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DOI: 10.1055/s-2002-33534
The Synthesis of a New Pyrazolo[3,4-c]pyridine C-Nucleoside, Structurally
Related
to Formycin B
Publication History
Publication Date:
17 September 2002 (online)
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Abstract
The first preparation of the 4-deaza analogue of formycin B is described, via the reaction of 3-acetamido-2-methoxy-4-methylpyridine with a suitably protected ribonolactone and subsequent ring closure to result in the 3-substituted pyrazolo[3,4-c]pyridine riboside 12.
Key words
lithiation - ring closure - heterocycles - C-nucleosides - pyrazolopyridine
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References
For hemiacetals 3 the prefix α refers to the position of the glycosidic OH group relative to the configuration at the reference C-atom (C-4′ in 3; i.e. the methylpyridinyl moiety is in the β-position). For C-glycosides (no glycosidic OH present), the prefix α-D refers to the alkyl (or aryl) position relative to the reference C-atom.
13Optimized procedure for the preparation of the hemiacetals 3: To a solution of 1 (0.5 g, 2.72 mmol) in dry THF (30 ml) at -78 °C was added under argon n-BuLi (4.3 ml, 6.95 mmol, 1.6 M solution in hexanes). The resulting light yellow solution was stirred at -78 °C for 15 min and the temperature then raised to -40 °C for 1 h. The orange-colored solution was then cooled to -78 °C, a solution of the d-ribonolactone 2 (1.4 g, 3.34 mmol) in dry THF (10 ml) was added dropwise and the resulting mixture was stirred at -78 °C for 1 h and at -40 °C for an additional 5 h. A saturated ammonium chloride solution was then added to the reaction mixture to quench the excess n-BuLi. The solvent was vacuum-evaporated, water was added to the residue and this was extracted with dichloromethane. The organic extracts were dried (Na2SO4) and concentrated to dryness to give an oil, which was purified by flash chromatography (silica gel 20 × 2 cm) using a mixture of cyclohexane-ethyl acetate, 7:3 v/v as the eluent to give the isomeric hemiacetals 3 (63%), together with 4 (6%).
14The configuration at C-1′ was assigned on the basis of nOe spectral data. In the case of the β-D anomer clear correlation peaks between the OH-1′ and the H-2′, H-3′ and H-5′ were observed.
15In the HMBC spectrum of 4, a strong correlation peak between the 5-aromatic H and the carbon of the methyl group (3 J coupling) is evident. The methyl group protons correlate also with the aromatic carbons 3,5 (3 J coupling) and 4 (2 J coupling). The methylene group protons possess a strong correlation with both the anomeric carbon and the carbonyl. NOESY data provided evidence for the β-conformation, since we observed correlation peaks between the methylene protons attached at the anomeric position with H-4′.
16Data of 3-[(1-hydroxy-2,3,5-tri- -benzyl)-β-d-ribofurano-syl)acetylamino]-2-methoxy-4-methylpyridine(4): Colorless oil. 1 -NMR (400 MHz, CDCl3) δ 1.95 (s, 3 H, 4-CH3), 2.78 (m, 2 H, COCH2), 3.38 (m, 2 H, H-5′), 3.73 (d, 1 H, J2 ′ ,3 ′ = 4.56 Hz, H-2′), 3.85 (m, 1 H, H-3′), 3.99 (s, 3 H, OCH3), 4.29 (m, 1 H, H-4′), 4.32-4.68 (m, 7 H, 3 × CH2-Ph, OH), 6.70 (d, 1 H, J 5,6 = 4.98 Hz, H-5), 7.2-7.4 (m, 15 H, 3 × C6H5), 7.84 (d, 1 H, J6,5 = 4.98 Hz, H-6), 7.94 (br s, 1 H, D2O exchangeable, NH). 13C NMR (50 MHz, CDCl3) δ 18.0 (4-CH3), 41.2 (COCH2), 53.3 (CH3O), 69.3 (C-5′), 72.3 (CH2-Ph), 72.6 (CH2-Ph), 73.3 (CH2-Ph), 76.9 (C-3′), 78.9 (C-2′), 80.4 (C-4′), 105.0 (C-1′), 118.9 (C-5), 119.1 (C-3), 127.7 [CH(Ph)], 128.0 [CH(Ph)], 128.2 [CH(Ph)], 128.3 [CH(Ph)], 128.6 [CH(Ph)], 136.7 [C(Ph)], 136.9 [C(Ph)], 137.6 [C(Ph)], 145.6 (C-4), 146.0 (C-6), 158.4 (C-2), 169.2 (C=O). Anal. Calcd. For C35H38N2O7: C: 70.21, H: 6.40, N: 4.68. Found: C: 70.10, H: 6.62, N: 4.63.
17The excess n-butyllithium (two equivalents) required for the lithiation of 1, induces the formation of an anion on the acetamide’s methyl group, which probably attacks the ribonolactone to provide 4.
18The use of acid labile protecting groups for the 5-OH of the lactone component (e.g. TBDMS) should be avoided, since the corresponding 1′,5′-anhydro derivative is obtained from this reaction as the major product, irrespective of the reaction conditions or of the Lewis acid used for catalysis.
20Preparation of 6: To a solution of the anomers 3 (0.5 g, 0.84 mmol) in dry CH2Cl2 (20 ml) at 0 °C was added under argon BF3Et2O (0.22 ml, 1.68 mmol). The solution was stirred at 5-10 °C for 5 h and then, was neutralized with a saturated NaHCO3 solution. The mixture was extracted with CH2Cl2 and the combined organic extracts were dried (Na2SO4) and concentrated to dryness. The residue was purified by flash chromatography (silica gel, 18 × 1 cm) using EtOAc as the eluent to give 0.46 g (95%) of an inseparable E/Z mixture of 5. This mixture was dissolved in absolute EtOH (20 mL) and hydrogenated (10% Pd/C, 90 mg) at 50 psi for 5 h. The catalyst was filtered off, washed with EtOH, the solvent was vacuum-evaporated and the residue was purified by column chromatography (CH2Cl2-MeOH, 97:3, silica gel, 18 × 1 cm), to give 6, together with the corresponding α-anomer(7). 3-Acetamido-2-methoxy-4-[(β-d-ribofuranosyl)methyl]py-ridine(6): White foam. 250 mg (29%). 1 NMR (400 MHz, CDCl3) δ 1.98 (s, 3 H, COCH3), 2.53 (dd, 1 H, J = 9.15 Hz, 14.64 Hz, pyCH2), 2.78 (dd, 1 H, J = 4.03 Hz, 14.64 Hz, pyCH2), 3.36 (d, 1 H, J4 ′ ,5 ′ = 5.12 Hz, J5 ′ ,5 ′ = 11.71 Hz, H-5′), 3.41 (d, 1 H, J4 ′ ,5 ′ = 4.03 Hz, J5 ′ ,5 ′ = 11.71 Hz, H-5′), 3.59 (m, 2 H, H-2′, H-4′), 3.81 (m, 5 H, H-1′, H-3′, OCH3), 4.88 (m, 1 H, OH, D2O exchangeable), 5.07 (m, 2 H, 2 × OH, D2O exchangeable), 6.96 (d, 1 H, J5,6 = 5.12 Hz, H-5), 7.93 (d, 1 H, J6,5 = 5.12 Hz, H-6), 9.21 (br s, 1 H, NHAc, D2O exchangeable). 13C NMR (50 MHz, CDCl3) δ 22.6 (CH3-CO), 36.0 (pyCH2), 53.6 (OCH3), 61.6 (C-5′), 70.8 (C-3′), 74.62 (C-2′), 81.45 (C-4′), 84.32 (C-1′), 118.7 (C-5), 120.1 (C-3), 143.9 (C-6), 147.6 (C-4), 159.7 (C-2), 169.0 (C=O). Anal. Calcd. for C14H20N2O6: C: 53.84, H: 6.45, N: 8.97. Found: C: 53.69, H: 6.17, N: 8.76. 3-Acetamido-2-methoxy-4-[(α-d-ribofuranosyl)methyl]pyridine(7): Mp: 187 °C (MeOH). 490 mg (57%). 1 NMR (400 MHz, CDCl3) δ 2.02 (s, 3 H, COCH3), 2.72 (dd, 1 H, J = 8.05 Hz, 14.27 Hz, pyCH2), 2.74 (dd, 1 H, J = 5.12 Hz, 14.27 Hz, pyCH2), 3.32 (d, 1 H, J4 ′ ,5 ′ = 5.13 Hz, J5 ′ ,5 ′ = 12.08 Hz, H-5′), 3.48 (d, 1 H, J4 ′ ,5 ′ = 2.20 Hz, J5 ′ ,5 ′ = 12.08 Hz, H-5′), 3.67 (m, 1 H, H-4′), 3.78 (m, 1 H, H-2′), 3.81 (s, 3 H, OCH3), 3.88 (m, 1 H, H-3′), 4.04 (m, 1 H, H-1′), 4.59 (m, 1 H, OH, D2O exchangeable), 4.84 (m, 2 H, 2 × OH, D2O exchangeable), 6.98 (d, 1 H, J5,6 = 5.12 Hz, H-5), 7.92 (d, 1 H, J6,5 = 5.12 Hz, H-6), 9.28 (br s, 1 H, NHAc, D2O exchangeable). 13C NMR (50 MHz, CDCl3) δ 22.7 (CH3CO), 31.3 (pyCH2), 53.3 (OCH3), 61.7 (C-5′), 71.9 (C-3′), 72.1 (C-2′), 79.1 (C-4′), 81.9 (C-1′), 118.7 (C-5), 120.0 (C-3), 143.5 (C-6), 148.0 (C-4), 159.5 (C-2), 168.5 (C=O). Anal. Calcd. for C14H20N2O6: C: 53.84, H: 6.45, N: 8.97. Found: C: 53.55, H: 6.39, N: 9.11.
21Data of 12: Yield: 90%. Mp: 253 °C (dec.) (EtOH). UV (CH3OH) λmax(nm) (ɛ × 10-3): 302 (8.17), 251 (6.62). 1 NMR (400 MHz, DMSO-d 6) δ 3.48 (m, 2 H, H-5′), 3.81 (m, 1 H, H-4′), 3.95 (m, 1 H, H-3′), 4.12 (m, 1 H, H-2′), 4.81 (d, 1 H, J1 ′,2 ′ = 6.95 Hz, H-1′), 4.82 (br s, 1 H, D2O exchangeable, OH-5′), 4.89 (br. s., 1 H, D2O exchangeable, OH-3′), 4.98 (br. s., 1 H, D2O exchangeable, OH-2′), 6.68 (d, 1 H, J4,5 = 6.22 Hz, H-4), 6.88 (d, 1 H, J5,4 = 6.22 Hz, H-5), 11.2 (br s, 1 H, D2O exchangeable, NH-6), 13.8 (br. s., 1 H, D2O exchangeable, NH-1). 13C NMR (50 MHz, DMSO-d 6) δ 62.2 (C-5′), 71.5 (C-3′), 75.1 (C-2′), 79.5 (C-1′), 85.5 (C-4′), 99.3 (C-4), 123.4 (C-3α), 132.8 (C-7α), 125.4 (C-5), 145.8 (C-3), 154.3 (C-7). Anal. Calcd. for C11H13N3O5: C: 49.44, H: 4.90, N: 15.72. Found: C: 49.60, H: 5.12, N: 15.51.