Parallel supported synthesis of polyamine-imidazole conjugates

Article abstract of DOI:10.1021/ol0480630

(Chemical Equation Presented) A small collection of nine polyamine-imidazole conjugates, potentially acting as RNases A mimics, has been synthesized on SynPhase lanterns using amino alcohols and diamines as building blocks. Couplings were performed via S<

Full text of DOI:10.1021/ol0480630

ORGANIC
LETTERS
2004
Vol. 6, No. 25
4711-4714
Parallel Supported Synthesis of
Polyamine Imidazole Conjugates
−
Sylvie Picard, Myriam Le Roch, Jacques Renault, and Philippe Uriac*
UniVersite´ Rennes 1, Institut de Chimie de Rennes, Synthe`se et Extraction de
Mole´cules a` Vise´e The´rapeutique, Faculte´ des Sciences Biologiques et Me´dicales,
2 aVenue du professeur Le´on Bernard, 35043 Rennes Cedex, France
philippe.uriac@uniV-rennes1.fr
Received September 23, 2004
ABSTRACT
A small collection of nine polyamine−imidazole conjugates, potentially acting as RNases A mimics, has been synthesized on SynPhase lanterns
using amino alcohols and diamines as building blocks. Couplings were performed via SN2 alkylation of methanesulfonates with amines. The
final introduction of N-4-nitrobenzyloxycarbonyldiamines allowed easy purification of the cleaved compounds.
The synthesis of molecules capable of cleaving RNA
nonrandomly has found a variety of important applications
in molecular biology1,2 including as probes for investigation
of nucleic acid structure in solution or in the development
of new antiviral therapeutics3 since RNA is the genetic
material of many pathogenic viruses.
Several approaches have been reported ranging from
antisense strategies4 to small molecules mimicking the active
center of ribonuclease A (RNase A).2,5-8 In a previous paper,9
we described the solution-phase synthesis of some polyamine-
imidazole conjugates acting as artificial RNases A. One of
these compounds exhibited a good selectivity (scission of
the RNA target at a unique location). However, the strategy
employed for the design of these conjugates resulted in
compounds that lacked diversity since it was limited to the
synthesis of symmetrical polyamine skeletons.
Thus, we decided to develop a parallel stepwise polyamine
synthetic pathway on SynPhase lanterns. The two main
methods of sequential synthesis of polyamine derivatives on
solid supports listed in the literature are based on reduc-
tion10,11 and alkylation12-16 methods. The latter seemed to
us to be the more suitable for generating the widest diversity,
especially using a repetitive reaction of nucleophilic dis-
placement of mesyl groups by an amine function. We
previously demonstrated the efficiency of this strategy,
developed on SynPhase lanterns, and which led to a small
combinatorial library of triamines functionalized on an
external nitrogen, starting from commercially available amino
alcohols and amines.15 In this way and on the basis of the
preceding results,9 we sought to synthesize polyamines of
different lengths with an imidazole cleaving group branched
(1) Garcia, A.; Giege´, R.; Behr, J. P. Nucl. Acids Res. 1990, 18, 89-95.
(2) Giege´, R.; Felden, B.; Zenkova, M. A.; Sil’nikov, V. N.; Vlassov,
V. V. Methods Enzymol. 2000, 318, 147-165.
(3) Phylactou, L. A. AdV. Drug DeliV. ReV. 2000, 44, 97-108.
(4) Kurreck, J. Eur. J. Biochem. 2003, 270, 1628-1644.
(5) Trawick, B. N.; Osiek, T. A.; Bashkin, J. K. Bioconjugate Chem.
2001, 12, 900-905.
(10) Manku, S.; Wang, F.; Hall, D. G. J. Comb. Chem. 2003, 5, 379-
391.
(6) Shinosuka, K.; Nakashima, Y.; Shimizu, K.; Sawai, H. Nucleosides
Nucleotides Nucleic Acids 2001, 20, 117-130.
(11) Jo¨nsson, D. Tetrahedron Lett. 2002, 43, 4793-4796.
(12) Kan, T.; Kobayashi, H.; Fukuyama, T. Synlett 2002, 8, 1338-1340.
(13) Manov, N.; Bienz, S. Tetrahedron, 2001, 57, 7893-7898.
(14) Hone, N. D.; Payne, L. J. Tetrahedron Lett. 2000, 41, 6149-6152.
(15) Renault, J.; Lebranchu, M.; Lecat, A.; Uriac, P. Tetrahedron Lett.
2001, 42, 6655-6658.
(7) Reynolds, M. A.; Beck, T. A.; Say, P. B.; Schwartz, D. A.; Dwyer,
B. P.; Daily, W. J.; Vaghefi, M. M.; Metzler, M. D.; Klem, R. E.; Arnold,
L. J. A., Jr. Nucl. Acids Res. 1996, 24, 760-765.
(8) Verbeure, B.; Lacey, C. J.; Froeyen, M.; Rozenki, J.; Herdewijn, P.
Bioconjugate Chem. 2002, 13, 333-350.
(9) Fouace, S.; Gaudin, C.; Picard, S.; Corvaisier, S.; Renault, J.; Carboni,
B.; Felden, B. Nucl. Acids Res. 2004, 32, 151-157.
(16) Olsen, C. A.; Witt, M.; Jaroszewski, J. W.; Franzyk, H. Org. Lett.
2003, 5, 4183-4185.
10.1021/ol0480630 CCC: $27.50
© 2004 American Chemical Society
Published on Web 11/09/2004

on an internal nitrogen. To obtain such compounds several
strategies are possible (Scheme 1).
We envisaged synthesizing a small pilot combinatorial
library of nine individualized polyamine-imidazole conju-
gates starting from commercial amino alcohols (2-amino-
ethanol, 3-aminopropan-1-ol, 4-aminobutan-1-ol), function-
alized 3-aminopropan-1-ol 2, and N-4-nitrobenzyloxycarbon-
yl diamines 3-5 (1,2-diaminoethane, 1,3-diaminopropane,
1,4-diaminobutane) (Figure 1) which were prepared accord-
ing to the literature.19,20
Scheme 1. Building Blocks and Strategies Leading to
Tetramine-Imidazole Compounds Functionalized on a Central
Nitrogen
Figure 1. Building blocks employed for the construction of the
small library.
Compound 2 was synthesized by the sequence shown in
Scheme 2. 3-[1-(Triphenylmethyl)-1H-imidazol-4-yl]propan-
Scheme 2. Preparation of Building Block 2
1-ol 1 was prepared from urocanic acid.21 Compound 1 was
then converted into the corresponding methanesulfonate
derivative. This latter was, without any further purification,
allowed to react with an excess of 1,3-diaminopropane to
afford 2 in 60% overall yield.
In strategy A, the imidazole cleaving group is linked to
the amine function of an amino alcohol (building block b).
The other building blocks are the commercial amino alcohols
a and the diamines c. For strategy B, the imidazole cleaving
group is introduced on the polyamine chain through the free
amine function of a monoprotected diamine d. The two other
building blocks are the commercial amino alcohols a.
We selected strategy A, which specifically afforded the
opportunity to introduce the functionality through building
block b that was easily obtained by the presence of two
distinct chemical functions.
Biological evaluation on RNA requires high compound
purities (g95%), and after a few preliminary attempts, we
were led to suggest a process beginning with solid-supported
steps (and the “split-pool” technique), followed by some
solution-phase steps, to purify each sample easily. Consider-
ing the extreme hydrophilicity of polyamine derivatives and
in order to facilitate purifications and to check reactions by
thin-layer chromatography, we decided to link the 4-ni-
trobenzyloxycarbonyl protecting group17 onto the diamine
c. This protecting group is stable under most aqueous acid
conditions and is easily removed by hydrogenolysis.18
Once in possession of building blocks (Figure 1), the solid-
supported steps of our strategyson polystyrene Hydroxy-
MethylPhenoxy (HMP) D-series SynPhase lanternsscould
be undertaken (Scheme 3). To facilitate the library synthesis,
a color encoding22 directed split-pool technique was em-
ployed. Sixty HMP lanterns were divided into three groups
of 20 lanterns, and each group was attached to colored stems
(one color for each commercial amino alcohol, three colors
in total). Each of the stem-attached lanterns was loaded with
a colored cog (one color for each monoprotected diamine,
three colors in total). For the first step, treatment of 60 HMP
lanterns with 4-nitrophenyl chloroformate,23 under conditions
previously established in our laboratory,24 gave 60 polymer-
bound carbonates 6.
(18) Dinh, T. Q.; Du, X.; Armstrong, R. W. J. Org. Chem. 1996, 61,
6606-6616.
(19) Huang, L.; Quada, J. C., Jr.; Lown, J. W. Bioconjugate Chem. 1995,
6, 21-33.
(20) Atwell, G. J.; Denny, W. A. Synthesis 1984, 12, 1032-1033.
(21) Stark, H.; Purand, K.; Hu¨ls, A.; Ligneau, X.; Garbarg, M.; Schwartz,
J.-C.; Schunack, W. J. Med. Chem. 1996, 39, 1220-1226.
(22) Wu, Z.; Ede, N. J. Org. Lett. 2003, 5, 2935-2938.
(23) Dressman, B. A.; Spangle, L. A.; Kaldor, S. W. Tetrahedron Lett.
1996, 37, 937-940.
(17) Shields, J. E.; McGregor, W. H.; Carpenter, F. H. J. Org. Chem.
1961, 26, 1491-1494.
(24) Tomasi, S.; Le Roch, M.; Renault, J.; Corbel, J. C.; Uriac, P. Pharm.
Pharmacol. Commun. 2000, 6, 155-159.
4712
Org. Lett., Vol. 6, No. 25, 2004

cleophilic agent at 50 °C for 6 h in anhydrous DMSO)
appeared to be inappropriate for our building block 2, despite
its nucleophilic secondary amine and probably because of a
steric obstruction. So, we had to increase temperature
(70 °C) and reaction time (32 h). Once again, one lantern of
each color was removed for cleaving for 1H NMR. Accept-
able yields (ranging from 45 to 62%) and purities (65-69%)
were obtained and are listed in the Supporting Information.
However, we noted that the shorter the mesylate analogues
of lantern-bound amino alcohols, the less the substitution
with building block 2 was effected (lower yield), probably
because of an unwanted cleavage of the carbamate bond
during the substitution step.
Scheme 3. Strategy A. Supported Steps
The lower yield was investigated with some additional
experiments (Scheme 4). During experiment a, one type 8b
Scheme 4. Formation of Compounds 16 and 17
lantern (bearing 3-aminopropan-1-ol converted into mesylate)
was submitted to conditions of substitution, but without any
nucleophilic agent: after treatment with TFA (releasing from
the lantern and removal of the mesyl group), compound 16
was obtained with a high purity (98%) but with a very low
yield (12%), confirming a loss of compound before TFA
cleavage. To analyze the product released by unexpected
cleavage in the reaction medium, experiment b was under-
taken: a type 8b lantern was then introduced in the same
conditions as during experiment a, but in dimethyl-d6
For the anchorage step, the polymer-bound carbonates 6
were converted into more stable polymer-bound carbam-
ates: the lanterns 6 with the same color stems were joined
together to react with one of the three commercial amino
alcohols (three reactions in total) under conditions described
in a previous paper.24 After this reaction, all lanterns 7a-c
were combined for washing and drying (repeated after each
step). At this level, one lantern of each batch was treated
individually with a solution of trifluoroacetic acid (TFA) in
CH2Cl2 containing 4-nitrophenol (35 µmol) as an internal
1
sulfoxide (DMSO-d6): compound 17 was identified by H
NMR and 13C NMR, and we assume it must be formed
through a rearrangement26 described in Scheme 4. The
isolation of such cyclic urethanes resulting from the thermal
instability of mesylate analogues of amino alcohols (prin-
cipally 2-aminoethanol and 3-aminopropan-1-ol) with amine
functions converted in carbamates has already been reported
in the literature.27,28 Fifty-four lanterns 9a-c were combined
1
standard. The H NMR spectrum of each cleaved product
was performed, and the integration ratio of the desired
compound to the internal standard allowed an evaluation of
the high yield (93-97%) and the high purity (>98%) of the
step. The 57 supports 7a-c were pooled for the mesylation
step15 to give lanterns 8a-c. To facilitate manipulations and
to reduce quantities of building block 2 to be introduced by
nucleophilic substitution,25 57 lanterns 8a-c were then
divided into three groups of 19. The conditions previously
established in our laboratory15 (1 M concentration of nu-
(25) Virgilio, A. A.; Schu¨rer, S. C.; Ellman, J. A. Tetrahedron Lett. 1996,
37, 6961-6964.
(26) Kempf, D. J.; Sowin, T. J.; Doherty, E. M.; Hannick, S. J.; Codavoci,
L.; Henry, R. F.; Green, B. E.; Spanton, S. G.; Norbeck, D. W. J. Org.
Chem. 1992, 57, 5692-5700.
Org. Lett., Vol. 6, No. 25, 2004
4713

for the second step of mesylation to give lanterns 10a-c.
For the next combinatorial step, the 54 lanterns 10 were
divided into three batches of 18 supports (each one consisted
of lanterns with the same color cogs): each batch was reacted
with one of the three monoprotected diamines (building
blocks 3, 4, 5; three batches and three reactions in total).
Each lantern-bound functionalized tetramine could be simply
identified by the color of the attached stems (for commercial
amino alcohols) and cogs (for monoprotected diamines).
Upon combined TFA cleavage of six identical lanterns
11, nine different tetramines 12aa-cc bearing an imidazole
cleaving group and protected with 4-nitrobenzyloxycarbonyl
were obtained.
products 13aa-cc by hydrogenation over Pearlman’s cata-
lyst18 generated the derivatives 14aa-cc which reacted with
hydrochloric acid to give the nine final functionalized
tetramines 15aa-cc. The whole library was analyzed by
1
HPLC and HRMS and fully characterized by H NMR and
13C NMR (for selected samples an indepth structural study
has been carried out using two-dimensional NMR; see the
Supporting Information). These analyses confirmed the
expected compounds with a good overall average yield
(18%), except for compounds 15ac and 15ba and a high
average purity (95%) (values of yields and purities are
reported in the Supporting Information). An interesting point
to note is that the best yields were obtained with compounds
built with four carbon chains (shorter polymer-bound car-
bamates 8 seemed likely to undergo unexpected cleavage,
and shorter N-4-nitrobenzyloxycarbonyl diamines could be
less reactive).
These tetramines could undergo a solution-phase treatment
(and particularly purifications) (Scheme 5). Each trifluoro-
In conclusion, an efficient protocol of parallel synthesis
of polyamine derivatives was developed and involved the
formation of the conjugate on D-series HMP SynPhase
lanterns, via consecutive alkylations, and easy solution-phase
purification of cleaved compounds thanks to a 4-nitrophen-
ylbenzyloxycarbonyl protecting group. This versatile meth-
odology has been used to synthesize a small library of 9
polyamine-imidazole conjugates which could be obtained in
the solution phase but certainly with much lower yields and
at the price of great purification effort.
Scheme 5. Strategy A. In-Solution Steps
The novel polyamine-imidazole conjugates are currently
undergoing in vitro biological evaluation in an RNA model.
Acknowledgment. We gratefully acknowledge Marc
Capet (Bioprojet-Biotech) for providing us with 3-[1-
(triphenylmethyl)-1H-imidazol-4-yl]propan-1-ol and Jean-
Francois Cupif and Jean-Guy Delcros for technical assistance
during this work. We thank the Conseil Re´gional de Bretagne
for financial support (PRIR No. 691).
acetic salt 12aa-cc was converted into its neutralized form.
Because of the presence of 4-nitrobenzyloxycarbonyl group,
purification by column chromatography on silica gel of each
of those residues was easily accomplished using MeOH/NH4-
OH (80:20-90:10). Compounds 13aa-cc were then typi-
cally obtained in 92% purity (estimated with HPLC) and 23%
yield based on the initial loading of lanterns (see the
Supporting Information). Deprotection of each of these nine
Supporting Information Available: Experimental pro-
cedures and spectrosopic data for compounds 1-5, 13aa-
cc, 15aa-cc, 16, and 17 and for products obtained by
separated cleavages from SynPhase lanterns are available.
By way of example, the in depth structural study using two-
dimensional NMR (COSY, HMBC, HMQC) of compound
15bc is reported. This material is available free of charge
via the Internet at http://pubs.acs.org.
(27) Kreye, P.; Kihlberg, J. Tetrahedron Lett. 1999, 40, 6113-6116.
(28) Banfi, L.; Guanti, G.; Riva, R. Tetrahedron: Asymmetry 1999, 10,
3571-3592.
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