Clean Synthesis of N-Pyrrolyl Azoles by Metal-Free Oxidative Cross-Coupling Using Recyclable Hypervalent Iodine Reagent

Abstract

The facile and clean oxidative coupling reaction of pyrroles with azoles has been achieved using the recyclable hypervalent iodine(III) reagents having adamantane structures. These iodine(III) reagents could be recovered from the reaction mixtures by a simple solid–liquid separation, i.e., filtration, for reuse.

The N-pyrrolyl azoles are important fragments in the molecules of biological systems or in many pharmaceuticals, insecticides, and functional materials, especially, fluorescent dyes.^1–4) In spite of this interest, the preparations of N-aryl azoles are severely restricted because the nitrogen heteroaromatics are sometimes not a good substrate to use in the traditional arylation methods such as the Ullmann coupling,^5–11) since the pyrrole-based metal and halide compounds are sometimes unstable. Katritzky et al. reported the synthesis of pyrrolyl-benzotriazoles by the Mannich condensation of o-phthalaldehyde and 2,5-dimethoxy-2,5-dihydrofuran with primary amines,¹²⁾ but this method was not effective for pyrroles, and the desired products were produced only in less than 10% yields. As a new approach, the oxidative C–H bond functionalization strategies of pyrroles^13–22) might be expected to be an attractive methodology for enabling rapid access to these molecules. Recently, we have developed a new hypervalent iodine-induced oxidative coupling of pyrroles at the N¹-positions of the azoles.²³⁾ In 2014, Chen and colleagues reported the oxidative coupling method of 1,2,3-triazoles with pyrroles using N-iodosuccinimide (NIS) as an oxidant, in which the reactions preferentially occurred at the N²-position of the 1,2,3-triazoles.²⁴⁾ However, these methods produced stoichiometric quantities of iodoarene or succinimide as a waste product after the reaction (Chart 1).

Chart 1. Reported Metal-Free Oxidative Cross-Coupling Reactions of Pyrroles with Azoles

Hypervalent iodine reagents have received much attention due to their mild oxidation abilities and low toxicity, easy handling, as alternatives to toxic heavy metal reagents.^25–32) For developing a new coupling for heteroaromatic compounds, we have been engaged in the study of the oxidative C–H bond arylations using hypervalent iodine reagents that utilize the new reactivities of the heteroaromatic iodonium intermediates.^33–37) As mentioned, we developed a hypervalent iodine-induced oxidative coupling of pyrroles at the N¹-positions of the azoles.²³⁾ This new method is versatile in allowing the reactions under metal-free and mild conditions, and tolerates a diverse series of functionalities of the pyrroles and azole coupling partners. We envisioned that a recycling version of this oxidative coupling should become feasible and provide an eco-friendly synthetic method (Chart 2) because a number of recent studies have demonstrated the utility of the recyclable hypervalent iodine reagents.^38–40)

Chart 2. C–N Coupling Reaction of Triazoles with Pyrroles Using Recyclable Hypervalent Iodine Reagent

Results and Discussion

The use of recyclable hypervalent iodine reagents in reactions should be a further promising and ecological approach for enhancing the practicability of the methods and for reducing the coproduction of stoichiometric amounts of iodoarenes as a result of the easy removal of the reagents from the reaction mixtures and their reuse.^41–59) Therefore, various types of recyclable hypervalent iodine reagents have been reported. First, the polymer-supported hypervalent iodine reagents, such as poly(diacetoxyiodo)styrene (PDAIS) and poly[bis(trifluoroacetoxyiodo)]styrene (PBTIS), incorporating phenyliodine(III) diacetate (PIDA) and phenyliodine(III) bis(trifluoroacetate) (PIFA) in their polymer backbones were developed.^41–48) Inspired by this study, other chemists have also reported new recyclable hypervalent iodine compounds having fluorous tags^49–52) and ionic supports.^53–55)

We have previously developed a structurally new recyclable hypervalent iodine reagent 1a, namely, 1,3,5,7-tetrakis[4-(diacetoxyiodo)phenyl]adamantane (Fig. 1), having a very similar reactivity to that of PIDA, which was easily recovered by precipitation utilizing the insolubility of the tetraiodide 2, stoichiometrically produced from 1a after the reactions, in common polar solvents.^56–59) Our recyclable hypervalent iodine(III) reagents 1 having an adamantane core have several advantages over the conventional polymer-supported reagents in reactivity and recyclability; they typically show higher reactivities compared to the polymer-supported reagents and no degradation of their backbones after repeated use, which are derived from the well-defined tetrahedral structures. Considering the background of the recyclable iodine reagent, we improved our coupling reaction conditions by using the recyclable iodine reagent and avoided the generation of a stoichiometric amount of iodobenzene waste from our initial conditions.

Fig. 1. Recyclable Hypervalent Iodine(III) Reagents Having Adamantane Cores

Based on our initial studies, we thus attempted the cross-coupling reaction of N-benzylpyrrole 3a with triazole 4a using the recyclable reagent 1a based on the standard reaction conditions in Cl(CH₂)₂Cl (DCE) in combination with bromotrimethylsilane (TMSBr) at 70°C, yielding the cross-product 5aa in 74% yield (Chart 3, Eq. 1). Other adamantane-based recyclable iodine(III) reagents 1b and c (Fig. 1) were less effective for the coupling reaction of N-benzylpyrrole 3a with triazole 4a than the recyclable reagent 1a. In addition, the less reactive polymer-supported reagent, the commercially available PDAIS, gave no coupling product 5aa under the given condition. As a result, the recyclable iodine(III) reagent 1a and TMSBr in DCE at 70°C gave the best result. The use of a highly polar, but low nucleophilic solvent, such as 1,1,1,3,3,3-hexafluoroisopropanol (HFIP, (CF₃)₂CHOH), trifluoroethanol (TFE), or other Lewis acids, such as BF₃·Et₂O and trimethylsilyl trifluoromethane sulfonate (TMSOTf) in DCE, resulted in no production of the desired coupling product.

Chart 3. Oxidative Cross-Coupling Reaction of Pyrrole 3a with Triazole 4a Using Recyclable Hypervalent Iodine(III) Reagents 1

The oxidants 1a could be easily separated from the reaction mixtures as the corresponding reduced forms, i.e., the tetraiodide 2 by a simple solid–liquid separation. The procedure to recover the tetraiodide started with the removal of the solvent under reduced pressure by a rotary evaporator. Methanol was then added to the resulting oily residues to precipitate the tetraiodide 2. As the tetraiodides are hardly soluble in methanol, they were simultaneously precipitated as a white powder by adding methanol and were collected by filtration to recover the tetraiodide 2. A series of recycling processes was finally completed by reoxidation of the recovered tetraiodide 2 to the initial reagents 1a using m-chloroperbenzoic acid (mCPBA). In this way, the reagents could be reproduced with almost the same purity and have been repeatedly used without any loss of activities.^58,59) Indeed, the reuse of the reagent 1a in the same reaction showed a comparable result in terms of the product yield and recovery of the tetraiodide 2.

With the reagent 1a (1×1/4 eq, 100 mol% of iodine(III) atom relative to the substrates), the reactions of the pyrroles 3 with azoles 4 smoothly proceeded under the homogeneous conditions in the presence of TMSBr in DCE (Chart 4). This method revealed a tolerance toward the N-protecting groups (see substrates 3b, c and d), and in most cases, good yields were obtained. In the case of the N-free pyrrole 3e, the coupling reaction efficiently proceeded and the corresponding coupling product 5ea was obtained in 62% yield. 2-Ethyl pyrrole 3f gave the desired coupling product 5fa in a significant yield. When we attempted the coupling reaction of the poly-substituted pyrroles, 3g, h and i, the coupling products, 5ga, ha and ia, were obtained in moderate yield, respectively. When the reaction was performed using 3-methylindole 3j, the reaction proceeded and gave the coupling product 5ja in 70% yield.

Chart 4. Scope of Reactions^{(a, b)}

Reaction conditions: (a) Pyrroles 3 (0.4 mmol), 1,2,3-triazole 4a (1.2 mmol), TMSBr (0.8 mmol), and iodine reagent 1a (0.4×1/4 mmol) in DCE (4 mL) at 70°C for 3 h. (b) Yields are those for the isolated N¹-selective coupling products. (c) Reaction performed at room temperature. Boc=tert-butoxycarbonyl.

Encouraged by these results, we applied this reaction to other azoles to synthesize various 2-(azol-1-yl)pyrrole and indole derivatives (Chart 5). The reactions of pyrroles 3a and e with different azoles 4b–e smoothly proceeded and the coupling products 5ab–ee were produced in good to excellent yields. The 4-butyl-1,2,3-triazole 4b also afforded the desired coupling adduct 5ab in 76% yield (Eq. 1). The reaction of N-benzylpyrrole 3a with pyrazole 4c and imidazole 4d smoothly proceeded to give 5ac and ad in good yields (Eq. 2). The N-free pyrrole 3e coupled with benzotriazole 4e afforded the coupling product 5ee in 69% yield (Eq. 3).

Chart 5. Cross-Coupling Reaction of Different Azoles with Pyrrole

Reaction conditions: Pyrroles 3 (0.4 mmol), azoles 4 (1.2 mmol), TMSBr (0.8 mmol), and iodine reagent 1a (0.4×1/4 mmol) in DCE (4 mL) at 70°C for 3 h.

Conclusion

We have demonstrated the facile and clean method for the oxidative C–N cross-coupling reaction of pyrroles and azoles using recyclable hypervalent iodine reagents. The present protocol provided easy and less waste access to N-functionalized azoles. The advantageous features of this coupling method are a metal-free procedure and mild reaction conditions. Further studies are underway to extend this coupling to other types of substrates.

Experimental

General Remarks

The ¹H- and ¹³C-NMR spectra were recorded by a JEOL JMN-400 spectrometer operating at 400 MHz in chloroform-d₃ (CDCl₃) at 25°C with tetramethylsilane as the internal standard. Data are reported as follows: chemical shift in ppm (δ), integration, multiplicity (s=singlet, d=doublet, t=triplet, q=quartet, br s=broad singlet, m=multiplet), coupling constant (Hz). The infrared spectra (IR) were obtained using a Hitachi 270-50 spectrometer, absorptions are reported in reciprocal centimeters. The mass spectra were obtained using a Shimadzu GCMS-QP 5000 instrument with ionization voltages of 70 eV. The high resolution mass spectra were performed by the Elemental Analysis Section of Osaka University. Column chromatography and TLC were carried out on Merck Silica gel 60 (230–400 mesh) and Merck Silica gel F254 plates (0.25 mm), respectively. The spots and bands were detected by UV irradiation (254, 365 nm).

Preparation of a Recyclable Hypervalent Iodine Reagent 1a

To a stirred solution of 1,3,5,7-tetrakis(4-iodophenyl)adamantane 2 (1.42 g, 1.5 mmol) in dichloromethane (150 mL)–acetic acid (150 mL) was added mCPBA (ca. 69% purity, 3.12 g, 18 mmol) at room temperature. The mixture was stirred for 12 h under the same reaction conditions during which the cloudy solution became clear. The resultant mixture was filtered, and dichloromethane was removed using a rotary evaporator. Hexane was added to the residue to precipitate the 1,3,5,7-tetrakis[4-(diacetoxyiodo)phenyl]adamantane 1a. After filtration, the almost pure product was obtained in nearly quantitative yield.

General Procedure for the Oxidative C–N Coupling Reaction of Azoles with Pyrroles Using a Recyclable Iodine Reagent 1a (Charts 3–5)

To a stirred solution of N-benzylpyrrole 3a (46 mg, 0.30 mmol) in DCE (3 mL) was added recyclable reagent 1a (106.2 mg, 0.30×1/4 mmol). TMSBr (92 mg, 0.6 mmol) and 1,2,3-triazole 4a (51 mg, 0.9 mmol) were then added at room temperature. The reaction mixture was stirred at 70°C for 3 h, then dichloromethane and saturated sodium hydrogen carbonate aqueous were successively added with stirring. The organic layer was then separated and evaporated to dryness. Methanol was added to the reaction mixture, and it was filtered to give the tetraiodide 2 (confirmed by ¹H-NMR analysis and TLC), which was washed several times with small portion of methanol (MeOH) for purification. The filtrate was evaporated and subjected to column chromatography (SiO₂, hexane) to give 1-(1-benzyl-1H-pyrrol-2-yl)-1H-1,2,3-triazole 5aa (57 mg, 74%).

1-(1-Benzyl-1H-pyrrol-2-yl)-1H-1,2,3-triazole (5aa)

¹H-NMR (400 MHz, CDCl₃) δ: 5.01 (2H, s), 6.27 (1H, t, J=4.0 Hz), 6.32–6.34 (1H, m), 6.80 (1H, t, J=4.0 Hz), 6.93–6.95 (2H, m), 7.23–7.27 (3H, m), 7.46 (1H, s), 7.70 (1H, s) ppm; ¹³C-NMR (100 MHz, CDCl₃) δ: 50.4, 105.7, 107.5, 121.9, 124.8, 126.8, 126.9, 127.7, 128.6, 133.1, 136.6 ppm; IR (KBr) ν 3126, 2927, 1709, 1573, 1497, 1323, 1068, 1013, 783, 719, 616 cm⁻¹; MS matrix assisted laser desorption/ionization-time of flight (MALDI-TOF) Calcd for C₁₃H₁₃N₄ m/z 225.11 [M+H]⁺, Found 224.2 (39), 225.2 (100).

1-(1-Methyl-1H-pyrrol-2-yl)-1H-1,2,3-triazole (5ba)

¹H-NMR (400 MHz, CDCl₃) δ: 3.49 (3H, s), 6.18 (1H, t, J=3.4 Hz), 6.24–6.26 (1H, m), 6.67 (1H, t, J=3.4 Hz), 7.73 (1H, s), 7.80 (1H, s) ppm; ¹³C-NMR (100 MHz, CDCl₃) δ: 33.5, 104.8, 107.1, 122.1, 125.1, 126.5, 133.2 ppm; IR (KBr) ν 3125, 2950, 1574, 1503, 1321, 1270, 1230, 1091, 1035, 1034, 787, 722, 611 cm⁻¹; MS (MALDI-TOF) Calcd for C₇H₉N₄ m/z 149.08 [M+H]⁺, Found: 148.1 (24), 149.2 (100).

tert-Butyl 2-(1H-1,2,3-Triazol-1-yl)-1H-pyrrole-1-carboxylate (5ca)

¹H-NMR (400 MHz, CDCl₃) δ: 1.32 (9H, s), 6.26 (1H, t, J=3.4 Hz), 6.41–6.42 (1H, m), 7.36 (1H, t, J=3.4 Hz), 7.74 (1H, s), 7.76 (1H, s) ppm; ¹³C-NMR (100 MHz, CDCl₃) δ: 27.3, 84.9, 109.2, 112.4, 121.8, 124.0, 127.4, 132.5, 147.1 ppm; IR (KBr) ν 3127, 2982, 1749, 1597, 1433, 1317, 1149, 957, 843, 736 cm⁻¹; MS (MALDI-TOF) Calcd for C₁₁H₁₄N₄O₂ m/z 234.11 [M]⁺, Found 234.3 (100), 235.3 (14).

1-(1-Phenyl-1H-pyrrol-2-yl)-1H-1,2,3-triazole (5da)

A brown solid; mp 105–107°C; ¹H-NMR (400 MHz, CD₂Cl₂) δ: 6.40 (1H, t J=3.4 Hz), 6.52–6.54 (1H, m), 7.00–7.01 (1H, m), 7.06–7.08 (2H, m), 7.29–7.32 (3H, m), 7.57 (1H, d, J=1.0 Hz), 7.64 (1H, d, J=1.0 Hz) ppm; ¹³C-NMR (100 MHz, CDCl₃) δ: 107.8, 108.4, 122.3, 124.6, 126.9, 127.8, 129.4, 133.3, 137.4 ppm; IR (KBr) ν 3125, 1596, 1477, 1457, 1354, 1260, 1237, 1038, 935, 765 cm⁻¹; high resolution (HR)-MS (FAB) Calcd for C₁₂H₁₁N₄ m/z 211.0984 [M+H]⁺, Found 211.0978.

1-(1H-Pyrrol-2-yl)-1H-1,2,3-triazole (5ea)

¹H-NMR (400 MHz, CDCl₃) δ: 6.24–6.25 (2H, m), 6.77–6.78 (1H, m), 7.78 (1H, s), 7.86 (1H, s), 9.26 (1H, br s) ppm; ¹³C-NMR (100 MHz, CDCl₃) δ: 98.1, 108.8, 116.9, 121.9, 126.0, 133.9 ppm; IR (KBr) ν 3151, 1589, 1499, 1299, 1050, 1028, 912, 743 cm⁻¹; MS (MALDI-TOF) Calcd for C₆H₇N₄ m/z 135.07; [M+H]⁺, Found: 133.9 (84), 134.9 (100).

1-(5-Ethyl-1H-pyrrol-2-yl)-1H-1,2,3-triazole (5fa)

¹H-NMR (400 MHz, CDCl₃) δ: 1.25 (3H, t, J=7.2 Hz), 2.65 (2H, q, J=7.2 Hz), 5.91 (1H, s), 6.12 (1H, t, J=3.0 Hz), 7.73 (1H, s), 7.81 (1H, s), 9.30 (1H, br s) ppm; ¹³C-NMR (100 MHz, CDCl₃) δ: 13.4, 20.7, 98.0, 104.6, 121.6, 124.4, 133.5, 133.7 ppm; IR (KBr) ν 3152, 2969, 1601, 1597, 1223, 1050, 1018, 776 cm⁻¹; MS (MALDI-TOF) Calcd for C₈H₁₁N₄ m/z 163.10 [M+H]⁺, Found 162.18 (92), 163.19 (100).

1-(4-Ethyl-3,5-dimethyl-1H-pyrrol-2-yl)-1H-1,2,3-triazole (5ga)

¹H-NMR (400 MHz, CDCl₃) δ: 1.07 (3H, t J=7.6 Hz), 2.01 (3H, s), 2.19 (3H, s), 2.41 (2H, q, J=1.7 Hz), 7.75–7.77 (2H, m), 8.37 (1H, br s) ppm; ¹³C-NMR (100 MHz, CDCl₃) δ: 8.7, 10.8, 15.5, 17.5, 109.4, 119.7, 121.2, 122.0, 123.7, 133.4 ppm; IR (KBr) ν 3195, 2961, 1619, 1542, 1450, 1384, 1284, 1035, 913, 782 cm⁻¹; MS (MALDI-TOF) Calcd for C₁₀H₁₅N₄ m/z 191.13 [M+H]⁺, Found 190.15 (64), 191.16 (100).

1-(3,5-Dimethyl-1H-pyrrol-2-yl)-1H-1,2,3-triazole (5ha)

¹H-NMR (400 MHz, CDCl₃) δ: 2.06 (3H, s), 2.25 (3H, s), 5.77 (1H, d, J=2.9 Hz), 7.765 (1H, s), 7.77 (1H, s), 8.56 (1H, br s) ppm; ¹³C-NMR (100 MHz, CDCl₃) δ: 10.5, 12.8, 108.42, 108.44, 110.3, 123.6, 126.4, 133.4 ppm; IR (KBr) ν 3148, 2925, 1618, 1538, 1326, 1224, 1094, 953, 793 cm⁻¹; MS (MALDI-TOF) Calcd for C₈H₁₁N₄ m/z 163.10 [M+H]⁺, Found 162.13 (56), 163.14 (100).

1-(1H-1,2,3-Triazole-1-yl)-4,5,6,7-tetrahydro-2H-isoindole (5ia)

A white solid; mp 127–129°C; ¹H-NMR (400 MHz, CDCl₃) δ: 1.70–1.81 (4H, m), 2.58–2.61 (4H, m), 6.47 (1H, d, J=2.5 Hz), 7.77 (1H, s), 7.81 (1H, s), 8.86 (1H, br s) ppm; ¹³C-NMR (100 MHz, CDCl₃) δ: 21.5, 22.0, 23.3, 23.4, 109.6, 111.6, 120.6, 121.8, 133.7 ppm; IR (KBr) ν 3148, 2925, 1714, 1533, 1325, 1230, 1034, 767, 592 cm⁻¹; MS (MALDI-TOF) Calcd for C₁₀H₁₃N₄ m/z 189.11 [M+H]⁺, Found 188.05 (40), 189.06 (100).

3-Methyl-2-[1,2,3]triazol-1-yl-1H-indole (5ja)⁵⁹⁾

¹H-NMR (400 MHz, CDCl₃) δ: 2.34 (3H, s), 7.14 (1H, dd, J=8.0, 1.0 Hz), 7.25 (1H, dd, J=8.0, 1.0 Hz), 7.37 (1H, d, J=8.0 Hz), 7.56 (1H, d, J=8.0 Hz), 7.83 (1H, s), 7.97 (1H, s), 9.44 (1H, br s) ppm; ¹³C-NMR (100 MHz, CDCl₃) δ: 8.4, 101.9, 111.4, 119.3, 120.3, 123.6, 123.7, 127.4, 127.8, 133.5, 133.9 ppm.

1-(1-Benzyl-1H-pyrrol-2-yl)-4-butyl-1H-1,2,3-triazole (5ab)

A brown solid; mp 73–77°C; ¹H-NMR (400 MHz, CDCl₃) δ: 0.95 (3H, t, J=7.6 Hz), 1.31–1.41 (2H, m), 1.59–1.67 (2H, m), 2.71 (2H, t, J=7.8 Hz), 5.01 (2H, s), 6.25 (1H, t, J=3.7 Hz), 6.29–6.31 (1H, m), 6.78–6.79 (1H, m), 6.94–6.97 (2H, m), 7.15 (1H, s), 7.24–7.29 (3H, m) ppm; ¹³C-NMR (100 MHz, CDCl₃) δ: 13.8, 22.1, 25.0, 31.3, 50.4, 105.4, 107.3, 121.6, 123.9, 125.3, 126.9, 127.7, 128.6, 136.8, 147.6 ppm; IR (KBr) ν 3111, 2956, 2859, 1950, 1576, 1496, 1453, 1274, 1230, 1274, 1070, 1007, 913, 722 cm⁻¹; MS (MALDI-TOF) Calcd for C₁₇H₂₁N₄ m/z 281.18 [M+H]⁺, Found 281.13 (100), 282.13 (20).

1-(1-Benzyl-1H-pyrrol-2-yl)-1H-pyrazole (5ac)

¹H-NMR (400 MHz, CDCl₃) δ: 4.96 (2H, s), 6.16 (1H, t, J=3.4 Hz), 6.19–6.21 (1H, m), 6.28 (1H, s), 6.64 (1H, s), 6.95 (2H, d, J=8.3 Hz), 7.18–7.23 (3H, m), 7.36 (1H, d, J=2.4 Hz), 7.69 (1H, s) ppm; ¹³C-NMR (100 MHz, CDCl₃) δ: 50.0, 104.4, 106.1, 107.0, 120.4, 127.0, 127.5, 128.5, 129.0, 132.7, 137.5, 141.1 ppm; IR (KBr) ν 3107, 3063, 2930, 1577, 1496, 1487, 1455, 1281, 1251, 1112, 1068, 996, 756, 714 cm⁻¹; HR-MS (FAB) Calcd for C₁₄H₁₄N₃ m/z 224.1188 [M+H]⁺, Found 224.1182.

1-(1-Benzyl-1H-pyrrol-2-yl)-1H-imidazole (5ad)

¹H-NMR (400 MHz, CDCl₃) δ: 4.79 (2H, s), 6.19–6.20 (2H, m), 6.71 (1H, t, J=2.4 Hz), 6.82 (1H, s), 6.87–6.90 (2H, m), 7.06 (1H, s), 7.20–7.29 (3H, m), 7.37 (1H, s) ppm; ¹³C-NMR (100 MHz, CDCl₃) δ: 49.5, 105.7, 107.3, 120.8, 121.7, 125.2, 126.3, 127.8, 128.7, 129.2, 137.0, 139.1 ppm; IR (KBr) ν 3110, 2924, 1708, 1574, 1490, 1348, 1311, 1235, 1060, 819, 720, 660 cm⁻¹; HR-MS (FAB) Calcd for C₁₄H₁₄N₃ m/z 224.1188 [M+H]⁺, Found 224.1182.

1-(1H-Pyrrol-2-yl)-1H-benzo[d][1,2,3]triazole (5ee)

A white solid; mp 110–113°C; ¹H-NMR (400 MHz, CDCl₃) δ: 6.36–6.38 (1H, m), 6.44–6.48 (1H, m), 6.86–6.88 (1H, m), 7.42 (1H, t, J=8.0 Hz), 7.56 (1H, t, J=8.0 Hz), 7.73 (1H, d, J=8.3 Hz), 8.09 (1H, d, J=8.3 Hz), 9.17 (1H, br s) ppm; ¹³C-NMR (100 MHz, CDCl₃) δ: 99.4, 109.0, 110.5, 116.8, 120.0, 124.6, 124.9, 128.5, 131.9, 145.6 ppm; IR (KBr) ν 3202, 2571, 1714, 1583, 1442, 1216, 1085, 1027, 938, 744, 658 cm⁻¹; HR-MS (FAB) Calcd for C₁₀H₉N₄ m/z 185.0826 [M+H]⁺, Found 185.0822.

Acknowledgments

This work was partially supported by Grants-in-Aid for Scientific Research (A) from the Japan Society for the Promotion of Science (JSPS), a Grant-in-Aid for Scientific Research on Innovative Areas “Advanced Molecular Transformation by Organocatalysts” from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan. K. M. also acknowledges support from the Grant-in-Aid for Young Scientists (B) from the JSPS.

Conflict of Interest

The authors declare no conflict of interest.

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