Peptide Chemistry
PAOLO GRIECO
The use of microwave
irradiation in peptide
chemistry
The introduction of high-throughput biological
screening and the accelerated discovery of new
biological targets has increased the demand on
synthetic chemists to produce new compounds for
testing. One response to this demand has been the
development of new techniques to greatly increase the
speed and efficiency of compound synthesis. In the case
of peptides combinatorial libraries containing large
numbers of individual components have afforded
high-affinity ligands and potent inhibitors to a variety of
targets. Recently there has been growing interest in
applying microwave heating for many chemical
applications requiring energy input1-3.
Although microwave techniques have been used in
organic chemistry since 1984, publications reporting
this nonconventional heating method have had an
exceptional increasing especially in latest 10 years.
In this paper, we will review the most interesting
examples of microwave-promoted reactions in peptides
and peptidomimetics synthesis.
reagents5. The
studies were
conducted in a
Smith
Synthesizer
(Figure 1) with a
single mode
microwave
cavity producing
continuous
irradiation with
monitoring of
temperature,
pressure and
Figure 1. Smith Microwave Synthesizer
irradiation
(Personal Chemistry)
power, making
the procedure
highly reproducible. However, this
synthesizer has been designed
specially for microwave-organic
synthesis (MOS).
Recently, microwave application in
SOLID-PHASE PEPTIDE SYNTHESIS UNDER
solid-phase peptide synthesis has
MICROWAVE IRRADIATION
been performed in a new automated
and dedicated synthesizer, Odyssey
The first example of microwave application in peptide
System (CEM) (Figure 2) that allows to
synthesis has been reported in 1992 by Yu et al.4. In
carry on the entire solid-phase
this paper a significant improvement of the coupling
peptide synthesis process including the
efficiency (2-4-fold), especially in side-chain-hindered
deprotection, coupling, and cleavage
amino acids, in solid-phase peptide synthesis, was
reaction6,7. In an optimized study the
described. The authors synthesized three fragments of
complete cycle, as well as final
acyl carrier protein (Acp) by stepwise coupling using
peptide cleavage, is performed in ten
pre-formed active ester in DMF and microwave
minutes.
irradiation. In each step the peptide bound formation
The system used a single mode cavity
Figure 2. Automated
Microwave Peptide
was completed within 4 min. Using microwave
and the coupling efficiency was
Synthesizer, Odyssey
irradiation no detectable racemization was observed
investigated using PyBOP and HBTU.
(CEM)
and a consistent reduced reaction time was obtained.
By this dedicate system has been
First studies on peptide synthesis has been performed
possible to synthesize a variety of
employed a domestic microwave oven that did not allow
peptide sequences8. Again, it was demonstrated that
accurate temperature measurements and, therefore, it
microwave irradiation can help in peptide synthesis to
may prove difficult to reproduce the results obtained.
drive difficult reactions to completion much faster
However, today this inconvenience have been overcame
comparing to conventional methodology9. Microwave
by availability of dedicated synthesizer suitable for
application has been also applied in difficult coupling of
peptide and organic chemistry.
_-aminobutyric acid (Aib) in the synthesis of several
An improvement on peptide synthesis has been reported
dipeptides using PyBOP/HOBt and HBTU/HOBt as
in 2002 where the authors performed a rapid
coupling reagents. The study demonstrated that this
solid-phase peptide synthesis by microwave-assisted
approach allows to obtain the desired compounds in
using sterically hindered Fmoc-amino acids. By this study
higher yields and in shorter reaction time when
optimized condition has been found to obtain di- and
compared with to conventional heating10.
tripeptides rapidly (1.5-20 min) and without
Microwave irradiation has proved to be useful in
racemization, in presence of a variety of coupling
increasing the efficiency of deprotection step of amino
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chimica oggi • Chemistry Today • July/August 2004
SYNTHESIS OF PEPTIDOMIMETICS
MICROWAVE-ASSISTED
Peptide Chemistry
group protected as Cbz and Bn.
Although these protecting groups have
today limited application, the methods
proposed are particularly suitable for the
synthesis of short peptides and can also
be carried out on appropriate molecular
supports11.
Figure 4. Macrocyclization under microwave irradiation
Microwave-irradiation methodology has
having particular attention in peptidomimetic chemistry.
It is known that peptidomimetic compounds are immune
to proteases and other modifying enzymes and the
synthesis of oligomeric combinatorial libraries of
peptidomimetics is usually more straightforward than
the creation of large libraries of molecules.
In particular the peptoids developed by Zuckerman, are
oligo(N-alkyl) glycines that differ from peptides in that
the side chain is connected to the amide nitrogen rather
than the R carbon atom (Figure 3). Peptoids have
resulted to
be
interesting
as ligands
of important
Figure 5. CombChem Microwave
target12.
Synthesizer (Mileston)
Recently a
small library
of these
These kinds of macrocyclic in
interesting
which the cystine bridge has
Figure 3. Native peptide (A) and
building
been replaced, have been the
Peptoid (B)
blocks have
subject of considerable study.
been
These novel macrocycles have
prepared on Rink MBHA amide resin using microwave
often improved pharmacokinetic
irradiation. To perform this library several amines were
and conformational properties.
used to construct various 9-residue peptoids that included
Peptide synthesis was
homo-oligomers and a hetero-oligomer (Scheme 1)13.
performed on ACT 348 1
synthesizer and
after the capping
with fluorobenzoic
acid the
macrocyclization
step was performed
on in a Milestone
Ethos CombChem
microwave
synthesizer
(Figure 5)14.
Macrocyclization
was performed at
Scheme 1. Synthesis of Peptoids by microwave irradiation
450 W, 50 °C, in
DMF for 10 min. These
In this study the microwave experiments were performed
conditions were suitable to
in a commercial microwave oven and the tests showed
provide more than 70%
that a reaction time of 30-40 s for each acylation and
conversion of all linear peptides
amine displacement step was sufficient to produce
in cyclic peptide without
9-residue peptoids with good yields and purities. The
decomposition of desired
yields and purities of the peptoids obtained using the
compounds.
microwave-accelerated protocol were comparable with
In our search of new Urotensinthose obtained at 37 °C but in a significant reduced time.
II analogues we have reported
For example, a 9-residue peptoid can be made in 3 h
a high-speed, one-pot protocol
Scheme 2. Urotensin-II analogues
synthesized under microwave
using the microwave with the protocol described by
for the generation of cyclic
irradiation
authors, compared to 20-32 h of the standard protocol.
peptides with a thiosulphide
Tandem combination of solid-phase peptide synthesis and
bridge from readily available
microwave-assisted reaction has been used to perform
building blocks precursors15. Our methodology involves
macrocyclization reactions to prepare peptidomimetics of
the use of iodide derivatives of serine for the synthesis of
the type showed in Figure 4.
cyclic peptide and subsequently to carry on the
chimica oggi • Chemistry Today • July/August 2004
19
Peptide Chemistry
thioalkylation reaction by microwave irradiation at 450
W, 50 °C, in DMF and DIEA for 10 min. Final one-step
cyclization was achieved after removing of the Fmoc and
OFm protecting groups and using HBTU/HOBt as
coupling reagent (Scheme 2). The final products were
isolated with good level of purity following cleavage from
the resin using a mixture of TFA/TES/H2O (95: 2.5: 2.5).
To demonstrate the real advantage of associating the
microwave-assisted reaction with the solid-phase
synthesis in peptide thioalkylation step, we compared the
results obtained with to conventional thermal heating.
Results demonstrated the undoubted advantages in yield
and reaction time with microwave heating, being the
reaction time reduced to 10 min from 8-12 h.
Several others examples of syntheses in which microwave
irradiation combined with peptide chemistry afford to
peptidomimetics as important building blocks are
reported in literature. Thus, the preparation of Fmocprotected isocyanates which can be used directly or via
carbamates to prepare urea peptides is an interesting
and useful application. These building blocks can be
prepared easily starting from powdered Fmoc-amino
acid azides, upon microwave irradiation, in almost
quantitative yields without any side product (Scheme 3).
Finally, as latest example to take in consideration, the
assistance of microwave irradiation has been also
applied in the synthesis of new 3,6-disubstituted-1,4diazepan-2,5-diones that can employed as new scaffolds
for combinatorial chemistry and as conformational
constrains for short peptides (Scheme 4).
The examples here reported confirms that microwave
irradiation combined with the peptide synthesis or solidphase peptide chemistry represents a powerful technique
for accelerating the synthesis of peptides and
peptidomimetics in a combinatorial chemistry context.
ACKNOWLEDGMENTS
I apologize to the many authors whose work was not
cited, in an attempt to limit the reference list. However,
several cited reviews contain more exhaustive coverage
of specific areas of research on microwave-assisted in
organic synthesis.
REFERENCES
1.
2.
3.
4.
5.
6.
7.
8.
Scheme 3. Synthesis of Fmoc-protected isocyanates
9.
10.
11.
12.
13.
14.
15.
16.
Scheme 4. Synthesis of 3,6-disubstituted-1,4-diazepam-2,5-diones
17.
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PAOLO GRIECO
Department of Pharmaceutical and Toxicological
Chemistry, University of Naples “Federico II”
Via D. Montesano, 49 - 80131 Naples, ITALY
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