Directed heterodimerization: Stereocontrolled assembly via solvent-caged unsymmetrical diazene fragmentation

Article abstract of DOI:10.1021/ja2057852

A general strategy for the directed and stereocontrolled assembly of carbon-carbon linked heterodimeric hexahydropyrroloindoles is described. The stepwise union of complex amines in the form of mixed diazenes followed by photoexpulsion of dinitrogen in a

Full text of DOI:10.1021/ja2057852

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pubs.acs.org/JACS
Directed Heterodimerization: Stereocontrolled Assembly via
Solvent-Caged Unsymmetrical Diazene Fragmentation
Mohammad Movassaghi,* Omar K. Ahmad, and Stephen P. Lathrop
Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States
S Supporting Information
b
and (ꢀ)-1 using our Co-promoted strategy provides the desired
ABSTRACT: A general strategy for the directed and
stereocontrolled assembly of carbonꢀcarbon linked hetero-
dimeric hexahydropyrroloindoles is described. The stepwise
union of complex amines in the form of mixed diazenes
followed by photoexpulsion of dinitrogen in a solvent cage
provides completely guided assembly at challenging Csp3ꢀCsp3
and Csp3ꢀCsp2 connections.
heterodimeric meso-2 in only 16% isolated yield (eq 1). The near-
statistical product mixture of 2 and 3 also contains the corre-
sponding disproportionation and related byproducts which
hampers the isolation of pure heterodimer 2.6 The low yield of
the desired product along with complications associated with
side product formation restricts the use of this chemistry in
preparative heterodimeric assembly. A maximally convergent
solution to heterodimeric molecules requires a method that
provides a single product with minimal influence of substrate
bias in the planned union.
imeric and oligomeric cyclotryptamine and cyclotrypto-
D
phan alkaloids constitute a large family of natural products
with diverse molecular architectures and a wide range of biolog-
ical activities.1 Nature can access an array of these alkaloids
containing quaternary stereocenters at C3a through the amalga-
mation of various monomers. In 2007, we reported a versatile
strategy for the concise and enantioselective synthesis of homo-
dimeric cyclotryptamine substructures.2 However, to date, there
are no reported methods for selective CꢀC3 bond construction
at the C3a quaternary stereocenter of two dissimilar cyclotrypt-
amine subunits, a synthetically challenging structural motif found
in many heterodimeric alkaloids (Figure 1).4 Herein we report a
strategy for completely stereoselective and directed union of
complex fragments at these sterically crowded linkages. We
demonstrate the utility of this chemistry in adjoining differing
monomers at CꢀC fusions common to this family of natural
products.
Inspired by the work of Bartlett, Engel, and Naumann,7 we
considered the possibility of using diazenes as traceless linkers8
and radical precursors for our desired heterodimerization chem-
istry. Dialkyl diazenes are known to undergo expulsion of
dinitrogen upon photoexcitation to generate two radical species.
However, in these cases,7b radical combination is accompanied
by varying amounts of disproportionation. Furthermore, photo-
excitation of unsymmetrical diazenes is often complicated by cross-
over products due to out-of-cage coupling,7f,l,m thus limiting their
utility in fragment assembly and complex molecule synthesis.
We envisioned the expulsion of dinitrogen from an unsymme-
trical diazene 5 (Scheme 1) to form a pair of carbon-centered
radicals whose directed union in a solvent cage9 would result in
selective formation of the desired heterodimer 4. The use of the
mixed sulfamide 6as the precursor10 to the unsymmetrical diazene 5
would provide a platform for the directed assembly of the two
monomeric amines, 7 and 70. Implementation of this strategy in
complex synthesis would require: (a) synthesis of cyclotryptamine-
based mixed sulfamides,11 (b) mild conditions for their conversion
to the corresponding unsymmetrical diazenes followed by fragmenta-
tion,12 (c) solvent-cage-controlled radical pair combination,9
and (d) minimization of out-of-cage coupling (homodimeriza-
tion) and disproportionation.7
Figure 1. Representative heterodimeric cyclotryptamine alkaloids.
Our laboratory seeks effective methodology for controlled
union of complex fragments for application in natural products
synthesis. While our Co(I)-promoted homodimerization of cyclo-
tryptamine derivatives has enabled concise total syntheses,2,5 we
found its extension to heterodimerization problematic. For
example, uncontrolled dimerization of tricyclic bromides (+)-1
Received: June 21, 2011
Published: July 16, 2011
r
2011 American Chemical Society
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Scheme 1. Directed Heterodimerization
Scheme 2. Homodimer Synthesis
Our initial studies focused on the evaluation of the use of
dialkyl diazenes in the context of homodimerization. We began
with the development of a diazene-based synthetic route to
homodimer (+)-3 (Scheme 2). We developed a versatile entry to
the necessary amines 7 (Scheme 1) by derivatization of the
corresponding benzylic bromides that had been utilized in our
Co-promoted dimerization studies. As illustrated in Scheme 2,
exposure of the bromide (+)-16 to tin tetrachloride and tri-
methylsilyl azide followed by reduction of the corresponding
azide13 provided the desired hexahydropyrroloindolyl amine14
(+)-7a (71%). Exposure of (+)-7a to sulfuryl chloride provided
the sulfamide (+)-6a in 81% yield. Under optimal conditions,
subsequent oxidation of sulfamide (+)-6a with N-chlorosuc-
cinimide in the presence of 2-tert-butylimino-2-diethylamino-
1,3-dimethylperhydro-1,3,2-diazaphosphorine on polystyr-
ene resin (BEMP) generated the desired diazene (+)-5a
in 61% isolated yield.15 The UV absorption at 355 nm and
the 13C NMR resonance of the C3a of (+)-5a were in accord
with previously reported data for dialkyl diazenes.16
Photoexcitation17 of (+)-5a led to expulsion of dinitro-
gen and formation of the desired dimeric hexacycle (+)-3
in 60% yield. The overall efficiency of the process is increased
(48% over two steps) when the freshly prepared crude
Scheme 3. Directed Assembly and Heterodimer Synthesis
1
diazene (∼99%, based on H NMR with internal standard)
is used in the following step without chromatographic
purification.
Having established the viability of using the sulfamide
(+)-6a as a precursor to homodimer (+)-3, we turned our
attention to the development of a general method for
directed heterodimerization. Stepwise sulfonylation of dif-
ferent hexahydropyrroloindolyl amines was expected to
provide a means for assembly of a heterodimeric structure
as the prelude to the construction of the desired linkage. The
selective synthesis of mixed sulfamide (+)-6b is illustrated in
Scheme 3. Treatment of amine (+)-7a with chlorosulfonic
acid followed by addition of sodium carbonate afforded the
corresponding sodium sulfamate salt (+)-12.11a In situ acti-
vation of 12 to form the sulfamoyl chloride 13 followed by
direct union with complex amine 7b provided the unsymme-
trical sulfamide (+)-6b (86% based on 7b). Exposure of (+)-
6b to N-chlorosuccinimide provided the corresponding un-
symmetrical diazene 5b, likely via the transient thiadiazir-
idine dioxide 14b. The crude diazene 5b was subjected to
photoinduced expulsion of dinitrogen to exclusively afford
the desired heterodimer (+)-4b in 68% yield from (+)-6b.
The optimal conditions involved irradiation using a medium-
pressure mercury vapor lamp in tert-butanol as solvent
in a Pyrex reaction vessel. Importantly, neither of the two
possible homodimeric products was observed by HPLC
analysis of the crude product mixture. Notably, the forma-
tion of heterodimer (+)-4b constitutes the first example of
directed and stereoselective CꢀC bond construction fusing two
different cyclotryptamine fragments at vicinal quaternary
stereocenters.
The exclusive formation of heterodimeric product (+)-4b
suggests exquisite control in the solvent-caged coupling
of the radical pair formed upon dinitrogen expulsion from
the dialkyl diazene 5b. We sought opportunities to probe the
level of control exerted by this strategy in the guided unifica-
tion of complex monomers. Exposure of an equal mixture
of symmetrical sulfamides 6a and 6c to the two-step seq-
uence for oxidation and fragmentation afforded only an
equal mixture of the respective homodimeric products 3
and 15 (55% and 51% yield, respectively, eq 2). Notably,
HPLC analysis of the crude product mixture against authentic
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samples of 3, 4b, and 15 did not reveal any of the heterodimeric
product 4b.6
Table 1. Directed Heterodimer Synthesis
Furthermore, photoexpulsion of dinitrogen from (+)-5a in the
presence of 1,4-cyclohexadiene (5.0 equiv), a H-atom donor,
resulted in an almost equal mixture of the desired dimeric
product (+)-3 (45% yield) and the corresponding monomeric
C3a-H reduction product (21% yield, ∼1:1 molar ratio).6
An increase in the amount of H-atom donor (20 equiv) afforded
a similar molar ratio of product (+)-3 (41% yield) to monomeric
C3a-H reduction product (20% yield). For comparison, under our
reported Co-mediated dimerization conditions,2 bromide (+)-1
exclusively provided the monomeric C3a-H reduction product in
the presence of 1,4-cyclohexadiene (5.0 equiv, 54%; 20 equiv,
82%).6 The formation of the dimer as the major product, even in
the presence of excess H-atom donor and at higher dilution under
our photochemical conditions, is consistent with solvent-cage-
directed radical-pair combination.
Having found conditions for the synthesis of the desired
heterodimeric product, we sought to further examine the scope
of this process. Gratifyingly, mixed sulfamides 6dꢀ6i (Table 1)
were readily prepared using the optimal conditions described
above (Scheme 3). In each case, the more readily available amine
was converted to the corresponding sulfamoyl chloride, allowing
for the directed assembly of the heterodimeric sulfamides.
Exposure of mixed sulfamides 6dꢀ6i to the optimized oxidative
diazene synthesis afforded the heterodimeric diazenes 5dꢀ5i
(Table 1). The crude diazenes were subjected to photochemical
expulsion of dinitrogen and gave the desired dimeric products
(+)-4dꢀ4i and meso-2.
a Mixed sulfamide synthesis: 7, 13, DMAP, Et3N, CH2Cl2, 0 f 23 °C.
Isolated % yield of 6 after chromatography. b Diazene synthesis: BEMP,
NCS, THF, 23 °C. Crude % yield of sensitive diazene 5 in parentheses.
c Heterodimer synthesis: t-BuOH, hν >280 nm, 23 °C, 5 h. Isolated %
yield of 4 after chromatography. Yield of 4 from 6 in brackets. d DBU,
NCS, MeOH, 0 f 23 °C. e hν 300 nm, 12 h.
Notably, this strategy allows access to complex heterodimers
such as products (+)-4e and (+)-4f. Specifically, heterodimer
(+)-4f results from the fusion of a tetracyclic diketopiperazine
with a cyclotryptamine moiety. Thus, the chemistry described
here offers the first solution for directed and exclusive hetero-
dimeric union of requisite dissimilar cyclotryptamine precursors.
Coupling the enantiomeric amines (+)-7a and (ꢀ)-7a afforded
the meso-sulfamide 6g (Table 1, entry 4), which upon oxidation
and photolysis provided cleanly and exclusively the correspond-
ing meso-dimer 2 (56% over two steps), a structural core found in
meso-chimonanthine,4a (+)-leptosin K,4d and many other cyclo-
tryptamine natural products.18 The exclusive formation of meso-2
in 28% overall yield from tricyclic bromides (+)-1 and (ꢀ)-1 can
be directly compared to the example described in eq 1.
Furthermore, we wanted to explore the applicability of this
methodology to the synthesis of C3a-aryl-substituted quaternary
stereocenters. The Csp3ꢀCsp2 connectivity between cyclotrypt-
amine substructures is found in many natural alkaloids (Figure 1).1
Notably, our Co-promoted dimerization chemistry is not
applicable to such unions. We were delighted to find that re-
placement of one of the amine components with an aniline
derivative provided access to mixed arylꢀcyclotryptamine sulf-
amides (Table 1, entries 5 and 6).6 Oxidation and photolysis
provided the corresponding arylated hexahydropyrroloindoles
(+)-4h and (+)-4i. The efficiency of the dinitrogen expulsion
from N-aryl-N0-cyclotryptaminyl diazenes 5h19 and 5i was on par
with that of mixed diazenes 5dꢀ5g. Current efforts are directed
at broadening the scope of this methodology by developing
milder methods for converting complex mixed arylꢀalkyl sulf-
amides to the corresponding diazenes.
We have developed a general strategy for the stereoselective
directed synthesis of dimeric substructures found in hexahydro-
pyrroloindole alkaloids. Our findings constitute the first con-
trolled coupling of different cyclotryptamine monomers at
quaternary carbons and is distinct from prior strategies based
on desymmetrization chemistry.20 The described protocol allows
for the synthesis of heterodimeric products with exquisite selec-
tivity in four operations from the corresponding amines while only
requiring purification of the mixed sulfamides and final products
after photolysis. This chemistry allows directed heterodimeriza-
tion at important substructure linkages, particularly the challen-
ging Csp3ꢀCsp3 connections, found in this family of hetero-
dimeric complex alkaloids. This completely stereocontrolled
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and directed fragment coupling draws on the versatility of
diazene chemistry7,21 and holds great potential for complex
molecule assembly.
J. Am. Chem. Soc. 1978, 100, 920. (i) Baldwin, J. E.; Adlington, R. M.;
Bottaro, J. C.; Kolhe, J. N.; Newington, I. M.; Perry, M. W. D. Tetra-
hedron 1986, 42, 4235. (j) Sumiyoshi, T.; Kamachi, M.; Kuwae, Y.;
Schnabel, W. Bull. Chem. Soc. Jpn. 1987, 60, 77. (k) Neuman, R. C., Jr.;
Grow, R. H.; Binegar, G. A.; Gunderson, H. J. J. Org. Chem. 1990,
55, 2682. (l) Engel, P. S.; Pan, L.; Ying, Y.; Alemany, L. B. J. Am. Chem.
Soc. 2001, 123, 3706. (m) Hoijemberg, P. A.; Karlen, S. D.; Snaramꢀe,
C. N.; Aramendía, P. F.; García-Garibay, M. A. Photochem. Photobiol. Sci.
2009, 8, 961. For relevant reviews, see: (n) Engel, P. S.; Steel, C. Acc.
Chem. Res. 1973, 6, 275. (o) Engel, P. S. Chem. Rev. 1980, 80, 99.
(8) We also explored the use of diacyl peroxides and diacyl diazenes:
(a) Bartlett, P. D.; Leffler, J. E. J. Am. Chem. Soc. 1950, 72, 3030.
(b) Leffler, J. E.; Bond, W. B. J. Am. Chem. Soc. 1956, 78, 335.
(c) Cramer, R. J. Am. Chem. Soc. 1957, 79, 6215. (d) Mackay, D.; Marx,
U. F.; Waters, W. A. J. Chem. Soc. 1964, 4793. (e) Feldhues, M.; Sch€afer,
H. J. Tetrahedron 1985, 41, 4213. (f) Spanttulescu, M. D.; Jain, R. P.;
Derksen, D. J.; Vederas, J. C. Org. Lett. 2003, 5, 2963.
’ ASSOCIATED CONTENT
S
Supporting Information. Experimental procedures, spectros-
b
copic data, and related mechanistic studies. This material is
available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
movassag@mit.edu
(9) (a) Nodelman, N.; Martin, J. C. J. Am. Chem. Soc. 1976, 98, 6597.
(b) Braden, D. A.; Parrack, E. E.; Tyler, D. R. Coordin. Chem. Rev. 2001,
211, 279.
(10) We also examined the oxidation of dialkyl ureas. For pioneering
work on diaziridinones, see: (a) Greene, F. D.; Stowell, J. C. J. Am. Chem.
Soc. 1964, 86, 3569. (b) Greene, F. D.; Stowell, J. C.; Bergmark, W. R.
J. Org. Chem. 1969, 34, 2254.
’ ACKNOWLEDGMENT
We acknowledge financial support by NIH-NIGMS (GM089732),
Amgen, and DuPont. M.M. is a Camille Dreyfus Teacher-Scholar.
O.K.A. acknowledges an Amgen summer graduate fellowship. We
thank Mr. Justin Kim, Dr. Nicolas Boyer, and Dr. Michael A. Schmidt
for helpful discussions.
(11) (a) Audrieth, L. F.; Sveda, M. J. Org. Chem. 1944, 9, 89. (b)
Hansen, N. C. Acta Chem. Scand. 1963, 17, 2141. (c) Weiss, G.; Schulze,
G. Liebigs Ann. Chem. 1969, 729, 40. (d) Kloek, J. A.; Leschinsky, K. L.
J. Org. Chem. 1976, 41, 4028. (e) Timberlake, J. W.; Ray, W. J., Jr.;
Stevens, E. D.; Cheryl, K. L. J. Org. Chem. 1989, 54, 5824.
(12) (a) Ohme, R.; Schmitz, E. Angew. Chem., Int. Ed. 1965, 4, 433.
(b) Golzke, F.; Oberlinner, G. A.; R€uchardt, C. Nouv. J. Chim. 1977,
1, 169. (c) Chang, H.-H.; Weinstein, B. J. Chem. Soc., Perkin Trans. 1
1977, 1601. (d) Ikeda, H.; Hoshi, Y.; Namai, H.; Tanaka, F.; Goodman,
J. L.; Mizuno, K. Chem.—Eur. J. 2007, 13, 9207.
’ REFERENCES
(1) (a) Cordell, G. A.; Saxton, J. E. In The Alkaloids: Chemistry and
Physiology; Manske, R. H. F., Rodrigo, R. G. A., Eds.; Academic Press:
New York, 1981; Vol. 20, pp 3ꢀ294. (b) Hino, T.; Nakagawa, M. In The
Alkaloids: Chemistry and Pharmacology; Brossi, A., Ed.; Academic Press:
New York, 1989; Vol. 34, pp 1ꢀ75. (c) Crich, D.; Banerjee, A. Acc.
Chem. Res. 2007, 40, 151. (d) Steven, A.; Overman, L. E. Angew. Chem.,
Int. Ed. 2007, 46, 5488.
(2) (a) Movassaghi, M.; Schmidt, M. A. Angew. Chem., Int. Ed. 2007,
46, 3725. (b) Movassaghi, M.; Schmidt, M. A.; Ashenhurst, J. A. Angew.
Chem., Int. Ed. 2008, 47, 1485. (c) Kim, J.; Ashenhurst, J. A.; Movassaghi,
M. Science 2009, 324, 238. (d) Kim, J.; Movassaghi, M. J. Am. Chem. Soc.
2010, 132, 14376.
(13) For base-promoted introduction of azide and p-MeC6H4 at C3a
of a related cyclotryptophan, see: Espejo, V. R.; Li, X.-B.; Rainier, J. D.
J. Am. Chem. Soc. 2010, 132, 8282.
(14) For a recent synthesis of C3a-amino cyclotryptamines, see:
Benkovics, T.;Guzei, I. A.;Yoon, T. P.Angew. Chem., Int. Ed. 2010, 49, 9153.
(15) Application of previously reported conditions was found to
suffer from incomplete conversion or low yield of the diazene.
(16) Key spectroscopic data for representative diazenes: R,R0-azo-
cumene, λmax = 367 nm,7b 13C NMR (CDCl3) δ 71.32 (R-carbon);12d
trans-N,N0-di(1-adamantyl)diazene, λmax(octane) = 368 nm.7k
(17) Photoexcitation at 23 °C was found to be superior to thermal
diazene fragmentation for our substrates. For example, diazene (+)-5a
was stable at 120 °C in DMSO-d6 but resulted in unproductive
decomposition at 150 °C.
(18) (a) Hart, N. K.; Johns, S. R.; Lamberton, J. A.; Summons, R. E.
J. Aust. Chem. 1974, 27, 639. (b) Libot, F.; Miet, C.; Kunesch, N;
Poisson, J. E.; Pusset, J.; Sꢀevenet, T. J. Nat. Prod. 1987, 50, 468.
(c) Verotta, L.; Pilati, T.; Tatø, M.; Eilsabetsky, E.; Amador, T. A.;
Nunes, D. S. J. Nat. Prod. 1998, 61, 392. (d) Jannic, V.; Guꢀeritte, F.;
Laprꢀevote, O.; Serani, L.; Martin, M.-T.; Sꢀevenet, T.; Potier, P. J. Nat.
Prod. 1999, 62, 838.
(19) Diazene 5h was found to undergo facile trans-to-cis isomeriza-
tion in solution (CD3CN) upon exposure to ambient light.
(20) For an elegant example of desymmetrization chemistry in a
related system, see: Kodanko, J. J.; Overman, L. E. Angew. Chem., Int. Ed.
2003, 42, 2528. Also see refs 2b, 5a, and 5d.
(21) For representative examples of intramolecular carbonꢀcarbon
bond formation using dialkyl diazene intermediates in natural product
synthesis, see: (a) Little, R. D.; Carroll, G. L.; Pettersen, J. L. J. Am. Chem.
Soc. 1983, 105, 928. (b) Little, R. D. Chem. Rev. 1996, 96, 93.
(c) Mascitti, V.; Corey, E. J. J. Am. Chem. Soc. 2004, 126, 15664.
(d) Wender, P. A.; Kee, J.-M.; Warrington, J. M. Science 2008, 320, 649.
(3) For inventive total syntheses of natural products employing a key
carbon(3a)ꢀnitrogen bond construction, see: (a) Newhouse, T.; Baran,
P. S. J. Am. Chem. Soc. 2008, 130, 10886. (b) Newhouse, T.; Lewis, C. A.;
Eastman, K. J.; Baran, P. S. J. Am. Chem. Soc. 2010, 132, 7119. (c) Espejo,
V. R.; Rainier, J. D. Org. Lett. 2010, 12, 2154. (d) Pꢀerez-Balado, C.; de
ꢀ
Lera, A. R. Org. Biomol. Chem. 2010, 8, 5179.
(4) (a) Eccles, R. G. Proc. Am. Pharm. Assoc. 1888, 84, 382. (b) Anet,
E. F. L. J.; Hughes, G. K.; Ritchie, E. Aust. J. Chem. 1961, 14, 173.
(c) Barrow, C. J.; Sedlock, D. M. J. Nat. Prod. 1994, 57, 1239. (d)
Takahashi, C.; Minoura, K.; Takeshi, T.; Numata, A.; Kushida, K.;
Shingu, T.; Hagishita, S.; Nakai, H.; Sato, T.; Harada, H. Tetrahedron
1995, 51, 3483. (e) Varoglu, M.; Corbett, T. H.; Valeriote, F. A.; Crews,
P. J. Org. Chem. 1997, 62, 7078.
(5) For other recent applications, see: (a) Pꢀerez-Balado, C.; de Lera,
A. R. Org. Lett. 2008, 10, 3701. (b) Pꢀerez-Balado, C.; Rodríguez-Gra~na,
P.; de Lera, A. R. Chem.—Eur. J. 2009, 15, 9928. (c) Iwasa, E.;
Hamashima, Y.; Fujishiro, S.; Higuchi, E.; Ito, A.; Yoshida, M.; Sodeoka,
M. J. Am. Chem. Soc. 2010, 132, 4078. (d) Foo, K.; Newhouse, T.; Mori,
I.; Takayama, H.; Baran, P. S. Angew. Chem., Int. Ed. 2011, 50, 2716.
(6) See Supporting Information for details.
(7) (a) Horner, L.; Naumann, W. Liebigs Ann. Chem. 1954, 587, 93.
(b) Nelsen, S. F.; Bartlett, P. D. J. Am. Chem. Soc. 1966, 88, 137.
(c) Nelsen, S. F.; Bartlett, P. D. J. Am. Chem. Soc. 1966, 88, 143.
(d) Timberlake, J. W.; Alender, J.; Garner, A. W.; Hodges, M. L.;
€
Ozmeral, C.; Szilagyi, S. J. Org. Chem. 1981, 46, 2082. (e) Hossain, M. T.;
Timberlake, J. W. J. Org. Chem. 2001, 66, 6282. For other pioneering
work in the area of diazene chemistry, see: (f) Porter, N. A.; Marnett, L. J.
J. Am. Chem. Soc. 1972, 95, 4361. (g) G€olitz, P.; de Meijere, A. Angew.
Chem., Int. Ed. 1977, 16, 854. (h) Porter, N. A.; Dubay, G. R.; Green, J. G.
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