Design, synthesis, and biological evaluation of novel oxadiazole- and thiazole-based histamine H3R ligands

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Design, Synthesis, and Biological Evaluation of Novel Oxadiazole- and Thia-

zole-Based Histamine H₃R Ligands

Mohammad A. Khanfar, David Reiner, Stefanie Hagenow, Holger Stark

PII:

S0968-0896(18)30764-8

https://doi.org/10.1016/j.bmc.2018.06.028

BMC 14420

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Reference:

Bioorganic & Medicinal Chemistry

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20 June 2018

Please cite this article as: Khanfar, M.A., Reiner, D., Hagenow, S., Stark, H., Design, Synthesis, and Biological

Evaluation of Novel Oxadiazole- and Thiazole-Based Histamine H₃R Ligands, Bioorganic & Medicinal

Chemistry (2018), doi: https://doi.org/10.1016/j.bmc.2018.06.028

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Design, Synthesis, and Biological Evaluation of Novel Oxadiazole-

and Thiazole-Based Histamine H₃R Ligands

Mohammad A. Khanfar^1,2,3, David Reiner¹, Stefanie Hagenow¹, Holger Stark^1*

¹Heinrich Heine University Düsseldorf, Institute of Pharmaceutical and Medicinal

Chemistry, Universitaetsstr. 1, 40225 Duesseldorf, Germany

²Faculty of Pharmacy, University of Jordan, P.O Box 13140, Amman 11942, Jordan.

³College of Pharmacy, Alfaisal University, Al Takhassusi Rd, Riyadh, 11533, Saudi

Arabia

*Corresponding author: Prof. Holger Stark, Tel.: +49 211 81-10478, Fax: +49 211 81-

13359, email: stark@hhu.de

1

Abstract

Histamine H₃receptor (H₃R) is largely expressed in the CNS and modulation of the

H₃R function can affect histamine synthesis and liberation, and modulate the release

of many other neurotransmitters. Targeting H₃R with antagonists/inverse agonists

may have therapeutic applications in neurodegenerative disorders, gastrointestinal and

inflammatory diseases. This prompted us to design and synthesize azole-based H₃R

ligands, i.e. having oxadiazole- or thiazole-based core structures. While ligands of

oxadiazole scaffold were almost inactive, thiazole-based ligands were very potent and

several exhibited binding affinities in a nanomolar concentration range. Ligands

combining 4-cyanophenyl moiety as arbitrary region, para-xylene or piperidine

carbamoyl linkers, and/or pyrrolidine or piperidine basic heads were found to be the

most active within this series of thiazole-based H₃R ligands. The most active ligands

were in silico screened for ADMET properties and drug-likeness. They fulfilled

Lipinski’s and Veber’s rules and exhibited potential activities for oral administration,

blood-brain barrier penetration, low hepatotoxicity, combined with an overall good

toxicity profile.

Keywords: histamine; H₃receptor; ligands; ADMET; thiazole; oxadiazole

2

1. Introduction

Histamine belongs to biogenic amines, which influence several intracellular

pathways, including its neurotransmission activities.^1-4Histamine’s regulatory

character in cellular activities is attributed to its binding to four subtypes of G-protein-

coupled receptors (GPCRs); H₁, H₂, H₃, and H₄that are differentially expressed in

several cell types.^{5, 6}

Histamine H₃receptor (H₃R) is a member of transmembrane class A of GPCR

family.^{7, 8}H₃R is largely expressed on the histaminergic neurons of the CNS, located

pre- and postsynaptically.⁹It plays an essential role in the biosynthesis and release of

histamine as well as in the modulation the release of different neurotransmitters (e.g.,

11

dopamine, serotonin, acetylcholine, noradrenaline, GABA, and glutamate.)^10,

Peripherally, H₃R is marginally distributed in the peripheral nervous system and

regulates the sympathetic effector systems and pain sensation.¹²Consequently,

modulation of the H₃R function can potentially affect histaminergic

neurotransmission. Therefore, targeting H₃R with antagonists/inverse agonists may

have therapeutic applications in neurodegenerative disorders, such as Parkinson's

disease, Alzheimer’s disease, as well as in depression, epilepsy, schizophrenia and in

sleep disorders. ^14-19

Recently, a number of H₃R antagonists/inverse agonists have entered late clinical

trials for the treatment of several CNS disorders.^{15, 20, 21}In March 2016, the European

Medicines Agency authorized marketing of pitolisant (Wakix^®) as the first H₃R

inverse agonist for the treatment of narcolepsy, designated as an orphan drug since

narcolepsy belongs to the rare diseases.^22-24Pitolisant is also currently in phase III

clinical trials for the treatment of hypersomnia.²⁵

3

First H₃R antagonists/inverse agonists reported in literature were imidazole-based

compounds followed by non-imidazoles in the next generation. They share a general

26, 27

pharmacophore generated from numerous chemical scaffolds (Figure 1).^15,

Prototype H₃R antagonists/inverse agonists pharmacophore consist of basic moiety

(mainly tertiary amine electrostatically interacting with Asp114 within H₃R binding

site) separated by a spacer to a central core (mainly electron-rich moiety)²⁸. The

distance between the basic head and the electron rich moiety is approximately 4-5 Å,

equivalent to 4 bonds in linear arrangement. The central core is usually connected to

an arbitrary region, mainly lipophilic, but polar or basic and even acidic moieties are

also tolerable.²⁹This arbitrary region modulates potency and pharmacokinetic

properties of H₃R antagonists/inverse agonists.^{27, 30}

Figure 1. Schematic presentation of the pharmacophore model of H₃R

antagonists/inverse agonists.

Oxadiazole and thiazole nuclei have attracted a wide attention in the search for new

therapeutic molecules. They are widely exploited in various applications because of

their versatile applicability in multiple therapeutic agents and their blood-brain barrier

32

(BBB) permeability;^31,

Five-membered heterocycles as core motifs in non-

4

imidazole H₃R ligands with structural resemblance to the presented structures were

explored in earlier research projects resulting in affine compounds (Figure 2).^{33, 34}

Among them ADS-531 that recently underwent in vivo examination upon effects on

food-intake and neurotransmitter systems^{35, 36}The former progress in the design of

potent oxadiazole- and thiazole-based H₃R ligands prompted our research group to

explore 2-arylthiazole-4-ylethers as analogues to ST-979 but differing in the

arrangement of the substituents.³⁷Furthermore, N-aryl-1,3,4-oxadiazole-2-amines and

3-phenyl-1,2,4-oxadiazole-5-carboxamides (cf. GSK247246 and ST-1095) were

designed to increase structural and linkage diversity in the H₃R central core motif ,

basically linked to our previous work on variations of polar azole core motifs in H₃R

ligands (Figure 2).^37-40Our synthetic effort combines various aliphatic and aromatic

spacers of variable length, and arbitrary region of electronically and sterically variable

substituents. Several H₃R analogues with binding affinities in low nanomolar

concentrations were obtained within this project.

Figure 2. Selected oxadiazole- and thiazole-based H₃receptor ligands serving as a

basis for design of new H₃R ligands ^aK_ivalues calculated from published pK_ivalues.

2. Results and Discussions

2.1 Hit-Identification and Hit-to-Lead-Optimization

5

Synthetic design strategy was driven by an initial structural hypothesis for various

azole-based H₃R ligands fitting the H₃R-pharmacophoric map (Figure 1).^37-39The

following structural modification strategies were applied to design potent H₃R

ligands: (1) employing oxadiazole (1,2,4 and 1,3,4) or thiazole rings as a polar,

electron-rich central core; (2) several cyclic and noncyclic amines were examined as a

basic head of the designed H₃R ligands including: pyrrolidine, piperidine, piperazine

and noncyclic aliphatic tertiary amines; (3) variable spacers of different chain length

connecting the central core and the basic head were examined using aliphatic and

aromatic linkers. Linkers of 2 to 5 elements of cyclic or noncyclic structure were

tested for optimal length; (4) substituted and unsubstituted aromatic and

heteroaromatic rings of variable size and electronic properties were placed on the

arbitrary region. Substantial modifications can be conducted on the arbitrary region to

modulate potency, solubility and pharmacokinetic properties. Initially, only a few

ligands of each compound series without changes in the arbitrary region were

synthesized and evaluated for hit identification. Hit-to-lead optimization was

conducted for those structures with promising results.

2.2 Chemistry.

An efficient one-pot synthesis of substituted 2-anilino-1,3,4-oxadiazoles was applied.

A carboxylic acid (1.0 eq.) and 4-phenyl-3-thiosemicarbazide (1.0 eq.) were mixed in

DCM at room temperature with three equivalents of N-(3-dimethylaminopropyl)-N¹-

ethylcarbodiimide (EDCI) as coupling reagent, producing the corresponding 2-

anilino- 1,3,4-oxadiazoles (2a-m) (Scheme 1). The thiosemicarbazides 1a-e were

readily prepared by reacting the corresponding isothiocyanate with hydrazine under

reflux in MeOH.

6

Scheme 1. Synthesis of 2-Anilino-1,3,4-oxadiazoles. Reagents and Conditions:

(a) (1) N₂H₄, MeOH, rt, 1 h. (2) 65-80 ^oC, 20 min (88-96%); (b) EDCI (3 eq), CH₂Cl₂, rt, 12 h

(50-84%). Structure of compounds 2a – 2m are listed in Table 1.

The synthesis of 1,2,4-oxadiazole derivatives started from commercially available

nitriles that were reacted with hydroxylamine in EtOH/H₂O in the presence of

NaHCO₃to give the corresponding phenylamidoximes 3a–b in good yields. The latter

were suspended in DCM and then refluxed for 5 h in pyridine with ethyl

chlorooxalate to provide 3-aryl-1,2,4-oxadiazole-5-carboxylates 4a–b. Then , the

obtained esters were eadily converted into the desired compounds 5a-c by reaction

with 3-substituted amine-1-yl propan-1-amine (Scheme 2).

Scheme 2. Synthesis of 3-phenyl-1,2,4-oxadiazole derivatives. Reagents and Conditions: (a)

NH₄OH, EtOH, NaHCO₃, reflux, 3 h (67-77%); (b) CHCl₃, ethyl chlorooxalate, rt for 30 min, then

refluxed for 5 h (79-82%); (c) EtOH, alkyl amine, reflux for 8 h (73-87%). Structure of compounds

5a – 5c are listed in Table 1.

The synthesis of 5-methyl thiazole analogues (7 - 36) was based on literature

procedure where the aryl / heteroaryl nitrile, α-mercaptocarboxylic acid, and pyridine

were reacted to form the corresponding hydroxymethylthiazole (6).⁴¹Subsequently,

the hydroxymethiazole was O-alkylated following a Williamson-type etherification

7

with K₂CO₃as the base to afford the final compounds (7 – 36) (Scheme 3). Analogues

39-42 were synthesized by first refluxing 4-hydroxythiazole with 4 equivalents of

α,α′-dibromo-p-xylene in acetone and K₂CO₃. After column chromatography, the

monoether products (38a-b) were reacted with either piperidine or pyrrolidine to

furnish the corresponding final compounds (39-42) (Scheme 4). The 5-ethyl thiazole

analogues (44-46) were synthesized by first mixing the corresponding

phenylthioamide, ethyl 2-bromobutanoate and pyridine under argon and slowly

heating up to 100-110 °C for 2 h to form hydroxyethylthiazole (43), which was O-

alkylated to afford the final compounds (44 – 46) (Scheme 5).⁴²

Scheme 3. Synthesis of 5-methyl thiazole derivatives. Reagents and Conditions:

(a) pyridine, 100 ^oC for 2 h (69-96%); (b) DMF (or acetone), Cs₂CO₃(or K₂CO₃) (3.0

eq.), KI (1.0 eq.), alkyl halide (0.9 eq.) (57-95%). Structure of compounds 7 – 37 are

listed in Table 2.

8

Scheme 4. Synthesis of xylene linker thiazole derivatives. Reagents and conditions:

o

(a) K₂CO₃, acetone, reflux, 12 h (74-86%); (b) alicyclic amine, K₂CO₃, DMF, 60 C,

12 h (84-90%).

Scheme 5. Synthesis of 5-ethyl thiazole derivatives. Reagents and Conditions:

o

(a) pyridine (1.0 eq.), 100-110 C, 3 h (76-98%); (b) DMF, K₂CO₃(3.0 eq.), KI (1.0

eq.), alkyl halide (0.9 eq.) (62-94%). Ligand structures of 44 – 46 are listed in Table

2.

2.3 Biological Evaluation

All final synthesized compounds were tested for their in vitro H₃R binding affinity in

a binding assay by competitive displacement of [³H]-N^α-methylhistamine as

radioligand. Their corresponding K_ivalues with 95% confidence intervals are listed in

Table 1 and Table 2.

2.3.1 Oxadiazole-based ligands

As it was previously demonstrated that oxadiazoles, e.g. linked either via oxy-

/thioethers or carboxamides to the basic amine moiety, are accepted as central core in

H₃R ligands, a small series of 1,2,4- and 1,3,4-oxadiazole derivatives were

designed.^{38, 43}

9

Analogues of 1,3,4-oxadiazole scaffold showed low binding affinities at H₃R (K_i>

1000 nM), thus, permitting only limited conclusions for structural variations to

improve H₃R affinity. The prototypes (2a - 2d) without modifications at the anilino

substructure resulted in ligand 2d showing weak affinity. Only a few variations of

these ligands were constructed to examine possibilities for improving this binding

behavior, resulting in 2e – 2m. Ligands with alicyclic basic head and 2-3 carbon

linkers showed comparable activity with 2d exhibiting low micromolar K_ivalues (2c

2g, 2j, 2k, and 2m). However, analogues with the same linker length but rigid linker

and basic head have negligible binding affinity (2b, 2f, 2i). This behavior implies the

necessity of flexibility to position the basic head in favorable orientation for salt

bridge formation with Asp114, described as crucial for H₃R antagonists/inverse

agonists binding.⁴⁴Moreover, ligands with noncyclic basic head and/or shorter linker

(2a, 2h, 2l) appeared inactive, except the 4-chlorine derivative showing micromolar

affinity (2e). These data suggest the need of a defined distance between the central

core and the cyclic basic head for these compounds. Nevertheless, the modifications

performed here were not able to improve affinity substantially, limiting any SAR

interpretations in this series.

The smaller series of 1,2,4-oxadiazole-5-carboxamides analogues structurally related

to previously described compounds FUB 654, ST-1095 or GSK247246were inactive

38,

(5a, 5b and 5c, Table 1).^33,

³⁹Therefore, they are considered as unsuitable

fundamental structure for further synthetic efforts, compared to 1,2,4-oxadiazole FUB

654 (Figure 2), anon-imidazole analogue of the highly affine Glaxo Wellcome

compound GR175737 (K_i= 2.5 nM).^{33, 45}Compared to previously published more

potent alkyl-/thioether-linked oxadiazole derivatives (Figure 2),³⁸N-phenyl-1,3,4-

oxadiazole-2-amines and 1,2,4-oxadiazole-5-carboxamides seem to bear a less

10

promising or more restricted core motif on substitution and linkage, respectively.

Subsequently, the more promising thiazole class has been followed in more detail.

Table 1. Oxadiazole-based ligands along with H₃R binding affinities.

Compd

No.

H₃R K_i(nM)^a

Scaffold

I

R¹

H

n

3

R²

[95% CI] ^a; (n)^b

> 5,000; (3)

> 5,000; (2)

2a

2b

I

H

0

2

2,000 [100; 72,200]; (2)

2c

I

H

3

0

1,300 [700; 2,500]; (2)

5,700 [300; 112,800]; (3)

> 5,000; (2)

2d

2e

2f

4-Cl

I

4-Cl

2

1

0

1,600 [100; 55,200]; (2)

> 5,000; (2)

2g

2h

2i

4-OCH₃

> 5,000; (2)

I

4-OCH₃

4-OCF₃

2

1,600 [700; 3,800]; (2)

2j

1,300 [100; 34,800]; (2)

11

2k

3,5-

dimethoxy

I

1

2

> 5,000 (2)

2l

3,5-

dimethoxy

1,100 [1,000; 1,200]; (2)

2m

II

H

3

> 5,000 (2)

> 5,000 (4)

> 5,000; (2)

5a

5b

5c

OCH₃

^aBinding affinity values (K_i) are expressed as mean with 95% confidence intervals;

^bNumber of experiments.

2.3.2 Thiazole-based ligands

The synthetic approach for thiazole-based ligands was supported by the previously

published structure, ST-979 (Figure 1).³⁷They can be considered as analogues were

the substituents were shifted by one carbon atom yielding 11 as initial structure with

moderate affinity. The resulting analogues exhibited superior binding affinities with

K_ivalues ranging from low micromolar to nanomolar concentrations. Similar to 1,3,4-

oxadiazole analogues, optimum binding activities are achieved with pyrrolidine or

piperidine basic heads. For example, compound 21 with pyrrolidine basic head and

three-carbon chain linker demonstrated a K_ivalue of 4 nM. Yet, analogues with para-

xylene (e.g., 41 and 42 with K_ivalues 7.0 nM and 72 nM, respectively) or piperidine

carbamoyl (e.g., 14 , 20, 23 with K_ivalues 5.4 nM, 54 nM and 42 nM, respectively)

linkers showed high binding affinities. Interestingly, the methylpiperazine carbamoyl

substituted thiazole 13 showed only weak affinity (K_i= 5,800 nM) compared to the

piperidine carbamoyl linked analogue 14 being more active by more than three orders

of magnitude. Different affinities of 2- and 3-carbon linked pyrrolidine or piperidine-

analogues were detected between the corresponding structures of (pyrrolidine-1-

12

yl)alkoxy linked 2-phenylthiazoles with K_iof 47 nM and 520 nM for dimethylen 8

and trimethylen analogue 10, respectively. Comparable binding behavior was detected

between the corresponding 4-chlorophenylthiazole analogues 16 and 18 as well as

between 17 and 19, both carrying a (piperidin-1-yl)alkoxy linker instead (Table

2).The arbitrary region (i.e. the phenyl moiety) showed wide electronic and steric

tolerability. Replacing the phenyl ring with pyridine was tolerable (or slightly less

active) but not with pyrimidine (except when it is coupled with para-xylene linker in

41 and 42). A cyano group in para-position generated the most active analogues (e.g.,

21, 22), which may indicate the favorable positioning of hydrogen bond acceptor at

this site presumably interacting with the Threonine residue identified by Roche et al.⁴⁶

Replacing the 5-methyl by an 5-ethyl substituent showed inconsistent pattern; it

improves the binding affinity as with 46 (K_i= 200 nM), which is about five fold more

active than its methyl counterpart (19, K_i= 930 nM), but not with 44 and 45.

Table 2. Thiazole-based ligands along with H₃R binding affinities.

Compd

No.

H₃R K_i[nM]

X

Y

Z

R¹

H

n

3

2

R²

R³

[95% CI]^a, (n)^b

4,100

[1,400; 12,200]; (3)

CH CH CH

CH₃

7

8

9

47

[20; 110]; (3)

200

[90; 490]; (3)

520

[340; 780]; (3)

CH CH CH

H

3

CH₃

10

11

870

[130; 5,710]; (5)

13

220

[70; 650]; (8)

CH CH CH

H

1

0

3

2

CH₃

12

13

14

15

16

5,800

[1,200; 27,000]; (4)

5.4

H

[2.2; 13.2]; (3)

280

4-Cl

[70; 1,170]; (4)

22

[5; 98]; (3)

44

CH CH CH

4-Cl

2

3

CH₃

17

18

19

[12; 167]; (3)

450

[180; 1,150]; (3)

930

[570; 1,540]; (4)

54

CH CH CH

4-Cl

0

3

CH₃

20

21

22

[22; 131]; (3)

4.2

4-CN

[2.6; 6.6]; (3)

21

CH CH CH

4-CN

3-Cl

3

0

2

3

CH₃

[2; 201]; (3)

42

23

24

25

[15; 122]; (4)

600

[310; 1,150]; (3)

430

[120; 1,550]; (3)

300

[130; 700]; (4)

26

27

28

29

98

3-Cl

H

0

3

2

CH₃

[56; 170]; (6)

110

[40; 300]; (4)

CH

N

CH

100

[30; 380]; (4)

H

14

57

CH

N

CH

H

2

3

CH₃

30

31

32

[42; 78]; (3)

410

[170; 1,010]; (4)

130

[20; 820]; (3)

5,900

[2,800; 12,600]; (4)

N

CH

N

H

3

2

3

0

CH₃

33

34

35

36

2,100

[800; 6,100]; (4)

1,100

[500; 2,400]; (6)

> 10,000

87

CH CH CH

3

1

3

CH₃

37

39

40

41

42

44

45

46

[16; 462]; (5)

450

4-CN

H

[190; 1,100]; (4)

190

[130; 280]; (3)

CH₃

7.0

N

CH

N

CH₃

[2.6; 18.4]; (4)

72

H

CH₃

[11; 492]; (4)

380

[180; 810]; (3)

CH CH CH

H

CH₂CH₃

170

[30; 870]; (4)

4-Cl

200

[100; 370]; (4)

4-Cl

^aBinding affinity values (K_i) are expressed as average along with 95% confidence

intervals; ^bNumber of experiments.

Accordingly, analogues combining optimum moieties, i.e. 4-cyanophenyl moiety as

arbitrary region, para-xylene or piperidine carbamoyl linkers, and/or pyrrolidine or

15

piperidine basic heads were found to be the most active within this series of thiazole-

based H₃R ligands.

2.6 In silico Molecular and ADMET properties of designed thiazole ligands.

Prediction of how much a ligand would have proper pharmacokinetics (ADME) and

pharmacodynamics (e.g., toxicological) properties (drug-like) is of great importance

during pre-clinical evaluation to assist drug discovery/development process, to guide

the optimization from a lead compound to a successful drug candidate, and to reduce

attrition rates during clinical trials, hence increasing the chance of reaching the

market.⁴⁷

Some important chemical descriptors correlate well with ADMET properties such as

polar surface area (PSA) and low molecular weight (MW) for high oral absorption.⁴⁸

Likewise, rapid renal clearance is correlated with hydrophilic and small compounds.

The hepatic metabolism of most drugs is associated with large and hydrophobic

compounds. Higher lipophilicity of compounds leads to increased metabolism and

poor absorption, along with an increased probability of binding to undesired

hydrophobic macromolecules, thereby increasing the potential for toxicity.⁴⁹

Table 3. Compliance of the most active ligands to Lipinski’s and Veber’s rules.

Compound AlogP

MW

(˂ 500)

386.5

421.0

314.4

411.5

421.0

367.5

No. HBA^aNo. HBD^bNo. rotatable Molecular PSA^c

(˂ 5)

(≤ 10)

(≤ 5)

bonds (≤ 10)

(˂ 140 Å)

2.33

3.00

1.80

2.21

3.00

1.56

5

4

6

5

1

4

5

4

6

75

79

99

75

81

14

20

21

23

27

41

^aNumber of Hydrogen-bond acceptors (HBA); ^bNumber of Hydrogen-bond donors

(HBD); ^cPolar Surface Area (PSA).

16

Lipinski’s⁴⁹and Veber’s⁵⁰rules are one of the most important measures to evaluate

the drug-likeness and to predict if a compound of certain chemical properties would

be orally bioavailable. In spite of some observed exceptions to Lipinski’s and Veber’s

rules, the vast majority (90%) of the orally bioavailable compounds are within their

cut-off limits. Lipinski's rule of five states that, generally, an orally bioavailable

compound should not violate the following criteria: ≤ 5 hydrogen bond donors

(HBD); ≤ 10 hydrogen bond acceptors (HBA); MW ˂ 500; and logP value of ˂ 5.

Alternatively, Veber’s findings described the role of PSA and number of rotatable

bonds as criteria to estimate the oral bioavailability. Veber’s rule stated that for a

compound to be orally bioavailable it should have either: a PSA ≤ 140 Å and ≤ 10

rotatable bonds, or ≤ 12 HBD and HBA in total and ≤ 10 rotatable bonds. Clearly

from Table 3, the most potent ligands (those with K_i˂ 100 nM) followed all

Lipinski’s and Veber’s rules without any exception, which highlight their drug-

likeness properties and their potential to pass the drug development process.

The most active ligands were in silico screened for predicted pharmacokinetics

properties (BBB penetration, absorption, solubility (Sw), hepatotoxicity, inhibition of

CYP2D6, and plasma protein binding (PPB)) being summarized in Table 4. The

intestinal absorption and BBB penetration were predicted by developing an ADME

model using descriptors 2D PSA and AlogP98 that include 95% and 99% confidence

ellipses. These ellipses define regions where well-absorbed compounds are expected

to be found. All active ligands showed medium (brain-blood ratio between 0.3:1 and

1:1) to high (brain-blood ratio between 1:1 and 5:1) penetration of BBB and good

intestinal absorption (at least 90% absorbed into bloodstream) within 95% confidence

ellipses (Table 4 and Figure 3). These results are therapeutically crucial for H₃R

ligands to be effective for targeting neurodegenerative diseases and their potential for

17

oral administration. Moreover, these ligands showed low (-6.0 ˂ logSw ˂ -4.0) to

good (-4.0 ˂ logSw ˂ -2.0) solubility, non-hepatotoxicity, and variable properties for

inhibition of CYP2D6 and PPB (Table 4). However, the compounds with highest

affinity in the series (14, 21 and 41) provide some deficiencies regarding BBB

permeability and display high PPB (PPB > 95% as with 14 and 41).

Table 4. Predicted ADME profiles of the most active ligands.

Compound

BBB^a

Absorption^bSolubility^cHepatotoxicity^d

CYP2D6^e

inhibitor

non-inhibitor

PPB^f

2

1

2

1

2

0

3

2

3

2

3

nontoxic

2 (˃ 95%)

0 (˂ 90%)

1 (˃ 90%)

0 (˂ 90%)

2 (˃ 95%)

14

20

21

23

27

41

^aPredicts ability of the compound to penetrate the blood brain barrier (BBB). Levels 0, 1, 2, 3, or 4

b

correspond to very high, high, medium, low, or undefined penetration, respectively; Predicts human

intestinal absorption after oral administration. Levels 0, 1, 2, or 3 correspond to good, moderate, poor,

c

or very poor absorption, respectively; Predicts the solubility of each compound in water at 25^oC.

Levels 0, 1, 2, 3, 4, 5, or 6 correspond to extremely low, very low, low, good, optimal, too soluble, or

d

unknown solubility, respectively; Predicts the occurrence of dose-dependent human hepatotoxicity.

f

^ePredicts cytochrome P450, 2D6 inhibition. Predicts the likelihood that a compound will be highly

bound to carrier proteins on the blood (PPB, plasma protein binding). Levels 0, 1, or 2 correspond to

binding ˂ 90%, binding ˃ 90%, or binding ˃ 95%, respectively.

As ligand efficiency measures emerge as increasingly important parameters in hit-to-

lead optimization, we calculated ligand efficiency (LE) and lipophilic ligand

efficiency (LLE) according to literature, based on in silico AlogP-values and in vitro

binding affinities.⁵¹LE values were within the commonly accepted limits with LE >

0.3 kcal per mole per non-H-atoms (nHA) (0.42, 0.50 and 0.41 kcal / mole / nHA for

compounds 14, 21 and 41 respectively).⁵²LLE values conform to the acceptance

criteria of LLE > 5, being 5.9, 6.6 and 6.6 for 14, 21 and 41, respectively.⁵³

18

Figure 3. Plot of PSA vs AlogP (calculated via “Calculate Molecular Properties”

module of Discovery Studio 2.5.5) for the most active ligands showing 95% and 99%

confidence limit ellipses corresponding to penetration of BBB and intestinal

absorption.

The United States Food and Drug Administration (US FDA) standard toxicity risk

predictor software TOPKAT (Discovery Studio, Accelrys, USA) locates fragments

within the compound that indicate a potential threat to toxicity risk. TOPKAT

(TOxicity Prediction by Komputer Assisted Technology) employs robust and cross-

validated Quantitative Structure Toxicity Relationship (QSTR) models for assessing

various measures of toxicity and utilizing the patented Optimal Predictive Space

validation method to assist in interpreting the results. Toxicity screening results of

TOPKAT for the most active H₃R ligands showed no risk of AMES mutagenicity, and

tolerable ocular and skin irritation (Table 5); however, they possess potential US FDA

rodent carcinogenicity, except against female mouse. Compound 14 is the only non-

skin sensitizing agent among the three most active compounds and appears as single-

carcinogen in male mouse and female rat, whereas the cyano-containing ligands 21

and 41 differentiate as skin sensitizing and multi-carcinogenic agents. Moreover,

particular attention was paid to the risk of aerobic biodegradability as 2-

phenylthiazoles are known to produce hepatotoxic and/or nephrotoxic thioamides and

19

55

1,2-dicarbonyl-species upon oxidative metabolism.^54,

Thus, exclusively in 5-

position alkylated thiazole derivatives were synthesized to achieve reduced oxidative

ring-scission and biodegradable probability, confirmed by the TOPKAT screening

results.⁵⁶

Besides the biological evaluation, this in silico screening provided additional

information that could be useful for further lead optimization of the presented

compounds (cf. additional in silico ADMET and toxicity information on compounds

8, 16, 17, 22, 30, 37, and 42 is provided in Supplementary Material).

20

Table 5. Toxicity profile of the most active ligands using toxicity prediction, extensible protocol of Accelrys Discovery studio 2.5.5.

Carcinogenicity_USFDA^d

Aerobic

AMES

Ocular

Skin

Compound

BioDegradability^a

Mutagenicity^b

Irritancy^c

Sensitizer^c

Female Mouse Male Mouse

Female rat

Male Rat

-

+

-

+

-

+

-

14

20

21

23

27

41

-

++

+

-

++

-

++

-

++

+

++

^aPrediction of oxidative degradability-properties of compounds. Indicators -, or + correspond to non-biodegradability, or degradability by bioorganisms, respectively;

^bPredicted mutagenicity in AMES mutagenicity test. Indicators -, or + correspond to non-mutagenicity, or mutagenicity, respectively; Prediction of organic toxicity.

c

d

Indicators -, +, or ++ correspond to none, mild, or severe / strong toxic properties, respectively; Prediction of carcinogenic properties. Indicators -, +, or ++ correspond to

classification as Non-Carcinogen, Single-Carcinogen, or Multi-Carcinogen, respectively.

21

3. Conclusions

A novel series of oxadiazole and thiazole derivatives as H₃R ligands are explored in

the present study. Oxadiazole ligands showed mediocre affinities unlike the thiazole,

which showed potent H₃R affinities in low nanomolar concentration range. The

highest affinities are observed with ligands of pyrrolidine or piperidine basic heads

and 2 – 3 carbon linkers. However, thiazole ligands of para-xylene or piperidine

carbamoyl linkers showed high binding affinities, too. The examined compounds

show excellent drug-like properties in compliance with Lipinski’s and Veber’s rules

and satisfactory in silico ADMET properties. The results from this study will be

helpful for further improvements of potent H₃R ligands.

22

4. Experimental

4.1. Chemistry

Reagents and solvents for synthesis were purchased from Sigma-Aldrich, VWR

Chemicals, Fisher Scientific, Alfa Aesar and Chemsolute and were used without

1

further purifications. H-NMR and ¹³C-NMR were recorded on a Bruker AMX

spectrometer (Bruker, Germany) at 300 and 75 MHz respectively, where CDCl₃or

DMSO-d₆was used as a solvent. Tetramethylsilane was used as standard and

chemical shifts are reported in part per million (ppm). Spin multiplicities are given as

s (singlet), d (doublet), dd (doublet of doublets), t (triplet), q (quintet) or m

(multiplet). Approximate coupling constants (J) in Hertz (Hz). Number and

assignment of protons (ax, axial; eq, equatorial; ph, phenyl; pip, piperidine; pyra,

pyrazine; pyr, pyridine; pyrr, pyrrolidine). Elementary analyses (C, H, N) were

measured on a CHN-Rapid (Heraeus, Germany) and all final compounds were within

0.4 % of the theoretical values. Electrospray ionization mass spectrometry (ESI-MS)

was performed on an amaZon speed (Bruker, Germany) in positive polarity. Data are

listed as mass number ([M+H⁺]). High-resolution mass spectra (HRMS) were run in

electrospray ionization (ESI) mode. Melting points (m.p., uncorrected) were

determined on a M-564 Büchi melting point apparatus (Büchi, Germany). Preparative

column chromatography was performed on silica gel 60 M, 0.04-0.063 mm

(Macherey-Nagel, Germany) and thin-layer chromatography (TLC) was carried out

using pre-coated silica gel 60 with fluorescence indicator at UV 254 nm (Macherey-

Nagel, Germany).

4.1.1. General Reaction Procedures.

General Procedure (A). Preparation of 4-Phenyl-3-thiosemicarbazide. The

phenylisothiocyanate (30 mmol) was added, dropwise, over a period of 1 h to a stirred

23

o

solution of hydrazine (30 mmol) in methanol (8 mL) at 65-80 C. The reaction

mixture was stirred for 20 min more at the same temperature. The solvent was

removed by evaporation in vacuo and the precipitate was collected by filtration and

washed with petroleum ether. The compound was used without further purification.

General Procedure (B). Preparation of 2-Anilino-5-alkyl-1,3,4-oxadiazole. A

carboxylic acid (0.33 mmol), a 4-phenyl-3-thiosemicarbazide (0.33 mmol), and EDCI

(182 mg, 1.0 mmol) were mixed in CH₂Cl₂(15 mL), and the reaction mixture was

stirred at room temperature for 12 h. The organic layer was washed three times with

saturated NaHCO₃, dried over MgSO₄, filtered, and concentrated. Chromatography

(5:95 MeOH/DCM, silica gel) of the residue afforded the title compound.

General Procedure (C). Preparation of Phenyl amidoximes. To a stirred solution

of phenyl nitrile (0.1 mol) in ethanol (100 mL), a solution of hydroxylamine (0,6 mol)

in H₂O (150 mL) was slowly added followed by NaHCO₃(0.3 mol). The resulting

mixture was refluxed for 3h. The solvent was evaporated in vacuo and the resulting

residue was poured into cold water. The formed precipitate was filtered off, washed

with water and dried in vacuo. The obtained product was recrystallized from

methanol.

General Procedure (D). Preparation of 1,2,4-Oxadiazole-5-carboxylates. Pyridine

(0.015 mol) was added to a solution of phenyl amidoxime (0.01 mol) dissolved in dry

CHCl₃(100 ml). Ethyl chloroxalate (0.025 mol) was then added to the solution under

vigorous stirring. The mixture was stirred at room temperature for 30 min, then

refluxed for 5 h, cooled to rt, washed with water and HCl (5%). Organic layer was

dried under MgSO4, concentrated in vacuo and the formed residue was purified by

flash-chromatography on silica gel.

24

General Procedure (E). Preparation of 1,2,4-Oxadiazole-5-carboxamides. A

primary amine (0.5 mmol.) was added to the carboxylate (0.5 mmol) dissolved in dry

ethanol (5 mL). The reaction mixture was refluxed for 12 h, and then cooled to rt. The

organic solvent was dried in vacuo and the crude product was purified by flash-

chromatography on silica gel to give the target compounds

General Procedure (F). Preparation of 4-Hydroxy-5-methylthiazole. Pyridine

(0.005 mol) was added to a mixture of thiolactic acid (0.02 mol) and nitrile (0.0.2

O

mol) at 23 C under argon. The reaction mixture was then heated at 100 ^OC for 2 h.

After cooling, the precipitate was collected, washed with absolute ethanol, and

recrystallized from methanol to afford the product.

General procedure (G). O-Alkylations of Hydroxy thiazoles

To a solution of thiazole (1.0 eq.) dissolved in DMF (4 mL), K₂CO₃(3.0 eq.) was

added, and the reaction mixture was left for 30 min at 60 °C. Then KI (1.0 eq.) and

the alkyl halide (0.9 eq.) was added and the reaction was left for an additional 8 h.

The reaction mixture was taken up in DCM, washed with Na₂CO₃(3x) and dried over

MgSO4 and concentrated in vacuo. The concentrate was purified by flash

chromatography yielding the desired compound.

General procedure (H). Preparation of 2-Phenyl-4-hydroxy-5-ethylthiazole.

Pyridine (5.83 mmol) was added to a mixture of ethyl 2-bromobutanoate (6.41 mmol)

O

and thiobenzamide (5.83 mmol) at 23 C under argon. The reaction mixture was then

heated at 100 ^OC for 2 h. After cooling, the precipitate was collected and washed with

absolute ethanol, and the recrystallized from methanol to afford the desired product.

General procedure (I). Preparation of Bromoethyl benzyl thiazole. 1,4-

Bis(bromomethyl)benzen (4.0 mmol), 4-hydroxythiazole (1.0 mmol) and K₂CO₃(2.0

mmol) dissolved in 50 mL acetone was refluxed for 12 h. After cooling, acetone was

25

evaporated and the crude product was constituted in DCM. The organic layer was

washed with NaHCO₃solution (3x) and dried over Na₂SO₃. The final compound was

purified with flash chromatography.

General procedure (J). Nucleophilic substitution of Bromoethyl benzyl thiazole.

The bromoethyl benzyl thiazole (1.0 eq.) was dissolved in DMF (4 mL) and K₂CO₃

(3.0 eq.) was added. The reaction mixture was left for 30 min at 60 °C, then the

piperidine or pyrolidine (0.9 eq.) was added and the reaction was left for an additional

8 h. The reaction mixture was taken up in DCM, washed with Na₂CO₃(3x) and dried

over MgSO₄and concentrated in vacuo. The concentrate was purified by flash

chromatography yielding the desired compound.

5-Methyl-2-phenyl-4-(2-(pyrrolidin-1-yl)ethoxy)thiazole (8). Compound 8 was

prepared according to procedure G from 5-methyl-2-phenylthiazol-4-ol and 1-(2-

1

chloroethyl)pyrrolidine to afford 62 mg of yellow solid (70%). H NMR (CDCl₃, 300

MHz) δ 7.77 (dd, J = 8.1, 1.8 Hz, 2H, ph-2,6H), 7.38 – 7.23 (m, 3H, ph-3,4,5H), 4.45

(t, J = 6.0 Hz, 2H, OCH₂), 2.87 (t, J = 6.0 Hz, 2H, OCH₂CH₂N), 2.73 – 2.50 (m, 4H,

13

pyrr-2,5H₂), 2.22 (s, 3H, CH₃), 1.77 (p, J = 1.8 Hz, 4H, pyrr-3,4H₂). C NMR (151

MHz, CDCl₃) δ 9.40, 23.53, 54.67, 55.32, 69.26, 107.09, 125.34, 128.78, 129.29,

134.00, 159.47, 159.53. HRMS m/z 289.1384 [M+H⁺] (calcd for C₁₆H₂₁N₂OS,

289.1375). Anal. Calcd for C₁₆H₂₀N₂OS: C 66.63, H 6.99, N 9.51, S 11.12. Found:

C 66.82, H 7.24, N 9.18, S 11.51.

5-Methyl-2-phenylthiazol-4-yl [1,4'-bipiperidine]-1'-carboxylate (14). Compound

14 was prepared according to procedure G from 5-methyl-2-phenylthiazol-4-ol and

1

[1,4'-bipiperidine]-1'-carbonyl chloride to afford 74 mg of yellow solid (80%). H

NMR (CDCl₃, 300 MHz) δ 7.79 – 7.74 (dd, J = 2.1, 1.8 Hz,, 2H, ph-2,6H), 7.31 (d, J

26

= 1.8 Hz, 1H, ph-4H), 7.29 (d, J = 2.1 Hz, 2H, ph-3,5H), 4.27 (dd, J = 30.2, 12.8 Hz,

2H, pip¹-2,6H_eq), 2.85 (m, 2H, pip¹-2,6H_ax), 2.55 – 2.32 (m, 5H, pip¹-4H, pip²-

2,6H_2ax/eq), 2.22 (s, 3H, CH₃), 1.94 – 1.72 (m, 2H, pip¹-3,5H_eq), 1.67 – 1.42 (m, 6H,

pip¹-3,5H_ax, pip²-3,5H_2ax/eq), 1.42 – 1.27 (m, 2H, pip²-4H_2ax/eq). ¹³C NMR (75 MHz,

CDCl₃) δ 10.03, 24.63, 26.21, 28.10, 44.40, 50.20, 62.36, 117.88, 125.72, 128.76,

129.81, 133.37, 151.79, 152.31, 161.20. HRMS m/z 386.1910 [M+H⁺] (calcd for

C₂₁H₂₈N₃O₂S, 386.1902). Anal. Calcd for C₂₁H₂₇N₃O₂S: C 65.43, H 7.06, N 10.90,

S 8.32. Found: C 66.18, H 7.12, N 10.84, S 8.38.

2-(4-Chlorophenyl)-5-methyl-4-(2-(pyrrolidin-1-yl)ethoxy)thiazole

(16).

Compound 16 was prepared according to procedure G from 2-(4-chlorophenyl)-5-

methylthiazol-4-ol and 1-(2-chloroethyl)pyrrolidine to afford 79 mg of yellow solid

1

(91%). H NMR (CDCl₃, 300 MHz) δ 7.66 (d, J = 8.6 Hz, 2H, ph-2,6H), 7.24 (d, J =

8.6 Hz, 2H, ph-3,5H), 4.40 (t, J = 6.0 Hz, 2H, OCH₂), 2.81 (t, J = 6.0 Hz, 2H,

OCH₂CH₂N), 2.63 – 2.47 (m, 4H, pyrr-2,5H₂), 2.19 (s, 3H, CH₃), 1.72 (p, J = 3.0 Hz,

4H, pyrr-3,4H₂). ¹³C NMR (75 MHz, DMSO) δ 9.28, 23.42, 54.59, 55.26, 69.42,

107.31, 126.37, 128.82, 132.38, 134.89, 157.85, 159.64. HRMS m/z 323.0991

[M+H⁺] (calcd for C₁₆H₁₉ClN₂OS, 323.0985). Anal. Calcd for C₁₆H₂₀ClN₂OS: C

59.52, H 5.93, N 8.68, S 9.93. Found: C 59.88, H 6.07, N 8.41, S 10.37.

2-(4-Chlorophenyl)-5-methyl-4-(2-(piperidin-1-yl)ethoxy)thiazole (17). Compound

17 was prepared according to procedure G from 2-(4-chlorophenyl)-5-methylthiazol-

1

4-ol and 1-(3-chloroethyl)piperidine to afford 43 mg of yellow solid (57%). H NMR

(CDCl₃, 300 MHz) δ 7.69 (d, J = 8.5 Hz, 2H, ph-2,6H), 7.28 (d, J = 8.5 Hz, 2H, ph-

3,5H), 4.44 (t, J = 6.0 Hz, 3H, OCH₂), 2.76 (t, J = 6.0 Hz, 3H, OCH₂CH₂N), 2.62 –

2.47 (m, 4H, pip-2,6H_2ax/eq), 2.21 (s, 3H, CH₃), 1.72 – 1.52 (m, 4H, pip-3,5H_2ax/eq),

27

1.45 – 1.32 (m, 2H, pip-4H_2ax/eq). ¹³C NMR (75 MHz, CDCl₃) δ, 9.73, 24.63, 26.34,

55.33, 58.66, 68.65, 107.37, 126.48, 128.93, 132.48, 134.99, 158.01, 159.78. HRMS

m/z 337.1148 [M+H⁺] (calcd for C₁₇H₂₂ClN₂OS, 337.1141). Anal. Calcd for

C₁₇H₂₁ClN₂OS: C 60.61, H 6.28, N 8.32, S 9.52. Found: C 61.08, H 6.51, N 8.41,

S 9.85.

2-(4-Chlorophenyl)-5-methylthiazol-4-yl [1,4'-bipiperidine]-1'-carboxylate (20).

Compound 20 was prepared according to procedure G from 2-(3-chlorophenyl)-5-

methylthiazol-4-ol and [1,4'-bipiperidine]-1'-carbonyl chloride to afford 84 mg of

yellow solid (89%). ¹H NMR (CDCl₃, 300 MHz) δ 7.69 (d, J = 8.6 Hz, 2H, ph-2,6H),

7.27 (d, J = 8.6 Hz, 2H, ph-3,5H), 4.27 (dd, J = 28.1, 13.3 Hz, 2H, pip¹-2,6H_eq), 2.86

(m, 2H, pip²-2,6H_eq), 2.62 – 2.34 (m, 5H, pip¹-4H, pip²-2,6H_2ax/eq), 2.22 (s, 3H, CH₃),

1.96 – 1.74 (m, 2H, pip¹-3,5H_eq), 1.68 – 1.45 (m, 6H, pip¹-3,5H_ax, pip²-3,5H_2ax/eq),

13

1.44 – 1.29 (m, 2H, pip²-4H₂). C NMR (75 MHz, CDCl₃) δ 10.02, 24.46, 25.96,

27.40, 27.97, 44.04, 44.33, 50.16, 62.39, 118.40, 126.90, 128.99, 131.86, 135.69,

151.88, 152.22, 159.79. HRMS m/z 420.1514 [M+H⁺] (calcd for C₂₁H₂₆ClN₃O₂S,

420.1513). Anal. Calcd for C₂₁H₂₇ClN₃O₂S: C 60.06, H 6.24, N 10.01, S 7.63. Found:

C 60.12, H 6.20, N 10.18, S 7.66.

4-(5-Methyl-4-(2-(pyrrolidin-1-yl)ethoxy)thiazol-2-yl)benzonitrile

(21).

Compound 21 was prepared according to procedure G from 2-(3-cyanophenyl)-5-

methylthiazol-4-ol and 1-(2-chloroethyl)pyrrolidine to afford 67 mg of yellow solid

(80%). ¹H NMR (CDCl₃, 300 MHz) δ 7.74 (d, J = 8.7 Hz, 2H, ph-2,6H), 7.49 (d, J =

8.7 Hz, 2H, ph-3,5H), 4.38 (t, J = 5.4 Hz, 2H, OCH₂), 2.84 (t, J = 5.5 Hz, 2H,

OCH₂CH₂N), 2.66 – 2.55 (m, 4H, pyrr-2,5H₂), 2.16 (s, 3H, CH₃), 1.80 – 1.64 (m, 4H,

28

pyrr-3,4H₂). ¹³C NMR (75 MHz, CDCl₃) δ 9.36, 31.25, 36.37, 54.98, 68.92, 109.48,

111.85, 118.48, 125.35, 132.44, 137.43, 156.31, 160.10. HRMS m/z 314.1332

[M+H⁺] (calcd for C₁₇H₂₀N₃OS, 314.1327). Anal. Calcd for C₁₇H₁₉N₃OS: C 66.03,

H 6.46, N 12.83, S 9.79. Found: C 66.18, H 6.51, N 12.79, S 9.72.

4-(5-Methyl-4-(3-(piperidin-1-yl)propoxy)thiazol-2-yl)benzonitrile

(22).

Compound 22 was prepared according to procedure G from 2-(3-cyanophenyl)-5-

methylthiazol-4-ol and 1-(3-chloropropyl)piperidine to afford 75 mg of yellow solid

(83%). ¹H NMR (CDCl₃, 300 MHz) δ 7.81 (d, J = 8.7 Hz, 2H, ph-2,6H), 7.55 (d, J =

8.7 Hz, 2H, ph-3-5H), 4.29 (t, J = 6.4 Hz, 2H, OCH₂), 2.45 – 2.40 (m, 2H,

OCH₂CH₂), 2.39 – 2.31 (m, 4H, pip-2,6H_2ax/eq), 2.21 (s, 3H, CH₃), 2.03 – 1.82 (m, 2H,

OCH₂CH₂CH₂N), 1.68 – 1.48 (m, 4H, pip-3,5H_2ax/eq), 1.45 – 1.27 (m, 2H, pip-

4H_2ax/eq). ¹³C NMR (75 MHz, CDCl₃) δ 9.40, 24.31, 25.80, 26.97, 54.54, 55.91, 69.11,

109.34, 112.00, 118.62, 125.45, 132.53, 137.66, 156.33, 160.53. HRMS m/z 342.1645

[M+H⁺] (calcd for C₁₉H₂₄N₃OS, 342.1640). Anal. Calcd for C₁₉H₂₃N₃OS: C 66.83,

H 6.79, N 12.31, S 9.39. Found: C 66.79, H 6.80, N 12.31, S 9.44.

2-(4-Cyanophenyl)-5-methylthiazol-4-yl [1,4'-bipiperidine]-1'-carboxylate (23).

Compound 23 was prepared according to procedure G from 4-(4-hydroxy-5-

methylthiazol-2-yl)benzonitrile and [1,4'-bipiperidine]-1'-carbonyl chloride to afford

1

91 mg of yellow solid (91%). H NMR (CDCl₃, 300 MHz) δ 7.85 (d, J = 8.7 Hz, 2H,

ph-2,6H), 7.59 (d, J = 8.2 Hz, 2H, ph-3,5H), 4.26 (dd, J = 31.1, 13.0 Hz, 2H, pip¹-

2,6H_eq), 2.84 (m, 2H, pip²-2,6H_eq), 2.56 – 2.44 (m, 5H, pip¹-4H, pip²-2,6H_2ax/eq), 2.26

(s, 3H, CH₃), 2.01 – 1.76 (m, 2H, pip¹-3,5H_eq), 1.66 – 1.47 (m, 6H, pip¹-3,5H_ax, pip²-

3,5H_2ax/eq), 1.45 – 1.30 (m, 2H, pip²-4H₂). ¹³C NMR (75 MHz, CDCl₃) δ 10.08, 24.41,

29

Article Doi

Design, synthesis, and biological evaluation of novel oxadiazole- and thiazole-based histamine H3R ligands

DOI: 10.1016/j.bmc.2018.06.028

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Article abstract of DOI:10.1016/j.bmc.2018.06.028

Full text of DOI:10.1016/j.bmc.2018.06.028

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