Efficient microwave-assisted Suzuki–Miyaura cross-coupling reaction of 3-bromo pyrazolo[1,5-a]pyrimidin-5(4H)-one: towards a new access to 3,5-diarylated 7-(trifluoromethyl)pyrazolo[1,5-a]pyrimidine derivatives

Abstract

A convenient and efficient synthetic route to C3-arylated 7-trifluoromethylpyrazolo[1,5-a]pyrimidin-5-one derivatives has been reported starting from 3-bromo-7-(trifluoromethyl)pyrazolo[1,5-a]pyrimidin-5-one through a Suzuki–Miyaura cross-coupling reaction. The arylation (heteroarylation) strategy can be performed using a wide variety of aryl and heteroaryl boronic acids and requiring a tandem catalyst XPhosPdG2/XPhos to avoid the debromination reaction. These optimized conditions were successfully extended to the synthesis of 7-, 8- and 9-arylated pyrimido[1,2-b]indazol-2-ones from their corresponding brominated starting materials. Furthermore, the second C-5 arylation of C3-arylated pyrazolo[1,5-a]pyrimidin-5-ones was achieved under standard Suzuki–Miyaura cross-coupling conditions, after activating the C–O bond of the lactam function with PyBroP, giving access to a small library of 3,5-diarylated 7-(trifluoromethyl)pyrazolo[1,5-a]pyrimidines in good to excellent yields. The interest of this approach has been highlighted by the synthesis of a known anti-inflammatory agent. Additionally, a preliminary biological evaluation has revealed that a number of derivatives display micromolar IC₅₀ values against monoamine oxidase B, an important target in the field of neurodegenerative disorders.

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Introduction

Transition metal-catalyzed cross-coupling reactions are widely recognized to be powerful synthetic tools for carbon–carbon and carbon-heteroatom bond formation, giving access to more complex molecules.¹ The Suzuki–Miyaura cross-coupling reaction is one of the most useful methods to create new carbon–carbon (C sp²–C sp²) bonds, thanks to the stability and the low toxicity of organoboron reagents.² Moreover, organoboron compounds provide high selectivity in cross-coupling reactions, proving that the Suzuki–Miyaura reaction is particularly suitable for the elegant synthesis of biologically active molecules and natural products.^3,4 The use of this coupling reaction in medicinal chemistry represents a considerable asset for inducing modification of the biochemical properties by introducing a functional diversification such as a vinyl, aryl or heteroaryl moiety.⁵ Despite the wide scope of this coupling reaction in various fields of chemistry, some difficulties and limitations may be encountered in the case of some heteroaromatic substrates. This is observed mainly for heterocycles having a free amino group, probably because of the complexation of the metal with NH function, leading to the inhibition or deactivation of the catalyst.⁶ For these reasons, the Pd-catalyzed Suzuki–Miyaura cross-coupling involving heteroaromatic substrates is therefore a major challenge for organic chemists⁷ and in the pharmaceutical discovery chemistry.⁸ On the other hand, pyrazolo[1,5-a]pyrimidine is a basic heterocycle of a variety of complex chemical compounds having large spectrum of biological properties,⁹ viz: COX-2 inhibitors,¹⁰ hepatitis C virus inhibitors,¹¹ kinase inhibitors,¹² anticancer agents,¹³ PET tumor imaging agents,¹⁴ anxiolytic agents,¹⁵ antimicrobial agents,¹⁶ and serotonin 5-HT6 receptor antagonists.¹⁷ In addition, pyrazolo[1,5-a]pyrimidine constitute the central core of several pharmaceutical drugs, such as Indiplon and Zaleplon, which are sedative-hypnotic, used to treat insomnia, and Ocinaplon which is an anxiolytic drug. Synthetically, a wide number of approaches giving access to pyrazolo[1,5-a]pyrimidine derivatives have been reported.¹⁸ The condensation of 3(5)-aminopyrazoles with 1,3-dicarbonyl compound appears to be the most general approach for the synthesis of these heterocycles.¹⁹ Other 1,3-bis electrophilic compounds, such as alkoxymethylene β-dicarbonyl derivatives, enaminonitriles and β-enaminones, have been used to generate this class of fused heterocycles.²⁰

Additionally, only few syntheses have been reported for pyrazolo[1,5-a]pyrimidin-5-one skeleton, involving essentially the condensation of aminopyrazoles with 1,3-dimethyluracil,^20,21 3-ethoxyacrylate²² or ethyl 4,4,4-trifluoroacetoacetate.²³

To date, despite significant progress in the development of palladium-catalyzed reactions of aminopyrazoles,²⁴ only one example of a Suzuki–Miyaura coupling reaction has been disclosed by Ellermann and co-workers, starting from 3-halogeno pyrazolo[1,5-a]pyrimidin-5-ones.²⁵ In this case, the cross-coupling was achieved using XPhosPdG3 (10 mol%) and XPhos (10 mol%) in THF at 80 °C for 16 h, providing the C3-arylated products in low yields ranging from 3 to 41%. Furthermore, no systematic study including reaction optimization conditions was reported. Consequently, the biological potential of the target arylated pyrazolo[1,5-a]pyrimidine derivatives and the lack of cross coupling exploration in this heterocycle led us to further develop the C3-arylation of fluorinated pyrazolo[1,5-a]pyrimidin-5-one 4a. Recently, Schmitt et al. have reported a two-pot synthesis of 7-(hetero)aryl pyrazolo[1,5-a]pyrimidin-5-ones from aryl (heteroaryl)halides and aminopyrazoles.²⁶ Nevertheless, these strategies have various drawbacks: the scope of the substrate is restricted and pyrazolo[1,5-a]pyrimidin-5-ones were obtained in low to moderate yields.

To the best of our knowledge, the Suzuki–Miyaura cross-coupling of fluorinated 3-bromo pyrazolo[1,5-a]pyrimidin-5-ones has never been described in the literature. This is somewhat surprising since it has been shown that the incorporation of fluorine atoms into compounds has a significant impact on their physical, chemical and biological properties.²⁷ For example, the presence of a CF₃ moiety contributes to increase the lipophilicity of biologically active molecules.²⁸ A versatile and flexible methodology for constructing 3-arylated pyrazolo[1,5-a]pyrimidin-5-ones, bearing a CF₃ group is thus of major importance. However, there have been few reports describing the synthesis of 3,5-disubstituted pyrazolo[1,5-a]pyrimidines bearing a trifluoromethyl group at C7 position.²⁹ Furthermore, selective and/or sequential arylation has never been studied on a pyrazolo[1,5-a]pyrimidin-5-one for the synthesis of substituted pyrazolo[1,5-a]pyrimidines.

New strategies are therefore necessary for easy and effective access to new variously substituted heterocycles targets, whose biological potential seems certain.

Recently, we reported the one-pot efficient regioselective synthesis of new fluorinated heterocycles via an addition/heterocyclization sequence using a fluorinated alkyne and 1,3-bisnucleophilic reagents.³⁰ Continuing our efforts to develop the synthesis of new potentially active heterocycles,³¹ we report herein the first examples of a general and efficient Suzuki–Miyaura C3-arylation and heteroarylation of 3-bromo-7-(trifluoromethyl)pyrazolo[1,5-a]pyrimidin-5-one 4a using various aryl and heteroaryl boronic acids (Scheme 1). Some of 3-arylated pyrazolo[1,5-a]pyrimidin-5-ones were subsequently used as building blocks to synthesize novel fluorinated 3,5-disubstituted pyrazolo[1,5-a]pyrimidines. Indeed and based on our recently published work (Scheme 1),³² an efficient and divergent method was developed to synthesize 3,5-diarylated pyrazolo[1,5-a]pyrimidines bearing a trifluoromethyl group at C7 position through C–O activation of the amide function.³³

Scheme 1 Convergent approach to arylation of fluorinated pyrazolo[1,5-a]pyrimidine and fluorinated pyrazolo[1,5-a]pyrimidin-5-one derivatives.

Results and discussion

The synthesis of 7-(trifluoromethyl)pyrazolo[1,5-a]pyrimidin-5-one 3 was accomplished according to our previously reported procedure through the condensation of commercially available 3-aminopyrazole 1 with ethyl 4,4,4-trifluorobutynoate 2 (Scheme 2).³² The reaction proceeded smoothly to provide regioselectively 7-(trifluoromethyl)pyrazolo[1,5-a]pyrimidin-5-one 3 in 63% yield, without any trace of the regioisomer pyrazolo[1,5-a]pyrimidin-7-one (Scheme 2). It is noteworthy that for several years in our laboratory, alkyne 2 has been the subject of investigations as a basic element to synthesize new fluorinated compounds.³⁴ Compound 3 was subsequently converted selectively into 3-bromo and 3-iodo pyrazolo[1,5-a]pyrimidin-5-ones 4a and 4b via NBS or NIS-mediated bromination or iodination in CH₂Cl₂ at room temperature in excellent yields [X = Br, 4a (94%), X = I, 4b (90%)] (Scheme 2).

Scheme 2 Synthesis of 3-halo pyrazolo[1,5-a]pyrimidin-5-ones 4a and 4b.

The resulting halides 4a and 4b could be a key building blocks to modify the C3 position through the Suzuki–Miyaura coupling reaction.^2a–c,35 This strategy is challenging because metal-catalyzed reactions present some problems and limitations especially when the used substrates contain an unprotected amino group.³⁶ First, we examined the coupling reaction between 3-bromo pyrazolo[1,5-a]pyrimidin-5-one 4a and p-methoxyphenylboronic acid, using PdCl₂(PPh₃)₂ (5 mol%) as a catalyst in the presence of Na₂CO₃ (2 equiv.) in dioxane at 110 °C (Table 1, entry 1). The analysis of the crude ¹H and ¹⁹F NMR spectrums showed 9% of the expected product 5a along with 91% of the undesired debrominated product 3 (Table 1, entry 1). To optimize the reaction conditions, the solvent, catalyst, ligand, base and temperature were carefully screened and all obtained results are summarized in Table 1. Using PdCl₂dppf instead of PdCl₂(PPh₃)₂ in the presence of Na₂CO₃ or K₂CO₃, increased slightly the yield of 5a to 17% (Table 1, entries 2 and 3).

Table 1 Optimization of Suzuki–Miyaura cross-coupling conditions between 4a and p-methoxyphenylboronic acid

Entry	Organic solvent	Base	Catalyst (5 mol%)	Ligand (10 mol%)	T (°C)	Ratio^a
						5a	3
a The ratio of mixture (5a/3) was determined from crude ^1H and ^19F NMR spectrums.b 3 equiv. of K2CO3 were used.c The reaction was carried out under microwave irradiation for 40 minutes.d Yield of isolated product 5a.e Reaction was performed with XPhosPdG2 (2.5 mol%) and XPhos (5 mol%).
1	Dioxane	Na2CO3	PdCl2(PPh3)2	—	110	9	91
2	Dioxane	Na2CO3	PdCl2dppf	—	110	11	89
3	Dioxane	K2CO3	PdCl2dppf	—	110	17	83
4	Dioxane	K2CO3	XPhosPdG2	XPhos	110	18	82
5	Dioxane	K2CO3	Pd2(dba)3	XPhos	110	4	96
6	EtOH	K2CO3	Pd2(dba)3	XPhos	110	45	55
7	EtOH	K2CO3	Pd(OAc)2	XPhos	110	52	48
8	EtOH	K2CO3	Pd(OAc)2	SPhos	110	41	59
9	EtOH	K2CO3	Pd(OAc)2	CPhos	110	24	76
10	EtOH	K2CO3	Pd(OAc)2	DavePhos	110	21	79
11	EtOH	K2CO3	Pd(OAc)2	Dppf	110	14	86
12	EtOH	K2CO3	Pd(OAc)2	BINAP	110	19	81
13	EtOH	K2CO3	Pd(OAc)2	XantPhos	110	20	80
14	EtOH	K2CO3	Pd(OAc)2	PPh3	110	18	82
15 (ref. 24)	THF	K3PO4	XPhosPdG3	Xphos	80	31	69
16	EtOH	K2CO3	XPhosPdG2	XPhos	110	92	8
17	EtOH	K2CO3	XPhosPdG2	XPhos	135	94	6
18^b	EtOH	K2CO3	XPhosPdG2	XPhos	110	78	22
19^c	EtOH	K2CO3	XPhosPdG2	XPhos	135	100(91)^d	0
20^c	EtOH	K2CO3	XPhosPdG2	—	135	45	55
21^c^,^e	EtOH	K2CO3	XPhosPdG2	XPhos	135	100(89)^d	0
22^c	H2O	K2CO3	XPhosPdG2	XPhos	135	100(86)^d	0

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Under the same conditions, replacing PdCl₂dppf by the tandem XPhosPdG2 (5 mol%) and XPhos (10 mol%) resulted in a similar 5a/3 ratio (Table 1, entry 4). No improvement in the yield of the coupling product 5a was observed when the reaction was carried out using Pd₂(dba)₃ (5 mol%) as a catalyst and XPhos (10 mol%) as a ligand (Table 1, entry 5). Using a polar protic solvent such as ethanol/H₂O instead of dioxane/H₂O increased the yield of the desired product 5a to 45%, decreasing the formation of by-product 3 to 55% (Table 1, entry 6). Encouraged by this result, the mixture of EtOH/H₂O (4 : 1) was chosen as solvents for further experiments. A similar result was recorded when the reaction was performed using Pd(OAC)₂/XPhos tandem as catalyst (Table 1, entry 7). Additional optimizations were undertaken, using the same catalyst [Pd(OAc)₂] and varying the ligand. In all cases, whatever the nature of the ligand employed in these reactions (Sphos, CPhos, DavePhos, dppf, BINAP, XantPhos and PPh₃), the side product 3 was obtained mainly at the expense of the target heterocycle 5a (Table 1, entries 8–14). Surprisingly, under Ellermann conditions (XPhosPdG3/XPhos, K₃PO₄, THF, 80 °C),²⁵ the yield of arylated adduct 5a did not exceed 31%, since the debrominated product 3 was also formed in 69% yield (Table 1, entry 15). Interestingly, when the coupling reaction was performed using the XPhos derived precatalyst XPhosPdG2 in EtOH/H₂O (4 : 1) at 110 °C, the starting material was completely consumed after 12 hours, and the yield of arylated product 5a was improved considerably to 92% (Table 1, entry 16). Nevertheless, the crude ¹H and ¹⁹F NMR spectrums showed the presence of 8% of by-product 3. Under the same reaction conditions and increasing the temperature to 135 °C, no significant difference in the result was observed (Table 1, entry 17). Using an excess of the base K₂CO₃ (3 equiv.) promoted the formation of the undesired product 3 (Table 1, entry 18). Gratifyingly, switching from conventional thermal heating to microwave irradiation at 135 °C, only the coupled product 5a was produced in 91% isolated yield (Table 1, entry 19). Most remarkably, the reaction time was reduced from 12 h to 40 min and no trace of the debrominated product 3 was detected in crude ¹H and ¹⁹F NMR spectrums. When the reaction was conducted without ligand, both heterocycles 5a and 3 were formed equally (Table 1, entry 20), indicating the crucial role of the ligand in avoiding the side debromination reaction. Finally, reducing both XPhosPdG2 (2.5 mol%) and XPhos (5 mol%) loading did not affect significantly the reaction yield, providing compound 5a in 89% yield (Table 1, entry 21). Very interestingly, when the reaction was carried out in water as the sole solvent, the arylation product 5a was the only isolated product in 86% yield (Table 1, entry 22). This result was very promising since water was the solvent of choice for green chemistry. Moreover, it is known that working with water remains tricky because of its limited chemical compatibility as well as the low aqueous solubility of a large number of reagents. It is noteworthy that the nature of the halo group present on the pyrazolo[1,5-a]pyrimidin-5-one 3 plays an important role in this arylation reaction. In fact, whatever the conditions used, including optimized ones, the deiodination reaction cannot be avoided when the substrate 4b was used as a starting material, presumably because of the easier reduction ability of the C–I bond to a C–H bond compared to C–Br bond.

Having established the optimized reaction conditions (Table 1, entry 22), the scope and limitations of the Suzuki–Miyaura cross-coupling reaction were examined with various aryl and heteroaryl boronic acids as coupling partners. The results are summarized in Scheme 3.

Scheme 3 Synthesis of 3-arylated(heteroarylated) pyrazolo[1,5-a]pyrimidin-5-ones by Suzuki–Miyaura cross-coupling reaction of 4a. ^a Isolated yield using water as solvent.

As shown in Scheme 3, in all cases the optimized conditions proved to be suitable for the coupling of 4a with a series of boronic acids, giving the expected C3-arylated products (5a–q) in good to excellent yields (67–89%). Unsubstituted aryls such as phenyl, biphenyl and naphthyl groups were efficiently introduced at C3 position of the pyrazolo[1,5-a]pyrimidin-5-one 4a in 74%, 79% and 85% yields, respectively (Scheme 3). A number of substituents (including functionalized ones) were tolerated on the boronic acids and very efficiently provided the desired products (5e–q). Arylboronic acid bearing an electron-donating group such as a methoxy group at para, meta or ortho position, reacted with 4a leading to the corresponding products 5a (89%), 5f (85%) and 5g (73%). As observed, it appears that the steric hindrance of the ortho-position has a significant effect on the reaction efficiency. The cross-coupling between 4a and 3,4-(ethylenedioxy)phenylboronic acid afforded the 3-arylated pyrazolopyrimidinone 5h in 67% yield. Boronic acids with electron-withdrawing substituent such as 4-acetyl and ethoxycarbonyl were tolerated, yielding the desired products 5i (87%) and 5j (72%), respectively. Boronic acid with a fluorine atom on the aromatic ring was also found to be compatible with this coupling reaction (5k, 76%). Moreover, the use of heteroarylboronic acids such as 2-thienyl, 3-thienyl, 2-furyl, 3-pyridyl was effective, providing the expected heterocycles in good yields [5l (80%), 5m (67%), 5n (84%) and 5o (80%)]. The introduction of a styryl group at C3 position was successfully achieved, affording the vinylation product 5q in 74% yield with retention of the double bond configuration. The presence of a free alcohol function on the boronic acid was tolerated and gave the expected product 5p in 73% yield. The success encountered during this arylation reaction allows easy diversification into biologically important biheterocyclic frameworks.

Encouraged by the efficiency of this Suzuki–Miyaura coupling reaction enabling the introduction of aryl, heteroaryl and vinyl groups at C3 position of pyrazol[1,5-a]pyrimidin-5-one 4a, we decided to extend our methodology to other electron-rich heterocycles such as pyrimido[1,2-b]indazol-2-ones. For this purpose, the tricyclic compounds 7a, 7b and 7c were first prepared by condensation of the corresponding brominated pyrimido[1,2-b]indazol-2-ones 6a, 6b and 6c with the fluorinated alkyne 2 in 64%, 58% and 60% yields, respectively (Scheme 4).

Scheme 4 Synthesis of 9, 8 and 7-brominated pyrimido[1,2-b]indazol-2-ones 7a–c.

A variety of substituted arylboronic acids (with either electron-donating or electron-withdrawing groups) were used to provide the expected coupling products 8a–d, 9a–d and 10a,b in good to excellent yields (61–89%) (Scheme 5). Unsubstituted phenyl boronic acid was successfully coupled with 7a and 7b giving the expected pyrimido[1,2-b]indazol-2-ones 8a and 9a in 70% and 72% isolated yields, respectively. This result demonstrated that the position of bromine on the aromatic ring has no significant effect on the effeciency of this Suzuki–Miyaura coupling. The coupling reaction of 7a and 7b with an heteroarylboronic acid such as 2-furylboronic acid produced the heteroarylated compounds 9d and 8d in 73% and 83% yields, respectively. Moreover, 3-pyridylboronic acid was effectively coupled with 7c, providing the C-7 heteroarylated heterocycle 10b in 68% yield (Scheme 5).

Scheme 5 Suzuki–Miyaura coupling reaction of brominated pyrimido[1,2-b]indazol-2-ones 7a–c. ^a Isolated yield using water as solvent.

The synthesized C3 heteroarylated pyrazolo[1,5-a]pyrimidin-5-ones 5a–q are attractive candidates for access to new 3,5-diarylated pyrazolo[1,5-a]pyrimidines because of their ability to undergo a transition-metal based coupling reaction.

The C3-arylated pyrazolo[1,5-a]pyrimidin-5-ones 5a, 5b, 5e, 5j and 5l were coupled with a variety of available boronic acids to give the desired 3,5-diarylated compounds using a combination of two conditions (Scheme 6), i.e., the phosphonium coupling condition [PyBrop (1.3 equiv.), Et₃N (3 equiv.), 1,4-dioxane, rt, 2 h] then the Suzuki–Miyaura cross-coupling conditions [PdCl₂(PPh₃)₂, Na₂CO₃, 110 °C, 12 h]. Direct arylation proceeded successfully providing the 3,5-diarylated-7-(trifluoromethyl)pyrazolo[1,5-a]pyrimidines 11a–h in good to excellent isolated yields (Scheme 6).

Scheme 6 Synthesis of 3,5-diarylated pyrazolo[1,5-a]pyrimidines 11a–h.

Arylboronic acid bearing an electron-donating group such as a methoxy group at para position was easily coupled with 5b, 5e, 5j and 5l, producing the corresponding diarylated products 11a (88%), 11e (90%), 11f (74%) and 11g (87%), respectively. Phenyl boronic acid substituted with a methoxy group at ortho position gave the expected coupling product 11b in only 66% yield. Steric hindrance could explain this result. Boronic acid with an electron-withdrawing fluorine atom was also tolerated, yielding the desired product 11d in 78% yield. Furthermore, heteroarylboronic acids such as 2-thienyl was successfully coupled with 5l to yield the C5-heteroarylation product 11c in 87% yield. It seems that the nature of the boronic acids used in this coupling reaction had a significant effect on the obtained results.

To demonstrate the versatility and the synthetic potential of our method, we focused our attention on the synthesis of compound having an interesting biological activity via the introduction of suitable aryls at C3 and C5 positions of the pyrazolo[1,5-a]pyrimidine core. For our delight, when the compound 5b was subjected to the conditions of C5-arylation with p-bromophenylboronic acid, the known heterocyclic compound 11h was isolated in excellent yield (91%) (Scheme 6). It is noteworthy that the 3,5-diarylated pyrazolo[1,5-a]pyrimidin-5-one 11h exhibited the comparable anti-inflammatory activity (83.4%) to the standard drug Indomethacin (84.2%).^29b,37

After successfully achieving stepwise sequential diarylation, we attempted to achieve this reaction sequence in a one-pot process to synthesize 3,5-diarylated pyrazolo[1,5-a]pyrimidines. The one-pot sequential arylation of 3-bromo pyrazolo[1,5-a]pyrimidin-5-one 4a was first performed with XPhosPdG2 (2.5 mol%)/XPhos (5 mol%) as catalyst in the presence of K₂CO₃ as base (2.0 equiv.) and p-methoxyphenylboronic acid (2.5 equiv.) in ethanol and water (4 : 1) at 135 °C under microwave irradiation for 40 min. Upon completion of the coupling (monitored by TLC), the solvents were evaporated before using a combination of two conditions to perform the second arylation: [PyBrop (1.3 equiv.), Et₃N (3 equiv.), 1,4-dioxane, rt, 2 h], then the Suzuki–Miyaura cross-coupling conditions [PdCl₂(PPh₃)₂, Na₂CO₃, 110 °C, 12 h] without isolating the intermediate product. Unfortunately, the sequential one-pot coupling reaction afforded the diarylated product 12 in low 25% overall yield (Scheme 7). Note that reversing the order of the arylation reactions using the same reaction conditions was unsuccessful, since only the debrominated product was formed and no trace of the expected diarylation product 12 was observed. This result shows that the stepwise sequential arylation of 4a, applying the earlier optimized conditions, was the best way to access 3,5-diarylated pyrazolo[1,5-a]pyrimidines in good yields.

Scheme 7 Sequential one-pot synthesis of fluorinated 3,5-diaryl pyrazolo[1,5-a]pyrimidine 12.

After successfully performing the C5-arylation providing 3,5-diarylated pyrazolo[1,5-a]pyrimidines, we decided to introduce greater diversity using this strategy. A C5-alkynylation using the Sonogashira coupling reaction was conducted,³⁸ wherein alkyne (1.5 equiv.), PdCl₂(PPh₃)₂ (5 mol%), CuI (10 mol%), Et₃N (5 equiv.) and dioxane were added after the prior addition of PyBroP and Et₃N at room temperature for 2 h. The mixture was subsequently heated at 80 °C for 12 h, providing 5-alkynyl-3-aryl(heteroaryl) pyrazolo[1,5-a]pyrimidines 13a–c in good to excellent yields (Scheme 8). In particular, unsubstituted phenylacetylene was coupled with 5b to afford 13a in 90% isolated yield. Phenylacetylene with an electron-donating group such as a methoxy group at para position was easily coupled with 5b, yielding the desired product 13b in 88% yield. Phenylacetylene bearing an electron-withdrawing group such as a chlorine atom at para position provided the desired product 13c in 78% yield. The electronic effect of the substituents on the aryl group seems to have a small effect on the yield of Sonogashira coupling reaction.

Scheme 8 Synthesis of 5-alkynyl-3-phenyl pyrazolo[1,5-a]pyrimidines 13a–c.

These successive C3-arylation/C5-arylation and C3-arylation/C5-alkynylation were carried out efficiently and open up interesting perspectives for the synthesis of diversely substituted and new potentially important biological bi (or tri)-heterocyclic frameworks.

In order to obtain further insight into the reaction mechanism of debromination, a series of control experiments were conducted to elucidate the role of each component in this reaction. The results obtained are presented in Table 2.

Table 2 Mechanistic study of the debromination reaction

Entry	Additive (X equiv.)	4a/3 (%)^a
a The ratio 4a/3 was determined from the crude ^1H NMR spectrum.
1	p-MeOPhB(OH)2 (1.5)	100/0
2	XPhos (0.05)	100/0
3	XPhosPdG2 (0.025)	97/3
4	K2CO3 (2.0)	0/100
5	Na2CO3 (2.0)	0/100
6	KOH (2.0)	0/100
7	—	100/0

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After examination of these results, we note that neither the presence of the boronic acid alone nor that of the ligand XPhos did not induce the formation of debrominated product 3, since only the starting material 4a was recovered (Table 2, entries 1 and 2). The use of XPhosPdG2 led to a low partial debromination (Table 2, entry 3). On the other hand, the treatment of compound 4a with K₂CO₃ (2 equiv.) at 135 °C resulted in the exclusive conversion of starting material 4a into the debrominated product 3 (Table 2, entry 4). The same result was obtained when K₂CO₃ was replaced by a base having a comparable basicity such as Na₂CO₃ or a stronger one such as KOH (Table 2, entries 5and 6). When the bromide 4a was heated to 135 °C without any additive, no trace of the debrominated product 3 has been detected (Table 2, entry 7), confirming without a doubt the involvement of the base in the promotion of this debromination reaction. These results are completely in agreement with those reported by Cankař et al. in the case of the Suzuki–Miyaura coupling reaction of brominated amino pyrazoles.³⁹

Basing on these experimental results and the previous literature report,³⁹ a possible mechanism is proposed as shown below in Scheme 9.

Scheme 9 Proposed mechanism for the debromination reaction.

The reaction proceeds first by a deprotonation leading to the formation of anion 14a, which can be transformed by delocalization of electrons into the anionic species 14b. The presence of a polar protic solvent allows a protonation of the compound 14b giving access to the intermediate 14c. A subsequent aromatization process occurred along with bromium (Br⁺) release, affording the anionic intermediate 14d, which after protonation, provides the debrominated product 3.

Pyrazolopyrimidinone derivatives are also known for their activities against AChE and BuChE cholinesterases.⁴⁰ It is worth mentioning that they showed moderate selectivity for BuChE over acetylcholinesterase (AChE). They are described as potential inhibitors of BuChE with micromolar IC₅₀ values. Their inhibitory potencies against BuChE were even higher than the anti-AD drug rivastigmine.

The prepared pyrazolopyrimidinones 5a–q were thus screened against both cholinesterases (AChE and BChE), MAO-B and MAO-A using established protocols as described in the experimental section. The results of the assays are summarized in Table 3.

Table 3 AChE, BChE and MAO inhibitory potencies of active compounds

	hAChE^a	hBChE^b	hMAO-A^c	hMAO-B^d
	RA^e at 100/10 μM	RA^e at 100/10 μM	RA^e at 100/10 μM	RA^e at 100/10 μM
a hAChE, human AChE.b hBChE, human BChE.c hMAO-A, human MAO-A.d hMAO-B, human MAO-B.e RA, residual activity at 100/10 μM compound concentration.f n.d., not determined.
5a	86/93	103/90	99/95	26/73
				IC50 = 35.0 ± 11.2 μM
5f	88/91	95/97	97/92	40/80
				IC50 = 101.1 ± 25.7 μM
5h	86/91	95/94	95/110	8/36
				IC50 = 5.36 ± 0.25 μM
5i	91/90	99/91	98/100	21/75
				IC50 = 26.2 ± 3.6 μM
Tacrine	IC50 = 0.115 ± 0.009 μM	IC50 = 0.023 ± 0.003 μM	n.d.^f	n.d.^f
Pargyline	n.d.^f	n.d.^f	IC50 = 3.97 ± 0.28 μM	IC50 = 0.20 ± 0.02 μM

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Interestingly, some of the tested amides showed interesting biological activities. The products which inhibit the studied targets are 7-(trifluoromethyl) pyrazolo[1,5-a]pyrimidin-5(4H)-ones derivatives 5a, 5f, 5h and 5i. They selectively inhibit hMAO-B in the micromolar range (Table 3). This suggests that phenyl substituted at the C-3 and/or C-4 positions by an OMe or COMe group is required for inhibition of hMAO-B.⁴¹ Compound 5h was the most potent hMAO-B inhibitor with an IC₅₀ value of 5.36 μM.

Conclusions

In conclusion, we have developed a catalytic system to synthesize a wide variety of new C3-arylated pyrazolo[1,5-a]pyrimidin-5-one derivatives via the Suzuki–Miyaura cross-coupling reaction of brominated pyrazolo[1,5-a]pyrimidin-5-one derivatives bearing a trifluoromethyl group, without the formation of any trace of the side debrominated product. The use of a catalytic amount of XPhosPdG2/XPhos tandem in the presence of K₂CO₃ in aqueous ethanol as green solvent allowed the efficient cross-coupling reaction of 3-bromo-7-(trifluoromethyl)pyrazolo[1,5-a]pyrimidin-5(4H)-one, 7-, 8- and 9-bromo 4-(trifluoromethyl)pyrimido[1,2-b]indazol-2(1H)-ones with a range of aryl, heteroaryl and styrylboronic acids, providing a rapid access to arylated and heteroarylated pyrazolo[1,5-a]pyrimidin-5-one derivatives in good to excellent yields. Some of synthesized C3-arylated pyrazolo[1,5-a]pyrimidin-5-ones were used as a building blocks for generating new 3,5-diarylated or 3-arylated/5-alkynylated pyrazolo[1,5-a]pyrimidines containing a trifluoromethyl group. The potential application of this environmentally protocol was also demonstrated by the synthesis of the known anti-inflammatory agent 11h, opening access for the design of new functionalized pyrazolo[1,5-a]pyrimidines, having other potential biological activities.

Experimental

General information

Melting points of samples were measured using open capillary tubes. ¹H, ¹³C and ¹⁹F NMR spectra were recorded at 300, 75 and 282 MHz respectively with a 300 MHz Bruker Avance FT-NMR spectrometer using CDCl₃, acetone-d6, THF-d8 or DMSO-d6 as the solvents. All chemical shifts are given in ppm and they are referenced to tetramethylsilane (TMS) as an internal standard. Electrospray ionization high-resolution mass spectrometry experiments were performed with a hybrid tandem quadrupole/time-of flight (Q-TOF) instrument, equipped with a pneumatically assisted electrospray (Zspray) ion source (Micromass, Manchester, U.K.) operated in positive mode. Reactions were monitored by thin-layer chromatography (TLC) analysis using silica gel (60 F254) plates. All compounds were visualized by UV irradiation (longwave at 365 nm or shortwave at 254 nm). All column chromatography was performed using silica gel 60 (230–400.13 mesh, 0.040–0.063 mm). All chemicals reagents purchased from commercial suppliers were used as received. Triethylamine was distilled over calcium hydride and the 1,4-dioxane over sodium and benzophenone.

The microwave-assisted reactions were performed using a Monowave 300 (microwave synthesis reactor: Anton Paar, 300 W maximum power). Microwave irradiation was carried out in sealed 10–30 mL vessels (borosilicate glass) with a PTFE-coated silicon septum and closed with a snap cap made of PEEK. The temperatures were measured on the surface of the vial with an IR sensor (measuring range: 30 to 300 °C; uncertainty: ±5 °C) and were measured with high precision in the reaction mixture with a ruby thermometer (measuring range: 30 to 300 °C; uncertainty: ±2 °C) that could be read directly from the instrument screen. The reaction time was measured from when the reaction mixture reached the stated temperature for temperature controlled experiments. The pressure was measured with a non-invasive pressure sensor located in the swiveling cover of the Monowave 300 (measuring range: 0 to 30 bar; uncertainty: ±1.5 bar).

Preparation and analytical data for 3-halo-7-(trifluoromethyl)pyrazolo[1,5-a]pyrimidin-5-one (4a–b)

NBS or NIS (1.05 equiv.) was added to a solution of pyrazolo[1,5-a]pyrimdin-5-one 3 (100 mg, 1 equiv.) in 8 mL of CH₂Cl₂ and the reaction was stirred under an argon atmosphere at room temperature for 12 h. The progress of the reaction was monitored by TLC (PE/EtOAc, 7/3). After completion of the reaction, evaporation of the solvent under reduced pressure provided the crude product 4a or 4b, which was purified by column chromatography.

3-Bromo-7-(trifluoromethyl)pyrazolo[1,5-a]pyrimidin-5-one (4a).^30c. The purification was carried out using (PE/EtOAc, 7.5/2.5) to afford 4a as white solid in 94% yield (147 mg). Mp: 209–211 °C; ¹H NMR (300 MHz, DMSO-d₆): δ 13.00 (br s, 1H), 8.07 (s, 1H), 6.74 (s, 1H); ¹³C NMR (75 MHz, DMSO-d₆): δ 159.7, 144.8, 140.5, 135.7 (q, J = 36.8 Hz), 119.2 (q, J = 274.6 Hz), 108.0, 75.1; ¹⁹F NMR (282 MHz, DMSO-d₆): δ −66.65; HRMS (ESI) m/z [M + H]⁺ calcd for C₇H₄BrF₃N₃O: 281.9484; found: 281.9474.

3-Iodo-7-(trifluoromethyl)pyrazolo[1,5-a]pyrimidin-5-one (4b). The purification was carried out using (PE/EtOAc, 8/2) to afford 4b as white solid in 90% yield (146 mg). Mp: 220–222 °C; ¹H NMR (300 MHz, DMSO-d₆): δ 12.76 (br s, 1H), 7.89 (s, 1H), 6.70 (s, 1H); ¹³C NMR (75 MHz, DMSO-d₆): δ 159.9, 149.1, 143.6, 135.6 (q, J = 37.0 Hz), 119.2 (q, J = 274.6 Hz), 107.9, 55.3; ¹⁹F NMR (282 MHz, DMSO-d₆): δ −66.70; HRMS (ESI) m/z [M + H]⁺ calcd for C₇H₄IF₃N₃O: 330.0198; found: 330.0173.

General procedure and analytical data for products 7a–c

In a microwave vial, 4(5 or 6)-bromo-1H-indazol-3-amine (0.47 mmol, 100 mg) was dissolved in 1,4-dioxane (4 mL). Ethyl 4,4,4-trifluorobut-2-ynoate (1.2 equiv.) was then added, and the mixture was degassed by argon bubbling for 10 min. The sealed tube was heated at 110 °C for 2 h. After cooling, MeONa (51.2 mg, 2 equiv.) was added and the reaction was stirred for 12 h at room temperature. The reaction mixture was neutralized with a solution of hydrochloric ether (2 M) and the solvent was removed under reduced pressure. The crude product was purified by silica-gel column chromatography.

8-Bromo-4-(trifluoromethyl)pyrimido[1,2-b]indazol-2(1H)-one (7a). The purification was carried out using (EtOAc 100%) to afford 7a as yellow solid in 64% of yield (100 mg). Mp 339–341 °C; ¹H NMR (300 MHz, DMSO-d₆): δ 13.78 (br s, 1H), 7.92–7.89 (m, 2H), 7.18–7.15 (m, 2H); ¹³C NMR (75 MHz, DMSO-d₆): δ 159.2, 150.9, 138.6, 135.9 (q, J = 36.9 Hz), 123.9, 123.6, 123.0, 119.5 (q, J = 274.4 Hz), 118.6, 111.2, 105.7; ¹⁹F NMR (282 MHz, DMSO-d₆): δ −66.71; HRMS (ESI) m/z [M + H]⁺ calcd for C₁₁H₆BrF₃N₃O: 331.9641; found: 331.9644.

9-Bromo-4-(trifluoromethyl)pyrimido[1,2-b]indazol-2(1H)-one (7b). The purification was carried out using (EtOAc 100%) to afford 7b as yellow solid in 58% yield (91 mg). Mp 345–347 °C; ¹H NMR (300 MHz, DMSO-d₆): δ 13.69 (br s, 1H), 8.15 (d, J = 1.4 Hz, 1H), 7.60 (d, J = 9.3 Hz, 1H), 7.49 (dd, J = 9.3 Hz, 1H), 7.12 (s, 1H); ¹³C NMR (75 MHz, DMSO-d₆): δ 159.5, 148.8, 138.0, 135.7 (q, J = 35.8 Hz), 133.2, 122.9, 119.5 (q, J = 275.2 Hz), 118.9, 111.8, 111.4, 108.1; ¹⁹F NMR (282 MHz, DMSO-d₆): δ −66.70; HRMS (ESI) m/z [M + H]⁺ calcd for C₁₁H₆BrF₃N₃O: 331.9641; found: 331.9644.

10-Bromo-4-(trifluoromethyl)pyrimido[1,2-b]indazol-2(1H)-one (7c). The purification was carried out using (EtOAc 100%) to afford 7c as yellow solid in 60% yield (94 mg). Mp 285–287 °C; ¹H NMR (300 MHz, DMSO-d₆): δ 13.06 (br s, 1H), 7.74 (d, J = 8.2 Hz, 1H), 7.45–7.36 (m, 1H); ¹³C NMR (75 MHz, DMSO-d₆): δ 160.5, 151.9, 143.1, 134.8 (q, J = 37.2 Hz), 130.9, 124.1, 119.7 (q, J = 274.1 Hz), 116.0, 114.1, 109.6, 105.3; ¹⁹F NMR (282 MHz, DMSO-d₆): δ −67.74; HRMS (ESI) m/z [M + H]⁺ calcd for C₁₁H₆BrF₃N₃O: 331.9641; found: 331.9639.

Pd-catalyzed arylation of 3-bromo 7-(trifluoromethyl)pyrazolo[1,5-a]pyrimdin-5-one 4a and C8(9 or 10)-bromo 4-(trifluoromethyl)pyrimido[1,2-b]indazol-2-one 7a–c with (hetero)aryl boronic acids.

A mixture of 4-bromo-7-(trifluoromethyl)pyrazolo[1,5-a]pyrimidin-5-one 4a or 8(9 or 10)-bromo-4-(trifluoromethyl)benzo[4,5]imidazo[1,2-a]pyrimidin-2-one 7a–c (100 mg, 1 equiv.), boronic acid (1.5 equiv.), and K₂CO₃ (2 equiv.) in EtOH (4 mL) and water (1 mL) was thoroughly degassed with a stream of argon. Then, XPhos (5 mol%) and XPhosPdG₂ (2.5 mol%) were added and the microwave vial containing the mixture was capped and inserted into microwave reactor. The reaction mixture was irradiated at 135 °C for 40 min. After that, the mixture was concentrated under reduced pressure. Purification of the crude product via column chromatography afforded desired 3-disubstituted pyrazolo[1,5-a]pyrimidin-5-one 5a–q and 8(9 or 10)- substituted benzo[4,5]imidazo[1,2-a]pyrimidin-2-ones 8a–d, 9a–d and 10a–b as solid, which were characterized by ¹H, ¹³C NMR, ¹⁹F NMR and HRMS. In all cases, products were recrystallized from diethyl ether or CH₂Cl₂.

3-(4-Methoxyphenyl)-7-(trifluoromethyl)pyrazolo[1,5-a]pyrimidin-5(4H)-one (5a)^30c. The purification was carried out using (PE/EtOAc: 8/2) to afford 5a as yellow solid in 91% yield (100 mg). Mp 227–229 °C; ¹H NMR (300 MHz, acetone-d₆): δ 11.20 (br s, 1H), 8.08 (s, 1H), 7.54 (d, J = 8.8 Hz, 2H), 7.01 (d, J = 8.8 Hz, 2H), 6.58 (s, 1H), 3.84 (s, 3H); ¹³C NMR (75 MHz, DMSO-d₆): δ 160.1, 158.5, 144.9, 144.0, 135.9 (q, J = 36.9 Hz), 128.6, 123.0, 119.5 (q, J = 274.4 Hz), 114.5 (3C), 107.6, 105.2, 55.6; ¹⁹F NMR (282 MHz, acetone-d₆): δ −68.39; HRMS (ESI) m/z [M + H]⁺ calcd for C₁₄H₁₁F₃N₃O₂: 310.0798; found: 310.0486.

3-Phenyl-7-(trifluoromethyl)pyrazolo[1,5-a]pyrimidin-5(4H)-one (5b)^23a. The purification was carried out using (PE/EtOAc: 8.5/1.5) to afford 5b as white solid in 74% yield (73 mg). Mp 240–242 °C; ¹H NMR (300 MHz, acetone-d₆): δ 11.27 (br s, 1H), 8.17 (s, 1H), 7.65 (d, J = 7.3 Hz, 2H), 7.44 (t, J = 7.3 Hz, 2H), 7.32 (tt, J = 7.3, 1.2 Hz, 1H), 6.62 (s, 1H); ¹³C NMR (75 MHz, DMSO-d₆): δ 160.4, 144.1, 136.0 (q, J = 36.6 Hz), 130.8, 129.1 (4C), 127.2, 126.9, 119.4 (q, J = 274.4 Hz), 105.6, 105.2; ¹⁹F NMR (282 MHz, acetone-d₆): δ −68.39; HRMS (ESI) m/z [M + H]⁺ calcd for C₁₃H₉F₃N₃O: 280.0692; found: 280.0682.

3-(Biphenyl-3-yl)-7-(trifluoromethyl)pyrazolo[1,5-a]pyrimidin-5(4H)-one (5c). The purification was carried out using (PE/EtOAc: 8.5/1.5) to afford 5c as yellow solid in 79% yield (100 mg). Mp 259–261 °C; ¹H NMR (300 MHz, acetone-d₆): δ 11.49 (br s, 1H), 8.27 (s, 1H), 7.91 (t, J = 1.5 Hz, 1H), 7.75 (d, J = 7.3 Hz, 2H), 7.63 (tt, J = 7.6, 1.5 Hz, 2H), 7.55 (d, J = 7.4 Hz, 1H), 7.52 (d, J = 2.9 Hz, 1H), 7.49 (d, J = 7.7 Hz, 1H), 7.4 (tt, J = 7.3, 1.5 Hz, 1H), 6.62 (s, 1H); ¹³C NMR (75 MHz, acetone-d₆): δ 160.4, 144.4, 141.1 (2C), 140.7, 136.0 (q, J = 36.7 Hz), 131.3, 129.7, 129.3 (2C), 128.0, 127.5 (2C), 126.5, 125.8, 125.4, 119.5 (q, J = 274.5 Hz), 106.0, 105.4; ¹⁹F NMR (282 MHz, acetone-d₆): δ −68.38; HRMS (ESI) m/z [M + H]⁺ calcd for C₁₉H₁₃F₃N₃O: 356.1005; found: 356.0994.

3-(Naphthalen-2-yl)-7-(trifluoromethyl)pyrazolo[1,5-a]pyrimidin-5(4H)-one (5d). The purification was carried out using (PE/EtOAc: 8.5/1.5) to afford 5d as yellow solid in 85% yield (100 mg). Mp 274–276 °C; ¹H NMR (300 MHz, acetone-d₆): δ 11.42 (br s, 1H), 8.31 (s, 1H), 8.18 (s, 1H), 7.99 (d, J = 8.5 Hz, 1H), 7.94 (d, J = 8.4 Hz, 1H), 7.93 (d, J = 6.8 Hz, 1H), 7.80 (dd, J = 8.5, 1.7 Hz, 1H), 7.58–7.49 (m, 2H), 6.65 (s, 1H); ¹³C NMR (75 MHz, DMSO-d₆): δ 160.4, 144.3, 136.0 (q, J = 36.65 Hz), 133.7, 132.1, 129.3, 128.5, 128.2, 128.0, 126.8, 126.1, 125.8, 125.3, 125.2, 119.5 (q, J = 274.6 Hz), 105.5, 105.1; ¹⁹F NMR (282 MHz, acetone-d₆): δ −68.35; HRMS (ESI) m/z [M + H]⁺ calcd for C₁₇H₁₁F₃N₃O: 330.0849; found: 330.0837.

3-(P-Tolyl)-7-(trifluoromethyl)pyrazolo[1,5-a]pyrimidin-5(4H)-one (5e). The purification was carried out using (PE/EtOAc: 8/2) to afford 5e as white solid in 88% yield (92 mg). Mp 257–259 °C; ¹H NMR (300 MHz, DMSO-d₆): δ 12.62 (br s, 1H), 8.32 (s, 1H), 7.61 (d, J = 7.9 Hz, 2H), 7.23 (d, J = 7.9 Hz, 2H), 6.77 (s, 1H), 2.33 (s, 3H); ¹³C NMR (75 MHz, DMSO-d₆): δ 160.3, 144.9, 144.0, 136.1, 135.9 (q, J = 36.6 Hz), 129.6 (4C), 127.8, 127.1, 119.5 (q, J = 274.5 Hz), 105.5, 88.5; ¹⁹F NMR (282 MHz, DMSO-d₆): δ −67.06; HRMS (ESI) m/z [M + H]⁺ calcd for C₁₄H₁₁F₃N₃O: 294.0849; found: 294.0847.

3-(3-Methoxyphenyl)-7-(trifluoromethyl)pyrazolo[1,5-a]pyrimidin-5(4H)-one (5f). The purification was carried out using (PE/EtOAc: 8/2) to afford 5f as yellow solid in 85% yield (93 mg). Mp 209–211 °C; ¹H NMR (300 MHz, acetone-d₆): δ 11.31 (br s, 1H), 8.18 (s, 1H), 7.35 (t, J = 8.2 Hz, 1H), 7.21–7.19 (m, 2H), 6.89 (dd, J = 8.2, 2.5 Hz, 1H), 6.63 (s, 1H), 3.86 (s, 3H); ¹³C NMR (75 MHz, acetone-d₆): δ 160.1, 159.0, 143.5, 138.6, 136.7 (q, J = 36.8 Hz), 131.5, 129.8, 119.4, 119.2 (q, J = 274.0 Hz), 112.8, 112.4, 105.8 (q, J = 4.2 Hz), 105.0, 54.6; ¹⁹F NMR (282 MHz, acetone-d₆): δ −68.41; HRMS (ESI) m/z [M + H]⁺ calcd for C₁₄H₁₁F₃N₃O₂: 310.0798; found: 310.0787.

3-(2-Methoxyphenyl)-7-(trifluoromethyl)pyrazolo[1,5-a]pyrimidin-5(4H)-one (5g). The purification was carried out using (PE/EtOAc: 8/2) to afford 5g as yellow solid in 73% yield (80 mg). Mp 187–189 °C; ¹H NMR (300 MHz, acetone-d₆): δ 10.64 (br s, 1H), 8.04 (s, 1H), 7.47 (dd, J = 7.5, 1.7 Hz, 1H), 7.36 (dd, J = 7.4, 1.7 Hz, 1H), 7.11 (d, J = 7.6 Hz, 1H), 7.03 (td, J = 7.5, 1.1 Hz, 1H), 6.54 (s, 1H), 3.93 (s, 3H); ¹³C NMR (75 MHz, acetone-d₆): δ 158.1, 156.2, 144.4, 137.9, 136.7 (q, J = 37.0 Hz), 129.7, 128.7, 121.1, 119.2 (q, J = 274.0 Hz), 118.8, 111.2, 106.5 (q, J = 4.4 Hz), 101.2, 55.0; ¹⁹F NMR (282 MHz, acetone-d₆): δ −68.16; HRMS (ESI) m/z [M + H]⁺ calcd for C₁₄H₁₁F₃N₃O₂: 310.0798; found: 310.0786.

3-(2,3-Dihydrobenzo[b][1,4]dioxin-6-yl)-7-(trifluoromethyl)pyrazolo[1,5-a]pyrimidin-5(4H)-one (5h). The purification was carried out using (PE/EtOAc: 8/2) to afford 5h as yellow solid in 67% yield (80 mg). Mp 245–247 °C; ¹H NMR (300 MHz, acetone-d₆): δ 11.31 (br s, 1H), 8.09 (s, 1H), 7.09 (s, 1H), 7.08 (dd, J = 8.8, 2.1 Hz, 1H), 6.88 (d, J = 8.6 Hz, 1H), 6.6 (s, 1H), 4.31–4.30 (m, 4H); ¹³C NMR (75 MHz, DMSO-d₆): δ 160.3, 144.0, 143.9, 142.7, 135.9 (q, J = 36.6 Hz), 123.9, 120.3, 119.3 (q, J = 274.6 Hz), 117.6 (2C), 116.0, 105.2, 105.0, 64.6; ¹⁹F NMR (282 MHz, acetone-d₆): δ −68.43; HRMS (ESI) m/z [M + H]⁺ calcd for C₁₅H₁₁F₃N₃O₃: 338.0747; found: 338.0735.

3-(4-Acetylphenyl)-7-(trifluoromethyl)pyrazolo[1,5-a]pyrimidin-5(4H)-one (5i). The purification was carried out using (PE/EtOAc: 6/4) to afford 5i as yellow solid in 87% yield (99 mg). Mp 262–264 °C; ¹H NMR (300 MHz, acetone-d₆): δ 11.36 (br s, 1H), 8.34 (s, 1H), 8.05 (d, J = 8.6 Hz, 2H), 7.87 (d, J = 8.6 Hz, 2H), 6.72 (s, 1H), 2.61 (s, 3H); ¹³C NMR (75 MHz, DMSO-d₆): δ 197.6, 161.0, 144.3, 136.0, 137.0 (q, J = 36.8 Hz), 134.8, 129.1 (3C), 126.6 (2C), 119.4 (q, J = 274.4 Hz), 105.0, 104.9, 27.0; ¹⁹F NMR (282 MHz, acetone-d₆): δ −68.47; HRMS (ESI) m/z [M + H]⁺ calcd for C₁₅H₁₁F₃N₃O₂: 322.0798; found: 322.0786.

Ethyl 4-(5-oxo-7-(trifluoromethyl)-4,5-dihydropyrazolo[1,5-a]pyrimidin-3-yl)benzoate (5j). The purification was carried out using (PE/EtOAc: 8/2) to afford 5j as white solid in 72% yield (90 mg). Mp 265–267 °C; ¹H NMR (300 MHz, acetone-d₆): δ 11.37 (br s, 1H), 8.34 (s, 1H), 8.06 (d, J = 8.4 Hz, 2H), 7.86 (d, J = 7.9 Hz, 2H), 6.72 (s, 1H), 4.37 (q, J = 7.1 Hz, 2H), 1.39 (t, J = 7.1 Hz, 3H); ¹³C NMR (75 MHz, DMSO-d₆): δ 166.0, 161.0, 144.3, 141.7, 136.0, 136.0 (q, J = 36.8 Hz), 129.9 (3C), 127.7, 126.6, 119.4 (q, J = 274.4 Hz), 105.1, 104.9, 61.1, 14.6; ¹⁹F NMR (282 MHz, acetone-d₆): δ −68.46; HRMS (ESI) m/z [M + H]⁺ calcd for C₁₆H₁₃F₃N₃O₃: 352.0903; found: 352.0891.

3-(4-Fluorophenyl)-7-(trifluoromethyl)pyrazolo[1,5-a]pyrimidin-5(4H)-one (5k). The purification was carried out using (PE/EtOAc: 8/2) to afford 5k as white solid in 76% yield (80 mg). Mp 277–279 °C; ¹H NMR (300 MHz, acetone-d₆): δ 11.23 (br s, 1H), 8.16 (s, 1H), 7.72–7.68 (m, 2H), 7.24–7.19 (m, 2H), 6.64 (s, 1H); ¹³C NMR (75 MHz, DMSO-d₆): δ 161.4 (d, J = 243.3 Hz), 160.5, 144.1, 135.9 (q, J = 36.7 Hz), 129.3, 129.2, 129.2, 127.3, 119.4 (q, J = 274.3 Hz), 116.0, 115.7, 105.4, 104.7; ¹⁹F NMR (282 MHz, acetone-d₆): δ −68.43, −116.93; HRMS (ESI) m/z [M + H]⁺ calcd for C₁₃H₈F₄N₃O: 298.0598; found: 298.059.

3-(Thiophen-2-yl)-7-(trifluoromethyl)pyrazolo[1,5-a]pyrimidin-5(4H)-one (5l). The purification was carried out using (PE/EtOAc: 8.5/1.5) to afford 5l as yellow solid in 80% yield (81 mg). Mp 219–221 °C; ¹H NMR (300 MHz, acetone-d₆): δ 11.20 (br s, 1H), 8.10 (s, 1H), 7.46 (d, J = 5.1 Hz, 1H), 7.34 (d, J = 3.5 Hz, 1H), 7.13 (dd, J = 5.1, 3.6 Hz, 1H), 6.66 (s, 1H); ¹³C NMR (75 MHz, acetone-d₆): δ 158.8, 143.3, 138.8, 136.6 (q, J = 37.2 Hz), 131.1, 127.7, 125.0, 124.6, 119.2 (q, J = 274.0 Hz), 105.9, 99.3; ¹⁹F NMR (282 MHz, acetone-d₆): δ −68.42; HRMS (ESI) m/z [M + H]⁺ calcd for C₁₁H₇F₃N₃OS: 286.0256; found: 286.0246.

3-(Thiophen-3-yl)-7-(trifluoromethyl)pyrazolo[1,5-a]pyrimidin-5(4H)-one (5m). The purification was carried out using (PE/EtOAc: 8.5/1.5) to afford 5m as yellow solid in 67% yield (68 mg). Mp 257–259 °C; ¹H NMR (300 MHz, acetone-d₆): δ 11.12 (br s, 1H), 8.20 (s, 1H), 7.74 (dd, J = 2.9, 1.3 Hz, 1H), 7.59 (dd, J = 5.0, 2.9 Hz, 1H), 7.50 (dd, J = 5.0, 1.3 Hz, 1H), 6.61 (s, 1H); ¹³C NMR (75 MHz, DMSO-d₆): δ 160.2, 144.2, 136.0 (q, J = 36.7 Hz), 130.6, 127.3, 126.7 (2C), 120.5, 119.4 (q, J = 274.5 Hz), 106.2, 101.5; ¹⁹F NMR (282 MHz, acetone-d₆): δ −68.39; HRMS (ESI) m/z [M + H]⁺ calcd for C₁₁H₇F₃N₃OS: 286.0256; found: 286.0246.

3-(Furan-2-yl)-7-(trifluoromethyl)pyrazolo[1,5-a]pyrimidin-5(4H)-one (5n). The purification was carried out using (PE/EtOAc: 8.5/1.5) to afford 5n as yellow solid in 84% yield (80 mg). Mp 247–249 °C; ¹H NMR (300 MHz, acetone-d₆): δ 11.03 (br s, 1H), 8.17 (s, 1H), 7.60 (dd, J = 1.8, 0.7 Hz, 1H), 6.73 (dd, J = 3.4, 0.7 Hz, 1H), 6.60 (s, 1H), 6.55 (dd, J = 3.4, 1.8 Hz, 1H); ¹³C NMR (75 MHz, DMSO-d₆): δ 159.9, 144.9, 142.2, 142.0, 137.9, 135.9 (q, J = 36.7 Hz), 119.3 (q, J = 274.5 Hz), 112.0, 107.1, 105.9, 97.3; ¹⁹F NMR (282 MHz, acetone-d₆): δ −68.29; HRMS (ESI) m/z [M + H]⁺ calcd for C₁₁H₇F₃N₃O₂: 270.0485; found: 270.0475.

3-(Pyridin-3-yl)-7-(trifluoromethyl)pyrazolo[1,5-a]pyrimidin-5(4H)-one (5o). The purification was carried out using (PE/EtOAc: 6.5/3.5) to afford 5o as yellow solid in 80% yield (80 mg). Mp 270–272 °C; ¹H NMR (300 MHz, acetone-d₆): δ 8.97 (s, 1H), 8.46 (s, 1H), 8.46–8.44 (m, 2H), 8.14 (d, J = 7.8 Hz, 1H), 7.44 (dd, J = 7.8, 4.8 Hz, 1H), 6.84 (s, 1H); ¹³C NMR (75 MHz, DMSO-d₆): δ 161.2, 147.9, 147.5, 143.9, 141.8, 135.7 (q, J = 36.7 Hz), 13.2, 127.3, 124.0, 119.5 (q, J = 274.3 Hz), 105.5 (q, J = 3.8 Hz), 102.3; ¹⁹F NMR (282 MHz, acetone-d₆): δ −67.11; HRMS (ESI) m/z [M + H]⁺ calcd for C₁₂H₇F₃N₄O: 281.0645; found: 281.0635.

3-[4-(Hydroxymethyl)phenyl]-7-(trifluoromethyl)pyrazolo[1,5-a]pyrimidin-5(4H)-one (5p). The purification was carried out using (PE/EtOAc: 6.5/3.5) to afford 5p as white solid in 73% yield (80 mg). Mp 241–242 °C; ¹H NMR (300 MHz, DMSO-d₆): δ 12.66 (br s, 1H), 8.36 (s, 1H), 7.69 (d, J = 6.0 Hz, 2H), 7.36 (d, J = 8.2 Hz, 2H), 6.80 (s, 1H), 5.21 (t, J = 4.9 Hz, 1H), 4.52 (d, J = 3.6 Hz, 2H); ¹³C NMR (75 MHz, DMSO-d₆): δ 160.4, 144.1, 141.2, 135.9 (q, J = 36.7 Hz), 129.1, 127.2 (3C), 126.9, 125.8, 119.5 (q, J = 274.4 Hz), 105.5, 105.4, 36.2; ¹⁹F NMR (282 MHz, DMSO-d₆): δ −67.05; HRMS (ESI) m/z [M + H]⁺ calcd for C₁₄H₁₁F₃N₃O₂: 310.0798; found: 310.0787.

(E)-3-Styryl-7-(trifluoromethyl)pyrazolo[1,5-a]pyrimidin-5(4H)-one (5q). The purification was carried out using (PE/EtOAc: 8.5/1.5) to afford 5q as yellow solid in 74% yield (80 mg). Mp 307–309 °C; ¹H NMR (300 MHz, DMSO-d₆): δ 12.88 (br s, 1H), 8.39 (s, 1H), 7.48 (d, J = 7.4 Hz, 2H), 7.41–7.35 (m, 3H), 7.23 (t, J = 7.2 Hz, 1H), 7.02 (d, J = 16.4 Hz, 1H), 6.65 (s, 1H); ¹³C NMR (75 MHz, DMSO-d₆): δ 152.3, 142.1, 138.7, 138.0, 136.0 (q, J = 36.6 Hz), 129.2 (2C), 127.5, 126.3, 126.1 (2C), 119.3 (q, J = 274.6 Hz), 116.1, 107.5, 102.7; ¹⁹F NMR (282 MHz, DMSO-d₆): δ −66.70; HRMS (ESI) m/z [M + H]⁺ calcd for C₁₅H₁₁F₃N₃O: 306.0849; found: 306.0838.

9-Phenyl-4-(trifluoromethyl)pyrimido[1,2-b]indazol-2(1H)-one (8a). The purification was carried out using (EtOAc: 100%) to afford 8a as yellow solid in 70% yield (70 mg). Mp 344–346 °C; ¹H NMR (300 MHz, acetone-d₆): δ 8.30 (s, 1H), 7.83 (dd, J = 9.2, 1.7 Hz, 1H), 7.73 (d, J = 7.4 Hz, 2H), 7.69 (d, J = 9.2 Hz, 1H), 7.50 (t, J = 7.4 Hz, 2H), 7.37 (t, J = 7.4 Hz, 1H), 6.98 (s, 1H); ¹³C NMR (75 MHz, DMSO-d₆): δ 159.0, 150.0, 140.6, 138.2, 136.0 (q, J = 36.5 Hz), 132.4, 130.3, 129.5 (2C), 127.5, 126.8 (2C), 119.1 (q, J = 274.6 Hz), 118.0, 117.2, 110.8, 107.2; ¹⁹F NMR (282 MHz, DMSO-d₆): δ −66.62; HRMS (ESI) m/z [M + H]⁺ calcd for C₁₇H₁₁F₃N₃O: 330.0849; found: 330.0838.

9-(4-Methoxyphenyl)-4-(trifluoromethyl)pyrimido[1,2-b]indazol-2(1H)-one (8b). The purification was carried out using (EtOAc: 100%) to afford 8b as yellow solid in 86% yield (93 mg). Mp 340–342 °C; ¹H NMR (300 MHz, acetone-d₆): δ 8.20 (s, 1H), 7.80 (dd, J = 9.2, 1.8 Hz, 1H), 7.66 (d, J = 8.8 Hz, 3H), 7.06 (d, J = 8.8 Hz, 2H), 6.96 (s, 1H), 3.86 (s, 3H); ¹³C NMR (75 MHz, DMSO-d₆): δ 160.3, 159.0, 150.0, 139.9, 135.5 (q, J = 36.0 Hz), 133.2, 131.5, 130.0, 127.9 (2C), 119.7 (q, J = 274.3 Hz), 117.2, 116.8, 114.9 (2C), 110.4, 107.8, 55.6; ¹⁹F NMR (282 MHz, acetone-d₆): δ −68.22; HRMS (ESI) m/z [M + H]⁺ calcd for C₁₈H₁₃F₃N₃O₂: 360.0954; found: 360.0941.

9-(4-Acetylphenyl)-4-(trifluoromethyl)pyrimido[1,2-b]indazol-2(1H)-one (8c). The purification was carried out using (EtOAc: 100%) to afford 8c as yellow solid in 72% yield (81 mg). Mp 346–348 °C; ¹H NMR (300 MHz, DMSO-d₆): δ 8.36 (s, 1H), 8.08 (d, J = 8.4 Hz, 2H), 7.85 (d, J = 8.4 Hz, 2H), 7.86 (dd, J = 9.2, 1.8 Hz, 1H), 7.72 (d, J = 9.2 Hz, 1H), 7.09 (s, 1H), 2.63 (s, 3H); ¹³C NMR (75 MHz, DMSO-d₆): δ 197.8, 159.8, 150.2, 145.0, 139.9, 135.7 (q, J = 36.8 Hz), 135.6, 134.7, 130.5, 129.8, 129.6, 127.4, 126.7, 119.6 (q, J = 274.1 Hz), 119.4, 117.2, 110.9, 107.7, 27.2; ¹⁹F NMR (282 MHz, DMSO-d₆): δ −66.69; HRMS (ESI) m/z [M + H]⁺ calcd for C₁₉H₁₃F₃N₃O₂: 372.0954; found: 372.0942.

9-(Furan-2-yl)-4-(trifluoromethyl)pyrimido[1,2-b]indazol-2(1H)-one (8d). The purification was carried out using (EtOAc: 100%) to afford 8d as yellow solid in 83% yield (80 mg). Mp 325–327 °C; ¹H NMR (300 MHz, acetone-d₆): δ 8.32 (t, J = 0.9 Hz, 1H), 7.84 (dd, J = 9.3, 1.7 Hz, 1H), 7.66 (dd, J = 1.8, 0.6 Hz, 1H), 7.63 (dd, J = 9.3, 0.9 Hz, 1H), 6.96 (s, 1H), 6.84 (d, J = 3.0 Hz, 1H), 6.59 (dd, J = 3.4, 1.8 Hz, 1H); ¹³C NMR (75 MHz, THF-d₈): δ 159.0, 154.9, 151.0, 142.7, 139.1, 137.3 (q, J = 37.1 Hz), 127.7, 124.1, 120.2 (q, J = 274.4 Hz), 118.0, 113.8, 112.3, 109.2, 107.8, 105.1; ¹⁹F NMR (282 MHz, acetone-d₆): δ −68.21; HRMS (ESI) m/z [M + H]⁺ calcd for C₁₅H₉F₃N₃O₂: 320.0641; found: 320.0629.

8-Phenyl-4-(trifluoromethyl)pyrimido[1,2-b]indazol-2(1H)-one (9a). The purification was carried out using (EtOAc: 100%) to afford 9a as yellow solid in 72% yield (71 mg). Mp 340–342 °C; ¹H NMR (300 MHz, DMSO-d₆): δ 13.74 (br s, 1H), 8.04 (d, J = 8.1 Hz, 1H), 7.88 (s, 1H), 7.80 (d, J = 6.5 Hz, 2H), 7.51–7.43 (m, 4H), 7.10 (s, 1H); ¹³C NMR (75 MHz, DMSO-d₆): δ 159.0, 151.2, 142.2, 140.6, 137.7, 136.0 (q, J = 36.8 Hz), 129.4 (2C), 128.4, 127.6 (2C), 121.5, 120.6, 119.6 (q, J = 274.5 Hz), 113.6, 110.6, 106.0; ¹⁹F NMR (282 MHz, DMSO-d₆): δ −66.67; HRMS (ESI) m/z [M + H]⁺ calcd for C₁₇H₁₁F₃N₃O: 330.0849; found: 330.0836.

8-[4-(Hydroxymethyl)phenyl]-4-(trifluoromethyl)pyrimido[1,2-b]indazol-2(1H)-one (9b). The purification was carried out using (EtOAc: 100%) to afford 9b as yellow solid in 61% yield (66 mg). Mp. 296–298 °C; ¹H NMR (300 MHz, DMSO-d₆): δ 13.70 (br s, 1H), 8.02 (d, J = 8.8 Hz, 1H), 7.84 (s, 1H), 7.75 (d, J = 8.2 Hz, 2H), 7.43 (d, J = 8.2 Hz, 2H), 7.39 (dd, J = 8.8, 1.2 Hz, 1H), 7.05 (s, 1H), 5.26 (br s, 1H), 4.56 (s, 2H); ¹³C NMR (75 MHz, DMSO-d₆): δ 159.3, 151.3, 142.8, 142.1, 139.0, 138.2, 135.8 (q, J = 36.5 Hz), 127.5 (2C), 127.3 (2C), 121.5, 120.4, 119.6 (q, J = 274.5 Hz), 113.2, 110.3, 106.1, 63.0; ¹⁹F NMR (282 MHz, DMSO-d₆): δ −66.74; HRMS (ESI) m/z [M + H]⁺ calcd for C₁₈H₁₃F₃N₃O₂: 360.0954; found: 360.0953.

(E)-8-Styryl-4-(trifluoromethyl)pyrimido[1,2-b]indazol-2(1H)-one (9c). The purification was carried out using (EtOAc: 100%) to afford 9c as yellow solid in 81% yield (87 mg). Mp >350 °C; ¹H NMR (300 MHz, DMSO-d₆): δ 13.63 (br s, 1H), 7.94 (d, J = 8.9 Hz, 1H), 7.69–7.63 (m, 3H), 7.47–7.39 (m, 5H), 7.30 (t, J = 7.4 Hz, 1H), 7.03 (s, 1H); ¹³C NMR (75 MHz, DMSO-d₆): δ 159.2, 151.2, 139.2, 137.9, 137.4, 135.9 (q, J = 36.6 Hz), 130.3, 129.4, 129.2 (2C), 128.4, 127.1 (2C), 121.0, 119.8 (q, J = 274.8 Hz), 118.5, 115.1, 110.3, 106.0; HRMS (ESI) m/z [M + H]⁺ calcd for C₁₉H₁₃F₃N₃O: 356.1005; found: 356.1004.

8-(Furan-2-yl)-4-(trifluoromethyl)pyrimido[1,2-b]indazol-2(1H)-one (9d). The purification was carried out using (EtOAc: 100%) to afford 9d as yellow solid in 73% yield (70 mg). Mp 329–331 °C; ¹H NMR (300 MHz, DMSO-d₆): δ 13.69 (br s, 1H), 7.99 (d, J = 8.8 Hz, 1H), 7.85 (dd, J = 6.2, 1.2 Hz, 2H), 7.44 (dd, J = 8.8, 1.0 Hz, 1H), 7.14 (d, J = 3.3 Hz, 1H), 7.05 (s, 1H), 6.66 (dd, J = 3.3, 1.8 Hz, 1H); ¹³C NMR (75 MHz, DMSO-d₆): δ 159.2, 153.4, 150.8, 144.1, 138.1, 135.9 (q, J = 36.8 Hz), 131.8, 121.7, 119.6 (q, J = 274.2 Hz), 117.4, 112.8, 110.4, 109.5, 108.2, 105.8; ¹⁹F NMR (282 MHz, DMSO-d₆): δ −66.72; HRMS (ESI) m/z [M + H]⁺ calcd for C₁₅H₉F₃N₃O₂: 320.0641; found: 320.0629.

10-(4-Methoxyphenyl)-4-(trifluoromethyl)pyrimido[1,2-b]indazol-2(1H)-one (10a). The purification was carried out using (EtOAc: 100%) to afford 10a as yellow solid in 89% yield (96 mg). Mp 264–266 °C; ¹H NMR (300 MHz, DMSO-d₆): δ 12.49 (br s, 1H), 7.68 (d, J = 8.3 Hz, 1H), 7.66 (d, J = 8.7 Hz, 2H), 7.56 (dd, J = 8.3, 6.9 Hz, 1H), 7.34 (s, 1H), 7.06 (d, J = 8.7, 2H), 7.04 (d, J = 6.9 Hz, 1H), 3.85 (s, 3H); ¹³C NMR (75 MHz, DMSO-d₆): δ 159.5, 159.3, 152.2, 142.8, 137.2, 134.6 (q, J = 36.8 Hz), 131.3, 130.8 (2C), 130.3, 121.3, 119.8 (q, J = 274.2 Hz), 115.0, 114.1 (2C), 107.9, 104.6, 55.6; ¹⁹F NMR (282 MHz, DMSO-d₆): δ −67.74; HRMS (ESI) m/z [M + H]⁺ calcd for C₁₈H₁₃F₃N₃O₂: 360.0954; found: 360.0953.

10-(Pyridin-3-yl)-4-(trifluoromethyl)pyrimido[1,2-b]indazol-2(1H)-one (10b). The purification was carried out using (EtOAc: 100%) to afford 10b as yellow solid in 68% yield (68 mg). Mp 274–276 °C; ¹H NMR (300 MHz, DMSO-d₆): δ 8.88 (s, 1H), 8.62 (d, J = 4.6 Hz, 1H), 8.13 (td, J = 7.9, 1.7 Hz, 1H), 7.74 (d, J = 8.6 Hz, 1H), 7.59 (dd, J = 8.6, 6.8 Hz, 1H), 7.51 (dd, J = 7.9, 4.6 Hz, 1H), 7.32 (s, 1H), 7.10 (d, J = 6.8 Hz, 1H); ¹³C NMR (75 MHz, DMSO-d₆): δ 160.7, 151.9, 149.7, 148.9, 143.9, 137.2, 134.6, 134.1 (q, J = 36.7 Hz), 133.9, 130.1, 123.6, 121.2, 119.9 (q, J = 273.9 Hz), 116.2, 107.8, 105.0; ¹⁹F NMR (282 MHz, DMSO-d₆): δ −67.74; HRMS (ESI) m/z [M + H]⁺ calcd for C₁₆H₁₀F₃N₄O: 331.0801; found: 331.0799.

General procedure for the direct C5 arylation of 5b, 5e, 5a, 5j and 5l with (hetero)aryl boronic acids via C–OH bond activation

To a stirred solution of aromatic lactam 5a, 5b, 5e, 5j or 5l (100 mg, 1.0 equiv.) in 1,4-dioxane (3 mL) at room temperature was added PyBroP (1.3 equiv.) and Et₃N (3 equiv.). The reaction mixture was stirred at room temperature for 2 h. The progress of the reaction was monitored by TLC. After completion of the first step, boronic acid (1.5 equiv.), Na₂CO₃ (5 equiv.), and PdCl₂(PPh₃)₂ (5 mol%) were added successively at room temperature. After the mixture had been stirred at 110 °C for 12 h, it was diluted with CH₂Cl₂, washed with saturated NH₄Cl solution, and dried with MgSO₄. The crude product was purified by column chromatography to give pure 3,5-diarylated 7-(trifluoromethyl)pyrazolo[1,5-a]pyrimidines 11a–h.

5-(4-Methoxyphenyl)-3-phenyl-7-(trifluoromethyl)pyrazolo[1,5-a]pyrimidine (11a). The purification was carried out using (PE/EtOAc: 9/1) to afford 11a as orange solid in 88% yield (116 mg). Mp 181–183 °C; ¹H NMR (300 MHz, CDCl₃): δ 8.55 (s, 1H), 8.19 (d, J = 8.9 Hz, 2H), 8.16 (d, J = 7.2 Hz, 2H), 7.63 (s, 1H), 7.52 (t, J = 7.8 Hz, 2H), 7.35 (t, J = 7.3 Hz, 1H), 7.09 (d, J = 8.9 Hz, 2H), 3.94 (s, 3H); ¹³C NMR (75 MHz, CDCl₃): δ 162.3, 154.7, 145.6, 143.9, 134.3 (q, J = 37.1 Hz), 131.5, 129.0 (2C), 128.8 (2C), 128.6, 126.7, 126.6 (2C), 119.6 (q, J = 274.6 Hz), 114.57 (2C), 111.6, 103.3 (q, J = 4.2 Hz), 55.5; ¹⁹F NMR (282 MHz, CDCl₃): δ −68.91; HRMS (ESI) m/z [M + H]⁺ calcd for C₂₀H₁₅F₃N₃O: 370.1162; found: 370.1150.

5-(2-Methoxyphenyl)-3-phenyl-7-(trifluoromethyl)pyrazolo[1,5-a]pyrimidine (11b). The purification was carried out using (PE/EtOAc: 9/1) to afford 11b as yellow solid in 66% yield (87 mg). Mp 198–200 °C; ¹H NMR (300 MHz, CDCl₃): δ 8.57 (s, 1H), 8.16–8.10 (m, 3H), 7.97 (s, 1H), 7.56–7.47 (m, 3H), 7.33 (tt, J = 7.4, 1.2 Hz, 1H), 7.19 (td, J = 7.5, 0.9 Hz, 1H), 7.09 (d, J = 8.3 Hz, 1H), 3.99 (s, 3H); ¹³C NMR (75 MHz, CDCl₃): δ 157.8, 155.1, 145.6, 143.4, 132.9 (q, J = 36.9 Hz), 132.3, 131.5, 131.4, 128.8 (2C), 126.7, 126.6 (2C), 126.0, 121.5, 119.8 (q, J = 274.3 Hz), 112.0, 111.6, 108.6 (q, J = 4.4 Hz), 55.7; ¹⁹F NMR (282 MHz, CDCl₃): δ −69.00; HRMS (ESI) m/z [M + H]⁺ calcd for C₂₀H₁₅F₃N₃O: 370.1162; found: 370.1160.

3-Phenyl-5-(thiophen-2-yl)-7-(trifluoromethyl)pyrazolo[1,5-a]pyrimidine (11c).³⁷. The purification was carried out using (PE/EtOAc: 9/1) to afford 11c as orange solid in 87% yield (107 mg). Mp 184–186 °C (Lit.³⁷ Mp: 120–125 °C); ¹H NMR (300 MHz, CDCl₃): δ 8.55 (s, 1H), 8.14 (dd, J = 8.3, 1.1 Hz, 2H), 7.8 (dd, J = 3.8, 1.0 Hz, 1H), 7.62 (dd, J = 5.0, 1.0 Hz, 1H), 7.54–7.49 (m, 3H), 7.35 (tt, J = 7.4, 1.1 Hz, 1H), 7.22 (dd, J = 5.0, 3.8 Hz, 1H); ¹³C NMR (75 MHz, CDCl₃): δ 150.3, 145.1, 143.9, 142.3, 134.3 (q, J = 37.3 Hz), 131.3, 131.1, 128.8 (2C), 128.6, 128.3, 126.8, 126.4 (2C), 119.4 (q, J = 274.7 Hz), 111.6, 102.9 (q, J = 4.2 Hz); ¹⁹F NMR (282 MHz, CDCl₃): δ −68.87; HRMS (ESI) m/z [M + H]⁺ calcd for C₁₇H₁₀F₃N₃S: 346.0620; found: 346.0609.

5-(4-Fluorophenyl)-3-(4-methoxyphenyl)-7-(trifluoromethyl)pyrazolo[1,5-a]pyrimidine (11d). The purification was carried out using (PE/EtOAc: 9.5/0.5) to afford 11d as red solid in 78% yield (98 mg). Mp 208–210 °C; ¹H NMR (300 MHz, CDCl₃): δ 8.53 (s, 1H), 8.22 (dd, J = 8.9, 5.3 Hz, 2H), 8.06 (d, J = 8.8 Hz, 2H), 7.61 (s, 1H), 7.28 (dd, J = 8.9, 5.3 Hz, 2H), 7.08 (d, J = 8.8 Hz, 2H), 3.91 (s, 3H); ¹³C NMR (75 MHz, CDCl₃): δ 164.7 (d, J = 252.6 Hz), 158.7, 153.5, 145.1, 143.7, 134.2 (q, J = 37.2 Hz), 132.4 (d, J = 3.1 Hz), 129.4, 129.3, 127.3 (2C), 123.7, 119.6 (q, J = 274.7 Hz), 116.4, 116.2, 114.4 (2C), 112.1, 103.2 (q, J = 4.1 Hz), 55.3; ¹⁹F NMR (282 MHz, CDCl₃): δ −69.04, −108.73; HRMS (ESI) m/z [M + H]⁺ calcd for C₂₀H₁₄F₄N₃O: 388.0995; found: 388.0996.

5-(4-Methoxyphenyl)-3-(p-tolyl)-7-(trifluoromethyl)pyrazolo[1,5-a]pyrimidine (11e). The purification was carried out using (PE/EtOAc: 9.5/0.5) to afford 11e as orange solid in 90% yield (118 mg). Mp 167–169 °C; ¹H NMR (300 MHz, CDCl₃): δ 8.52 (s, 1H), 8.17 (d, J = 8.9 Hz, 2H), 8.03 (d, J = 8.1 Hz, 2H), 7.59 (s, 1H), 7.33 (d, J = 8.1 Hz, 2H), 7.08 (d, J = 8.9 Hz, 2H), 3.93 (s, 3H), 2.44 (s, 3H); ¹³C NMR (75 MHz, CDCl₃): δ 162.2, 154.4, 145.5, 143.8, 136.5, 134.2 (q, J = 36.4 Hz), 129.5 (2C), 128.9 (2C), 128.7, 128.5, 126.5 (2C), 119.6 (q, J = 274.8 Hz), 114.5 (2C), 111.7, 103.2, 55.5, 21.3; ¹⁹F NMR (282 MHz, CDCl₃): δ −68.94; HRMS (ESI) m/z [M + H]⁺ calcd for C₂₁H₁₇F₃N₃O: 384.1318; found: 384.1317.

Ethyl4-(5-(4-methoxyphenyl)-7(trifluoromethyl)pyrazolo[1,5-a]pyrimidin-3-yl)benzoate (11f). The purification is carried out using (PE/EtOAc: 8.5/1.5) to afford 11f as orange solid in 74% yield (93 mg). Mp 212–214 °C; ¹H NMR (300 MHz, CDCl₃): δ 8.61 (s, 1H), 8.27 (d, J = 8.6 Hz, 2H), 8.26 (d, J = 8.9 Hz, 2H), 8.20 (d, J = 8.6 Hz, 2H), 7.67 (s, 1H), 7.11 (d, J = 8.9 Hz, 2H), 4.43 (q, J = 7.1 Hz, 2H), 3.95 (s, 3H), 1.45 (t, J = 7.1 Hz, 3H); ¹³C NMR (75 MHz, CDCl₃): δ 166.5, 162.5, 155.4, 146.0, 144.1, 136.0, 134.5 (q, J = 37.2 Hz), 130.1 (2C), 129.1 (2C), 128.3, 128.3, 126.0 (2C), 119.5 (q, J = 274.7 Hz), 114.7 (2C), 110.5, 103.6 (q, J = 4.1 Hz), 60.90, 55.5, 14.4; ¹⁹F NMR (282 MHz, CDCl₃): δ −68.80; HRMS (ESI) m/z [M + H]⁺ calcd for C₂₃H₁₉F₃N₃O₃: 442.1373; found: 442.1360.

5-(4-Methoxyphenyl)-3-(thiophen-2-yl)-7-(trifluoromethyl)pyrazolo[1,5-a]pyrimidine (11g). The purification was carried out using (PE/EtOAc: 9/1) to afford 11g as orange solid in 87% yield (114 mg). Mp 177–179 °C; ¹H NMR (300 MHz, CDCl₃): δ 8.46 (s, 1H), 8.22 (d, J = 8.9 Hz, 2H), 7.66 (dd, J = 3.6, 1.0 Hz, 1H), 7.62 (s, 1H), 7.35 (dd, J = 5.1, 1.0 Hz, 1H), 7.17 (dd, J = 5.1, 3.6 Hz, 1H), 7.10 (d, J = 8.9 Hz, 2H), 3.94 (s, 3H); ¹³C NMR (75 MHz, CDCl₃): δ 162.4, 154.7, 144.7, 143.0, 134.2 (q, J = 37.2 Hz), 133.0, 129.0 (2C), 128.3, 127.4, 124.0, 123.4, 119.5 (q, J = 274.6 Hz), 114.5 (2C), 107.5, 103.3 (q, J = 4.1 Hz), 55.5; ¹⁹F NMR (282 MHz, CDCl₃): δ −68.85; HRMS (ESI) m/z [M + H]⁺ calcd for C₁₈H₁₃F₃N₃OS: 376.0726; found: 376.0714.

5-(4-Bromophenyl)-3-phenyl-7-(trifluoromethyl)pyrazolo[1,5-a]pyrimidine (11h)³⁷. The purification was carried out using (PE/EtOAc: 9.5/0.5) to afford 11h as orange solid in 91% yield (136 mg). Mp 174–176 °C (Lit.³⁷ Mp: 174–176 °C); ¹H NMR (300 MHz, CDCl₃): δ 8.59 (s, 1H), 8.13 (dd, J = 7.4, 1.2 Hz, 2H), 8.08 (d, J = 8.6 Hz, 2H), 7.72 (d, J = 8.6 Hz, 2H), 7.63 (s, 1H), 7.53 (t, J = 7.4 Hz, 2H), 7.37 (tt, J = 7.4, 1.2 Hz, 1H); ¹³C NMR (75 MHz, CDCl₃): δ 153.9, 144.3, 135.0, 134.7 (q, J = 37.4 Hz), 132.5 (2C), 133.1, 129.2, 128.9 (2C), 128.8 (2C), 127.0, 126.7 (2C), 126.1, 119.5 (q, J = 274.7 Hz), 112.5, 103.3 (q, J = 4.2 Hz); ¹⁹F NMR (282 MHz, CDCl₃): δ −68.94; HRMS (ESI) m/z [M + H]⁺ calcd for C₁₉H₁₂BrF₃N₃: 418.0161; found: 418.0161.

3,5-Bis(4-methoxyphenyl)-7-(trifluoromethyl)pyrazolo[1,5-a]pyrimidine (12). The purification was carried out using (PE/EtOAc: 9.5/0.5) to afford 12 as orange solid in 25% yield (136 mg). Mp 171–173 °C; ¹H NMR (300 MHz, CDCl₃): δ 8.49 (s, 1H), 8.18 (d, J = 8.9 Hz, 2H), 8.08 (d, J = 8.9 Hz, 2H), 7.6 (s, 1H), 7.09 (d, J = 8.9 Hz, 2H), 7.07 (d, J = 8.9 Hz, 2H), 3.94 (s, 3H), 3.90 (s, 3H); ¹³C NMR (75 MHz, CDCl₃): δ 162.2, 158.6, 154.2, 145.2, 143.4, 134.1 (q, J = 36.9 Hz), 128.8 (2C), 128.7, 127.7 (2C), 124.0, 119.6 (q, J = 274.7 Hz), 114.3 (2C), 111.5, 103.1 (q, J = 4.2 Hz), 55.5, 55.3; ¹⁹F NMR (282 MHz, CDCl₃): δ −68.98; HRMS (ESI) m/z [M + H]⁺ calcd for C₂₁H₁₇F₃N₃O₂: 400.1267; found: 400.1268.

General procedure for the direct alkynylation of 5b with alkynes via C–OH bond activation

To a stirred solution of aromatic lactam 5b (1.0 equiv.) in 1,4-dioxane (3 mL) at room temperature was added PyBroP (1.3 equiv.) and Et₃N (3 equiv.). The reaction mixture was stirred at room temperature for 2 h. The progress of the reaction was monitored by TLC. After completion of the first step, alkyne (1.5 equiv.), PdCl₂(PPh₃)₂ (5 mol%) and CuI (10 mol%) were added successively at room temperature. After the mixture had been stirred at 110 °C for 12 h, it was diluted with CH₂Cl₂, washed with saturated NH₄Cl solution, and dried with MgSO₄. The crude product was purified by column chromatography to give pure 3,5-disubstituted 7-(trifluoromethyl)pyrazolo[1,5-a]pyrimidines 13a–c.

3-Phenyl-5-(phenylethynyl)-7-(trifluoromethyl)pyrazolo[1,5-a]pyrimidine (13a). The purification was carried out using (PE/EtOAc: 9.8/0.2) to afford 13a as orange solid in 90% yield (117 mg). Mp 180–182 °C; ¹H NMR (300 MHz, CDCl₃): δ 8.59 (s, 1H), 8.07 (dd, J = 8.3, 1.2 Hz, 2H), 7.70 (dd, J = 7.5, 1.8 Hz, 2H), 7.54–7.42 (m, 5H), 7.36 (s, 1H), 7.35 (tt, J = 7.5, 1.2 Hz, 1H); ¹³C NMR (75 MHz, CDCl₃): δ 145.5, 144.2, 141.1, 133.8 (J = 37.0 Hz), 132.4 (2C), 130.6, 130.1, 128.9 (2C), 128.6 (2C), 127.2, 126.9 (2C), 121.0, 119.3 (q, J = 274.7 Hz), 112.9, 109.4, 94.6, 87.5; ¹⁹F NMR (282 MHz, CDCl₃): δ −69.07; HRMS (ESI) m/z [M + H]⁺ calcd for C₂₁H₁₃F₃N₃: 364.1056; found: 364.1055.

5-[(4-Methoxyphenyl)ethynyl]-3-phenyl-7-(trifluoromethyl)pyrazolo[1,5-a]pyrimidine (13b). The purification was carried out using (PE/EtOAc: 9.8/0.2) to afford 13b as red solid in 88% yield (124 mg). Mp 163–165 °C; ¹H NMR (300 MHz, CDCl₃): δ 8.57 (s, 1H), 8.07 (dd, J = 8.3, 1.2 Hz, 2H), 7.64 (d, J = 8.9 Hz, 2H), 7.51 (t, J = 7.4 Hz, 2H), 7.35 (tt, J = 7.4, 1.2 Hz, 1H), 7.33 (s, 1H), 6.96 (d, J = 8.9 Hz, 2H), 3.89 (s, 3H); ¹³C NMR (75 MHz, CDCl₃): δ 161.2, 145.5, 144.1, 141.5, 134.2 (2C), 133.7 (q, J = 37.7 Hz), 130.8, 128.9 (2C), 127.2, 126.9 (2C), 119.31 (q, J = 274.7 Hz), 114.4 (2C), 112.9, 112.7, 109.35 (q, J = 4.2 Hz), 95.52, 87.02, 55.42; ¹⁹F NMR (282 MHz, CDCl₃): δ −69.05; HRMS (ESI) m/z [M + H]⁺ calcd for C₂₂H₁₅F₃N₃O: 394.1162; found: 394.1161.

5-[(4-Chlorophenyl)ethynyl]-3-phenyl-7-(trifluoromethyl)pyrazolo[1,5-a]pyrimidine (13c). The purification was carried out using (PE/EtOAc: 9.8/0.2) to afford 13c as orange solid in 78% yield (111 mg). Mp 152–154 °C; ¹H NMR (300 MHz, CDCl₃): δ 8.60 (s, 1H), 8.06 (dd, J = 8.3, 1.2 Hz, 2H), 7.63 (d, J = 8.6 Hz, 2H), 7.51 (t, J = 7.4 Hz, 2H), 7.43 (d, J = 8.6 Hz, 2H), 7.36 (tt, J = 7.4, 1.2 Hz, 1H), 7.35 (s, 1H); ¹³C NMR (75 MHz, CDCl₃): δ 145.5, 144.3, 140.8, 136.5, 133.9 (q, J = 38.0 Hz), 133.6 (2C), 130.6, 129.1 (2C), 128.9 (2C), 127.3, 126.9 (2C), 119.5, 119.2 (q, J = 274.9 Hz), 113.1, 109.2 (q, J = 4.1 Hz), 93.2, 88.3; ¹⁹F NMR (282 MHz, CDCl₃): δ −69.08; HRMS (ESI) m/z [M + H]⁺ calcd for C₂₁H₁₂ClF₃N₃: 398.0666; found: 398.0668.

Protocols of inhibition

Inhibition of hAChE and hBChE. Ellman's method was used to determine the inhibitory potencies of the compounds as described previously.⁴² Briefly, compounds at 100 μM (1% DMSO, final concentration) were incubated with hBChE or hAChE (final concentrations of 1 nM or 50 pM, respectively) in 333 μM 5,5ʹ-dithiobis (2-nitrobenzoic acid) for 5 min. The reactions were started by addition of the substrate (butyrylthiocholine and acetylthiocholine iodide, 500 μM final concentrations). The increases of absorbances (λ = 412 nm) were measured, and the initial velocities were calculated. The residual activities were calculated with respect to 1% DMSO. Appropriate serial dilutions of the active compounds (RAs <50% at 100 mM test compound) were made and assayed to determine residual activities, which were further used to calculate the IC₅₀ with GraphPad Prism 6 software (GraphPad Software, CA, USA).

Inhibition of hMAO-A and hMAO-B. The inhibitory potencies were determined as described previously.⁴³ For screening purposes, the compound at 100 μM was incubated with hMAO-A or hMAO-B in 50 mM phosphate buffer (pH 7.4, 0.05% [v/v] Triton X-114) for 15 min at 37 °C. The enzyme reaction was started by adding final concentrations of 250 μM Amplex Red reagent, 4 U mL⁻¹ horseradish peroxidase, and 1 mM p-tyramine (final volume, 200 μL). The increase in the fluorescence intensity (λ_ex = 530 nm, λ_em = 590 nm) was measured at 37 °C over a period of 20 min, and velocities were calculated thereof. For control experiments, DMSO was used instead of the compounds. To determine the blank value, a phosphate-buffered solution replaced the enzyme solution. The residual activities were calculated from the velocities measured. Appropriate serial dilutions of the active compound were made and assayed to determine residual activities, which were further used to calculate the IC₅₀ with GraphPad Prism 6 software (GraphPad Software, CA, USA).

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We thank the “Département d'analyses Chimiques et Médicales” (Tours, France) for chemical analyses. We would also like to thank Dr Damijan Knez from Ljubljana University for his help in carrying out the first biological tests.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d0ra07959f

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