Lewis acid mediated tandem reaction of propargylic alcohols to tetrazoles involving C-O- and C-C-bond cleavage reactions and a C-N-bond formation

Article abstract of DOI:10.1002/chem.201403752

A novel and direct synthesis of 1-aryl-5-arylvinyl-tetrazoles from easily prepared propargylic alcohols and TMSN₃ is developed in the presence of TMSCl under mild conditions (TMS=trimethylsilyl). The process involves an allenylazide intermediate, followed by a C-C-bond cleavage and C-N-bond formation to afford the desired products. Moreover, this method offers a good functional-group applicability and can be scaled-up to grams (yield up to 85%).

Full text of DOI:10.1002/chem.201403752

DOI: 10.1002/chem.201403752
Communication
&
Synthetic Methods
Lewis Acid Mediated Tandem Reaction of Propargylic Alcohols to
Tetrazoles Involving CÀO- and CÀC-Bond Cleavage Reactions and
a CÀN-Bond Formation
Xian-Rong Song, Ya-Ping Han, Yi-Feng Qiu, Zi-Hang Qiu, Xue-Yuan Liu, Peng-Fei Xu,* and
Yong-Min Liang*[a]
Abstract: A novel and direct synthesis of 1-aryl-5-arylvinyl-
tetrazoles from easily prepared propargylic alcohols and
TMSN3 is developed in the presence of TMSCl under mild
conditions (TMS=trimethylsilyl). The process involves an
allenylazide intermediate, followed by a CÀC-bond cleav-
age and CÀN-bond formation to afford the desired prod-
ucts. Moreover, this method offers a good functional-
group applicability and can be scaled-up to grams (yield
up to 85%).
1,5-Disubstituted tetrazoles represent a key molecular motif
with widespread occurrence in medicinal chemistry and agro-
chemicals, as well as in important synthetic intermediates.[1]
Scheme 1. New strategies for the synthesis of 1,5-disubstituted tetrazoles.
With the wide application of 1,5-disubstituted tetrazoles in
pharmaceutical research, the exploration of efficient synthetic
strategies towards these compounds has attracted much at-
tention among synthetic chemists. In this respect, numerous
efforts have been made to prepare 1,5-disubstituted tetra-
zoles.[2–9] Among various methods that introduce a nitrogen
source, using azide-containing compounds, which show a high
chemical reactivity towards functionalizing the starting materi-
als by different types of reaction modes, has been proven to
be efficient. For example, compounds such as ketones,[2]
amides,[3] oximes,[4] imidoyl,[5] imidoylbenzotriazoles,[6] and ni-
triles[7] could generate 1,5-disubstituted tetrazoles with azide-
containing compounds. Moreover, the Jiao and Prabhu groups
recently reported the Cu-catalyzed direct transformation of
simple hydrocarbon or allylic alcohols in the presence of 2,3-di-
chloro-5,6-dicyano-1,4-benzoquinone (DDQ) as oxidant to form
1,5-disubstituted tetrazoles, respectively (Scheme 1a).[8] In
2013, Echavarren and co-workers developed a gold-catalyzed
reaction of alkynes with TMSN3 for the synthesis of 1,5-disub-
stituted tetrazoles by CÀC-bond cleavage (Scheme 1b).[9] Al-
though a great number of pioneering methodologies have
been established, new practical and efficient approaches to
1,5-disubstituted tetrazoles from readily available starting ma-
terials are still extremely attractive and desirable.
Propargylic alcohols have served as versatile synthons for
the construction of various synthetic intermediates, allenes in
particular.[10] Our continuous interest in the potential reactivity
of propargylic alcohols stimulated us to employ them as pre-
cursor of allenyl cations. Recently, our group[11] and others re-
ported[12] that the dehydration of propargylic alcohols by Lewis
acids generates allenyl-cation intermediates, which could fur-
ther undergo divergent reactions with various nucleophiles.
Thus, we envisioned whether TMSN3 could be used as a nucleo-
phile in the reaction with propargylic alcohols to produce the
1,5-disubstituted tetrazole nucleus through CÀO- and CÀC-
bond cleavage reactions and CÀN-bond formation. To the best
of our knowledge, the direct transformation of propargylic al-
cohols to form 1,5-disubstituted tetrazoles has not been dis-
closed yet. Herein, we report the TMSCl-mediated reaction of
propargylic alcohols with TMSN3 as the nitrogen source to pro-
duce 1-aryl-5-arylvinyl-tetrazoles with high chemoselectivity,
which tolerates a variety of propargylic alcohol substrates
under mild conditions.
[a] Dr. X.-R. Song, Y.-P. Han, Y.-F. Qiu, Z.-H. Qiu, X.-Y. Liu, Prof. Dr. P.-F. Xu,
Prof. Dr. Y.-M. Liang
State Key Laboratory of Applied Organic Chemistry
Lanzhou University, Lanzhou 730000 (China)
Fax: (+86)931-891-2582
Initially, 0.1 mmol of propargylic alcohol 1a with TMSN3
(3.0 equiv) and TMSCl (1.0 equiv) was used in the presence of
molecular sieves (13X) in 1,2-dichloroethane (DCE) at 808C; to
our delight, the desired product 5-(2,2-diphenylvinyl)-1-(4-me-
thoxyphenyl)-1H-tetrazole (2a) was obtained in 40% yield after
E-mail: xupf@lzu.edu.cn
liangym@lzu.edu.cn
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201403752.
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Communication
could be isolated in excellent yields (2I and j), indicating the
compatibility of this transformation with electron-deficient
substituents. Moreover, with substituents at the ortho-position
on the aryl ring, the desired products were obtained in good
yields (2c, 2g, 2k), which indicates that steric hindrance has
no influence on the efficiency of this reaction. Notably, sub-
strates containing a chloro or bromo group on the aryl ring
(1l–n) could give the corresponding products in excellent
yields (2l–n), which provides an opportunity for further trans-
formations through orthogonal cross-coupling reactions. Fur-
thermore, sensitive functional groups, such as nitro and ester
groups, were tolerated and afforded the desired products in
excellent yields (2o and p), again indicating that this reaction
has good functional groups tolerance. When a substrate con-
taining several alkyl substituents on the aryl ring was em-
ployed, the reaction proceeded well and gave higher yields
(2q). Fortunately, the heteroaryl-substituted substrate 1r con-
taining a 2-thienyl group could give the desired product 2r. In
addition, alkyl and halogen substituents (R1, R2) on aryl rings
attached to the benzylic position (1s–u) were compatible in
this transformation, and good yields were obtained in all cases.
When unsymmetric tertiary propargylic alcohols (1v-w) were
employed in this reaction, the regioisomers 2v and w were ob-
tained in high yields under the optimal conditions. However,
alkyl-substituted tertiary propargylic alcohol 1x did not afford
the desired product after 4.0 h. This might be owing to the
fact that the alkyl group can not migrate to the nitrogen atom
in the rearrangement process. When changing the tertiary
propargylic alcohol from a twofold aryl-substituted to a 1-
methyl-1-phenyl-substituted one (1y), the desired product 2y
could not be observed under the standard conditions. Thus, it
was confirmed that the 1-methyl-1-phenyl-substituted tertiary
propargylic alcohol is more likely to undergo an intramolecular
dehydration to afford the 1,3-enyne compound 5y.[15]
Table 1. Optimization of the reaction conditions.[a]
Entry
Acid
([equiv])
TMSN3
[equiv]
Solvent
Yield
[%][b]
1
2
3
4
5
6
7
8
9
10[c]
11[d]
12[e]
13
14
TMSCl (1.0)
TfOH (1.0)
3.0
3.0
3.0
3.0
4.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
DCE
DCE
DCE
DCE
DCE
DCE
CH3NO2
1,4-dioxane
MeCN
CH3NO2
CH3NO2
CH3NO2
CH3NO2
CH3NO2
40
<5
<5
<5
75
85
90
20
70
trace
20
25
50
88
p-TsOH (1.0)
MsOH (1.0)
TMSCl (1.0)
TMSCl (1.0)
TMSCl (1.0)
TMSCl (1.0)
TMSCl (1.0)
TMSCl (1.0)
TMSCl (1.0)
TMSCl (1.0)
TMSCl (0.5)
TMSCl (1.2)
[a] Unless otherwise noted, all reactions were performed with 1a
(0.1 mmol), TMSN3 (0.4 mmol), and molecular sieves (13X; 30 mg) in sol-
vent (1.0 mL) at 808C. [b] Isolated yield. [c] The reaction was carried out in
the absence of molecular sieves (13X). [d] Reaction performed at room
temperature. [e] 4 ꢁ molecular sieves was used instead of 13X molecular
sieves. TMS=trimethylsilyl, MsOH=mesylic acid, TfOH=trifluoromethane-
sulfonic acid, p-TsOH=p-toluenesulfonic acid.
2.0 h (Table 1, entry 1). Subsequently, several representative
protic acids, such as TfOH, p-TsOH, and MsOH, were screened,
but no superior results were obtained (Table 1, entries 2–4).
Next, by increasing the load of TMSN3 to five equivalents, 2a
was obtained in 85% yield (Table 1, entries 5 and 6). CH3NO2
proved to be the most efficient solvent in this transformation,
which increased the yield of 2a to 90% (Table 1, entries 7–9).
Only a trace amount of 2a was obtained in the absence of mo-
lecular sieves (13X, Table 1, entry 10), which might be owing to
the molecular sieve acting as a solid acid to activate the dehy-
dration of the propargylic alcohol.[13] 2a was obtained only in
a lower yield when molecular sieves 4 ꢁ was used instead of
molecular sieves 13X (Table 1, entry 12). A higher yield was not
obtained by adjusting the amount of TMSCl (Table 1, entries 13
and 14). Ultimately, the use of TMSCl (1.0 equiv) in the pres-
ence of molecular sieves (13X) in CH3NO2 at 808C proved to be
the most efficient, which was identified as the standard condi-
tions.
To further explore the scope of this method, the compatibili-
ty with secondary propargylic alcohols under the optimal con-
ditions was investigated, as shown in Table 3. Various secon-
dary propargylic alcohols with electron-rich and electron-poor
substituents on the aryl substituent reacted efficiently to
afford the corresponding products 4a–d in excellent yields.
Notably, when unsymmetric secondary propargylic alcohols
were employed in this reaction, the desired products (4a–d)
could be obtained with high E stereoselectivity.
An obvious advantage of this method is that the reaction
can be scaled-up to gram scale; when the easily prepared sec-
ondary propargylic alcohol 3b was used as substrate under
the standard conditions, the desired product (E)-1-(4-methoxy-
phenyl)-5-styryl-1H-tetrazole (4b) was obtained in a high yield
of 85%, which provides potential application in organic syn-
thesis or, prospectively, even in industry (Scheme 2).
With the optimized conditions in hand, we explored the
scope of this reaction for the transformation of tertiary propar-
gylic alcohols with TMSN3, as shown in Table 2. Various tertiary
propargylic alcohols 1a–w could be easily converted into the
corresponding products 2a–w in moderate to excellent yields.
The structure of 2a was confirmed by X-ray diffraction (see the
Supporting Information).[14] Firstly, substrates containing a me-
thoxy or alkyl groups on the aryl substituent could give the
corresponding products in high yields (2a and b, 2d–f, 2h).
Fluoro-substituted propargylic alcohols were also tolerated
under the optimal conditions, and the corresponding products
Finally, we explored the mechanism of this transformation. It
is generally approved that propargylic alcohols can easily rear-
range to the corresponding a,b-unsaturated carbonyl com-
pounds through a Meyer–Schuster rearrangement under acidic
conditions.[16] We assumed that one possible pathway would
be a tandem sequence involving a Lewis acid mediated
Meyer–Schuster rearrangement of the propargylic alcohols to
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starting material to undergo this transformation
under the standard conditions; however, the desired
product 2a was not observed (Scheme 3). These re-
sults clearly indicate that a,b-unsaturated ketones
and amides are not involved in this novel transforma-
tion.
Table 2. Reaction scope for the transformation of tertiary propargylic
alcohols.[a,b]
On the basis of the above observations and previ-
ous reports[8,12], we propose a plausible mechanism
for this tandem transformation (Scheme 4). Initially,
dehydration of the propargylic alcohol in the pres-
ence of TMSCl and molecular sieves (13X) gives the
propargylic cation intermediate A, which can gener-
ate an allenyl-cation intermediate B through tauto-
merization of the propargyl cation. Subsequent
attack of the azide anion on the allenyl cation affords
allenylazide intermediate C. Protolysis of intermediate
C forms intermediate D, which can release nitrogen
gas through a highly chemoselective 1,2-aryl shift to
form intermediate E. Then, intermediate E is attacked
by another azide anion through a [3+2] cycloaddi-
tion[18] to generate the desired 1-aryl-5-arylvinyl-tetra-
zole product.
In conclusion, a novel and direct synthesis of 1-
aryl-5-arylvinyl-tetrazoles from easily prepared prop-
argylic alcohols and TMSN3 has been developed in
the presence of TMSCl. The process may involve an
allenylazide intermediate, followed by a CÀC-bond
cleavage and CÀN-bond formation to afford the de-
sired products. This transformation offers a new syn-
thetic strategy for the synthesis of important nitro-
gen-containing compounds and, furthermore, ex-
tends the application of azides to organic transforma-
tion of alkynols. The synthetic utility of this tandem
reaction has been demonstrated by its applicability
to a wide range of propargylic alcohol substrates and
its scale-up to grams (yield up to 85%).
Experimental Section
General procedure for the synthesis of 1-aryl-5-ar-
ylvinyl-tetrazoles
Propargylic alcohol 1a (31.4 mg, 0.1 mmol), molecular
sieves (13X; 30.0 mg), TMSCl (9.0 mL, 0.1 mmol), and azi-
dotrimethylsilane (75 mL, 0.5 mmol) in CH3NO2 (1.0 mL)
were stirred at 808C under air. After 2 h, as monitored by
TLC, the mixture was concentrated and purified by flash
chromatography on silica gel (petroleum ether/EtOAc=
8:1, add Et3N) to afford 2a as a white solid (32.0 mg,
yield 90%).
[a] Unless otherwise noted, all reactions were performed with 1a (0.1 mmol), TMSN3
(0.5 mmol), and molecular sieves (13X; 30 mg) in CH3NO2 (1.0 mL) at 808C. [b] Isolated
yield. [c] For 4.0 h.
the a,b-unsaturated ketones, then followed by a Schmidt reac- Acknowledgements
tion to form the amides,[17] which subsequently react with
TMSN3 to generate the products.[3] To verify this hypothesis,
two possible key intermediates, that is, the a,b-unsaturated
ketone 6 and cinnamamide 7, were prepared and used as
We thank the National Science Foundation (NSF21072081) and
the National Basic Research Program of China (973 Program)
2010CB8–33203 for financial support. We also acknowledge
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Table 3. Reaction scope for the transformation of secondary propargylic
alcohols.[a,b]
Keywords: 1-aryl-5-arylvinyl-tetrazoles · CÀC-bond cleavage ·
CÀN-bond formation · Lewis acids · propargylic alcohols
[1] a) D. R. Armour, K. M. L. Chung, M. Congreve, B. Evans, S. Guntrip, T.
Hubbard, C. Kay, D. Middlemiss, J. E. Mordaunt, N. A. Pegg, M. V. Vinad-
er, P. Ward, S. P. Watson, Bioorg. Med. Chem. Lett. 1996, 6, 1015; b) D.
Moderhack, J. Prakt. Chem./Chem.-Ztg. 1998, 340, 687; c) D. Potts, P. J.
Stevenson, N. Thompson, Tetrahedron Lett. 2000, 41, 275; d) A. Fettes,
E. M. Carreira, Angew. Chem. 2002, 114, 4272; Angew. Chem. Int. Ed.
2002, 41, 4098; e) P. Jankowski, K. Plesniak, J. Wicha, Org. Lett. 2003, 5,
2789; f) D. Enders, A. Lenzen, M. Muller, Synthesis 2004, 1486; g) L. M. T.
Frija, I. V. Khmelinskii, M. L. S. Cristiano, Tetrahedron Lett. 2005, 46, 6757;
h) J. Quintin, G. Lewin, J. Nat. Prod. 2004, 67, 1624; i) M. J. Schnermann,
D. L. Boger, J. Am. Chem. Soc. 2005, 127, 15704; j) L. V. Myznikov, A. Hra-
balek, G. I. Koldobskii, Chem. Heterocycl. Compd. 2007, 43, 1; k) B. J. Al-
Hourani, S. K. Sharma, J. Y. Mane, J. Tuszynski, V. Baracos, T. Kniess, M.
Suresh, J. Pietzsch, F. Wuest, Bioorg. Med. Chem. Lett. 2011, 21, 1823.
[2] a) A. L. Tçkꢂs, G. Litkei, Synth. Commun. 1993, 23, 895; b) H. Suzuki, Y. S.
Hwang, C. Nakaya, Y. Matano, Synthesis 1993, 1218; c) A. S. El-Ahl, S. S.
Elmorsy, H. Soliman, F. A. Amer, Tetrahedron Lett. 1995, 36, 7337; d) A. G.
Schultz, A. Wang, C. Alva, A. Sebastian, S. D. Glick, D. C. Deecher, J. M.
Bidlack, J. Med. Chem. 1996, 39, 1956.
[3] R. N. Butler, D. A. O. Donoghue, J. Chem. Res., Synop. 1983, 18.
[4] a) J. Zabrocki, J. B. Dunbar, Jr., K. W. Marshall, M. V. Toth, G. R. Marshall, J.
Org. Chem. 1992, 57, 202; b) E. W. Thomas, Synthesis 1993, 767; c) A. Le-
Tiran, J. P. Stables, H. Kohn, Bioorg. Med. Chem. 2001, 9, 2693; d) J. Xiao,
X. Zhang, D. Wang, C. Yuan, J. Fluorine Chem. 1999, 99, 83; e) A. F. He-
garty, N. M. Tynan, S. Fergus, J. Chem. Soc. Perkin Trans. 2 2002, 1328.
[5] T. V. Artamonova, A. B. Zhivich, M. Yu. Dubinskii, G. I. Koldobskii, Synthe-
sis 1996, 1428.
[a] Unless otherwise noted, all reactions were performed with 1a
(0.2 mmol), TMSN3 (1.0 mmol), and molecular sieves (13X; 60 mg) in
CH3NO2 (2.0 mL) at 808C. [b] Isolated yield.
[6] A. R. Katritzky, C. Cai, N. K. Meher, Synthesis 2007, 1204.
[7] a) D. Cantillo, B. Gutmann, C. O. Kappe, J. Am. Chem. Soc. 2011, 133,
4465; b) Angew. Chem. 2011, 123, 3587; Angew. Chem. Int. Ed. 2011, 50,
3525; c) F. Couty, F. Durrat, D. Prim, Tetrahedron Lett. 2004, 45, 3725;
d) F. Himo, Z. P. Demko, L. Noodleman, K. B. Sharpless, J. Am. Chem. Soc.
2003, 125, 9983; e) Z. P. Demko, K. B. Sharpless, Angew. Chem. 2002,
114, 2217; Angew. Chem. Int. Ed. 2002, 41, 2113.
[8] a) F. Chen, C. Qin, Y. Cui, N. Jiao, Angew. Chem. 2011, 123, 11689; Angew.
Chem. Int. Ed. 2011, 50, 11487; b) B. V. Rokade, K. Gadde, K. R. Prabhu,
Adv. Synth. Catal. 2014, 356, 946.
Scheme 2. Scale-up experiment.
support from the “111” Project and Program for Changjiang
[9] M. Gaydou, A. M. Echavarren, Angew. Chem. 2013, 125, 13710; Angew.
Chem. Int. Ed. 2013, 52, 13468.
[10] a) X. Zhang, W. T. Teo, Sally, P. W. H. Chan, J. Org. Chem. 2010, 75, 6290;
b) X. Zhang, W. T. Teo, P. W. H. Chan, Org. Lett. 2009, 11, 4990; c) W.
Huang, Q. Shen, J. Wang, X. Zhou, J. Org. Chem. 2008, 73, 1586; d) K.
Lee, P. H. Lee, Org. Lett. 2008, 10, 2441; e) R. Sanz, D. Miguel, A. Marti-
nez, J. M. Alvarez-Gutierrez, F. Rodriguez, Org. Lett. 2007, 9, 727.
[11] a) K.-G. Ji, J. Chen, H.-T. Zhu, F. Yang, A. Shaukat, Y.-M. Liang, Chem. Eur.
J. 2011, 17, 305; b) K.-G. Ji, H.-T. Zhu, F. Yang, A. Shaukat, Xiao-F. Xia, Y.-F.
Yang, X.-Y. Liu, Y.-M. Liang, J. Org. Chem. 2010, 75, 5670; c) K.-G. Ji, X.-Z.
Shu, S.-C. Zhao, H.-T. Zhu, Y.-N. Niu, X.-Y. Liu, Y.-M. Liang, Org. Lett. 2009,
11, 3206.
Scheme 3. Control experiments.
Scholars and innovative Research Team in University (IRT1138).
[12] H. Zhang, H. Tanimoto, T. Morimoto, Y. Nishiyama, K. Kakiuchi, Org. Lett.
2013, 15, 5222.
[13] G. Busca, Chem. Rev. 2007, 107,
5366.
[14] CCDC 993234 (2a) contains the
supplementary
crystallographic
data for this paper (see the Sup-
porting Information). These data
can be obtained free of charge
from The Cambridge Crystallo-
graphic
Data
Centre
via
www.ccdc.cam.ac.uk/data_request/
cif
[15] 1-Methyl-1-phenyl-substituted ter-
tiary propargylic alcohol 1y tends
to undergo an intramolecular de-
hydration under the standard con-
Scheme 4. Proposed mechanism.
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ditions to afford the 1,3-enyne compound 5y, see: S. Swaminathan,
K. V. Narayanan, Chem. Rev. 1971, 71, 429.
M. G. Unthank, P. Turner, T. D. Sheppard, J. Org. Chem. 2011, 76, 1479;
h) D. Wang, Y. Zhang, A. Harris, L. N. S. Gautam, Y. Chen, X. Shi, Adv.
Synth. Catal. 2011, 353, 2584.
[17] a) K. F. Schmidt, Z. Angew. Chem. 1923, 35, 511; b) H. Wolff, Org. React.
1946, 3, 307; c) G. R. Krow, Tetrahedron 1981, 37, 1283; d) J. Aube, G. L.
Milligan, J. Am. Chem. Soc. 1991, 113, 8965; e) S. Brꢃse, C. Gil, K. Knep-
per, V. Zimmermann, Angew. Chem. 2005, 117, 5320; Angew. Chem. Int.
Ed. 2005, 44, 5188; f) S. Lang, J. A. Murphy, Chem. Soc. Rev. 2006, 35,
146; g) S. Grecian, J. Aubꢂ, in Organic Azides: Syntheses and Applications
(Eds.: S. Brꢃse, K. Banert), Wiley, Chichester 2010, p. 191; h) A. Wrobleski,
T. C. Coombs, C. W. Huh, S.-W. Li, J. Aubꢂ, Org. React. 2012, 78, 1.
[18] a) M. I. K. Amer, B. L. Booth, J. Chem. Res., Synop. 1993, 4; b) E. Cubero,
M. Orozco, F. J. Luque, J. Am. Chem. Soc. 1998, 120, 4723; c) T. Lioux, G.
Gosselin, C. Mathꢂ, Eur. J. Org. Chem. 2003, 3997.
´
[16] For reviews and recent typical examples, see: a) V. Cadierno, S. E. Garca-
Garrido, J. Gimeno, Adv. Synth. Catal. 2006, 348, 101; b) D. A. Engel, G. B.
Dudley, Org. Lett. 2006, 8, 4027; c) M. Stefanoni, M. Luparia, A. Porta, G.
Zanoni, G. Vidari, Chem. Eur. J. 2009, 15, 3940; d) D. A. Engel, G. B.
Dudley, Org. Biomol. Chem. 2009, 7, 4149; e) V. Cadierno, P. Crochet,
´
S. E. Gaca-Garrido, J. Gimino, Dalton Trans. 2010, 39, 4015; f) B. M. Trost,
Received: May 30, 2014
X. Luan, Y. Miller, J. Am. Chem. Soc. 2011, 133, 12824; g) M. N. Pennell,
Published online on August 8, 2014
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