G. Zischinsky et al. / Bioorg. Med. Chem. Lett. 20 (2010) 380–382
381
moiety like in 2 (Fig. 1).13 As commonly accepted, the reduction of
O
O
the number of rotatable bonds and molecular weight should result
in molecules with improved pharmacokinetic profiles.16
All pharmacophoric key elements of 1 and 2 are still present in
the newly designed compounds 3. Therefore these bicyclic scaf-
folds should exhibit potentially high integrin binding affinity.
Different strategies were applied to construct the bicyclic ring
system in compounds 3a–j. Briefly, the phenylalanine derivatives
4–9 were prepared by the reaction of mesitoyl chloride with the
corresponding, commercially available amino acid derivatives
(Scheme 1). The resulting intermediates were then reacted in cyclo
additions, carbon–carbon or nitrogen–carbon bond formation reac-
tions, Schemes 2–4, respectively, to yield the bicyclic core motives
3a–j.
a
b, c
b, c
OH
3d,f
NH
B
X
O
8
OH
X =S, O
O
O
X
O
13a X = O
13b X = S
O
O
O
d
3g
NH
7
S
S
O
14
Scheme 3. Reagents and conditions: (a) 8, Pd(PPh3)4, NaHCO3, DME/H2O, 80 °C; (b)
4-methyl-pyridin-2-ylamine, Ti(OiPr)4, NaHB(OAc)3, DCE, rt; (c) TFA; (d) 7, NaNO2,
HCl, H2O, 0 °C, then thiophene-3-carbaldehyde, CuCl2, dioxane, rt.
Beginning with the amino acid derivatives 4–6, the correspond-
ing aldehydes 10–12 were obtained by cyclo addition reaction. The
final compounds 3b, 3c, and 3e were obtained by reductive amina-
tion and the removal of the protecting groups.
integrin a5b1 as compared to the other selected integrins. In addi-
tion, the five-membered aromatic heterocycles in 3b–g and the
six-membered ring systems in rac-3h–j were shown to serve as
suitable scaffolds in the current synthetic strategy (Table 2).
As discussed, integrin selectivity of the hydroxypyrrolidine
The synthesis of compounds 3d and 3f, utilized a Suzuki cou-
pling17 while the carbon–carbon bond formation in compound 3g
was achieved via diazonium salt formation.18 The resulting alde-
hydes 13a, 13b, and 14 were transformed into the final compounds
3d, 3f, and 3g by applying the reaction sequence used in the forma-
tion of 3b, 3c, and 3e. In the preparation of compound 3a, the pre-
cursor 16 was prepared via 15 from itaconic acid.19 The
introduction of the 4-methyl-pyridin-2-ylamine group was per-
formed by alkylation followed by palladium catalyzed amidation20
to give compound 3a (Scheme 4).
based integrin
a5b1 antagonists, that is, 1, is mainly controlled
by the amide of the 2-amino acid moiety.12–15 Thus, the mesitoyl
amide was used in all derivatives of 3 and, as predicted, resulted
in excellent integrin selectivities of more than one order of
magnitude.
All 1,3-substituted five-membered aromatic heterocycles
showed good to excellent affinities for integrin a5b1, ranging from
1.5 nM (3c) to 79 nM (3d). However, the affinity of the 1,2-substi-
tuted derivative 3g drops apparently due to the different geometric
orientation of both substituents compared to the 1,3-substitution
pattern.
A comparison of the substitution vectors of planar five- and six-
membered ring systems revealed that the 10,30-substitution of the
five-membered ring orientated the 30-vector between the 3- and
4-vectors of the 1,3- and 1,4-substituted six-membered ring
(Fig. 2a). This orientation is supportive of the comparable affinities
observed for the meta- and para-substituted biphenyl derivatives
rac-3i and rac-3j.
For the synthesis of rac-3h, an Ulmann type reaction21 was ap-
plied. The resulting biphenyl derivatives rac-3i and rac-3j were
prepared by a Suzuki reaction of boronic acid rac-9 and the corre-
sponding aromatic bromides 23a and 23b (Scheme 4).
Pyrrolidinone derivative 3a possess low-nanomolar affinity and
more than two orders of magnitude higher selectivity towards
a
O
NH2
However, the affinities of the five-membered ring derivatives
3a, 3c, and 3e are significantly higher. Given that 1,3-substituted
X
NH
O
O
X
O
O
five-membered rings provide better scaffolds for integrin
a5b1
4: X = N3
5: X = C CH
6: X = CN
7: X = NH2
8: X = I
9: X = B(OH)2
antagonists than meta- or para-substituted six-membered rings,
it is reasonable to assume the same three dimensional orientation
of substituents can be achieved by means of a 1,4-substituted
cyclohexane scaffold in the chair conformation (Fig. 2b). Indeed,
the related piperidine based compound, rac-3h, proved to have
Scheme 1. Reagents and conditions: (a) mesitoyl chloride, DCM, NEt3, 0 °C.
high affinity for integrin
structure.
a5b1 and therefore provides a new core
Pharmacokinetic profile of selected compounds show low sys-
temic clearances (3d and 3e) and long terminal half-lifes after iv
administration in male Wistar rat (Table 3). Furthermore promising
oral bioavailabilities were also observed for these compounds.
In summary, the discovery of a series of new heterocycle based
integrin a5b1 antagonists is described. It was shown that the com-
bination of 2-pyridyl amino methyl substituted heterocycles (com-
pound 1) with a phenylalanine moiety (compound 2) results in a
O
O
a, b
c, d
3b
NH
NH
S
NC
O
O
O
N
O
O
O
6
10
O
e
c, d
c, d
NH
NH
3c
3e
N3
N
N N
O
O
O
O
O
O
O
O
4
5
11
new series of potent integrin
a5b1 scaffolds. In addition, it was
shown that both aromatic and non-aromatic five- and six-mem-
O
O
f, g, h
NH
NH
N O
O
O
12
The compounds were administered into two groups of male rats (4 animals in
each group) at 1 mg/kg iv (group 1) and 10 mg/kg. po (group 2). Eight blood samples
were taken from each animal. The aliquots of the plasma samples were transferred,
precipitated with methanol containing a structural analogue of the analyte as internal
standard and analyzed by HPLC–MS/MS for any taken time point to determine time/
concentration values. The pharmacokinetic parameters were calculated by using
WINNONLIN version 5.1.
Scheme 2. Reagents and conditions: (a) H2S, DIPEA, DMF, 50 °C; (b) 2-bromo-
malonaldehyde, dioxane, 60 °C; (c) 4-methyl-pyridin-2-ylamine, Ti(OiPr)4, NaB-
H(OAc)3, DCE, rt; (d) TFA; (e) 3,3-diethoxy-propyne, Cu(OAc)2, dioxane/H2O, 10%
HCl, rt; (f) chloro-hydroxyimino-acetic acid ethyl ester, NEt3, DCM, 0 °C; (g) NaBH4,
LiCl, THF/EtOH, 0 °C; (h) Dess–Martin-periodinane, DCM, rt.