Substituted guanidines: Introducing diversity in combinatorial chemistry

Article abstract of DOI:10.1021/ol006322p

(matrix presented) The guanidine moiety is an important motif present in many biologically active compounds. Fully substituted guanidines are of key importance for the development of bioactive molecules. The present paper reports on an efficient procedure for the direct solid-phase conversion of amines to fully substituted guanidines under very mild conditions.

Full text of DOI:10.1021/ol006322p

ORGANIC
LETTERS
2000
Vol. 2, No. 23
3539-3542
Substituted Guanidines: Introducing
Diversity in Combinatorial Chemistry
Montserrat del Fresno,† Ayman El-Faham,‡,§ Louis A. Carpino,§
,†,|
,†
Miriam Royo,* and Fernando Albericio*
Departament de Qu´ımica Orga`nica, UniVersitat de Barcelona,
08028 Barcelona, Spain, Department of Chemistry, Faculty of Science,
UniVersity of Alexandria, 2132 Alexandria, Egypt, and Department of Chemistry,
UniVersity of Massachusetts, Amherst, Massachusetts 01003
albericio@qo.ub.es
Received July 11, 2000
ABSTRACT
The guanidine moiety is an important motif present in many biologically active compounds. Fully substituted guanidines are of key importance
for the development of bioactive molecules. The present paper reports on an efficient procedure for the direct solid-phase conversion of
amines to fully substituted guanidines under very mild conditions.
Guanidine functions are an important motif often present in
natural products as well as in many compounds having
therapeutic activity.1 This moiety is fully protonated under
physiological conditions due to its strongly basic character,
a fact which is crucial for specific ligand-receptor interac-
tions.2
Consequently, procedures that allow for the preparation
of guanidine-derived products in high yield under mild
conditions are of great interest in medicinal chemistry.
Although fully substituted guanidines are not common,
their presence can facilitate binding to complex receptors
and therefore can be of key importance for the development
of bioactive molecules.2c,3 Many methods have been de-
scribed for the preparation of functionalized guanidines either
in solution4 or by solid-phase methods,1f-h,2c,3c,5 but they do
not allow the formation of pentasubstituted guanidines.
The present paper reports on a new and convenient method
for the solid-phase preparation of pentasubstituted guanidines
which involves the use of aminium/uronium salt-based
reagents. These compounds have been used mainly as
† Universitat de Barcelona.
‡ University of Alexandria.
§ University of Massachusetts.
| E-mail: miriam@qo.ub.es.
(3) (a) Frederic, M.; Scherman, D.; Byk, G. Tetrahedron Lett. 2000, 41,
675-679. (b) Le, V.-D.; Wong, C.-H. J. Org. Chem. 2000, 65, 2399-
2409. (c) Wilson, L. J.; Klopfenstein, S. R.; Li, M. Tetrahedron Lett. 1999,
40, 3999-4002. (d) Ostresh, J. M.; Schoner, C. C.; Hamashin, V. T.; Nefzi,
A.; Meyer, J.-P.; Houghten, R. A. J. Org. Chem. 1998, 63, 8622-8623.
(e) Drewry, D. H.; Gerritz, C. W.; Linn, J. A. Tetrahedron Lett. 1997, 38,
3377-3380.
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Ogbu, C. Synlett. 1999, 2, 193-194. (c) Ko, S. Y.; Lerpiniere, J.; Christofi,
A. M. Synlett 1995, 815-816. (d) Kim, K. S.; Qian, L. Tetrahedron Lett.
1993, 34, 7677-7680. (e) Kim, K. S.; Qian, L. Tetrahedron Lett. 1993,
34, 7677-7680. (f) Poss, M. A.; Iwanowicz, E.; Reid, J. A.; Lin, J.; Gu, Z.
Tetrahedron Lett. 1992, 33, 5933-5936.
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57-67. (b) McAlpine, I. J.; Armstrong, R. W. Tetrahedron Lett. 2000, 41,
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2663-2666. (h) Lin, P.; Ganesan, A. Tetrahedron Lett. 1998, 39, 9789-
9792. (i) Dodd, D. S.; Kozikowski, A. P. Tetrahedron Lett. 1994, 35, 977-
980.
(2) (a) Linton, B. R.; Carr, A. J.; Orner, B. P.; Hamilton, A. D.; J. Org.
Chem. 2000, 65, 1566-1568. (b) Linton, B.; Hamilton, A. D.; Tetrahedron
1999, 55, 6027-6038. (c) Schneider, S. E.; Bishop, P. A.; Salazar, M. A.;
Bishop, O. A.; Anslyn, E. V. Tetrahedron 1998, 54, 15063-15086 (d)
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10.1021/ol006322p CCC: $19.00 © 2000 American Chemical Society
Published on Web 10/21/2000

coupling reagents in peptide synthesis by activating the
carboxyl group of the amino acid.6 However, during the
much slower activation of hindered amino acids, protected
peptide segments, or carboxylic acids involved in cyclization,
the aminium/uronium salt may undergo reaction with the
amino component to give the corresponding guanylated
derivative.7,8
Scheme 2. Synthesis of the Guanylated Peptides
We have taken advantage of this side reaction and used it
for the synthesis of pentasubstitued guanidines. By treatment
of an amine function with one of these aminium/uronium
salts under basic conditions, the guanidine derivatives are
obtained in high yields and purities.
The synthesis of the various guanylating agents bearing
different substituents (Rn) can be performed by modification
of standard methods previously described for the correspond-
ing HBTU and HATU analogues (Scheme 1).9-11 This
Scheme 1. General Procedure for the Synthesis of Guanylating
Agents
TOF-MS. With 5 equiv of the reagent (entry 3), a slightly
higher purity (96%) was obtained but increasing the reaction
time from 0.5 to 2 h (entries 3 vs 4) gave no change in the
yield or the HPLC profile. Because of the known instability
of some of these reagents under basic conditions, an
experiment in which fresh reagent was added after 1 h of
reaction time was also performed, but no major changes were
detected (entries 2 and 5).
From these preliminary experiments, it is clear that the
reaction occurs with very high efficiency, since a very high
yield of pure product is obtained with only 1.2 equiv of
reagent. Second, the series of guanylating agents shown in
Figure 1was used to test the versatility of the reaction. This
series includes reagents with different substituents in the
dialkyl amino function (Rn) and different leaving groups (Y).
The purity yields for this series of reactions are shown in
Table 2. Formation of the guanylated derivative proceeded
in almost quantitative yields in all cases, and no remarkable
method allows the preparation of a broad range of reagents.
As a model target, the tripeptide Boc-Lys(Fmoc)-Phe-Ala-
amide-linker-resin was chosen.12 After removal of the Fmoc
group with piperidine, the ꢀ-amino function of lysine was
converted to its guanylated form (Scheme 2). The guanylation
reaction was first performed under various reaction conditions
with commercial HATU in order to establish the efficiency
of these compounds as guanylating agents (Table 1).
When treated with 1.2 equiv of the reagent for 2 h (entry
1, Table 1), a 92% purity was achieved with a very clean
HPLC profile and the correct mass as shown by MALDI-
Table 1. Study of the Number of Equivalents of HATU and
the Reaction Time for the Guanylation of
Boc-Lys-Phe-Leu-AM-MBHA
entry reagents (equiv) solvent
time, h
purity of 4 (%)a
1
2
3
4
5
6
HATU (1.2)
DIEA (2.4)
HATU (1.2 + 1.2) DMF
DIEA (2.4)
HATU (5)
DMF
2
92
(7) (a) Albericio, F.; Bofill, J. M.; El-Faham, A.; Kates, S. A. J. Org.
Chem. 1998, 63, 9678-9683. (b) Story, S. C.; Aldrich, J. V. Int. J. Pept.
Protein Res. 1994, 43, 292-296. (c) Gausepohl, H.; Pieles, U.; Frank, R.
W. In Peptides-Chemistry and Biology: Proceedings of the 12th American
Peptide Symposium; Smith, J. A., Rivier, J. E., Eds.; ESCOM, Science,
Leiden, 1992; 523-524.
(8) Chloroformamidinium salts were used by Barton et al. (Barton, D.
H. R.; Elliot, J. D.; Gero, S. D. J. Chem. Soc., Perkin Trans. 1 1982, 2085-
2090) for the preparation of sterically hindered guanidine bases in a strategy
similar to that the described in this work. The use of the aminium/uronium
salts derived from either HOBt or HOAt has the advantage of the great
shelf stability of these reagents, which allows them to be stored for long
periods of time.
1 + 1b
0.5
94
96
96
96
96
DMF
DMF
DMF
DMF
DIEA (10)
HATU (5)
2
DIEA (10)
HATU (5 + 5)
DIEA (10)
HATU (5 + 5)
DIEA (10)
1 +1b
overnight
(9) (a) El-Faham, A. Bull. Fac. Sci. Alex. UniV. 1996, 36, 73-80. (b)
Knorr, R.; Trzeciak, A.; Bannwarth, W.; Gillessen, D. Tetrahedron Lett.
1989, 30, 1927-1930. (c) Dourtoglou, V.; Gross, B.; Lambropoulou, V.;
Zioudrou, C. Synthesis 1984, 572-574 (d) Dourtoglou, V.; Ziegler, J.-C.;
Gross, B. Tetrahedron Lett. 1978, 1269-1272.
a As determined from integration of the HPLC chromatogram. b The resin
was first treated with the guanylating reagent and DIEA and allowed to
react for 1 h, and then fresh guanylating reagent was added.
(10) Among these reagents HATU is commercially available.
3540
Org. Lett., Vol. 2, No. 23, 2000

Table 2. Study of Guanylation by Reagents Other Than
HATUa
reagents
entry
1
(equiv)
solvent
DMF
purity of 4 (%)b
HAPyU (5)
DIEA (10)
TOPPipUc(5)
DIEA (10)
HAMDU (5)
DIEA (10)
HBMDU (5)
DIEA (10)
M2PA (5)
DIEA (10)
M2PB (5)
DIEA (10)
E2PA (5)
DIEA (10)
HATPU (5)
DIEA (10)
HBTPU (5)
DIEA (10)
E4A (5)
97
2
3
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
91
92
91
99.9
90
93
93
96
97
94
4
5
6
7
8
9
10
11
DIEA (10)
E4B (5)
DIEA (10)
a All reactions were carried out overnight by adding the solvent and the
base to the resin and adding the guanylating agent as a solid. b As determined
from integration of the HPLC chromatogram. All products were charaterized
by MALDI-TOF-MS. c This reagent is known to be less reactive than others;
therefore the reaction was performed by adding HOAt as an additive.
difference is observed when changing the leaving group (Y).
Most efficient appeared to be the reagent M2PA (entry 5,
Table 2), but the generality of this effect for the dimethyl-
amino/pyrrolidinyl reagent will require further study.
(11) Abbreviations used for amino acids and the designations of peptides
follow the rules of the IUPAC-IUB Commission of Biochemical Nomen-
clature in J. Biol. Chem. 1972, 247, 977-983. The following additional
abbreviations are used: Boc, tert-butyloxycarbonyl; DIEA, N,N′-diisopro-
pylethylamine; DIPCDI, N,N′-diisopropylcarbodiimide; DMF, N,N-di-
methylformamide; E4A, O-(7-azabenzotriazol-1-yl)-1,1,3,3-tetraethyluro-
nium hexafluorophosphate; E4B, O-(benzotriazol-1-yl)-1,1,3,3-tetraethyl-
uronium hexafluorophosphate; E2PA, O-(7-azabenzotriazol-1-yl)-1,1-diethyl-
3,3-dimethyleneuronium hexafluorophosphate; Fmoc, 9-fluorenylmethoxy-
carbonyl; HAMDU, O-(7-azabenzotriazol-1-yl)-1,3-dimethyl-1,3-dimeth-
yleneuronium hexafluorophosphate; HAPyU, 1-(1-pyrrolidinyl-1H-1,2,3-
triazolo[4,5-b]pyridin-1-ylmethylene)pyrrolidinium hexafluorophosphate N-
oxide; HATU, N-[(dimethylamino)-1H-1,2,3-triazolo[4,5-b]pyridino-1-
ylmethylene]-N-methylmethanaminium hexafluorophosphate N-oxide; HATPU,
O-(7-azabenzotriazol-1-yl)-1,1,3-trimethyl-3-phenyluronium hexafluoro-
phosphate; HBMDU, O-(benzotriazol-1-yl)-1,3-dimethyl-1,3-dimethylene-
uronium hexafluorophosphate; HBTPU, O-(benzotriazol-1-yl)-1,1,3-tri-
methyl-3-phenyluronium hexafluorophosphate; HOBt, 1-hydroxybenzo-
triazole; HPLC, high performance liquid chromatography; Linker AM,
p-[R,S]-R-[1-(9H-fluoren-9-yl)methoxyformamido]-2,4-dimethoxybenzyl]-
phenoxyacetic acid; MALDI-TOF MS, matrix-assisted laser desorption
ionization/time-of-flight mass spectrometry; TFA, trifluoroacetic acid; M2-
PA,O-(7-azabenzotriazol-1-yl)-1,1-dimethyl-3,3-dimethyleneuroniumhexafluoro-
phosphate; M2PB, O-(benzotriazol-1-yl)-1,1-dimethyl-3,3-dimethyleneuro-
nium hexafluorophosphate; TOPPipU, 2-[2-oxo-1(2H)-pyridyl]-1,1,3,3-bis-
(pentamethylene)uronium tetrafluoroborate. Amino acid symbols denote
L-configuration unless indicated otherwise.
(12) The first two amino acids were introduced using a standard Fmoc/
tBu protocol. Fmoc-amide-linker-resin was washed with DMF (5 × 0.5
min), and Fmoc removal was accomplished with piperidine-DMF [(1: 4),
1 × 1 min, 2 × 5 min], followed by washes with DMF (5 × 0.5 min).
Figure 1. Structures of aminium/uronium salt-based reagents.
Org. Lett., Vol. 2, No. 23, 2000
3541

In summary, a new synthetic method for the preparation
of pentasubstituted guanidines is presented. The reaction
consists of an attack of an amino function on the guanylating
agent. The reaction occurs under mild conditions and without
producing any significant side products.
As part of our solid-phase combinatorial library synthesis
program, we are currently applying this methodology to the
preparation of libraries of solid phases based on 2,5-
diketopiperazine and hydantoins as scaffolds.
Acknowledgment. This work was partially supported by
the Universitat de Barcelona (MdelF), Ministerio de Edu-
cacio´n y Ciencia (M.R), DGICYT (PB96-1490), and Gen-
eralitat de Catalunya. Reagent syntheses carried out in
Amherst were supported by the National Institutes of Health
(GM-09706) and the National Science Foundation (NSF-
CHE-9707651).
Fmoc-amino acids (5 equiv) were activated with DIPCDI (5 equiv) in the
presence of HOBt (5 equiv) in DMF, and the coupling was carried out for
2 h at 25 °C. In all cases, the Kaiser test (Kaiser, E.; Colescott, R. L.;
Bosinger, C. D.; Cook, P. Anal. Biochem. 1970, 34, 595-598) was negative.
The last amino acid was introduced in its NR-Boc, Nꢀ-Fmoc form following
the same procedure. After removal of the Fmoc group of Lys, DIEA (2
equiv with respect to the guanylating agent) was added in DMF and then
the aminium/uronium salt. The reaction was also followed by the Kaiser
test and after the reaction the test was negative. After all reactions, the
resin was washed with DMF (5 × 0.5 min) and DCM (5 × 0.5 min). Finally,
treatment of the resin with TFA-H2O (19:1) caused simultaneous elimina-
tion of the Boc group and cleavage of the peptide from the resin.
OL006322P
3542
Org. Lett., Vol. 2, No. 23, 2000