Synthesis and characterization of metabolites and potential impurities of the antiulcerative drug tenatoprazole

Article abstract of DOI:10.1080/00397910801986192

Tenatoprazole (Ulsacare) is a recently developed antiulcerative drug used for the treatment of both erosive and nonerosive gastroesophageal reflux disease. During the bulk synthesis of tenatoprazole, we have observed four impurities (tenatoprazole N-oxide, tenatoprazole sulfone N-oxide, N-methyl tenatoprazole, and desmethoxy tenatoprazole) and two metabolites (tenatoprazole sulfide and tenatoprazole sulfone). The present work describes the synthesis and characterization of these impurities. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/00397910801986192

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Synthesis and Characterization of Metabolites
and Potential Impurities of the Antiulcerative
Drug Tenatoprazole
Ganta Madhusudhan Reddy a b , V. V. N. K. V. Prasada Raju a , J. Moses Babu
a , Ch. Praveen a , Mayur Khunt a , K. Mukkanti b & Padi Pratap Reddy a
a Research and Development API, Innovation Plaza, Dr. Reddy's Laboratories
Ltd., Hyderabad, Andhra Pradesh, India
b Institute of Science and Technology, J. N. T. University, Hyderabad, India
Published online: 09 May 2008.
To cite this article: Ganta Madhusudhan Reddy , V. V. N. K. V. Prasada Raju , J. Moses Babu , Ch. Praveen ,
Mayur Khunt , K. Mukkanti & Padi Pratap Reddy (2008): Synthesis and Characterization of Metabolites and
Potential Impurities of the Antiulcerative Drug Tenatoprazole , Synthetic Communications: An International
Journal for Rapid Communication of Synthetic Organic Chemistry, 38:11, 1725-1736
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Synthetic Communicationsw, 38: 1725–1736, 2008
Copyright # Taylor & Francis Group, LLC
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910801986192
Synthesis and Characterization
of Metabolites and Potential Impurities
of the Antiulcerative Drug Tenatoprazole
Ganta Madhusudhan Reddy,1,2 V. V. N. K. V. Prasada Raju,1
J. Moses Babu,1 Ch. Praveen,1 Mayur Khunt,1 K. Mukkanti,2
and Padi Pratap Reddy1
1Research and Development API, Dr. Reddy’s
Laboratories Ltd., Hyderabad, Andhra Pradesh, India
2Institute of Science and Technology, J. N. T. University, Hyderabad,
India
Abstract: Tenatoprazole (Ulsacarew) is a recently developed antiulcerative drug used
for the treatment of both erosive and nonerosive gastroesophageal reflux disease.
During the bulk synthesis of tenatoprazole, we have observed four impurities (tenatopra-
zole N-oxide, tenatoprazole sulfone N-oxide, N-methyl tenatoprazole, and desmethoxy
tenatoprazole) and two metabolites (tenatoprazole sulfide and tenatoprazole sulfone).
The present work describes the synthesis and characterization of these impurities.
Keywords: Impurities, metabolites, sulfide and sulfone, synthesis, tenatoprazole
INTRODUCTION
Proton pump inhibitors (PPIs) are now widely used for the treatment of
both erosive and nonerosive gastro-esophageal reflux disease.[1] Current
PPIs used clinically are substituted pyridylmethylsulfinyl benzimidazole
Received October 12, 2007
DRL Communication No. IPDO-IPM-000091.
Address correspondence to Padi Pratap Reddy, Research and Development API,
Innovation Plaza, Dr. Reddy’s Laboratories Ltd., Survey Nos. 42, 45, 46 & 54, Bachu-
pally, Quthubullapur Mandal, Hyderabad 500 072, Andhra Pradesh, India. E-mail:
prataprp@drreddys.com
1725

1726
G. M. Reddy et al.
prodrugs. Tenatoprazole, 5-methoxy-2-(4-methoxy-3,5-dimethyl-pyridin-2-
ylmethanesulfinyl)-1H-imidazo[4,5-b]pyridine (1), is a novel proton pump
inhibitor, which structurally resembles omeprazole. Tenatoprazole belongs
to the class of covalent proton pump inhibitors and inhibits gastric (Hþ,
Kþ)-ATPase with potency similar to that of omeprazole. However, the anti-
secretary and antiulcer effects of tenatoprazole were reported to be two to
four times more potent than those of omeprazole with long-lasting effects
on gastric acid secretion.[1–3] In human studies, tenatoprazole was more
potent than esomeprazole. Particularly, the pH . 4 holding time was
higher during the night for tenatoprazole than for esomeprazole.[4]
The presence of impurities in an active pharmaceutical ingredient
(API) can have a significant impact on the quality and safety of the
drug products. Therefore, it is necessary to study the impurity profile of
the API to be used in the manufacturing of a drug product. International
Conference on Harmonization (ICH) guidelines recommend identifying
and characterizing all impurities that are present at a level of 0.10%
or more.[5,6] In this context, a comprehensive study was undertaken to
synthesize and characterize the following four impurities:[7] 5-methoxy-2-
(4-methoxy-3,5-dimethyl-1-oxide-2-pyridinylmethanesulfinyl)-1H–imidazo [4,
5-b]pyridine (tenatoprazole N-oxide, 2); 5-methoxy-2-(4-methoxy-3,5-
dimethyl-1-oxide-2-pyridinylmethanesulfonyl)-1H-imidazo[4,5-b]pyridine
(tenatoprazole sulfone N-oxide, 3); 5-methoxy-2-(3,5-dimethylpyridin-2-
ylmethanesulfinyl-1H-imidazo[4,5-b]pyridine (desmethoxy tenatoprazole,
4); and 5-methoxy-2-(4-methoxy-3,5-dimethylpyridin-2-ylmethanesulfinyl-
1-methyl-1H-imidazo[4,5-b]pyridine (N-methyl tenatoprazole, 5); and two
metabolites, 5-methoxy-2-(4-methoxy-3,5-dimethyl-2-pyridinylmethanethio)-
1H–imidazo[4,5-b]pyridine (tenatoprazole sulfide, 6) and 5-methoxy-2-(4-
methoxy-3,5-dimethyl-pyridin-2-ylmethanesulfonyl)-1H-imidazo[4,5-b]pyridine
(tenatoprazole sulfone, 7). Results are presented in this article. Except for
sulfide 6 and sulfone 7, which were earlier reported as metabolites,[8]
other impurities 2, 3, 4, and 5 are hitherto not known.
RESULTS AND DISCUSSION
Synthesis of tenatoprazole 1 involves condensation of chloro methyl pyridine
derivative 8 with the mercapto compound 9 in the presence of NaOH and
oxidation of the resulting compound 6 with m-CPBA (Fig. 1).
5-Methoxy-2-(4-methoxy-3,5-dimethyl-1-oxide-2-pyridinylmethane
sulfinyl)-1H-imidazo[4,5-b]pyridine (Tenatoprazole-N-oxide, 2)
Preparation of tenatoprazole-N-oxide commenced from 3,5-dimehtyl-2-
chloromethyl pyridine hydrochloride (8). Treatment of 8 with alkali and

Metabolites and Impurities in Tenatoprazole
1727
Figure 1. Synthesis of tenatoprazole.
subsequent oxidation using meta-chloro perbenzoic acid (m-CPBA) gave
the corresponding N-oxide 11. Condensation of 11 with mercapto deriva-
tive 9 in aqueous alkali followed by m-CPBA oxidation of resulting
sulfide 12 furnished the desired N-oxide impurity 2 in 72% yield
(Fig. 2). The protonated molecular ion of 2 appeared as the base peak at
m/z 363.1 and sodium adduct at 385.0 in the EI spectrum. High-resolution
mass spectrometry (m/z calcd. for C16H18N4O4SNa (Mþ þ Na): 385.0946;
found: 385.0943), IR (NH 3448 cm21, S55O 1064 cm21, arylalkylether
1
1030 cm21), and H NMR (d ppm, 13.8, s, NH; 3.66, s, 3H, 3.92, s, 3H,
O-CH3, 2.09, s, 3H, py-CH3 and 2.21, s, 3H, py-CH3) further support the
assigned structure of 2.
Figure 2. Synthesis of tenatoprazole-N-oxide.

1728
G. M. Reddy et al.
Figure 3. Synthesis of tenatoprazole sulfone N-oxide.
5-Methoxy-2-(4-methoxy-3,5-dimethyl-1-oxide-2-pyridinmethane
sulfonyl)-1H-imidazo[4,5-b]pyridine (Tenatoprazole Sulfone
N-oxide, 3)
Tenatoprazole sulfide 6 on treatment with 3 equiv. of m-CPBA in chloroform
smoothly afforded tenatoprazole sulfone N-oxide 4 (Fig. 3). The sodium
adduct appeared as the base peak at m/z 401.0. HRMS (m/z calcd. for
C16H19N4O5S (Mþ þ H): 379.1076; found: 379.1062) is consistent with the
assigned structure of 1. IR spectrum showed the presence of NH (3429
cm21), O55S55O (1330 cm21), and arylalkylether (1136 cm21) stretchings.
1
In the H NMR spectrum, a deuterium exchangeable singlet at 14.28 corre-
sponds to NH function. The O-CH3 protons appeared as singlets at 3.71 and
3.92. Singlets at 2.16 and 2.26, each integrating for three protons, are due to
two methyl groups of pyridine ring.
5-Methoxy-2-(3,5-dimethylpyridin-2-ylmethanesulfinyl-1H-
imidazo[4,5-b]pyridine (Desmethoxy Tenatoprazole, 4)
2,3,5-Trimethyl pyridine (13) was chosen as the starting material for the
synthesis of 4. Hydrogen peroxide–mediated oxidation of 13 and subsequent
transformation of the resulting 14 into 2-hydroxy methyl pyridine derivative
15 in the presence of acetic anhydride, followed by reaction with thionyl
chloride, provided the required key intermediate 16. Condensation of 16
with 2-mercapto derivative 9 in the presence of alkali in methanolic
medium and concomitant oxidation with m-CPBA yielded the desmethoxy
tenatoprazole 4 (Fig. 4). HRMS (EI) (m/z calcd for C15H17N4O2S
(Mþ þ H): 317.1072; found: 317.1081), IR (NH 3445 cm21
, S55O
1060 cm21 and arylalkylether 1257, 1060 cm21), and 1H NMR (d ppm,
14.00, s, NH; 3.90, s, 3H O-CH3, 2.24, s, 3H and 2.29, s, 3H py-CH3)
spectral data of 4 are in conformity with the assigned structure (Figure 5).
5-Methoxy-2-(4-methoxy-3,5-dimethylpyridin-2-ylmethanesulfinyl-
1-methyl-1H-imidazo[4,5-b]pyridine (N-Methyltenatoprazole, 5)
Methylation of tenatoprazole 1 using 1.2 equiv of methyl iodide in the
presence of potassium carbonate in dichloromethane yielded N-methyl

Metabolites and Impurities in Tenatoprazole
1729
Figure 4. Synthesis of desmethoxy tenatoprazole.
tenatoprazole, whose molecular formula in high resolution mass spectra
(HRMS) corresponds to C17H21N4O3S with Mþ þ H at 361.1332 (Calculated
1
361.1334). H-NMR spectrum of 5 is characterized by the presence of five
singlets corresponding to five methyl groups (d ppm ¼ 3.90, 3.89, 3.67,
2.19, and 2.16).
5-Methoxy-2-(4-methoxy-3,5-dimethyl-2-pyridinylmethanethio)-
1H-imidazo[4,5-b]pyridine (Tenatoprazole Sulfide, 6)
Tenatoprazole sulfide 6 was obtained using the synthetic sequence followed
for tenatoprazole (Fig. 1). In the EI mass spectrum of 6, the potassium
adduct ion appeared as the base peak at m/z 369.1, HRMS (EI) (m/z calcd.
for C16H19N4O2S (Mþ þ H): 331.1229; found: 331.1227) data of 6 is consist-
ent with that of sulfide. IR and spectral data of 6 are identical with those of
reference sample.
Figure 5. Synthesis of N-methyl tenatoprazole.

1730
G. M. Reddy et al.
Figure 6. Synthesis of tenatoprazole sulfone.
5-Methoxy-2-(4-methoxy-3,5-dimethyl-pyridin-2-ylmethanesulfonyl)
-1H-imidazo[4,5-b] pyridine (Tenatoprazole Sulfone, 7)
Tenatoprazole sulfone 7 was prepared by the controlled oxidation of tenator-
prazole 1, using an optimal amount of m-CPBA (Fig. 6). The protonated
molecular ion of 7 appeared as the base peak at m/z 363.1, in HRMS (EI,
m/z calcd. for C16H19N4O4S (Mþ þ H): 363.1127; found: 363.1132). IR
[NH (3435 cm21), O55S55O (1328 cm21), arylalkylether (1080 cm21)] and
1H NMR spectral data (d ppm, 13.0, s, NH; 3.92, s, 3H, O-CH3; 3.69, s,
3H, O-CH3; 2.23, s, 3H, CH3; 2.16, s, 3H, CH3) of 7 are similar to those of
tenatoprazole 1 (Figure 6).
CONCLUSION
Information of the different possible impurities, metabolites, and their
synthetic routes is prerequisite for thorough understanding of the impurity
formation pathway of the antiulcerative drug, tenatoprazole. Keeping in
view this regulatory importance of tentatoprazole impurities, the process-
related impurities, and metabolites in tenatoprazole bulk drug were identified,
synthesized, and characterized using mass (both EI and HRMS), IR, and NMR
techniques.
EXPERIMENTAL
1
The H NMR spectra data were recorded at 200 MHz on a Varian Gemini-
2000 FT NMR spectrometer; the chemical shifts were reported on d parts
per million (ppm) relative to TMS. The infrared spectra were obtained
using a Perkin-Elmer Spectrum One FT IR spectrophotometer, with sub-
stances being pressed in a KBr pellet. The mass analysis has been
performed on AB-4000 Q-trap LC-MS/MS mass spectrometer (MDS
SCIEX, Applied Bio systems, California, USA). The HRMS analysis has
been performed on the Micromass LCT Premier XE mass spectrometer
equipped with an ESI Lock spray source for accurate mass values (Water

Metabolites and Impurities in Tenatoprazole
1731
Corporation, Milford, MA, USA). Solvent removal was accomplished by a
rotary evaporator operating at house vacuum (40–50 torr). The solvents and
reagents were used without further purification.
5-Methoxy-2-(4-methoxy-3,5-dimethyl-1-oxide-2-pyridinylmethane
sulfinyl)-1H-imidazo[4,5-b]pyridine (Tenatoprazole-N-oxide, 2)
4-Methoxy-3,5-dimehtyl-2-chloromethylpyridine-1-oxide (11)
Aq. NaOH (2.0 g, 0.050 mol) was added to a solution of 4-methoxy-3,5-
dimethyl-2-chloromethylpyridine hydrochloride (10.0 g, 0.045 mol, 8) in
CHCl3 (50.0 mL) and H2O (50.0 mL) for pH adjustment and stirred for
30 min. The chloroform layer was separated and washed with water
(50.0 mL). m-CPBA (13.0 g, 0.052 mol, 70%) in CHCl3 (50.0 mL) was
added to this solution at 55–60 8C for 1 h and stirred for 30 min for
reaction completion. Water (50.0 mL) was added to the reaction mass, and
pH was adjusted to 8.5–9.0 with aq. NaOH solution. The organic and
aqueous layers were separated. The organic layer was washed with H2O
(2 ꢀ 25.0 mL) and concentrated under reduced pressure at less than 45 8C
to get the residue. To the residue, Ethyl acetate (25.0 mL) was added and
stirred for 30 min. The isolated solid was filtered, washed with chilled ethyl
acetate (10.0 mL), and dried at 40–45 8C to yield 11 (yield 8.2 g, 90.4%).
IR (KBr, cm21): 3050 (Py-H), 2943 (Ali-H), 1247, 1054 (arylalkylether),
1
1460 (aromatic C55C); H NMR (CDCl3, d ppm): 2.2 (s, 3H, Py-CH3), 2.3
(s, 3H, Py-CH3), 3.71 (s, 3H, O-CH3), 4.94 (s, 2H, -CH2Cl), 8.17 (s, 1H,
Py-H); MS m/z (EI): 201.9 (Mþþ H), 223.9 (Mþ þ Na), 425 (2Mþ þ Na).
5-Methoxy-2-(4-methoxy-3,5-dimethyl-1-oxide-2-
pyridinylmethanethio)-1H-imidazo [4,5-b]pyridine (12)
4-Methoxy-3,5-dimehtyl-2-chloromethyl-pyridine-1-oxide (5 g, 0.026 mol,
11) in MeOH (50 mL) was added slowly to a solution of 2-mercapto-5-meth-
oxypyridine imidazole (5 g, 0.027 mol, 9) and NaOH (2 g, 0.05 mol) in H2O
(100 mL) and stirred for 1 h. After completion of the reaction, the reaction
mixture was extracted with CHCl3 (2 ꢀ 30 mL) and washed with water
(50 mL). The organic layer was distilled off completely and charged with
n-hexane (10 mL). The isolated solid was filtered, washed with n-hexane
(10 mL), and dried at 35–40 8C to get a white solid 12 (yield 8.1 g, 87%).
IR (KBr, cm21): 2941 (Ali-H), 1227, 1031 (arylalkylether), 1608, 1590
1
(C55C); H NMR (DMSO-d6, 200 MHz, d ppm): 2.17 (s, 3H, Py-CH3), 2.28
(s, 3H, Py-CH3), 3.71 (s, 3H, O-CH3), 4.94 (s, 2H, Py-CH2Cl), 8.17 (s, 1H,
Py-H); MS m/z (EI): (Mþ þ H) 347.1, (Mþ þ Na) 369.1, (Mþ þ K) 385.0.

1732
G. M. Reddy et al.
5-Methoxy-2-(4-methoxy-3,5-dimethyl-1-oxide-2-
pyridinylmethanesulfinyl)-1H-imidazo [4,5-b]pyridine (2)
m-CPBA (1.1 g, 4.4 mmol) in CHCl3 (15 mL) at 210 to 215 8C was added
slowly to a cooled solution of 5-methoxy-2-(4-methoxy-3,5-dimethyl-1-
oxide-2-pyridinylmethanethio)-1H–imidazo[4,5-b]pyridine (2 g, 5.5 mmol,
12) in CHCl3 (15 mL) over a period of 30 min. Reaction mass was decomposed
with the solution of NaOH (1 g, 0.025 mol) in water (25 mL), and pH was
adjusted to 8–8.5 using AcOH. The organic and aqueous layers were
separated. Compounds were extracted into the aq. NaOH solution (0.5 g in
25 mL of water) from the organic layer, the resulting aq. layer was washed
with CHCl3 (2 ꢀ 15 mL) to remove unreacted sulfide, and pH was adjusted
with acetic acid to 8.5–9.0. It was washed with CHCl3 (2 ꢀ 15 mL) and the
aq. layer pH was adjusted to 7.0 with AcOH and then extracted with CHCl3
(2 ꢀ 10 mL). The combined organic layers were concentrated under reduced
pressure. The product was isolated from MeOH (10 mL) and dried at 45–
50 8C to yield a white solid 3 (yield 1.5 g, 72%). IR (KBr, cm21): 3448 (N-
H), 3057 (Py-H), 2993, 2938 (Ali-H), 1274, 1030 (arylalkylether), 1609,
1
1586 (C55C), 1064 (S55O); H NMR (DMSO-d6, 200 MHz, d ppm): 2.09
(s, 3H, Py-CH3), 2.21 (s, 3H, Py-CH3), 3.66 (s, 3H, O-CH3), 3.92 (s, 3H,
O-CH3), 4.85 (s, 2H, Py-CH2), 6.79 (d, J ¼ 8.6, 1H, Py-H), 7.97 (d, J ¼ 8.8,
1H, Py-H), 8.21 (s, 1H, Py-H), 8.26 (s, 1H, Py-H); HRMS (EI); m/z calcd.
for (Mþ þ Na) C16H18N4O4SNa: 385.0946; found: 385.0943 (20.8 ppm).
5-Methoxy-2-(4-methoxy-3,5-dimethyl-1-oxide-2-pyridinmethane
sulfonyl)-1H-imidazo[4,5-b]pyridine (Tenatoprazole Sulfone
N-Oxide, 3)
m-CPBA (2.3 g, 9.3 mmol, 70%) in CHCl3 (10 mL) was added to a cooled
solution of 5-methoxy-2-(4-methoxy-3,5-dimethyl-pyridin-2-ylmethanethio)-
1H-imidazo[4,5-b]pyridine (1 g, 3 mmol, 1) in CHCl3 (15 mL) and MeOH
(5 mL) slowly at 210 to 215 8C over a period of 20 min. The reaction was
maintained for 30 min for the completion of the reaction. The reaction mass
was decomposed into the solution of NaOH (1 g, 0.025 mol) in H2O
(25 mL), and pH was adjusted to 8–8.5 using AcOH (1.4 mL). The organic
and aqueous layers were separated. Compounds were extracted into the aq.
NaOH solution (0.3 g in 25 mL of H2O) from the organic layer. MeOH
(5 mL) was charged to the aq. layer, and pH was adjusted to 8.5–9 with
AcOH (0.4 mL). The isolated solid was filtered, washed with mixture of
H2O and MeOH (1:1, 3 mL), and dried at 45–50 8C to yield a white solid 4
(yield 1.05 g, 92%). IR (KBr, cm21): 3106 (Py-H), 2989 (Ali-H), 1276,
1
1027 (arylalkylether), 1610, 1585 (C55C), 1330 (O55S55O); H NMR
(DMSO, 200 MHz, d ppm): 2.16 (s, 3H, Py-CH3), 2.26 (s, 3H, Py-CH3),
3.71 (s, 3H, O-CH3), 3.92 (s, 3H, O-CH3), 5.32 (s, 2H, Py-CH2), 8.03

Metabolites and Impurities in Tenatoprazole
1733
(s, 1H, Py-H), 14.28 (s, 1H, benzimidazole-NH); MS: þve ESI: (Mþ þ 1) 379,
(Mþ þ Na) 401, 2ve ESI: (M2 2H) 377; HRMS (EI): m/z calcd. for
(Mþ þ H) C16H19N4O5S: 379.1076; found: 379.1062. (23.7 ppm).
5-Methoxy-2-(3, 5-dimethyl-2-pyridinylmethanesulfinyl)-1H-
imidazo[4,5-b]pyridine (Desmethoxy Tenatoprazole, 4)
2,3,5-Trimethylpyridine-1-oxide (14)
H2O2 (9 g, 264 mmol) was added to a solution of 2,3,5-trimethylpyridine
(10 g, 55 mmol, 13) in AcOH (15 mL) at 90–95 8C, for 10–15 min.
Reaction was maintained at 90–95 8C for 8–9 h, and aq. HCl (10 mL) was
added to the reaction mass at 90–95 8C for 10 min. The reaction mass was
distilled off completely under vacuum at less than 90–95 8C, and the solid
was isolated in pet. ether (30 mL). The solid was dried at 40–45 8C to
constant weight to yield 14 (9.7 g, 85%). IR: (KBr, cm21): 3051 (Py-H),
1
1255 (N-O); H NMR (CDCl3, 200 MHz, d ppm): 2.22 (s, 3H, Py-CH3),
2.29 (s, 3H, Py-CH3), 2.46 (s, 3H, Py-CH3), 6.89 (s, 1H, Py-H), 8.02 (s, 1H,
Py-H); MS m/z: (Mþ þ H) 138.0.
2-Hydroxymethyl-3,5-dimethylpyridine (15)
2,3,5-Trimethylpyridine-1-oxide (10 g, 50 mmol, 14) was added slowly to
liquid Ac2O (40 mL) at 25–30 8C. The reaction mass was heated to 90–
95 8C and maintained at 90–95 8C for 3 h. Ac2O was distilled off completely
to obtain the crude product. To the reaction mixture, a solution of caustic lye
(4 mL) in water (70 mL) at 15–20 8C was added, and the reaction mass was
maintained for 1 h. The reaction mass was extracted with DCM
(3 ꢀ 20 mL), and combined neat organic layers were distilled off completely
under vacuum to obtain the crude material 15 (8.6 g, 85%). IR (neat, cm21):
1
3391 (O-H), 3019 (Py-H), 2952 (Ali-H), 1606, 1575 (aromatic C55C);
H
NMR (CDCl3, 200 MHz, d ppm): 2.17 (s, 3H, Py-CH3), 2.30 (s, 3H, Py-
CH3), 4.64 (s, 2H, Py-CH2-O), 4.78 (broad s, 1H, CH2-OH), 7.28 (s, 1H,
Py-H), 8.21 (s, 1H, Py-H); MS m/z: (Mþ þ H) 138.0.
2-Chloromethyl-3,5-dimethylpyridine hydrochloride (16)
SOCl2 (9.5 mL, 130 mmol) was added slowly to a solution of 2-hydroxy-
methyl-3,5-dimethylpyridine (15 g, 76 mmol, 15) in DCM (45 mL) at 0–
5 8C for 45 min. The reaction was maintained at 0–5 8C for 1 h. The
reaction mass was distilled off completely under reduced pressure, and a
solid was isolated from ethyl acetate (45 mL). The solid was dried at 45 8C
to yield 16 (16.3 g, 85%). IR (KBr, cm21): 3005 (Py-H), 2968 (Ali-H),
1
1594, 1503, 1450 (aromatic C55C); H NMR (CDCl3, 200 MHz, d ppm):

1734
G. M. Reddy et al.
2.55 (s, 3H, Py-CH3), 2.61 (s, 3H, Py-CH3), 5.15 (s, 2H, Py-CH2-Cl), 8.03
(s, 1H, Py-H), 8.42 (s, 1H, Py-H); MS m/z: (Mþ þ H) 156.0.
5-Methoxy-2-(3,5-dimethyl-2-pyridinylmethanesulfinyl)-1H-imidazo
[4,5-b]pyridine (Desmethoxy Tenatoprazole, 4)
A solution of 2-chloromethyl-3,5-dimethylpyridine hydrochloride (17.2 g,
95 mmol, 16) in H2O (60 mL) was added slowly to the solution of
2-mercapto-5-methoxypyridineimidazole (20 g, 104 mmol, 9) in H2O
(100 mL), acetone (60 mL), and NaOH (10 g, 250 mmol) at 25–30 8C for
45 min. The reaction was maintained at the same temperature for 30 min
and then extracted with CHCl3 (2 ꢀ 50 mL), and the combined organic
layers were washed with water (2 ꢀ 50 mL).
Per acetic acid (45 mL, 88.8 mmol, assay: 15%) was added slowly to the
cooled solution (110 mL) at 225 to 230 8C for 1 h. The reaction mass was
decomposed into a basic aqueous solution (15 g of NaOH in water 100 mL
of water), pH was adjusted with AcOH to 8.0, and the layers were
separated. The CHCl3 layer was extracted with alkaline solution (5 g of
NaOH in 100 mL of H2O) and then given CHCl3 washings (2 ꢀ 25 mL) to
the aq. layer. To the aq. layer, acetone (60 mL) was added, and pH was
adjusted to 8.0 with AcOH. The separated solid was filtered, washed with
acetone (15 mL), and dried at 45–50 8C for 5 h to yield 5 as a white
solid (yield 23.7 g, 72%). IR (KBr, cm21): 3445 (N-H) 2982 (Ali-H), 1257
1
(arylalkylether), 1611, 1585, 1466 (aromatic C55C), 1060 (S55O); HNMR
(DMSO-d6, 200 MHz, d ppm): 2.24 (s, 3H, Py-CH3), 2.29 (s, 3H, Py-CH3),
3.90 (s, 3H, O-CH3), 4.74 (dd, J ¼ 14, J0 ¼ 24, 2H, Py-CH2), 6.79
(d, J ¼ 8, 1H, Py-H), 7.45 (s, 1H, Py-H), 7.98 (d, J ¼ 8, 1H, Py-H), 8.16
(s, 1H, Py-H), 13.9 (broad s, 1H, benzimidazole N-H); MS m/z: (Mþ þ 1)
317.0, (2Mþ þ 1) 655.2; HRMS (EI) m/z calcd. for C15H17N4O2S
(Mþ þ H): 317.1072; found: 317.1081 (2.8 ppm).
1-Methyl-5-methoxy-2-(4-methoxy-3,5-dimethyl-2-pyridinmethane
sulfinyl)-1H-imidazo[4,5-b]pyridine (N-Methyltenatoprazole, 5)
To a mixture of 5-methoxy-2-(4-methoxy-3,5-dimethyl-pyridin-2-ylmethane-
sulfinyl)-1H-imidazo[4,5-b]pyridine (10 g, 28.9 mmol, 2), K2CO3 (30 g,
217 mmol), and
a catalytic amount of N,N-(dimethylamino)pyridine
(DMAP) in DCM (100 mL) at 25–30 8C, MeI (4.92 g, 34.6 mmol) was
slowly added over a period of 20 min. The reaction was maintained for 10 h
for the completion of reaction. The reaction mass was filtered through the
hyflow bed and washed with DCM (25 mL); the filtrate was washed with
H2O (3 ꢀ 100 mL) and dried over anhydrous Na2SO4. The organic layer was
concentrated under reduced pressure, isolated from ethyl acetate (25 mL),
and dried at 45–50 8C to yield 6 as a white solid (yield 9.5 g, 91.3%). IR

Metabolites and Impurities in Tenatoprazole
1735
(KBr, cm21): 2929 (Ali-H), 1269 (arylalkylether), 1603, 1588 (C55C), 1048
1
(S55O); H NMR (DMSO, 200 MHz, d ppm): 2.16 (s, 3H, Py-CH3), 2.19
(s, 3H, Py-CH3), 6.67 (s, 3H, N-CH3), 3.89 (s, 3H, O-CH3), 3.90 (s, 3H,
O-CH3), 4.91 (dd, J ¼ 13.8, J0 ¼ 19.6, 2H, Py-CH2), 6.84 (dd, J ¼ 10,
J0 ¼ 12, 1H, Py-H), 8.10 (d, J ¼ 8,1H, Py-H), 8.10 (s, 1H, Py-H). MS m/z:
(Mþ þ H) 361.2, (Mþ þ Na) 383.1, (2Mþ þ Na) 743.3; HRMS (EI) m/z
calcd. for C17H21N4O3S (Mþ þ H): 361.1334; found: 361.1332. (20.6 ppm).
5-Methoxy-2-(4-methoxy-3,5-dimethyl-2-pyridinylmethanethio)-
1H-imidazo[4,5-b]pyridine (Tenatoprazole sulfide, 6)
A solution of 3,5-dimehtyl-2-chloromethyl pyridine hydrochloride (10 g,
45 mmol, 8) in H2O (50 mL) was added to a solution of 2-mercapto-5-meth-
oxypyridine imidazole (9.72 g, 54 mmol, 9), NaOH (4.5 g, 112 mmol) in H2O
(100 mL) and MeOH (25 mL) at 25–30 8C over a period of 30–45 min. The
isolated solid was filtered, washed with a 1:1 mixture of H2O and MeOH
(30 mL), and dried at 45–50 8C to yield 6 (yield 13.2 g, 88.8%). IR (KBr,
cm21): 3218 (NH), 2942 (Ali-H), 1265, 1042 (arylalkylether), 1598, 1481
1
(aromatic C55C). H NMR (DMSO, 200 MHz, d ppm): 2.20 (s, 3H, Py-
CH3), 2.27 (s, 3H, Py-CH3), 3.72 (s, 3H, O-CH3), 3.86 (s, 3H, O-CH3), 4.66
(s, 2H, Py-CH2), 6.60 (d, J ¼ 8.4, 1H, Py-H), 7.78 (broad s, 1H, Py-H),
8.17 (s, 1H, Py-H), 13.01 (broad s, 1H, NH); MS m/z: (Mþ þ H) 331.0,
(Mþ þ Na) 353.1, (Mþ þ K) 369.1; HRMS (EI) m/z calcd. for (Mþ þ H)
C16H19N4O2S 331.1229; found: 331.1227 (20.6 ppm).
5-Methoxy-2-(4-methoxy-3,5-dimethyl-pyridin-2-ylmethane
sulfonyl)-1H-imidazo[4,5-b]pyridine (Tenatoprazole Sulfone, 7)
m-CPBA (6.5 g, 0.026 mol, 70%) in CHCl3 (25 mL) was added slowly to a
cooled solution of 5-methoxy-2-(4-methoxy-3,5-dimethyl-2-pyridinylmetha-
nethio)-1H-imidazo[4,5-b]pyridine (5 g, 0.015 mol, 6) in CHCl3 (25 mL), at
210 to 215 8C over a period of 30 min. The reaction was maintained for
30 min for the maximum completion of reaction. The reactionmass was decom-
posed into the solution of NaOH (5 g, 0.125 mol) in H2O (25 mL), and pH was
adjusted to 8–8.5 using AcOH. The organic and aqueous layers were separated.
Compounds were extracted into the aq. NaOH solution (2.5 g in 25 mL of
water) from the organic layer. CHCl3 (20 mL) was charged to the aq. layer,
pH was adjusted to 8.5–9 with AcOH, and organic and aqueous layers were
separated. pH was adjusted to 7.0 with AcOH, and then aq. layer was
extracted with CHCl3 (2 ꢀ 20 mL), dried over anhydrous Na2SO4, and
distilled off completely under reduced pressure. The isolated solid in MeOH
(10 mL) was filtered, washed with MeOH (5 mL), and dried at 45–50 8C to
yield a white solid 7 (yield 3.9 g, 71%). IR (KBr, cm21): 3021 (Py-H), 2939

1736
G. M. Reddy et al.
(Ali-H), 1233, 1020 (arylalkylether), 1612, 1510, 1575 (aromatic C55C), 1328,
1142 (O55S55O in sulfone); 1H NMR (DMSO, 200 MHz, d ppm): 2.16 (s, 3H,
Py-CH3), 2.23 (s, 3H, Py-CH3), 3.69 (s, 3H, O-CH3), 3.92 (s, 3H, O-CH3), 5.01
(s, 2H, Py-CH2), 6.87 (d, J ¼ 10, 1H, Py-H), 8.00 (s, 1H, Py-H), 8.03
(d, J ¼ 11.6, 1H, Py-H), 13.00 (broad s, 1H, NH); MS m/z: (Mþ þ H) 363.1,
(Mþ þ Na) 385.0, (Mþ þ K) 401.0, (2Mþ þ Na) 747.1; HRMS (EI) m/z
calcd. for C16H19N4O4S (Mþ þ H) 363.1127; found: 363.1132 (1.4 ppm).
ACKNOWLEDGMENTS
The authors are grateful to their colleagues at the Analytical Research Depart-
ment of Integrated product developement operations (IPDO), Discovery
Research, and the management of Dr. Reddy’s Laboratories Ltd. for
providing the necessary support.
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