Synthesis of novel tetrahydro-1H-pyrazolo[4,3-c]pyridines via intramolecular nitrilimine cycloaddition

Article abstract of DOI:10.1248/cpb.c110498

The preparation of novel tetrahydro-1H-pyrazolo[4,3-c]pyridines is reported. Pivotal to the synthesis of these compounds was the development of mild reaction conditions to generate a highly functionalized nitrilimine capable of undergoing an intramolecular cycloaddition with a tethered alkyne. The desired cycloadduct was formed as an equal mixture of diastereomers.

Full text of DOI:10.1248/cpb.c110498

August 2012
Note
Chem. Pharm. Bull. 60(8) 1063–1066 (2012)
1063
Synthesis of Novel Tetrahydro-1H-pyrazolo[4,3-c]pyridines via
Intramolecular Nitrilimine Cycloaddition
Albert William Garofalo,a Jacek Jerzy Jagodzinski,b Andrei William Konradi,a
,a,†
Raymond Anthony Ng,a Christopher Michael Semko,* Hing Leung Sham,a Minghua Sun,a and
Xiaocong Michael Yea
b
a Department of Medicinal Chemistry, Élan Pharmaceuticals; and Department of Process and Analytical Chemistry,
Élan Pharmaceuticals; 800 Gateway Boulevard, South San Francisco, CA 94080, U.S.A.
Received November 26, 2011; accepted May 15, 2012
The preparation of novel tetrahydro-1H-pyrazolo[4,3-c]pyridines is reported. Pivotal to the synthesis of
these compounds was the development of mild reaction conditions to generate a highly functionalized nitril-
imine capable of undergoing an intramolecular cycloaddition with a tethered alkyne. The desired cycload-
duct was formed as an equal mixture of diastereomers.
Key words intramolecular cycloaddition; nitrilimine; pyrazole; γ-secretase inhibitor
Alzheimer’s disease (AD) is the most prevalent form of instead yielded pyrazolopyridine 8 (Chart 2). The isolation of
senile dementia affecting the elderly.1) A diagnostic hallmark 8 can be explained by the decomposition of 6 via loss of sul-
of AD is the presence of neuritic plaques composed of ag- finic acid followed by tautermization of 7 to observed product
gregated amyloid β-peptides (Aβ). This observation has led 8. Having encountered this result, an alternate synthetic ap-
to the amyloid cascade hypothesis which asserts that accumu- proach was investigated.
lated Aβ in the brain is responsible for the neurodegeneration
Intramolecular 1,3-dipolar cycloaddition of nitrilimines with
associated with the disease.2) Aβ peptides are derived from alkynes is a well documented method for the preparation of
the sequential cleavage of the transmembrane amyloid pre- fused polycyclic pyrazoles.10,11) While nitrilimines have been
cursor protein (APP) by β-secretase and γ-secretase. Thus, obtained by a number of means including the thermolysis of
γ-secretase inhibitors offer the potential as disease modify- 2,5-disubstituted tetrazoles12) or 1,3,4-oxadiazolin-5-ones,13)
ing agents for the treatment of AD.3) In addition to process- the photolysis of sydnones,14) and the oxidation of aldehyde
ing APP, γ-secretase is also responsible for the cleavage of hydrazones,15) they are most conveniently derived from the
a number of other transmembrane proteins including Notch. reaction of hydrazonoyl chlorides with base or silver salts.16)
Notch cleavage by γ-secretase releases the Notch intracellular This approach required the preparation of acyl hydrazide 16 as
domain (NICD) and following translocation to the nucleus the illustrated in Chart 3. The synthesis of α,α-difluoro-β-amino
NICD serves as a transcription cofactor.4) Since it has been ester 12 was modified from methodology reported by work-
well established that inhibition of the Notch signaling pathway ers at Merck.17) Thus, the Ti(OEt)4 mediated condensation
leads to gastrointestinal and immune system related toxici- of (S)-t-butylsulfinamide with 4-chlorobenzaldehyde gave
ties, a goal of this research was to identify an APP selective sulfinimine 10. Stereoselective addition of the Reformatsky
γ-secretase inhibitor.5,6)
reagent derived from ethyl bromodifluoroacetate to 10 gave
We recently reported the discovery of tetrahydro-1H- propanoate 11 in a diastereomeric ratio of 9:1 as determined
1
pyrazolo[4,3-c]pyridines, including 1, as a novel class of by H-NMR. Following removal of the chiral auxiliary under
γ-secretase inhibitors7,8) (Fig. 1). Compound 1 is a highly acidic conditions, the resulting hydrochloride 12 was smoothly
potent and metabolically stable inhibitor which demonstrated converted to sulfonamide 13. Mitsunobu coupling with the
robust in vivo activity. Additionally, 1 exhibited 69-fold APP appropriate racemic propargyl alcohol gave ester 14 as a 1:1
selectivity in a cellular based assay.9) Because efforts in this mixture of diastereomers. Subsequent hydrazinolysis and to-
series suggested replacement of the C6 cyclopropyl group with sylation afforded the requisite acyl hydrazides 16.
halophenyl functionality might well impart cellular selectivity
Chlorination of 16 with mesyl chloride in the presence of
of greater than 200-fold, we were motivated to prepare ana- Hünig’s base gave the desired hydrazonoyl chlorides 17 (Chart
logs 2a and 2b.
4). The reaction mixture was then treated with additional
base, in an attempt to generate the nitrilimine, but failed to
Results and Discussion
Chemistry Initially, we attempted to apply the methods
developed for synthesis of 1 to the syntheses of 2a and 2b.
Key to the preparation of 1 was the intramolecular cycload-
dition of diazoketone 3 to yield ketopyrazole 4 followed by
treatment with neat bis-(2-methoxyethyl)aminosulfur tri-
fluoride (BAST) (Chart 1). Unfortunately, attempts to prepare
ketopyrazole 6 failed to give any of the desired product but
† Present address: Thermo Fisher Scientific; 46360 Fremont Blvd., Fre-
mont, CA 94538, U.S.A.
Fig. 1. Tetrahydro-1H-pyrazolo[4,3-c]pyridines
*To whom correspondence should be addressed. e-mail: semko@sbcglobal.net
© 2012 The Pharmaceutical Society of Japan

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Reagents and conditions: (a) EtOH, 80°C; (b) BAST, 80°C.
Chart 1
Reagents and conditions: (a) EtOH, 80°C.
Chart 2
Fig. 2. ORTEP Diagram of 23
Reagents and conditions: (a) (S)-t-butylsulfinamide, Ti(OEt)4, THF, 89%; (b) BrCF2CO2Et, Zn powder, sonication, THF; (c) 4N HCl, dioxane–EtOH, 71% 2 steps; (d)
3-chlorosulfonyl-6-trifluoromethylpyridine, pyridine, 88%; (e) 1-cyclopropylprop-2-yn-1-ol, PPh3, DIAD, THF, 57%; (f) NH2NH2·H2O, EtOH, quant.; (g) TsCl, pyridine,
50°C, 45%.
Chart 3
yield any of cycloadduct 18. Treating 17 with Hünig’s base in be the result of attack of lutidine on the triflate or the nitril-
refluxing toluene gave an unidentifiable mixture of products. imine, the use of a more sterically hindered base was warrant-
Recognizing the stability of a highly functionalized nitril- ed. The reaction was conducted as before but utilizing 2,6-di-
imine at elevated temperature might be problematic, a differ- tert-butyl-4-methylpyridine (DTBMP) as the base (Chart 5).
ent approach was explored to promote nitrilimine formation Gratifyingly, LC/MS analysis of the reaction mixture showed
and cycloaddition under mild reaction conditions. Hydrazide two products with the desired mass, consistent with cycload-
16 was treated with triflic anhydride in the presence of excess dition of isomeric nitrilimine, with either configuration at the
2,6-lutidine in CH2Cl2 at −15°C. Analysis of the reaction mix- propargyl amine stereocenter. Careful silica gel chromatog-
ture by LC/MS suggested inner salt 19 had been formed. For- raphy achieved separation of the diastereomeric cycloadducts
mation of inner salts similar to 19, from hydrazonoyl chlorides 18 and 20. Finally, removal of the tosyl group with hydrazine
and pyridines, has been reported.18)
Since the formation of 19 could reasonably be assumed to pyrazoles 2b and 25 were prepared from hydrazide 22.
yielded pyrazoles 2a and 21. Utilizing the same methodology,

August 2012
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Reagents and conditions: (a) MsCl, i-Pr2NEt, THF, 60°C; (b) i-Pr2NEt, THF, 60°C; (c) i-Pr2NEt, toluene, reflux; (d) Tf2O, 2,6-lutidine, CH2Cl2, −15°C.
Chart 4
Reagents and conditions: (a) Tf2O, DTBMP, CH2Cl2, −15°C, 18, 18%; 20, 24%; 23, 11%; 24, 12%; (b) NH2NH2, EtOH, 2a, 65%; 21, 47%; 2b, 72%; 25, 75%.
Chart 5
The syn configuration of 23 was determined by X-ray
Conclusion
In summary, we have described the preparation of novel
crystallography19) (Fig. 2). This result provided for the stereo-
1
chemical assignment of 24, 2b and 25. Analysis of the H- tetrahydro-1H-pyrazolo[4,3-c]pyridines via the intramolecu-
NMR data for compounds 23 and 24 revealed unique coupling lar cycloaddition of a nitrilimine with a tethered alkyne.
data for the C-6 methine protons. For compound 23, a doublet Generating the nitrilimine directly from a hydrazide at low
(J=14.7Hz) was observed at 5.75ppm, while for 24 a doublet temperature, using the very powerful dehydrating agent triflic
of doublets (J=7.3, 9.6Hz) was observed at 5.91ppm. Similar anhydride and the very non-nucleophilic base DTBMP, was a
data was obtained for compounds 18 and 20, respectively. key to success. This work is a further example of the powerful
Thus, the combination of NMR data and the X-ray crystal re- synthetic utility of dipolar cycloaddition in the construction of
sult allowed for the assignment of syn stereochemistry to com- heterocyclic ring systems.
pounds 18 and 2a and anti stereochemistry to compounds 20
and 21. Compounds 2a and 2b were evaluated in the cellular
selectively assay and unfortunately possessed APP selectivity
of less than 100-fold.
Experimental
General Flash chromatography was performed on
a
Teledyne Isco CombiFlash Rf automated chromatography sys-
1
tem. H- (400MHz) and 13C- (100MHz) NMR spectra were

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Vol. 60, No. 8
obtained on a Bruker Avance spectrometer using tetrameth- J=24.0Hz), 147.5, 148.6, 151.3 (q, J=35.0Hz), 161.9, 164.6;
ylsilane as internal standard with chemical shifts reported in High-resolution MS Calcd for C28H23F6N4O4S2 [M+H]+:
ppm. LC/MS were recorded on an Agilent 1100-1946D LC- 657.1067, Found: 657.1059.
MS system (electrospray ionization (ESI), 3000V) employing
a Phenomenex Luna C18 column (30×4.60mm). High resolu-
tion mass spectra were obtained by Professor Chad Nelson,
Mass Spectrometry and Proteomics Facility, University of
Utah utilizing a Thermo Electron LTQ-FT instrument (ESI,
spray voltage, 2.8kV; heated capillary voltage, 120V). X-
Ray crystallographic analysis was performed by Dr. Hu Pan,
Department of Molecular Design, Élan Pharmaceuticals. The
cellular selectivity assay was performed by Lany Ruslim, De-
partment of Biology, Élan Pharmaceuticals.
References and Notes
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(2010).
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9) Cellular Selectivity: SNC Cell Notch ED50/Aβ ED50.
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19) For 23: Crystal data: space group P212121 (No. 19), a=14.132(2)Å,
b=16.807(2)Å, c=25.556(4)Å, V=6070(15)Å3, Z=8, final R factors:
R=0.0668, Rw=0.1741, and R1=0.0592 for I>2.0σ(I) data. CCDC
847895 contains complete supplementary crystallographic data for
this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via http://www.ccdc.cam.
ac.uk/data_request/cif.
Preparation of 23 and 24 To 1.63g 22 and 4.96g 2,6-di-
tert-butyl-4-methyl-pyridine in 60mL CH2Cl2 at −15°C was
added dropwise, with stirring, a solution of triflic anhydride
(2.73g) in 10mL CH2Cl2. The resulting mixture was stirred
at −15°C for 2h, and then the reaction was quenched by ad-
dition of 100mL of half-saturated aqueous NaHCO3. The two
phases were stirred together vigorously, and then the CH2Cl2
layer was separated. The aqueous layer was then extracted
twice with CH2Cl2. The combined CH2Cl2 extracts were dried
over Na2SO4, filtered, and evaporated. Flash chromatography
of the residue (120g silica; 0–15% EtOAc–hexane) afforded
184mg (11%) of 23 and 200mg (12%) of 24. For 23: Rf=0.65
(9:1 hexane–ethyl acetate on silica); 1H-NMR (CDCl3) δ:
0.04–0.11 (1H, m), 0.27–0.40 (2H, m), 0.47–0.56 (1H, m),
0.88–0.96 (1H, m), 2.50 (3H, s), 4.21 (1H, d, J=9.7Hz), 5.75
(1H, d, J=14.7Hz), 6.95 (2H, t, J=8.6Hz), 7.25–7.31 (2H,
m), 7.42 (2H, d, J=8.1Hz), 7.76 (1H, d, J=8.3Hz), 7.97 (2H,
d, J=8.4Hz), 8.05 (1H, s), 8.28 (1H, d, J=8.2Hz), 8.97 (1H,
d, J=1.8Hz); 13C-NMR (CDCl3) δ: 5.5, 5.9, 19.3, 21.8, 57.8,
60.3 (t, J=29Hz), 115.6, 115.8, 120.6 (m), 121.1 (m), 120.6 (q,
J=275Hz), 127.6, 128.7, 129.0 (m), 130.4, 130.8 (m), 132.7,
136.9 (d, J=3.4Hz), 139.7, 145.6 (dd, J=22.5, 28Hz), 147.1,
148.0, 151.0 (q, J=35.5Hz), 161.4, 163.9; High-resolution MS
Calcd for C28H23F6N4O4S2 [M+H]+: 657.1067, Found: 657.1059.
For 24: Rf=0.50 (9:1 hexane–ethyl acetate on silica); 1H-
NMR (CDCl3) δ: 0.31–0.44 (2H, m), 0.61–0.69 (1H, m),
0.70–0.85 (2H, m), 2.47 (3H, s), 4.11 (1H, d, J=8.6Hz), 5.91
(1H, dd, J=7.3, 9.6Hz), 6.86 (2H, t, J=8.6Hz), 7.13 (2H, dd,
J=5.3, 8.5Hz), 7.37 (2H, d, J=8.3Hz), 7.73 (1H, d, J=8.3Hz),
7.91 (2H, d, J=8.3Hz), 8.04 (1H, s), 8.24 (1H, dd, J=2.0,
8.2Hz), 9.15 (1H, d, J=1.7Hz); 13C-NMR (CDCl3) δ: 5.4, 9.7,
17.7, 22.2, 58.3, 63.5 (t, J=29Hz), 116.0, 116.2, 120.8, 121.5
(q, J=273Hz), 122.2 (m), 128.0, 128.9, 130.5, 131.1 (dd, J=2.1,
9.9Hz), 133.3, 136.7, 142.2, 146.7 (d, J=21.0Hz), 147.1 (d,