostly inhibited; so it was concluded that [OH]
– species may be formed during the reaction, which may induce partial racemization of the product.
R
5
R
4
R
R
2
1
1
R
3
COOH
+
HO R
9
R
4
5
5 mol% PdCl
DMA, 30 o
C
2
R
3
R
1
O
R
2 O
10 (52

82%)
R
1
= Me, Et; R
2
= H, Me, (CH
2
)
5
, Ph; R
3
= H, Me, Et; R
4
= H, Me, (CH
2
)
5
, Ph; R
5
= H, Me
Bn
Ph
Et
Me
COOH
+
HO
Bn
5 mol% PdCl
2
DMA, 30 o
C
Me
Ph
Et
O
(82%)
O
Scheme 5. PdCl
2
-catalyzed cross coupling reaction of 2,3-allenoic acids and 2,3allenols.
Ph Me
H COOH
( R )1a
98%ee
+ Me
HO Me
9a
Me
Me
Me
5 mol% PdCl
2
DMA, 30 o
C Ph
O
O
10a no TFA (58%, 36% ee)
0.8 equiv. TFA (62%, 98% ee)
Scheme 6. Cyclization-coupling reaction of an optically active 2,3-allenoic acid in presence or absence of TFA.
A possible mechanism to explain the cross-coupling cyclization reaction of 1a and 9a is drawn in Scheme 7.
Regioselective carbopalladation of 2,3-allenol 9a with intermediate 11 would regioselectively form the

-allylic palladium intermediate 12 . Subsequent

-hydroxide elimination would afford butenolide 10a and XPd + [OH
–
],which may induce partial racemization of the product when this reaction was conducted in absence of TFA. Finally, XPd + [OH
–
] is converted to the catalytically active species, PdX
2
, by reaction with H + .
However, it may be inferred from the catalytic cycle of Scheme 7 that the [OH
–
] group is protonated with HX and consequently it would be come off as water, becoming apparent that no free [OH
–
] anion is available in the reaction medium. It may be also taken into account that butenolide 10a is an intrinsically easy isomerizable compound. As a consequence, more TFA (Scheme 6)
3

would simply suppress the isomerization of the acidic hydrogen at
C5.
1a
H
2
O
PdX
2
HX
PdX Me
XPd
+
[OH

]
Ph
O
11
O
XPd
10a Me
9a
OH
Ph
O
12
O
Scheme 7. Possible mechanism for the cross-coupling cyclization reaction of allenoic acid 1a and allenol 9a .
In 2006, the first cross-coupling reaction between two different

-allenol derivatives was reported, namely

-allenols 13 and protected

-allenols 14 to afford 2,3,4-trifunctionalized 2,5dihydrofurans.
[8] It was discovered that the treatment of

-allenols
13 with PdCl
2
(5 mol %) in dimethylformamide, in the absence of any oxidant, led to the cyclization adducts 15 when protected

allenols 14 were used as the coupling partners. No homodimerization products were detected (Scheme 8). The domino cyclization reaction is totally regioselective, with exclusive formation of the five-membered oxacycle. The free

allenol component undergoes heterocyclization to give a 2,5dihydrofuran, and the protected

-allenol cross-coupling partner becomes attached to the C4 carbon atom of the oxacycle as a substituted buta-1,3-diene functionality. This reaction is applicable to

-allenols with a wide range of substitution.

-Allenols 13 can bear alkyl, heteroalkyl, aryl and heteroaryl substituents. When the cross-coupling process was evaluated for substrates that contain stereocenters, the stereochemical integrity was conserved at the carbinolic atom of the allenol as well as at the distal stereocenters.
This coupling sequence was also extrapolated to sterically more encumbered tertiary allenic alcohols.
[9] All substrates reacted efficiently to afford high yields of the corresponding spiro adducts, such as the oxindole shown in Scheme 8 which is obtained in a
76% isolated yield. All products were obtained as before as single isomers, that is, with complete E selectivity with regard to the newly established C=C double bond. In order to get optimal results, three equivalents of the protected allenol 14 are required, which does not look attractive from an atom economy perspective.
Besides, the reaction is apparently limited to terminal allenes where chirality transfer is not involved.
R
1
OH
13
R
2
R
3
OCOR
4
Me
14
5 mol% PdCl
2 O
Me
+
DMF, RT
R 1 R 2
15 (47

90%)
R 3
R
1
, R
3
= alkyl, heteroalkyl, aryl, heteroaryl; R
2
= Me, Ph; R
4
= alkyl, aryl
PMPCOO
H H
OH
Me
+
O
O
N
Bn
O
OAc
Me 5 mol% PdCl
2
DMF, RT, 4h
O
PMPCOO
H H
O
Me
N
Bn
(59%)
Me
O
O
HO
Me
+ MeC
6
H
4
OAc
Me 5 mol% PdCl
2
O Me
N
Me
O
DMF, RT, 2h
N
Me
O
Me
(76%)
C
6
H
4
Me
Scheme 8. Palladium-catalyzed heterocyclization/cross-coupling of

-allenol derivatives 13 and 14 .
The catalytic cycle shown in Scheme 9 was proposed.
Regioselective palladium-mediated intramolecular oxypalladation of the free allenol component 13 generates a palladadihydrofuran intermediate 16 , which then undergoes a cross-coupling reaction with the protected allenol partner 14 . The coupling of vinyl palladium intermediates 16 with protected allenols 14 to give species 17 takes place regioselectively at the central allene carbon atom of 14 . Finally, trans-

-deacyloxypalladation generates a buta-1,3-dienyl dihydrofuran 15 in a highly stereoselective manner with exclusive formation of the E isomer and concomitant regeneration of the Pd II species.
R
5
COOH
Pd
II
Cl
2
OH
R
1
R
2
13
R
3
R
1
R
2
R
3
O
R
4
15
ClPd(OCOR
5
)
Me
HCl
R
1
R
2
R
3
O
R
5
COO
Me
H
R
4
Pd
II
Cl
17
R
1
R
2
R
3
O
16
Pd
II
Cl
R
4
OCOR
5
14
Me
Scheme 9. Mechanistic explanation for the Pd II -catalyzed heterocyclization/crosscoupling of

-allenols and protected

-allenols.
In 2008, Ma and coworkers reported the PdI
2
-catalyzed dimeric coupling-cyclization reaction of two different 2,3-allenols to afford 4-(1’,3’-dien-2’-yl)-2,5-dihydrofuran derivatives 18 . 2-
Substituted 2,3-allenols 13 cyclized to form the dihydrofuran ring, whereas the 2-unsubstituted 2,3-allenol 9 provided the 1,3-diene functionalization at the 4-position (Scheme 10).
[10] These are the first examples of a dimeric coupling-cyclization reaction with two different 2,3-allenols. High stereoselectivities for the formation of the C=C bond were observed, giving products ( E )18 when secondary 2,3-allenols 9 were used. Besides, it is possible to perform the synthesis of optically active

-(4-(1’,3’-dien-2’-yl)-
2,5-dihydrofuran derivatives 18 from optically active 2,3-allenols by a highly efficient chirality transfer under the standard reaction conditions. The tandem double-cyclization reaction of 1,

-
4

bisallenols 19 to form 2,5-dihydrofuran-fused bicyclic skeletons
20 was also developed.
[11] The reaction of symmetric substrates
(R 1 = R 2 ) was performed under the catalysis of 5 mol % of PdCl
2 and DMF as solvent (Scheme 11). The bisallenols could bear different substituents: alkyl, aryl and ether functional groups as well as different tether: NTs, carbon and sulfone group. However, the addition of 0.5 equivalent of NaI was necessary to carry out the reaction on using a bisallenol with a carbon tether. When the reaction was carried out using unsymmetric (R 1 ≠ R 2 ) 1,6bisallenols 19 , the reaction afforded a mixture of inseparable bicyclic crossed adducts. Fortunately, if one of the hydroxyl groups was protected, the reaction afforded a single bicycle as the product in excellent yield (Scheme 12).
R
3
HO
R
2
R
1
+
13
HO
R
4
9
R
3
5 mol% PdI
2
BF
3
.Et
2
O
DMSO, 80 o
C
0.2 M, 1h
R
4
R
2
O
R
18 (49

81%)
1
R
1
= alkyl, aryl; R
2
= n Bu, Ph, allyl, CO
2
Me; R
3
, R
4
= H, Et
HO n Bu p -NO
2
C
6
H
4
+
HO
Bn
5 mol% PdI
2
BF
3
.Et
2
O
DMSO, 80 o
C
0.2 M, 1h
Bn n Bu
O
(81%) p -NO
2
C
6
H
4
Scheme 10. PdI
2
-catalyzed dimeric coupling-cyclization reaction of two different 2,3allenols 9 and 13 .
R
2
OH
R
2
X
H
5 mol% PdCl
2
X
DMF, 0.1 M
R
1
O
R
1 OH
19
20 (R
1
= R
2
, 51

81%)
Scheme 11. Pd
II
-catalyzed coupling cyclization reaction of 1,

-bisallenols 19 . p -ClC
6
H
4
LG
1
2 n -C
4
H
9
LG
2
LG
1
= LG
2
= OH
LG 1 = OAc, LG 2 = OH
LG
1
= OH, LG
2
= OAc
5 mol% PdCl
2
NaI (0.5 equiv)
DMF, 0.1 M n -C
4
H
9
O
32%
77%
 p -ClC
6
H
4
H
+ p -ClC
6
H
4
O
50%

94% n -C
4
H
9
H
Scheme 12. Pd II -catalyzed coupling cyclization reaction of protected unsymmetric
1,

-bisallenols.
As a new methodology, Ma and coworkers developed a cyclization reaction of 2,3-allenoic acids 1 in the presence of nonactivated alkyl or aryl-substituted allenes 21 , in which a lactone ring was formed from the 2,3-allenoic acid and the alkyl or arylallene yielded a stereodefined allylic bromide moiety in the final bromoalk-2’(Z)-en-2’-yl)furan-2(5H)-one 22 (Scheme 13).
[12]
After some screening, it was observed that the reaction of 1 with
21 in the presence of 5 mol % Pd(OAc)
2
, 110 mol % of benzoquinone (BQ) and 200 mol % of LiBr.H
2
O in HOAc afforded Z 22 in good yields. The reaction tolerates different groups in the 2,3-allenoic acid, like phenyl, p -halophenyl,

naphthyl, cyclopropyl as well as the fully substitution. The monosubstituted allenes could bear alkyl and aryl groups as well as some functional groups such as malonate, acetoxy and phthalic amide which provide opportunity for further synthetic elaboration.
The highly selective formation of Z -isomers is remarkable because most reported carbopalladation reactions of monosubstituted allenes gave a mixture of E and Z isomeric products with low selectivity.
[13] This fact may be explained by face-selective coordination of allene 21 with the palladium atom in 23 : The palladium atom coordinates to the terminal C=C double bond of allene 21 from the face opposite to the substituent group to avoid steric congestion (Scheme 14). An alternative explanation may arise from the fact that the two above complexes (left edge,
Scheme 14) should be equivalent energetically. However, there is energetic difference on comparing species anti 23 and syn 23 . The intermediate syn 23 is higher in energy than the complex anti 23 because of the steric repulsion between R 4 and R 1 or R 3 . As a consequence, Z 22 is obtained via anti 23 preferentially. Besides, it appears that Z 22 is the product where allylic 1,3-strain is minimized. Apparently, the present protocol is restricted to the synthesis of

-lactones, because 3,4-allenoic acids or superior homologues have not been successfully implemented. In addition, the reaction may be sensitive to steric hindrance, because the extrapolation of the coupling sequence to disubstituted allenes in place of monosubstituted allenes 21 was not considered.
R
1
R
3
1
R
2
COOH
+
21
R
4
5 mol% Pd(OAc)
2
LiBr.H
2
O, BQ
HOAc, 60 o
C
R
4
Br
R
2
R
1
O
R
3
O
Z 22 (44

88%)
R
1
= aryl; R
2
= alkyl; R
3
= H, Et; R
4
= alkyl, Ph
Ph Me
5 mol% Pd(OAc)
2
Ph Br
Me
Ph
H
COOH
+
LiBr.H
2
O, BQ
HOAc, 60 o
C
Ph
O
(88%)
O
Scheme 13. Coupling-cyclization reaction of 2,3-allenoic acids 1 with simple allenes
21 .
O
R
1
R
3
Pd OAc
O
H
R
2
H
R 4 H
O
O
R 1
R
3
R
4
Pd
OAc anti 23
R
2
Br

R
4
R
1
R
3
O
O
+ Pd
0
R 2
Br
Z 22
Benzoquinone
H
+
Hydroquinone
Pd
II
O
R
1
R 3
Pd OAc
O
R
4
R 2
H
H H
O
O
R
R 1
4
R
3
Pd
OAc
R
2 syn 23
Br

R
1
R 4
R 3
O
E 22
Br
R
2
O
Scheme 14. Rationale of the stereoselectivity for the preparation of 22 .
In summary, the cyclizative Pd-catalyzed coupling reactions using two different allene derivatives is a powerful method for the direct synthesis of functionalized butenolides, furanimines, and 2,5dihydrofuran derivatives. The directness of this allene-based approach which permits great flexibility in selection of different coupling partners, coupled with the versatility of the reactions are impressive advantages that should facilitate the synthesis of many useful molecules. However, it exists a limitation from an atom
5

economy point of view because the majority of the processes require a stoichiometric excess of one of the coupling partners
(from 2 to 5 equivalents). Besides, only the preparation of fivemembered heterocycles has been accomplished to date. On the other hand, there are unsolved problems in the area of the selective formation of heterocycles arising from a cross-coupling process which involves non-equivalent allene derivatives, and also in expanding the number and type of allenic moieties that may be used.
The search for even more broadly applicable and more selective catalysts should bring about more information in the use of two different allenic moieties as versatile coupling agents in organic synthesis.
Support for this work by the DGI-MICINN (Project CTQ2009-09318), CAM (Project
S2009/PPQ-1752), and UCM-BSCH (Grant GR58/08) are gratefully acknowledged.
[1] a) The Chemistry of Allenes (Ed.: S. R. Landor), Academic: London, 1982; b)
Modern Allene Chemistry (Eds.: N. Krause, A. S. K. Hashmi), Wiley-VCH,
Weinheim, 2004; c) Cumulenes and Allenes In Science of Synthesis (Ed.: N.
Krause), Georg Thieme Verlag: Stuttgart, Germany, 2008, Vol. 44.
[2] For selected reviews, see: a) S. Ma, Chem. Rev.
2005 , 105 , 2829; b) B.
Alcaide, P. Almendros, Eur. J. Org. Chem.
2004 , 3377; c) R. Zimmer, C. U.
Dinesh, E. Nandanan, F. A. Khan, Chem. Rev.
2000 , 100 , 3067.
[3] For selected reviews on transition metal-catalyzed cyclization of functionalized allenes bearing a nucleophilic center, see: a) M. Brasholz, H.-U.
Reissig, R. Zimmer, Acc. Chem. Res.
2009 , 42 , 45; b) N. Bongers, N. Krause,
Angew. Chem. 2008 , 120 , 2208; Angew. Chem. Int. Ed.
2008 , 47 , 2178; c) R.
A. Widenhoefer, X. Han, Eur.
J. Org. Chem.
2006 , 4555; d) A. Hoffmann-
Röder, N. Krause, Org. Biomol. Chem.
2005 , 3 , 387; e) S. Ma, Acc. Chem.
Res.
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