Oxadiazoles in medicinal chemistry

Article abstract of DOI:10.1021/jm2013248

Oxadiazoles are five-membered heteroaromatic rings containing two carbons, two nitrogens, and one oxygen atom, and they exist in different regioisomeric forms. Oxadiazoles are frequently occurring motifs in druglike molecules, and they are often used with the intention of being bioisosteric replacements for ester and amide functionalities. The current study presents a systematic comparison of 1,2,4- and 1,3,4-oxadiazole matched pairs in the AstraZeneca compound collection. In virtually all cases, the 1,3,4-oxadiazole isomer shows an order of magnitude lower lipophilicity (log D), as compared to its isomeric partner. Significant differences are also observed with respect to metabolic stability, hERG inhibition, and aqueous solubil ity, favoring the 1,3,4-oxadiazole isomers. The difference in profile between the 1,2,4 and 1,3,4 regioisomers can be rationalized by their intrinsically different charge distributions (e.g., dipole moments). To facilitate the use of these heteroaromatic rings, novel synthetic routes for ready access of a broad spectrum of 1,3,4-oxadiazoles, under mild conditions, are described.

Full text of DOI:10.1021/jm2013248

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pubs.acs.org/jmc
Oxadiazoles in Medicinal Chemistry
Jonas Bostrom,* Anders Hogner, Antonio Llinas, Eric Wellner, and Alleyn T. Plowright
̀
̈
AstraZeneca R&D Molndal, S-431 83 Molndal, Sweden
̈
̈
S
* Supporting Information
ABSTRACT: Oxadiazoles are five-membered heteroaromatic rings
containing two carbons, two nitrogens, and one oxygen atom, and
they exist in different regioisomeric forms. Oxadiazoles are frequently
occurring motifs in druglike molecules, and they are often used with
the intention of being bioisosteric replacements for ester and amide
functionalities. The current study presents a systematic comparison of
1,2,4- and 1,3,4-oxadiazole matched pairs in the AstraZeneca com-
pound collection. In virtually all cases, the 1,3,4-oxadiazole isomer
shows an order of magnitude lower lipophilicity (log D), as compared
to its isomeric partner. Significant differences are also observed with
respect to metabolic stability, hERG inhibition, and aqueous solubil-
ity, favoring the 1,3,4-oxadiazole isomers. The difference in profile between the 1,2,4 and 1,3,4 regioisomers can be rationalized
by their intrinsically different charge distributions (e.g., dipole moments). To facilitate the use of these heteroaromatic rings,
novel synthetic routes for ready access of a broad spectrum of 1,3,4-oxadiazoles, under mild conditions, are described.
for carbonyl containing compounds such as esters, amides,
carbamates, and hydroxamic esters.14−16
INTRODUCTION
■
Compounds containing heterocyclic ring systems are of great
importance both medicinally and industrially. As an example,
five-membered ring heterocycles containing two carbon
atoms, two nitrogen atoms, and one oxygen atom, known as
oxadiazoles (Figure 1a), are of considerable interest in different
areas of medicinal and pesticide chemistry and also polymer
and material science.1 The level of interest is clearly shown,
as over the past 9 years the number of patent applications
containing oxadiazole rings has increased considerably (100%),
to a total of 686 (Figure 1b). Within drug discovery and devel-
opment, a number of compounds containing an oxadiazole
moiety are in late stage clinical trials, including zibotentan (1)
as an anticancer agent2 and ataluren (2) for the treatment of
cystic fibrosis3 (Figure 2). So far, one oxadiazole containing
compound, raltegravir (3),4 an antiretroviral drug for the treat-
ment of HIV infection, has been launched onto the market-
place. It is clear that oxadiazoles are having a large impact on
multiple drug discovery programs across a variety of disease
areas, including diabetes,5 obesity,6 inflammation,7 cancer,8 and
infection.9
Oxadiazole rings have been introduced into drug discovery
programs for several different purposes. In some cases, they
have been used as an essential part of the pharmacophore,
favorably contributing to ligand binding.10 In other cases,
oxadiazole moieties have been shown to act as a flat, aromatic
linker to place substituents in the appropriate orientation,11 as
well as modulating molecular properties by positioning them in
the periphery of the molecule.12 It has also recently been shown
that significant differences in thermodynamic properties can be
achieved by influencing the water architecture within the aldose
reductase active site by using two structurally related oxadiazole
regioisomers.13 Finally, oxadiazoles have been used as replacements
Oxadiazole rings can exist in different regioisomeric forms;
two 1,2,4-isomers (if asymmetrically substituted), a 1,3,4-
isomer, and a 1,2,5-isomer (Figure 1). The 1,2,5-regioisomer
is significantly less common (Figure 1) and orients the side
chains (R1 and R2, Figure 1) in different positions relative
to the other three isomers. The two 1,2,4- and the 1,3,4
regioisomeric oxadiazoles all present the R1 and R2 side
chains with essentially the same exit vector arrangement,
thus placing the side chains in very similar positions. The
consequence is that matched pairs will show the same overall
molecular shapes and are thus expected to bind in a similar
fashion.17 Moreover, oxadiazoles display interesting hydrogen
bond acceptor properties, and it will be shown that the
regioisomers display significantly different hydrogen bonding
potentials.
While searching for new inhibitors of an enzyme in an in-
house drug discovery program, we were interested in syn-
thesizing the 1,3,4- and the two 1,2,4-oxadiazole regioisomers in
an efficient manner to explore if and how different electronic
effects influenced ligand binding, while maintaining the direc-
tionality of the substituents. Interestingly, the 1,3,4-regioisomer
showed an improved lipophilicity profile, as well as more
favorable ADME properties. To probe whether this was a
general effect, we searched the entire AstraZeneca compound
collection for sets of oxadiazole containing compounds that
only differed in their regioisomeric form. A data set was ob-
tained and was subject to thorough analysis. A rationalization
for the significant differences between oxadiazole regioisomers
Received: October 4, 2011
Published: December 19, 2011
© 2011 American Chemical Society
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Journal of Medicinal Chemistry
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Figure 1. (a) Oxadiazole rings can exist in different regioisomeric forms. One 1,3,4-isomer and one 1,2,5-isomer and two 1,2,4-isomers (if asym-
metrically substituted). (b) The number of patent applications containing oxadiazoles has increased significantly between 2000 and 2008. The 1,2,5-
regioisomer is less common than the other regioisomers.
Figure 2. Structures of oxadiazole containing compounds in late stage clinical development (1 and 2) or launched drug (3).
using high-level quantum molecular calculations is presented.
Finally, a novel, fast, and mild synthetic approach to attain
structurally diverse 1,3,4-oxadiazoles is described.
1,2,4-Oxadiazoles (7) are most frequently prepared from nitriles
(4) or other amidoxime precursors. After acylation of 5 to the
O-acylamidoxime 6, cyclization is mainly achieved by heating
at ca. 100 °C or in the presence of coupling reagents.18 One
of the most common building blocks for the synthesis of 1,3,
4-oxadiazoles (10) are hydrazide derivatives (9), which can be
cyclized under various conditions. Most of the preparation methods
involve strong acidic conditions at elevated temperatures, which
narrows the convenient access of derivatized 1,3,4-oxadiazoles.18,19
Milder reaction conditions can be achieved via the genera-
tion of phosphonium intermediates or by using 2-chloro-1,3-
dimethylimidazolinium chloride (DMC), leading to activation
of the monoacylhydrazide moiety, followed by cyclization to give
the 1,3,4-oxadiazole (10). Such methods, however, generally
require longer reaction times to go to completion. None of the
existing protocols were considered sufficiently robust to generate
a wide variety of oxadiazoles at low temperature with short reac-
tion times to optimize the accessibility of the 1,3,4-isomers. The
oxophilicity of the phosphonium agent is often used to initiate
the dehydration process. A mild method for the dehydration
CHEMISTRY
■
The preparation of the 1,2,4-oxadiazole (7) as well as the 1,3,4-
isomer (10) is well described in the literature; see Scheme 1.18
a
Scheme 1. Common Cyclization Reactions of Oxadiazoles
a
Reagents and conditions: (a) EtOH, NH2OH; (b) acylating reagent,
DIPEA, DCM; (c) pyridine, reflux; (d) hydrazine hydrate, rt; (e)
acylating reagent, DIPEA, DCM; (f) POCl3, 100 °C; (g) 2-chloro-
1,3-dimethylimidazolinium chloride, triethylamine, DCM, room temp,
16 h.
́
of acylhydrazides (9) using PPh3 under Mukaiyamas redox-
condensation is known to generate 1,3,4-oxadiazoles (10) with
some limitations depending on the substitution pattern.20 Kelly
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and co-workers have investigated the dehydrocyclization of
the exception of the phenol substituted compound 16g, possi-
bly due to reaction of the phosphonium salt 11 with the phenol
precursor 14g.
thiazolines by employing the PV-reagent 11 (Figure 3).21 Their
Attempts were made to understand the necessity of the
reagents in the cyclization step (c) in Scheme 2. Carrying out
the reaction in the absence of triphenylphosphine oxide (just
triflic anhydride) did not yield any oxadiazole products, in
contrast to the described procedure by Liras et al.22 However, it
was found that the triphenylphosphine oxide can be replaced
with diphenyl-2-pyridylphosphine oxide, which is readily avail-
able by oxidation of the 2-pyridylphosphine.23 Despite some
loss in reaction yields (approximately 20−50%), the use of this
reagent can be advantageous, particularly in a parallel setting
where HPLC purification of the compounds is automated and
there is a risk for coelution of the product with triphenyl-
phosphine oxide.
Figure 3. The PV-reagent 11, employed for dehydrocyclization of
thiazolines.21
method tolerates the presence of protective groups which are
stable under mild acidic conditions, such as esters and carbamates,
and showed excellent stereocontrol. Our interest was focused
on the feasibility of utilizing this procedure to generate 1,3,4-
oxadiazoles with a variety of substitution patterns. To inves-
tigate the scope of the reaction, fast and easy access to diverse
building blocks with varying functionalities was needed. Hence,
starting from acylhydrazides as versatile starting materials, amino-
semicarbazoles or 1,2-diacylated hydrazides are readily accessed
by reaction with isocyanates or acylating agents, respectively
(Scheme 2). After generation of the bistriphenylphosphonium
COMPUTATIONAL RETRIEVAL OF MATCHED
PAIRS
■
The current work describes differences between the most
common oxadiazole regioisomers, the 1,2,4- and 1,3,4- oxadia-
zoles, and not the 1,2,5-oxadiazoles. That is, the focus is on
regioisomers which differentiate only by ring atom positions
and not in the relative position of their corresponding R1
and R2 substituents. The data set of 1,2,4-oxadizole and 1,3,4-
oxadiazole matched pairs was generated in an automated fashion
as follows. The AstraZeneca corporate collection was queried for
all 1,2,4-oxadiazole containing compounds, where the attachment
atoms of R1 and R2 were allowed to be either carbon, nitrogen,
or hydrogen (Figure 1), using the pattern matching function
(OESubSearch) implemented in the OEChem Python Toolkit.24
In a subsequent step, each 1,2,4-oxadiazole containing mole-
cule was transformed into the corresponding 1,3,4-oxadiazole
matched pair using the OEChem reaction processing function
(OEUniMolecularRxn), as shown in Figure 4.24 The SMILES
strings for the transformed structures were standardized (cano-
nicalized) and used to search the AstraZeneca corporate collec-
tion, to computationally efficiently identify all exact matched
pairs. In this study, exact matched pairs are defined as pairs
of molecules only differing by oxadiazole regioisomers; the
remaining parts (R1 and R2) of the molecules are identical
within a matched pair. There are many methods for molecular
manipulations and for performing matched molecular pairs
analysis (MMPA).25 The advantage of the query-based ap-
proach described herein is that it is tailored to provide detailed
insights on one specific transformation, in contrast to other
MMPA methods.25 Figure 4 depicts a schematic overview of
the process of retrieval of all oxadiazole matched pairs in the
AstraZeneca corporate collection.
Scheme 2. Preparation of 1,3,4-Oxadiazoles Using
a
Phosphonium Salt 11
a
Reagents and conditions: (a) R2NCO (1 equiv), ethanol, room temp;
(b) R2COCl (1.1 equiv), triethylamine (1.5 equiv), DCM, room temp;
(c) Ph3PO (3 equiv), Tf2O (1.5 equiv), DCM, 0 °C to room temp.
ditriflate 11, the cyclization proceeds by dehydration of the
hydrazides (13 and 14) in a one-pot procedure at room tem-
perature.
Reaction times are typically in the range 1−30 min with a
tendency of longer reaction times for electron deficient
R1-substituents. Yields are moderate to good (Table 1), with
Table 1. Syntheses of Aminooxadiazoles 15a−e and
Oxadiazoles 16a−i
a
acylhydrazide
R1
R2
product yield
13a
13b
13c
13d
13e
14a
14b
14c
14d
14e
14f
phenyl
phenyl
ethyl
15a
15b
15c
15d
15e
16a
16b
16c
16d
16e
16f
72%
77%
49%
52%
82%
96%
82%
68%
70%
84%
78%
26%
52%
78%
phenyl
3-pyridyl
n-propyl
ethyl
phenyl
DATA SET
■
5-bromothiophen-2-yl phenyl
The entire AstraZeneca compound collection was used as a
source for the systematic retrieval of a comprehensive data set
of exact matched pairs of 1,2,4- and 1,3,4-oxadiazole regio-
isomers (Figure 4). This resulted in 770 exact matched pairs.
A subset of 148 of the 770 pairs displayed satisfactory purity
and quantity, and was found to be externally known and thus
available for public disclosure. The 148 matched pairs include
282 compounds and define the data set used in the current
study. There are 16 triplets in the data set, meaning that
the 1,3,4-regioisomer and both of the two corresponding 1,2,4-
regioisomers are present.
phenyl
phenyl
phenyl
p-tolyl
phenyl
p-chlorophenyl
benzyl
phenyl
p-chlorophenyl
phenyl
ethyl
3-pyridyl
ethyl
14g
14h
14i
4-hydroxyphenyl
16g
16h
16i
5-bromothiophen-2-yl iso-propyl
phenyl N,N-dimethyl-4-
aminophenyl
a
The yields are based on isolated compounds.
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Figure 4. Automated procedure used to identify all regioisomeric oxadiazole matched pairs in the AstraZeneca corporate collection. Step 1. A python
script, using the OEChem tool-kit, is used to process a MDL reaction (.rxn) file, and the 1,2,4-oxadiazole substructure is used to search a database
containing molecules (step 2), with their associated data (e.g., log D, solubility, HLM CLint, cytochrome P450, hERG inhibition, etc). The molecules
in the database matching the 1,2,4-oxadiazole query pattern are transformed to (virtual) 1,3,4-oxadiazole compounds using the OEChem reaction
processing function (step 3) and are subsequently used to search the standardized SMILES strings stored in the database by computational efficient
string comparisons (step 4). All exact molecular matches are recorded and stored in a tabulated format.
The structural diversity of the compounds in the data set is
reflected in their activity against different target proteins. The
data set was matched against GVKBio’s Medicinal Chemistry
and Target Databases26 containing SAR data extracted from
medicinal chemistry journals and patents; 82 compounds were
identified that are claimed for 16 target proteins, belonging
to several different target-classes (e.g., enzymes, GPCR, and
ion channels). In addition, the molecules in the data set were
evaluated against five calculated physicochemical properties
widely used in medicinal chemistry (ClogP, molecular weight,
the number of rotatable bonds, and the number of hydrogen
bond acceptors and donors). The compounds essentially show
the same characteristics as compounds with typical druglike
properties. It should also be mentioned that the data set covers
molecules with different ionization states at physological pH;
253 neutral, 25 basic, and 4 acidic compounds. The data set was
also subjected to a near-neighbor analysis, and the 282 compounds
were found to cover several different structural series: 25 clusters
according to lingo similarities,27 using the recommended similarity
cutoff of 0.5.28
In summary, an exhaustive and systematic matched pair anal-
ysis of AstraZeneca’s corporate compound collection resulted
in a set of 148 1,2,4-oxadiazole and 1,3,4-oxadiazole matched
pairs. The relatively large and unbiased set spanned a broad
spectrum of physical and chemical properties and should there-
fore allow general conclusions to be drawn. The lipophilicity
(log D) values were determined for all compounds by a HPLC
LC/MS method.29 Aqueous solubility, hERG inhibition, pKa
values, cytochrome P450 (CYP) inhibition, metabolic stability
(HLM CLint), time-dependent inhibition (TDI), and CYP
Reaction Phenotyping (CRP) data are reported for a significant
number of example compounds. The data set, including results
as well as detailed descriptions for the experimental methods,
can be found in the Supporting Information.
RESULTS AND DISCUSSION
■
Our interest in the observed differences in physicochemical and
pharmacological properties between a small set of 1,2,4- and
1,3,4-oxadiazole regioisomers in an internal drug discovery pro-
ject prompted us to embark on a more general analysis. Thus,
all oxadiazole matched pairs were extracted from the Astra-
Zeneca compound collection to provide a larger data set. The
main goal was to develop a better understanding for the gene-
rality of the observed differences and attempt to rationalize the
observed effects.
Oxadiazole Isomers Influence on Lipophilicity. The
role of lipophilicity in determining the overall quality of can-
didate drug molecules has recently been described as of para-
mount importance.30 For example, lipophilicity has been shown
to significantly impact oral bioavailability,31 lead to increased
promiscuity,32 and increase the risk of toxicity.33 Lipophilic
compounds are also likely to be rapidly metabolized, to show
low solubility, and to display poor oral absorption.34 As a
consequence, medicinal chemists generally follow the mantra
“reduce lipophilicity” when encountering any such issue.35
A common design strategy to reduce lipophilicity is to introduce
polar atoms, while maintaining the overall molecular shape of
lead molecules, in order to improve the odds of retaining the
affinity to the target protein. This approach has, for example,
been successfully shown in terms of affording “me-too” drugs.36
The lipophilicity of compounds is often assessed by calcu-
lations, and as such ClogP37 calculations are typically first in line
when choosing a method. Chemical series in drug discovery
projects often include charged moieties, and this is a potential
drawback when using log P calculations, since they do not expli-
citly take account of different ionic states and their potential
influence on compound lipophilicity. On the other hand, log D
calculations are designed to account for pH dependencies
and are, thus, frequently used when predicting lipophilicity for
ionizable compounds.
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Lipophilicity is a deceptively simple system to model com-
pared to other systems encountered in drug discovery (such as
blood brain barrier penetration or whole organs). Despite a
large amount of experimental data available, it is still not a
trivial task to achieve precise predictions.38 This is illustrated in
Figure 5, where experimentally determined log D values are
approaches to modeling log D is matched molecular pairs
analysis.40 The idea of matched molecular pairs analysis is to
systematically identify pairs of molecules with minor structural
differences and to determine the difference in property change
associated, rather than predicting their absolute values.40
When comparing the experimentally determined log D values
for the data set of 148 oxadiazole matched pairs, a remarkable
correlation is revealed, as shown in Figure 6a. The correlation
Figure 5. Experimentally determined log D values plotted against
calculated ClogP values (a) and ACDlogD7.4 values (b) for the data
set of oxadiazoles. The correlation coefficients (R2) are 0.85 and 0.66,
respectively. The higher the experimentally determind log D, the less
predictive the ClogP model is. The ACD logD7.4 predictions deviate
from experiments to a relatively large extent. The 1:1 lines are shown
for clarity.
Figure 6. (a) Experimentally determined log D values plotted for the
matched-pairs of 1,2,4- and 1,3,4-oxadiazoles. The 1,3,4-regioisomer is,
in all cases, less lipophilic. The 1:1 line is shown for clarity. (b) A box-
plot displaying the log D differences between the regioisomers. The
median log D value for the 1,2,4-isomers is 4.4, whereas it is 3.2 for the
1,3,4-isomers.
plotted against calculated ClogP values (Figure 5a) or ACD
logD7.439 values (Figure 5b) for the current data set of oxa-
diazoles (N = 282). The high correlation coefficient (R2 = 0.85)
for ClogP is satisfactory, and most compounds are correctly
ranked. However, the correlation deviates from the 1:1 line,
which results in less accurate predictions for lipophilic com-
pounds. Thus, the higher the logD, the less predictive the ClogP
model becomes. The accuracy in the ACD logD7.4 predictions is
suboptimal. For example, compounds where the experimentally
determined log D values are around 3.0 show a range of four log
units in the predictions (Figure 5b). Given this lack of precision,
it is clear that further method development efforts are desirable
to better predict log D values. A recent example that is markedly
different to linear models using fragment-based parametrized
coefficient (R2) is as high as 0.97. The 1,3,4-oxadiazole regio-
isomers consistently show lower log D values, as compared to
their 1,2,4-oxadiazole matched pair. The box-plot in Figure 6b
shows that the median log D difference for the regioisomers is
1.2 log D units. This important information can be successfully
employed when designing compounds with the aim of lowering
lipophilicity.
It should be noted that, although not as pronounced, a
similar correlation to the one shown in Figure 6a is obtained
when using calculated ClogP values; 1,3,4-oxadiazole regio-
isomers are consistently predicted to be lower in lipophilicity
(ΔClogP = 0.86). The corresponding correlation plot for ACD
logD7.4 predictions shows a mixed picture; almost half of the
regioisomeric pairs are erroneously predicted to have the same
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value. The ACD logD7.4 mispredictions are independent of
molecular charge (i.e., neutral or charged species at physio-
logical pH) and do not pick up the observed difference in log D
within a matched pair to a satisfactory level of detail.
The differences in log D between the 1,3,4- and the 1,2,4-
regioisomers seem to be general and wide-ranging; no signifi-
cant difference is observed between the oxadiazole compounds
with aliphatic and aromatic substituents. Furthermore, little
or no difference in log D is observed between the two 1,2,4
regioisomers in the 16 triplets in the data set. The different
regioisomers affect the basicity of nearby ionizable functional
groups to a very similar extent, as evidenced by experimentally
determined pKa values for a subset of test compounds (vide
infra).
Oxadiazole Isomers Influence on Aqueous Solubility.
Drug discovery compounds with low solubility carry a higher
risk of failure due to the fact that insufficient solubility may, for
example, compromise pharmacokinetic and pharmacodynamic
properties and/or mask other undesirable properties. In some
cases, poor solubility can stop promising drugs from reaching
the market,41 and regulatory authorities currently require addi-
tional investigations on low soluble compounds when the aim is
oral administration.
Figure 7. Experimentally determined log D values plotted against
molar solubility values for the set of oxadiazoles (N = 116). The cor-
relation coefficient is low (R2 = 0.4), indicating that other factors than
lipophilicity (log D) are important for the aqueous solubility of the
compounds. The 1:1 line is shown for clarity.
As noted above, lipophilicity can influence many other molec-
ular properties,30 and perhaps the most obvious connection
exists between lipophilicity and solubility. For example, aqueous
solubilities are often reported to be adversely influenced by
lipophilicity.42 Poor aqueous solubility can also be the result of
solid phase contributions, such as strong intermolecular inter-
actions, making dissolution energetically unfavorable. There are
several experimental approaches available to tackle the issue of
poor aqueous solubility. For example, appropriate formulation
work, formation of pro-drugs, polymorph selection, and the
generation of salts (of acidic and basic drugs) can be used
to overcome solubility related problems. However, these ap-
proaches can be expensive and do not guarantee success. Thus,
the preferred approach to improve solubility is by molecular
design, in particular at the early stage of a drug discovery pro-
gram. The design strategy of improving solubility has suc-
cessfully been used in the context of “me-too” drugs,36 where it
has been shown that minor atomic variations (e.g., matched
pairs) can cause drastic changes in solubility and, thus, result in
significant therapeutic advantages.43
To assess the extent to which the aqueous solubilities, for
the present set of oxadiazole containing compounds, depend on
their experimentally determined log D values, a subset of the
data set (N = 116, 55 matched pairs) was investigated. Figure 7
displays the measured log D values versus the measured aque-
ous solubility values. The correlation is poor, and it is evident
that other factors than log D (e.g., intermolecular interactions)
influence the aqueous solubility for this set of oxadiazole con-
taining compounds. Although a linear relationship is not observed,
Figure 7 reveals a trend; the more soluble compounds (−log
solubility <4.5) also include all the hydrophilic compounds (log
D < 2.0).
Figure 8. Experimentally determined molar solubility values plotted
for the matched-pairs. The 1,3,4-oxadiazoles are in general more
soluble. The 1:1 line is shown for clarity.
higher aqueous solubility than their 1,2,4 isomers, and there
is only one single example where the 1,2,4-oxadiazole isomer
is more soluble than its regioisomeric partner (Figure 8). As
indicated above, the difference in aqueous solubility can only
be observed for the more lipophilic compounds (log D > 2.0).
Figure 9 shows an illustrative example where 2-[(5-phenyl-
A number of lipophilic compounds display high solubility.
A significant fraction of these compounds include a charged
moiety. The charge is hypothesized to compensate for the high
lipophilicity and thus lead to high solubility.
The observed differences in lipophilicity between the oxadia-
zole regioisomeric pairs are reflected in their aqueous solubility
measurements (Figure 8). That is, in the majority of the
56 matched pairs, the 1,3,4-oxadiazole regioisomers display a
Figure 9. Matched pair example where the less lipophilic 1,3,4-
oxadiazole containing compound 17 is sixteen times more soluble than
the corresponding 1,2,4-oxadiazole 18.
1,3,4-oxadiazol-2-yl)methylsulfanyl]-4-propyl-1H-pyrimidin-6-
one (17) is sixteen times more soluble than the more lipophilic
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2-[(3-phenyl-1,2,4-oxadiazol-5-yl)methylsulfanyl]-4-propyl-1H-
pyrimidin-6-one (18).
In the majority of matched pairs (36 out of 55), the dif-
ference in aqueous solubility between the regioisomers is within
the experimental error. Since the difference in log D is more
than one log unit for most of these pairs, they consequently do
not adhere to the commonly observed lipophilicity/solubility
trend. This may be due to different crystal packing effects. Un-
fortunately, the estimation of crystal lattice energies remains to
be the main challenge in the prediction of solubilities.44 How-
ever, in an attempt to shed some light on this matter, the Cam-
bridge Structural Database (CSD) was examined.45 A larger
fraction of compounds were found to match the 1,3,4-oxadiazole
substructure, as compared to compounds matching the 1,2,4-
oxadiazole substructure, 183 and 114, respectively. This
relationship is opposite of what one would expect considering
the frequency of occurrence of oxadiazole regioisomers in
patent applications (Figure 1). When performing the same
substructure search in SciFinder,46 the frequency of occurrence
is also reversed (1,2,4-oxadiazoles, 491 994; 1,3,4-oxadiazoles,
381 252). A speculative conclusion is that 1,3,4-oxadiazoles are
more likely to crystallize and that the two effects (lipophilicity
and intermolecular interactions) can cancel each other out. But
there could, of course, be many other reasons for this obser-
vation. For example, it could be a direct consequence of the
stronger dipole interactions (vide infra) between the more
polar 1,3,4-oxadiazole units in the crystal. One single matched
pair of oxadiazoles could be found (CSD reference codes:
UJUVIE and UJUVOK). The nature of the crystal packing for
this particular example was not sufficient to provide a general
rationalization of 1,2,4- and 1,3,4-oxadiazoles and their differ-
ence in aqueous solubility.47
Oxadiazole Isomers Influence on Metabolic Stability
and CYP450 Inhibition. A frequently occurring challenge
in drug discovery projects is to balance target potency and
metabolic stability. Low metabolic stability is associated with
issues such as high hepatic clearance, short half-life, and poor
in vivo exposure. A typical design strategy to improve compounds
metabolic stability is to reduce lipophilicity by incorporating
polar atoms while retaining affinity to the target protein. A dif-
ferent design approach is to block metabolically labile positions
or to incorporate metabolically stable functional groups, which
can in fact be of lipophilic nature.48
Metabolic intrinsic clearance (CLint) data measured in human
liver microsomes (HLM) for 68 oxadiazole containing
compounds (34 matched pairs) were determined to investigate
if significant effects in metabolic stability were observed in the
entire data set as well as between the oxadiazole regioisomers.
Although the metabolic stability of a molecular subunit clearly
depends on the structural context in which it is embedded, the
overall lipophilicity of a compound can often serve as an indicator
of metabolic stability. However, in this data set, no correlation was
obtained when comparing HLM CLint versus log D values (data
not shown). The matched pair analysis, however, reveals a trend
for the regioisomers. That is, in more than half of the pairs (19
out of 34), the 1,3,4-oxadiazoles show better metabolic stability, in
terms of lower HLM CLint values (Figure 10).
Figure 10. Experimentally determined HLM CLint values plotted for
the matched-pairs. The 1,3,4-oxadiazoles are in general more stable.
The 1:1 line is shown for clarity.
compounds containing the 1,2,4-oxadiazole versus the 1,3,4-
oxadiazole moiety differs. Since cytochrome P450 (CYP) en-
zymes play a major role in HLM metabolism, we hypothesized
that compounds containing the 1,3,4-oxadiazole moiety either
bind more weakly to the most common CYP enzymes and are
therefore less metabolized or, alternatively, bind more strongly
to the CYP enzymes, leading ultimately to stronger inhibition
and a decrease in CLint values.
To investigate both hypotheses, recombinant CYP inhibition
data was collected for a set of 33 matched pairs. In none of the
matched pairs was a strong CYP inhibition (<1.0 μM) observed.
Figure 11 displays the frequency of occurrence data for binned
IC50 values for six common cytochrome P450 enzymes (CYP1A2,
CYP2C8, CYP2C9, CYP2C19, CYP2D6, and CYP3A4) for the
1,2,4- and 1,3,4-oxadiazole containing compounds.
In all cases, save one (CYP2C8), the 1,2,4-oxadiazole con-
taining compounds display a larger fraction of high-affinity
compounds. The observed data is in line with the general chara-
cteristics of human P450 substrates.49 That is, oxadiazole regio-
isomeric differences are observed at CYP3A4, which is known
to favor lipophilic molecules, and at CYP1A2, which is asso-
ciated to favor planar heterocyclic compounds (Figure 11).49
Overall, this indicates that 1,2,4-oxadiazole derivatives are more
frequently recognized by cytochrome P450 enzymes, leading
to a higher metabolic turnover in HLM. While the parent
compounds in general did not show strong CYP inhi-
bition, it is possible that their putative metabolites could lead
to CYP inhibition. By investigating time-dependent inhibition
(TDI) for the different CYP enzymes, it is possible to rule out
the formation of reactive metabolites leading to strong CYP
inhibition, which in turn could result in an apparent increase in
metabolic stability. Table 2b shows that, in general, the 1,3,4-
oxadiazoles display either no TDI or the same TDI profile as
their 1,2,4-regioisomeric matched pairs (Figure 12). These
findings support the hypothesis that the increase in metabolic
stability for the 1,3,4-regioisomers is originating from weaker
coordination to the CYP enzymes rather than stronger CYP
inhibition by the parent compound or possible reactive metabo-
lites. Assuming a dissimilarity in coordination strength of the
different regioisomers, we were interested if a regioisomeric
swap of the oxadiazole scaffold could influence the overall CYP
recognition pattern. Thus, to investigate the possible changes in
the contributions of different CYP enzymes to the total metabolic
The higher intrinsic clearance for the 1,2,4-regioisomers
could originate from a higher metabolic instability of the 1,2,
4-oxadiazole moiety. However, since the oxadiazole is not
the primary site of metabolism (as observed by an in-house
metabolite identification assay in HLM), it seems likely that
the molecular recognition of the metabolizing enzymes for
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Figure 11. Diagrams showing the frequency of occurrence data for binned IC50 values for the six most common cytochrome P450 enzymes,
CYP1A2, CYP2C8, CYP2C9, CYP2C19, CYP2D6, and CYP3A4, for 1,2,4- and 1,3,4-oxadiazole containing compounds. In all cases, except one
(CYP2C8), the 1,2,4-oxadiazole compounds show a larger fraction of high-affinity compounds. CYP inhibition values show the inhibition of
metabolic degradation of the corresponding substrate by human recombinant cytochrome P450s at 37 °C (the percent of inhibition is determined at
five different concentrations and is reported as IC50).
clearance, a CYP reaction phenotyping (CRP)50 study was
performed on five matched pairs. Most interestingly, a
substantial change in the CRP profile for some of the matched
pairs was observed, underlining the ability to influence molec-
ular recognition of CYP enzymes through minimal structural
changes (Tables 2 and 3). This is a significant piece of infor-
mation, since it is desirable for a compound to be cleared
by more than one excretion route and elimination pathway to
reduce the risk of drug−drug interactions (DDI).
Oxadiazole Isomers Influence on hERG Inhibition.
Lipophilic compounds often show greater promiscuity with
unintended effects on other biological targets and are therefore
generally more liable to side effects than hydrophilic com-
pounds. One such example of an unwanted side effect is the
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a
Table 2. HLM Clint and CYP Inhibition Data for Five Matched Pairs
CYP
2C9 IC50 (μM)
no.
hMics Clint (μL/min/mg)
3A4 IC50 (μM)
2D6 IC50 (μM)
2C8 IC50 (μM)
2C19 IC50 (μM)
1A2 IC50 (μM)
19
20
21
22
23
24
25
26
27
28
>300
8
14.2
>20
14.6
>20
10.2
>20
>20
>20
>20
>20
>20
>20
9.4
>20
>20
>20
>20
>20
>20
>20
>20
>20
>20
>20
>20
>20
>20
1.3
>20
>20
>20
>20
2.4
93
2.2
<5.0
83
5.7
1.7
9.8
10
17.6
1.2
17
>20
>20
>20
10.2
>20
>20
1.3
>20
>20
>20
>20
>20
50
>20
>20
NV
>20
>20
>20
>20
>20
<5.0
9
4
a
HLM CLint values were determined as the rate of disappearance in human liver microsomes, measured from 45 min incubation with human hepatic
liver microsomes (1 mg/mL at 37 °C). CYP inhibition values show the inhibition of metabolic degradation of the corresponding substrate by human
recombinant cytochrome P450s at 37 °C (the percent of inhibition is determined at five different concentrations and reported as IC50).
Figure 12. Structures of the five matched-pairs discussed in the metabolic stability and CYP450 inhibition section. The data associated with
compounds 19−28 are shown in Tables 2 and 3
a
Table 3. Time-Dependent Inhibition (TDI) and CYP Reaction Phenotyping (CRP) Data for Five Matched Pairs.
TDI
2C9 (%)
CRP
2C9 (%)
no.
3A4 (%)
2D6 (%)
1A2 (%)
2C19 (%)
2C8 (%)
3A4 (%)
2D6 (%)
1A2 (%)
2C19 (%)
2C8 (%)
19
20
21
22
23
24
25
26
27
28
30
<7
42
<7
<7
<7
<7
<7
9
<11
<11
<11
<11
<9
<10
<10
<10
<10
<7
<19
<19
<19
<19
<19
<19
<19
<19
<19
NT
<12
<12
<12
<12
<12
13
<14
<14
<14
<14
<13
<14
<13
<13
<13
NT
99.5
100
0.0
0.0
0.2
0.0
0.0
0.0
0.1
0.0
4.7
2.4
0.3
0.0
1.0
1.2
0.1
0.0
0.2
0.0
0.0
0.0
1.5
1.5
0.0
0.0
0.9
1.5
99.8
0.0
0.1
0.0
0.1
100
8.6
0.0
0.0
17.7
11.7
91.5
52.2
92.1
92.0
29.5
68.5
0.2
38.0
14.8
7.9
<11
<9
<10
<7
1.1
<19
<12
<12
NT
0.1
<9
<7
2.4
0.0
45.4
2.2
<9
<7
2.3
1.5
NT
NT
NT
1.7
0.0
3.2
a
TDI shows the %-inhibition of human recombinant cytochrome P450s with/without NADPH after 30 min preincubation at 37 °C. TDI is
performed at two test concentrations (10 and 50 μM). Only results for the 50 μM concentration are shown here. CRP data is expressed as the
percent contribution to the metabolism from each CYP isoform, relative to the other isoforms included in the test, at 37 °C.
inhibition of the hERG potassium ion channel. Inhibition of
the hERG channel causes a slower outflow of potassium ions,
lengthening the time required to repolarize the cell, which can
trigger Torsades de Pointes (TdP) and arrhythmia. Results from
in vitro hERG assays are therefore often used as a predictor of a
compound producing cardiac arrhythmic side effects, and this
has resulted in a number of drugs being withdrawn from the
market.51 Inhibition of the hERG ion-channel is thus often seen
as a major concern and represents a significant regulatory
hurdle for new molecular entities in drug development. There
are several derivative studies suggesting the use of simple rules
for overcoming hERG inhibition in drug design.52 The most
recurrent rule suggests that hERG inhibition can be reduced by
decreasing lipophilicity.53
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Table 4. Molecular Structures, Experimental log D, pKa, hERG Inhibition, and Calculated ClogP
a
Patch clamp assay using IonWorks technology in hERG-expressing CHO cells.
Inhibition data against the hERG channel was collected for
a set of 11 oxadiazole matched pairs. The hERG IC50 patch
clamp assay used was run in the IonWorks high-throughput
mode (see Supporting Information).53 The values for both oxa-
diazole regioisomers in four of the 11 matched pairs were deter-
mined to be above the highest assay concentration (>33 μM).
These four pairs were excluded from the comparison. Table 4
shows that in the remaining seven matched pairs the 1,2,4-
oxadiazoles display higher affinity toward the hERG receptor than
their 1,3,4-oxadiazole partner. As previously shown (Figure 6),
the 1,2,4-oxadiazoles are in general one log unit more lipophilic
than their 1,3,4-regioisomer matched pair, and this is thus also the
case for these 11 matched pairs. The results shown in Table 4 are
in line with the finding by Jamieson et al., who stated that “should
compounds have ClogP values <3 and still exhibit hERG potency
then structural modification is required, and on average 1 log unit
reduction in ClogP leads to 0.8 log unit reduction in hERG activity”.52
Although this hERG data set is on the smaller side and the
differences in affinities are not vast, a pattern is emerging. The
more lipophilic 1,2,4-isomers are also more potent against the
hERG channel. Moreover, experimentally measured pKa values
for the seven matched pairs in Table 4 illustrate that the oxadia-
zole regioisomers do not have a significant effect on the basicity of
nearby ionizable functional groups; the pKa values are nearly
identical within a matched-pair. This is an interesting observation
in itself, since most ligands that inhibit the hERG channel include
a basic nitrogen likely to be protonated at physiological pH.52
Consequently, it seems not advisible to use various oxadiazole
regioisomers in the design strategy of reducing hERG inhibition
by lowering the pKa (and thus reducing the proportion of mole-
cules in their protonated form). It could therefore be concluded
that the increase in hERG inhibition observed for the 1,2,4
regioisomers is most likely driven by lipophilicity.
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Dipole Moments and Minimized Molecular Electro-
static Potentials. An attempt to rationalize the observed
difference in profile between the 1,2,4 and 1,3,4 regioisomers
was conducted. Experimentally determined dipole moments in
solution (benzene), which provide information on molecular
electronic distributions, were found for two compounds in our
matched pair data set: the diphenyl substituted 1,3,4-oxadiazole
(16a) and the corresponding diphenyl substituted 1,2,4-oxadia-
zole (41).54 The reported dipole moment for compound 16a is
3.5 D, whereas it was reported to be significantly lower for com-
pound 41, 1.8 D.54 High-level ab initio calculations (LMP2/cc-
pVTZ(-f)) are in excellent agreement with the experimental
results; the dipole moment for the 1,3,4-oxadiazole 16a is calcu-
lated to be 3.1 D as compared to 1.8 D for the 1,2,4-oxadiazole
matched pair 41 (Figure 13). It is proposed that the different
CONCLUSIONS
■
Oxadiazoles can exist in different regioisomeric forms and are
frequently occurring motifs in druglike molecules, often used as
bioisosteres for ester and amide functionalities. The most
common regioisomeric forms are the two 1,2,4-isomers and the
1,3,4-isomer. These isomers are geometrically virtually identical,
thus orienting possible substituents into the same space.
Novel computational methods were used to search the Astra-
Zeneca corporate collection and to retrieve all exact matched
pairs of 1,2,4-oxadiazole and 1,3,4-oxadiazole regioisomers. 148
such matched pairs were identified, spanning structural and
biological space, allowing for an in-depth analysis and the ability
to draw general conclusions. Several parameters frequently used
for decision-making in drug discovery were experimentally deter-
mined. The subsequent analysis revealed systematic trends,
regardless of substitution patterns. That is, in virtually all
cases, the 1,3,4-oxadiazole isomer shows an order of magnitude
lower lipophilicity (measured log D), as compared to the 1,2,4
isomer. This favorable effect is also observed in increased aque-
ous solubility, a decrease of hERG inhibition, and improved
metabolic stability of the 1,3,4-oxadiazole isomers. Moreover, it
is suggested that the 1,2,4-derivatives coordinate more frequently
to the heme-moiety in cytochrome P450 enzymes, leading to a
higher metabolic turnover in HLM. Changes between regioiso-
mers can also significantly affect the profile of CYP recognition
responsible for metabolism of the parent compound. These ob-
servations provide the medicinal chemist with a tool to shift the
CRP pattern and influence potential drug−drug interactions.
The distinct profile difference between the 1,2,4- and 1,3,4-
oxadiazole regioisomers is likely to be caused by their intrinsi-
cally different charge distributions, as evidenced by experiment
and high level quantum mechanical calculations.
Figure 13. Illustration showing that the 1,3,4-oxadiazole containing
compound 16a displays a significantly larger dipole vector than its
1,2,4-oxadiazole matched pair 41: 3.1 and 1.8 D (LMP2/cc-pVTZ-
(-f)), respectively. The two nitrogens in compound 16a display
stronger calculated hydrogen bond acceptors (large negative Vmin
values). These electronic effects are believed to be the reason for the
fact that compound 16a is significantly less lipophilic and more soluble
than compound 41.
charge distributions can be used to rationalize why 1,3,4-oxadia-
zole containing compounds show improved molecular properties
as compared to their 1,2,4-regioisomeric matched pairs. The
greater the dipole moment, the greater is the polarity, and this
is the reason for the lower lipophilicity as well as the increased
solubility of 16a (log D, 4.0; solubility, 26 μM), as compared to
that of 41 (log D, 5.9; solubility, 7 μM).
The observations reported herein can successfully be em-
ployed when designing compounds with the aim of lowering
lipophilicity, by only making minor structural modifications.
The results presented can also be used to further develop com-
putational approaches to improve the accuracy of calculated
predictions (e.g., log D).
A second type of calculation was performed. Hydrogen bond
acceptor and donor strengths have previously been linked to
lipophilicity and can be studied at the atomic level of detail
using quantum mechanical calculations.55−57 More specifically,
the value of the electrostatic potential at a minimum (Vmin) has
been found to be an effective predictor of hydrogen bond
acceptor strength.55−57 Figure 13 also shows the calculated
hydrogen bond acceptor strengths (described in kcal/mol). It
can be seen that the hydrogen bond acceptor strengths differ in
magnitude as it moves around between the heterocyclic atoms.
The two nitrogen atoms in the 1,3,4-oxadiazole are calculated
to be very favorable hydrogen bond acceptors (in terms of low
negative values), whereas the oxygen atom is predicted to be a
comparably poorer hydrogen bond acceptor. In comparison,
the nitrogens in the 1,2,4-oxadiazole are not as strong hydrogen
bond acceptors, but the oxygen is calculated to be slightly
better than that in the 1,3,4-oxadiazole case. These calculated
results support the case that electrostatic differences are respon-
sible for the discrepancy in lipophilicity between oxadiazole regio-
isomers. The calculations are also in line with the very recent
work by Leach et al., who assessed the difference in hydrogen
bond accepting potentials both with measurements (Δ log P) and
with calculations.58
Finally, novel chemistry has been developed, allowing straight-
forward high-throughput synthesis of a diverse array of sub-
stituted 1,3,4-oxadiazoles, under mild conditions.
EXPERIMENTAL SECTION
■
Computational Methods for Dipoles and Electrostatic
Potentials. The minimized electrostatic potentials (Vmin) were
calculated using the Gaussian 09 program59 as described by Kenny
et al. (HF/6-31G*).57 The dipole moments were calculated with the
Jaguar program60 using the cc-pVTZ(-f) basis set at the LMP2 level of
theory (in vacuo).
Synthetic Chemistry. General Methods. Preparative HPLC was
performed using a reversed-phase C-18 column (FractionLynx III;
mobil phase, gradient 5−95% acetonitrile in 0.2% NH3, pH 10;
column, Xbridge Prep C18 5 μm OBD 19 mm × 150 mm), with the
above-mentioned gradient at a flow rate of 20.0 mL min−1. Product
collection was carried out by mass triggered fraction collection on
[M + 1]. Reversed-phase HPLC was performed using a C-18 column
(Acquity; mobil phase, gradient 2−95% acetonitrile, pH 10; column,
Acquity BEH C18, 1.7 μm, 2.1 mm × 100 mm) with the above-
mentioned gradient at a flow rate of 1.0 mL min−1. Purity was deter-
mined by UV percentage of the observed mass peak using reversed-
phase HPLC (instrument, Acquity; mobil phase, gradient 2−95%
acetonitrile, pH 10; column, Acquity BEH C18, 1.7 μm, 2.1 mm ×
100 mm). All compounds reported have UV purity >95%, and all
yields are based on the isolated product.
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The 1H NMR spectra were recorded on a Varian Inova 600 spectro-
meter operating at a magnetic field of 14.1 T equipped with a 5 mm
1H{13C, 15N} triple resonance gradient probe, a Varian Inova 500 spec-
trometer operating at a magnetic field of 11.7 T equipped with a 5 mm
1H{13C} gradient probe or a Varian Inova 400 spectrometer operating
at a magnetic field of 9.4 T equipped with a 5 mm autoswitchable
multi nuclei gradient probe.
N-Phenyl-5-propyl-1,3,4-oxadiazol-2-amine (15d). 1H NMR
(600 MHz, d6-DMSO) δ 0.93 (t, 3H), 1.66 (tq, 2H), 2.61−2.78 (m,
2H), 6.94 (ddd, 1H), 7.29 (dd, 2H), 7.51 (dd, 2H), 10.30 (s, 1H).
APCI MS m/z: 204.1 (M + 1).
5-(5-Bromothiophen-2-yl)-N-phenyl-1,3,4-oxadiazol-2-amine
1
(15e). H NMR (400 MHz, d6-DMSO) δ 6.98 (t, 1H), 7.28−7.38
(m, 3H), 7.42 (d, 1H), 7.50−7.59 (m, 2H), 10.71 (s, 1H). APCI MS
m/z: 322.0 (M + 1).
Typical Preparation Procedure for 1,3,4-Oxadiazoles. Trifluor-
omethanesulfonic anhydride (50 μL, 0.3 mmol) was added slowly to
a solution of triphenylphosphine oxide (167 mg, 0.6 mmol) in dry
CH2Cl2 (2 mL) at 0 °C. The reaction mixture was stirred for 5 min at
0 °C and then adjusted to room temperature, followed by addition of
the acylated semicarbazide (0.2 mmol). The reaction was monitored
by LCMS. The reaction mixture was quenched with 10% aqueous
NaHCO3 solution. The aqueous layer was extracted with CH2Cl2, and
the combined organic layers were dried over Na2SO4, filtered, and
concentrated. The resultant crude product was purified by automated
HPLC (mobil phase, gradient 5−95% acetonitrile in 0.2% NH3, pH
10; column, Xbridge Prep C18 5 μm OBD 19 mm × 150 mm).
2,5-Diphenyl-1,3,4-oxadiazole (16a). 1H NMR (500 MHz, CDCl3)
δ 4.75 (s, 2H), 7.37−7.62 (m, 3H), 8.14 (s, 2H).
ASSOCIATED CONTENT
■
S
* Supporting Information
The oxadiazole matched pair data set, including all measured
data reported in the article, together with a detailed description
of the experimental methods used to measure log D, solubility,
hERG inhibition, pKa values, CYP inhibition, HLM CLint, TDI,
and CRP. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*Telephone number: +46 31 7065251. E-mail: jonas.bostrom@
astrazeneca.com.
2-(4-Methylphenyl)-5-phenyl-1,3,4-oxadiazole (16b). 1H NMR
(600 MHz, d6-DMSO) δ 7.56−7.67 (m, 6H), 8.06−8.16 (m, 4H). APCI
MS m/z: 223.1 (M + 1).
2-(4-Chlorophenyl)-5-phenyl-1,3,4-oxadiazole (16c). 1H NMR
(600 MHz, d6-DMSO) δ 3.00 (s, 6H), 6.84 (d, 2H), 7.55−7.02 (m,
2H), 7.89 (d, 2H), 8.03−8.08 (m, 2H). APCI MS m/z: 266 (M + 1).
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1
(16i). H NMR (600 MHz, d6-DMSO) δ 7.58−7.66 (m, 3H), 7.67−
7.72 (m, 2H), 8.09−8.16 (m, 4H). APCI MS m/z: 257 (M + 1).
Typical Preparation Procedure for 2-amino-1,3,4-oxadiazoles.
Trifluoromethanesulfonic anhydride (50 μL, 0.3 mmol) was added
slowly to a solution of triphenylphosphine oxide (167 mg, 0.6 mmol)
in dry CH2Cl2 (2 mL) at 0 °C. The reaction mixture was stirred for
5 min at 0 °C and then adjusted to room temperature, followed by
addition of the aminosemicarbazole (0.2 mmol). The reaction was
monitored by LCMS. The reaction mixture was quenched with 10%
aqueous NaHCO3 solution. The aqueous layer was extracted with
CH2Cl2, and the combined organic layers were dried over Na2SO4,
filtered, and concentrated. The resultant crude product was purified
by automated HPLC (mobil phase, gradient 5−95% acetonitrile in
0.2% NH3, pH 10; column, Xbridge Prep C18 5 μm OBD 19 mm ×
150 mm).
1
N,5-Diphenyl-1,3,4-oxadiazol-2-amine (15a). H NMR (600 MHz,
d6-DMSO) δ 6.95−7.05 (m, 1H), 7.29−7.39 (m, 2H), 7.53−7.57 (m,
3H), 7.57−7.62 (m, 2H), 7.83−7.91 (m, 2H). APCI MS m/z: 238.1
(M + 1).
N-Ethyl-5-phenyl-1,3,4-oxadiazol-2-amine (15b). 1H NMR (600 MHz,
d6-DMSO) δ 1.11−1.23 (m, 3H), 3.24 (qd, 2H), 7.45−7.52 (m, 3H), 7.72
(t, 1H), 7.76−7.80 (m, 2H). APCI MS m/z: 190.1 (M + 1).
1
N-Ethyl-5-(pyridin-3-yl)-1,3,4-oxadiazol-2-amine (15c). H NMR
(600 MHz, d6-DMSO) δ 1.14 (dt, 3H), 3.25 (qd, 2H), 7.53 (ddd,
1H), 7.85 (t, 1H), 8.13 (ddd, 1H), 8.65 (dt, 1H), 8.95 (dt, 1H). APCI
MS m/z: 191.1 (M + 1).
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