T. Sifferlen et al. / Bioorg. Med. Chem. Lett. 23 (2013) 3857–3863
CO2H CO2H CHO
3859
CHO
b
c,d
a
Ri
Ri
Ri
Ri
10
11
12
b,g
13
Ri = F, Cl, CF3, OCHF2, OCF3, CN
CO2Bu
NH2
Br
Br
Br
e
f
f
h
Ri
Ri
Ri
OCHF2
OH
14
15
16
18
17
O
OH
O
CO2Me
OH
O
O
j
i
k
Ri
Ri
Ri
Ri
19
20
21
22
Scheme 2. Synthetic routes towards substituted aldehydes 13 and 22. Reagents and conditions: (a) malonic acid, pyridine, piperidine, 75 °C (84–96%); (b) standard
conditions: H2 (1 atm), 10% Pd/C (10% in weight), MeOH, rt (86–100%); for chloro-substituted aryl derivatives: H2 (1 atm), 10% Pd/C (6% in weight), ZnBr2 (0.2 equiv), AcOEt, rt
(85–92%); (c) 1 M BH3 in THF, THF, 0 °C to rt (88–99%); (d) pyridinium chlorochromate, CH2Cl2, 0 °C to rt (52–93%); (e) CuBr2, tert-butyl nitrite, MeCN, 45 °C (70–86%); (f) n-
butyl acrylate, DABCO, K2CO3, Pd(OAc)2, DMF, 120 °C (53–97%); (g) 1 M NaOH, MeOH, H2O, rt (94–100%); (h) ClCF2CO2Na, K2CO3, DMF, H2O, 100 °C (74%); (i) methyl
bromoacetate, K2CO3, acetone, reflux (98–100%); (j) LiAlH4, THF, 0 °C (93–99%); (k) oxalyl chloride, DMSO, Et3N, CH2Cl2, À78 °C (95–99%).
trite, CuBr2, MeCN).15 Difluoromethoxy-substituted aldehydes 13
were efficiently prepared by heating the bromo-phenols 17 in
presence of sodium chlorodifluoroacetate.16 Finally, substituted
2-phenoxyacetaldehydes 22 were obtained from the related phe-
nols 19 after O-alkylation with methyl bromoacetate, reduction
of the methyl ester, and subsequent oxidation of the resulting
primary alcohol under Swern conditions.17
containing mono-substituted phenyl rings, the influence of poly-
substitution was scrutinized. The addition of a meta-fluorine atom
on the para-trifluoromethylphenyl was beneficial, with a twofold
increase in hOX1R potency, and afforded derivative 30 which is
one of the most potent DORA identified in this series. The influence
of the stereochemistry was investigated with the potent (S;R) ste-
reoisomer 30 in comparison to the corresponding (R;R) isomer 31,
which was significantly less potent against both orexin receptors
(30- and 32-fold decrease, respectively, in hOX1R and hOX2R po-
tency with 31). For potent affinities with the orexin receptors, it
is mandatory that the compounds possess the (R)-configuration
at the phenylglycine moiety (see structure associated to Table 1).
Thus, the two additional stereoisomers corresponding to 30 and
31, and that contain the (S)-configured phenylglycine moiety, were
The antagonistic activity of the novel 1-chloro-3-ethyl-5,6,7,8-
tetrahydroimidazo[1,5-a]-pyrazines 9 with both orexin receptors
was determined with a cell-based FLIPR assay (fluorometric imag-
ing plate reader) measuring Ca2+ flux as a functional determinant
of orexin binding.18 The results of the structure–activity relation-
ship studies of the substituted phenyl derivatives are summarized
in Table 1. Preliminary investigations were devoted towards deriv-
atives containing a mono-substituted phenyl ring. Thus, moving
the electron-withdrawing trifluoromethyl moiety present in almo-
rexant from the para-position (stereoisomer 23) to the meta-posi-
tion (24) induced an almost twofold loss in hOX1R potency while
maintaining the affinity for hOX2R. The importance of the linker
between the 1-chloro-3-ethyl-5,6,7,8-tetrahydroimidazo[1,5-
a]pyrazine and the trifluoromethylphenyl ring was as well investi-
gated. Thus, the CH2O linker led to a substantial loss of affinity for
hOX1R as illustrated with the potent selective orexin 2 receptor
antagonists 25 and 26. Additional electron-withdrawing substitu-
ents were evaluated for comparison with trifluoromethyl. A para-
cyano group (stereoisomer 27) resulted in strongly decreased
activity for the hOX1R (100-fold loss in potency) while high
potency at hOX2R was maintained. Compared to para-trifluoro-
methyl, a para-difluoromethoxy moiety resulted in an almost
twofold loss in hOX1R potency while allowing a ninefold increase
in potency for hOX2R (28). The electron-withdrawing para-triflu-
oromethoxy residue as well resulted in a significant loss of activity
towards the hOX1R as shown with the preferential orexin 2
receptor antagonist 29. After this initial exploration of derivatives
essentially inactive (IC50 >1 lM with both orexin receptors). Com-
pared to meta-fluorine in 30, a meta-chloro substituent on the
para-trifluoromethylphenyl (derivative 32) was equally beneficial
for the hOX1R potency but chlorine generated an almost threefold
drop in hOX2R affinity. The association of an ortho-fluorine atom
and a para-trifluoromethyl group did not alter the interactions
with the orexin receptors as shown with the equipotent DORAs
23 and 33. Difluorination of the para-trifluoromethylphenyl was
well tolerated (stereoisomers 34–36) as shown with compound
36. A trifluorinated phenethyl moiety (stereoisomer 37) allowed
potent interactions with both orexin receptors, and the trifluoro-
methyl group is therefore not a mandatory structural feature for
obtaining potent DORAs. In a next step, additional substitution of
the meta-trifluoromethylphenyl was further investigated. Thus,
the addition of a para-fluorine atom resulted in lower potency
for the hOX1R as shown for the selective orexin 2 receptor antago-
nist 38. On the other hand, the insertion of an ortho-fluorine atom
on the meta-trifluoromethylphenyl proved to be beneficial as
shown with the significant increase in hOX1R potency for DORA
39 as compared to compound 38.