3-Formylpyrazolo[1,5-a]pyrimidines as Key ... - ACS Publications

Article Cite This: J. Org. Chem. 2018, 83, 10887−10897

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3‑Formylpyrazolo[1,5‑a]pyrimidines as Key Intermediates for the Preparation of Functional Fluorophores Juan-Carlos Castillo,†,‡ Alexis Tigreros,† and Jaime Portilla*,† †

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Bioorganic Compounds Research Group, Department of Chemistry, Universidad de los Andes, Carrera 1 No. 18A-10, Bogotá, Colombia ‡ Escuela de Ciencias Químicas, Facultad de Ciencias, Universidad Pedagógica y Tecnológica de Colombia UPTC, Avenida Central del Norte, Tunja, Colombia S Supporting Information *

ABSTRACT: A one-pot route for the regioselective synthesis of 3-formylpyrazolo[1,5-a]pyrimidines 4a−k in good yields through a microwave-assisted process is provided. The synthesis proceeds via a cyclocondensation reaction between βenaminones 1 with NH-3-aminopyrazoles 2, followed by formylation with an iminium salt moiety (Vilsmeyer−Haack reagent). These N-heteroaryl aldehydes 4 were successfully used as strategic intermediates for the preparation of novel functional fluorophores with yields up to 98%. The structures of the products obtained and regioselectivity of the reactions were determined on the basis of NMR measurements and X-ray diffraction analysis. Since pyrazolo[1,5-a]pyrimidines (PPs) 3 have shown an important fluorescence, photophysical properties of four 2-methylderivatives substituted at position 7 with different acceptor (A) or donor (D) groups were investigated. The compounds evaluated exhibited large Stokes shift in different solvents, but only the substituted p-methoxyphenyl (4-An) showed a strong fluorescence intensity with quantum yields up to 44% due to its greater ICT. Therefore, hybrid systems based on pyrazolo[1,5-a]pyrimidines could be used as fluorescent probes to detect biologically or environmentally relevant species.

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INTRODUCTION Functionalized N-heterocycles are of notable interest to organic chemists to both uncover novel derivatives and explore new applications in drug delivery and biomedical research. The functional groups may expand the applicability and scope of such compounds.1 The synthesis of N-heteroaryl aldehydes is an important focus of research for synthetic organic chemists due to their presence in biologically active compounds, as well as other functional molecules.2 In addition, the carbonyl group can undergo a variety of transformations such as C−C, C−N, and C−O coupling reactions and also participate in redox reactions;3 thus, the biological and physicochemical effect of such postfunctionalized compounds would be improved. In particular, pyrazolo[1,5-a]pyrimidine derivatives have received augmented interest in recent years due to their broad range of biological and pharmacological activities.4 For example, the hypnotic drug Indiplon (I),5a diamide derivative (II) (good anticancer activity),5b and anxiolytic agent Ocinaplon (III)5c have this structural scaffold 7-aryl substituted as well as diverse carbonyl derivatives at position 3 (Figure 1). In addition, © 2018 American Chemical Society

pyrazolo[1,5-a]pyrimidines (PPs) have found applications in material sciences because of their interesting photophysical properties.6 Consequently, the development of efficient and sustainable methods for the synthesis and derivatization of pyrazolo[1,5a]pyrimidines are still highly desirable. Synthetic approaches to PPs have largely relied on the condensation reaction between NH-3-aminopyrazoles with 1,3-bis-electrophilic compounds (e.g., β-dicarbonyl compounds,7 β-enaminones,8 β-ketonitriles,9 β-haloenones,10 among others11). Interestingly, microwave12 and ultrasonic13 irradiation using β-enaminones as precursor have been successfully used to facilitate the reaction. A less obvious approach was the KOtBu-induced condensation reaction between 1,3,5-triaryl-pentane-1,5-diones with NH-3aminopyrazoles, which proceeds with loss of one molecule of aryl methyl ketone.14 Besides other stepwise approaches using single bond-forming reactions, a couple of tricomponent Received: June 22, 2018 Published: July 27, 2018 10887

DOI: 10.1021/acs.joc.8b01571 J. Org. Chem. 2018, 83, 10887−10897

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The Journal of Organic Chemistry

Figure 1. Examples of biologically active 3-funtionalized 7-arylpyrazolo[1,5-a]pyrimidines.

Scheme 1. Synthesis of Functionalized Pyrazolo[1,5-a]pyrimidines under Mild Conditions

methods for the syntheses of N-heterocycles,20 we proposed the microwave-assisted consecutive synthesis of 3formylpyrazolo[1,5-a]pyrimidines via a Vilsmeyer−Haack formylation (Scheme 1c). It is important to mention that few examples of such compounds were reported in the literature, and multistep procedures were used for their selective preparation in all cases in medium−good yields (50−81%).21 Moreover, we were unable to find any examples for the direct and regioselective formylation of pyrazolo[1,5a]pyrimidines at position 3. To the best of our knowledge, this is the first report of using a microwave-assisted one-pot reaction for the synthesis of 3-formylpyrazolo[1,5-a]pyrimidines.

procedures have been developed in recent years to prepare PPs.15 In spite the existence of diverse pyrazolo[1,5-a]pyrimidines synthesis, efficient methods which can introduce a broad range of functional groups on the ring in one step or a single pot have remained largely unexplored. Most of these methods involve the synthesis of halo derivatives by multistep sequences that proceed with the preformation of the pyrazolo[1,5-a]pyrimidine core and subsequent regioselective halogenation.16 However, the incorporation of others functional groups such as hydrazone, carboxyl, enone, and nitroso have been poorly explored. In fact, most of the existing reports are pharmaceutical patents. Thus, developing an operationally simple and time-efficient method for preparing functionalized PPs by employing a single-step or one-pot procedure is desirable. Our lab previously reported a microwave-assisted synthesis of 6-aryldiazenylpyrazolo[1,5-a]pyrimidin-7-amines from αarylhydrazinylidene-β-ketonitriles and their use in the preparation of pyrazolo[5,1-b]purine derivatives (Scheme 1a).17 More recently, we reported our initial results on the timeefficient synthesis of 3-halo and 3-nitro-pyrazolo[1,5-a]pyrimidines in a stepwise and one-pot manner. The reactions were carried out from β-enaminones and NH-3-aminopyrazoles, followed by a regioselective electrophilic substitution with easily available reagents in the same reaction vessel (Scheme 1b).18 Over the past several years we have been quite interested in the synthesis of heteroaryl aldehydes19 due to the usefulness for their use in numerous transformations such as condensations or redox reactions.3 Inspired by these results and our continuing interest in the search for novel and greener

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RESULTS AND DISCUSSION Given our previous results regarding the consecutive synthesis of 3-halo- and 3-nitro-pyrazolo[1,5-a]pyrimidines (Scheme 1b),18 we planned to study an analogous approach toward 3formylpyrazolo[1,5-a]pyrimidines 4 in a single reaction vessel. The synthesis was carried out starting from β-enaminones 1 and NH-3-aminopyrazoles 2 and finishing with the addition of 2 equiv of Vilsmeier−Haack reagent generated in situ from POCl3 and DMF. It is important to mention that the first step in the reaction was performed using conditions of our previously developed protocol, where the crude intermediate 3a was obtained almost pure, according to 1H NMR analysis, by high-vacuum removal of all volatiles of the reaction mixture.18 In this way the second step was optimized in the same microwave tube as indicated below (Table 1). We carried 10888


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60 and 80 °C for 10 min. Pleasingly, the highest yield (91%) was achieved at 80 °C, while at lower temperatures, the formylated product 4a was obtained in acceptable yields together with the intermediate 3a (Table 1, entries 2−4). Reaction times longer than 10 min resulted in lesser yields of desired product 4a (Table 1, entries 5 and 6). In order to determine the specific microwave effect on accelerating the reaction versus conventional heating, a preheated sand bath was used as a source of heat in comparative experiments (Table 1, entry 4 vs 7). Pleasingly, the heteroaryl aldehyde 4a was also successfully obtained using conventional heating at 80 °C for 60 min, but this method was not completely satisfactory due to loss of intermediate mass during the transfer process from the microwave tube to the round bottom. Consequently, we corroborated the feasibility of our hypothesis to perform a reliable cyclocondensation−formylation sequence in the same microwave tube without further isolation of 3a. Once the reaction conditions have been optimized (Table 1, entry 4), we then examined the scope of this synthesis with a variety of preformed pyrazolo[1,5-a]pyrimidines 3a−k. The results are summarized in Scheme 2. In general, the microwave-assisted reaction of β-enaminones 1a−g with NH3-aminopyrazoles 2a−d afforded the crude intermediate 3a−k, which were successfully formylated under microwave irradiation to provide the heteroaryl aldehydes 4a−k in good yields. These results evidenced the high electron density of the carbon atom at position 3 of intermediates 3a−k (see 13C NMR spectra of PPs 3 in Supporting Information),18 from which some representative derivatives were isolated in this work. The previously reported PPs 3a, 3d, and 3f were obtained18 to complement the subsequent photophysical studies, while the novel derivatives 3c, 3e, 3g, and 3i were synthesized and completely characterized accordingly. The structure of the 3formylpyrazolo[1,5-a]pyrimidine 4a was solved by singlecrystal X-ray diffraction analysis.22 It was found that the pyrazolyl group well tolerated either alkyl or aryl substituents

Table 1. Optimization of the Reaction Parameters for the One-Pot Synthesis of 3-Formylpyrazolo[1,5-a]pyrimidine 4aa

entry 1 2 3 4 5 6 7d

T (°C) b

40 60b 70b 80b 80b 80b 80c

time t 10 10 10 10 15 20 60

min min min min min min min

yield (%) 45 64 77 91 83 77 86

Reaction conditions: β-enaminone 1a (0.50 mmol) and 5-amino-3methyl-1H-pyrazole (2a, 0.50 mmol) at 180 °C under microwave irradiation for 2 min, after cooling by airflow addition of 2 equiv of preformed Vilsmeier−Haack reagent (2 equiv of POCl3 and 4 equiv of DMF). bRun in 10 mL sealed tubes at a power of 300 W. c Conventional heating with a sand bath. dRun in a 10 mL roundbottom flask with 0.5 mL of anhydrous DMF at 80 °C for 60 min (not one-pot procedure). a

out the optimization by varying the temperature and testing the effect of microwave irradiation in contrast to conventional heating. As a test reaction, we submitted an equimolar mixture of the β-anaminone 1a and 5-amino-3-methyl-1H-pyrazole (2a) at 180 °C under microwave irradiation for 2 min in a sealed tube; after cooling by airflow addition of 2 equiv of preformed Vilsmeier−Haack reagent at 40 °C for 10 min provided the desired product 4a in 45% yield as the only detectable regioisomer (Table 1, entry 1). The structure of 4a was confirmed by NMR and mass spectroscopy. We continue our study using microwave irradiation at temperatures between

Scheme 2. Microwave-Assisted Synthesis of 3-Formylpyrazolo[1,5-a]pyrimidines 4a−ka

Reaction conditions: β-enaminone 1 (0.50 mmol) and NH-3-aminopyrazole 2 (0.50 mmol) at 180 °C for 2 min; after cooling by airflow addition of 2 equiv of Vilsmeier−Haack reagent at 80 °C under microwave irradiation for 10 min. (See the Experimental Section for details.) a

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The Journal of Organic Chemistry Scheme 3. Synthesis of Postfunctionalized Pyrazolo[1,5-a]pyrimidines 6 and 8a

a Reaction conditions: (a) 4 (0.50 mmol), malononitrile (5, 0.50 mmol), and piperidine (0.10 mmol) in EtOH (2.0 mL); (b) 4 (0.50 mmol) and 2hydrazinylpyridine (7, 0.50 mmol) in EtOH (2.0 mL). (See the Experimental Section for details.)

Scheme 4. Synthesis of Postfunctionalized Pyrazolo[1,5-a]pyrimidines 10 and 11a

a

Reaction conditions: (a) 4 (0.50 mmol), p-chloroacetophenone (9, 0.55 mmol) and 20% aqueous NaOH (0.20 mL) in MeOH (5.0 mL); (b) 4 (0.50 mmol) and Oxone (0.50 mmol) in DMF (3.0 mL). (See the Experimental Section for details.)

(R1), and almost no loss of efficiency was observed for the βenaminones tested, which indicated the low electronic influence of the aryl (or heteroaryl) group on their reactivity. Albeit when the β-enaminone 4-pyridyl-substituted 1f was used, the one-pot reaction afforded the lowest yields toward the formation of the products 4f (72%) and 4k (74%). This novel synthetic methodology to obtain the heteroaryl aldehydes 4a−k was distinguished by its operational simplicity, short reaction times, and eco-compatibility in terms of waste and energy. Once 3-formylpyrazolo[1,5-a]pyrimidines 4 were obtained we developed some postfunctionalization reactions to generate novel PPs derivatives, knowing that the nitrogenous heteroaryl aldehydes are important intermediates in organic and medicinal chemistry.19,23 The formyl group can be converted into hydrazone, imine, enone, alcohol, and carboxylic acid among other important functional groups, though the incorporation of such functional groups on the pyrazolo[1,5a]pyrimidine core has been poorly explored. Therefore, we study the possibility of chemically manipulating the formyl group in the representative aldehydes 4a, 4b, and 4j by an environmentally friendly synthetic protocol. As expected, the Knoevenagel condensation of 4 with malononitrile (5) using piperidine as catalyst in absolute ethanol at 80 °C under microwave irradiation for 10 min afforded the dicyanovinyl derivatives 6 in nearly quantitative yields (Scheme 3a). Equally, when 2-hydrazinylpyridine (7) was used as nucleophile under analogous conditions without catalyst, heteroaryl aldehydes 4 were efficiently converted to 2-pyridilhydrazones 8 in excellent yields (Scheme 3b). Curiously, novel functional derivatives 6 and 8 could be used as fluorescent probes for selective recognition of cyanide or for the detection of metal ions, respectively, somewhat similar to our previous reports with analogous structures (Scheme 3, gray).19b,c The formyl moiety of 4 also suffered a condensation reaction using p-chloroacetophenone (9), allowing the incorporation of enone functional group. The Claisen−Schmidt reaction of heteroaryl aldehydes 4a,b with 9 using sodium hydroxide as

base at room temperature led to the formation of the expected heterochalcones 10a,b in high yields (Scheme 4a). These chalcones 10 bearing a pyrazolo[1,5-a]pyrimidine moiety could be used as a building block for the development of potential antitumor agents, among other valuable compounds.24 On the other hand, we investigated the transitionmetal-free oxidation of the formyl group of 4a,b to carboxylic acids 11a,b with Oxone in DMF at room temperature. Oxone is a trade name for the potassium triple salt with formula 2KHSO5·KHSO4·K2SO4, commonly used as an oxidizing agent for numerous organic transformations and whose active ingredient is potassium peroxymonosulfate (KHSO5). This salt may be used as an alternative to common transition-metal oxidants for the transformation of aldehydes to carboxylic acids.25 As expected, the 3-formylpyrazolo[1,5-a]pyrimidines 4a,b were oxidized effortlessly to their carboxylic acids 11a,b in good yields using 1 equiv of Oxone (Scheme 4b). This transformation resulted in a protocol where (a) there is no need for rigorous exclusion of air or moisture, (b) the oxidation process is highly efficient, and (c) the replacement of expensive and toxic transition-metal oxides by environmentally benign reagent Oxone is made.25 It is important to note that pyrazolo[1,5-a]pyrimidine-3-carboxylic acid 11 could be used as a reagent for the synthesis of amide derivatives with potential biological activity.5b,21c In addition, Yoshida and coworkers also reported a transition-metal-free oxidation of 3formylpyrazolopyrimidine to the corresponding carboxylic acid but using a large excess of oxidizing reagent (NaClO2:NaH2PO4, ∼10:5 equiv).21c Given our interesting synthetic results (obtaining 4, 6, 8, 10, and 11), we consider carrying out photophysical studies of some nonfunctionalized PPs 3 in order to establish the scope of the pyrazolo[1,5-a]pyrimidine core as a new organic fluorophore. This study was done to categorize N-heteroaryl aldehydes 4 as key intermediates of novel functional fluorophores, since they have the respective heterocyclic core. Functional fluorophores are sophisticated structures for photophysical applications, often difficult to access by 10890


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Figure 2. Structure of fluorophores 3a and 3d−f. Relative quantum yields were obtained with anthracene (ϕF = 0.28 in ethanol)30 as reference. Photograph was taken using 20 μM solutions in DCM. Hand-held UV lamp under long wavelength (λ = 365 nm) was used.

economy. Pleasingly, aldehydes 4 readily engage in nucleophilic addition reactions with malononitrile, 2-hydrazinylpyridine, and p-chloroacetophenone for the synthesis of dicyanovinyl 6, hydrazone 8, and heterochalcone 10 derivatives, respectively, in excellent yields (90−98%). Likewise, we developed an efficient and simple protocol for the Oxone-induced oxidation of 4 to the respective carboxylic acids 11 in good yields, which is a valuable alternative to traditional metal-mediated oxidations. Besides the great synthetic and biological interest of the novel synthesized compounds, its photophysical importance is highlighted because the pyrazolo[1,5-a]pyrimidine core proved to be an ICT modulable fluorophore. Consequently, these compounds could be used to design fluorescent probes in the detection of various relevant species; thus, we expect to extend our studies toward the synthesis of chemosensors based on PPs for anions or metal ion detection.

conventional synthetic strategies from simple starting materials.26 In this way we selected compounds 3a and 3d−f to carry out the corresponding studies because their structures are substituted at position 7 with different electron-donor (D) and electron-acceptor (A) groups (Figure 2). This structural feature is crucial in fluorescence phenomena that involve charge transfer (CT) and has been often observed in various N-heterocycles suitably substituted.27 The UV−vis and fluorescence emission spectra of PPs 3a and 3d−f were achieved in toluene (PhMe), dichloromethane (DCM), acetonitrile (ACN), dimethyl sulfoxide (DMSO), and ethanol (EtOH) as solvents of different polarity (See Supporting Information, Figures S2−S6 and Table S1). Fluorophores 3a and 3d−f exhibited a large Stokes shift (7187−9240 cm−1) and without any tendency of solvatochromic shift in the different solvents evaluated. In addition, compound 3d exhibited the strongest fluorescence intensity with quantum yields (ϕF) up to 44%. This fluorophore (D−A) is quite significant versus that of 3a and 3e−f due to the presence of a strong donating group like the 4-methoxyphenyl that favors a higher intramolecular charge transfer (ICT).19c,28 The quantum yields of 3d in PhMe, DCM, ACN, DMSO, and EtOH are 34%, 44%, 17%, 23%, and 19%, respectively. Compounds 3a and 3e have similar fluorescence intensity between them but lower than fluorophore 3d, while 3f presented a very weak fluorescence due to the electronic effect of the 7-aryl substituent. The difference between 3d and 3e is because the m-methoxy substituents only contribute to inductive effects (A), whereas the p-methoxy substituent contributes both inductive (A) and resonance effects (D).29 These findings confirm that the fluorescence phenomenon of these compounds is governed by an ICT mechanism sensitive to the electronic nature of the 7aryl substituent group on the pyrazolo[1,5-a]pyrimidine core (Figure 2). Therefore, these fluorophores could be used as building blocks of novel fluorescent probes to detect biologically or environmentally relevant species. In fact, the heteroaryl aldehydes 4a−k retain the fluorescent characteristics of their precursors 3 (Figures S7).

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EXPERIMENTAL SECTION

General Information. All reagents were purchased from commercial sources and used without further purification unless otherwise noted. All starting materials were weighed and handled in air at room temperature. The reactions were monitored by TLC visualized by a UV lamp (254 or 365 nm). Flash chromatography was performed on silica gel (230−400 mesh). All reactions under microwave irradiation were performed using a sealed reaction vessel (10.0 mL, max pressure = 300 psi) containing a Teflon-coated stir bar (obtained from CEM). Microwave-assisted reactions were performed in a CEM Discover SP focused microwave (ν = 2.45 GHz) reactor equipped with a built-in pressure measurement sensor and a vertically focused IR temperature sensor. Controlled temperature, power, and time settings were used for all reactions. NMR spectra were recorded at 400 MHz ( 1 H) and 101 MHz ( 13 C) at 298 K using tetramethylsilane (0 ppm) as the internal reference. NMR spectroscopic data were recorded in CDCl3 or DMSO-d6 using as internal standards the residual nondeuteriated signal for 1H NMR and the deuteriated solvent signal for 13C NMR spectroscopy. DEPT spectra were used for the assignment of carbon signals. Chemical shifts (δ) are given in ppm, and coupling constants (J) are given in Hertz. The following abbreviations are used for multiplicities: s = singlet, d = doublet, t = triplet, q = quartet, dd = doublet of doublets, and m = multiplet. Melting points were collected using a capillary melting point apparatus and are uncorrected. High-resolution mass spectra (HRMS) were recorded using a Q-TOF spectrometer via electrospray ionization (ESI). Crystallographic data were recorded on a diffractometer using graphite-monochromated Mo Kα radiation (λ= 0.71073 Å). Structures were solved using an interactive algorithm,31a subsequently completed by a difference Fourier map, and refined using the program SHELXL2014,31b and the graphic material was prepared using the Mercury 3.8 sofware.31c Noncommercially available NH-3-aminopyrazoles 2 were prepared using known procedures.32 The noncommercially available 2-hydrazinylpyridine 7 was prepared by a method developed in our laboratory.19a The electronic absorption and fluorescence emission spectra were recorded in quartz cuvettes having a path length of 1 cm. UV−vis

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CONCLUSIONS In summary, we developed an efficient and expeditious microwave-assisted synthesis of 3-formylpyrazolo[1,5-a]pyrimidine 4a−k in good to excellent yields with the formation of three new bonds in a one-pot manner. The construction of heteroaryl aldehydes 4 via a sequential cyclocondensation reaction of β-enaminones 1 with NH-3-aminopyrazoles 2 followed by formylation with Vilsmeyer−Haack reagent in a single reaction vessel has not been previously reported. This one-pot sequence offers improvements in terms of broad substrate scope, operational simplicity, short reaction times, high yield, environmentally sustainable, and high atom 10891


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The Journal of Organic Chemistry and fluorescence measurements were performed at room temperature (20 °C). For fluorescence measurements, both the excitation and the emission slit widths were 5 nm. Synthesis and Characterization. General Procedure for the Synthesis of β-Enaminones 1a−g. A mixture of the corresponding methyl ketone (8.0 mmol) and N,N-dimethylformamide dimethyl acetal (DMF−DMA, 8.8 mmol) was subjected to microwave irradiation under solvent-free conditions at 150 °C (160 W, monitored by an IR temperature sensor) and maintained at this temperature for 15 min in a sealed tube containing a a Teflon-coated magnetic stir bar. The resulting reaction mixture was cooled to 55 °C by airflow, and the excess of DMF−DMA was removed under reduced pressure. The resulting clean crude product was used without further purification following the method developed in our laboratory.18 However, herein we report the synthesis and characterization of β-enaminones 1c, 1e, and 1f, which were obtained by this new method.18 (E)-1-(4-Bromophenyl)-3-(dimethylamino)prop-2-en-1-one 1c. By following the general procedure at 160 °C and maintaining that temperature for 15 min in the reaction with 4-bromoacetophenone (1642 mg, 8.25 mmol) and DMF−DMA (1661 μL, 12.5 mmol), the β-enaminone 1c was obtained as a yellow solid (2034 mg, 97%). Mp 81−82 °C (amorphous) (Lit.33 82−83 °C). 1H NMR (400 MHz, CDCl3): δ = 2.92 (s, 3H), 3.14 (s, 3H), 5.64 (d, J = 12.4 Hz, 1H), 7.53 (d, J = 8.4 Hz, 2H), 7.75−7.81 (m, 3H) ppm. 13C{1H} NMR (101 MHz, CDCl3): δ = 37.2 (CH3), 45.1 (CH3), 91.7 (CH), 125.4 (C), 129.1 (CH), 131.3 (CH), 139.3 (C), 154.5 (CH), 187.2 (C) ppm. HRMS (ESI+): calcd for C11H13BrNO+ 254.0181 [M + H]+; found 254.0189. These NMR data matched previously reported data.33 (E)-3-(Dimethylamino)-1-(3,4,5-trimethoxyphenyl)prop-2-en-1one 1e. By following the general procedure at 160 °C and maintaining that temperature for 15 min in the reaction with 3,4,5trimethoxyacetophenone (1713 mg, 8.15 mmol) and DMF−DMA (1621 μL, 12.2 mmol), the β-enaminone 1e was obtained as a yellow solid (2076 mg, 96%). Mp 127−129 °C (amorphous) (Lit.34 128− 130 °C). 1H NMR (400 MHz, CDCl3): δ = 2.94 (s, 3H), 3.14 (s, 3H), 3.87 (s, 3H), 3.91 (s, 6H), 5.64 (d, J = 12.2 Hz, 1H), 7.15 (s, 2H), 7.80 (d, J = 12.3 Hz, 1H) ppm. 13C{1H} NMR (101 MHz, CDCl3): δ = 37.2 (CH3), 45.0 (CH3), 56.2 (CH3), 60.8 (CH3), 91.6 (CH), 104.8 (CH), 136.0 (C), 140.5 (C), 152.7 (C), 154.2 (CH), 187.6 (C) ppm. HRMS (ESI+): calcd for C14H20NO4+ 266.1392 [M + H]+; found 266.1401. These NMR data matched previously reported data.34 (E)-3-(Dimethylamino)-1-(thiophen-2-yl)prop-2-en-1-one 1g. By following the general procedure at 160 °C and maintaining that temperature for 15 min in the reaction with 4-acetylthiophene (873 μL, 8.08 mmol) and DMF−DMA (1621 μL, 12.2 mmol), the βenaminone 1g was obtained as a yellow solid (1435 mg, 98%). Mp 100 °C (amorphous) (Lit.35 101 °C). 1H NMR (400 MHz, CDCl3): δ = 2.91 (s, 3H), 3.12 (s, 3H), 5.62 (d, J = 12.3 Hz, 1H), 7.07 (t, J = 4.4 Hz, 1H), 7.46 (d, J = 4.7 Hz, 1H), 7.62 (d, J = 4.2 Hz, 1H), 7.77 (d, J = 12.2 Hz, 1H) ppm. 13C{1H} NMR (101 MHz, CDCl3): δ = 37.2 (CH3), 44.9 (CH3), 91.5 (CH), 127.5 (CH), 128.3 (CH), 130.2 (CH),147.3 (C), 153.5 (CH), 180.7 (C) ppm. HRMS (ESI+): calcd for C9H12NOS+ 182.0640 [M + H]+; found 182.0648. These NMR data matched previously reported data.35 General Procedure for the Synthesis of 7-Aryl-2-methylpyrazolo[1,5-a]pyrimidines 3a, 3c−g, and 3i. A mixture of β-enaminone 1 (0.50 mmol) and NH-3-aminopyrazole (2, 0.50 mmol) was subjected to microwave irradiation under solvent-free conditions at 180 °C (190 W, monitored by an IR temperature sensor) and maintained at this temperature for 2 min in a sealed tube containing a Teflon-coated magnetic stirring bar. The resulting reaction mixture was cooled to 55 °C by airflow, and the precipitated product formed upon addition of cold EtOH/H2O (1:1, 1.0 mL) was filtered off, washed, and dried to give the pure product 3. The previously reported compounds by us 3a, 3d, and 3f, the novel derivatives 3e and 3i, and compounds made by our method 3c and 3g are representative compounds in this work.

These seven pyrazolopyrimidines were isolated and characterized following the method developed in our laboratory.18 7-(4-Bromophenyl)-2-methylpyrazolo[1,5-a]pyrimidine 3c. The general procedure at 180 °C and maintaining that temperature for 2 min in the reaction with 1c (140 mg, 0.55 mmol) and 2a (54 mg, 0.56 mmol) afforded product 3c as a yellow solid (152 mg, 96%). Mp 149−150 °C (amorphous) (Lit.16d 148−150 °C). 1H NMR (400 MHz, CDCl3): δ = 2.52 (s, 3H), 6.57 (s, 1H), 6.79 (d, J = 4.5 Hz, 1H), 7.69 (d, J = 8.5 Hz, 2H), 7.96 (d, J = 8.5 Hz, 2H), 8.44 (d, J = 4.5 Hz, 1H) ppm.13C{1H} NMR (101 MHz, CDCl3): δ = 14.7 (CH3), 96.5 (CH), 106.2 (CH), 125.4 (C), 130.0 (C), 130.7 (CH), 131.9 (CH), 144.9 (C), 148.5 (CH), 150.6 (C), 155.1 (C) ppm. HRMS (ESI+): calcd for C13H11BrN3+ 288.0136 [M + H]+; found 288.0140. 2-Methyl-7-(3,4,5-trimethoxyphenyl)pyrazolo[1,5-a]pyrimidine 3e. The general procedure at 180 °C and maintaining that temperature for 2 min in the reaction with 1e (141 mg, 0.53 mmol) and 2a (54 mg, 0.56 mmol) afforded product 3e as a yellow solid (155 mg, 98%). Mp 124−126 °C (amorphous). 1H NMR (400 MHz, CDCl3): δ = 2.54 (s, 3H), 3.94 (s, 9H), 6.57 (s, 1H), 6.80 (d, J = 4.5 Hz, 1H), 7.33 (s, 2H), 8.44 (d, J = 4.5 Hz, 1H) ppm.13C{1H} NMR (101 MHz, CDCl3): δ = 14.8 (CH3), 56.2 (CH3), 60.8 (CH3), 96.2 (CH), 106.2 (CH), 106.7 (CH), 126.2 (C), 140.2 (C), 145.8 (C), 148.5 (CH), 150.7 (C), 153.1 (C), 154.9 (C) ppm. HRMS (ESI +): calcd for C16H18N3O3+ 300.1348 [M + H]+; found 300.1351. 2-Methyl-7-(thiophen-2-yl)pyrazolo[1,5-a]pyrimidine 3g. The general procedure at 180 °C and maintaining that temperature for 2 min in the reaction with 1g (92 mg, 0.51 mmol) and 2a (51 mg, 0.53 mmol) afforded product 3g as a yellow solid (101 mg, 92%). Mp 120−121 °C (amorphous) (Lit.16d 106−108 °C). 1H NMR (400 MHz, CDCl3): δ = 2.61 (s, 3H), 6.56 (s, 1H), 7.14 (d, J = 4.6 Hz, 1H), 7.26 (t, J = 4.5 Hz, 1H), 7.71 (d, J = 5.0 Hz, 1H), 8.34 (d, J = 3.8 Hz, 1H), 8.42 (d, J = 4.6 Hz, 1H) ppm. 13C{1H} NMR (101 MHz, CDCl3): δ = 14.8 (CH3), 96.1 (CH), 102.9 (CH), 127.5 (CH), 131.2 (CH), 131.5 (C), 132.0 (CH), 139.2 (C), 147.8 (CH), 150.5 (C), 154.8 (C) ppm. HRMS (ESI+): calcd for C11H10N3S+ 216.0595 [M + H]+; found 216.0593. 2-(4-Bromophenyl)-7-(4-methoxyphenyl)pyrazolo[1,5-a]pyrimidine 3i. The general procedure at 180 °C and maintaining that temperature for 2 min in the reaction with 1d (105 mg, 0.51 mmol) and 2c (126 mg, 0.53 mmol) afforded product 3i as a yellow solid (184 mg, 95%). Mp 165−166 °C (amorphous). 1H NMR (400 MHz, CDCl3): δ = 3.92 (s, 3H), 6.88 (d, J = 4.3 Hz, 1H), 7.01 (s, 1H), 7.10 (d, J = 8.8 Hz, 2H), 7.57 (d, J = 8.3 Hz, 2H), 7.88 (d, J = 8.3 Hz, 2H), 8.19 (d, J = 8.7 Hz, 2H), 8.46 (d, J = 4.4 Hz, 1H) ppm. 13C{1H} NMR (101 MHz, CDCl3): δ = 55.5 (CH3), 93.4 (CH), 106.5 (CH), 114.0 (CH), 123.0 (C), 128.1 (CH), 131.2 (CH), 131.5 (C), 131.8 (CH), 146.1 (C), 149.1 (CH), 151.3 (C), 154.5 (C), 161.9 (C) ppm. HRMS (ESI+): calcd for C19H15BrN3O+ 380.0398 [M + H]+; found 380.0405. General Procedure for the Synthesis of Pyrazolo[1,5-a]pyrimidine-3-carbaldehydes 4a−j. A mixture of β-enaminone 1 (0.50 mmol) and NH-3-aminopyrazole (2, 0.50 mmol) was subjected to microwave irradiation under solvent-free conditions at 180 °C (190 W, monitored by an IR temperature sensor) and maintained at this temperature for 2 min in a sealed tube containing a Teflon-coated magnetic stirring bar. Later cooling to room temperature, 300 μL of anhydrous N,N-dimethylformamide cooled to 0 °C was added into the tube, and subsequently, the formylating agent solution was added dropwise with stirring at 0 °C―this reagent was prepared by adding phosphoryl chloride (2.0 mmol, 187 μL) to 310 μL of cool anhydrous N,N-dimethylformamide (4.0 mmol) over a period of 10 min, maintaining the temperature at 0 °C for an additional 30 min. After stirring the formylation for 15 min at 0 °C, the reaction mixture was warmed to room temperature and stirred for additional 15 min. Then the solution was irradiated with microwaves at 80 °C (90 W, monitored by an IR temperature sensor) and maintained at this temperature for 10 min in a sealed tube containing a Teflon-coated magnetic stirring bar, and the resulting reaction mixture was cooled to 55 °C by airflow. The pH of the solution was maintained at 7 by 10892


Article


2-Methyl-7-(pyridin-4-yl)pyrazolo[1,5-a]pyrimidine-3-carbaldehyde 4f. Following the general procedure in the reaction between βenaminone 1f (88 mg, 0.50 mmol), NH-3-aminopyrazole 2a (50 mg, 0.52 mmol), N,N-dimethylformamide (310 μL, 4.00 mmol), and phosphoryl chloride (187 μL, 2.00 mmol) afforded compound 4f as a yellow solid (86 mg, 72%). Mp 233−234 °C (amorphous). 1H NMR (400 MHz, CDCl3): δ = 2.75 (s, 3H), 7.16 (d, J = 4.4 Hz, 1H), 7.94 (d, J = 4.7 Hz, 2H), 8.78 (d, J = 4.4 Hz, 1H), 8.89 (d, J = 4.7 Hz, 2H), 10.44 (s, 1H) ppm.13C{1H} NMR (101 MHz, CDCl3): δ = 14.5 (CH3), 109.1 (C), 109.5 (CH), 123.1 (CH), 137.4 (C), 144.5 (C), 150.6 (CH), 151.8 (C), 152.2 (CH), 157.7 (C), 183.9 (CH) ppm. HRMS (ESI+): calcd for C13H11N4O+ 239.0933 [M + H]+; found 239.0940. 2-Methyl-7-(thiophen-2-yl)pyrazolo[1,5-a]pyrimidine-3-carbaldehyde 4g. Following the general procedure in the reaction between β-enaminone 1g (100 mg, 0.55 mmol), NH-3-aminopyrazole 2a (55 mg, 0.57 mmol), N,N-dimethylformamide (310 μL, 4.00 mmol), and phosphoryl chloride (187 μL, 2.00 mmol) afforded compound 4g as a yellow solid (120 mg, 90%). Mp 196−197 °C (amorphous). 1H NMR (400 MHz, CDCl3): δ = 2.81 (s, 3H), 7.30 (t, J = 5.0 Hz, 1H), 7.38 (d, J = 4.7 Hz, 1H), 7.80 (d, J = 5.0 Hz, 1H), 8.38 (d, J = 3.5 Hz, 1H), 8.64 (d, J = 5.0 Hz, 1H), 10.41 (s, 1H) ppm. 13C{1H} NMR (101 MHz, CDCl3): δ = 14.6 (CH3), 105.7 (CH), 108.5 (C), 128.0 (CH), 130.4 (C), 132.6 (CH), 133.9 (CH), 140.6 (C), 151.4 (CH), 151.8 (C), 157.1 (C), 184.0 (CH) ppm. HRMS (ESI+): calcd for C12H10N3OS+ 244.0545 [M + H]+; found 244.0535. 2-(4-Chlorophenyl)-7-(4-methoxyphenyl)pyrazolo[1,5-a]pyrimidine-3-carbaldehyde 4h. Following the general procedure in the reaction between β-enaminone 1d (98 mg, 0.48 mmol), NH-3aminopyrazole 2b (97 mg, 0.50 mmol), N,N-dimethylformamide (310 μL, 4.00 mmol), and phosphoryl chloride (187 μL, 2.00 mmol) afforded compound 4h as a yellow solid (148 mg, 85%). Mp 227−228 °C (amorphous). 1H NMR (400 MHz, CDCl3): δ = 3.93 (s, 3H), 7.11 (d, J = 8.8 Hz, 2H), 7.18 (d, J = 4.6 Hz, 1H), 7.47 (d, J = 8.5 Hz, 2H), 8.08 (d, J = 8.5 Hz, 2H), 8.16 (d, J = 8.8 Hz, 2H), 8.75 (d, J = 4.6 Hz, 1H), 10.46 (s, 1H) ppm. 13C{1H} NMR (101 MHz, CDCl3): δ = 55.5 (CH3), 107.5 (C), 109.4 (CH), 114.3 (CH), 121.7 (C), 128.6 (CH), 129.9 (C), 130.9 (CH), 131.5 (CH), 136.1 (C), 147.3 (C), 152.5 (CH), 152.9 (C), 156.4 (C), 162.4 (C), 183.5 (CH) ppm. HRMS (ESI+): calcd for C20H15ClN3O2+ 364.0853 [M + H]+; found 364.0861. 2-(4-Bromophenyl)-7-(4-methoxyphenyl)pyrazolo[1,5-a]pyrimidine-3-carbaldehyde 4i. Following the general procedure in the reaction between β-enaminone 1d (113 mg, 0.55 mmol), NH-3aminopyrazole 2c (138 mg, 0.58 mmol), N,N-dimethylformamide (310 μL, 4.00 mmol), and phosphoryl chloride (187 μL, 2.00 mmol) afforded compound 4i as a yellow solid (195 mg, 87%). Mp 230−231 °C (amorphous). 1H NMR (400 MHz, CDCl3): δ = 3.93 (s, 3H), 7.11 (d, J = 9.0 Hz, 2H), 7.19 (d, J = 4.6 Hz, 1H), 7.63 (d, J = 8.5 Hz, 2H), 8.01 (d, J = 8.4 Hz, 2H), 8.16 (d, J = 8.9 Hz, 2H), 8.76 (d, J = 4.5 Hz, 1H), 10.46 (s, 1H) ppm. 13C{1H} NMR (101 MHz, CDCl3): δ = 55.5 (CH3), 107.5 (C), 109.4 (CH), 114.3 (CH), 121.7 (C), 124.5 (C), 130.3 (C), 131.2 (CH), 131.5 (CH), 131.6 (CH), 147.3 (C), 152.5 (CH), 152.9 (C), 156.5 (C), 162.4 (C), 183.5 (CH) ppm. HRMS (ESI+): calcd for C20H15BrN3O2+ 408.0348 [M + H]+; found 408.0350. 2,7-Bis(4-methoxyphenyl)pyrazolo[1,5-a]pyrimidine-3-carbaldehyde 4j. Following the general procedure in the reaction between βenaminone 1d (103 mg, 0.50 mmol), NH-3-aminopyrazole 2d (98 mg, 0.52 mmol), N,N-dimethylformamide (310 μL, 4.00 mmol), and phosphoryl chloride (187 μL, 2.00 mmol) afforded compound 4j as a yellow solid (149 mg, 83%). Mp 171−172 °C (amorphous). 1H NMR (400 MHz, CDCl3): δ = 3.86 (s, 3H), 3.90 (s, 3H), 7.01 (d, J = 8.9 Hz, 2H), 7.07 (d, J = 8.9 Hz, 2H), 7.12 (d, J = 4.4 Hz, 1H), 8.07 (d, J = 8.9 Hz, 2H), 8.16 (d, J = 8.9 Hz, 2H), 8.70 (d, J = 3.7 Hz, 1H), 10.43 (s, 1H) ppm. 13C{1H} NMR (101 MHz, CDCl3): δ = 55.3 (CH3), 55.4 (CH3), 107.2 (C), 109.1 (CH), 113.8 (CH), 114.1 (CH), 121.8 (C), 123.7 (C), 131.1 (CH), 131.5 (CH), 147.0 (C), 152.1 (CH), 152.6 (C), 157.6 (C), 161.0 (C), 162.3 (C), 183.7 (CH)

adding an aqueous solution of sodium bicarbonate (20%), and the reaction mixture was vigorously stirred at room temperature for 30 min. The resulting precipitated was filtered, washed with cold water (2 × 5.0 mL), and purified by flash chromatography on silica gel (eluent CH2Cl2) to afford the corresponding heteroaryl aldehydes 4. 2-Methyl-7-phenylpyrazolo[1,5-a]pyrimidine-3-carbaldehyde 4a. Following the general procedure in the reaction between βenaminone 1a (88 mg, 0.50 mmol), NH-3-aminopyrazole 2a (49 mg, 0.51 mmol), N,N-dimethylformamide (310 μL, 4.00 mmol), and phosphoryl chloride (187 μL, 2.00 mmol) afforded compound 4a as a yellow solid (108 mg, 91%). Mp 167−168 °C (amorphous). 1H NMR (400 MHz, CDCl3): δ = 2.74 (s, 3H), 7.10 (d, J = 4.5 Hz, 1H), 7.58− 7.61 (m, 3H), 8.02−8.06 (m, 2H), 8.71 (d, J = 4.5 Hz, 1H), 10.43 (s, 1H) ppm. 13C{1H} NMR (101 MHz, CDCl3): δ = 14.5 (CH3), 108.8 (C), 109.4 (CH), 128.8 (CH), 129.4 (CH), 129.9 (C), 131.7 (CH), 147.4 (C), 152.0 (C), 152.2 (CH), 157.3 (C), 183.9 (CH) ppm. HRMS (ESI+): calcd for C14H12N3O+ 238.0980 [M + H]+; found 238.0981. 7-(4-Chlorophenyl)-2-methylpyrazolo[1,5-a]pyrimidine-3-carbaldehyde 4b. Following the general procedure in the reaction between β-enaminone 1b (111 mg, 0.53 mmol), NH-3-aminopyrazole 2a (53 mg, 0.55 mmol), N,N-dimethylformamide (310 μL, 4.00 mmol), and phosphoryl chloride (187 μL, 2.00 mmol) afforded compound 4b as a yellow solid (128 mg, 89%). Mp 212−213 °C (amorphous). 1H NMR (400 MHz, CDCl3): δ = 2.74 (s, 3H), 7.07 (d, J = 4.5 Hz, 1H), 7.57 (d, J = 8.7 Hz, 2H), 8.02 (d, J = 8.7 Hz, 2H), 8.70 (d, J = 4.6 Hz, 1H), 10.4 (s, 1H) ppm. 13C{1H} NMR (101 MHz, CDCl3): δ = 14.5 (CH3), 108.9 (C), 109.2 (CH), 128.3 (C), 129.1 (CH), 130.8 (CH), 138.0 (C), 146.2 (C), 152.0 (C), 152.1 (CH), 157.4 (C), 183.9 (CH) ppm. HRMS (ESI+): calcd for C14H11ClN3O+ 272.0591 [M + H]+; found 272.0594. 7-(4-Bromophenyl)-2-methylpyrazolo[1,5-a]pyrimidine-3-carbaldehyde 4c. Following the general procedure in the reaction between β-enaminone 1c (124 mg, 0.49 mmol), NH-3-aminopyrazole 2a (49 mg, 0.51 mmol), N,N-dimethylformamide (310 μL, 4.00 mmol), and phosphoryl chloride (187 μL, 2.00 mmol) afforded compound 4c as a yellow solid (125 mg, 81%). Mp 221−222 °C (amorphous). 1H NMR (400 MHz, CDCl3): δ = 2.74 (s, 3H), 7.08 (d, J = 4.5 Hz, 1H), 7.74 (d, J = 8.6 Hz, 2H), 7.94 (d, J = 8.6 Hz, 2H), 8.72 (d, J = 4.6 Hz, 1H), 10.43 (s, 1H) ppm. 13C{1H} NMR (101 MHz, CDCl3): δ = 14.6 (CH3), 108.9 (C), 109.2 (CH), 126.5 (C), 128.7 (C), 130.9 (CH), 132.1 (CH), 146.3 (C), 152.0 (C), 152.2 (CH), 157.4 (C), 183.9 (CH) ppm. HRMS (ESI+): calcd for C14H11BrN3O+ 316.0085 [M + H]+; found 316.0089. 7-(4-Methoxyphenyl)-2-methylpyrazolo[1,5-a]pyrimidine-3-carbaldehyde 4d. Following the general procedure in the reaction between β-enaminone 1d (103 mg, 0.50 mmol), NH-3-aminopyrazole 2a (51 mg, 0.52 mmol), N,N-dimethylformamide (310 μL, 4.00 mmol), and phosphoryl chloride (187 μL, 2.00 mmol) afforded compound 4d as a white solid (104 mg, 78%). Mp 181−182 °C (amorphous). 1H NMR (400 MHz, CDCl3): δ = 2.75 (s, 3H), 3.91 (s, 3H), 7.07−7.11 (m, 3H), 8.09 (d, J = 9.0 Hz, 2H), 8.66 (d, J = 4.6 Hz, 1H), 10.42 (s, 1H) ppm. 13C{1H} NMR (101 MHz, CDCl3): δ = 14.6 (CH3), 55.5 (CH3), 108.6 (CH), 108.6 (C), 114.3 (CH), 122.0 (C), 131.3 (CH), 147.2 (C), 152.0 (CH), 152.2 (C), 157.1 (C), 162.4 (C), 183.9 (CH) ppm. HRMS (ESI+): calcd for C15H14N3O2+ 268.1086 [M + H]+; found 268.1080. 2-Methyl-7-(3,4,5-trimethoxyphenyl)pyrazolo[1,5-a]pyrimidine3-carbaldehyde 4e. Following the general procedure in the reaction between β-enaminone 1e (143 mg, 0.54 mmol), NH-3-aminopyrazole 2a (55 mg, 0.57 mmol), N,N-dimethylformamide (310 μL, 4.00 mmol), and phosphoryl chloride (187 μL, 2.00 mmol) afforded compound 4e as a yellow solid (161 mg, 91%). Mp 216−217 °C (amorphous). 1H NMR (400 MHz, CDCl3): δ = 2.75 (s, 3H), 3.94 (s, 6H), 3.95 (s, 3H), 7.10 (d, J = 4.5 Hz, 1H), 7.30 (s, 2H), 8.70 (d, J = 4.5 Hz, 1H), 10.42 (s, 1H) ppm.13C{1H} NMR (101 MHz, CDCl3): δ = 14.7 (CH3), 56.4 (CH3), 60.9 (CH3), 107.0 (CH), 108.8 (C), 109.3 (CH), 124.8 (C), 140.9 (C), 147.2 (C), 152.1 (C), 152.2 (CH), 153.3 (C), 157.3 (C), 184.0 (CH) ppm. HRMS (ESI+): calcd for C17H18N3O4+ 328.1297 [M + H]+; found 328.1297. 10893


Article

The Journal of Organic Chemistry ppm. HRMS (ESI+): calcd for C21H18N3O3+ 360.1348 [M + H]+; found 360.1351. 2-(4-Methoxyphenyl)-7-(pyridin-4-yl)pyrazolo[1,5-a]pyrimidine3-carbaldehyde 4k. Following the general procedure in the reaction between β-enaminone 1f (92 mg, 0.52 mmol), NH-3-aminopyrazole 2d (104 mg, 0.55 mmol), N,N-dimethylformamide (310 μL, 4.00 mmol), and phosphoryl chloride (187 μL, 2.00 mmol) afforded compound 4k as a yellow solid (127 mg, 74%). Mp 181−182 °C (amorphous). 1H NMR (400 MHz, CDCl3): δ = 3.88 (s, 3H), 7.03 (d, J = 8.9 Hz, 2H), 7.27 (d, J = 4.3 Hz, 1H), 8.05 (d, J = 8.8 Hz, 2H), 8.12 (d, J = 4.9 Hz, 2H), 8.86 (d, J = 4.4 Hz, 1H), 8.92 (d, J = 5.0 Hz, 2H), 10.48 (s, 1H) ppm. 13C{1H} NMR (101 MHz, CDCl3): δ = 55.4 (CH3), 107.9 (C), 110.2 (CH), 114.0 (CH), 123.2 (C), 123.7 (CH), 131.2 (CH), 143.8 (C), 149.4 (CH), 149.5 (C), 152.2 (CH), 152.4 (C), 158.2 (C), 161.4 (C), 183.7 (CH) ppm. HRMS (ESI+): calcd for C19H15N4O2+ 331.1195 [M + H]+; found 331.1199. General Procedure for the Synthesis of Dicyanovinyl Derivatives 6a,b. A solution of pyrazolo[1,5-a]pyrimidine-3-carbaldehyde 4 (0.50 mmol), malononitrile (5, 0.50 mmol), and piperidine (0.10 mmol) in 2.0 mL of ethanol absolute was irradiated with microwaves at 80 °C (90 W, monitored by an IR temperature sensor) and maintained at this temperature for 10 min in a sealed tube containing a Tefloncoated magnetic stirring bar, and the resulting reaction mixture was cooled to 55 °C by airflow and concentrated under reduced pressure; the residue was directly purified by flash chromatography on silica gel (eluent CH2Cl2) to afford the pure product 6. 2-((7-(4-Chlorophenyl)-2-methylpyrazolo[1,5-a]pyrimidin-3-yl)methylene)-malononitrile 6a. Following the general procedure at 80 °C and maintaining that temperature for 10 min in the reaction with pyrazolo[1,5-a]pyrimidine-3-carbaldehyde 4b (135 mg, 0.50 mmol), malononitrile 5 (34 mg, 0.52 mmol), and piperidine (10 μL, 0.10 mmol) in 2.0 mL of ethanol absolute afforded compound 6a as a yellow solid (153 mg, 96%). Mp 237−238 °C (amorphous). 1H NMR (400 MHz, CDCl3): δ = 2.67 (s, 3H), 7.14 (d, J = 4.2 Hz, 1H), 7.59 (d, J = 8.2 Hz, 2H), 7.95 (s, 1H), 8.00 (d, J = 8.2 Hz, 2H), 8.78 (d, J = 4.2 Hz, 1H) ppm. 13C{1H} NMR (101 MHz, CDCl3): δ = 15.0 (CH3), 78.6 (C), 104.5 (C), 109.9 (CH), 113.9 (C) 115.5 (C), 127.9 (C), 129.3 (CH), 130.9 (CH), 138.5 (C), 146.7 (C), 148.6 (CH), 149.5 (C), 152.0 (CH), 156.9 (C) ppm. HRMS (ESI+): calcd for C17H11ClN5+ 320.0703 [M + H]+; found 320.0700. 2-((2,7-bis(4-Methoxyphenyl)pyrazolo[1,5-a]pyrimidin-3-yl)methylene)-malononitrile 6b. Following the general procedure at 80 °C and maintaining that temperature for 10 min in the reaction between pyrazolo[1,5-a]pyrimidine-3-carbaldehyde 4j (179 mg, 0.50 mmol), malononitrile 5 (34 mg, 0.52 mmol), and piperidine (10 μL, 0.10 mmol) in 2.0 mL of ethanol absolute afforded compound 6b as a yellow solid (200 mg, 98%). Mp 204−205 °C (amorphous). 1H NMR (400 MHz, CDCl3): δ = 3.90 (s, 3H), 3.92 (s, 3H), 7.05−7.11 (m, 4H), 7.19 (d, J = 4.7 Hz, 1H), 7.61 (d, J = 8.6 Hz, 2H), 7.85 (s, 1H), 8.15 (d, J = 8.9 Hz, 2H), 8.78 (d, J = 4.6 Hz, 1H) ppm. 13C{1H} NMR (101 MHz, CDCl3): δ = 55.4 (CH3), 55.5 (CH3), 80.2 (C), 102.5 (C), 109.4 (CH), 113.7 (C), 114.3 (CH), 114.7 (CH), 115.6 (C), 121.8 (C), 123.1 (C), 130.9 (CH), 131.6 (CH), 147.7 (C), 149.1 (C), 149.8 (CH), 151.4 (CH), 158.1 (C), 161.4 (C), 162.5 (C) ppm. HRMS (ESI+): calcd for C24H18N5O2+ 408.1460 [M + H]+; found 408.1463. General Procedure for the Synthesis of 2-Pyridilhydrazone Derivatives 8a,b. A solution of pyrazolo[1,5-a]pyrimidine-3-carbaldehyde 4 (0.50 mmol) and 2-hydrazinylpyridine (7, 0.50 mmol) in 2.0 mL of ethanol absolute was irradiated with microwaves at 80 °C (90 W, monitored by an IR temperature sensor) and maintained at this temperature for 10 min in a sealed tube containing a Tefloncoated magnetic stirring bar, the resulting reaction mixture was cooled to 55 °C by airflow, and the precipitated product formed was filtered off, washed with cold EtOH (2 × 1.0 mL), and dried to afford the pure product 8. (E)-7-(4-Chlorophenyl)-2-methyl-3-((2-(pyridin-2-yl)hydrazono)methyl)pyrazolo[1,5-a]pyrimidine 8a. Following the general procedure, the reaction between pyrazolo[1,5-a]pyrimidine-3-carbaldehyde 4b (130 mg, 0.48 mmol) and 2-hydrazinylpyridine 7 (56 mg, 0.51

mmol) in 2.0 mL of ethanol absolute afforded compound 8a as an orange solid (165 mg, 95%). Mp 240−241 °C (amorphous). 1H NMR (400 MHz, DMSO-d6): δ = 2.69 (s, 3H), 6.71 (t, J = 5.9 Hz, 1H), 7.13 (d, J = 8.3 Hz, 1H), 7.23 (d, J = 4.0 Hz, 1H), 7.61−7.69 (m, 3H), 8.08 (d, J = 4.1 Hz, 1H), 8.16 (d, J = 8.3 Hz, 2H), 8.47 (s, 1H), 8.59 (d, J = 4.0 Hz, 1H), 10.7 (br s, 1H) ppm. 13C{1H} NMR (101 MHz, DMSO-d6): δ = 15.4 (CH3), 104.2 (C), 105.8 (CH), 107.9 (CH), 114.1 (CH), 128.5 (CH), 129.1 (C), 131.1 (CH), 132.2 (CH), 135.8 (C), 137.8 (CH), 144.1 (C), 147.5 (CH), 147.6 (C), 149.8 (CH), 151.6 (C), 157.1 (C) ppm. HRMS (ESI+): calcd for C19H16ClN6+ 363.1125 [M + H]+; found 363.1134. (E)-2,7-bis(4-Methoxyphenyl)-3-((2-(pyridin-2-yl)hydrazono)methyl)pyrazolo[1,5-a]pyrimidine 8b. Following the general procedure, the reaction between pyrazolo[1,5-a]pyrimidine-3-carbaldehyde 4j (180 mg, 0.50 mmol) and 2-hydrazinylpyridine 7 (56 mg, 0.51 mmol) in 2.0 mL of ethanol absolute afforded compound 8b as an orange solid (209 mg, 93%). Mp 223−224 °C (amorphous). 1H NMR (400 MHz, CDCl3): δ = 3.85 (s, 3H), 3.87 (s, 3H), 6.71 (t, J = 3.4 Hz, 1H), 7.00−7.17 (m, 5H), 7.31 (t, J = 4.9 Hz, 1H), 7.61 (t, J = 3.0 Hz, 1H), 7.99−8.07 (m, 3H), 8.26 (d, J = 7.8 Hz, 2H), 8.53 (s, 1H), 8.63 (d, J = 3.5 Hz, 1H), 10.83 (br s, 1H) ppm. 13C{1H} NMR (101 MHz, CDCl3): δ = 55.2 (CH3), 55.4 (CH3), 102.8 (C), 106.3 (CH), 107.8 (CH), 113.4 (CH), 113.9 (CH), 114.1 (CH), 122.1 (C), 125.1 (C), 130.7 (CH), 131.2 (CH), 132.7 (CH), 138.0 (CH), 145.1 (C), 146.8 (CH), 148.6 (C), 149.9 (CH), 152.6 (C), 156.7 (C), 159.8 (C), 161.5 (C) ppm. HRMS (ESI+): calcd for C26H23N6O2+ 451.1882 [M + H]+; found 451.1885. General Procedure for the Synthesis of Chalcones 10a,b. A mixture of pyrazolo[1,5-a]pyrimidine-3-carbaldehyde 4 (0.50 mmol), p-chloroacetophenone 9 (0.55 mmol) and 20% aqueous NaOH (0.20 mL) in MeOH (5.0 mL) was stirred at room temperature for 12 h. After the reaction was complete (monitored by TLC), the volume of the reaction mixture was reduced to 1.0 mL under reduced pressure and water (5.0 mL) was added. The aqueous solution was extracted with ethyl acetate (2 × 5.0 mL), the combined organic extracts were dried over anhydrous Na2SO4, and the solvent was removed under reduced pressure. The crude product was purified by flash chromatography on silica gel employing CH2Cl2/MeOH (40:1, v/ v) as eluent to afford the corresponding chalcone 10. (E)-1-(4-Chlorophenyl)-3-(2-methyl-7-phenylpyrazolo[1,5-a]pyrimidin-3-yl)prop-2-en-1-one 10a. Following the general procedure, the reaction between pyrazolo[1,5-a]pyrimidine-3-carbaldehyde 4a (118 mg, 0.50 mmol), p-chloroacetophenone 9 (71 μL, 0.55 mmol), and 20% aqueous NaOH (200 μL) in 5.0 mL of MeOH afforded compound 10a as a yellow solid (174 mg, 93%). Mp 201− 203 °C (amorphous). 1H NMR (400 MHz, CDCl3): δ = 2.65 (s, 3H), 7.00 (d, J = 4.2 Hz, 1H), 7.48 (d, J = 8.0 Hz, 2H), 7.58−7.62 (m, 3H), 8.04−8.11 (m, 5H), 8.24 (d, J = 15.3 Hz, 1H), 8.67 (d, J = 4.3 Hz, 1H) ppm. 13C{1H} NMR (101 MHz, CDCl3): δ = 13.3 (CH3), 105.4 (C), 108.4 (CH), 119.0 (CH), 128.7 (CH), 128.8 (CH), 129.4 (CH), 129.9 (CH), 130.4 (C), 131.4 (CH), 134.5 (CH), 137.2 (C), 138.6 (C), 147.0 (C), 148.6 (C), 150.4 (CH), 156.7 (C), 189.3 (C) ppm. HRMS (ESI+): calcd for C22H17ClN3O+ 374.1060 [M + H]+; found 374.1057. (E)-1-(4-Chlorophenyl)-3-(7-(4-chlorophenyl)-2-methylpyrazolo[1,5-a]pyrimidin-3-yl)prop-2-en-1-one 10b. Following the general procedure, the reaction between pyrazolo[1,5-a]pyrimidine-3-carbaldehyde 4b (133 mg, 0.49 mmol), p-chloroacetophenone 9 (71 μL, 0.55 mmol), and 20% aqueous NaOH (200 μL) in 5.0 mL of MeOH afforded compound 10b as a yellow solid (180 mg, 90%). Mp 259− 260 °C (amorphous). 1H NMR (400 MHz, CDCl3): δ = 2.66 (s, 3H), 6.99 (d, J = 4.3 Hz, 1H), 7.49 (d, J = 8.3 Hz, 2H), 7.58 (d, J = 8.4 Hz, 2H), 8.02−8.09 (m, 5H), 8.24 (d, J = 15.3 Hz, 1H), 8.68 (d, J = 4.4 Hz, 1H) ppm. 13C{1H} NMR (101 MHz, CDCl3): δ = 13.3 (CH3), 105.5 (C), 108.2 (CH), 119.4 (CH), 128.7 (CH), 129.1 (CH), 129.9 (CH), 130.8 (CH), 134.4 (CH), 137.1 (C), 137.8 (C), 144.2 (C), 145.8 (C), 148.6 (C), 150.4 (CH), 153.6 (C), 156.8 (C), 189.3 (C) ppm. HRMS (ESI+): calcd for C22H16Cl2N3O+ 408.0670 [M + H]+; found 408.0675. 10894


Article

The Journal of Organic Chemistry General Procedure for Oxidation of Aldehyde to Carboxylic Acid 11a,b. A mixture of pyrazolo[1,5-a]pyrimidine-3-carbaldehyde 4 (0.50 mmol) and Oxone (0.50 mmol) in 3.0 mL of anhydrous DMF was stirred at room temperature for 12 h. Then 1.0 mL of 1 N HCl was added to dissolve the salts, and the reaction mixture was stirred at room temperature for 30 min. The desired product was extracted with ethyl acetate (2 × 5.0 mL), the combined organic extracts were washed with brine (1 × 5.0 mL) and dried over anhydrous Na2SO4, and the solvent was removed under reduced pressure. The crude product was purified by flash chromatography on silica gel employing CH2Cl2/MeOH (30:1, v/v) as eluent to afford the corresponding carboxylic acid 11. 2-Methyl-7-phenylpyrazolo[1,5-a]pyrimidine-3-carboxylic Acid 11a. Following the general procedure, the reaction between heterocyclic aldehyde 4a (116 mg, 0.49 mmol) and oxone (154 mg, 0.50 mmol) in 3.0 mL of anhydrous DMF afforded compound 11a as an orange solid (105 mg, 85%). Mp 173−174 °C (amorphous). 1H NMR (400 MHz, CDCl3): δ = 2.54 (s, 3H), 6.65 (d, J = 4.4 Hz, 1H), 7.54−7.56 (m, 3H), 8.03−8.06 (m, 2H), 8.27 (d, J = 4.4 Hz, 1H), 9.31 (br s, 1H) ppm. 13C{1H} NMR (101 MHz, CDCl3): δ = 11.3 (CH3), 105.2 (CH), 128.2 (C), 128.6 (CH), 129.1 (CH), 131.0 (C), 131.1 (CH), 136.9 (C), 143.7 (C), 145.7 (CH), 145.7 (C), 146.5 (C) ppm. HRMS (ESI+): calcd for C14H12N3O2+ 254.0930 [M + H]+; found 254.0925. 7-(4-Chlorophenyl)-2-methylpyrazolo[1,5-a]pyrimidine-3-carboxylic Acid 11b. Following the general procedure, the reaction between heterocyclic aldehyde 4b (130 mg, 0.48 mmol) and oxone (154 mg, 0.50 mmol) in 3.0 mL of anhydrous DMF afforded compound 11b as a yellow solid (110 mg, 80%). Mp 204−205 °C (amorphous). 1H NMR (400 MHz, CDCl3): δ = 2.44 (s, 3H), 6.83 (d, J = 4.0 Hz, 1H), 7.55 (d, J = 8.3 Hz, 2H), 8.00 (d, J = 8.3 Hz, 2H), 8.45−8.49 (m, 2H) ppm. 13C{1H} NMR (101 MHz, CDCl3): δ = 11.7 (CH3), 107.0 (CH), 120.8 (C), 128.5 (C), 129.0 (CH), 130.6 (CH), 137.4 (C), 140.5 (C), 145.1 (C), 146.5 (C), 148.9 (CH), 158.7 (C) ppm. HRMS (ESI+): calcd for C14H11ClN3O2+ 288.0540 [M + H]+; found 288.0536. Photophysical Properties. UV−Vis Absorption and Fluorescence Studies. The solvochromic studies of compounds 3a and of 3d−f were carried out with 10 μM solutions in toluene (PhMe), dichloromethane (DCM), acetonitrile (ACN), dimethyl sulfoxide (DMSO), and ethanol (EtOH). Fluorescence response in photographs was at an excitation of 365 nm using a UV lamp. The relative quantum yields were obtained using anthracene (ϕF = 0.28 in ethanol at 340 nm)30 as reference and calculated according to the following equation19b,c,30,36 ϕf , x = ϕf , st ·

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Department of Chemistry and Vicerrectoriá de Investigaciones at the Universidad de los Andes for financial support. Our gratitude to the Colombian Institute for Science and Research (COLCIENCIAS) for the financial support and for the postdoctoral fellowships conferred to Dr. Alexis Tigreros (Con. 784 of 2017). We also acknowledge B.Sc. Hernán-Alejandro Rosero for preliminary experiments, Edwin ́ Guevara for acquiring the mass spectra, and Dr. Mario Macias (Universidad de los Andes) for his help with the analysis of the X-ray crystallographic data.

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(1) (a) Majumdar, P.; Pati, A.; Patra, M.; Behera, R. K.; Behera, A. K. Acid Hydrazides, Potent Reagents for Synthesis of Oxigen-, Nitrogen-, and/or Sulfur-Containing Heterocyclic Rings. Chem. Rev. 2014, 114, 2942−2977. (b) Allais, C.; Grassot, J.-M.; Rodriguez, J.; Constantieux, T. Metal-Free Multicomponent Synthesis of Pyridines. Chem. Rev. 2014, 114, 10829−10868. (c) Chen, J.-R.; Hu, X.-Q.; Lu, L.-Q.; Xiao, W.-J. Formal [4 + 1] Annulation Reactions in the Synthesis of Carbocyclic and Heterocyclic Systems. Chem. Rev. 2015, 115, 5301−5365. (d) Aguilar, E.; Sanz, R.; Fernández-Rodríguez, M. A.; García-García, P. 1,3-Dien-5-ynes: Versatile Building Blocks for the Synthesis of Carbo- and Heterocycles. Chem. Rev. 2016, 116, 8256−8311. (2) (a) Zhang, J.; Wu, D.; Chen, X.; Liu, Y.; Xu, Z. CopperCatalyzed Oxidative Cyclization of 1,5-Enynes with Concomitant C-C Bond Cleavage: An Unexpected Access to 3-Formyl-1-indenone Derivatives. J. Org. Chem. 2014, 79, 4799−4808. (b) Wood, J. M.; Furkert, D. P.; Brimble, M. A. Synthesis of the 2-Formylpyrrole Spiroketal Pollenopyrroside A and Structural Elucidation of Xylapyrroside A, Shensongine A and Capparisine B. Org. Biomol. Chem. 2016, 14, 7659−7664. (c) Pradipta, A. R.; Taichi, M.; Nakase, I.; Saigitbatalova, E.; Kurbangalieva, A.; Kitazume, S.; Taniguchi, N.; Tanaka, K. Uncatalyzed Click Reaction between Phenyl Azides and Acrolein: 4-Formyl-1,2,3-triazolines as ″Clicked″ Markers for Visualizations of Extracellular Acrolein Released from Oxidatively Stressed Cells. ACS Sens. 2016, 1, 623−632. (d) Rao, C.; Mai, S.; Song, Q. CuCatalyzed Synthesis of 3-Formyl Imidazo[1,2-a]pyridines and Imidazo[1,2-a]pyrimidines by Employing Ethyl Tertiary Amines as Carbon Sources. Org. Lett. 2017, 19, 4726−4729. (3) (a) Elangovan, S.; Topf, C.; Fischer, S.; Jiao, H.; Spannenberg, A.; Baumann, W.; Ludwig, R.; Junge, K.; Beller, M. Selective Catalytic Hydrogenations of Nitriles, Ketones, and Aldehydes by Well-Defined Manganese Pincer Complexes. J. Am. Chem. Soc. 2016, 138, 8809− 8814. (b) Li, Z.; Fang, M.; LaGasse, M. K.; Askim, J. R.; Suslick, K. S. Colorimetric Recognition of Aldehydes and Ketones. Angew. Chem., Int. Ed. 2017, 56, 9860−9863. (c) Liang, G.; Wang, A.; Li, L.; Xu, G.; Yan, N.; Zhang, T. Production of Primary Amines by Reductive Amination of Biomass-Derived Aldehydes/Ketones. Angew. Chem., Int. Ed. 2017, 56, 3050−3054. (d) Greene, L. E.; Lincoln, R.; Krumova, K.; Cosa, G. Development of a Fluorogenic Reactivity Palette for the Study of Nucleophilic Addition Reactions Based on meso-Formyl BODIPY Dyes. ACS Omega 2017, 2, 8618−8624. (4) (a) Khanapur, S.; Paul, S.; Shah, A.; Vatakuti, S.; Koole, M. J. B.; Zijlma, R.; Dierckx, R. A. J. O.; Luurtsema, G.; Garg, P.; van Waarde, A.; Elsinga, P. H. Development of [18F]-Labeled Pyrazolo[4,3-e]1,2,4-triazolo[1,5-c]pyrimidine (SCH442416) Analogs for the Imaging of Cerebral Adenosine A2A Receptors with Positron Emission Tomography. J. Med. Chem. 2014, 57, 6765−6780. (b) Xu, Y.; Brenning, B. G.; Kultgen, S. G.; Foulks, J. M.; Clifford, A.; Lai, S.; Chan, A.; Merx, S.; McCullar, M. V.; Kanner, S. B.; Ho, K.-K. Synthesis and Biological Evaluation of Pyrazolo[1,5-a]pyrimidine Compounds as Potent and Selective Pim-1 Inhibitors. ACS Med.

2 Fx 1 − 10−Ast ηx · · Fst 1 − 10−Ax ηst2

where x and st indicate the sample and standard solution, respectively, ϕ is the quantum yield, F is the integrated area of the emission, A is the absorbance at the excitation wavelength, and η is the index of refraction of the solvents.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.joc.8b01571. 1 H and 13C{1H} NMR spectra for all compounds and spectroscopic properties (PDF) CIF for compound 4a (CIF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] ORCID

Jaime Portilla: 0000-0002-8206-7481 10895


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