J. Jin et al. / Bioorg. Med. Chem. Lett. 18 (2008) 5481–5486
5485
1) a, b, or c
O
O
2) d
H
N
H
N
H
N
H
N
N+
R2
NH
R1
N
N
H
O
O
O
O
DMHB
O
O
t-BuO
HO
16
17
Scheme 3. Reagents and conditions: (a) i—allyl bromide, CH3CN, rt; ii—3-hydroxybenzaldehyde, Na(OAc)3BH, 10% of HOAc in NMP, rt; (b) i—cyclopropylcarboxyaldehyde,
Na(OAc)3BH, 10% of HOAc in NMP, rt; ii—cyclopropylmethylbromide, CH3CN, 75 °C; (c) 1,6-hexyldibromide, CH3CN, 75 °C; (d) 50% of TFA in DCE, rt.
Table 6
We then further explored the quaternary ammonium salt moi-
ety. In addition to N-methyl quaternary ammonium salt 2a, other
quaternary ammonium salts exemplified by N-allyl ammonium
salt 16a had excellent M3 potency (pA2 = 10.0) and good subtype
selectivity (greater than 10-fold selective for M3 over M2 and
100-fold selective for M3 over M1) (Table 6). Symmetrical quater-
nary ammonium salts such as N,N-dicyclopropylmethyl compound
16b also possessed high M3 potency and good subtype selectivity
for M3 over M2 and M1, but were less potent compared to 2a and
16a. In addition, compound 16c, which possesses a spiro quater-
nary ammonium center, was less potent and less selective for M3
over M2 and M1 compared to 16b, but still showed good M3 po-
tency with a pA2 of 8.5. Symmetrical quaternary ammonium salts
exemplified by 16b and 16c eliminated the chiral center at the
quaternary ammonium nitrogen. Synthesis of compounds 16a,
16b, and 16c is outlined in Scheme 3. Resin-bound piperidine 17
was prepared according to Scheme 1. Reductive amination of inter-
mediate 17, alkylation of the resulting tertiary amines and subse-
quent resin cleavage and simultaneous removal of the tert-butyl
protecting group afforded the desired compounds 16a and 16b.
Compound 16c was prepared via alkylation of 17 with 1,6-hexyl-
dibromide, resin cleavage and protecting group removal.
Further optimization of the quaternary ammonium salt moiety
O
H
N
H
N
N+
R2
R1
N
H
O
O
O
HO
a
Compound
FLIPR pA2
M2
N+
R2
M3
M1
R1
N+
N+
2a
16a
16b
9.9
9.0
8.8
8.3
8.3
7.8
7.8
7.4
7.1
OH
10.0
9.2
OH
In summary, SAR exploration of multiple regions of the HTS hit
1a led to the identification of key structural motifs necessary for
achieving high M3 potency and good subtype selectivity. Further
optimization of this series resulted in highly potent M3 antagonists
such as 2a and 10b with greater than 100-fold subtype selectivity
for M3 over M1.
N+
N+
16c
8.5
Acknowledgments
a
Means of at least two determinations with standard deviation of < 0.3.
We thank Bing Wang for NMR, Qian Jin for LC/MS, and Carl
Bennett for purification.
similar M3 potency to isopropyl benzoate 2a. Further optimization
of the ester group resulted in cyclohexyl ester 10b, which was the
most potent M3 antagonist to date with a pA2 of 10.7. Compared to
2a, compounds 10a and 10b were slightly less selective for M3 over
M2, but maintained good subtype selectivity for M3 over M1.
Synthesis of compounds 7, 8, 9, 10a, and 10b is outlined in Scheme
2. Urea formation from resin-bound primary amine 11, prepared
according to Scheme 1, and commercially available 4-amino-N-iso-
propyl benzamide and anilines 13, 14, 15a, and 15b (vide infra),
followed by nosyl removal and reductive amination, afforded
resin-bound intermediates 12. Alkylation of tertiary amines 12,
followed by resin cleavage and simultaneous removal of the
tert-butyl protecting group, produced the desired quaternary
ammonium salts 7, 8, 9, 10a, and 10b. Anilines 13 and 14 were
synthesized in a 3-step sequence—formation of amidoximes from
nitriles and hydroxylamine,18 cyclization of amidoximes with acid
chlorides to form oxadiazoles,19 and reduction of the nitro group.20
2-Amino-5-thiophenecarboxylates 15a and 15b were prepared
from commercially available 2-nitro-5-thiophenecarboxylic acid
under standard ester formation and hydrogenation conditions.
References and notes
1. Caulfield, M. P.; Birdsall, N. J. M. Pharmacol. Rev. 1998, 50, 279.
2. Eglen, R. M. Prog. Med. Chem. 2005, 43, 105.
3. Hulmes, E. C.; Birdsall, N. J. M.; Buckley, N. J. Ann. Rev. Pharmacol. Toxicol. 1990,
30, 633.
4. Caulfield, M. P. Pharmacol. Ther. 1993, 58, 319.
5. Eglen, R. M. Auto. Autacoid Pharmacol. 2006, 26, 219.
6. Lee, A. M.; Jacoby, D. B.; Fryer, A. D. Curr. Opin. Pharmacol. 2001, 1, 223.
7. (a) Barnes, P. J. Thorax 1989, 44, 161; (b) Gater, P. J.; Alabastar, V. J.; Piper, I. Pul.
Pharmacol 1989, 2, 87.
8. Faulkner, D.; Fryer, A. D.; MacLagan, J. Br. J. Pharmacol. 1986, 88, 181.
9. Lammers, J. W. J.; Minnette, P. A.; Mc Custer, M.; Barnes, P. J. Annu. Rev. Respir.
Diseases 1989, 139, 446.
10. Fryer, A. D.; Adamko, D. J.; Yost, B. L.; Jacoby, D. B. Life Sci. 1999, 64, 449.
11. Jin, J.; Wang, Y.; Shi, D.; Wang, F.; Davis, R. S.; Jin, Q.; Fu, W.; Foley, J. J.; Webb, E.
F.; Dehaas, C. J.; Berlanga, M.; Burman, M.; Sarau, H. M.; Morrow, D. M.; Rao, P.;
Kallal, L. A.; Moore, M. L.; Rivero, R. A.; Palovich, M.; Salmon, M.; Belmonte, K.
E.; Busch-Petersen, J. J. Med. Chem. 2008, 51, 4866.
12. Measuring inhibition of acetylcholine-induced [Ca2+]i-mobilization in
Chinese Hamster Ovary (CHO) cells stably expressing human recombinant
M3 receptor.