C. Garino et al. / Bioorg. Med. Chem. Lett. 16 (2006) 1995–1999
1997
Br
Br
O
O
N
O
X \ Ar
(i)
H2N
Ar
N
H
MeO
O
O
N
X
N
X
Ar-COOH
NBoc
8a
8c
9a
10a
10c
NH.TFA
9c
2a: X=NBoc
a: X=NBoc
c: X=NH.TFA
(ii)
O
N S
O
S
(iii)
8d
9d
d: X=NSO2-2-Thiophene
Ar: Heteroaromatic moiety
Scheme 3. Reagents and conditions: (i) POCl3, pyridine, À20 °C to rt; (ii) TFA, CH2Cl2, rt, 12 h; (iii) Cl-SO2-2-thiophene, NaHCO3, CH2Cl2, rt, 4 h.
salts 8c, 9c and 10c followed by sulfonylation of 8c and
9c with 2-thiophene sulfonyl chloride20 in the presence
of NaHCO3 in methylene chloride to give, respectively,
compounds 8d and 9d.
vation could be of interest to design new series of ana-
logues in which various lengths of linkers could be
introduced between the heterocycles and the carbonyl
of the amide group.
All intermediates and final compounds of both series
Another observation stands on the influence of the posi-
tion by which the coumarine moiety is linked to the
amide group. Position 4 in the case of analogue 8a ap-
pears to be the most favourable, while position 3 for
compounds bearing a N-Boc group on the piperazine
ring (6a and 7a) is a slightly less active inhibitor.
1
were fully characterized by H NMR, 13C NMR and
MS (data available on request).
All the compounds were assayed as BACE-1 inhibitors,
using a fluorescence resonance energy transfer (FRET)
assay, which uses purified baculovirus-expressed
(BACE-1) and
a
specific substrate (Rh-EVNL-
It could be also observed that at least in this in vitro
enzymatic assay, the hydrophobicity of the derivatives
exemplified by ClogP does not appear to be a determi-
native parameter in the BACE-1 inhibitory activities. In-
deed, no correlation between ClogP values and
inhibitory activities can be deduced from the obtained
results. Nevertheless, for a hypothetical therapeutical
use, taking into account the well-known rule of five of
Lipinsky et al.,21 compounds with ClogP values lower
than 5 should have improved drug ability properties
compared to more hydrophobic analogues (ClogP
greater than 5).
DAEFK-quencher) based on the Swedish mutation of
the amyloid precursor protein (APP). This peptidic sub-
strate becomes highly fluorescent upon enzymatic
cleavage.
IC50 inhibition values were determined at least three
times. The IC50 values obtained for both naphthyl
and coumarinyl series of compounds are summarized
in Table 1.
In order to compare the BACE-1 inhibitory activities of
the new heterocyclic derivatives 6a–c, 7a–c, 8a,c,d, 9a,c,d
and 10a,c with those of described Vertex derivatives, we
synthesized as bench-marking some Vertex compounds
3a–c according to the known procedures.9
From these results it could be now possible to envisage
the design of new BACE-1 inhibitors:
Coumarinyl analogues in which the coumarinyl moiety
should be linked through different lengths of linkers to
the amide function.
The linkers between the coumarin nucleus and the amide
function should be preferably branched at the 4-position
of the coumarinyl moiety.
Substituents on the nitrogen atom of the piperazine ring
(H; Boc; benzyl; and SO2-2-thiophene) could be also
important features to consider in the design of optimized
BACE-1 inhibitor candidates.
From the obtained results it can be seen that rough
replacement of the naphthyl group by chromone, cou-
marin or quinoline moieties does not significantly impair
the inhibitory activity of the resulting derivatives since
the IC50 values for the whole series of compounds ran-
ged from 0.09 to 5 lM.
Furthermore, some of the new synthesized analogues
belonging to the coumarinyl series (6b,c) were found
to be more active than the corresponding Vertex
analogues. The most potent inhibitor 8a with an
IC50 of 0.093 lM is one log more active than its cor-
responding naphthyl analogue 3a (IC50 value of
1.7 lM).
In conclusion, the obtained data suggest that starting
from an initial structure (Vertex like analogues), there
is some latitude with regard to functional group identity
(coumarin, chromene and quinoline) that allows further
refinement of inhibitory potency relative to b-secretase
(BACE-1). BACE inhibitor 8a is at this moment a typi-
cal example of refined structure.
Moreover, the coumarinyl inhibitor 8a is linked to the
amide function through a methylene spacer. This obser-