Reductive amination products containing naphthalene and indole moieties bind to melanocortin receptors

Article abstract of DOI:10.1016/S0960-894X(02)00088-4

Presumed pharmacophoric groups of melanocortin peptides (naphthalene, amino or guanidine, and indole moieties) were combined in mimetics molecules looking for their favorable location for activity at melanocortin (MC) receptors. Twenty-two compounds were prepared and tested. The best of these displayed micromolar affinities for the MC receptors.

Full text of DOI:10.1016/S0960-894X(02)00088-4

Bioorganic & Medicinal Chemistry Letters 12 (2002) 1035–1038
Reductive Amination Products Containing Naphthalene and Indole
Moieties Bind to Melanocortin Receptors
Felikss Mutulis, Ilze Mutule, Maris Lapins and Jarl E. S. Wikberg*
Department of Pharmaceutical Biosciences, Division of Pharmacology, Uppsala Biomedical Center, PO Box 591,
Uppsala University, S-751 24 Uppsala, Sweden
Received 22 June 2001; revised 17 December 2001; accepted 31 January 2002
Abstract—Presumed pharmacophoric groups of melanocortin peptides (naphthalene, amino or guanidine, and indole moieties)
were combined in mimetics molecules looking for their favorable location for activity at melanocortin (MC) receptors. Twenty-two
compounds were prepared and tested. The best of these displayed micromolar affinities for the MC receptors. # 2002 Elsevier
Science Ltd. All rights reserved.
Five subtypes of melanocortin receptors, MC1À5R, are
known.1À3 The MC1R regulates skin pigmentation and
the immune system. The MC2R (ACTH receptor) con-
trols steroid production. The MC3R might be involved
in regulation of central sexual behavior, the MC4R
controls feeding behavior and the MC5R has a role for
regulating exocrine gland secretion.1À3 The melanocor-
tic peptides are the natural ligands for the MCRs
and consist of the melanotropins a-MSH, b-MSH
and g-MSH, and the adrenocorticotropin ACTH.
These peptides are not very suited for therapeutic
applications and there exists a need for non-peptide
ligands. Some peptoids,4 isoquinolines5 and b-turn
related heterocycles6 were reported to show moderate
activity on MCRs. We recently found some MCR active
substances whose preparation included reductive ami-
nation.7 Here we report a new series synthesized in a
related way.
Ethylenediamine derivative 6a was prepared on solid
phase from carboxylated Wang polymer attached with
ethylenediamine 5a. Piperazine derivatives 6b and 6c
were similarly synthesized using piperazine-attached
polymer. After reductive amination, cleavage from the
polymer was achieved using a trifluoroacetic acid based
cocktail. Guanidation of both compounds proceeded as
described above, giving 7a and 7b.
The secondary amine 10a was prepared from N-Boc-3-
formyl-indole 1b and 1-aminomethylnaphthalene 8a by
reductive amination, and deblocking the indole moiety
of the intermediate 9a with trifluoroacetic acid. Simi-
larly, another secondary amine 9b was obtained from
2-naphthaldehyde 1a and tryptamine 8b. Compound 9c
of this type contained two indole moieties and was syn-
thesized from unprotected 3-indolylaldehyde 1c and
tryptamine 8b. The Schiff base was formed in suspen-
sion by stirring for 24 h at room temperature. Com-
pound 9d was similarly synthesized from 2-
naphthaldehyde 1a and 1-aminomethylnapthalene 8a.
The diamines 3a,b were synthesized as shown in Scheme
1by reduction of Schiff bases with NaCNBH in tri-
3
methylorthoformate in the presence of AcOH (4%).
Compound 3b was prepared starting from N-Boc-3-for-
myl-indole 1b.8 The HCl treatment at the end removed
Boc groups. Guanidines 4a,b were prepared from 3a,b
introducing the guanidine function by using guanylpyr-
azole (equimolar quantity) in DMF.9 The products were
isolated by reversed phase HPLC using acetonitrile–
water–0.1% TFA as eluent and freeze drying.
The tertiary amide 11a was obtained from Boc pro-
tected secondary amine 9a. The coupling reaction pro-
ceeded by a low yield. The product was Boc deprotected
and isolated by preparative HPLC and freeze dryed.
Compounds 11b and 11d were prepared in a similar way
as 11a. Derivative 11c was obtained from Fmoc-d-
Arg(Pbf)–OH. At the end the Fmoc group was removed
with piperidine followed by removal of Pbf with tri-
fluoroacetic acid and scavengers. Compound 11e was
obtained from Fmoc-g-aminocaproic acid. Synthesis of
the tertiary amide 11f proceeded similarly by addition of
*Corresponding author. Tel.: +46-18-471-4238; fax: +46-18-559718;
e-mail: jarl.wikberg@farmbio.uu.se
0960-894X/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved.
PII: S0960-894X(02)00088-4

1036
F. Mutulis et al. / Bioorg. Med. Chem. Lett. 12 (2002) 1035–1038
Scheme 1. Reagents and conditions: (a) trimethylorthoformate, 1h; (b) NaCNBH 3, AcOH, 10 min; (c) HCl/dioxane; (d) 1H-pyrazole 1-carbox-
.
amidine hydrochloride, DIEA, DMF, 20 h; (e) TFA–1,2-ethanedithiol–triisopropylsilane–water (925:25:25:25), 2 h; (f) Boc-Arg-OH HCl, TFFH,
DIEA, DMF, 20 h; (g) Fmoc-amino acid, TFFH, DIEA, DMF, 20 h; (h) 20% piperidine/DMF, 30 min.
Fmoc-Glu(OtBu)–OH and cleavage with piperidine fol-
lowed by trifluoroacetic acid.
Table 1. Binding activities (Ki mM) of compounds for human
recombinant MCRs.a
The double reductive amination product 16a was pre-
Compd
MC1R
MC3R
MC4R
MC5R
pared by loading imidazole carboxylate Wang resin with
tetramethylenediamine. The polymeric monoacylated
diamine formed was further reacted with Fmoc-Lys(A-
loc)–OH (Scheme 2). The polymer 13a obtained was
subjected to palladium derivative deprotection (Scheme
2). The polymeric Lys derivative obtained containing a
free a-amino group was further reacted with Boc-indo-
lyl-3-aldehyde. The Schiff base formed was reduced with
NaCNBH3 giving secondary amine 14a. The Fmoc
group was then removed deblocking the a-aminofunc-
tion of the polymeric lysine derivative. The amino group
was introduced into the reaction with 2-naphthaldehyde
and the product reduced. The polymer 15a formed was
cleaved by trifluoroacetic acid in the presence of sca-
vengers. The resulting raw product was purified by
HPLC yielding pure 16a (LC/MS and NMR).
3a
3b
4a
4b
6a
6b
6c
7a
7b
9b
9c
9d
10a
11a
11b
11c
11d
11e
11f
16a
16b
16c
>1000
28
19
57
145
31
45
nb
>1000
>1000
>1000
>1000
59
94
104
56
107
8.8
43
150
36
47
83
141
256
74
12
nb
>1000
>1000
>1000
>1000
16
>1000
>1000
68
>1000
118
161
>1000
161
70
>1000
14
18
32
86
>1000
26
11
>1000
214
52
100
38
12
2.5
2.7
10
4.1
45
0.69
9
18
19
33
294
167
58
5.4
2.1
15
2
89
24
44
27
1.2
1.5
1.5
46
9.8
27
0.7
173
28
36
Compounds 16b and 16c were synthesized in a similar
way starting from ethylenediamine or trimethylenedia-
mine attached to carboxylated Wang polymer. Fmoc-l-
Dap(Aloc)–OH was used for 16b and 16c; for 16b 2-
naphthaldehyde was introduced twice. Yields generally
ranged over 25–90%.
56
aReceptor binding was assessed using radioligand binding on human
MCR subtypes10 (mean values from at least two measurements,
deviation between them did not exceed 30%). nb, no binding up to 0.5
mM.

F. Mutulis et al. / Bioorg. Med. Chem. Lett. 12 (2002) 1035–1038
1037
situation. On the other hand, guanidation of the less
rigid 3a,b increased the binding affinity (c.f., 4a,b, Table
1).
Acylation of secondary amines with amino acids con-
tributed much to the activity. This can, for example, be
seen by comparing the data for 9d with the data for 11b
(Table 1). Quite unexpectedly 11b, containing a naph-
thalene-naphthalene combination, showed better bind-
ing compared to the 11a,d containing naphthalene and
indole moieties. It was unexpected also that 11c, con-
taining a d-arginine residue showed approximately the
same activity as 11b, the latter that was derived from l-
arginine. Compound 11e that has an e-aminocaproyl
group shows also comparable activity to the above
mentioned arginine derivatives. However, introduction
of the amphoteric l-glutamic acid residue (11f) led to a
considerable reduction in activity.
As a result of simulated annealing molecular dynamics
calculations11 we found a low energy conformation of
11b (Fig. 1). Important features of it are the parallel
interacting naphthalene ring systems, with the guanidine
group being placed at a far distance. The conformation
seems useful for explaining structure–affinity relation-
ship of analogues of 11b. Obviously, both naphthalene
groups and the guanidine function, but not the arginine
residue a-amino group, are interacting with the MCRs.
Such a situation would explain why neither different
positions of the a-amino group (i.e., the change of con-
figuration of the asymmetric carbon atom in 11b and
11c) nor the removal of this function (11e) affects the
binding affinity significantly. On the other hand, the
importance of a distantly located basic function is illu-
strated by the lower affinity of 11f, which contains a
carboxylic group instead of a guanidino (11b, 11c) or an
amino group (11e).
Scheme 2. Reagents and conditions: (a) NH2(CH2)nNH2, DMF, 2 h,
(b) Fmoc-amino acid, HATU, DIEA, DMF, 1h; (c) tetra-
kis(triphenylphosphine)-palladium(0) in CHCl3+5% AcOH+ 2.5%
NMM, 2 h; (d) aldehyde, trimethylorthoformate, 1h; (e) NaCNBH ,
3
trimethylorthoformate, AcOH, 10 min; (f) 20% piperidine/DMF, 30 min;
(g) TFA–1,2-ethanedithiol–triisopropylsilane–water (925:25:25:25), 2 h.
Reductive amination followed by acylation allowed us
to prepare a series of melanocortin mimetics. They all
contain naphthalene/indole and amino/guanidino
groups. The secondary and tertiary amines exert rela-
tively low affinity (Ki>15 mM) for the MCRs (Table 1).
The secondary amines containing indole and naphtha-
lene functions (10a) are better binders than the ones
containing two naphthalenes (9d), or one naphthalene
and one additional primary amine function (3a, 6a).
Piperazine derivatives (6b,c, 7a,b) show very low activ-
ity. Obviously, these small and rigid molecules do not fit
well to the MCR binding sites. Guanidation of the sec-
ondary amine function (7a,b) did not improve the
On comparing the data for substances 16a–c, it is seen
that 16c shows considerably higher affinity on the
MC1R than on the other HMCRs. Obviously the geo-
metry of 16c fits better to the MC1R than 16a,b.
In conclusion, we have here shown that a wide array of
substances exhibit MC receptor binding affinity. The
structure–activity relationships obtained will be useful
for further developments of MCR subtype selective
compounds.
Acknowledgements
This work was supported by a grant from Melacure
Therapeutics. Biological parts of studies were supported
by the Swedish Medical Research Council (04X-05957).
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