An engineered linker capable of promoting on-resin reactions for microwave-assisted solid-phase organic synthesis

Article abstract of DOI:10.1002/anie.200602523

An active link: A diglycine-containing linker (cat·linker) was fabricated on Rink amide resin for dual functions: a) attachment of a scaffold and b) capture of metal ions for promoting on-resin reactions (see picture). The metal-catching feature of the li

Full text of DOI:10.1002/anie.200602523

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A Metal-Catching Linker
complexes has been advanced in recent years, to allow easy
recovery of resin-bound chiral catalysts from the reaction
solution (Figure 1B).[6] In such heterogeneous catalysis, the
substrate is not loaded onto the resin and conventional
purification is required for product separation. Despite the
fact that many metal-catalyzed reactions have been carried
out on resin-bound scaffolds,[1d] there is virtually no example
of a solid-phase reaction promoted by a metal species that is
covalently bound in close proximity to the scaffold. We report
herein an engineered linker (cat·linker) with the dual
functions of a normal linker for attachment of a scaffold
and a promoter for facilitating the reaction occurring on the
tethered scaffold (Figure 1C). It has been proven essential for
performing CuII-mediated heteroannulation on a cat·linker-
conjugated scaffold under microwave irradiation.[7]
In connection with our previous solid-phase synthesis of
an indole library[8] through PdII-[9] or CuII-catalyzed[10] intra-
molecular heteroannulation under controlled microwave
heating,[11] we designed the solid-phase synthesis of 2,5-
disubstituted indoles 4 by anchoring 2-bromo-4-nitroaniline
onto Rink amide resin through a diacid spacer (Scheme 1).
DOI: 10.1002/anie.200602523
An Engineered Linker Capable of Promoting On-
Resin Reactions for Microwave-Assisted Solid-
Phase Organic Synthesis**
Li-Ping Sun and Wei-Min Dai*
Solid-phase organic synthesis (SPOS)[1] has emerged as one of
the key tools in combinatorial chemistry[2] for generating
libraries of small organic molecules. In order to transfer the
versatile reaction types established in solution to solid-phase
synthesis, numerous classes of linkers[1–3] have been devel-
oped, including traceless and multifunctional linkers.[4] The
latter enable the generation of diversity in the end products
upon cleavage (R in Figure 1A). If a linker possesses
elements of chirality, reactions occurring on the tethered
scaffold are induced to form chiral molecules.
In a different approach from the resin-bound chiral
auxiliaries,[5] the immobilization of chiral ligands and metal
Figure 1. Schematic representations of A) solid-phase synthesis, B) so-
lution synthesis by using supported metal complexes, and C) solid-
phase synthesis with a novel linker capable of on-resin activation.
M=metal ion, cat·linker=a linker capable of catching metal ions and
promoting on-resin reactions.
Scheme 1. Initial attempts to form 1-acyl indoles 4 byusing diacid-
modified linkers. DIC=N,N-diisopropylcarbodiimide, DMF=N,N-
dimethylformamide, HOBt=N-hydroxybenzotriazole, MW=micro-
wave, NMP=N-methylpyrrolidinone, TFA=trifluoroacetic acid.
[*] Dr. L.-P. Sun, Prof. W.-M. Dai
Department of Chemistry
The Hong Kong Universityof Science and Technology
Clear Water Bay, Kowloon, Hong Kong SAR (P.R. China)
Fax: (+852)2358-1594
E-mail: chdai@ust.hk
Homepage: http://ihome.ust.hk/~chdai/index.html
This approach differed from our reported solid-phase syn-
thesis employing resin-bound 1-alkynes.[8] We envisaged that
the N-acyl chains in 4 could be easily removed during
postcleavage modification upon exposure to a base,[12] which
would result in an indirect traceless synthesis.[4]
We found that the diacid chain length influenced the solid-
phase reactions at different stages. First, the succinic acid
derived 1a failed[13] to couple with the Rink amide resin while
[**] This work was supported in part bythe Innovation and Technology
Fund (ITS/119/00) from the Innovation and TechnologyCommis-
sion and an Earmarked Research Grant (600904) from the Research
Grants Council, The Hong Kong Special Administrative Region, P.R.
China. Support from the Department of Chemistry, The Hong Kong
Universityof Science and Technology, is also acknowledged.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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1b and 1c, with their longer chains, could be successfully
Fmoc in 5–7 followed by coupling with the acid 1b furnished
8a–c in excellent overall yields.
With 8a–c in hand, we synthesized the resin derivatives
9a–c by a) Sonogashira cross-coupling, b) nitro reduction,
and c) sulfonamide formation (Scheme 3).[8] Upon exposure
loaded onto the resin. For example, cleavage of 2b supplied
the resin-free product in 90% yield. From 2b and 2c, the
resin-bound 2-alkynyl anilides 3b and 3c were prepared in
good purities and yields by following our established proce-
dures.[8] To our surprise, the CuII-mediated cyclization under
controlled microwave heating at 2008C[8] did not produce
indoles 4b and 4c, as confirmed by cleavage of the materials
from the resin. We reasoned that the failure in the cyclization
might result from an inferior contact of CuII with the resin-
bound alkynes.
In order to enhance the efficiency of solid-phase reactions
by running them in a more “solution-like” environment,
poly(ethylene glycol) (PEG) grafted polystyrene supports
and other resins have been introduced.[1a] We took a different
approach to improve the heterogeneous reaction profiles by
building a metal-capture unit onto the linker in close
proximity to the scaffold. Scheme 2 illustrates our design
and the fabrication of the polyglycine-derived engineered
linkers. We built the polyglycine chains on the Rink amide
resin (0.70 mmolgÀ1) through microwave-assisted polypep-
tide synthesis.[14] A 93% yield was estimated for 5 based on a
loading of 0.65 mmolgÀ1. By iterative peptide synthesis, the
glycine-modified resins 6 and 7 were prepared. Removal of
Scheme 3. Effect of glycine units on cat·linker performance.
of 9a–c to Cu(OAc)2 in NMP under controlled microwave
heating at 2008C for 10 min, the desired product 10b was
obtained in 90% yield after cleavage from the resin. By
contrast, compound 10a was hardly detected in the resin
cleavage mixture and 10c was a trace component of a
complex mixture. The remarkable effect of the built-in
glycine units on the heteroannulation might be explained by
a metal-catching and activation mechanism. That is, the
dipeptide moiety in 9a was not efficient for fishing CuII from
the solution onto the resin, while the coordination sites of CuII
were mostly occupied by the tetrapeptide in the case of 9c,
thereby rendering activation of the alkyne difficult.[15] As
depicted in the structure of 11, the tripeptide unit of 9b may
form a CuII complex through the amide carbonyl oxygen
donors[16] and the Cu ion also chelates with the neighboring
alkyne to promote the heteroannulation. It should be
emphasized that the formation of stable CuII complexes is
not necessary for activating the alkyne moiety toward
heteroannulation because excess CuII was used for the solid-
phase reaction. Instead, any factor, such as diffusion, which
improves transfer of CuII onto the resin is beneficial to the
heterogeneous reaction.
In order to gain support for the above assumption, we
carried out control experiments with the resin-bound alkynes
9d–f, prepared from 8a–c. First, 9d–f were subjected to
microwave heating in NMP at 2008C for 10 min in the absence
of Cu(OAc)2. After cleavage, the resin-free alkynes were
Scheme 2. Design and synthesis of three examples of the new system
cat·linker (polyglycine-derived segment+Rink amide linker).
Fmoc=(9-fluorenylmethyloxy)carbonyl.
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Chemie
essentially recovered, as confirmed by 1H NMR spectroscopy
(see S37–S39 in the Supporting Information). In a separate set
of experiments, the resin-bound alkynes 9d–f were sopped in
an NMP solution of Cu(OAc)2 at room temperature for 24 h,
thereby allowing CuII to diffuse onto the resin. After washing
and drying, the CuII-treated alkynes were subjected to the
same microwave heating in NMP at 2008C for 10 min without
additional Cu(OAc)2. Formation of indole 10e was clearly
demonstrated by 1H NMR analysis of the crude reaction
mixture. Indoles 10d and 10 f were also formed but to a much
lesser extent (see S35–S36 in the Supporting Information). On
the basis of these findings, we can conclude that: a) a
combination of microwave heating and CuII is essential for
the heteroannulation,[8,10] b) once CuII is sopped up onto the
resin, heteroannulation takes place upon heating, and c) the
diglycine-derived cat·linker in 8b is superior for promoting
on-resin heteroannulation, although the exact mode of Cu
intake may be subject to further discussion.
Table 1: Synthesis of a 16-member indole library.
14: R; Ar
Yield [%][a]
Purity[%] [b]
a: 4-MeC6H4; 4-FC6H4
b: 4-MeC6H4; 4-MeOC6H4
c: 4-MeC6H4; 4-iPrC6H4
d: 4-MeC6H4; 2-thienyl
e: 4-MeOC6H4; 4-FC6H4
f: 4-MeOC6H4; 4-MeOC6H4
g: 4-MeOC6H4; 4-iPrC6H4
h: 4-MeOC6H4; 2-thienyl
i: Ph; 4-FC6H4
j: Ph; 4-MeOC6H4
k: Ph; 4-iPrC6H4
l: Ph; 2-thienyl
m: nBu; 4-FC6H4
49
38
45
45
48
60
53
43
51
44
56
46
47
51
43
59
93[c]
98
97
93
90[c]
96
97
98
95[c]
89
97
98
96[c]
100
99
n: nBu; 4-MeOC6H4
o: nBu; 4-iPrC6H4
p: nBu; 2-thienyl
99
[a] Calculated based on the loading of 5 (0.65 mmolgÀ1). [b] Determined
byHPLC. The structures were characterized by 1H NMR spectroscopy
and MS. [c] The fluorine atom was replaced bypyrrolidine.
As an application of the cat·linker, we synthesized a 16-
member library of indoles 14 from the scaffold 8b by using 4
terminal alkynes and 4 arylsulfonyl chlorides. The results are
summarized in Scheme 4 and Table 1. The solid-phase syn-
indoles 14 are 38–60%, as calculated from the loading of 5
(0.65 mmolgÀ1). To our delight, the purities of 14 are excellent
(89–100%), as determined by LC–MS analysis (Table 1).[17]
In summary, we have developed an engineered linker with
dual functions for anchoring the scaffold onto a solid support
and for promoting a metal-mediated reaction at an appro-
priate stage of the solid-phase synthesis. Incorporation of the
CuII-capturing tripeptide segment into the so-called cat·linker
proves essential for the microwave-assisted indole synthesis
through heteroannulation. We consider that diffusion of CuII
from the solution onto the support may be a main path, but
distribution of CuII on the resin surface is influenced by the
linker structure. Only CuII species located close to the
attached scaffold and capable of complexation with the
alkyne can promote the heteroannulation. In addition to easy
on-resin fabrication, the cat·linker has no influence on the
Pd0–CuI-catalyzed Sonogashira cross-coupling reaction or the
SnCl2·H2O reduction of NO2. The cat·linker-modified poly-
styrene support retains the same resin profile and can be used,
as in the current study, for IRORI MicroKan reactor-based
Rf-encoded split-pool combinatorial synthesis.
Received: June 22, 2006
Published online: October 6, 2006
Scheme 4. Synthesis of a library of indoles 14 (see Table 1).
Keywords: heteroannulation · heterocycles · heterogeneous
.
catalysis · microwave-assisted reactions · solid-phase synthesis
thesis of 12 was carried out by using the IRORI radio
frequency (Rf)-encoded MicroKan reactors.[8] Then, an indi-
vidual library member 12 was transferred from the MicroKan
reactor along with the Rf tag to a 10-mL pressurized process
vial for the CuII-mediated heteroannulation, heated with a
technical microwave reactor. Attempts to directly release
indole 14 from the resin-bound product were not successful.
Therefore, 13 was cleaved from the support at the site of the
Rink amide linker and was converted into 14 by treatment
with a mixed pyrrolidine/THF solution at 608C for 12 h. The
peptide residue could be easily removed from the product by
filtration through a short silica gel plug. The overall yields of
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[17] The LC–MS charts can be found in the Supporting Information.
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