HETEROCYCLES

Short Paper | Regular issue | Vol. 89, No. 4, 2014, pp. 1025-1034
Received, 6th January, 2014, Accepted, 5th February, 2014, Published online, 7th February, 2014.
DOI: 10.3987/COM-14-12931

■ An Efficient Synthetic Route towards Novel Furo- and Thieno-triazolopyridines

Kazuhiro Tomoike, Hiroshi Maruoka,* Fumi Okabe, Eiichi Masumoto, Toshihiro Fujioka, and Kenji Yamagata

Faculty of Pharmaceutical Sciences, Fukuoka University, 8-19-1 Nanakuma, Jonan-ku, Fukuoka 814-0180, Japan

Abstract

An efficient method for the synthesis of novel nitrogen-containing heterotricycles is described. 4,5-Dihydro-3-furan- and -3-thiophene-carbonitriles 1a,b and 2a,b having an active methylene group at C-2 position served as the precursor of enamines 3a,b and 4a,b, which were followed by an exchange reaction of amines, such as acetohydrazide and benzohydrazide, and subsequent tandem intramolecular cyclization reaction to lead the corresponding furo- and thieno-triazolopyridines 5a,b, 6a,b, 7a,b, and 8a,b.

Pyridines and their analogues, especially fused pyridines, are important because of their incidence in nature,1 their biological properties,2 and their utilities as intermediates for the design of biologically active compounds.3 Therefore, a large number of general methods for the preparation of pyridine derivatives have recently been reported.4 On the other hand, heterocycles containing triazole ring systems also occur in a wide variety of natural and biologically active compounds.5 Triazole moieties are useful building blocks in chemistry and can be modified to exhibit important roles in pharmacological applications. For these reasons, efficient methods for the synthesis of nitrogen-containing molecules merits further investigations.6
In the course of our investigation of the synthesis of heterobicycles,7 we have shown the synthesis of fused thiopyranthiones,8a thiophenes,8a pyridines,8b and pyridinones8b from 4,5-dihydro-3-furan- and -3-thiophene-carbonitriles A having an active methylene group at C-2 position as versatile starting materials (Figure 1). Our general point of interest goes to the synthesis of 4,5-annulated furo- and thieno-pyridines with nitrogen-containing heterocycles, thus providing easy strategies towards the synthesis of novel furo- and thieno-pyridine fused structures. To further extend the utility of A, we herein describe an efficient procedure for the synthesis of furo- and thieno-triazolopyridine derivatives 5−8 from key starting materials 1 and 2.

Initially, we examined condensation reaction of methyl 3-cyano-4,5-dihydro-2-furan- and -2-thiophene- acetates 1a and 2a with N,N-dimethylformamide dimethyl acetal9 (DMFDMA). Compounds 1a and 2a were easily prepared by Wittig reaction of tetrahydro-2-oxo-3-furan- and -3-thiophene-carbonitriles with methyl (triphenylphosphoranylidene)acetate according to our previous procedure.8a Thus, the reaction of compounds 1a,b and 2a,b with DMFDMA resulted in the formation of enamines 3a,b and 4a,b with 57−76% isolated yields (Scheme 1 and Table 1).

In the next step, we attempted an exchange reaction of amines and subsequent intramolecular cyclization of enamines 3a,b and 4a,b with hydrazides (Scheme 1). As a consequence, the reaction of enamines 3a,b and 4a,b with acetohydrazide (2 equiv.) and/or benzohydrazide (1 equiv.) in refluxing acetic acid for 0.5 h led to the corresponding furo- and thieno-triazolopyridines 5b, 6a,b, 7a,b, and 8a,b in moderate to good yields (Table 2). In the case of the reaction of 3a with acetohydrazide under the same condition, the desired 5a could not be isolated at all and the reaction was not clean. Fortunately, we found the reaction condition under which compound 5a could be isolated. Indeed, when a mixture of 3a and acetohydrazide (2 equiv.) in the presence of acetic acid (1 equiv.) in methanol was refluxed for 3 h, the desired furotriazolopyridine 5a was obtained in 79% yield.

Elemental analyses, MS spectra, 1H and 13C NMR spectra of compounds 5−8 are consistent with the assigned structures (see experimental section). For example, the IR spectrum of 5a reveals a band at 1713 cm-1 due to an ester carbonyl group. The 1H NMR spectrum of 5a in CDCl3 exhibits a three-proton singlet at δ 2.56 assignable to the methyl protons and a three-proton singlet at δ 3.96 assignable to the methyl protons of the methyl ester. The 13C NMR spectrum of 5a in CDCl3 shows a signal at δ 14.5 because of the methyl carbon, a signal at δ 52.5 because of the methyl carbon of the methyl ester, a signal at δ 151.0 because of the C-9b carbon, a signal at δ 163.1 because of the ester carbonyl carbon, and a signal at δ 166.7 because of the C-2 carbon.
A plausible mechanism for the formation of the furo- and thieno-triazolopyridines 5−8 is shown in Scheme 2. Thermal treatment of enamines 3 and 4 with hydrazides probably causes an exchange reaction of amines to give the non-isolable intermediate B, which underwent in situ cyclization to result in the formation of the fused pyridines C. Subsequently, a ring-closure/dehydration reaction of C easily occurs in the presence of acetic acid and then the corresponding furo- and thieno-triazolopyridines 5−8 would be produced.

To confirm the structures of furo- and thieno-triazolopyridines, we synthesized compounds 6a and 8a by an alternative route (Scheme 3).10 Thus, the reaction of enamines 3a and 4a with hydrazine monohydrochloride in refluxing methanol gave the corresponding 1,2-diaminopyridinium salts 9 (88%) and 10 (64%). Compounds 9 and 10 were readily reacted with benzaldehyde in the presence of sodium methoxide at room temperature to yield 6a (60%) and 8a (69%), which were identical with authentic samples prepared according to Scheme 1 on the basis of a comparison of the melting points, IR, and NMR spectra.

In conclusion, we have demonstrated the synthesis of novel furo- and thieno-triazolopyridines 5−8 starting from 4,5-dihydro-3-furan- and -3-thiophene-carbonitriles 1 and 2 having an active methylene group at C-2 position through a domino reaction. The key exchange reaction of amines and subsequent tandem intramolecular cyclization of enamines 3 and 4 with acetohydrazide and/or benzohydrazide proceeds smoothly to furnish the corresponding furo- and thieno-triazolopyridines 5−8. Functionalized nitrogen-containing heterotricycles are important synthons in organic synthesis and for the preparation of biologically active compounds with interest in medicinal chemistry.

EXPERIMENTAL
All melting points are uncorrected. The IR spectra were recorded on a JASCO FT/IR-4100 spectrometer. The 1H and 13C NMR spectra were measured with a JEOL JNM-A500 spectrometer at 500.00 and 125.65 MHz, respectively. The 1H and 13C chemical shifts (δ) are reported in parts per million (ppm) relative to TMS as internal standard. Positive(+) or negative(-) FAB MS spectra were obtained on a JEOL JMS-700T spectrometer. Elemental analyses were performed on YANACO MT-6 CHN analyzer. The starting compounds 1a,b and 2a,b were prepared in this laboratory according to the procedure reported in literature.8a

The preparation of enamines 3a and 4a from 1a and/or 2a and DMFDMA. A solution of 1a and/or 2a (20 mmol) and DMFDMA (2.86 g, 24 mmol) was stirred at 80 °C for 2 h. After removal of MeOH in vacuo, the residue was purified by column chromatography on alumina with Et2O as the eluent to afford 3a and 4a.
Methyl 3-cyano-4,5-dihydro-α-[(dimethylamino)methylene]-2-furanacetate (3a): Colorless columns (2.59 g, 58%), mp 123−124 °C (acetone/petroleum ether); IR (KBr): ν 2200 (CN), 1686 cm-1 (CO); 1H NMR (CDCl3): δ 3.02 [br s, 6H, N(CH3)2] , 2.96 (t, J = 9.6 Hz, 2H, 4-H), 3.71 (s, 3H, CO2CH3), 4.51 (t, J = 9.6 Hz, 2H, 5-H), 7.60 (s, 1H, olefin H); 13C NMR (CDCl3): δ 30.4 (C-4), 39.2, 46.8 [N(CH3)2], 51.4 (CO2CH3), 70.4 (C-5), 85.6 [C=CHN(CH3)2], 85.7 (C-3), 116.9 (CN), 153.9 [C=CHN(CH3)2], 167.47 (CO), 167.51 (C-2); FAB(+) MS: m/z 223 [M+H]+. Anal. Calcd for C11H14N2O3: C, 59.45; H, 6.35; N, 12.60. Found: C, 59.26; H, 6.33; N, 12.41.
Methyl 3-cyano-4,5-dihydro-α-[(dimethylamino)methylene]-2-thiopheneacetate (4a): Pale yellow prisms (2.76 g, 58%), mp 95−96 °C (dec.) (Et2O); IR (KBr): ν 2202 (CN), 1685 cm-1 (CO); 1H NMR (CDCl3): δ 3.04 [s, 6H, N(CH3)2], 3.06 (br s, 2H, 4-H), 3.34 (br s, 2H, 5-H), 3.70 (s, 3H, CO2CH3), 7.56 (s, 1H, olefin H); 13C NMR (CDCl3): δ 32.5 (C-4), 36.1 (C-5), 42.4 [N(CH3)2], 51.3 (CO2CH3), 87.7 (C-3), 103.2 [C=CHN(CH3)2], 116.2 (CN), 152.5 [C=CHN(CH3)2], 159.1 (C-2), 167.6 (CO); FAB(+) MS: m/z 239 [M+H]+. Anal. Calcd for C11H14N2O2S: C, 55.44; H, 5.92; N, 11.76. Found: C, 55.38; H, 5.99; N, 11.74.
General procedure for the preparation of furo- and thieno-triazolopyridines 5−8 from enamines 3 and/or 4 and acetohydrazide and/or benzohydrazide.
Procedure A. A mixture of 3a (1.11 g, 5 mmol), acetohydrazide (0.74 g, 10 mmol), and AcOH (0.30 g, 5 mmol) in MeOH (10 mL) was refluxed for 3 h. After removal of the solvent in vacuo, cold water was added to the residue. The precipitate was collected by filtration, washed with water, dried, and recrystallized from MeOH to give 5a.
Procedure B. A solution of 3b or 4a,b (5 mmol) and acetohydrazide (0.74 g, 10 mmol) in AcOH (10 mL) was refluxed for 0.5 h. After work-up as described above, 5b and 7a,b were obtained.
Procedure C. A solution of 3a,b or 4a,b (5 mmol) and benzohydrazide (0.68 g, 5 mmol) in AcOH (10 mL) was refluxed for 0.5 h. After work-up as described above, 6a,b and 8a,b were obtained.
Methyl 8,9-dihydro-2-methylfuro[3,2-c][1,2,4]triazolo[1,5-a]pyridine-6-carboxylate (5a): Colorless needles (0.93 g, 79%), mp 198−199 °C (MeOH); IR (KBr): ν 1713 (CO) cm-1; 1H NMR (CDCl3): δ 2.56 (s, 3H, CH3), 3.46−3.51 (m, 2H, 9-H), 3.96 (s, 3H, CO2CH3), 4.91−4.95 (m, 2H, 8-H), 8.97 (s, 1H, 5-H); 13C NMR (CDCl3): δ 14.5 (CH3), 27.0 (C-9), 52.5 (CO2CH3), 74.0 (C-8), 105.8 (C-6), 109.4 (C-9a), 131.7 (C-5), 151.0 (C-9b), 160.0 (C-6a), 163.1 (CO), 166.7 (C-2); FAB(+) MS: m/z 234 [M+H]+. Anal. Calcd for C11H11N3O3: C, 56.65; H, 4.75; N, 18.02. Found: C, 56.49; H, 4.77; N, 18.05.
8,9-Dihydro-2-methylfuro[3,2-c][1,2,4]triazolo[1,5-a]pyridine-6-carbonitrile (5b): Colorless needles (0.36 g, 36%), mp 270−271 °C (MeOH); IR (KBr): ν 2237 (CN) cm-1; 1H NMR (DMSO-d6): δ 2.44 (s, 3H, CH3), 3.43 (t, J = 9.2 Hz, 2H, 9-H), 4.92 (t, J = 9.2 Hz, 2H, 8-H), 9.46 (s, 1H, 5-H); 13C NMR (DMSO-d6): δ 13.8 (CH3), 26.9 (C-9), 74.4 (C-8), 86.2 (C-6), 108.8 (C-9a), 112.8 (CN), 134.2 (C-5), 150.2 (C-9b), 159.1 (C-6a), 165.9 (C-2); FAB(+) MS: m/z 201 [M+H]+. Anal. Calcd for C10H8N4O: C, 59.99; H, 4.03; N, 27.99. Found: C, 59.78; H, 4.09; N, 27.92.
Methyl 8,9-dihydro-2-phenylfuro[3,2-c][1,2,4]triazolo[1,5-a]pyridine-6-carboxylate (6a): Colorless needles (1.24 g, 84%), mp 206−208 °C (MeOH); IR (KBr): ν 1710 (CO) cm-1; 1H NMR (DMSO-d6): δ 3.46 (t, J = 9.2 Hz, 2H, 9-H), 3.87 (s, 3H, CO2CH3), 4.89 (t, J = 9.2 Hz, 2H, 8-H), 7.52−7.55 (m, 3H, aryl H), 8.17−8.19 (m, 2H, aryl H), 9.20 (s, 1H, 5-H); 13C NMR (DMSO-d6): δ 26.6 (C-9), 52.3 (CO2CH3), 73.7 (C-8), 105.9 (C-6), 109.9 (C-9a), 126.9, 128.8, 130.1, 130.4 (C aryl), 131.9 (C-5), 150.9 (C-9b), 159.4 (C-6a), 162.5 (CO), 165.2 (C-2); FAB(+) MS: m/z 296 [M+H]+. Anal. Calcd for C16H13N3O3: C, 65.08; H, 4.44; N, 14.23. Found: C, 64.93; H, 4.55; N, 14.24.
8,9-Dihydro-2-phenylfuro[3,2-c][1,2,4]triazolo[1,5-a]pyridine-6-carbonitrile (6b): Colorless needles (0.68 g, 52%), mp 239−240 °C (acetone); IR (KBr): ν 2241 (CN) cm-1; 1H NMR (DMSO-d6): δ 3.51 (t, J = 9.5 Hz, 2H, 9-H), 4.95 (t, J = 9.5 Hz, 2H, 8-H), 7.51−7.54 (m, 3H, aryl H), 8.15−8.18 (m, 2H, aryl H), 9.59 (s, 1H, 5-H); 13C NMR (DMSO-d6): δ 27.0 (C-9), 74.5 (C-8), 87.0 (C-6), 109.5 (C-9a), 112.7 (CN), 126.8, 128.6, 129.6, 130.4 (C aryl), 134.7 (C-5), 150.8 (C-9b), 159.5 (C-6a), 165.4 (C-2); FAB(+) MS: m/z 263 [M+H]+. Anal. Calcd for C15H10N4O: C, 68.69; H, 3.84; N, 21.36. Found: C, 68.56; H, 3.96; N, 21.33.
Methyl 8,9-dihydro-2-methylthieno[3,2-c][1,2,4]triazolo[1,5-a]pyridine-6-carboxylate (7a): Colorless needles (0.57 g, 45%), mp 160−161 °C (CH2Cl2/petroleum ether); IR (KBr): ν 1712 (CO) cm-1; 1H NMR (CDCl3): δ 2.46 (s, 3H, CH3), 3.48 (s, 4H, 8- and 9-H), 3.89 (s, 3H, CO2CH3), 9.10 (s, 1H, 5-H); 13C NMR (CDCl3): δ 13.9 (CH3), 31.98 (C-9), 32.07 (C-8), 52.3 (CO2CH3), 111.9 (C-6), 123.8 (C-9a), 130.1 (C-5), 144.7 (C-6a), 148.9 (C-9b), 163.7 (CO), 165.4 (C-2); FAB(+) MS: m/z 250 [M+H]+. Anal. Calcd for C11H11N3O2S: C, 53.00; H, 4.45; N, 16.86. Found: C, 52.84; H, 4.56; N, 16.78.
8,9-Dihydro-2-methylthieno[3,2-c][1,2,4]triazolo[1,5-a]pyridine-6-carbonitrile (7b): Colorless needles (0.72 g, 67%), mp 253−254 °C (CH2Cl2); IR (KBr): ν 2232 (CN) cm-1; 1H NMR (DMSO-d6): δ 2.47 (s, 3H, CH3), 3.56−3.60 (m, 2H, 9-H), 3.71−3.75 (m, 2H, 8-H), 9.56 (s, 1H, 5-H); 13C NMR (DMSO-d6): δ 14.0 (CH3), 33.4 (C-9), 33.6 (C-8), 93.8 (C-6), 115.0 (CN), 124.4 (C-9a), 134.0 (C-5), 144.8 (C-6a), 148.9 (C-9b), 165.7 (C-2); FAB(+) MS: m/z 217 [M+H]+. Anal. Calcd for C10H8N4S: C, 55.54; H, 3.73; N, 25.91. Found: C, 55.58; H, 3.71; N, 25.89.
Methyl 8,9-dihydro-2-phenylthieno[3,2-c][1,2,4]triazolo[1,5-a]pyridine-6-carboxylate (8a): Colorless needles (1.40 g, 90%), mp 169−170 °C (acetone); IR (KBr): ν 1719 (CO) cm-1; 1H NMR (DMSO-d6): δ 3.49−3.57 (m, 4H, 8- and 9-H), 3.89 (s, 3H, CO2CH3), 7.51−7.54 (m, 3H, aryl H), 8.16−8.18 (m, 2H, aryl H), 9.20 (s, 1H, 5-H); 13C NMR (DMSO-d6): δ 32.1 (C-8), 32.2 (C-9), 52.4 (CO2CH3), 112.5 (C-6), 124.4 (C-9a), 126.8, 128.6, 129.9, 130.3 (C aryl), 130.6 (C-5), 145.3 (C-6a), 149.4 (C-9b), 163.6 (CO), 164.9 (C-2); FAB(+) MS: m/z 312 [M+H]+. Anal. Calcd for C16H13N3O2S: C, 61.72; H, 4.21; N, 13.50. Found: C, 61.62; H, 4.26; N, 13.42.
8,9-Dihydro-2-phenylthieno[3,2-c][1,2,4]triazolo[1,5-a]pyridine-6-carbonitrile (8b): Colorless needles (0.86 g, 62%), mp 249−250 °C (CH2Cl2); IR (KBr): ν 2234 (CN) cm-1; 1H NMR (DMSO-d6): δ 3.64−3.67 (m, 2H, 9-H), 3.74−3.77 (m, 2H, 8-H), 7.52−7.55 (m, 3H, aryl H), 8.14−8.17 (m, 2H, aryl H), 9.66 (s, 1H, 5-H); 13C NMR (DMSO-d6): δ 33.5 (C-8), 33.7 (C-9), 94.4 (C-6), 114.9 (CN), 125.0 (C-9a), 127.0, 128.9, 129.5, 130.7 (C aryl), 134.4 (C-5), 145.4 (C-6a), 149.4 (C-9b), 165.0 (C-2); FAB(+) MS: m/z 279 [M+H]+. Anal. Calcd for C15H10N4S: C, 64.73; H, 3.62; N, 20.13. Found: C, 64.77; H, 3.61; N, 20.05.

The preparation of diaminopyridinium salts 9 and 10 from 3a and/or 4a and hydrazine monohydrochloride. A mixture of 3a or 4a (5 mmol) and hydrazine monohydrochloride (0.45 g, 6.5 mmol, in the case of the reaction of 3a) or (0.69 g, 10 mmol, in the case of the reaction of 4a) in MeOH (10 mL) was refluxed for 2 h (in the case of 3a) or for 4 h (in the case of 4a). After removal of the solvent in vacuo, Et2O was added to the residue. The precipitate was collected by filtration, washed with Et2O, dried, and recrystallization from MeOH to give 9 and 10.
4,5-Diamino-2,3-dihydro-7-(methoxycarbonyl)furo[3,2-c]pyridinium chloride (9): Colorless prisms (1.08 g, 88%), mp 281−283 °C (dec.) (MeOH); IR (KBr): ν 3271, 3186 (NH2), 1732 (CO) cm-1; 1H NMR (DMSO-d6): δ 3.13 (t, J = 9.3 Hz, 2H, 3-H), 3.82 (s, 3H, CO2CH3), 4.92 (t, J = 9.3 Hz, 2H, 2-H), 6.80 (br s, 2H, NH2), 8.46 (br s, 2H, NH2), 8.53 (s, 1H, 6-H); 13C NMR (DMSO-d6): δ 28.4 (C-3), 55.8 (CO2CH3), 79.2 (C-2), 106.9 (C-7), 111.3 (C-3a), 150.4 (C-6), 156.3 (C-4), 166.4 (CO), 170.0 (C-7a); FAB(-) MS: m/z 244 [M-H]-. Anal. Calcd for C9H12ClN3O3: C, 44.00; H, 4.92; N, 17.10. Found: C, 43.97; H, 4.90; N, 16.98.
4,5-Diamino-2,3-dihydro-7-(methoxycarbonyl)thieno[3,2-c]pyridinium chloride (10): Pale yellow prisms (0.72 g, 64%), mp 283−285 °C (dec.) (MeOH); IR (KBr): ν 3287, 3243 (NH2), 1715 (CO) cm-1; 1H NMR (DMSO-d6): δ 3.26 (t, J = 8.9 Hz, 2H, 3-H), 3.51 (t, J = 8.9 Hz, 2H, 2-H), 3.86 (s, 3H, CO2CH3), 6.89 (br s, 2H, NH2), 8.54 (br s, 2H, NH2), 8.57 (s, 1H, 6-H); 13C NMR (DMSO-d6): δ 31.8 (C-3), 32.1 (C-2), 52.7 (CO2CH3), 109.2 (C-7), 121.3 (C-3a), 143.6 (C-6), 149.6 (C-4), 158.0 (C-7a), 162.7 (CO); FAB(+) MS: m/z 226 [M-HCl+H]+. Anal. Calcd for C9H12ClN3O2S: C, 41.30; H, 4.62; N, 16.05. Found: C, 41.18. H, 4.58; N, 16.10.
The preparation of furo- and thieno-triazolopyridines 6a and 8a from 9 and/or 10 and benzaldehyde. A mixture of 9 or 10 (5 mmol), benzaldehyde (1.06 g, 10 mmol), and sodium methoxide (0.41 g, 7.5 mmol) in MeOH (50 mL) was stirred at rt for 1 h. After removal of the solvent in vacuo, cold water was added to the residue. The precipitate was collected by filtration, washed with water, dried, and recrystallized from an appropriate solvent to afford 6a (0.89 g, 60%) and 8a (1.08 g, 69%). The melting points, IR, and NMR spectra of these compounds coincided with authentic samples prepared from 3a and/or 4a and benzohydrazide.

ACKNOWLEDGEMENTS
The authors thank Hiroshi Hanazono and Yukiko Iwase for obtaining MS and NMR spectra and Junko Honda for her valuable help with elemental analyses.

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