Discovery of novel 5,6,7,8-tetrahydro[1,2,4]triazolo[4,3-a]pyridine derivatives as γ-secretase modulators

Bioorganic & Medicinal Chemistry Letters 25 (2015) 4245–4249

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Discovery of novel 5,6,7,8-tetrahydro[1,2,4]triazolo[4,3-a]pyridine derivatives as c-secretase modulators Takafumi Takai ⇑, Yasutaka Hoashi, Yoshihide Tomata, Sachie Morimoto, Minoru Nakamura, Tomomichi Watanabe, Tomoko Igari, Tatsuki Koike Pharmaceutical Research Division, Takeda Pharmaceutical Company Ltd, 26-1, Muraokahigashi 2-chome, Fujisawa, Kanagawa 251-8555, Japan

a r t i c l e

i n f o

Article history: Received 13 May 2015 Revised 27 July 2015 Accepted 30 July 2015 Available online 7 August 2015 Keywords: c-Secretase modulators Alzheimer’s disease Amyloid beta

a b s t r a c t Novel 5,6,7,8-tetrahydro[1,2,4]triazolo[4,3-a]pyridine derivatives were designed, synthesized, and evaluated as c-secretase modulators (GSMs). An optimization study of this series resulted in the identification of (R)-11j, which showed a potent Ab42-lowering effect, high bioavailability and good blood–brain barrier permeability in mice. Oral administration of (R)-11j significantly reduced brain Ab42 in mice at a dose of 10 mg/kg. Ó 2015 Elsevier Ltd. All rights reserved.

Alzheimer’s disease (AD) is an age-associated neurodegenerative disorder characterized by impaired memory and cognition.1 A characteristic pathology of AD is the formation of plaques with the accumulation of amyloid beta (Ab), which is produced by stepwise processing of the amyloid precursor protein (APP) by b-secretase and c-secretase.2 Because c-secretase also cleaves other substrates in addition to APP, such as Notch,3 it was reported that c-secretase inhibitor induced several toxicities in clinical trials.4 c-Secretase modulators (GSMs) lower pathogenic Ab by cleavage shift without affecting Notch signal and have been proposed as a potential therapeutic agent for AD.5 We recently reported the potent piperazine derivative 1 as a GSM, which selectively reduced brain Ab42 levels in normal mice (Fig. 1).6 In the study, the oxazolylphenyl moiety in 1 was validated as a suitable surrogate for the imidazolylphenyl moiety, a highly conserved component across GSMs from several research laboratories.5 To identify a novel class of GSM, the oxazolylphenyl moiety was introduced into various right side fragments in reported GSMs, such as the 3-benzyl-1,2,4-triazole in compound 2.7 As a result, we observed that the 1,2,4-triazole derivative 3 showed moderate Ab42-lowering activity (Fig. 1). A recent analysis of GSMs indicated that their increased lipophilicity enhanced their potency but reduced their drug-likeness.8 Here we describe lead generation from 3 using ligandlipophilicity efficiency (LLE) as a drug-likeness guideline, and the

⇑ Corresponding author. Tel.: +81 466 32 1133; fax: +81 466 29 4454. E-mail address: [email protected] (T. Takai). http://dx.doi.org/10.1016/j.bmcl.2015.07.101 0960-894X/Ó 2015 Elsevier Ltd. All rights reserved.

structure–activity relationship (SAR) of novel 5,6,7,8-tetrahydro [1,2,4]triazolo[4,3-a]pyridines. Monocyclic 1,2,4-triazole derivative 3 was synthesized as shown in Scheme 1. Bromide 126 was converted to nitrile 13, which was coupled with hydrazide 14 and subsequently cyclized to afford 3. The synthetic route for the preparation of the tetrahydro[1,2,4]triazolo[1,5-a]pyridine derivative 4 is illustrated in Scheme 2. Hydrolysis of the cyano group of 13 yielded benzoic acid 15. Coupling 15 with t-butyl carbazate and subsequent N-Boc deprotection afforded hydrazide 16a as an HCl salt form. Hydrazide 16b was also directly obtained as a free form by condensation of 15 with hydrazine monohydrate. Imidate 19 was prepared by alkylation of phenylacetonitrile 17, followed by ethanolysis of 18 under acidic conditions. Finally, ring formation using 16a and 19 in the presence of imidazole gave the tetrahydro[1,2,4]triazolo[1,5-a]pyridine derivative 4. Tetrahydro[1,2,4]triazolo[4,3-a]pyridine and related bicyclic analogs 5 and 7–10 were synthesized as shown in Scheme 3. Alkylation of the benzyl position of phenylacetic acid 20 with 1-bromoalkanes 21a–c afforded 22a–c. Benzohydrazide 16a or 16b was acylated with 22a–d to give diacyl compounds 23a–d, which were cyclized by treatment with CCl4 and PPh3 to yield oxadiazoles 24a–d. Alkylchloride 24a–d were subjected to nucleophilic azidation, followed by reduction of the azide group to give the primary amines 25a–d. Intracyclization of 25a–d proceeded smoothly under acidic conditions to provide the annulate derivatives 5, 7, 8, and 10. Synthesis of dihydro[1,2,4]triazolo

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Figure 1. Piperazine derivative 1, and 1,2,4-triazole derivatives 2 and 3.

Scheme 1. Reagents and conditions: (a) Zn(CN)2, Pd2(dba)3, DavePhos, DMF, 130 °C, 88%; (b) K2CO3, n-BuOH, 120 °C, 12%.

Scheme 2. Reagents and conditions: (a) n-BuOH, 8 M NaOH aq., reflux, 74%; (b) For 16a, (i) t-butyl carbazate, DEPC, Et3N, DMF, rt, 73%; (ii) 4 M HCl/EtOAc, EtOAc, rt, 96%; For 16b, hydrazine monohydrate, CDI, THF, 92%; (c) 1-bromo-3-chloropropane, t-BuOK, THF, 10 °C to rt, 35%; (d) AcCl, EtOH, 0 °C to rt, 76%; (e) 16a, imidazole, MeOH, rt to 65 °C, 46%.

Scheme 3. Reagents and conditions: (a) 1.6 M n-BuLi/hexane, THF; (b) 16a, HATU, Et3N, DMF, or 16b, DEPC, Et3N, DMF; (c) CCl4, PPh3, MeCN or Burgess reagent, THF; (d) (i) NaN3, DMSO; (ii) PPh3, H2O, THF; (e) AcOH, reflux; (f) NaH, DMF, air, rt, 39%; (g) TsOH
H2O, toluene, reflux, 94%.

T. Takai et al. / Bioorg. Med. Chem. Lett. 25 (2015) 4245–4249

[4,3-a]pyridine 9 was achieved by oxidation of the benzyl position of 5 under basic condition and subsequent dehydration of 26 with p-toluene sulfonic acid. Tetrahydro[1,2,4]triazolo[4,3-a]pyridines 11a–d, 30, and 11f–j were synthesized by a manner similar to that described for 5 as shown in Scheme 4. Carboxylic acids 28e and 28h were obtained from the corresponding esters 31e and 31h by alkylation and hydrolysis. In the case of preparation of 28g, direct alkylation of the benzyl position of phenylacetic acid 27g did not proceed well. The desired carboxylic acid 28g was smoothly obtained by alkylation of the protected compound 32 and the following deprotection. A palladium-catalyzed cross coupling reaction of morpholine with bromide 30 afforded 4-morpholine derivative 11e. The synthetic route to tetrahydro[1,2,3]triazolo[1,5-a]pyridine 6 is illustrated in Scheme 5. The Sonogashira coupling of the bromide 12 with 5-hexyn-1-ol, followed by oxidation of primary alcohol 33 yielded the aldehyde 34. A coupling reaction of 34 with 3,4-dichlorophenylmagnesium bromide gave the secondary alcohol 35, which was treated with mesylchloride to yield 36. The 4,5,6,7-tetrahydro[1,2,3]triazolo[1,5-a]pyridine ring of 6 was constructed by nucleophilic substitution of 36 with sodium azide and a subsequent intramolecular [3+2] cycloaddition in one pot. Triazole derivatives 3–10 were evaluated for inhibitory activity against Ab42 production in rat primary neuronal cells, lipophilicity (log D at pH 7.4),9 and LLE (=pIC50 log D) as shown in Table 1. We initially introduced an additional ring between the benzyl position and the triazole ring of 3 to identify the active conformation. Enhancement of potency was observed with both the bicyclic derivatives 4 and 5. The LLE value of 4 was equal to that of the monocyclic triazole 3. This suggests that the increase of lipophilicity of 4 from 3 may simply result in the enhancement of potency. However, a marked improvement in LLE was achieved in 5. Conformational constraint in 5 could effectively fix the active conformation of 3 and contribute to the increased LLE value. Although

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replacing the 1,2,4-triazole ring of 5 with the 1,2,3-triazole ring of 6 retained the potency, the increased lipophilicity of 6 resulted in a decrease in the LLE value. Expanding the tetrahydropyridine ring gave the triazoloazepine analog 7 with a negative effect on LLE, and ring contraction to a five-membered ring (8) also reduced potency and the LLE value. Introduction of an olefin moiety (9) into the tetrahydropyridine ring of 5 was unfavorable for the activity. These results suggest that the ring size of the bicyclic system and the direction of the distal phenyl group affect GSM activity and that the conformation of 5 is the most desirable for activity in this series. Triazolooxazine analog 10, which is expected to have a conformation similar to that of 5, showed decreased potency and LLE, indicating that the polarity of the oxygen atom of the triazolooxazine core in 10 exerted an adverse effect on the activity. We accordingly selected the tetrahydro[1,2,4]triazolo[4,3-a]pyridine ring as a core scaffold. We next investigated the SAR of the distal phenyl moiety with monosubstituted analogs 11a–g and disubstituted analogs 11h–j (Table 2). Removal of the chlorine atom at the 3-position in the 3,4-dichlorobenzene of 4 resulted in a slight decrease of potency, but the LLE of 11a was retained by this modification. Compound 11b containing a 4-trifluoromethyl group showed potency and LLE similar to those of 11a. The less lipophilic 4-fluoro analog 11c decreased the potency while retaining the same LLE as that of 11a. This tendency was observed when polar groups, such as a methoxy group (11d) or morpholino group (11e), were introduced at the 4-position of the phenyl group. The regioisomeric trifluoromethyl analogs 11f (3-CF3) and 11g (2-CF3) showed the potency similar to that of 11b. A strong correlation between potency and log D value was observed among monosubstituted analogs (Fig. 2, indicated by black circles). This result suggests that GSM activity is dominated by lipophilicity in the tetrahydro[1,2,4]triazolo [4,3-a]pyridine series, and that various substituents on the distal phenyl ring would be tolerable for LLE. We accordingly prepared

Scheme 4. Reagents and conditions: (a) n-BuLi, 1-bromo-3-chloropropane; (b) (i) 16a or 16b, HATU or DEPC; (ii) CCl4 or CCl3CN, PPh3, (c) (i) NaN3; (ii) PPh3, H2O; (iii) AcOH; (d) (i) NaH, 1-bromo-3-chloropropane, DMF; (ii) NaOH, H2O, MeOH, THF; (e) 1,1-di-tert-butoxytrimethylamine, toluene, 80 °C, 81%; (f) (i) NaH, 1-chloro-3-iodopropane, DMF, rt, 91%; (ii) TFA, rt, 99%; (g) morpholine, DavePhos, Pd2(dba)3, t-BuONa, toluene, 100 °C, 20%.

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Scheme 5. Reagents and conditions: (a) 5-hexyn-1-ol, PdCl2(PPh3)2, CuI, Et3N, 70 °C, 83%; (b) Dess–Martin periodinane, MeCN, DMSO, rt, 83%; (c) 3,4-dichlorophenylmagnesium bromide, THF, 78 °C, 41%; (d), MsCl, Et3N, THF, 0 °C, 97%; (e) sodium azide, DMSO, 110 °C, 48%.

Table 1 Ab42-lowering effects of 3–10, log D values and LLE

a b c

Compound

Ab42 IC50a (nM)

log Db

LLEc

3

2000

4.1

1.6

4

860

4.4

1.7

5

210

3.2

3.5

6

210

4.2

2.5

7

220

3.5

3.2

8

630

3.3

2.9

9

600

3.6

2.6

10

530

3.1

3.2

circles), and the most lipophilic 4-chloro-2-trifluoromethyl analog 11j showed the most potent Ab42-lowering effects, as we expected. Finally, we obtained each enantiomer of 11j.10 The chiral configuration greatly affected the potency and the eutomer (R)-11j exhibited 14-fold more potent activity than the distomer (S)-11j. Compound (R)-11j exhibited the most potent Ab42-lowering effect in this series and a moderate Ab40-lowering effect (IC50 = 490 nM) in the same cellular assay. In addition, (R)-11j was inactive against Notch signal (IC50 >10 lM). In vivo pharmacokinetic and pharmacological profiles of (R)-11j were evaluated in mice. Sufficient exposure of (R)-11j in brain and plasma was confirmed at 3 h after oral administration at a dose of 10 mg/kg (Table 3). Furthermore, compound (R)-11j showed a statistically significant reduction of Ab42 in both brain and plasma at the same dosage (Table 4). In conclusion, we designed and synthesized bicyclic analogs to fix the active conformation of 3. By screening various bicyclic systems for lead generation, the novel tetrahydro[1,2,4]triazolo[4,3-a] pyridine core scaffold with enhanced potency and LLE was

Table 2 Ab42-lowering effects of 11a-j, (S)-11j and (R)-11j, their log D values and their LLE

IC50 values are means of triplicate measurements. log D at pH 7.4.9 LLE = pIC50 log D.

disubstituted analogs 11h–j to further enhance potency by increasing lipophilicity. Definite correlation between potency and log D value was also observed with 11h–j (Fig. 2, indicated by white

a b c

Compound

R

Ab42 IC50a (nM)

log Db

LLEc

11a 11b 11c 11d 11e 11f 11g 11h 11i 11j (S)-11j (R)-11j

4-Cl 4-CF3 4-F 4-OMe 4-morpholine 3-CF3 2-CF3 3-CF3, 4-Cl 2-CF3, 5-CF3 2-CF3, 4-Cl 2-CF3, 4-Cl 2-CF3, 4-Cl

570 510 1500 1900 2300 370 380 220 300 84 830 60

2.8 2.9 2.4 2.4 2.3 2.8 3.0 3.2 3.3 3.5 3.5 3.5

3.4 3.4 3.4 3.3 3.3 3.6 3.4 3.5 3.2 3.6 2.6 3.7

IC50 values are means of triplicate measurements. log D at pH 7.4.9 LLE = pIC50 log D.

T. Takai et al. / Bioorg. Med. Chem. Lett. 25 (2015) 4245–4249

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discussion. We are indebted to Mr. Motoo Iida for structural determination by single-crystal X-ray analysis. We express our gratitude to Dr. Makoto Kamata and Dr. Takanobu Kuroita for helpful discussion during the preparation of the manuscript. We also thank Mr. Tomohiro Onishi, Dr. Yasuko Takahashi, Dr. Koji Murakami, Ms. Yayoi Doken, Dr. Tatsuya Hayama and Dr. Shigeru Morita for their contribution in the biological evaluation. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.bmcl.2015.07. 101. These data include MOL files and InChiKeys of the most important compounds described in this article. References and notes Figure 2. Plot of pIC50 against log D for 3–5 and 11a–j.

Table 3 Cortex and plasma concentration of (R)-11j in C57BL/6 J mice (male, 9 w.o.) Cortexa (lg/g)

Plasmaa (lg/mL)

2.332

6.450

a

Means of concentrations at 3 h after p.o. administration at dose of 10 mg/kg, n = 6 (cortex), 5 (plasma).

Table 4 In vivo profile of (R)-11j in C57BL/6 J mice (male, 9 w.o.) Brain Aba (%)

a b c d

Plasmaa Ab (%)

Ab42

Ab40

Ab42

Ab40

50d

75b

31c

63c

Expressed as % vehicle at 3 h after p.o. administration at dose of 10 mg/kg, n = 6. p <0.05, p <0.01, p <0.001 Welch test.

discovered. In subsequent SAR studies of the tetrahydro[1,2,4]triazolo[4,3-a]pyridines, strong correlation between potency and log D value was observed, and increasing lipophilicity of compounds helped in identification of (R)-11j as a potent GSM. Compound (R)-11j exhibited a significant Ab42-lowering effect with high plasma and brain exposures by oral administration in mice. These results show that the tetrahydro[1,2,4]triazolo[4,3-a]pyridine derivative (R)-11j has the potential of being a therapeutic agent for AD. Further profiling of (R)-11j is in progress and will be reported in due course. Acknowledgements We thank Mr. Yuichi Kajita, Mr. Hitoaki Nishikawa, and Mr. Tetsuya Tsukamoto for compound synthesis and helpful

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