Optimization of the 4-aryl group of 4-aryl-pyridine glucagon antagonists: Development of an efficient, alternative synthesis

Article abstract of DOI:10.1016/S0960-894X(02)00143-9

A narrow structure-activity relationship was established for the 4-aryl group in 4-aryl-pyridine glucagon antagonists, with only small substituents being well-tolerated, and only at the 3′- and 4′-positions. However, substitution with a 2′-hydroxy group gave a ca. 3-fold increase in activity (e.g., 4′-fluoro-2′-hydroxy analogue 33, IC₅₀=190 nM). For efficient preparation of 2′-substituted phenylpyridines, a novel synthesis via pyrones and 4-methoxy-pyridines was developed.

Full text of DOI:10.1016/S0960-894X(02)00143-9

Bioorganic & Medicinal Chemistry Letters 12 (2002) 1303–1306
Optimization of the 4-Aryl Group of 4-Aryl-pyridine Glucagon
Antagonists: Development of an Efficient, Alternative Synthesis
Roger A. Smith,a,* Donald L. Hertzog,a Martin H. Osterhout,a Gaetan H. Ladouceur,a
a
Mary Korpusik,a MarkA. Bobko, J. Howard Jones,a Kathleen Phelan,a
Romulo H. Romero,a Thomas Hundertmark,a Margit L. MacDougall,b
James N. Livingstonb and William R. Schoena
aDepartment of Chemistry Research, Bayer Research Center, 400 Morgan Lane, West Haven, CT 06516, USA
bDepartment of Metabolic Disorders Research, Bayer Research Center, 400 Morgan Lane, West Haven, CT 06516, USA
Received 13 December 2001; accepted 14 February 2002
Abstract—A narrow structure–activity relationship was established for the 4-aryl group in 4-aryl-pyridine glucagon antagonists,
with only small substituents being well-tolerated, and only at the 30- and 40-positions. However, substitution with a 20-hydroxy
group gave a ca. 3-fold increase in activity (e.g., 40-fluoro-20-hydroxy analogue 33, IC50=190 nM). For efficient preparation of 20-
substituted phenylpyridines, a novel synthesis via pyrones and 4-methoxy-pyridines was developed. # 2002 Elsevier Science Ltd.
All rights reserved.
As the incidence of Type 2 diabetes increases toward
200 million cases worldwide by 2010,1 the search for
effective therapies continues in earnest along several
mechanistic strategies.2 For example, the peptide hor-
mone glucagon functions to trigger hepatic glucose
production, and a strong body of evidence has promp-
ted the pursuit of small-molecule glucagon receptor
antagonists for the treatment of Type 2 diabetes.3
Recently, we reported the discovery of 4-phenyl-pyri-
dines as a novel class of selective glucagon antagonists
[e.g., screening hit 1, human glucagon receptor (hGR)
binding affinity IC50=7 mM].4 Investigation of sub-
stituents on the pyridine ring led to significantly more
potent analogues such as 2 (hGR binding affinity
IC50=0.70 mM; cAMP production IC50=0.40 mM) and
3 (hGR binding affinity IC50=0.11 mM; cAMP produc-
tion IC50=0.065 mM).4 In this article, we describe the
optimization of the 4-aryl group in this compound ser-
ies, and the development of an effective, alternative
synthetic route to these analogues.
Preparation of analogues of 2 having a variety of 4-aryl
groups was achieved via application of the Hantzsch
dihydropyridine synthesis.5 Oxidation to the corre-
sponding pyridine diester,6 followed by mono-reduction
to ester–alcohol, oxidation to ester–aldehyde, Wittig
olefination, reduction and hydrogenation provided the
desired analogues in generally excellent overall yields
(Scheme 1).4
Following the Topliss sequential method to guide lead
optimization,7 the parent 4 and 4-chloro analogue 9
were first prepared, and were found to be somewhat less
potent as glucagon antagonists than the lead 2 (Table 1).8
The subsequent analogues prompted by the Topliss
paradigm (e.g., 11, 22, 12, or 13, 10, 14, 15, 27, 24)
provided no significant gain in potency, relative to the
parent (4). Pursuing a more extensive variety of ana-
logues, it was found that small substituents (fluoro,
*Corresponding author. Tel.: +1-203-812-2154; fax: +1-203-812-
6182; e-mail: roger.smith.b@bayer.com
0960-894X/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved.
PII: S0960-894X(02)00143-9

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R. A. Smith et al. / Bioorg. Med. Chem. Lett. 12 (2002) 1303–1306
Table 1. Glucagon antagonist activities8 of analogues of 2
Compd
X
% inhibition (20 mM)
IC50 (mM)
2
4
5
6
7
8
4-F
H
3-F
2-F
3,4-F2
2,4-F2
0.70
1.4
3.0
9.0
1.1
11
9
4-Cl
3-Cl
3,4-Cl2
2,4-Cl2
4-CH3
3-CH3
2-CH3
4-Et
4-iPr
4-Ph
0.8
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
49
39a
8
Scheme 1. Synthesis of analogues of 2.
1.0
chloro, and methyl) were well tolerated at the 40-posi-
tion (2, 9, and 13), but gave rise to significant losses in
activity when bonded at the 30- or 20-positions. A steep
reduction in activity was seen as substituent size
increased at the 40-position, from methyl to ethyl and
phenyl (13, 16, and 18). The substituent size limitations
were also found to apply to electron-withdrawing and
-donating substituents (e.g., 22–27). With these results
defining a rather tight binding pocket, it was gratifying
to find that substitution by hydroxy at the 20-position
(30) afforded a ca. 3-fold improvement over the parent
compound (4). Substitution at this position with amino,
representing another H-bond donor/acceptor, was also
quite well tolerated (31). A larger H-bond donor group
at this position, however, was not (32). Finally, the
beneficial effects of the 40-F and 20-OH substituents were
found to be additive, with analogue 33 exhibiting a fur-
ther increase in potency: hGR binding affinity
IC50=190 nM; cAMP production IC50=120 nM.
Although the factors responsible for the favorable 20-
OH substitution are not fully understood, consideration
of molecular models indicates that this group may
engage in hydrogen-bonding with the hydroxymethyl
moeity on the pyridyl ring. It is noteworthy that slight
displacement of the hydroxy group position gives rise to
considerably less potent compounds (34 and 35). Also,
the 20-hydroxy moiety can not act as a surrogate for the
3-hydroxymethyl group, as 36 is relatively inactive (11%
inhibition at 20 mM). Finally, it should be noted that the
20-substituted analogues (e.g., 30 and 33) are chiral
(racemates), by virtue of atropisomerism9 owing to
restricted rotation about the phenyl–pyridine bond.
46
33
42
34
2
2,3-(CH)4
3,4-(CH)4
3,4-(CH2)4
4-CF3
6
16
37
35
25
14a
37
48
27
24
3-CF3
4-SO2CH3
4-OCH3
3-OCH3
2-OCH3
4-OH
3-OH
2-OH
6.0
0.50
2.3
2-NH2
2-NHCOCH3
4-F-2-OH
2-CH2OH
4-F-3-CH2OH
13
44
0.19
19a
aThe E-pent-1-enyl rather than pentyl derivative was tested.8
quite problematic (Scheme 2). Indeed, the literature is
replete with examples of low-yielding Hantzsch syn-
theses utilizing 2-substituted benzaldehydes.10 We
therefore embarked upon an alternative route to diester
37, in which a pyrone would be converted to a 4-halo-
pyridine, and then undergo a nucleophilic displacement
by a carbon nucleophile such as an aryl Grignard
reagent.
At the outset of this work, a survey of the literature
revealed very few examples of pyrones having ester
groups at the 3,5-positions and any carbon substituents
at the 2,6-positions. Half of these examples had per-
fluoroalkyl groups at the 2- and/or 6-positions, and
none had the required 2,6-diisopropyl pattern. Synthetic
Identification of the favorable 20-hydroxy substituent
presented
Hantzsch reaction afforded high yields in most cases,
a new challenge, however. While the
synthesis of the diester 37 (a precursor to 33)4b was
Scheme 2. Hantzsch route to intermediate 37.

R. A. Smith et al. / Bioorg. Med. Chem. Lett. 12 (2002) 1303–1306
1305
procedures reported for this class include a process via
the anhydrous magnesium complex of diethyl acetone-
1,3-dicarboxylate11 or the condensation of copper ethyl
acetoacetate with phosgene.12 After investigation of a
variety of reaction conditions, we found that the puta-
tive magnesium complex of dimethyl-1,3-ace-
tonedicarboxylate could be formed in situ and then
converted to the 2,6-diisopropyl pyrone 38, by an effi-
cient, one-pot process in 64% yield13 (Scheme 3). In
related examples, the 2,6-dimethyl and 2,6-dicyclohexyl
analogues were prepared in similar yields, whereas the
2,6-di-tert-butyl and 2,6-diphenyl analogues were not
obtained in significant yield through this procedure.
Treatment of pyrone 38 with ammonia (gaseous or
aqueous) readily gave the 4-hydroxy-pyridine 39,14 from
which could be prepared the 4-chloro-pyridine 40.15
Unfortunately, however, treatment of 40 with aryl
Grignard reagents such as 41 did not provide the
desired coupled products.
In summary, variation of the 4-aryl group in 4-aryl
pyridine glucagon antagonists was investigated, and a
wide variety of substituents was found to give reduced
activity, as compared to the 40-fluorophenyl lead 2.
Substitution by a 20-hydroxy moiety was surprisingly
well tolerated (30), and the most potent analogue iden-
tified in this study was a 40-fluoro-20-hydroxy-phenyl
analogue (33, IC50=190 nM). To permit the efficient
preparation of such 20-substituted phenyl-pyridines, a
novel synthesis via pyrones and 4-methoxy-pyridines
was developed, that proceeds in high yield from readily
available starting materials. In a future article, we will
describe the effect of combining the optimized substitu-
tions on the 4-aryl group (such as in 33) and the
pyridine ring (such as in 3).
Acknowledgements
We would like to thank David Hartsough for compu-
tational studies, and members of the analytical
chemistry group for their valuable support.
We reasoned that an alternative leaving group on the
pyridine diester might facilitate the coupling reaction.
Noting that Meyers and colleagues have made extensive
use of the methoxy group as nucleofuge in their elegant
biaryl syntheses,16 we speculated that the methoxy
group might serve as an efficient leaving group in our
own system, and were encouraged by the fact that it
could be readily accessed from 39. Methylation of 39
proved straightforward,17 and, gratifyingly, warming
diester 42 with Grignard reagent 41 afforded phe-
nylpyridine 37 in 78% yield.18 Moreover, purification of
compound 37 involved only a simple trituration from
hexane to afford analytically pure product. Curiously,
we observed no addition of the Grignard reagent to the
methyl ester groups in 42, a consideration that had
caught our attention at the outset.
References and Notes
1. Amos, A. F.; McCarty, D. J.; Zimmet, P. Diabetic Med.
1997, 14, S7.
2. (a) Zhang, B. B.; Moller, D. E. Curr. Opin. Chem. Biol.
2000, 4, 461. (b) Cobb, J.; Dukes, I. Annu. Rep. Med. Chem.
1998, 33, 213.
3. (a) Livingston, J. N.; Schoen, W. R. Annu. Rep. Med.
Chem. 1999, 34, 189. (b) Connell, R. D. Exp. Opin. Ther. Pat.
1999, 9, 701. (c) Cascieri, M. A.; Koch, G. E.; Ber, E.;
Sadowski, S. J.; Louizides, D.; De Laszlo, S. E.; Hacker, C.;
Hagmann, W. K.; MacCoss, M.; Chicchi, G. G.; Vicario, P. P.
J. Biol. Chem. 1999, 274, 8694. (d) Madsen, P.; Knudsen, L. B.;
Wiberg, F. C.; Carr, R. D. J. Med. Chem. 1998, 41, 5150.
4. (a) Ladouceur, G. H.; Cook, J. H.; Doherty, E. M.;
Schoen, W. R.; MacDougall, M. L.; Livingston, J. N. Bioorg.
Med. Chem. Lett. 2002, 12, 461. (b) Schoen, W. R.; Ladou-
ceur, G. H.; Cook, J. H.; Lease, T. G.; Wolanin, D. J.;
Kramss, R. H.; Hertzog, D. L.; Osterhout, M. H. US Patent
6,218,431, 2001; Chem. Abstr. 2001, 134, 311111.
5. Jones, G. In Comprehensive Heterocyclic Chemistry: The
Structure, Reactions, Synthesis and Uses of Heterocyclic Com-
pounds; Katritzky, A. R., Rees, C. W., Booulton, A. J.,
McKillop, A., Eds.; Pergamon: New York, 1984; Vol. 2, Part
2A, p 482.
6. Pfister, D. Synthesis 1990, 689.
7. Topliss, J. G. J. Med. Chem. 1972, 15, 1006.
8. All data reported herein reflect purified and characterized
(1H NMR, MS) samples. The human glucagon receptor bind-
ing assay was carried out as decribed previously.4a Selected
compounds were also tested in a functional cell assay,4a and
were determined to be antagonists. For many of the analogues
listed in Table 1, the 3-(1-E-pent-1-enyl)-pyridyl derivatives
were also tested, and these were found to have glucagon
binding activities comparable to or very slightly less than those
of the corresponding 3-(1-pentyl)-pyridyl derivatives.
9. Lloyd-Williams, P.; Giralt, E. Chem. Soc. Rev. 2001, 30,
145.
10. (a) Alajarin, R.; Vaquero, J. J.; Garcia Navio, J. L.;
Alvarez-Builla, J. Synlett 1992, 297. (b) Loev, B.; Goodman,
M. M.; Snader, K. M.; Tedeschi, R.; Macko, E. J. Med. Chem.
Scheme 3. Alternative synthesis of intermediate 37.

1306
R. A. Smith et al. / Bioorg. Med. Chem. Lett. 12 (2002) 1303–1306
1974, 19, 956. (c) Weissberger, A.; Klingsberger, E. In The
Chemistry of Heterocyclic Compounds; Interscience: New
York, 1960; Vol. 14, Part 1, p 510.
11. (a) Yamato, M.; Kusunki, Y. Chem. Pharm. Bull. 1981,
29, 1214. (b) Lee, L. F.; Miller, M. L. Eur. Pat. Appl. 182,769,
1986; Chem. Abstr. 1986, 105, 190960.
well-stirred ice-water mixture (200 mL), and was then stirred
for an additional 30 min. Dilute aqueous HCl (10%, 125 mL)
was then slowly added, and the mixture stirred for 10 min. The
mixture was extracted with diethyl ether (3ꢁ), and the com-
bined organic phases were washed with brine, dried (MgSO4),
and rotary evaporated to provide the product as crystals (7.12
g, 84%): mp 110–111 C (EtOAc–hexane); H NMR (CDCl3,
300 MHz) d 4.0 (s, 6H), 3.0 (sept, 2H), 1.3 (d, 12H); GC/EIMS
m/z 313 (M+).
ꢀ
1
12. Conrad, M.; Guthzeit, M. Chem. Ber. 1886, 19, 19.
13. Dimethyl 2,6-diisopropyl-4-oxo-4H-pyran-3,5-dicarboxy-
late (38). To a mixture of dimethyl 1,3-acetone-dicarboxylate
(28.7 mL, 0.2 mol) in 200 mL of DCE (dichloroethane) was
added magnesium bromide (37.6 g, 0.4 mol), with stirring at
room temperature. The mixture was cooled to 0 ꢀC, anhydrous
pyridine (65 mL, 0.8 mol) was gradually added, and the mix-
ture was stirred for 30 min. Isobutyryl chloride (90 mL, 0.6
mol) was then added dropwise at 0 ꢀC, and then the mixture
was allowed to warm to room temperature as it was stirred
overnight. The mixture was again cooled to 0 ꢀC, 100 mL of
1 N HCl was gradually added, and the mixture was stirred for
30 min. The aqueous phase was separated and extracted with
DCE (2ꢁ). The combined organic phases were washed with
brine (2ꢁ), dried (MgSO4), and rotary evaporated to provide
43.3 g (73%) of crude product, which was recrystallized from
EtOAc–hexane to provide pure 38 as white needles (37.6 g,
16. Meyers, A. I.; Lutomski, K. A. J. Am. Chem. Soc. 1982,
104, 879.
17. Dimethyl 2,6-diisopropyl-4-methoxy-3,5-pyridine-dicarboxy-
late (42). To a mixture containing 39 (10.0 g, 33.9 mmol) in
acetone (100 mL) was added potassium carbonate (7.0 g, 50.8
mmol) at room temperature. The resultant mixture was stirred
as iodomethane (5.29 g, 2.3 mL, 37.3 mmol) was added. The
mixture was heated at reflux overnight and was then cooled to
rt. Most of the solvent was then removed under reduced pres-
sure. The resultant residue was diluted with EtOAc and then
washed with water. The organic phase was washed sequen-
tially with 10% hydrochloric acid, saturated aqueous sodium
bicarbonate, and brine, dried (Na2SO4), and concentrated
under reduced pressure to provide 42 as a white solid (10.0 g,
64%): mp 100–102 ꢀC; H NMR (CDCl3, 300 MHz) d 3.9 (s,
95%): H NMR (CDCl3, 300 MHz) d 3.9 (s, 6H), 3.8 (s, 3H),
1
1
6H), 3.1 (sept, 2H), 1.3 (d, 12H); GC/EIMS m/z 296 (M+).
14. Dimethyl 2,6-diisopropyl-4-hydroxy-3,5-pyridine-dicarboxy-
late (39). A solution of 38 (9.10 g, 30.7 mmol) in methanol
(100 mL) was cooled to 0 ꢀC, and then anhydrous ammonia
gas was bubbled into the solution for 5–10 min. The solution
was then stirred at 0 ꢀC for 3–6 h, during which time the pyr-
one was converted to the less polar pyridone, as monitored by
TLC. After complete conversion, the volatiles were removed
by rotary evaporation to provide 39 as a white powder (13.5 g,
92%): mp 157–158 ꢀC (EtOAc); 1NMR (CDCl3 300 MHz) d
11.85 (bs, 1H), 4.0 (s, 6H), 3.4 (sept, 2H, J=6.6 Hz), 1.2 (d,
12H, J=6.6 Hz); GC/EIMS m/z 295 (M+).
15. Dimethyl 2,6-diisopropyl-4-chloro-3,5-pyridinedicarboxy-
late (40). To a flaskcontaining phosphorus oxychloride (30
mL) was gradually added 39 (8.00 g, 27 mmol) with stirring,
followed by the dropwise addition of 2,6-lutidine (3.53 mL, 30
mmol). The solution was stirred at reflux (POCl3 bp 106 ꢀC)
overnight, and then the mixture was cooled to room tempera-
ture. The mixture was slowly and cautiously poured into a
2.9 (sept, 2H), 1.2 (d, 12H); GC/EIMS m/z 309 (M+).
18. Dimethyl 2,6-diisopropyl-4-(20-benzyloxy-40-fluoro-phenyl)-
3,5-pyridinedicarboxylate (37). A solution containing dimethyl
2,6-diisopropyl-4-methoxy-3,5-pyridinedicarboxylate (42) (52.0 g,
0.168 mol, 1 equiv) in THF (125 mL) was added to a 1.5 M
solution of 2-benzyloxy-4-fluorophenylmagnesium bromide
(150 mL, 0.269 mol, 1.6 equiv) at reflux. The reaction was
heated at reflux for 1 h. The mixture was then cooled to 0 ꢀC
and quenched by dropwise addition of saturated aqueous
ammonium chloride. The mixture was then extracted with
EtOAc and the combined organic layer was washed sequen-
tially with water, saturated aqueous ammonium chloride, and
brine. The organic layer was then dried (Na2SO4), and rotary
evaporated to provide a yellow solid. The solid was triturated
from hexane to provide pure 37 as a beige solid (62.5 g, 78%).
1H NMR (CDCl3, 300 MHz) d 7.20–7.40 (m, 5H), 7.08–7.18
(m, 1H), 6.59–6.78 (m, 2H), 5.02 (s, 2H), 3.51 (s, 6H), 3.17
(sept, 2H, J=6.6 Hz), 1.24–1.38 (m, 12H); FAB/LSIMS m/z
480 (MH+).