Imidazopiperidine amides as dipeptidyl peptidase IV inhibitors for the treatment of diabetes

Article abstract of DOI:10.1016/j.bmcl.2007.08.030

A series of substituted imidazopiperidine amides has been prepared and evaluated for inhibition of dipeptidyl peptidase IV (DPP-4). Substitution at the 1- and 3-positions produced increased selectivity for DPP-4 relative to DPP-8 and DPP-9. Compounds in this series had IC₅₀ values as low as 5.8 nM for inhibition of DPP-4.

Full text of DOI:10.1016/j.bmcl.2007.08.030

Bioorganic & Medicinal Chemistry Letters 17 (2007) 5853–5857
Imidazopiperidine amides as dipeptidyl peptidase IV inhibitors
for the treatment of diabetes
Ping Chen,a,* Charles G. Caldwell,a Robert J. Mathvink,a Barbara Leiting,b
Frank Marsilio,b Reshma A. Patel,b Joseph K. Wu,b Huaibing He,a Kathryn A. Lyons,a
Nancy A. Thornberryb and Ann E. Webera
aDepartment of Medicinal Chemistry, Merck Research Laboratories, PO Box 2000, Rahway, NJ 07065, USA
bDepartment of Metabolic Disorders-Diabetes, Merck Research Laboratories, PO Box 2000, Rahway, NJ 07065, USA
Received 13 July 2007; revised 10 August 2007; accepted 13 August 2007
Available online 19 August 2007
Abstract—A series of substituted imidazopiperidine amides has been prepared and evaluated for inhibition of dipeptidyl peptidase
IV (DPP-4). Substitution at the 1- and 3-positions produced increased selectivity for DPP-4 relative to DPP-8 and DPP-9. Com-
pounds in this series had IC50 values as low as 5.8 nM for inhibition of DPP-4.
Published by Elsevier Ltd.
Glucagon-like peptide-1 (GLP-1) and glucose-depen-
dent insulinotropic peptide (GIP) are important incretin
hormones released from intestine in response to nutrient
ingestion. They both stimulate insulin secretion in a glu-
cose-dependent manner. In addition, GLP-1 also sup-
presses glucagon secretion, slows gastric emptying, and
reduces food intake, while stimulating pancreatic b-cell
growth.1,3 Continuous subcutaneous infusion of GLP-
1 in diabetic patients for six weeks resulted in reduction
of blood glucose and hemoglobin A1c levels.2
Among the structurally novel b-amino acid-based DPP-
4 inhibitors reported from these laboratories, the triazol-
opyrazine sitagliptin phosphate (1), also called Januvia,
is a potent, selective, orally active DPP-4 inhibitor
recently approved for treatment of type 2 diabetes.4 In
a related series of piperazine derivatives, represented
by structures 2–4, introduction of a 2-methyl substituent
improved DPP-4 potency by more than two fold, while
the benzyl group of compound 4 gave a 27-fold increase
GLP-1 and GIP are rapidly inactivated in the circula-
tion by the enzyme dipeptidyl peptidase IV (DPP-4), a
serine peptidase which cleaves an N-terminal dipeptide
from polypeptides having a proline or alanine residue
at the P1 site.1 Inhibition of DPP-4 prevents the degra-
dation of GLP-1 and GIP, resulting in enhancement of
their physiological effects.1,3 Clinical studies have shown
that small molecule DPP-4 inhibitors reduce both pran-
dial and fasting glucose levels significantly, leading to
lowering of hemoglobin A1c levels in type 2 diabetics.3
Thus, DPP-4 inhibition has emerged as a new therapeu-
tic approach for treatment of type 2 diabetes.
F
F
NH2
O
R
NH2
O
N
N
N
N
F
NH
F
N
·H3PO4
CF3
DPP-4 IC50
1 sitagliptin phosphate
DPP-4 IC50 = 0.018 μM
2
3
4
R = H
R = CH3 1.6
R = Bn 0.139 μM
3.7
μM
μM
NH2
O
N
N
F
N
H
Keywords: Dipeptidyl peptidase IV inhibitors; Diabetes; Glucagon-like
peptide-1 (GLP-1); Incretin hormones; Imidazopiperidine amides.
5
DPP-4 IC50 = 0.97 μM
*
Corresponding author. Tel.: +1 732 594 4263; fax: +1 732 594
5790; e-mail: ping_chen@merck.com
Figure 1. b-Amino acid DPP-4 inhibitors.
0960-894X/$ - see front matter Published by Elsevier Ltd.
doi:10.1016/j.bmcl.2007.08.030

5854
P. Chen et al. / Bioorg. Med. Chem. Lett. 17 (2007) 5853–5857
NH2
N
N
nopyridine (6) and 3-amino-4-(methylamino)pyridine
(7).8 Treatment with trifluoroacetic acid as described
by Threadgill yielded the 2-(trifluoromethyl)imidazo-
pyridines 8 and 9.9 Hydrogenation then generated imi-
dazopiperidine intermediates 12 and 13. Protection of
the piperidine nitrogen, alkylation on the imidazole ring,
and removal of the Boc protecting group produced 1-
and 3-alkyl imidazopiperidines 13–16, and 17–20.
c
a or b
N
HN
N
R'
R'
N
R
N
R
NHR
6 R = H
7 R = CH3
8 R = H, R' = CF3
9 R = CH3, R'= CF3
10 R = CH3, R'= c-Pr
11 R = CH3, R'= t-Bu
12 R = H, R' = CF3
13 R = CH3, R' = CF3
R
N
Synthesis of the 2-cyclopropyl and 2-tert-butyl substi-
tuted imidazopyridines 10 and 11 was accomplished by
acylation of 7 followed by acid-catalyzed cyclization of
the intermediate amides (Scheme 1). These derivatives,
as well as the 2-trifluoromethyl analogs described above,
were used in acylpyridinium reactions10 to produce the
4-substituted 4,5-dihydro intermediates 21–27 (Scheme
2). Treatment with potassium tert-butoxide converted
the N-phenoxycarbonyl group of these intermediates
into a Boc group prior to hydrogenation.11 In most
cases, enantiomers of the Boc-protected hydrogenation
products were separated using a chiral HPLC column.
Finally, removal of the N-Boc group using methanolic
HCl gave imidazopiperidine intermediates 28–34. An
early attempt to synthesize 28 using a one-pot hydroge-
nation and deprotection of the N-Cbz intermediate 35
resulted in formation of aromatization product 36. Imi-
dazopyridine 36 was later converted into 4,4-dimethyl
analog 37 following a reaction route similar to that used
to prepare imidazopiperidine 28 from intermediate 9.
N
d-f
HN
HN
12
+
CF3
CF3
N
N
R
13 R = CH3
14 R = C2H5
15 R = i-Bu
17 R = CH3
18 R = C2H5
19 R = i-Bu
20 R = 4-F-Bn
16 R = 4-F-Bn
Scheme 1. Reagents and conditions: (a) TFA, reflux (R0 = CF3); (b)
R0COCl, Et3N, CH2Cl2, then TsOH, xylene (R0 = t-Bu) or AcOH
(R0 = c-Pr); (c) H2 (1000 psi), PtO2, EtOH, 40 °C; (d) (Boc)2O,
CH2Cl2; (e) R-I, NaH, THF; (f) HCl, CH3OH.
in potency.5 The first imidazopiperidine analog we pre-
pared was the unsubstituted derivative 5, which showed
modest potency as a DPP-4 inhibitor (Fig. 1). We initi-
ated an investigation of the effect of substitution in this
series of heterocyclic compounds. The work described
here involves the synthesis and biological evaluation of
imidazopiperidine-based b-amino acid derivatives.
Introduction of a trifluoromethyl group at the 4-posi-
tion was accomplished by a condensation reaction of
histamine (38) with trifluoroacetaldehyde ethyl hemiace-
tal (Scheme 3).12 The mono-protected intermediate 39
was generated by reaction with 2 equiv of di-tert-butyl
dicarbonate, followed by treatment with ammonia to
remove the more reactive imidazole Boc group. Methyl-
ation, HPLC separation, and deprotection gave the
enantiomers of intermediate 42.
Determinations of IC50 values for inhibition of DPP-4
and quiescent prolyl peptidase (QPP/DPP-II) were car-
ried out as described previously.6 Compounds were also
tested against DPP-8 and DPP-9, two enzymes which
have been associated with significant toxicity in animal
studies.7
An initial synthetic route to imidazopiperidine interme-
diates having substitution at the 1-, 2- or 3-position is
depicted in Scheme 1. Chemistry began with 3,4-diami-
Synthesis of imidazopiperidines 47 and 48 via 3,4-diam-
inopiperidine intermediate 46 is outlined in Scheme 4.
Epoxidation of 1,2,3,6-tetrahydropyridine derivative
4313 followed by opening with sodium azide afforded
O
R'
R'
N
N
a
N
b-e
N
N
N
N
R
PhO
N
HN
R
R
CH3
CH3
CH3
CF3
·2HCl
9 R = CF3
10 R = -Pr
21 R = CF3, R' = CH3
22 R = CF3, R' = C2H4
28 R = CF3, R' = CH3
29 R = CF3, R' = C2H4
N
N
a-c
H2N
d
Boc
N
N
c
N
11 R =
t-Bu
23 R = CF3, R' =
24 R = CF3, R' = Bn,
25 R = CF3, R' = 4-F-Bn
c
-Pr
30 R = CF3, R' =
31 R = CF3, R' = Bn,
32 R = CF3, R' = 4-F-Bn
c-Pr
H
H
38
39
33 R =
34 R =
c
-Pr, R' = CH3
26 R =
27 R =
c-Pr, R' = CH3
f
t-Bu, R' = CH3
t-Bu, R' = CH3
CF3
CF3
CH3
CF3
CH3
Boc
CH3
N
Boc
N
N
CH3 CH3
e, f
N
N
N
N
HN
+
Cbz
N
N
c
N
N
N
HN
N
N
CF3
CF3
CF3
CH3
N
CH3
N
CH3
CH3
40
41
42
CH3
35
36
37
Scheme 3. Reagents and conditions: (a) CF3CH(OH)OC2H5, NaOH,
H2O, reflux; (b) 2 equiv (Boc)2O, CH2Cl2, THF; (c) NH3, CH3OH; (d)
CH3I, KN(TMS)2, THF, ꢀ20 °C; (e) separate isomers; (f) HCl,
CH3OH.
Scheme 2. Reagents: (a) PhOCOCl, R0MgX, THF; (b) t-BuOK, THF;
(c) H2, 10% Pd/C, EtOH; (d) HPLC resolution; (e) HCl, CH3OH; (f)
Cbz–Cl, CH3MgBr, THF.

P. Chen et al. / Bioorg. Med. Chem. Lett. 17 (2007) 5853–5857
5855
Boc
Boc
OH
N3
Boc
N3
opiperidine intermediates 50 was successful, providing
DPP-4 inhibitors 51 after removal of the Boc group.
For the less reactive imidazopiperidines leading to
gem-dimethyl derivative 66 and the 4-trifluoromethyl
derivatives 67a–b, it was preferable to activate acid 49
as a mixed anhydride with pivaloyl chloride prior to
condensation with 50 in the presence of 4-(dimethyl-
amino)pyridine and N-methylmorpholine in dichloro-
methane (Scheme 5).
a, b
N
N
N
+
OH
43
44
45
Boc
NH2
NH2
Boc
N
N
N
N
c-e
f, g
R
44
H
Table 1 lists data for an initial series of DPP-4 inhibitors
having substitution on the imidazole ring. While all of
these compounds have submicromolar IC50 values as
DPP-4 inhibitors, relatively little variation in potency
is observed. It is noteworthy that compounds 52 and
61, the two examples lacking N-substitution on the
imidazole ring, show the lowest selectivity for DPP-4
over DPP-8 and DPP-9. i-Butyl and 4-fluorobenzyl
groups at the 3-position produce high levels of selectivity
(e.g., 58 and 60), but these analogs are slightly less po-
tent as DPP-4 inhibitors. The 1-methyl derivative 53,
with its balance of activity and selectivity as a DPP-4
inhibitor, was selected for further modification to exam-
ine the effects of substitution at the 4-position.
46
47 R = Ph
48 R = 4-F-Ph
Scheme 4. Reagents and condition: (a) m-CPBA, CH2Cl2; (b) NaN3,
NH4Cl, EtOH, H2O; (c) MsCl, Et3N, CH2Cl2; (d) NaN3, DMF; (e) H2,
10% Pd/C, EtOH; (f) PhC(@NH)OEt HCl or 4-FC6H4C(@NH)SMe
HCl, EtOH, reflux; (g) (COCl)2, DMSO, Et3N, CH2Cl2.
F
X
NHBoc
CO2H
F
F
49
X
R5 R4
O
a, b
R3
NH2
N
N
R2
R5 R4
R3
N
Table 2 displays enzyme assay data for 1-methylimidaz-
opiperidine derivatives having substitution at 2- and 4-
positions. For diastereomeric isomers 63a–b, 65a–b,
and 67a–b to 72a–b, one stereoisomer is 2- to 130-fold
more potent than the other as a DPP IV inhibitor. X-
ray crystal structure determination of these active dia-
stereomers was not conducted in this series. However,
a related analog having 2-substituted triazolopiperazine
subunit on the right-hand side was studied and the abso-
lute stereochemistry of 2-methyl triazolopiperazine
derivative was determined by X-ray crystal structure,18
in which R-isomer (IC50 = 25 nM) is 120-fold more po-
tent than S-isomer (IC50 = 274 nM). We assume that
the active stereoisomer of 63b, 65b, 67b–72b have (R)
configuration while 63a, 65a, 67a–72a have (S) stereo-
chemistry. The 2,4,5-trifluorophenyl analog 64 is the
R-diastereomer, prepared from the same enantiomer of
F
N
R1
51
HN
R2
N
R1
50
Scheme 5. Reagents: (a) EDC, HOBt, Et3N, DMF; (b) HCl, CH3OH.
separable azido alcohols 44 and 45. Conversion of 44
into the corresponding mesylate,14 displacement by
azide, and hydrogenation afforded diamine 46. Cycliza-
tion with benzimidate (for R = Ph)15 or thiobenzimidate
(for R = 4-C6H4F)16 produced an imidazoline which was
oxidized to the imidazole using Swern conditions.17
Finally, in most cases EDC-mediated coupling of Boc-
protected b-amino acids 49 (X = H or F) with imidaz-
Table 1. Substitution on the imidazole ring
F
R3
NH2
O
N
N
R2
F
N
R1
Compound
R1
R2
R3
DPP-4 IC50 (lM)
QPP IC50 (lM)
DPP-8 IC50 (lM)
DPP-9 IC50 (lM)
52
53
54
55
56
57
58
59
60
61
62
H
CF3
CF3
CF3
CF3
CF3
CF3
CF3
CF3
CF3
Ph
—
—
0.14
0.15
0.13
0.18
0.23
0.22
0.26
0.25
0.25
0.13
0.28
>100
92
80
9.4
68
51
>100
50
CH3
—
CH3
—
50
58
C2H5
—
>100
76
49
>100
63
C2H5
—
77
54
i-Bu
—
>100
>100
>100
>100
9.4
i-Bu
—
85
64
>100
23
4-F–Bn
—
4-F–Bn
—
—
56
33
>100
6.6
26
H
CH3
4-C6H4F
58
73

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P. Chen et al. / Bioorg. Med. Chem. Lett. 17 (2007) 5853–5857
Table 2. Substitution at the 4-position
F
X
R2 R3
O
NH2
N
N
N
R1
F
CH3
Compound
X
R1
R2
R3
DPP-4 IC50 (lM)
QPP IC50 (lM)
DPP-8 IC50 (lM)
DPP-9 IC50 (lM)
63a
63b
64
H
H
F
F
F
F
F
F
F
F
F
F
F
F
F
F
F
F
CF3
CF3
CF3
CF3
CF3
CF3
H
CH3
CH3
CH3
C2H5
C2H5
CH3
CF3
H
H
H
H
H
CH3
H
H
H
H
H
H
H
H
H
H
H
H
0.19
>100
>100
39
64
77
>100
>100
>100
>100
89
0.065
0.045
0.61
58
>100
55
65a
65b
66
93
45
0.060
0.57
0.38
28
76
4.0
>100
39
15
67a
67b
68a
68b
69a
69b
70a
70b
71a
71b
72a
72b
>100
>100
>100
>100
ND
H
CF3
c-Pr
0.050
0.34
0.11
38
CF3
CF3
c-Pr
c-Pr
t-Bu
t-Bu
CF3
CF3
CF3
CF3
>100
37
90
>100
85
65
c-Pr
CH3
CH3
CH3
CH3
Bn
0.49
0.25
30
37
33
80
ND
ND
0.63
0.37
19
46
21
ND
0.57
100
70
>100
>100
>100
>100
Bn
4-F–Bn
4-F–Bn
0.0084
0.78
0.0058
20
39
100
46
15
Table 3. Pharmacokinetic properties in rats
67b showed a decrease in oral bioavailability and no
improvement in other parameters. The more potent ben-
zyl derivative 71b had similar pharmacokinetic
properties.
64
67b
71b
Dosage
iv (mg/kg)
po (mg/kg)
1
2
1
2
1
2
In conclusion, a series of substituted imidazopiperidine
amides has been prepared and evaluated as DPP-4
inhibitors. Substitution at the 1- and 3-positions pro-
duced increased selectivity for DPP-4 relative to DPP-
8 and DPP 9, although improved potency against DPP
4 was not observed. Introduction of a substituent at
the 4-position of the imidazopiperidine unit significantly
improved DPP-4 inhibition, resulting in compounds
with IC50 values as low as 5.8 nM.
Clp (ml/min/kg)
Vdss (L/kg)
t1/2 (h)
81
93
64
8.0
1.2
0.098
40
8.3
1.4
0.050
16
6.5
1.5
0.072
16
Cmax (po dosage, lM)
Oral bioavailability (%)
piperidine 28 used in the synthesis of the 2,5-difluor-
ophenyl compound 63b. Relative to 63b, the 2,4,5-triflu-
orophenyl analog 64 shows a small increase in potency
as a DPP-4 inhibitor, but is less selective versus QPP.
Introduction of an ethyl or cyclopropyl group at the
4-position (samples 65b and 68b) does not result in an
improvement relative to the 4-methyl derivative 64
(DPP-4 IC50 = 45 nM). A benzyl or 4-fluorobenzyl sub-
stituent at the 4-position (samples 71b and 72b), how-
ever, produces compounds having DPP-4 IC50 values
below 10 nM. The 4,4-dimethyl compound 66 shows in-
creased potency against DPP-8 and DPP-9 while losing
activity against DPP-4. The 4-trifluoromethyl analog
67b is a DPP-4 inhibitor with an IC50 value of 50 nM.
Acknowledgments
The authors thank Dr. Joseph Laquidara, Mr. Glenn
Reynolds, Mr. Joseph Simeone, and Mr. Frederick
Wong for large-scale preparation of several intermedi-
ates used in this work.
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