Covalent double level dynamic combinatorial libraries: Selectively addressable exchange processes

Article abstract of DOI:10.1039/b808565j

Hydrazones and disulfides have been combined in one dynamic system: hydrazones were exchanged by acid catalysis in the presence of disulfide and a thiol group without interference; neutralization of the reaction medium turns off the exchange of hydrazones and, at the same time, activates thiolate-disulfide exchange. The Royal Society of Chemistry.

Full text of DOI:10.1039/b808565j

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COMMUNICATION
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Covalent double level dynamic combinatorial libraries: selectively
addressable exchange processesw
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A. Gaston Orrillo, Andrea M. Escalante and Ricardo L. E. Furlan*
Received (in Cambridge, UK) 20th May 2008, Accepted 16th July 2008
First published as an Advance Article on the web 30th September 2008
DOI: 10.1039/b808565j
Hydrazones and disulfides have been combined in one dynamic
system: hydrazones were exchanged by acid catalysis in the
presence of disulfide and a thiol group without interference;
neutralization of the reaction medium turns off the exchange of
hydrazones and, at the same time, activates thiolate–disulfide
exchange.
Dynamic combinatorial libraries (DCLs)1 are mixtures of
compounds that interconvert as part of an equilibrium that
can be shifted by molecular recognition of a specific template
molecule ‘‘ideally’’ toward the library member that is best
Scheme 1 Mono- and bifunctionalized building blocks used to
evaluate the exchange processes.
bound. These libraries have become a useful source of new
receptors, ligands for biomacromolecules, and catalysts.2
A key issue in designing dynamic combinatorial libraries is
the choice of a reversible chemical reaction to interconvert the
library components.3 Ideally, a rapid reversible reaction is
required that proceeds under mild conditions and that is
tolerant of a wide range of functional groups and recognition
events. Furthermore, it should be possible to turn the reaction
off in order to isolate and handle selected members of the
library individually. Within the various reactions tested to date
by different research groups, two of the most frequently
used are thiol/disulfide exchange and hydrazide/hydrazone
exchange.
of covalently assembled double level dynamic combinatorial
libraries.
It has been reported that the exchange of hydrazones in
chlorinated solvents can be achieved by acid catalysis with
trifluoroacetic acid (TFA)6 whereas disulfide exchange
proceeds smoothly in the presence of organic bases like
triethylamine (TEA).7,8
In order to evaluate the compatibility of both exchange
processes several building blocks were prepared incorporating
either one disulfide bond and one protected aldehyde group
(1), one hydrazide group (2 and 3), or one thiol group (4)
(Scheme 1).
The combination of different reversible reactions in one
dynamic system can enhance the level of diversity achieved.
Two types of multilevel dynamic libraries have been described
to date: ‘‘orthogonal’’, when the reversible processes operate
independently, and ‘‘communicating’’, when the processes
cross over. Eliseev, Lehn et al. have reported the use of
double-level ‘‘orthogonal’’ libraries where hydrazone
exchange and ligand exchange around a cobalt ion can be
addressed independently.4 Otto, Sanders et al. reported that
structural diversity within a DCL can be expanded by main-
taining two exchange processes simultaneously generating a
double-level ‘‘communicating’’ library based on exchange of
disulfide and thioester linkages.5 In this work, we report the
first example of the combination of two selectively addressable
reversible reactions that can be alternated for the preparation
Initially we studied the effect of increasing amounts of TEA
on the exchange of hydrazones in chloroform solutions
acidified with 15 equivalents of TFA with respect to the
concentration of hydrazones. The general procedure for the
exchange experiments entailed dissolution of the building
blocks 1 and 2 (5 mM) in CHCl3 containing TFA (75 mM).
The reaction was stirred at room temperature for 24 h. TEA
(an appropriate amount) followed by one equivalent of build-
ing block 3 were added and the reaction was kept stirring at
room temperature. The exchange of hydrazide 3 and hydra-
zone 5 can be detected by HPLCz after 24 h of reaction only if
there is an excess of TFA (Fig. 1). When the amount of TEA
equals or surpasses the amount of TFA, formation of the
hydrazone 6 is not detected by HPLC.
The effect of increasing amounts of TEA on the exchange of
disulfide 5 and thiol 4 was also studied with a similar experi-
mental setup. The general procedure involved dissolution of
the building blocks 1 and 2 (5 mM) in CHCl3 containing TFA
(75 mM). The reaction was stirred at room temperature for
24 h. TEA (an appropriate amount) followed by one equiva-
lent of building block 4 were added and the reaction was kept
stirring at room temperature. In this case the formation of
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Farmacognosia, Facultad de Ciencias Bioquımicas y Farmaceuticas,
Universidad Nacional de Rosario, Rosario, Argentina.
E-mail: rfurlan@fbioyf.unr.edu.ar; Fax: +54 (0)3414-375315;
Tel: +54 (0)3414-375315
w Electronic supplementary information (ESI) available: Materials
and experimental procedures; LC data for preparation of libraries
starting with disulfide exchange followed by hydrazone exchange and
vice versa. See DOI: 10.1039/b808565j
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Fig. 1 Proportion of 5 (’) and 6 (K) after 24 h of reaction in
chloroform in the presence of 15 equivalents of TFA and different
amounts of TEA.
Fig. 3 HPLC traces recorded at l = 290 nm for the mixture prepared
from 5, 3 and 4 (a) in the presence of 15 equivalents of TFA and 5
equivalents of TEA, and (b) in the presence of 15 equivalents of TFA
and 20 equivalents of TEA.
products resulting from the exchange is observed after 24 h
either when TEA is in excess or when the amount of TEA
equals the amount of TFA (Fig. 2).
temperature for 24 h. TEA (an appropriate amount) followed
by one equivalent of building blocks 3 and 4 were added and
the reaction was kept stirring at room temperature. As
expected, in the presence of an excess of 10 or more equiva-
lents of TFA only products of hydrazone exchange are
observed after 24 h of reaction (Fig. 3a), whereas in the
presence of excess of TEA, only the products of disulfide
exchange are observed (Fig. 3b).
The results suggest that each of these exchange processes
can be activated whereas the other is inactive by changing the
relative amounts of TFA and TEA. This was proved by adding
hydrazide 3 and thiol 4 to a series of chloroform solutions of 5
containing 15 equivalents of TFA plus different amounts of
TEA. The general procedure involved dissolution of the
building blocks 1 and 2 (5 mM) in CHCl3 containing TFA
(75 mM, 15 equivalents). The reaction was stirred at room
When the ratio TFA : TEA is in the range 10 : 15 to 15 : 15,
both exchange processes are activated, however they are so slow
that the amount of library members that are products of both
exchange processes cannot be detected by HPLC after 24 h of
reaction. Four days after the reaction was started they still
represent less than 1.5% of the library. Consequently, in
practice the exchange processes are not completely independent
since the conditions that are necessary to produce hydrazones at
a convenient rate (high proportion of TFA) affect the thiol/
disulfide exchange, whereas under the conditions at which
disulfide exchange is fast enough, hydrazones do not exchange.
These results show that two selectively addressable dynamic
chemistries can be combined for the preparation of fully
covalent double level dynamic combinatorial libraries from
building blocks appropriately functionalized to participate in
both exchange processes. This process can be carried out in
both directions: disulfide exchange followed by hydrazone
exchange and vice versa (see Fig. S1 in ESIw).
This work was supported by CONICET, ANPCYT,
Universidad Nacional de Rosario. G.O. thanks CONICET
for his fellowship. A.E. and R.F. are CONICET Researchers.
We thank Dr Daniel Wunderlin from the Universidad
Nacional de Cordoba for his help with the LC-MS analysis.
Fig. 2 Proportion of 5 (K), and proportion of the sum of products of
disulfide exchange 7–9 containing hydrazones (’) after 24 h of
reaction in chloroform in the presence of 15 equivalents of TFA and
different amounts of TEA. Compound 4 and its dimer are not detected
by the analytical system.
Notes and references
z HPLC analysis was carried out using a Hewlett-Packard 1050
instrument, coupled to a HP 1050 DAD; data were analysed using
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HP ChemStation. Reverse phase HPLC separations were carried out
using a 25 cm, 4.6 mm i.d. 5 mm particle size, symmetry column using
acetonitrile and water gradients.
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This journal is The Royal Society of Chemistry 2008
5300 | Chem. Commun., 2008, 5298–5300