HETEROCYCLES

Short Paper | Regular issue | Vol. 87, No. 1, 2013, pp. 147-154
Received, 8th October, 2012, Accepted, 14th November, 2012, Published online, 15th November, 2012.
DOI: 10.3987/COM-12-12601

■ A New Synthesis of Amino Substituted Azolo[1,3,5]triazines via Reaction of N1,N1-Dimethyl-N2-azolylformamidines with Cyanamide

Dmitrii V. Kalinin, Svetlana A. Kalinina, and Anton V. Dolzhenko*

School of Pharmacy, Curtin University of Technology, GPO Box U1987, Perth, Western Australia 6845, Australia

Abstract

The amino substituted triazine ring was annelated to aminoazoles using a new effective synthetic procedure. The method of preparation involved initial formation of azolylformamidines in the reaction of aminoazoles with N,N-dimethylformamide dimethyl acetal followed by the triazine ring closure with cyanamide affording therefore fused aminotriazines.

Hetarylformamidines prepared from N,N-dimethylformamide dimethyl acetal2 have been well recognized as valuable building blocks for construction of various fused heterocyclic systems.3 One of the most explored type of transformations of N1,N1-dimethyl-N2-azolylformamidines (1) has been an annelation of a pyrimidine ring to the azoles (1) via reaction of the electrophilic amidine carbon atom with methylene active compounds followed by intramolecular ring closure upon reaction of the azole nitrogen atom with the reactive group introduced by the methylene compounds. In case of methylene active nitriles, amino substituted azolopyrimidines (2) were formed as products of the reaction (Scheme 1).4

A similar reaction of 1 utilizing N-nucleophiles with attached reactive electrophilic group (e.g. cyanamide or urethanes) has not been explored. These reactions are particularly interesting as they may lead to fused 1,3,5-triazines isosteric to biologically active purines, e.g. 1,2,4-triazolo[1,5-a][1,3,5]triazines (5-azapurines)5 and pyrazolo[1,5-a][1,3,5]triazines (5-aza-9-deazapurines).6
Hosmane and Leonard reported7 an example of the triazine ring closure observed when formamidine 3 was heated with cyanamide in isopropanol for 24 h (Scheme 2). The reaction was proposed to proceed via intermediate 4, which was not isolated. Compound 5, the product of subsequent addition of cyanamide to 4, was obtained in 13% yield. No further investigations of this type of reactions have been carried out.

Herein we report new reactions of N1,N1-dimethyl-N2-azolylformamidines with cyanamide affording formation of amino substituted azolo[1,3,5]triazines.
Initially we prepared N1,N1-dimethyl-N2-1,2,4-triazolyl-3(5)-formamidine (7a) and attempted to involve it in the reaction with equimolar quantity of cyanamide (Scheme 3). Heating the reaction mixture in methanol for 24 h allowed preparation of 5-aza-adenine (8a) with 10% yield. Some improvement of the reaction yield up to 15% was achieved when two equivalents of cyanamide were used. Further increase of the yield to 32% was observed upon addition of sodium methoxide to the reaction.

Analogously, N1,N1-dimethyl-N2-1,2,4-triazolyl-5(3)-formamidines (7b,c) were prepared from amines 6b,c by treatment with N,N-dimethylformamide dimethyl acetal and then converted to corresponding 5-aza-adenines (8b,c) with the yields substantially higher than that for 8a. Similarly to 5(3)-amino-1,2,4-triazoles,8 annular tautomerism in the triazole ring of formamidines 7 resulted in the equilibrium between two tautomeric forms 7 and 7′ (Scheme 3). Tautomeric preferences and rate of tautomeric exchange were found to be highly dependent on the substutuents on the triazole ring, physical state, solvent, temperature etc.9 Signals of two tautomeric forms 7 and 7′ were observed in the NMR spectra (DMSO-d6 solution) of phenyl substituted 7b. The tautomer 7′ was found to be minor in the equilibrium (ΔG298 = 7.8 kJ/mol).
The developed method was further extended to the annelation of amino-1,3,5-triazine to 5(3)-aminopyrazoles (9) and 2-aminobenzimidazole (14) using the same sequence of the reactions with N,N-dimethylformamide dimethyl acetal and then cyanamide (Schemes 4 and 5).

4-Aminopyrazolo[1,5-a][1,3,5]triazines (11) were successfully synthesized from aminopyrazoles (9) via N1,N1-dimethyl-N2-pyrazolyl-5(3)-formamidines (10) (Scheme 4). The NMR spectroscopy data for 10a in DMSO-d6 solution revealed that two tautomers 10 and 10′ were almost equistable (ΔG298 = 0.35 kJ/mol). Changing methyl group in the pyrazole ring to phenyl moiety (compound 10b) shifted the tautomeric equilibrium towards tautomeric form 10, which was substantially preferred over 10′ (ΔG298 = 3.2 kJ/mol).
A reaction of amines with N1,N1-dimethyl-N2-pyrazolyl-5(3)-formamidines bearing a cyano group in position 4 of the pyrazole ring has been established to proceed via a pyrimidine ring closure with subsequent Dimroth rearrangement.10 This methodology has been successfully utilized for the preparation of bioactive pyrazolo[3,4-d]pyrimidines.11 Therefore treatment of 10c with cyanamide might theoretically result in the formation of pyrazolo[3,4-d]pyrimidines (12 or/and 13) alternative to 4-amino-8-cyanopyrazolo[1,5-a][1,3,5]triazine (11c). We found that heating 10c with cyanamide in methanol in the presence of sodium methoxide regioselectively provided 11c.
The amino-1,3,5-triazine ring was also successfully annelated to 2-aminobenzimidazole (14) using condensation with N,N-dimethylformamide dimethyl acetal and subsequent treatment of the resulted 15 with cyanamide affording therefore 4-amino-1,3,5-triazino[1,2-a]benzimidazole (16) (Scheme 5).

It should be noted that despite using excess of cyanamide, no products of cyanamide addition to the amino group at the annelated triazine ring were isolated for any of the explored substrates (cf. Scheme 2).
Our attempts to prepare oxo-analogues of the synthesized fused aminotriazines (8, 11, and 16) using urethane instead of cyanamide in a similar reaction with the N1,N1-dimethyl-N2-azolylformamidines (7, 10, and 15) were unsuccessful and furnished isolation of the starting formamidines.
In summary, new practical and general method of the amino-1,3,5-triazine annelation to aminoazoles was successfully developed and exemplified by preparation of 7-amino-1,2,4-triazolo[1,5-a][1,3,5]triazines (8), 4-aminopyrazolo[1,5-a][1,3,5]triazines (11) and 4-amino-1,3,5-triazino[1,2-a]benzimidazole (16).

EXPERIMENTAL
General Methods. Melting points (uncorrected) were determined on a Gallenkamp melting point apparatus. 1H and 13C NMR spectra were recorded on a Bruker Avance III spectrometer (400 MHz), using DMSO-d6 as a solvent and TMS as an internal reference. 5(3)-Amino-1,2,4-triazoles (1b,c) were prepared according to previously reported methods;8 5(3)-amino-3(5)-phenylpyrazole (5) was synthesized using known method.12 Other reagents were purchased from Alfa Aesar.
General method for preparation of N1,N1-dimethyl-N2-azolylformamidines (7, 10, and 15).
A mixture of aminoazoles (6, 9 or 14, 10 mmol) with N,N-dimethylformamide dimethyl acetal (2.0 mL, 15 mmol) in toluene (10 mL) was heated under reflux. Upon completion of the reaction, the mixture was cooled and precipitated formamidines (7, 10 or 15) were filtered and washed with hexane. The compounds were sufficiently pure and were used for the next step without further purification; analytical samples were recrystallized from toluene.
N1,N1-Dimethyl-N2-1,2,4-triazolyl-3(5)-formamidine (7a)
Reaction time: 3 h. Yield 71%; mp 159-161 °C (toluene). 1H NMR (400 MHz, DMSO-d6): δ 2.95 (3H, s, NMe), 3.07 (3H, s, NMe), 7.57 (1H, br. s, H-5/3), 8.40 (1H, s, N=CH-NMe2), 12.86 (1H, br. s, N(1)H). 13C NMR (100 MHz, DMSO-d6): δ 34.0 (NMe), 40.0 (NMe), 148.9 (C-5/3), 157.2 (N=CH-NMe2), 160.7 (C-3/5). Anal. Calcd for C5H9N5: C, 43.15; H, 6.52; N, 50.33. Found: C, 43.12; H, 6.57; N, 50.28.
N1,N1-Dimethyl-N2-[3(5)-phenyl-1,2,4-triazol-5(3)-yl]formamidine (7b)
Reaction time: 1.5 h. Yield 93%; mp 199-201 °C (toluene). 1H NMR (400 MHz, DMSO-d6): δ 2.99 (3H, s, NMe), 3.12 (3H, s, NMe), 7.35 (1H, t, J = 7.3 Hz, H-4′), 7.42 (2H, t, J = 7.5 Hz, H-3′ and H-5′), 7.99 (2H, d, J = 7.5 Hz, H-2′ and H-6′), 8.41* and 8.52 (1H, s, N=CH-NMe2), 12.97 and 13.67* (1H, s, N(1)H); *- signals of minor tautomeric form (KT = 23.4). 13C NMR (100 MHz, DMSO-d6): δ 34.1 (NMe), 40.1 (NMe), 125.3 (C-3′ and C-5′), 128.2 (C-4′), 128.9 (C-2′ and C-6′), 132.3 (C-1′), 157.3 (N=CH-NMe2), 158.7 (C-5), 160.9 (C-3). Anal. Calcd for C11H13N5: C, 61.38; H, 6.09; N, 32.54. Found: C, 61.41; H, 6.12; N, 32.46.
N1,N1-Dimethyl-N2-[3(5)-phenylamino-1,2,4-triazol-5(3)-yl]formamidine (7c)
Reaction time: 10 min. Yield 96%; mp 207-209 °C (toluene). 1H NMR (400 MHz, DMSO-d6): δ 2.96 (3H, s, NMe), 3.07 (3H, s, NMe), 6.73 (1H, t, J = 7.3 Hz, H-4′), 7.17 (2H, t, J = 7.9 Hz, H-3′ and H-5′), 7.53 (2H, d, J = 8.6 Hz, H-2′ and H-6′), 8.34 (1H, s, N=CH-NMe2), 8.76 (1H, s, NHPh), 12.13 (1H, s, N(1)H). 13C NMR (100 MHz, DMSO-d6): δ 34.1 (NMe), 40.1 (NMe), 115.4 (C-2′ and C-6′), 118.3 (C-4′), 128.4 (C-3′ and C-5′), 142.5 (C-1′), 157.0 (N=CH-NMe2), 158.1 and 158.3 (C-3 and C-5). Anal. Calcd for C11H14N6: C, 57.38; H, 6.13; N, 36.50. Found: C, 57.40; H, 6.22; N, 36.33.
N1,N1-Dimethyl-N2-[3(5)-methylpyrazol-5(3)-yl]formamidine (10a)
Reaction time: 1 h. Yield 70%; mp 127-128 °C (toluene), lit.:13 120-122 °C. 1H NMR (400 MHz, DMSO-d6): δ 2.08 (3H, br. s, Me), 2.86 (3H, br. s, NMe), 2.96 (3H, br. s, NMe), 5.56 (1H, br. s, H-4), 7.89 (1H, s, N=CH-NMe2), 11.62 (1H, br. s, N(1)H). 13C NMR (100 MHz, DMSO-d6): δ 10.9* and 14.0 (br. s, Me), 33.8 (NMe), 39.5 (NMe), 88.8 and 94.2* (br. s, C-4), 138.3* (C-5), 146.7 (C-3), 152.6 (C-5), 154.2* and 154.8 (br. s, N=CH-NMe2), 159.3* (C-3); *- signals of minor tautomeric form (KT = 1.15). Anal. Calcd for C7H12N4: C, 55.24; H, 7.95; N, 36.81. Found: C, 55.27; H, 8.12; N, 36.60.
N1,N1-Dimethyl-N2-[3(5)-phenylpyrazol-5(3)-yl]formamidine (10b)
Reaction time: 1 h. Yield 96%; mp 177-179 °C (toluene). 1H NMR (400 MHz, DMSO-d6): δ 2.92 (3H, s, NMe), 3.02 (3H, s, NMe), 6.24 and 6.29* (1H, br. s, H-4), 7.25 (1H, br. t, J = 7.4 Hz, H-4′), 7.37 (2H, br. t, J = 7.5 Hz, H-3′ and H-5′), 7.72 (2H, br. d, J = 7.6 Hz, H-2′ and H-6′), 8.04 (1H, s, N=CH-NMe2), 12.20 and 12.43* (1H, br. s, N(1)H); *- signals of minor tautomeric form (KT = 3.6). 13C NMR (100 MHz, DMSO-d6): δ 33.8 (NMe), 39.6 (NMe), 86.1 (C-4), 134.5 (C-3′ and C-5′), 126.8 (C-4′), 128.3 (C-2′ and C-6′), 134.6 (C-1′), 149.6 (C-3), 153.4 (C-5), 155.5 (N=CH-NMe2). Anal. Calcd for C12H14N4: C, 67.27; H, 6.59; N, 26.15. Found: C, 67.18; H, 6.70; N, 26.02.
N1,N1-Dimethyl-N2-[4-cyanopyrazol-5(3)-yl]formamidine (10c)
Reaction time: 7 h. Yield 84%; mp 162-164 °C (toluene), lit.:14 155-157 °C. 1H NMR (400 MHz, DMSO-d6): δ 2.96 (3H, s, NMe), 3.06 (3H, s, NMe), 7.81 (1H, br. s, H-3/5), 8.01 (1H, s, N=CH-NMe2), 12.77 (1H, br. s, N(1)H). 13C NMR (100 MHz, DMSO-d6): δ 33.9 (NMe), 40.0 (NMe), 82.9 (C-4), 115.9 (CN), 136.4 (CH-5/3), 155.0 (N=CH-NMe2), 156.2 (C-3/5). Anal. Calcd for C7H9N5: C, 51.52; H, 5.56; N, 42.92. Found: C, 51.45; H, 5.66; N, 42.84.
N1,N1-Dimethyl-N2-benzimidazolyl-2-formamidine (15)
Reaction time: 10 min. Yield 94%; mp 242-244 °C (toluene). 1H NMR (400 MHz, DMSO-d6): δ 3.01 (3H, s, NMe), 3.12 (3H, s, NMe), 6.87-7.03 (2H, m, H-5 and H-6), 7.12-7.34 (2H, m, H-4 and H-7), 8.67 (1H, s, N=CH-NMe2), 11.62 (1H, br. s, N(1)H). 13C NMR (100 MHz, DMSO-d6): δ 34.2 (NMe), 40.1 (NMe), 109.2 (br. s, C-7), 116.0 (br. s, C-4), 119.3 (br. s, C-5), 120.1 (br. s, C-6), 133.7 (br. s, C-7a), 143.0 (br. s, C-3a), 157.5 (N=CH-NMe2), 158.5 (C-2). Anal. Calcd for C10H12N4: C, 63.81; H, 6.43; N, 29.77. Found: C, 63.77; H, 6.50; N, 29.62.
General method of preparation of 7-amino-1,2,4-triazolo[1,5-a][1,3,5]triazines (8), 4-aminopyrazolo[1,5-a][1,3,5]triazines (11), and 4-amino-1,3,5-triazino[1,2-a]benzimidazole (16).
Metallic sodium (0.138 g, 6 mmol) was dissolved in MeOH (10 mL) and mixed with N1,N1-dimethyl-N2-azolylformamidines (7, 10, or 15, 3 mmol) and cyanamide (0.25 g, 6 mmol); the mixture was heated under reflux for 24 h. The solvent was evaporated under reduce pressure, the residue was dissolved in water (20 mL) and acidified with HClconc. to pH 3. After stirring at 0 °C for 30 min, the precipitate formed was filtered, washed with cold water and recrystallized from suitable solvent.
7-Amino-1,2,4-triazolo[1,5-a][1,3,5]triazine (8a)
Yield 32%; mp 328-330 °C (water), lit.:15 320 °C. 1H NMR (400 MHz, DMSO-d6): δ 8.33 (1H, s, H-5), 8.50 (1H, s, H-2), 8.75 (1H, br. s, NH), 9.06 (1H, br. s, NH). 13C NMR (100 MHz, DMSO-d6): δ 151.8 (C-7), 154.8 (C-2), 156.9 (C-3a), 159.0 (C-5). Anal. Calcd for C6H7N5: C, 35.30; H, 2.96; N, 61.74. Found: C, 35.19; H, 3.08; N, 61.55.
7-Amino-2-phenyl-1,2,4-triazolo[1,5-a][1,3,5]triazine (8b)
Yield 72%; mp 358-360 °C (DMF), lit.:16 290-293 °C. 1H NMR (400 MHz, DMSO-d6): δ 7.53-7.58 (3H, m, H-3′, H-4′ and H-5′), 8.18-8.24 (2H, m, H-2′ and H-6′), 8.34 (1H, s, H-5), 8.72 (1H, br. s, NH), 9.09 (1H, br. s, NH). 13C NMR (100 MHz, DMSO-d6): δ 127.0 (C-3′ and C-5′), 128.9 (C-2′ and C-6′), 130.1 (C-1′), 130.7 (C-4′), 151.6 (C-7), 157.6 (C-3a), 159.2 (C-5), 163.5 (C-2). Anal. Calcd for C6H7N5: C, 56.60; H, 3.80; N, 39.60. Found: C, 56.51; H, 3.94; N, 39.44.
7-Amino-2-phenylamino-1,2,4-triazolo[1,5-a][1,3,5]triazine (8c)
Yield 70%; mp 319-320 °C (DMF/water). 1H NMR (400 MHz, DMSO-d6): δ 6.92 (1H, t, J = 7.3 Hz, H-4′), 7.29 (2H, dd, J = 7.4 Hz, J = 8.5 Hz, H-3′ and H-5′), 7.79 (2H, d, J = 8.6 Hz, H-2′ and H-6′), 8.21 (1H, s, H-5), 8.28 (1H, br. s, NH), 8.81 (1H, br. s, NH), 9.78 (1H, s, NHAr). 13C NMR (100 MHz, DMSO-d6): δ 117.0 (C-2′ and C-6′), 120.4 (C-4′), 128.6 (C-3′ and C-5′), 140.6 (C-1′), 150.3 (C-7), 155.8 (C-3a), 158.6 (C-5), 161.8 (C-2). Anal. Calcd for C10H9N7: C, 52.86; H, 3.99; N, 43.15. Found: C, 52.80; H, 4.07; N, 43.08.
4-Amino-7-methylpyrazolo[1,5-a][1,3,5]triazine (11a)
Yield 42%; mp 207-209 °C (EtOH), lit.:17 188 °C. 1H NMR (400 MHz, DMSO-d6): δ 2.41 (3H, s, Me), 6.24 (1H, s, H-8), 8.02 (1H, s, H-2), 8.22 (1H, br. s, NH), 8.54 (1H, br. s, NH). 13C NMR (100 MHz, DMSO-d6): δ 14.2 (Me), 94.8 (C-8), 149.4 and 150.4 (C-7 and C-8a), 153.6 (C-2), 154.6 (C-4). Anal. Calcd for C6H7N5: C, 48.32; H, 4.73; N, 46.95. Found: C, 48.34; H, 4.77; N, 46.78.
4-Amino-7-phenylpyrazolo[1,5-a][1,3,5]triazine (11b)
Yield 85%; mp 307-309 °C (DMF), lit.:18 283-286 °C. 1H NMR (400 MHz, DMSO-d6): δ 6.97 (1H, s, H-8), 7.46 (1H, t, J = 7.3 Hz, H-4′), 7.52 (2H, t, J = 7.3 Hz, H-3′ and H-5′), 8.09 (2H, d, J = 7.3 Hz, H-2′ and H-6′), 8.10 (1H, s, H-2), 8.33 (1H, br. s, NH), 8.76 (1H, br. s, NH). 13C NMR (100 MHz, DMSO-d6): δ 92.3 (C-8), 126.3 (C-3′ and C-5′), 128.7 (C-2′ and C-6′), 129.2 (C-4′), 132.1 (C-1′), 149.8 and 150.6 (C-7 and C-8a), 153.8 (C-2), 155.2 (C-4). Anal. Calcd for C11H9N5: C, 62.55; H, 4.29; N, 33.16. Found: C, 62.48; H, 4.35; N, 33.10.
4-Amino-8-cyanopyrazolo[1,5-a][1,3,5]triazine (11c)
Yield 37%; mp 293-295 °C, decomp. (DMF/water). 1H NMR (400 MHz, DMSO-d6): δ 8.30 (1H, s, H-2), 8.69 (1H, s, H-7), 8.91 (1H, br. s, NH), 9.21 (1H, br. s, NH). 13C NMR (100 MHz, DMSO-d6): δ 80.1 (C-8), 113.2 (CN), 146.9 (C-7), 151.1 (C-8a), 152.4 (C-4), 157.3 (C-2). Anal. Calcd for C6H4N6: C, 45.00; H, 2.52; N, 52.48. Found: C, 44.92; H, 2.65; N, 52.32.
4-Amino-1,3,5-triazino[1,2-a]benzimidazole (16)
Yield 53%; mp 313-315 °C (DMF), lit.:16 296-298 °C. 1H NMR (400 MHz, DMSO-d6): δ 7.39 (1H, t, J = 7.8 Hz, H-7), 7.53 (1H, t, J = 7.7 Hz, H-8), 7.76 (1H, d, J = 7.9 Hz, C-9), 8.28 (1H, s, H-2), 8.43 (1H, d, J = 8.2 Hz, C-6), 8.60 (2H, br. s, NH2). 13C NMR (100 MHz, DMSO-d6): δ 114.1 (C-6), 118.4 (C-9), 121.3 (C-7), 125.0 (C-5a), 125.8 (C-8), 143.2 (C-9a), 152.0 (C-10a), 153.4 (C-4), 159.1 (C-2). Anal. Calcd for C9H7N5: C, 58.37; H, 3.81; N, 37.82. Found: C, 58.25; H, 3.92; N, 37.63.

ACKNOWLEDGEMENTS
Authors acknowledge financial support of Russian Ministry of Education and Science.

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