Synthesis and physicochemical characterization of the process-related impurities of olmesartan medoxomil. Do 5-(Biphenyl-2-yl)-1-triphenylmethyltetrazole intermediates in sartan syntheses exist?

Article abstract of DOI:10.3390/molecules201219762

During the process development for multigram-scale synthesis of olmesartan medoxomil (OM), two principal regioisomeric process-related impurities were observed along with the final active pharmaceutical ingredient (API). The impurities were identified as N-1- and N-2-(5-methyl-2-oxo-1,3-dioxolen-4-yl)methyl derivatives of OM. Both compounds, of which N-2 isomer of olmesartan dimedoxomil is a novel impurity of OM, were synthesized and fully characterized by differential scanning calorimetry (DSC), infrared spectroscopy (IR), nuclear magnetic resonance spectroscopy (NMR) and high-resolution mass spectrometry/electrospray ionization (HRMS/ESI). Their ¹H, ¹³C and ¹⁵N nuclear magnetic resonance signals were fully assigned. The molecular structures of N-triphenylmethylolmesartan ethyl (N-tritylolmesartan ethyl) and N-tritylolmesartan medoxomil, the key intermediates in OM synthesis, were solved and refined using single-crystal X-ray diffraction (SCXRD). The SCXRD study revealed that N-tritylated intermediates of OM exist exclusively as one of the two possible regioisomers. In molecular structures of these regioisomers, the trityl substituent is attached to the N-2 nitrogen atom of the tetrazole ring, and not to the N-1 nitrogen, as has been widely reported up to the present. This finding indicates that the reported structural formula of N-tritylolmesartan ethyl and N-tritylolmesartan medoxomil, as well as their systematic chemical names, must be revised. The careful analysis of literature spectroscopic data for other sartan intermediates and their analogs with 5-(biphenyl-2-yl)tetrazole moiety showed that they also exist exclusively as N-2-trityl regioisomers.

Full text of DOI:10.3390/molecules201219762

Article
Synthesis and Physicochemical Characterization of the
Process-Related Impurities of Olmesartan Medoxomil.
Do 5-(Biphenyl-2-yl)-1-triphenylmethyltetrazole
Intermediates in Sartan Syntheses Exist?
Iwona Dams 1,*, Anna Ostaszewska 1, Maria Puchalska 1, Justyna Chmiel 1, Piotr Cmoch 1,2
Iwona Bujak 1, Agata Białon´ska 3 and Wojciech J. Szczepek 1
,
Received: 23 October 2015 ; Accepted: 19 November 2015 ; Published: 1 December 2015
Academic Editor: Jean Jacques Vanden Eynde
1
Pharmaceutical Research Institute, Rydygiera 8, Warsaw 01-793, Poland; a.ostaszewska@ifarm.eu (A.O.);
m.puchalska@ifarm.eu (M.P.); j.chmiel@ifarm.eu (J.C.); piotr.cmoch@icho.edu.pl (P.C.);
i.bujak@ifarm.eu (I.B.); w.szczepek@ifarm.eu (W.J.S.)
Institute of Organic Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland
Faculty of Chemistry, University of Wrocław, Joliot-Curie 14, 50-383 Wrocław, Poland;
agata.bialonska@chem.uni.wroc.pl
2
3
*
Correspondence: i.dams@ifarm.eu; Tel.: +48-22-456-3929
Abstract: During the process development for multigram-scale synthesis of olmesartan medoxomil
(OM), two principal regioisomeric process-related impurities were observed along with the
final active pharmaceutical ingredient (API). The impurities were identified as N-1- and
N-2-(5-methyl-2-oxo-1,3-dioxolen-4-yl)methyl derivatives of OM. Both compounds, of which N-2
isomer of olmesartan dimedoxomil is a novel impurity of OM, were synthesized and fully
characterized by differential scanning calorimetry (DSC), infrared spectroscopy (IR), nuclear magnetic
resonance spectroscopy (NMR) and high-resolution mass spectrometry/electrospray ionization
1
(HRMS/ESI). Their H, 13C and 15N nuclear magnetic resonance signals were fully assigned.
The molecular structures of N-triphenylmethylolmesartan ethyl (N-tritylolmesartan ethyl) and
N-tritylolmesartan medoxomil, the key intermediates in OM synthesis, were solved and refined using
single-crystal X-ray diffraction (SCXRD). The SCXRD study revealed that N-tritylated intermediates
of OM exist exclusively as one of the two possible regioisomers. In molecular structures of these
regioisomers, the trityl substituent is attached to the N-2 nitrogen atom of the tetrazole ring, and not
to the N-1 nitrogen, as has been widely reported up to the present. This finding indicates that the
reported structural formula of N-tritylolmesartan ethyl and N-tritylolmesartan medoxomil, as well as
their systematic chemical names, must be revised. The careful analysis of literature spectroscopic
data for other sartan intermediates and their analogs with 5-(biphenyl-2-yl)tetrazole moiety showed
that they also exist exclusively as N-2-trityl regioisomers.
Keywords: crystal structure; impurities; medoxomil; NMR spectroscopy; olmesartan; prodrugs;
regioisomers; sartans; structure; synthesis
1. Introduction
Elevated blood pressure is one of the most important causes of death and disability worldwide,
accounting for 7.6 million premature deaths and 92 million disability-adjusted life years annually [1].
Controlling blood pressure and prevention of its complications such as coronary heart disease, stroke,
renal failure and eye damage are the main objectives for the treatment of hypertension [2,3]. In April
2002, the U.S. Food and Drug Administration (FDA) approved OM (
7, Scheme 1) for the treatment of
Molecules 2015, 20, 21346–21363; doi:10.3390/molecules201219762
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Molecules 2015, 20, 21346–21363
hypertension. The seventh in a growing class of antihypertensive agents known as the angiotensin
II receptor blockers (Figure 1), the drug works by inhibiting effects of angiotensin II, a potent
vasoconstrictor and one of the key contributors to cardiovascular and renal disease [
a prodrug containing an ester moiety, which is rapidly and completely cleaved to release the active
metabolite olmesartan ( ) during absorption from the gastrointestinal tract. Potential advantages of
4–8]. OM is
8
this drug include once-daily dosing, an absence of significant adverse reactions, a well-tolerated side
effect profile, and a cost-effective average wholesale price.
Scheme 1. Synthesis of OM (7) and the impurities 9 and 10. Conditions: (1) K2CO3, KI, DMF, 24 h at r.t.,
83%; (2) KOH, DMF, 54–56 ˝C for 22 h; (3) K2CO3, KI, DMF, 22 h at r.t., 96%; (4) H2SO4-H2O, Me2CO,
50–55 ˝C for 2 h; 80%; (5) NaOH, MeOH, 24 h at r.t.; (6) AcOH, H2O; 96%; (7) K2CO3, KI, DMF, 22 h
at r.t.
Figure 1. Structures of angiotensin II receptor blockers used as APIs in the treatment of hypertension.
OM contains a biphenyl moiety substituted with an imidazol-1-ylmethyl and a tetrazol-5-yl
group at C-4 and C-21, respectively. Aside from OM, four other sartans used in clinical practice,
i.e., losartan, valsartan, irbesartan and candesartan, are 5-(biphenyl-2-yl)tetrazole derivatives.
[21-(N-Triphenylmethyltetrazol-5-yl)biphenyl-4-yl]methyl bromide [
9] (2) is a common intermediate
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Molecules 2015, 20, 21346–21363
used in the synthesis of these sartan drugs. Despite its wide synthetic application, the available
literature data and chemical databases are not in agreement on its chemical structure, giving formulas
of the two bromides that differ substitution with trityl group at tetrazole N-1 (CAS 124750-51-2) and
N-2 (CAS 133051-88-4). The similar structural dualism is observed for the N-tritylated intermediates
(CAS 144690-33-5 for N-1 tritylated and CAS 172875-59-1 for N-2 tritylated) and (CAS 144690-92-6 for
N-1 tritylated and CAS 1020157-01-0 for N-2 tritylated) in the synthesis of OM ( ). As intermediates
3
6
7
in active pharmaceutical ingredient (API) synthesis often afford numerous impurities affecting the
quality of the final drug product, their structure explanation is essential for impurities identification
and characterization. A complete physicochemical characterization, not only for an API, but also of its
key synthetic intermediates, has recently become a requirement of both the U.S. FDA and the European
Medicine Agency (EMA). Therefore, the real structure of the starting bromide
2 and the intermediate
esters 3 and 6 should be unambiguously clarified.
The protection and deprotection of the tetrazole N-atom are essential steps during the synthesis
of the sartan drugs containing 5-(biphenyl-2-yl)tetrazole moiety, and are often accompanied by side
reactions and process-related impurities formation, influencing the high quality of the final active
pharmaceutical ingredient (API). The available literature data indicate that tetrazolic acids of sartans,
formed as a result of undesired tetrazole N-atom deprotection from trityl group, may exist as mixtures
of 1H- and 2H- tautomeric forms that subsequently undergo adverse alkylation reactions to afford
mixtures of N-1 and N-2-alkyl regioisomeric impurities. Alkylation of some sartan tetrazolic acids
with primary alkyl halides, i.e., methyl iodide, iodomethyl pivalate, medoxomil chloride, bromide
ethyl iodide, under different reaction conditions afforded mixtures of regioisomeric N-1- and N-2-alkyl
derivatives [10 12]. It should be noted that Rádl et al. wrongly assigned the position of trityl group in
2, or
–
1
their [2 -(1-trityl-1H-tetrazol-5-yl)biphenyl-4-yl]methyl substituent [11].
Recently, we have developed an improved, scalable, cost-effective and environment-friendly
technology for the industrial-scale synthesis of OM (7, Scheme 1) based on the general route described
by Yanagisawa et al. [13 14] During the process development, the two regioisomeric impurities
,
9
and
10 of OM were observed along with the final API samples on the levels 0.03%–0.18% and 0.02%–0.13%,
respectively. The impurities were identified as N-1- and N-2-(5-methyl-2-oxo-1,3-dioxolen-4-yl)methyl
derivatives of OM. To the best of our knowledge, the isomeric N-2-medoxomil impurity
9 is a new
compound that has never been identified. The determination of a drug substance impurity profile,
including potential degradation products and process-related impurities, is critical for the safety
assessment of API and manufacturing process thereof. According to the guidelines issued by the
International Conference on Harmonization (ICH) and European Pharmacopoeia it is mandatory to
identify and characterize the impurities in a pharmaceutical product if present above the accepted
limits of 0.1% [15
only for OM API but also its regioisomeric impurities
and 6 to comply with current FDA and EMA requirements.
Herein, we discussed the origin of formation, synthesis, identification and characterization of
both N-1- and N-2-(5-methyl-2-oxo-1,3-dioxolen-4-yl)methyl regioisomeric impurities and 10 of OM
), respectively. On the basis of analytical data (IR, NMR and SCXRD), we disproved the structural
dualism of the tetrazole tritylated compounds and , existing in the literature data and chemical
,16]. Therefore, there was a need for a complete physicochemical characterization not
9
and 10 and the key synthetic intermediates
3
9
(
7
2,
3
6
databases, and determined their real structure as N-2-tritylated tetrazole regioisomers. Additionally,
the careful analysis of literature spectroscopic data for other tritylated sartan intermediates with
5-(biphenyl-2-yl)tetrazole moiety showed that they also exist exclusively as N-2-trityl derivatives.
2. Results and Discussion
2.1. Synthesis of Olmesartan Medoxomil (7) and N-1- and N-2-Medoxomil Impurities 9 and 10
The medoxomil ester
of ethyl ester
6 (Scheme 1) is synthesized in a one-pot process comprising hydrolysis
3
with KOH and alkylation with medoxomil chloride (
5) in N,N-dimethylformamide
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Molecules 2015, 20, 21346–21363
(DMF) or dimethyl sulfoxide (DMSO). Deprotection of tetrazole nitrogen from trityl group during
basic hydrolysis of ethyl ester
acid ( ) resulted is immediately involved in the next alkylation step of a one-pot process giving a
mixture of N-1- and N-2-medoxomil regioisomers and 10. The effective synthesis of intermediate
from the in situ prepared potassium salt always requires the excess of medoxomil chloride . The use
of an equimolar amount of does not prevent impurities formation, as the alkylation of tetrazole unit
3 to potassium salt 4 is very important, as dipotassium salt of olmesartan
8
9
6
4
5
5
is faster than alkylation of carboxyl group [10].
The content of dipotassium salt of olmesartan acid (
8) in the purified ester 6 samples, detected as
acid
of the ester
10) were also observed. The same compounds, on the comparable levels 0.15%–0.64% (RRT 2.22,
and 0.12%–0.97% (RRT 1.86, 10), were detected in the crude OM ( ) samples as potential impurities
8, ranged from 0.13% to 1.28% (RRT 0.47) as detected by HPLC (Table 1). However, in the samples
6
two other impurities on the levels 0.16%–0.67% (RRT 0.61, ) and 0.13%–1.06% (RRT 0.53,
9
9
)
7
of the final pharmaceutical substance (Table 2). The purification attempts indicated that removal of
both impurities from the final product, below the level of 0.1% accepted by ICH guidelines, to achieve
pharmaceutical grade purity OM could be problematic. Since the European and the United States
Pharmacopoeias on OM also specify none of the detected impurities [17
had to be identified, synthesized and characterized thoroughly.
,18], both compounds 9 and 10
Table 1. Synthesis and HPLC data of N-tritylolmesartan medoxomil (6) samples.
HPLC (RRT) (%)
Purification
Extraction/Crystallization
Yield
(%)
Example
0.47
8
0.53
10
0.61
9
0.88
4
0.98
3
1.00
6
Solvent
1
2
3
4
5
DMF
DMF
DMSO
DMF
DMF
CH2Cl2/i-PrOH–H2O
AcOEt/AcOEt–hexanes
AcOEt/EtOH–H2O
CH2Cl2/Me2CO–H2O
CH2Cl2/MeCN–H2O
0.13
0.16
1.28
0.16
0.14
0.56
1.06
0.24
0.15
0.13
0.56
0.67
0.31
0.18
0.16
-
0.08 97.11
0.01 97.08
0.01 97.21
0.04 98.88
0.02 97.75
96
68
86
87
92
0.06
0.02
0.03
0.04
Table 2. Synthesis and HPLC data of olmesartan medoxomil (OM, 7) samples.
HPLC purity (RRT) (%)
Solvent
Acid
Yield
(%)
Example
Work up
Crystallization
1.00
7
1.86
10
2.22
9
Me2CO
H2O/filtration
Crude
Me2CO–AcOEt
Crude
Me2CO
Crude
THF–Me2CO–H2O
Crude
Me2CO–H2O
Crude
CH3CN–H2O
98.63
99.20
97.97
99.22
97.88
98.24
99.15
99.47
99.23
99.46
0.51
0.08
0.97
0.10
0.21
0.13
0.12
0.02
0.13
0.02
0.64
0.04
0.61
0.08
0.34
0.18
0.15
0.03
0.16
0.03
-
80
-
65
-
67
-
86
-
1
2
3
4
5
H2SO4–H2O CH2Cl2/Na2CO3[aq]
Me2CO H2O/filtration
H2SO4–H2O CH2Cl2/Na2CO3[aq]
Me2CO H2O/filtration
H2SO4–H2O CH2Cl2/Na2CO3[aq]
Me2CO H2O/filtration
H2SO4–H2O CH2Cl2/NaHCO3[aq]
Me2CO H2O/filtration
H2SO4–H2O CH2Cl2/Na2CO3[aq]
90
During a one-pot process of the medoxomil ester
acid ( ) detected by thin layer chromatography (TLC) in the reaction mixture after basic hydrolysis of
the ethyl ester almost disappeared in the next alkylation step with medoxomil chloride ( ). This fact,
in accordance with the available literature data on isomeric impurities of sartans [10 12], threw our
suspicion to the possibility of dimedoxomil impurities formation by O- and N-alkylations of olmesartan
acid ( ). The synthesis of regioisomeric dimedoxomil impurities and 10 was achieved according to the
route described in the Scheme 1. Thus, OM ( ) was subjected to basic hydrolysis with sodium hydroxide
(NaOH) in methanol at ambient reaction conditions to afford after acidification the olmesartan acid (
6 synthesis, the dipotassium salt of olmesartan
8
3
5
–
8
9
7
8
)
in 96% yield. The structure of
8
was confirmed by spectral analysis. Alkylation of the acid
8 with the
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Molecules 2015, 20, 21346–21363
excess of medoxomil chloride (5) in DMF in the presence of K2CO3 provided a mixture of dimedoxomil
compounds
by column chromatography on silica gel with 50%–100% AcOEt/hexanes gradient elution to afford
N-2-medoxomil derivative (Rf = 0.75 for 30% MeOH/AcOEt) and N-1-medoxomil derivative 10
9 and 10 in the ratio of 42.56%:57.44% by HPLC. The isomers were separated and purified
9
(Rf = 0.64 for 30% MeOH/AcOEt). The structure of regioisomers was established on the basis of
spectroscopic methods.
Formation of two regioisomeric dimedoxomil derivatives of olmesartan (8) is in good agreement
with previous observations that alkylation of compounds possessing 5-(biphenyl-2-yl)tetrazole
moiety with primary alkyl halides under different reaction conditions gives the mixtures of two
regioisomeric N-1- and N-2-alkyl derivatives. The observed ratio of N-1- to N-2-alkyl regioisomer was
75.4:24.6 for methyl (-CH3) [10], 75:25 for pivaloyloxymethyl (-CH2-OOC-tert-Bu) [10], 60.2:39.8 for
[2
(-CH2CH3) [12]. In contradiction with these data was the result of the alkylation of olmesartan (
medoxomil chloride ( ) in acetone, in the presence of K2CO3 and tetra-n-butylammonium bromide,
'
-(2-trityl-2H-tetrazol-5-yl)biphenyl-4-yl]methyl [11], 60:40 for medoxomil [10] and 47.3:52.7 for ethyl
8) with
5
described by Venkanna et al. [19] The authors presented the false information that the reaction leads to
a single crystalline product with the yield 93% and they attributed the structure of N-1-medoxomil
derivative to the prepared compound without any evidence. In our hands, the reproduction of the
reaction according to Venkanna procedure gave a mixture of dimedoxomil derivatives with the ratio
of N-1- to N-2-regioisomer 43.5:56.5 and acetone condensation products. The attempts to isolate and
purify the N-1- (10) regioisomer by the Authors’ procedure were unsuccessful, affording a dark-brown
oily mixture of N-2- (9) and N-1- (10) medoxomil derivatives and acetone condensation products.
As the contents of dimedoxomil isomers
OM ( ) samples were on the comparable levels, we checked the stability of both impurities under
detritylation of OM conditions. Thus, the pure isomers and 10 were heated with a three-fold excess
of H2SO4 in an acetone-water solution. After being stirred for 2 h, the reaction mixture contained the
unreacted isomer or 10 as the main product, and the trace amounts of OM ( ) and the olmesartan
acid ( ). The experiments confirmed that both impurities and 10 are hardly decomposable under
9 and 10 in the purified medoxomil ester 6 and the crude
7
9
9
7
8
9
acidic detritylation conditions, thus contaminating the crude final product.
2.2. Determination of the Structure of the Bromide 2 and the Intermediates 3 and 6
Alkylation of 5-phenyltetrazole with trityl chloride under alkaline conditions as well as with
triphenylmethanol under acidic conditions should lead exclusively to the 5-phenyl-2-trityltetrazole [20
,21].
Therefore, [21-(N-triphenylmethyltetrazol-5-yl)biphenyl-4-yl]methyl bromide (
2), the commercially
available in kg scale intermediate to the syntheses of sartans, should be tritylated at the 2-position of
tetrazole. The crystal structure of [21-(N-triphenylmethyltetrazol-5-yl)biphenyl-4-yl]methyl bromide
obtained according to the procedure given by Aldrich was solved using a single crystal grown
from butan-2-one [22]. Although the structural formula shown in the paper is incorrect (tetrazole
connected phenyl substituted with p-bromophenyl at metha position), the presented molecular structure
unequivocally proves the presence of the trityl substituent on the nitrogen N-2 atom of tetrazole.
Zhao et al. using the single crystals grown from ethyl methyl ketone and ethyl acetate confirmed the
same position of trityl substituent in two polymorphs of
13C-NMR data recorded for our [21-(2-triphenylmethyltetrazol-5-yl)biphenyl-4-yl]methyl bromide
) and bromides described in the literature in many cases shows differences (see Supplementary
Materials; Tables S1–S3, respectively); however, no documented case of a compound with the trityl
2
later [23]. The comparison of the 1H- and
(2
2
substituent at the N-1-positi1on of 5-(biphenyl-2-yl)tetrazole was found. Thus, the starting bromide
2
for syntheses of sartans is [2 -(2-triphenylmethyl-2H-tetrazol-5-yl)biphenyl-4-yl]methyl bromide.
Treatment of
the ethyl ester . The SCXRD measurement of this product showed that the molecule contains
tetrazole ring tritylated at N-2 position (Figure 2, left). Thus, the intermediate compound is ethyl
1 with the bromide 2 in the presence of K2CO3 and KI in DMF (Scheme 1) gave
3
3
1
4-(1-hydroxy-1-methylethyl)-2-propyl-1-[2 -(2-triphenylmethyl-2H-tetrazol-5-yl)biphenyl-4-yl]methyl-
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Molecules 2015, 20, 21346–21363
1H-imidazole-5-carboxylate. The next intermediate in the synthesis of OM, N-tritylolmesartan
medoxomil (
the potassium salt
As expected, its SCXRD also showed tritylation of the tetrazole at N-2 position and the
6), is obtained in two steps from
3
(Scheme 1). Alkaline hydrolysis of
3 generates
4. Treatment of the in situ formed salt
4
with yields medoxomil ester
5
6
.
correct product name is (5-methyl-2-oxo-1,3-dioxolen-4-yl)methyl 4-(1-hydroxy-1-methylethyl)-2-
propyl-1-[21-(2-triphenylmethyl-2H-tetrazol-5-yl)biphenyl-4-yl]-methyl-1H-imidazole-5-carboxylate
(Figure 2, right). The results described above strongly prove retention of N-2 trityl position
in the course of sartan synthesis. In fact, the trityl losartan obtained by condensing
(2-butyl-4-chloro-1H-imidazol-5-yl)methanol with the bromide
2 in the presence of K2CO3 in
DMF, reported in chemical databases as both N-1- (CAS 124751-00-4) and N-2-trityl (CAS 133909-99-6),
is tritylated at N-2 of tetrazole, which was unambiguously proved by SCXRD [24].
Figure 2. Molecular structure of N-tritylolmesartan ethyl (
(6, right).
3, left) and N-tritylolmesartan medoxomil
The structures of the starting bromide
2 and the intermediates 3 and 6 in the synthesis of OM, as
well as the impurities
9
and 10, were confirmed on the basis of the detailed analysis of the 1H- and
13C-NMR results. A careful analysis of the 1H- and 13C-NMR chemical shifts and 1H/13C gradient
1
selected HSQC and HMBC (g-HSQC and g-HMBC) correlations allowed to correctly assign H-
and 13C-NMR signals. Identification of H-31 protons of these compounds was done based on the
analysis of the 1H-13C g-HMBC long-range 1H-13C correlations. It was essential from the viewpoint
of tetrazole substitution. In the case of 2, 3 and 6
H-31 proton at ca. 7.80–8.00 ppm gave correlation
with carbon at ca. 164 ppm, identifying tetrazole carbon signal as characteristic for 2,5-disubstituted
tetrazoles. According to the literature data, their C-5 tetrazole (C-5T) carbon atom signal lies in the
range of 159.0–167.8 ppm (except for 5-tert-butyl substituent) [10
signal of 1,5-disubstituted tetrazoles (except for 5-tert-butyl, 5-(2-furyl) and 5-(2-thienyl) substituents)
appears within the 150.2–156.6 ppm range [10 26 29 30]. The observed difference of the chemical
shifts for the two isomeric compounds calculated as ∆δ
δ2,5´isomer ´ δ1,5´isomer lies in the range
of 10.6–12.0 ppm 26 28
29]. Moreover, it has been also shown that the 13C-NMR chemical shifts
of tetrazole N-Cα (TN-Cα) carbon atom of 2,5-disubstituted tetrazoles are higher compared to the
corresponding resonances in the 1,5-disubstituted isomers [10 26 31 32]. Therefore, comparison of
13C-NMR data of
and 10 with those presented for 2,5- and 1,5-disubstituted tetrazoles allows
,25–28]. By contrast, C-5T carbon atom
,
, ,
=
[
, ,
,
, ,
2, 3, 6, 9
unambiguously identifying and differentiating isomeric N-1- and N-2-substituted tetrazoles. The same
rules were applied by Kubo et al. [10] in the structure assignment of the N-1- and N-2-alkyl isomeric
products obtained by alkylation of candesartan derivative CV-11194 with methyl iodide, iodomethyl
pivalate and medoxomil chloride.
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Molecules 2015, 20, 21346–21363
2.3. Determination of the Structure of Dimedoxomil Impurities 9 and 10
The [M + H]+ values, m/z 671.2454 and 671.2463, obtained for both impurities
to C34H35N6O9. The elemental composition responds to a dimedoxomil derivative of olmesartan (
9
and 10 correspond
).
and 10 have different melting points, IR and 1H-NMR data than
8
However, both isolated products
9
the unknown compound depicted by Venkanna et al. [19] as N-1-medoxomil impurity 10. It may be
assumed that the compound described by these Authors exists in a different crystalline form. Then the
two samples of compound 10 may exhibit different melting points and IR data. However, the 1H-NMR
spectra recorded in the same solvent should be the same or very similar. A careful comparison of the
1H-NMR data (300 MHz, CDCl3) recorded by the Authors for the supposed N-1-medoxomil compound
with ours 1H-NMR data (200 MHz or 600 MHz in CDCl3) obtained for both dimedoxomil isomers
and 10 shows essential differences indicating that the compound synthesized by Venkanna et al. 19] is
9
[
none of the N-1- and N-2-medoxomil regioisomers (Table 3). Without any reliable proof, the authors
drew the structure of the supposed dimedoxomil compound by analogy to the incorrect structures of
the starting bromide
the paper. Since the structures of compounds
structure of the unknown compound was also determined as N-1-medoxomil substituted. It should be
noticed that the structure of N-1-medoxomil compound 10 was disclosed in the patent literature [33 34],
2
and the intermediates, i.e., ethyl ester
3 and medoxomil ester 6, also presented in
2,
3
and were reported as tritylated at N-1 position, the
6
,
synthetic article [35] and analytical article [36]. However, synthetic procedures and physicochemical
data (m.p., IR, 1H- and 13C-NMR) allowing preparation, identification and characterization of this
compound were not revealed. It is striking that in all of these papers the structural formulas of
N-tritylated compounds show trityl substituent at wrong N-1 position.
Table 3. 1H Chemical shifts (δH/ppm) for the unknown compound [19] and compounds
9 and 10 of
current work recorded in CDCl3 solution.
N-2-Substituted 9
600 MHz
Unknown Comp. [19]
N-1-Substituted 10
200 MHz
0.97 (3H, t)
1.62 (6H, s)
1.74 (2H, m)
2.08 (3H, s)
2.19 (3H, s)
2.72 (2H, t)
4.90 (2H, s)
5.44 (2H, s)
5.45 (2H, s)
5.61 (1H, br, OH)
6.82 (2H)
300 MHz
0.96 (3H, t)
1.54 (6H, s)
1.66 (2H, m)
1.7–1.98 (6H, s)
200 MHz
0.95 (3H, t)
1.61 (6H, s)
1.69 (2H, m)
1.89 (3H, s)
2.13 (3H, s)
2.63 (2H, t)
4.66 (2H, s)
4.91 (2H, s)
5.41 (2H, s)
5.57 (1H, s, OH)
6.87 (2H)
600 MHz
0.95 (3H, t)
1.61 (6H, s)
1.70 (2H, m)
1.89 (3H, s)
2.12 (3H, s)
2.63 (2H, t)
4.67 (2H, s)
4.91 (2H, s)
5.40 (2H, s)
5.53 (1H, s, OH)
6.87 (2H)
0.98 (3H, t)
1.62 (6H, s)
1.74 (2H, m)
2.08 (3H, s)
2.19 (3H, s)
2.72 (2H, t)
4.90 (2H, s)
5.44 (2H, s)
5.46 (2H, s)
2.55 (2H, t)
4.40 (2H, s)
4.92 (2H, s)
5.19 (2H, s)
6.82 (2H)
7.11 (2H)
6.98 (2H, d)
7.28 (2H, dd)
7.11 (2H)
7.12 (2H)
7.12 (2H)
7.44 (1H, dd)
7.49 (1H, td)
7.56 (1H, td)
7.84 (1H, dd)
7.41–7.61 (3H, m)
7.45–7.80 (4H, m)
7.50–7.77 (4H, m)
7.53 (1H, dd)
7.58 (1H, td)
7.63 (1H, dd)
7.72 (1H, td)
7.84 (1H, dd)
The structures of
9 and 10 could be determined only on the basis of the detailed analysis of
the multinuclear magnetic resonance results since in their case the attempts of obtaining crystals
1
suitable for SCXRD measurements failed. For these compounds careful analysis of the H-, 13C-,
15N-NMR chemical shifts and 1H/13C (1H/15N) gradient selected HSQC and HMBC (g-HSQC and
g-HMBC) correlations were done. Differentiation of
from 1H/13C g-HMBC experiment of methylene group of medoxomil substituent at the tetrazole ring.
In the case of methylene protons at 5.46 ppm (in CDCl3) correlate with carbons at 9.3, 130.7 and
9 and 10 was possible using cross-peaks coming
9
140.5 ppm (Figure 3, left). It means that only three carbons of medoxomil can be identified. Similarly,
in the case of 10 (Figure 3, right) methylene protons at 4.67 ppm (in CDCl3) correlate with medoxomil
21352

Molecules 2015, 20, 21346–21363
carbons at 8.9, 130.3 and 140.0 ppm. However, other additional correlation is observed in 2D g-HMBC
experiment. The fourth cross-peak at 4.67 ppm/154.6 ppm identifies tetrazole carbon atom and allows
10 to assign the structure of 1,5-disubstituted isomer [10,26,29,30]. Thus, the impurity 9 should be
2,5-disubstituted tetrazole.
Analyzing the results obtained, we have found another regularity for isomeric 2,5- and
1,5-disubstituted tetrazoles, with tetrazole ring at 21-position of the biphenyl system. For these
compounds most important are some aromatic 1H-NMR signals corresponding to the H-31 and H-51
1
protons. Comparison of the corresponding parts of H-NMR spectra in CDCl3 solution (Figure 4,
left) and DMSO-d6 solution (Figure 4, right) indicates that compounds 2, 3, 6 and 9 should be
structurally similar because they show the same sequence of signals. This is probably caused by
very similar inductive effect of the trityl (benzyl-type) and medoxomil (allyl-type) substituents. In
contrast, the 1H-NMR spectra for 10 differ significantly pointing to other tetrazole ring substitution by
the medoxomil substituent.
Figure 3. 1H/13C g-HMBC NMR spectrum of N-2 substituted dimedoxomil derivative
solution (left) and N-1 substituted dimedoxomil derivative 10 in CDCl3 solution (right).
9 in CDCl3
Figure 4. 1H-NMR spectra of
range (left) and in DMSO-d6 solutions at 7.4–7.9 ppm range (right).
2 (A), 3 (B), 6 (C), 9 (D) and 10 (E) in CDCl3 solutions at 7.3–8.0 ppm
According to another rule, proton N-Cα
H
signal of alkyl group at tetrazole is more deshielded for
1
2,5-disubstituted isomer compared to its 1,5-disubstituted counterpart [10
shifts of tetrazole medoxomil N-CαH2 of
whereas those of 10 at 5.19 ppm and at 4.67 ppm, respectively. Thus, the rule is fulfilled and
2,5-disubstituted tetrazole, whereas 10 1,5-disubstituted. Applying the above rule Kubo et al. [10
,26
,
30
,37]. H-NMR chemical
9
are in DMSO at 5.90 ppm and in CDCl3 at 5.46 ppm,
is
9
]
correctly assigned the ratio of N-1- to N-2-medoxomil isomer (δ 4.54 and δ 5.41, respectively, in CDCl3)
21353

Molecules 2015, 20, 21346–21363
in the isolated mixture obtained by alkylation of candesartan derivative CV-11194 with medoxomil
chloride (
N-2-substituted isomer (
derivatives of CV-11194.
5
) in DMF in the presence of K2CO3. The same rule allowed them to assign the ratio of N-1- to
δ
5.39 and 6.36, respectively, in CDCl3) in a mixture of N-pivaloyloxymethyl
δ
Finally, incomplete sets of the 15N chemical signals for
9 and 10 in DMSO coming from
1H-15N g-HMBC spectra are fully in accordance with data presented earlier for different 2,5- and
1,5-disubstituted tetrazoles [26] (see Table 4).
Based on the above-mentioned results it is possible to state that compounds 2, 3, 6 and 9 are
2,5-disubstituted tetrazoles possessing trityl/medoxomil group localized at tetrazole N-2 atom. Thus,
they belong to the 2H tetrazole series. Beyond any doubt, compound 10 is 1,5-disubstituted tetrazole
possessing medoxomil group localized at tetrazole N-1 atom and it belongs to the 1H tetrazole series.
Table 4. Comparison of 15N-NMR data for some 2,5- and 1,5-disubstituted tetrazoles.
2,5-Disubstituted Tetrazoles
Lit. Comp.
R
Ph-
Me-, CP-
PhCH2-
PhCH2CH2-
o-subst. Ph-
R2
Me-
N-2
´102.1
from
N-1
´79.5
N-3
+3.2
N-4
´51.8
[26] a
from
to ´71.8
´
81.0
from +1.2
to +6.0
from
to ´46.2
´
53.5
[26] a
Me-
´105.1 to
´99.9
9 b
Med
´99.2
´76.7
0.0
–
1,5-Disubstituted Tetrazoles
Lit. Comp.
R
Ph-
Me-, CP-
PhCH2-
PhCH2CH2-
o-subst. Ph-
R1
Me-
N-1
´156.2
from
´154.2 to
´151.2
´149.4
N-4
´50.4
N-2
´5.3
N-3
+11.4
[26] a
[26] a
10 b
from
to ´44.1
´
58.9
from ´7.7
from +9.2
to +14.6
Me-
to ´2.4
Med
–
´11.1
–
a
b
in CDCl3, relative to external CH3NO2; in DMSO-d6, relative to CH3NO2. Me—methyl, Ph—phenyl,
CP—cyclopropyl, Med—medoxomil.
3. Experimental Section
3.1. General Information
Ethyl 4-(1-hydroxy-1-methylethyl)-2-propylimidazole-5-carboxylate
(1;
97.0%), and
4-chloromethyl-5-methyl-1,3-dioxolen-2-one (medoxomil chloride,
from Shanghai FWD Chemicals Ltd. [21-(2-Triphenylmethyl-2H-tetrazol-5-yl)biphenyl-4-yl]methyl
bromide (
; 96.79%) marketed as N-(triphenylmethyl)-5-[41-(bromomethylbiphenyl)-2-yl]tetrazole,
i.e., without specified trityl position, was purchased from Huanggang Luban Pharmaceutical Co.
Ltd. (Huanggang, China). Potassium dihydrogen phosphate (V) ( 99.5%, for HPLC), phosphoric
5; 92.9%) were purchased
2
ě
acid (V) (85 wt % in H2O), acetonitrile and tetrahydrofuran of HPLC grade were purchased from
Sigma-Aldrich (Munich, Germany) and POCh S.A. (Gliwice, Poland) chemical companies. Deionized
water was prepared using MilliQ plus purification system (Millipore, Bradford, PA, USA). Potassium
bromide (FT-IR grade), deuterated chloroform and deuterated dimethylsulfoxide were purchased
from Merck KGaA (Darmstad, Germany). The course of all reactions and the purity of products
were checked by thin-layer chromatography (TLC). Analytical TLC was performed on silica gel
DC-Alufolien Kieselgel 60 F254 (Merck KGaA, Darmstad, Germany), with mixtures of hexanes, ethyl
acetate, dichloromethane and methanol, in various ratios as developing systems. Compounds were
21354

Molecules 2015, 20, 21346–21363
visualized by UV light (
λ = 254 nm). Column chromatography was carried out on silica gel (Kieselgel
60, 40–63 m, 230–400 mesh, Merck) with mixtures of ethyl acetate, methanol and dichloromethane in
µ
varying ratios as eluents.
3.2. Melting Point
Melting points were determined from DSC thermograms performed on a Mettler Toledo DSC822E
differential scanning calorimeter (Mettler-Toledo, Columbus, OH, USA).
3.3. FT-IR Spectroscopy
FT-IR spectra were taken for KBr pellets on a Nicolet Impact 410 FT-IR spectrophotometer (Thermo
Fisher Scientific, Waltham, MA, USA).
3.4. Mass Spectrometry
HRMS spectra were recorded on an AMD 604 Inectra Gmbh (AMD Inectra GmbH, Harpstedt,
Germany) and a Mariner PE Biosystem ESI-TOF (PerSeptive Biosystems/Applied Biosystems,
Waltham, MA, USA) spectrometers.
3.5. X-ray Analysis
The X-ray diffraction data for ethyl 4-(1-hydroxy-1-methylethyl)-2-propyl-1-[21-(2-triphenyl
methyl-2H-tetrazol-5-yl)biphenyl-4-yl]methyl-1H-imidazole-5-carboxylate (
3
) and (5-methyl-2-oxo-1,3-
dioxolen-4-yl)methyl 4-(1-hydroxy-1-methylethyl)-2-propyl-1-[2 -(2-triphenylmethyl-2H-tetrazol-5-yl)
biphenyl-4-yl]methyl-1H-imidazole-5-carboxylate ( ) were collected on an Oxford Diffraction X-calibur
(Oxford Diffraction: Wrocław, Poland) with Ruby detector (Mo-K radiation; = 0.71073 Å).
Monocrystals of and , suitable for the XRD experiment, were obtained from acetone-methanol and
1
6
α
λ
3
6
acetone solution, respectively. The data were collected at 100 K using an Oxford Cryosystem device
(Table 5). Data reduction and analysis were carried out with the CrysAlisPro program [38]. The space
group was determined using the XPREP program [39]. Structures were solved by direct methods using
the SHELXS program and refined using all F2 data, as implemented by the SHELXL program [40]. In
˝
3
, the imidazole unit reveals two positioned disorder (in ratio of 0.92:0.08) by rotation of 180 around
the C11–N71 bond (CH2-imidazole N1 bond). In , hydrogen atoms bonded to methyl groups (C85,
6
C100 and C120 i.e., methyl of medoxomil group and two methyls of acetone) are disordered over two
positions. The ratio was refined assuming that the sum of both components of the disorder is equal
to 1. Non-hydrogen atoms were refined with anisotropic displacement parameters. SIMU, SADI and
ISOR restraints were applied for the disordered atom. All H atoms were found in ∆ρ maps or placed
at calculated positions. Before the last cycle of refinement, all H atoms were fixed and were allowed to
ride on their parent atoms. For clarity, the minor component of the disordered imidazole units in
the acetone molecule in solvate, are omitted in Figures 2 and 3 respectively. Crystal structures of
were visualized with Mercury CDS 3.3 software [41]. Crystallographic data for (CCDC 1059380)
acetone solvate (CCDC 1059381) have been deposited with the Cambridge Crystallographic
3, and
6
3
and
and
6
6
3
Data Centre. Copies of this information may be obtained free of charge from the Director, CCDC,
12 UNION Road, Cambridge 1EZ. UK (fax: +44-1223-336033; e-mail: deposit@ccdc.cam.ac.uk or
http://www.ccdc.cam.ac.uk.
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Molecules 2015, 20, 21346–21363
Table 5. Summary of crystallographic data and structure refinement for 3 and 6 acetone solvate.
Identification Code
3
6 Acetone Solvate
Chemical formula
Molecular weight
Temperature
C45H44N6O3
716.86 g/mol
100(2) K
C48H44N6O6¨ C3H6O
858.97 g/mol
100(2) K
Wavelength
0.71073 Å
0.71073 Å
Crystal system, space group
Triclinic, P-1
Orthorhombic, Pca21
a = 13.493(3) Å
b = 11.100(3) Å
c = 29.124(4) Å
α = 90˝
a = 9.531(2) Å
b = 10.196(3) Å
c = 20.049(3) Å
α = 77.46(3) ˝
β = 80.09(3) ˝
γ = 78.98(3) ˝
1849.4(8) Å3
Unit cell dimensions
β = 90˝
γ = 90˝
4362.0(16) Å3
4, 1.308 mg/m3
0.088
Volume
Z, Calculated density
Absorbtion coefficient
F(000)
2, 1.287 mg/m3
0.082
760
1816
Crystal size
Theta range for data collection
0.41 ˆ 0.32 ˆ 0.16 mm
2.826˝–28.857˝
´12 ď h ď 10
´13 ď k ď 13
´25 ď l ď 26
13,774/8397
0.71 ˆ 0.49 ˆ 0.20 mm
2.798˝–28.929˝
´18 ď h ď 16
´9 ď k ď 15
Limiting indices
´38 ď l ď 27
13,935/7992
Reflections collected/unique
Completeness to Thmax
0.867
0.887
Absorption correction
None
None
Maximum and minimum transmission
0.987, 0.967 (shelx estimated)
the full-matrix least-squares
method using all F2 data
8397/643/188
1.028
0.983, 0.940 (shelx estimated)
the full-matrix least-squares
method using all F2 data
7992/577/1
Refinement method
Data/restraints/parameters
Goodness-of-fit on F2
1.046
Final R1/wR2 indices [I > 2sigma(I)]
R1/wR2 indices (all data)
0.057/0.110
0.037/0.077
0.093/0.129
0.044/0.081
0.301 and ´0.300 e/Å3
0.213 and ´0.247 e/Å3
Largest diff. peak and hole
3.6. NMR Spectroscopy
The NMR spectra of all the compounds studied were measured in CDCl3 or DMSO-d6 solutions
with a Varian-NMR-vnmrs600 (Varian Inc., Palo Alto, CA, USA) (at 298 K) equipped with a 600 MHz
PFG Auto XID (1H/15N-31P 5 mm) indirect probehead. Standard experimental conditions and standard
Varian programs (ChemPack 4.1) were used. To assign the structures under consideration, the following
1D and 2D experiments were employed: the 1D selective NOESY and 2D:COSY, 1H-13C HSQC and
1H-13C HMBC (in gradient version). The 15N-NMR chemical shifts were obtained on a basis of the 2D
1H-15N gradient selected HMBC experiment, optimized for nJ(N-H) = 6 Hz. The 1H- and 13C-NMR
chemical shifts were given relative to the TMS signal at 0.0 ppm, whereas neat nitromethane at
0.0 ppm was used as standard for 15N-NMR chemical shifts. Concentration of all solutions used for
measurements was about 20–30 mg of compounds in 0.6 mL of solvent.
3.7. High Performance Liquid Chromatography
Analytical HPLC were performed on a Waters Alliance 2695 system (Waters Chromatography
Division, Milford, MA, USA) equipped with a Waters W2489 dual
W2690/5 quaternary pump, autosampler, 100 L syringe, degasser and column oven. The data were
analyzed using the Empower 2 software package (built 2154, Waters Company, Milford, MA, USA).
λ absorbance detector, a Waters
µ
21356

Molecules 2015, 20, 21346–21363
3.7.1. N-Tritylolmesartan Medoxomil (6) Purity Determination
A new method was developed to estimate the purity of N-tritylolmesartan medoxomil (6) samples.
The chromatographic separations were performed on a Kinetex C18 100A (150
ˆ
4.6 mm i.d., particle
size 2.6 m) analytical column manufactured by Phenomenex (Phenomenex Inc., Torrance, CA, USA).
µ
˝
˝
The column oven temperature was set at 30 C and autosampler was kept at 5 C. All chromatographic
runs were carried out in a gradient elution mode, t (min)/B (%): 0/5; 3.5/40; 5/40; 13.5/75; 16/80;
19/95; 25/95; 26/5; 35/5, at a flow rate of 1.0 mL/min. Separations were achieved using a mobile
phase consisting of eluent A (0.1% H3PO4) and a mixture of CH3CN–THF (80:20, v/v) as an eluent B.
UV detection was performed at 250 nm and the injection volume was set as 10 µL. Individual stock
solutions of standards and samples were prepared in acetonitrile at the concentration of 0.5 mg/mL.
3.7.2. Olmesartan Medoxomil (OM, 7) Purity Determination
Determination of dimedoxomil impurities
method described by pharmacopoeia monographs [17
Q2(R1) guidelines, for the parameters like system suitability, specificity, linearity, precision, accuracy,
limit of detection (LOD) and limit of quantitation (LOQ) [42 43]. A typical HPLC chromatogram
of a reference standard solution comprising the medoxomil ester ( ), OM ( ), olmesartan acid (
and regioisomieric impurities and 10 at the concentration of 0.1 mg/mL, used for analyses of OM
batches is given in the Figure 5. The chromatographic separations were carried out on a Symmetry
C8 (100 ˆ 4.6 mm i.d., particle size 3.5 m) analytical column manufactured by Waters (Waters
9
and 10 in OM (7) samples was performed using a
,
18]. The method was validated as per the ICH
,
6
7
8)
9
µ
Corporation, Milford, MA, USA). The column oven temperature was set at 40 ˝C and autosampler
˝
was kept at 20 C. All chromatographic runs were carried out in gradient elution mode, t (min)/B (%):
0/25; 10/25; 35/100; 45/100; 46/100; 50/25, at a flow rate of 1.0 mL/min. Separations were achieved
using a mobile phase consisting of an eluent A [KH2PO4 buffer (2.04 g/L, pH 3.4):CH3CN, 8:2, v/v]
and an eluent B [KH2PO4 buffer (2.04 g/L, pH 3.4):CH3CN, 2:8, v/v]. UV detection was performed at
250 nm and the injection volume was set as 10 µL. Individual stock solutions of standards and samples
were prepared in acetonitrile at the concentration of 1.0 mg/mL.
Figure 5. Chromatogram of a reference standard solution containing OM (7) and the impurities 6, 8, 9,
and 10 at the concentration of 0.1 mg/mL.
3.8. Syntheses
The synthesis of OM (
7) and olmesartan (
8
) from commercially available substrates and
1
,
2
5 is
shown in Scheme 1. The detailed descriptions of the preparation of compounds
3
and 6–8 as well as
their physical and spectroscopic data are provided in the Supplementary Materials.
21357

Molecules 2015, 20, 21346–21363
1
3.8.1. (5-Methyl-2-oxo-1,3-dioxolen-4-yl)methyl 4-(1-hydroxy-1-methylethyl)-2-propyl-1-{2 -[2-(5-me-
thyl-2-oxo-1,3-dioxolen-4-yl)methyl-2H-tetrazol-5-yl]biphenyl-4-yl}methyl-1H-imidazole-5-carboxylate
1
(9) and (5-methyl-2-oxo-1,3-dioxolen-4-yl)methyl 4-(1-hydroxy-1-methylethyl)-2-propyl-1-{2 -[1-(5-
methyl-2-oxo-1,3-dioxolen-4-yl)methyl-1H-tetrazol-5-yl]biphenyl-4-yl}methyl-1H-imidazole-5-
carboxylate (10)
K2CO3 (1.95 g, 14.11 mmol, 2.1 eq), KI (0.67 g, 4.03 mmol, 0.6 eq) and medoxomil chloride
(
5
) (2.20 g, 14.78 mmol, 2.2 eq) were added to a suspension of olmesartan (
1.0 eq) in DMF (60 mL). After being stirred at room temperature for 22 h, TLC analysis (30%
MeOH/AcOEt) indicated the disappearance of the starting material and formation of two reaction
products characterized by similar, but different from OM ( ), polarity. The reaction mixture was
diluted with H2O (50 mL) and the products were extracted with AcOEt (2 50 mL). The combined
8, 3.00 g, 6.72 mmol,
8
7
ˆ
organic phases were dried over anhydrous MgSO4, filtered and concentrated under reduced pressure
to give an oily residue. The crude mixture of products (6.36 g, according to the HPLC analysis
9
:
10—42.56%:57.44%) was purified by column chromatography over silica gel with 50%–100%
AcOEt/hexanes gradient elution to afford N-2-medoxomil derivative (glassy oil containing ethyl
acetate, 0.82 g, Rf = 0.76 for 10% MeOH/AcOEt), a mixture of N-2- and N-1-medoxomil 10 derivatives
9
9
(2.09 g) and N-1-medoxomil derivative 10 (1.36 g, Rf = 0.70 for 10% MeOH/AcOEt). Total reaction
yield was 95%.
9
: FT-IR (KBr)
ν: 3438, 2968, 2933, 2874, 1828, 1738, 1708, 1675, 1529, 1463, 1433, 1392, 1308, 1227, 1131,
´1
1033, 1066, 765 cm
.
1
1H-NMR (CDCl3, 600 MHz)
δ: 7.84 (1H, dd, J = 7.7 and 1.0 Hz, biphenyl H-3 ), 7.56 (1H, td, J = 7.5 Hz
and 1.4 Hz, biphenyl H-51), 7.49 (1H, td, J = 7.6 and 1.3 Hz, biphenyl H-41), 7.44 (1H, dd, J = 7.7
and 0.9 Hz, biphenyl H-61), 7.11 (2H, BB1 of AA1BB1 system, biphenyl H-2 and H-6), 6.82 (2H, AA1
of AA1BB1 system, biphenyl H-3 and H-5), 5.46 (2H, s, tetrazole -CH2-N<), 5.44 (2H, s, imidazole
>N-CH2-), 4.90 (2H, s, -CH2O-), 2.72 (2H, t, J = 7.2 Hz, -CH2CH2CH3), 2.19 (3H, s, medoxomil CH3-5
at N-2 of tetrazole), 2.08 (3H, s, medoxomil CH3-5 of ester group), 1.74 (2H, m, -CH2CH2CH3), 1.62
(6H, s, -C(OH)(CH3)2), 0.98 (3H, t, J = 7.2 Hz, -CH2CH2CH3).
13C-NMR (CDCl3, 150 MHz)
δ: 165.9 (tetrazole C-5), 160.8 (>C=O), 160.0 (imidazole C-4), 152.3
(imidazole C-2), 151.8 (medoxomil >C=O of ester group), 151.3 (medoxomil >C=O at N-2 of tetrazole),
141.3 (biphenyl C-11), 140.6 (medoxomil C-4 or C-5 of ester group), 140.5 (medoxomil C-4 or C-5 at
N-2 of tetrazole), 140.0 (biphenyl C-1), 135.7 (biphenyl C-4), 132.8 (medoxomil C-4 or C-5 of ester
group), 130.8 (biphenyl C-61), 130.7 (medoxomil C-4 or C-5 at N-2), 130.4 (biphenyl C-51), 129.6 (2C,
biphenyl C-2 and C-6), 127.8 (biphenyl C-41), 125.5 (biphenyl C-21), 125.0 (2C, biphenyl C-3 and C-5),
116.0 (imidazole C-5), 70.4 (-
-CH2-N<), 29.2 (2C, -C(OH)(
9.3 (medoxomil CH3-5 at N-2 of tetrazole), 9.3 (medoxomil CH3-5 of ester group).
C
C
(OH)(CH3)2), 54.0 (-CH2O-), 49.2 (imidazole >N-CH2-), 45.1 (tetrazole
H3)2), 29.1 (- H2CH2CH3), 21.4 (-CH2 H2CH3), 13.8 (-CH2CH2 H3),
C
C
C
1
1H-NMR (DMSO-d6, 600 MHz)
δ: 7.77 (1H, dd, J = 7.8 and 1.2 Hz, biphenyl H-3 ), 7.63 (1H, td, J = 7.5
and 1.2 Hz, biphenyl H-51), 7.55 (1H, td, J = 7.5 and 1.2 Hz, biphenyl H-41), 7.47 (1H, dd,1J = 7.8 1and
1
1
1
1
1
1.2 Hz, biphenyl H-6 ), 7.05 (2H, BB of AA BB system, biphenyl H-2 and H-6), 6.84 (2H, AA of AA BB
system, biphenyl H-3 and H-5), 5.90 (2H, s, tetrazole -CH2-N<), 5.42 (2H, s, imidazole >N-CH2-), 5.21
(1H, s, -OH), 5.07 (2H, s, -CH2O-), 2.65 (2H, t, J = 7.2 Hz, -C 2CH2CH3), 2.17 (3H, s, medoxomil CH3-5
H
at N-2 of tetrazole), 2.08 (3H, s, medoxomil CH3-5 of ester group), 1.63 (2H, m, -CH2CH2CH3), 1.48
(6H, s, -C(OH)(CH3)2), 0.90 (3H, t, J = 7.2 Hz, -CH2CH2CH3).
13C-NMR (DMSO-d6, 150 MHz)
δ: 164.7 (tetrazole C-5), 160.7 (>C=O), 157.6 (imidazole C-4), 151.7
(medoxomil >C=O of ester group), 151.5 (medoxomil >C=O at N-2 of tetrazole), 151.0 (imidazole
C-2), 140.9 (biphenyl C-11), 140.4 (medoxomil C-4 or C-5 of ester group), 140.4 (medoxomil C-4 or
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Molecules 2015, 20, 21346–21363
C-5 at N-2 of tetrazole), 139.0 (biphenyl C-1), 136.2 (biphenyl C-4), 132.8 (medoxomil C-4 or C-5 of
ester group), 131.2 (medoxomil C-4 or C-5 at N-2 of tetrazole), 130.8 (biphenyl C-61), 130.5 (biphenyl
C-511), 130.3 (biphenyl C-31), 129.1 (2C, biphenyl C-2 and C-6), 127.8 (biphenyl C-41), 125.4 (biphenyl
C-2 ), 125.2 (2C, biphenyl C-3 and C-5), 116.1 (imidazole C-5), 69.6 (-
C(OH)(CH3)2), 54.1 (-CH2O-), 48.0
(imidazole >N-CH2-), 44.9 (tetrazole -CH2-N<), 29.7 (2C, -C(OH)(
CH3)2), 28.3 (-CH2CH2CH3), 20.6
(-CH2
CH2CH3), 13.6 (-CH2CH2CH3), 8.7 (medoxomil CH3-5 of ester group), 8.6 (medoxomil CH3-5 at
N-2 of tetrazole).
15N-NMR (DMSO-d6, 60 MHz)
´120.0 (imidazole N-3), ´211.6 (imidazole N-1).
δ
: 0.0 (tetrazole N-3),
´76.7 (tetrazole N-1),
´99.2 (tetrazole N-2),
HRMS (ESI) m/z 671.2454 (calcd. for C34H35N6O9 [M + H]+ 671.2466).
˝
˝
˝
10: mp 143.84–151.62 C (AcOEt/hexanes), peak 147.57 C, heating rate 10.00 C/min (white crystals).
˝
Lit. mp 126–130 C (acetone) [19].
FT-IR (pellets, KBr) : 3383, 3028, 2967, 2935, 2875, 1836, 1821, 1808, 1743, 1676, 1533, 1467, 1397, 1371,
ν
1313, 1230, 1192, 1151, 1085, 1003, 957, 769.
Lit. IR (KBr) ν: 3385, 2931, 2875, 1816, 1742, 1531 [19].
1H-NMR (CDCl3, 600 MHz)
δ: 7.72 (1H, td, J = 7.6 and 1.4 Hz, biphenyl H-5 ), 7.63 (1H, dd, J = 7.8 and
1
1
1
0.8 Hz, biphenyl H-6 ), 7.58 (1H, td, J = 7.5 and 1.2 Hz, biphenyl H-4 ), 7.53 (1H, dd, J = 7.7 and 1.1 Hz,
biphenyl H-31), 7.12 (2H, BB1 of AA1BB1 system, biphenyl H-2 and H-6), 6.87 (2H, AA1 of AA1BB1
system, biphenyl H-3 and H-5), 5.53 (1H, s, -OH), 5.40 (2H, s, imidazole >N-CH2-), 4.91 (2H, s, -C
H2O-),
4.67 (2H, s, tetrazole -CH2-N<), 2.63 (2H, t, J = 7.2 Hz, -C 2CH2CH3), 2.12 (3H, s, medoxomil CH3-5 of
H
ester group), 1.89 (3H, s, medoxomil CH3-5 at N-1 of tetrazole, 1.70 (2H, m, -CH2CH2CH3), 1.61 (6H, s,
-C(OH)(CH3)2), 0.95 (3H, t, J = 7.2 Hz, -CH2CH2CH3).
13C-NMR (150 MHz, CDCl3)
δ: 160.7 (>C=O), 160.6 (imidazole C-4), 154.6 (tetrazole C-5), 152.4
(imidazole C-2), 151.7 (medoxomil >C=O of ester group), 151.0 (medoxomil >C=O at N-1 of tetrazole),
141.0 (biphenyl C-11), 140.6 (medoxomil C-4 or C-5 of ester group), 140.0 (medoxomil C-4 or C-5 at
N-1 of tetrazole), 137.7 (biphenyl C-1), 137.1 (biphenyl C-4), 132.8 (medoxomil C-4 or C-5 of ester
group), 132.2 (biphenyl C-51), 131.1 (biphenyl C-31), 130.6 (biphenyl C-61), 130.3 (medoxomil C-4 or
C-5 at N-1 of tetrazole), 129.0 (2C, biphenyl C-2 and C-6), 128.4 (biphenyl C-41), 126.2 (2C, biphenyl
C-3 and C-5), 121.6 (biphenyl C-21), 115.9 (imidazole C-5), 70.4 (-
(imidazole >N-CH2-), 39.8 (tetrazole -CH2-N<), 29.2 (- H2CH2CH3), 29.1 (2C, -C(OH)(
(-CH2 H2CH3), 13.8 (-CH2CH2 H3), 9.3 (medoxomil CH3-5 of ester group), 8.9 (medoxomil CH3-5 at
N-1 of tetrazole).
C
(OH)(CH3)2), 53.9 (-CH2O-), 48.9
C
CH3)2), 21.2
C
C
1H-NMR1(DMSO-d6, 600 MHz)
δ
1: 7.76 (1H, m, ΣJ « 16.8 Hz, biphenyl H-5
'), 7.64 (3H, m, biphenyl
1
1
1
1
1
1
1
H-3 , H-4 and H-6 ), 7.04 (2H, BB of AA BB system, biphenyl H-2 and H-6), 6.85 (2H, AA of AA BB
system, biphenyl H-3 and H-5), 5.40 (2H, s, imidazole >N-CH2-), 5.21 (1H, s, -OH), 5.19 (2H, s, tetrazole
-CH2-N<), 5.04 (2H, s, -CH2O-), 2.58 (2H, t, J = 7.2 Hz, -CH2CH2CH3), 2.08 (3H, s, medoxomil CH3-5
of ester group), 1.78 (3H, s, medoxomil CH3-5 at N-1 of tetrazole), 1.56 (2H, m, -CH2CH2CH3), 1.47
(6H, s, -C(OH)(CH3)2), 0.86 (3H, t, J = 7.2 Hz, -CH2CH2CH3).
13C-NMR (DMSO-d6, 150 MHz)
δ: 160.7 (>C=O), 157.6 (imidazole C-4), 154.0 (tetrazole C-5), 151.6
(medoxomil >C=O of medoxomil ester), 151.2 (medoxomil >C=O at N-1 of tetrazole), 151.0 (imidazole
C-2), 141.1 (biphenyl C-11), 140.4 (medoxomil C-4 or C-5 of ester group), 139.7 (medoxomil C-4 or
21359

Molecules 2015, 20, 21346–21363
C-5 at N-1 of tetrazole), 137.2 (biphenyl C-1), 137.1 (biphenyl C-4), 132.8 (medoxomil C-4 or C-5
of ester group), 131.8 (biphenyl C-51), 131.0 (2C, biphenyl C-31 and C-61), 130.7 (medoxomil C-4 or
C-5 at N-1 of tetrazole), 128.5 (2C, biphenyl C-2 and C-6), 128.0 (biphenyl C-41), 125.6 (2C, biphenyl
C-3 and C-5), 121.6 (biphenyl C-21), 116.2 (imidazole C-5), 69.6 (-
(imidazole >N-CH2-), 39.8 (tetrazole -CH2-N<), 29.6 (2C, -C(OH)(
(-CH2 H2CH3), 13.5 (-CH2CH2 H3), 8.7 (medoxomil CH3-5 of ester group), 8.2 (medoxomil CH3-5 at
C
(OH)(CH3)2), 54.1 (-CH2O-), 47.9
C
H3)2), 28.2 (- H2CH2CH3), 20.5
C
C
C
N-1 of tetrazole).
15N-NMR (DMSO-d6, 60 MHz)
´212.3 (imidazole N-1).
δ:
´11.1 (tetrazole N-2),
´119.6 (imidazole N-3),
´149.4 (tetrazole N-1),
HRMS (ESI) m/z 671.2463 (calcd. for C34H35N6O9 [M + H]+ 671.2466).
3.8.2. Reaction of Olmesartan (8) with Medoxomil Chloride (5) According to the Procedure Described
by Venkanna et al.
K2CO3 (2.23 g, 16.17 mmol, 1.95 eq), n-Bu4NBr (cat.) and medoxomil chloride
mmol, 2.0 eq) were added to a solution of olmesartan ( , 3.7 g, 8.29 mmol, 1.0 eq) in Me2CO (40 mL).
After heating at reflux for 12 h, TLC analysis (30% MeOH/AcOEt) indicated the disappearance of the
starting material and formation of two reaction products characterized by similar, but different from
OM ( ) polarity. The reaction mixture was cooled to room temperature and filtered. The resulting
5 (2.71 g, 18.24
8
8
7
solid was washed with Me2CO (10 mL). The combined filtrate and washings were concentrated under
reduced pressure, and the residue was portioned between AcOEt (50 mL) and H2O (50 mL). The
acetate layer was dried over anhydrous Na2SO4, filtered and concentrated under reduced pressure
to give a dark-brown oily residue (6.64 g, according to the HPLC analysis
9:10—56.5%:43.5%). An
unsuccessful attempt to purify the crude mixture of N-2- ( ) and N-1- (10) medoxomil derivatives and
9
acetone condensation products by column chromatography on silica gel with 10% MeOH/CH2Cl2
elution was made. A dark-brown oily starting material was recovered. After the addition of Me2CO,
no solid of dimedoxomil derivatives precipitated out of solution.
4. Conclusions
Two principal process-related impurities of olmesartan medoxomil API (OM,
the olmesartan acid ( ) were described. The impurities were identified as isomeric N-2- (
10) (5-methyl-2-oxo-1,3-dioxolen-4-yl)methyl derivatives of OM. The N-2-medoxomil impurity
7
) formed from
) and N-1-
is
8
9
(
9
a new compound that has never been synthesized, identified and characterized. The structure of
regioisomeric N-1-medoxomil impurity 10 was disclosed in the literature. However, either there
were no synthetic procedures and physicochemical data, or presented data do not prove the revealed
structure. Thus, both compounds
structures of the impurities, the substrate
9
and 10 were synthesized, separated and fully characterized. The
and the intermediates and in the synthesis of OM
2
3
6
were determined by crystallographic and/or spectroscopic methods. The careful analysis of ours and
reported analytical data (m.p., IR, NMR and SCXRD) allowed disproving the structural dualism of the
tetrazole tritylated compounds 2, 3 and 6, existing in the literature data and chemical databases, and
undoubtedly established their exact structure as N-2-tritylated tetrazole regioisomers. The analysis
also evidently shows that the tritylated intermediates of other sartans with 2-(tetrazol-5-yl)biphenyl
moiety possess triphenylmethyl substituent at tetrazole N-2 position.
Supplementary Materials: The detailed descriptions of the preparation of
data (IR, 1H-NMR, 13C-NMR, HRMS) and DSC thermograms of the starting bromide
synthesized and are provided in the Supporting Materials. Supplementary materials can be accessed
at: http://www.mdpi.com/1420-3049/20/12/19762/s1.
3
and
6–
8
as well as the spectral
2
and compounds
3
6–8
Acknowledgments: The National Centre for Research and Development provided financial support under
the INNOTECH Programme (INNOTECH-K1/IN1/14/159003/NCBR/12). The authors would like to kindly
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Molecules 2015, 20, 21346–21363
acknowledge A. Witkowska, K. Kuziak and K. Trzcin´ska from the R & D Analytical Department, Pharmaceutical
Research Institute, for recording of IR spectra and DSC thermograms.
Author Contributions: Iwona Dams and Wojciech J. Szczepek designed and carried out the synthetic experiments,
analyzed the data and wrote the paper. Anna Ostaszewska carried out synthetic experiments. Maria Puchalska,
Justyna Chmiel and Iwona Bujak performed HPLC analyses. Piotr Cmoch carried out NMR experiments and
interpreted the spectra. Agata Białon´ska performed X-ray analyses and resolved the molecular structure of the
compounds 3 and 6.
Conflicts of Interest: The authors declare no conflict of interest.
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©
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