One pot synthesis of selenocysteine containing peptoid libraries by Ugi multicomponent reactions in water

Article abstract of DOI:10.1039/b514597j

Selenocysteine containing peptoids and peptide-peptoid conjugates were synthesized by combinatorial Ugi-MCRs (multicomponent reactions) in water: for the first time, an acetal (selenoacetal 2a) was used in Ugi-MCR to furnish selenocysteine peptoids in one

Full text of DOI:10.1039/b514597j

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One pot synthesis of selenocysteine containing peptoid libraries by Ugi
multicomponent reactions in water{
Muhammad Abbas, John Bethke and Ludger A. Wessjohann*
Received (in Cambridge, UK) 14th October 2005, Accepted 22nd November 2005
First published as an Advance Article on the web 15th December 2005
DOI: 10.1039/b514597j
method. For the first time, we apply the Ugi four component
reaction (Ugi-4CR) to the synthesis of model selenopeptoids.
These selenopeptoids are supposed to have a similar short-range
environment and show similar properties as the selenoprotein
portion but are easy to synthesize and to study.
Selenocysteine containing peptoids and peptide–peptoid con-
jugates were synthesized by combinatorial Ugi-MCRs (multi-
component reactions) in water: for the first time, an acetal
(selenoacetal 2a) was used in Ugi-MCR to furnish selenocys-
teine peptoids in one step as model compounds for selenocys-
teine peptides and proteins.
In all studies, a strategy was followed in which the seleno moiety
is embedded in the carbonyl building unit, which has to be one of
the four components in the Ugi-4CR, and is the only one to place
the methylselenol side chain in the a-position of the dipeptoid
formed. Since unprotected 2-selenoacetaldehyde is not available
and the corresponding diselenide is unreactive or misbehaved in
Ugi reactions, other protected forms had to be used.
Diselenodiacetal 1 was synthesized from KSeCN and 1,1-
diethoxy-2-bromoacetate in DMF (Scheme 1).11 Reductive
alkylation of diselenide 1 gave the selenylacetals 2a and 2b which
can be used directly in case of aqueous reaction medium. For the
Ugi reaction in organic solvents the acetals 2a and 2b were
deprotected under acidic conditions to obtain the selenylaldehydes
3a and 3b, respectively (Scheme 1).12 The selenoaldehyde 3c is
more conveniently prepared from vinyl acetate and PhSeCl
(Scheme 1).13
Selenium has been shown to be a nutritionally essential trace
element for mammals, including humans.1 It is an integral part of
thioredoxin-reductases2 and glutathione peroxidases (GPx), anti-
oxidant enzymes, as well as of several other selenoproteins.3,4 Until
now, more than 30 selenoproteins and selenopeptides are identified
but the function of several is still unknown. To examine the
differences of cysteine (Cys, C) versus selenocysteine (Sec, U) in
proteins, efforts have been made to replace cysteine with
selenocysteine.5,6 Unfortunately, the conditions necessary for
selenocysteine incorporation by the cell’s protein assembly
mechanism are very specific. As a result, the preparation of non-
natural selenocysteine containing peptides and proteins by
molecular biology methods is difficult, although some recent
successful approaches have been documented.7 Complex proteins
provide limited access for certain selective measurements (e.g.
redox potential)8 which need to be correlated to small changes in
the selenocysteine vicinity. In proteins, other residues, buried Sec-
sites, and folding can complicate direct measurements or data
interpretation. Small Sec–peptide fragments can provide clearer
answers, but often suffer from lengthy syntheses, or are not readily
available as libraries from larger-scale preparations. There are only
very few methods available for the synthesis of natural selenium
compounds, especially of selenocysteine due to its lability towards
oxidation.5,9 This leads to quite limited chemical studies on
biologically relevant organoselenium compounds. Most available
data are exclusively based upon aromatic selenides (i.e. phenyl-
selenenyl derivatives) in organic solvents, which is of limited
relevance under physiological conditions.10 Thus, syntheses of
aliphatic selenols are rare and always designed for only one target
compound (e.g. selenocysteine).9
Scheme 1 Synthesis of 2-selenoacetaldehyde building blocks.
In contrast to such singular approaches, in this communication
a combinatorial one is reported, which produces a variety of
selenocysteine analogs through a broadly applicable and fast
Leibniz Institute of Plant Biochemistry, Department of Bioorganic
Chemistry, Weinberg 3, D-06120 Halle (Saale), Germany.
E-mail: wessjohann@ipb-halle.de; Fax: +49 345 5582 1309;
Tel: +49 345 5582 1301
{ Electronic supplementary information (ESI) available: Procedure for the
synthesis of selenocysteine peptoids under water and microwave condi-
tions and spectral data of some selected products. See DOI: 10.1039/
b514597j
Scheme 2 One pot combinatorial synthesis of selenocysteine peptoids by
Ugi-4-component reaction of 2-selenoacetal building blocks.
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Chem. Commun., 2006, 541–543 | 541

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Compounds 2a, 3b, and 3c were used as model carbonyl
Table 1 Selected Se-Ugi-4CRs under microwave or aqueous
conditions
building blocks for the Ugi-4CR (Scheme 2). The other
components (i.e. amines, acids, and isonitriles) are commercially
available, except isonitrile 6c which was prepared from the
corresponding formamide.14
Isolated
Yield (%)
Aldehyde Amine Acid
R1 (4) R2 (5) R3
Isocyanide
(6) R4
of Ugi
Solvent product (7)
In preliminary experiments, we used aldehydes 3a–c under
classical Ugi-4CR conditions (stirring the four components in
methanol or CHCl3 at room temperature). The yields of the
products were very low. However, under conventional and
especially microwave heating, the products could be obtained in
up to 61% yield (Table 1). Because the results were still
unsatisfactory, we carried out the Ugi-4CR in different solvents.
Interestingly, we found that in water the reaction worked smoothly
at room temperature and gives better yields (Table 1, 7b–f).15
Excess water can have a negative impact on the Ugi reaction
because of a less favorable equilibrium for the initial Schiff-base
formation, and thus the competing Passerini reaction of unreacted
aldehyde may dominate. This was not observed here, instead the
7a Me (3a) Ph
Me (2a)
7b Me (3a) Ph
Me (2a)
Me
Me
c-C6H12
t-Bu
CHCl3 61a
H2O
CHCl3 58a
H2O
82b
CHCl3 4a
85b
7c Bn (3b)
Bn (3b)
7d Ph (3c)
Ph (3c)
7e Ph (3c)
Ph (3c)
Ph
Ph
Ph
N-Boc–Gly c-C6H12
H2O
44b
Me t-Bu
CHCl3 43a
H2O
N-Boc–Gly CH2CO2Et CHCl3 27a
H2O
65b
DMBd N-Boc–Gly CH2CO2Et CHCl3 6a
H2O
35c
87b
7f Ph (3c)
Ph (3c)
a
b
c
Microwave, 160 uC, CHCl3. Water, rt. Water, Yb2(SO4)3.
DMB = 2,4-dimethoxy benzyl.
d
Scheme 3 A selection of starting materials (acids, amines and isonitriles) and protected selenopeptoid products with isolated yields of purified products.
Ph = phenyl, PMP = 4-methoxy phenyl, MNP = 3-nitro phenyl, Tr = trityl.
542 | Chem. Commun., 2006, 541–543
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added. The reaction mixture is stirred overnight. 10 ml Ethylacetate is
added to dissolve the gummy product. The water layer is washed with
ethylacetate three times, combined, dried over Na2SO4 and concentrated to
gummy product which is purified by chromatography on silica gel, usually
with petrol ether : ethyl acetate (ca. 3 : 1).
formation of solid reaction products, which separate from the
aqueous solution is likely to drive peptoid product formation.{
Also, under these conditions protected aldehyde 2a can be used
directly in the Ugi-4CR. This is an important advantage, as a
selenofunctionality in an unprotected aldehyde is detrimental for
Ugi reactivity. The yields obtained with aniline as the amine in
water, varying the acid and the isonitrile components (65–87%)
were superior to the ones obtained earlier. In the case of 2,4-
dimethoxybenzyl (DMB) amine, however, no reaction could be
observed. In order to activate the intermediate imines, different
Lewis acids were tested of which Yb2(SO4)3 6 8H2O (10 mol%)
was the most suitable one for this purpose.
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Next, the suitability of the Ugi-4CR protocol for selenocysteine
incorporation into peptides was tested, using several amino acid
residues as part of the different components of this four-
component four-centre reaction. As can be seen from the results
in Table 1 and Scheme 3, the yields of Ugi products are highly
dependent on the amine used. In the case of anilines, the yields are
satisfactory, whereas in the case of electron rich DMB-amine the
yields are quite low (7f, 7j, 7n and 7r). The reactivity of the amine
unit plays a key role in the selenocysteine analog syntheses. Amine
units from an amino acid mostly remain inactive under the
reaction conditions used and usually give Passerini products (7t).
However, the isonitrile derived from glycine ethyl ester (6b),
usually problematic because of side reactions,16 worked well in
Ugi-4CR under aqueous conditions (in Ugi products 7e and 7f).
PMP-amine (in Ugi products 7g, 7i, 7k, 7o and 7p), DMB-amine
(in Ugi products 7f, 7j, 7n and 7r), and trityl amine (in Ugi product
7s) can be used in the selenopeptoid synthesis reactions as they can
be removed under acidic (Tr)17 or oxidative conditions (PMP18
and DMB19) to give selenopeptides. Acetic acid (7a, 7b, 7d and 7o)
and N-Boc–Gly–OH (in Ugi products 7c, 7e, 7f, 7h–n, and 7q)
gave the best results. However N-Boc–(S–tBu)–Cys–OH (in Ugi
product 7g, 7r and 7s) gave moderate yields of cysteine–
selenocysteine (–Cys-Sec–) dipeptoids. Cys-Sec-derivatives (in Ugi
products 7g, 7r and 7s) can lead to the formation of a selenenyl
sulfide (–S–Se–) bridge which is a crucial intermediate in the
catalytic cycle of some selenoproteins.2,20 With a tripeptide
(N-Boc–Gly-Gly-Gly–OH) as acid building block, the tetrapeptoid
7p is obtained in reasonable yield.
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In summary, we have developed a very straightforward and
short synthesis of selenocysteine and/or selenomethionine peptoids
in aqueous medium. These selenocysteine peptoids will be further
used for electrochemical and physiological studies.
We gratefully acknowledge financial support from Deutsche
Forschungsgemeinschaft as part of the Selenoprotein Schwerpunkt
DFG-SPP 1087 (We-1467/4-1), and thank Prof. Dr. B.
Westermann and Dr. W. Brandt for valuable suggestions and
discussion.
Notes and references
{ Procedure for selenopeptoid synthesis in aqueous medium: To selenylalde-
hyde 3b–c (1.3 mmol) or acetal 2a (2 mmol) in 5 ml of degassed water,
amine (1.3 mmol) is added at room temp. At this point a non-reactive acid
catalyst [e.g. Yb2(SO4)3-hydrate] can be added. The mixture is stirred for
20 min. Then 1.3 mmol of isonitrile followed by 1.0 mmol of acid are
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Chem. Commun., 2006, 541–543 | 543