Synthesis, SAR, and X-ray structure of novel potent DPPIV inhibitors: Oxadiazolyl ketones

Article abstract of DOI:10.1016/j.bmcl.2007.05.046

Synthesis of a novel series of DPPIV inhibitors with 1,2,4- and 1,3,4-oxadiazolyl ketone derivatives and its structure-activity relationships are discussed. Compound 18h showed good inhibitory activity against DPPIV and favorable pharmacokinetic properties. In vivo pharmacodynamic efficacy and co-crystal structure of compound 18h with DPPIV is also described.

Full text of DOI:10.1016/j.bmcl.2007.05.046

Bioorganic & Medicinal Chemistry Letters 17 (2007) 4167–4172
Synthesis, SAR, and X-ray structure of novel potent
DPPIV inhibitors: Oxadiazolyl ketones
Ki Dong Koo, Min Jung Kim, Sungsub Kim, Kyoung-Hee Kim, Sang Yong Hong,
Gwong-Cheung Hur, Hyeon Joo Yim, Geun Tae Kim, Hee Oon Han, O Hwan Kwon,
Tae Sik Kwon, Jong Sung Koh* and Chang-Seok Lee*
LG Life Sciences, Ltd/R&D Park, 104-1 Munji-dong, Yuseong-gu, Daejeon 305-380, Republic of Korea
Received 15 December 2006; revised 27 April 2007; accepted 17 May 2007
Available online 21 May 2007
Abstract—Synthesis of a novel series of DPPIV inhibitors with 1,2,4- and 1,3,4-oxadiazolyl ketone derivatives and its structure–
activity relationships are discussed. Compound 18h showed good inhibitory activity against DPPIV and favorable pharmacokinetic
properties. In vivo pharmacodynamic efficacy and co-crystal structure of compound 18h with DPPIV is also described.
Ó 2007 Elsevier Ltd. All rights reserved.
GLP-1 (glucagon-like peptide 1) has been intensively
studied as a treatment of type 2 diabetes mellitus
(T2DM).1 GLP-1 is an incretin hormone secreted by
intestinal L-cells in response to food intake. However
GLP-1 is rapidly degraded in plasma by the action of
dipeptidyl peptidase IV (DPPIV). Inhibition of DPPIV
has been reported to increase the level of GLP-12 which
increases pancreatic b-cell proliferation and survival.
For this reason, DPPIV inhibitors are emerging as
new potential drugs of T2DM,3 with Sitagliptin 14
already in the market. Extensive research has resulted
in a series of potent DPPIV inhibitors, and several
DPPIV inhibitors are under late-stage clinical develop-
ment including Vildagliptin 25 (Fig. 1).
A series of 1,2,4-oxadiazolyl ketone pyrrolidines was
synthesized according to Scheme 1. 1,2,4-Oxadiazolyl
ketone substituted compounds 10 were prepared from
hydroxy proline 3 as starting material (Scheme 1). Fluo-
rination of compound 3 using (diethylamino)sulfur tri-
fluoride (DAST) in the presence of dichloromethane
gave fluoropyrrolidine 4. Reduction of compound 4
with DIBAL-H, followed by cyanide addition, provided
cyanohydrin 5. Sequential treatment of cyanohydrin 5
with ethyl vinyl ether in the presence of PPTS and
hydroxyl amine produced amidoxime 6, which was
reacted with proper carboxylic acid ‘R1CO2H’ to give
5-alkyl-oxadiazol-3-yl pyrrolidine 7.7 Removal of eth-
oxyethyl group of pyrrolidine 7 in the presence of PPTS
and Dess–Martin oxidation of the resultant alcohol gave
1,2,4-oxadiazolyl ketone 8. Conversion of the tert-but-
oxy residue of 8 to a bromoacetyl bromide residue to
produce 9 was carried out by treatment with hydrochlo-
ric acid in ethyl acetate, followed by coupling reaction of
the deprotected pyrrolidine nitrogen with bromoacetyl
bromide. And the subsequent reaction of bromide 9 with
proper amine ‘R2NH2’ in the presence of potassium
carbonate gave the desired compounds 10a–i.
In a recent study, DPPIV inhibitors utilizing (S)-2-
cyanopyrrolidine as a key scaffold showed good activity
against dipeptidyl peptidase IV. Our search for a unique
scaffold led us to keto aryl pyrrolidines, which were used
as a common surrogate among serine protease inhibi-
tors.6 Same applied for DPPIV inhibitors.6b We report
the synthesis of oxadiazolyl ketone derivatives that are
highly effective and exhibit long-acting in vivo pharma-
codynamic activity.
The inhibitors 13a–13e were also prepared as shown in
Scheme 2. In this case, adamantyl amine groups were
first introduced from compound 4 in Scheme 1 as in
the usual method. Then compound 11 was converted
to compound 13 in a manner similar to Scheme 1.
1,3,4-Oxadiazolyl compounds 18a–i were synthesized
from ester 14 as shown in Scheme 3. Hydrolysis of the
Synthesis of the compounds listed in Tables 1–3 is out-
lined in Schemes 1–3.
Keywords: Oxadiazolyl ketones; DPPIV inhibitors.
*
Corresponding authors. Tel.: +82 42 866 2261; fax: +82 42 861
2566; e-mail: csleed@lgls.co.kr
0960-894X/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2007.05.046

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K. D. Koo et al. / Bioorg. Med. Chem. Lett. 17 (2007) 4167–4172
F
OH
F
NH2
O
N
N
N
N
N
H
F
N
O
N
CF3
1
2
Figure 1. Known DPPIV inhibitors.
Table 1. DPPIV inhibitors with 1,2,4-oxadiazolyl ketone
X
R2
N
N
H
N
O
O
O
N
R1
a
Compound
R2
X
R1
IC50 (lM)
DPPVIII
DPPIV
0.077
DPPII
5.47
Elastase
>500
10a
10b
10c
10d
10e
10f
10g
10h
10i
H
F
CH3
t-Bu
t-Bu
CH3
H
67.3
69.9
6.59
—
HO
HO
0.067
0.223
7.939
0.035
0.063
0.015
0.095
0.21
70.1
10
>400
—
H
H
H
F
HO
HO
—
—
O
0.20
35.1
<0.39
<0.39
—
1.45
5.72
<0.39
<0.39
—
—
F
t-Bu
338
>400
>400
—
H
H
H
p-Fluorophenyl
1-Methylcyclohexyl
Me
Adamantyl
a All IC50 were measured with human DPPIV enzyme.
proline ester 14, followed by coupling with N,O-di-
methyl hydroxylamine, gave Weinreb amide 15. After
preparation of Grignard reagent of compound 15a,7
Weinreb amide 15 was added to the resultant solution
to give 1,3,4-oxadiazolyl ketone 16. The compounds
18a–i were synthesized in a similar manner as shown
in Scheme 1 from compound 16.
mantyl substituent (10i). Also, selectivities against elas-
tase are excellent (at least >300 lM). In case where R2
is tert-butyl group, p-fluorophenyl (10g) and 1-meth-
ylcyclohexyl (10h) substituents at R1 provided less selec-
tivity compared to tert-butyl (10f) substituents.
Fluorinated pyrrolidine compound 10b displayed better
DPPII and DPPVIII selectivities than compounds 10a,c
which have no fluorine on pyrrolidine ring. When aro-
matic ring is substituted at R2 (10e) it showed good
DPPIV activity but exhibited unfavorable selectivity
against DPPII and DPPVIII enzymes. Compound 10b
(hydroxyl on tert-butyl substituent) and compound 10f
Biological evaluation of novel compounds in Tables 1–3
was done based on IC50 values for inhibition of DPPIV.
Generally in Table 1, DPPIV activities are good except
when R2 has carboxylic acid substituent (10d) or ada-

K. D. Koo et al. / Bioorg. Med. Chem. Lett. 17 (2007) 4167–4172
4169
Table 2. Adamantyl derivatives with 1,3,4-oxadiazolyl ketone
in Table 2 showed good activity and selectivity espe-
cially against DPPVIII.
X
B
In Table 2, adamantyl derivatives of 1,3,4-oxadiazolyl
substituted compounds 13a–e showed acceptable
DPPIV activities but displayed low selectivity against
DPPII and DPPVIII enzymes. For example, investiga-
tion of thiazolidine 13a, difluoropyrrolidine 13d, and
hydroxyadamantyl 13e led to insufficient inhibition over
DPPIV enzyme. Pyrrolidine 13b and cis-fluoropyrroli-
dine 13c showed good DPPIV inhibitory effect but rela-
tively lower selectivity against DPPII and DPPVIII.
These results suggested that the further optimization
should be concentrated on the modification of the ada-
mantyl substituent and of the 5-membered 1,3,4-
oxadiazole derivatives (see Table 2).
N
A
N
H
N
O
N
O
O
Me
Compound
A
B
X
Inhibition, IC50 (lM)
DPPIV DPPII DPPVIII
13a
13b
13c
13d
13e
H
S
—
H
0.224
0.084
3.24
2.07
2.67
1.25
10
5.8
H
C
C
C
C
0.808
1.44
3.26
4.4
H
F (cis) 0.053
H
OH
di-F
H
0.680
0.339
Most of the compounds in Table 3 showed good activity
against DPPIV and exhibited excellent selectivity against
elastase. A variety of substituents at R1 (18a–e) resulted
in no differences in DPPIV inhibition. While phenyl
(18c) or cyclohexyl substituents (18d) at R1 gave low
(no hydroxyl on tert-butyl substituent) showed similar
activity. However selectivity against DPPII and
DPPVIII is higher of compound 10b than that of com-
pound 10f. Most 1,2,4-oxadiazolyl substituted inhibitors
Table 3. DPPIV inhibitors with 1,3,4-oxadiazolyl ketones
A
R2
N
N
H
O
N
N
O
O
R1
Compound
R2
A
R1
Inhibition, IC50 (lM)
DPPIV
0.020
DPPII
DPPVIII
Elastase
222
18a
18b
18c
F
97.2
<0.39
H
H
t-Bu
0.056
0.017
>10
1.29
—
<0.39
<0.39
>400
18d
18e
18f
18g
18h
18i
F
0.030
0.010
0.255
0.047
0.022
0.025
<0.39
8.66
1.38
60
<0.39
<0.39
3.10
>400
238
H
H
H
F
Me
Me
—
HO
HO
t-Bu
t-Bu
t-Bu
0.93
>400
227.8
—
145.8
1.71
1.15
HO
F
F
1.09

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K. D. Koo et al. / Bioorg. Med. Chem. Lett. 17 (2007) 4167–4172
X
X
X
d, e
t-BuO
N
b, c
t-BuO
N
t-BuO
N
N
O
OH
O
O
O
OMe
CN
NH2
O
HO
Me
OEt
X = (R)-Hydroxy (3)
X = (S)-Fluoride (4)
6
5
a
X
X
t-BuO
N
O
g, h
t-BuO
N
f
N
R1
N
R1
O
O
O
N
O
Me
N
O
OEt
8
7
X
X
R2
i, j
k
N
O
N
Br
N
N
N
H
O
O
O
O
O
N
N
R1
R1
(X = H or F)
9
10
Scheme 1. Reactions and conditions: (a) DAST, CH2Cl2, À78 °C; (b) DIBAL-H, THF, 0 °C; (c) NaHSO3, NaCN, H2O, 0 °C; (d) ethyl vinyl ether,
PPTS, CH2Cl2; (e) NH2OHÆHCl, MeOH, NaHCO3, reflux; (f) R 1CO2H, CDI, DMF, 90 °C; (g) PPTS, EtOH, 40 °C; (h) Dess–Martin reagent,
CH2Cl2; (i) 4 N HCl in EtOAc; (j) BrCH2COBr, i-Pr2NEt, CH2Cl2, 0 °C; (k) R 2NH2, K2CO3, CH2Cl2.
X
X
X
B
B
B
c
a, b
N
N
N
A
A
N
A
N
N
Me
H
H
N
H
O
O
OMe
O
N
N
O
O
O
OMe
O
Me
13
A = OH or H,
B = C or S and X = H or F
11
12
Scheme 2. Reactions and conditions: (a) LiOH, MeOH, THF, H2O; (b) HNMe(OMe)ÆHCl, EDC, HOBt, Et3N, DMF; (c) n-BuLi, 2-methyl-1,3,4-
oxadiazole, MgBr2ÆOEt2, THF, À78 °C.
selectivities against DPPII and DPPVIII, bulkier sub-
stituents 20a or 20e at R1 exhibited high selectivity
against DPPII but low selectivity against DPPVIII. Sim-
ilar to compound 10e in Table 4, phenyl substituted
compound 18i at R2 also gave low selectivity. Fluori-
nated pyrrolidine compound 18h gave not only better
activity against DPPIV, but also displayed better DPPII
and DPPVIII selectivity than compounds 18f,g which
lack the fluorine on pyrrolidine ring.
dogs. Plasma clearance and volume of distribution of
compound 18h were much reduced in monkeys com-
pared to rats and dogs. Based on these data and long
half-lives in monkey, even though it is very difficult to
predict human PK profiles, 18h is expected for once-daily
dosing in human.
In pharmacodynamic studies, plasma DPPIV activity
of 18h was evaluated in Sprague–Dawley rats
(Fig. 2). Upon the oral administration of the com-
pound 18h, plasma DPPIV activity was kept sup-
pressed by more than 50% for 24 h after dosing
even at 0.03 mg/kg (Fig. 2). This long duration of
activity can be explained by the kon À koff rate and
X-ray crystallography.
Compound 18h was selected for pharmacokinetic
screening since it showed excellent activity and selectiv-
ity (Table 4). Compound 18h exhibited good pharmaco-
kinetic profiles across the species in rat, dog, and
monkey. Bioavailability of 18h was good especially on

K. D. Koo et al. / Bioorg. Med. Chem. Lett. 17 (2007) 4167–4172
4171
N
N
A
A
A
R1
O
a, b
17a
N
N
boc
N
boc
boc
N
Me
c
N
N
O
OMe
O
O
O
OMe
R1
15
14
16
A
A
f
R2
d, e
N
N
N
Br
N
N
H
O
O
N
N
O
O
O
O
R1
R1
A = H or F
17
18
Scheme 3. Reactions and conditions: (a) LiOH, MeOH, THF, H2O; (b) HNMe(OMe)ÆHCl, EDC, HOBt, Et3N, DMF; (c) n-BuLi, 17a, MgBr2ÆOEt2,
THF, À78 °C, then added 15; (d) HCl in EtOAc; (e) BrCH2COBr, Et3N, 0 °C; (f) R 2NH2, K2CO3, CH2Cl2.
Ser630. The activated carbonyl oxygen atom is stabi-
lized through the interaction with the hydroxyl group
of Tyr547. The proline is located in the S1 pocket
and its trans-4-fluoride atom makes close contacts with
the side chain of Tyr631. Two nitrogen atoms of the
oxadiazole group make hydrogen bonds to the side
chains of Arg125 and His740, respectively. The
nitrogen atom of the secondary amine group makes
hydrogen bonds with the side chains of Glu205 and
Glu206.
Table 4. Pharmacokinetic data of compound 18h
Rat
Dog
Monkey
Dosagea (mg/kg)
Dosageb (mg/kg)
Cmaxb (lg/ml)
Clp (ml/min/kg)
Vdss (ml/kg)
5
5
5
5
5
5
0.03
241
14027
249
39
0.64
71
0.21
18
4867
88
2421
278
35
b
t1/2 (min)
Bioavailability (%)
87
a iv.
b po.
The kinetics of DPPIV with 18h was measured using
BiaCore 300 (Table 5). It showed relatively small Kd,
which suggested the slow release of 18h from DPPIV.
Crystal structure of 18h with DPPIV is shown in
Figure 3.10 The ketone group of the compound 18h
makes a covalent bond with the hydroxyl group of
Tyr631
Tyr547
Vehicle
Tyr666
Tyr662
Plasma DP-IV with 18h
0.03mpk
0.1 mpk
1 mpk
Ser630
120.0
100.0
80.0
60.0
40.0
20.0
0.0
10mpk
Glu206
Glu205
Arg125
Figure 3. Crystal structure of DPPIV complexed with compound 18h.
˚
Compound 18h is shown in green; residues within 4 A of the
compound are labeled.9
0
5
10
15
20
25
30
Time (hour)
Table 5. Kinetics data of compound 18h
Inhibition from basal plasma DPP IV Activity (%)
Dose (mg/kg)
18h
1 hr after dosing
24 hr after dosing
0.03
0.1
1
66
76
82
82
46
50
68
74
Ka (1/Ms)
Kd (1/s)
KA (1/M)
KD (M)
Chi
2.31E+05
5.69EÀ05
4.05E+09
2.47EÀ10
0.165
10
Figure 2. Effects of 18h on plasma DPPIV activity in male Sprague–
Dawley rats.8

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K. D. Koo et al. / Bioorg. Med. Chem. Lett. 17 (2007) 4167–4172
3. Kim, D.; Wang, L.; Beconi, M.; Eiermann, G. J.; Fisher,
M. H.; He, H.; Hickey, G. J.; Kowalchick, J. E.; Leiting,
B.; Lyons, K.; Marsilio, F.; McCann, M.; Patel, R. A.;
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Ki of 18h is 0.247 nM calculated by Kd/Ka which is equal
to koff/kon.
A novel series of DPPIV inhibitors with oxadiazolyl
ketone have been discovered. They are highly potent
DPPIV inhibitors. Representative inhibitor 18h has
good potency against DPPIV, and good selectivity
against DPPII, DPPVIII, and elastase. The compound
18h also resulted in good pharmacokinetic properties
especially on monkeys compared to rats and dogs. It
has shown good pharmacodynamic profile in SD rats.
Further studies on compounds 18h will be reported
elsewhere.
5. (a) Villhauer, E. B.; Brinkman, J. A.; Naderi, G. B.; Burkey,
B. F.; Dunning, B. E.; Prasad, K.; Mangold, B. L.; Russell,
M. E.; Hughes, T. E. J. Med. Chem. 2003, 46, 2774; (b)
McIntyre, J. A.; Castan’ er, J. Drugs Future 2004, 29, 887.
6. (a) Eduards, P. D.; Wolannin, D. J.; Andisik, D. W.;
Davis, M. W. J. Med. Chem. 1995, 38, 76; (b) Ferraris, D.;
Ko, Y.-S.; Calvin, D.; Chiou, T.; Lautar, S.; Thomas, B.;
Wozniak, K.; Rojas, C.; Kalish, V.; Belyakov, S. Bioorg.
Med. Chem. Lett. 2004, 14, 5579.
Acknowledgments
We thank Dr. Hee Bong Lee for advice on synthesis. We
are also grateful to Dr. Sung-Joo E. Lee for reviewing
the manuscript.
7. Gerenal synthesis of 1,2,4-oxadiazole; Tracy, L. D; The-
odore, J. N; Diane, C; Denise, E. P; John, A. P. J. Bioorg.
Med. Chem. Lett. 1999, 9, 209.
8. Each animal received its oral dosing solution at time 0 h.
Vehicle was water. Blood samples were collected through
tail vein just before (time 0) and 1, 3, 6, 12, 24, and 48 h
after dosing. Data are means SE (n = 3).
9. Oefner, C.; D’Arcy, A.; Sweeney, A. M.; Pierau, S.;
Gardiner, R.; Dale, G. E. Acta Cryst. 2003, D59, 1206.
10. The coordinate will be deposited in the Protein Data
Bank: PDB ID.
References and notes
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