Synthetic studies toward the development of novel minoxidil analogs and conjugates with polyamines

Article abstract of DOI:10.1016/j.tetlet.2010.02.037

Syntheses of novel polyamine-modified minoxidil analogs (PMMs) and minoxidil-polyamine conjugates (MPCs) are described in an effort to improve the biological activity and selectivity of minoxidil and its poor solubility in water.

Full text of DOI:10.1016/j.tetlet.2010.02.037

Tetrahedron Letters 51 (2010) 1989–1993
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Synthetic studies toward the development of novel minoxidil analogs and
conjugates with polyamines
*
George E. Magoulas, Stavros E. Bariamis, Constantinos M. Athanassopoulos, Dionissios Papaioannou
Laboratory of Synthetic Organic Chemistry, Department of Chemistry, University of Patras, 26504 Patras, Greece
a r t i c l e i n f o
a b s t r a c t
Article history:
Syntheses of novel polyamine-modified minoxidil analogs (PMMs) and minoxidil–polyamine conjugates
(MPCs) are described in an effort to improve the biological activity and selectivity of minoxidil and its
poor solubility in water.
Received 23 November 2009
Revised 1 February 2010
Accepted 5 February 2010
Available online 11 February 2010
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
Minoxidil
Polyamine conjugates
Nucleophilic aromatic substitution
N,N0-Carbonyldiimidazole
Pyrimidines
N-Oxides
Minoxidil (MNX, Fig. 1) is an antihypertensive agent and a
peripheral vasolidator. The antihypertensive activity of MNX is
due to the rapid relaxation of vascular smooth muscle by its sulfated
metabolite (MNXS).1 Formation of MNXS is catalyzed by sulfotrans-
ferase enzymes.2 MNXS is one of several chemically unrelated drugs,
which cause opening of plasma membrane adenosine triphosphate
(ATP)-sensitive potassium channels (KATP channels) and it is likely
that this property explains its relaxant effect on vascular smooth
muscle.3 KATP channels have been identified in a variety of tissues
(muscle cells, pancreatic b-cells, and neurons) and exhibit important
physiological roles in cellular signaling processes that regulate heart
rate, smooth muscle contraction, insulin secretion, and neurotrans-
mitter release.4 MNX is the drug of choice in the clinical manage-
ment of male pattern baldness, also known as androgenic alopecia.
Its use is associated with hirsutism and hypertrichosis.5 A number
of in vitro effects of minoxidil have shown that it is involved in cell
proliferation, collagen synthesis, and prostaglandin synthesis. How-
ever, neither these activities nor the correlation of MNXS with KATP
channels explains how it promotes hair growth.6
Naturally occurring polyamines (PAs), or their conjugates with
other biomolecules, show interesting biological activities. Further-
more, conjugation of biologically interesting molecules with PAs is
a means of either improving the activity of the non-PA moiety or
combining the activities of the conjugated molecules.7 We there-
fore considered it interesting to examine the possibility of incorpo-
* Corresponding author. Tel.: +30 2610962954; fax: +30 2610962956.
E-mail address: dapapaio@chemistry.upatras.gr (D. Papaioannou).
Figure 1. Compounds relevant to this work.
0040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.02.037

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G. E. Magoulas et al. / Tetrahedron Letters 51 (2010) 1989–1993
Scheme 1. Syntheses of PMMs 10 and 14.
rating MNX and PA molecules in a single compound. We also antic-
ipated that these conjugates would improve the solubility of MNX
in water8 and thus lead to an efficient liquid-vehicle delivery to the
hair follicle. It has been recently reported that MNX topical foam,
free of the irritant solvent propylene glycol traditionally used for
its solubilization, is more advantageous over the corresponding
MNX topical solution.9
respectively, in 75–95% yield (Scheme 1). The latter were N-oxi-
17d
dized17 using phthalic anhydride/H2O2
at ꢀ10 °C to ambient
temperature for 2.5–8 h to afford the desired pyrimidine N-oxides
8 and 12, respectively, in 53–58% yield. This system gave the high-
est yields of N-oxides compared to other methods involving H2O2
or m-CPBA as the oxidant. The obtained N-oxides were treated
with anhydrous piperidine (Pip) in PhMe at 110 °C for 2.5–5 h to
afford the corresponding compounds 9 and 13, respectively, in
75–83% yield. Finally, routine deprotection with 1.5 M HCl in EtOH
afforded the PMMs 10 and 14, as the hydrochloride salts, in 85–
93% yield.
Although several analogs of MNX have been synthesized,10
none incorporating PAs have been so far reported. We envisaged
that such modification could be accomplished through the follow-
ing two molecular entities:
a polyamine-modified minoxidil
(PMM) and a minoxidil–polyamine conjugate (MPC).11 In the pres-
ent study, we chose to use putrescine (PUT), spermidine (SPD), and
spermine (SPM) as PAs for the preparation of the PMMs and MPCs.
These PAs were to be introduced into the projected molecules
through their derivatives 1,12 2,13 3,13a 4,14 and 5.15 MNX analogs
10, 14, 19, and 23 (Schemes 2 and 3) were envisaged as examples
of PMMs. Furthermore, mono-substituted MNXs such as 26, 28,
and 30 and the symmetric conjugate 32 are considered examples
of MPCs (Scheme 3).
We anticipated that the synthesis of PMMs 10, 14, 19, and 23
could be realized through the sequential amination16 of the com-
mercially available functionalized pyrimidines, such as 2-amino-
4,6-dichloropyrimidine (6), (Scheme 1) and 2,4,6-trichloropyrimi-
dine (15) (Scheme 2). Initially, 6 was reacted with the selectively
protected linear PAs N1-tert-butoxycarbonylputrescine (1) and
N1,N5,N9-tris(tert-butoxycarbonyl)spermine (4) in n-BuOH, in the
presence of N,N-diisopropylethylamine (DIEA), at 116 °C for 4 h
to give the anticipated mono-substituted derivatives 7 and 11,
Next, 15 was reacted with the PA derivative 1 or 4 in CH2Cl2, in
the presence of triethylamine, at ambient temperature for 2 h to
give a mixture of the two anticipated regioisomers in the ratio
55:45 in the case of 1 (see Supplementary data) which could be
separated by flash column chromatography, if so desired. However,
for the purpose of the present study, the mixture of regioisomers
was used as such in the next experiment, after evaporation of
the solvent and flash column chromatography to eliminate base-
line impurities. Treatment of the above-mentioned regioisomeric
mixture with further 1 or 4, respectively, in n-BuOH, in the pres-
ence of DIEA, at 116 °C for 12 h, finally gave the disubstituted com-
pounds 16 and 20, respectively, in 64–74% overall yield. These
were N-oxidized using m-CPBA in CHCl3 at ꢀ10 °C to ambient tem-
perature, for 0.5–12 h to give the desired pyrimidine N-oxides 17
and 21, respectively, in 20–22% yield. In these cases, m-CPBA pro-
vided better yields, compared to the phthalic anhydride/H2O2 sys-
tem. The obtained N-oxides were then treated with an anhydrous
piperidine in PhMe at 110 °C for 3–4 h, to afford compounds 18

G. E. Magoulas et al. / Tetrahedron Letters 51 (2010) 1989–1993
1991
Scheme 2. Syntheses of PMMs 19 and 23.
and 22, in 65–88% yield. Finally, deprotection with 1.5 M HCl in
EtOH gave the PMMs 19 and 23, as the hydrochloride salts, in
83–88% yield.
after crystallization from MeCN. The formation of both isomers
with the observed preference for isomer 24a, under the present
reaction conditions, might be explained by initial acylation of the
negatively charged O atom of MNX followed by an internal nucle-
ophilic attack on the activated carbonyl function by the more
nucleophilic C-6 amino group.
Treatment of intermediate 24a with suitably protected linear
PAs (Scheme 3), such as N1-tritylputrescine (2), 4, N1,N8-bistrityl-
spermidine (3), and N4,N9-bistritylspermine (5), in DMF at temper-
atures varying from ambient to 70 °C provided the anticipated
protected MPCs 25, 27, 29, and 31, respectively, in 45–77% yield.
Finally, deprotection afforded conjugates 26, 28, 30, and 32, as
the corresponding hydrochloride salts, in 88–94% yield. It should
be noted that attempts to activate, for example, compound 25 with
CDI, in order to attach a second same or different PA molecule,
were unsuccessful leading only to the recovery of the starting
material. In comparison with the parent molecule MNX, the new
compounds exhibited, as expected, significantly improved solubil-
ities in H2O (see Supplementary data).
For the attachment of PA molecules on MNX we reasoned,
based on the literature findings18 that an intermediate such as 24
(Scheme 3) would serve the purpose of activating MNX toward
amine nucleophiles. This kind of intermediate had been previously
obtained through pyrolysis of suitable carbamates, which in turn
were obtained by alkylchloroformate treatment of o-amino-substi-
tuted pyridine or pyrimidine N-oxides.19 We envisaged that N,N0-
carbonyldiimidazole (CDI) would activate MNX under mild reac-
tion conditions. Indeed, when MNX was reacted with CDI at
60 °C in an anhydrous DMF for 3 h the activated intermediate 24
was obtained in 75% yield, as a mixture of the two expected iso-
mers in the ratio 24a:24b = 80:20 (LC–MS and 1H NMR). We based
our identification of the isomers on the chemical shift of H-519
which for 24a (proton is ortho to the carbamate function) was at
d 5.73 ppm and that for 24b (proton is ortho to the free amino func-
tion) at d 5.32 ppm. The main isomer 24a could be obtained pure

1992
G. E. Magoulas et al. / Tetrahedron Letters 51 (2010) 1989–1993
Scheme 3. Syntheses of MNX-intermediate 24 and MPCs 26, 28, 30, and 32.
In conclusion, MPCs were readily obtained through CDI-medi-
NMR spectra) associated with this article can be found, in the on-
line version, at doi:10.1016/j.tetlet.2010.02.037.
ated activation of MNX, followed by coupling with suitably pro-
tected linear PAs, and finally N-deprotection. On the other hand,
PMMs were assembled through sequential replacement of the chlo-
rine atoms of suitable, commercially available, chloropyrimidines
with selectively protected linear PAs, followed by N-oxidation,
piperidine introduction and finally N-deprotection. All the com-
pounds showed significantly improved solubilities in water com-
pared to MNX. Applications of the above-described protocols to
the preparation of other MPCs and PMMs, suitable for structure–
activity relationship studies, as well as the biological evaluation
of all new compounds to various cellular targets are currently in
progress. It should be noted that the incorporation of PAs can also
increase the toxicity (SPM to a greater extent than SPD or PUT).20
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Acknowledgments
We thank the Research Committee of the University of Patras
(Greece) for funding the present work in the form of a fellowship
(S.E.B.) and consumables, in the context of the basic research Pro-
gram ‘Constantin Carathéodory’. Also, we would like to thank Pro-
fessor G. W. Francis (Chemistry Department, University of Bergen,
Bergen, Norway) for the kind provision of HRMS analyses.
Supplementary data
Supplementary data (experimental procedures, physical, ana-
lytical and spectroscopic data and selected copies of 1H and 13C

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