Solid-phase library synthesis of polyynes similar to natural products

Article abstract of DOI:10.1002/anie.200703208

(Figure Presented) High in polyunsaturates: The combinatorial synthesis of a 65-membered polyyne library has been achieved by using an iterative acetylene homologation strategy combined with a solid support (see scheme). The library was evaluated against

Full text of DOI:10.1002/anie.200703208

Communications
DOI: 10.1002/anie.200703208
Polyyne Library
Solid-Phase Library Synthesis of Polyynes Similar to Natural
Products**
Seonwoo Lee, Taeho Lee, Yun Mi Lee, Deukjoon Kim, and Sanghee Kim*
Naturally occurring polyynes are a structurally diverse class of
compounds that contain two or more conjugated triple bonds
(Scheme 1).[1] To date,more than a thousand polyyne com-
followed by a cross-coupling reaction with a trialkylsilylace-
tylene.
The importance of natural products for medicinal appli-
cations has meant that the demand for the generation of
libraries of molecules based on natural products has grown
significantly.[9] Given the attractive and versatile biological
activities of polyyne natural products,it would be interesting
to obtain a library of compounds containing the common
structural motif of natural polyynes. Such libraries might be
useful for discovering ligands or probes for either unknown or
discovered biological targets. To the best of our knowledge,
however,there has been no literature precedent related to the
systematic synthesis of a polyyne library. Therefore,we
focused our attention on the construction of a library of
natural-product-like polyynes by employing solid-phase com-
binatorial synthesis techniques and our iterative acetylene
homologation strategy.
One of the salient features of our iterative protocol in the
synthesis of a library of compounds is that it allows at least
three points of diversity,including the number of incorpo-
rated acetylene units. Furthermore,the reaction conditions of
our iterative process are mild enough to tolerate a variety of
functional groups and are suitable for application to solid-
phase synthesis. Herein,we report the first solid-phase
synthesis of a combinatorial library of natural-product-like
polyynes.
Scheme 1. Structures of representative polyyne natural products.
pounds have been isolated from various natural sources.[2]
This structurally unique class of compounds exhibit a variety
of biological effects,including antimicrobial,antitumor,and
antiangiogenic activities.
As a consequence of their biological significance and
interesting physicochemical properties,[1,3] natural polyynes
have been the subject of numerous synthetic studies,most of
which have been well-summarized in recent reviews.[2,4] Most
natural polyynes pose a substantial synthetic challenge
because of their highly unsaturated nature. The method
most commonly used for the preparation of unsymmetrical
diyne and polyyne natural products is the Cadiot–Chodkie-
wicz coupling reaction,[4,5] that is,the metal-catalyzed cross-
coupling of a 1-haloalkyne with a terminal alkyne. The major
limitation of this coupling reaction is that the vast majority of
terminal diynes and higher polyynes required as coupling
partners or precursors of 1-haloalkynes are often unstable.[3,6]
Many elegant alternative methods have been explored to
overcome this challenge.[3,7] Within this context,we previ-
ously developed an iterative strategy for the synthesis of
unsymmetrically substituted polyynes.[8] Our iterative strat-
egy entails a two-step acetylene homologation sequence,
namely,in situ desilylative bromination of alkynylsilanes
To test the feasibility of the process on a solid support[10]
we focused our initial studies on the preparation of the triyne
natural product (S)-(E)-15,16-dihydrominquartynoic acid (1,
Scheme 1),[11] which we had previously synthesized by using
the solution-phase iterative acetylene homologation strat-
egy.[8] We chose a 2-Cl-trityl resin because 1 could be cleaved
from this solid support under mild conditions with high
efficiency. Furthermore,the resin was compatible with the
reactions employed during the synthesis.
Our successful solid-phase synthesis of 1 is depicted in
Scheme 2. Thus,9-decynoic acid was loaded onto the
commercially available 2-Cl-trityl resin 2 (1.3 mmolgÀ1)
with high efficiency to give the resin-bound terminal alkyne
3 (loading level = 1.10 mmolgÀ1,85%). Conversion of the
terminal alkyne 3 into bromoalkyne 4 was achieved readily in
excellent yield (96%)[12] with NBS/AgNO3 in DMF. The
resulting resin-bound bromoalkyne 4 was anticipated to react
with triisopropylsilyl-protected acetylene similarly to the
[*]S. Lee, Dr. T. Lee, Y. M. Lee, Prof. D. Kim, Prof. S. Kim
College of Pharmacy
Seoul National University
San 56-1, Shilim, Kwanak, Seoul 151-742 (Korea)
Fax: (+82)2-762-8322
E-mail: pennkim@snu.ac.kr
solution-phase reaction under the modified Sonogashira
[**]This work was supported by the Korea Science and Engineering
Foundation (KOSEF) through the National Research Laboratory
Program funded by the Ministry of Science and Technology
(M10500000055-06J0000-05510).
[13]
conditions ([PdCl2(PPh3)2] CuI, iPr2NH,THF).
However,
the use of the solution-phase reaction conditions did not
afford the desired cross-coupling product 5 with sufficient
purity and yield. Unexpectedly,substantial amounts of the
homocoupled product[14]—the dimer of a 9-decynoic acid—
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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(Scheme 3). The resin-bound terminal alkyne 10A was
treated with NBS/AgNO3 to give bromoalkyne 11A. The
resulting bromoalkyne 11A was homologated by one acety-
lene unit through the above-mentioned two-step sequence to
afford bromodiyne 12A/B2. Repeating the two-step homo-
logation sequence generated the homologated bromotriyne
12A/B3.
Scheme 2. Solid-phase synthesis of (S)-(E)-15,16-dihydrominquartynoic
acid (1). a) 9-Decynoic acid (1.2 equiv), DIPEA (4.0 equiv), CH2Cl2, RT,
6 h; b) NBS (8.0 equiv), AgNO3 (0.2 equiv), DMF, RT, 0.5 h; c) TIPSA
(4.0 equiv), [PdCl2(PPh3)2](0.4 equiv), CuI (0.4 equiv), THF/ iPr2NH
(1:1), RT, 3 h; d) NBS (8.0 equiv), AgF (4.0 equiv), DMF, RT, 2 h; e) 8
(4.0 equiv), [Pd(PPh3)4](0.4 equiv), CuI (0.4 equiv), DMF, RT, 3 h;
f) 1% TFA/CH2Cl2, RT, 10 min, 23% (overall, from 3). DIPEA=N,N-
diisopropylethylamine, NBS=N-bromosuccimide, TIPSA=(triisopro-
pylsilyl)acetylene.
was also observed after cleavage with TFA. The pseudodilu-
tion effect of the polymer matrix was considered not to work
properly in this operation. We surmised that this outcome
could be the result of poor swelling of the resin. Therefore,the
resin loaded with bromoalkyne 4 was swelled with THF/
iPr2NH (1:1) before the addition of TIPS-protected acetylene
and coupling catalysts. This mixed solvent system resulted in a
better swelling of the resin and led to a high yield (87%) and
purity of the desired cross-coupling product 5. In this case,no
homocoupled by-product was detected.
Under our recently developed in situ AgF-mediated
desilylative bromination conditions (NBS/AgF in DMF),[15]
the resultant TIPS-diyne 5 was successfully converted into the
corresponding bromodiyne 6 in high yield (93%). At this
stage,a washing step with pyridine was needed to completely
release any impurities and metal salts out of the polymer
matrix. With the optimal reaction conditions in hand,we
performed the second iteration in the same manner as just
described. We encountered no difficulties when converting
the resin-bound bromodiyne 6 into bromotriyne 7 through
this two-step sequence (63% over 2 steps). The cross-
coupling reaction of bromotriyne 7 with vinylstannane 8,[16]
mediated by [Pd(PPh3)4] and CuI,successfully gave enetriyne
9. Finally,the final product was cleaved from the solid support
using 1% TFA in CH2Cl2 and then purified by column
chromatography on silica gel to give the desired 1 in 23%
overall yield (based on the resin-loading level of 3).
With the successful development of a solid-phase syn-
thesis of natural product 1,we sought to evaluate the
versatility of the sequence toward a combinatorial library
synthesis by introducing three points of diversity: the starting
terminal alkynes,the number of incorporated acetylene units,
and the final cross-coupling partners. Thus,five terminal
alkynes A (A1–A5) of two different types,which bear a
carboxylic acid or hydroxy group at the tail,were anchored to
resin 2 through an ester or ether linkage to give 10A
Scheme 3. Solid-phase combinatorial synthesis of a library of natural-
product-like polyynes. a) DIPEA (4.0 equiv), CH2Cl2, RT; b) NBS
(8.0 equiv), AgNO3 (0.2 equiv), DMF, RT; c) TIPSA (4.0 equiv), [PdCl2-
(PPh3)2](0.4 equiv), CuI (0.4 equiv), THF/ iPr2NH (1:1), RT; d) NBS
(8.0 equiv), AgF (4.0 equiv), DMF, RT; e) C1–C4 (4.0 equiv), [PdCl2-
(PPh3)2], (0.4 equiv), CuI (0.4 equiv), THF/iPr2NH (1:1), RT or C5
(4.0 equiv), [Pd(PPh3)4](0.4 equiv), CuI (0.4 equiv), DMF, 60 8C; f) 1%
TFA/CH2Cl2, RT, 10 min.
The obtained bromoalkyne 11A (identical to 12A/B1 in
Scheme 3),bromodiyne 12A/B2,and bromotriyne 12A/B3
were cross-coupled with five different coupling partners C
(C1–C5) in a combinatorial manner to give 13A/B/C. In this
coupling process,a uniform set of Sonogashira conditions was
employed throughout except for the case of vinylstannane C5,
for which the Stille coupling conditions was used. The
resulting resin-bound product 13A/B/C was cleaved from
the solid support using 1% TFA in CH2Cl2 to provide the
polyyne products A/B/C. In most cases,one major product
was detected by TLC and isolated by chromatography.
However,in the cases of tetraynes derived from the coupling
of bromotriyne 12A/B3 with C3 and C4,TLC showed the
presence of a complex mixture with no major product. This
result could be attributed to the markedly lower stability of
those tetraynes,which discouraged us from isolating the
compounds. Therefore,we were able to obtain 65 compounds
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Communications
Table 1: (Continued)
out of the planned 75 (87% success rate) with an average
purity higher than 95%. Most of the obtained polyynes were
stable under refrigerator conditions for at least two weeks,
while A5/B1/C4 and A5/B2/C4 decomposed rapidly at room
temperature under air.
Product
Yield [%][a] Purity [%][d] Product
Yield [%][a] Purity [%][d]
A1/B2/
C5
A1/B3/
C1
A1/B3/
C2
A1/B3/
C3
A1/B3/
C4
A1/B3/
C5
A2/B1/
C1
A2/B1/
C2
A2/B1/
C3
A2/B1/
C4
A2/B1/
C5
A2/B2/
C1
A2/B2/
C2
A2/B2/
C3
A2/B2/
C4
A2/B2/
C5
A2/B3/
C1
A2/B3/
C2
A2/B3/
C3
A2/B3/
C4
A2/B3/
C5
A3/B1/
C1
A3/B1/
C2
A3/B1/
C3
A3/B1/
C4
A3/B1/
C5
A3/B2/
C1
A3/B2/
C2
38
22
27
97
97
99
–
A4/B1/
C3
A4/B1/
C4
A4/B1/
C5
A4/B2/
C1
97
58
37
89
64
74
37
58
48
32
97
96
94
97
94
96
94
94
90
96
–
Table 1 shows the compounds obtained and their yields
1
and purities. H NMR spectroscopy and MS were used to
confirm the identity of each isolated compound.[17] The
overall yields ranged from 12% to 97%,based on the resin-
loading level of 10A. As expected,the overall yield decreased
as the number of incorporated acetylene units increased.
Other variables did not significantly affect the yield.
[b]
–
[b]
–
–
A4/B2/
C2
23
76
68
53
65
54
56
47
50
53
43
21
27
94
98
99
97
98
90
97
98
96
98
98
95
99
–
A4/B2/
C3
A4/B2/
C4
A4/B2/
C5
A4/B3/
C1
A4/B3/
C2
A4/B3/
C3
A4/B3/
C4
A4/B3/
C5
A5/B1/
C1
A5/B1/
C2
A5/B1/
C3
A5/B1/
C4
A5/B1/
C5
A5/B2/
C1
A5/B2/
C2
Among the various biological properties,cytotoxic activ-
ity is one of the most intriguing properties of natural polyynes.
Therefore,as a demonstration of its usefulness and a
preliminary evaluation of the polyyne library,members of
this obtained library were individually evaluated[18] for
cytotoxic activity against various cancer cells by using the
sulforhodamine B method.[19] It was found that the cytotoxic
potency was highly dependent on the type of cells and the
chemical composition of the polyynes. As a representative
example,the cytotoxic activities of the library members
against HCT116 human colon cancer cells are shown in
Figure 1,where the cells were treated with 10 mm of each
compound. The observed data revealed important structural
requirements of the polyynes for cytotoxicity.[20] In general,a
higher number of acetylene units elicited a higher cytotox-
icity. Building blocks A4, A5,and C1 caused enhanced
cytotoxic potency. The most active members of the library had
IC50 values in the micromolar range,with the most potent
compound being A5/B3/C1 (IC50 1.2 mm).[21] This biological
study implies that our polyyne libraries can be used as
biochemical tools or as the source of new lead compounds for
future development.
[b]
–
[b]
–
–
13
92
79
79
58[c]
71
82
68
68
37[c]
57
19
27
85
97
98
98
96
90
98
96
80
87
93
96
96
–
[b]
–
In conclusion,we have developed a facile solid-phase
synthetic pathway to generate a library of natural-product-
like polyynes that might be difficult to obtain by other means.
Sixty-five polyynes out of the planned seventy-five could be
[b]
–
–
25
77
69
67
67
60
55
47
58
95
99
97
98
94
98
96
99
96
A5/B2/
C3
A5/B2/
C4
A5/B2/
C5
A5/B3/
C1
A5/B3/
C2
A5/B3/
C3
A5/B3/
C4
A5/B3/
C5
Table 1: A combinatorial library of natural-product-like polyynes.
Product
Yield [%][a] Purity [%][d] Product
Yield [%][a] Purity [%][d]
A1/B1/
C1
A1/B1/
C2
A1/B1/
C3
A1/B1/
C4
A1/B1/
C5
A1/B2/
C1
A1/B2/
C2
A1/B2/
C3
77
70
65
63
67
55
43
54
53
96
98
94
94
94
98
94
90
98
A3/B2/
C4
A3/B2/
C5
A3/B3/
C1
A3/B3/
C2
A3/B3/
C3
A3/B3/
C4
A3/B3/
C5
A4/B1/
C1
50
40
14
12
98
98
96
98
–
[b]
–
[b]
–
–
27
87
[b]
–
A3/B2/
C3
[b]
–
–
17
96
86
95
98
98
[a]Yield of isolated product after column chromatography (calculated
from the loading of 10 A). [b]Product was not obtained. [c]The isolated
compound was unstable to air at room temperature. [d]After isolation,
the purity was determined immediately by reverse-phase HPLC by
measuring the peak area, with monitoring by UV at 220 nm (gradient,
20–100% MeOH/H2O with 0.1% TFA, 30 or 60 min).
A1/B2/
C4
A4/B1/
C2
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2001, 3,2883 – 2886; b) A. L. K. Shi Shun,E. T. Chernick,S.
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[7] For selected examples,see a) E. MØtay,Q. Hu,E.-i. Negishi,
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[8] S. Kim,S. Kim,T. Lee,H. Ko,D. Kim, Org. Lett. 2004, 6,3601 –
3604.
[9] For some more recent reviews on this topic,see a) D. G. Hall,S.
Manku,F. Wang, J. Comb. Chem. 2001, 3,125 – 150; b) P. Arya,
R. Joseph,D. T. H. Chou, Chem. Biol. 2002, 9,145 – 156;
c) A. M. Boldi, Combinatorial Synthesis of Natural Product-
Based Libraries,CRC Press,Taylor & Francis Group,Boca
Raton,FL. 2006.
[10] For a previous example of the successful synthesis of unsym-
metrical diynes on a solid support,see J. M. Montierth,D. R.
Figure 1. Cytotoxic activities of library members against HCT116
human colon cancer cells. The assay was carried out in triplicate at
10 mm.
DeMario,M. J. Kurth,N. E. Schore,
11741 – 11748.
Tetrahedron 1998, 54,
[11] A. Ito,B. Cui,D. Chꢀvez,H.-B. Chai,Y. G. Shin,K. Kawanishi,
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J. M. Pezzuto,A. D. Kinghorn, J. Nat. Prod. 2001, 64,246 – 248.
[12] The yields in Scheme 2 were calculated after cleavage from the
support with 1% TFA/CH2Cl2 followed by column chromatog-
raphy.
[13] a) K. Sonogashira, J. Organomet. Chem. 2002, 653,46 – 49; b) K.
Sonogashira in Handbook of Organopalladium Chemistry for
Organic Synthesis, Vol. 1 (Ed: E.-i. Negishi),Wiley,New York,
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obtained using the iterative acetylene homologation strategy.
The library was evaluated against cancer cells to demonstrate
its possible utility as a chemical tool to unravel cellular
processes or as a platform for drug discovery and design. The
development of this solid-phase method considerably reduced
the time required for the construction of the polyyne library,
and thus will allow for the preparation of a more diversified
and expanded library.
[14] For a previous example of the formation of a homocoupled
product in the Sonogashira reaction on a solid support,see Z.
Yang,Y. Liao,R. Fathi,M. Reitman,Y. Zhang, Tetrahedron Lett.
2001, 42,1815 – 1818.
Received: July 18,2007
Published online: September 25,2007
[15] T. Lee,H. R. Kang,S. Kim,S. Kim, Tetrahedron 2006, 62,4081 –
4085.
Keywords: combinatorial chemistry · cross-coupling ·
natural products · polyynes · solid-phase synthesis
.
[16] T. Lee,S. Kim, Tetrahedron: Asymmetry 2003, 14,1951 – 1954.
[17] The randomly selected library members (A3/B1/C1, A2/B3/C2,
and A1/B3/C5) as well as one series of polyyne compounds (A2/
B1–B3/C1) were further characterized by 13C NMR spectrosco-
py to ensure the identity and purity (see the Supporting
Information).
[1] J. Lam,H. Breteler,T. Arnason,L. Hansen, Chemistry Biology
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[18] A5/B1/C4 and A5/B2/C4 were excluded because of their
instability.
[19] For the protocol of the SRB method,see S. K. Lee,K. Nam,Y.
Heo, Planta Med. 2003, 69,21 – 25.
[20] For a previous study that explored the structural basis of
polyynes for neurotoxicity,see K. Uwai,K. Ohashi,Y. Takaya,T.
Ohta,T. Tadano,K. Kisara,K. Shibusawa,R. Sakakibara,Y.
Oshima, J. Med. Chem. 2000, 43,4508 – 4515.
[21] The exact mechanism of the cytotoxicity of the active members is
under investigation. For this study and in vivo testing,ca. 200 mg
of the most active members, A5/B3/C1 and A2/B3/C1,were
individually prepared without difficulty by the solid-phase
method described.
[2] A. L. K. Shi Shun,R. R. Tykwinski, Angew. Chem. 2006, 118,
1050 – 1073; Angew. Chem. Int. Ed. 2006, 45,1034 – 1057.
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[5] P. Cadiot,W. Chodkiewicz in Chemistry of Acetylenes (Ed.:
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[6] For recent papers that include a discussion of this topic,see
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Angew. Chem. Int. Ed. 2007, 46, 8422 –8425
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