B. Liu et al. / Bioorg. Med. Chem. Lett. 16 (2006) 1864–1868
1865
NO2
group of 2 to other groups (hydroxyl, methylamino,
NO2
NO2
NO2
d
O
O
methyl, etc.) also resulted in inactive analogs (data not
shown). In this letter, we will describe the SAR studies
on the diaminopyrimidine 4- and 6-position side chain
modification to achieve high ghrelin antagonist potency
and good DHFR selectivity.
a
b
c
O
O
HO
HO
O
O
O
O
N
Br
O
Br
9
10
11
O
O
S
NO2
e,f
O
H
NO2
g,h,i
R1 R2
N
O
The diaminopyrimidine 6-position side chain modifica-
tions (4a–4t) were prepared in a similar fashion as de-
scribed previously.7 Briefly, 4-nitrophenyl-acetonitrile
was first acylated with appropriate acid chlorides
(Scheme 1). Subsequent methylation, cyclization with
guanidine, and reduction of the nitro group yielded ani-
line 3. Reductive amination of 3 with p-methylsulfonyl
benzaldehyde provided the ghrelin antagonists 4a–4t.
Carboxamides 4u and 4v were synthesized from basic
hydrolysis of ethyl ester in 4t, followed by condensing
the resulting acid with corresponding benzyl amines.
Cl
N
N
N
N
H2N
N
H2N
N
H2N
12
14a-14l
13
Scheme 3. Reagents and conditions: (a) 1—SOCl2, 2—BrCH2CH2OH;
(b) CH3CH2COCl, Et3N, 0 °C-rt, 43% over 3 steps; (c) NaH, DMF, rt;
(d) guanidine, EtONa, reflux, 70% over 2 steps; (e) TMSBr, dioxane,
150 °C; (f) POCl3, 100 °C, 85% over 2 steps; (g) HNR1R2, dioxane,
170 °C, 20 min; (h) 10% Pd/C, MeOH, rt; (i) 4-methanesulfonylbenzal-
dehyde, NaBH3CN, MeOH/AcOH/NaOAc, rt, ca. 35% over three steps.
The 4-alkylaminopyrimidine analogs 14a–14l were pre-
pared as described in Scheme 3. Condensation of
bromoethyl alcohol and an acid chloride made in situ
from p-nitrophenyl acetic acid and thionyl chloride led
to ester 9. Acylation of 9 with propionyl chloride pro-
vided enol ester 10. Treatment of 10 with NaH in
DMF produced a dienolate intermediate, which under-
went an intramolecular O-alkylation reaction followed
by a facile cleavage of the enol ester to provide ketene
acetal 11. Condensation between 11 and guanidine gave
pyrimidine 12, which was converted to 4-chloropyrimi-
dine 13 in a 2-step sequence. Reaction of chloropyrimi-
dine 13 with a variety of primary and secondary amines
gave 4-akylamino pyrimidines. Reduction of the nitro
group, followed by a reductive amination step, produced
final analogs 14a–14l.
The 6-ether side chain modifications (7a–7h) were
achieved starting from the previously described benzyl
ether 5.7 Debenzylation and bromination of 5 generated
benzyl bromide 6 (Scheme 2). A small amount of water
in the reaction mixture prevented the formation of ani-
line acetamide during this process. Williamson ether
synthesis using 6 with various alcohols provided 6-ether
analogs 7a–7h. Analogously, 6-aminoalkyl-substituted
pyrimidines 8a–8p could be synthesized through dis-
placement of benzyl bromide 6 with different amines.
NH2
NH2
a,b,c,d
e
R
Cl
N
+
O2N
O
R
H2N
N
N
3
O
O
S
S
Speculating that the 6-ethyl group of diaminopyrimidine
2 might occupy the same binding site as the octanoyl
group of ghrelin, we replaced the ethyl group of 2 with
different lipophilic chains and found that this position
is quite tolerant of different substitution. As shown in
Table 1, both straight and branched aliphatic chain
(3–8 carbons) substituents led to analogs (4a–4h) with
low nanomolar binding IC50 values comparable to those
of lead compound 2.9 Unfortunately, the longer aliphat-
ic chains (4f–4h) did not provide any FLIPR potency
boost as expected. Only 6-isopropyl diaminopyrimidine
4a was 2-fold more potent in both binding and FLIPR
assays than 2. Consistent with the beneficial effect of
a-branching, all the 6-cycloalkyl-substituted diamino-
pyrimidines (4i–4l) exhibited equal or a slightly better
FLIPR activity. In contrast, 6-bicycloalkyl substituents
led to compounds (4m–4o) with 2- to 8-fold-loss in
FLIPR potency. Further functionalization of the ali-
phatic chain provided some highly potent analogs, such
as propionyl benzyl amides 4u and 4v, both of which
have single-digit nanomolar potency in FLIPR assay
and sub-nanomolar binding IC50 for 4u.
O
O
H
N
H
N
f,g
NH2
N
NH2
N
4t as
starting
material
N
N
NHR
R
H2N
H2N
4a-4t
4u-4v
O
Scheme 1. Reagents and conditions: (a) Et3N, DMAP, CH2Cl2, rt; (b)
TMSCHN2, CH2Cl2, rt; (c) guanidine, EtOH, reflux, 45–80% over
three steps; (d) H2, Pd(OH)2, MeOH, 95%; (e) p-methanesulfonyl-
benzaldehyde, NaBH3CN, MeOH/HOAc/NaOAc, rt, 70%; (f) LiOH,
THF/MeOH, rt; (g) H2NR, TBTU, i-Pr2NEt, DMF, rt, 82% over two
steps.
O
S
O
O
O
O
O
S
S
H
H
N
H
N
N
NH2
N
NH2
N
NH2
N
a
b
N
N
N
OBn
7a-7h
Br
O
H2N
H2N
H2N
R
5
6
O
O
c
S
H
N
NH2
N
N
NR1R2
8a-8p
H2N
Previous SAR studies have illustrated the importance
of the 6-benzyloxymethyl ether in 1 for maintaining
ghrelin activity.7 Not surprisingly, this ether in
combination with 4-methylsulfonyl benzyl amine
Scheme 2. Reagents and conditions: (a) 30% HBr/AcOH with 5%
H2O, reflux, 4 h, 72–93%; (b) ROH, NaH, THF, DMPU, rt, 12–25%;
(c) HNR1R2, i-Pr2NEt, THF, rt, 25–32%.