The Synthesis of a Novel C-Nucleoside Designed as Guanosine Analogue

Abstract

The syntheses of a novel C-nucleoside which can be viewed as 8-aza-3,9-dideazaguanosine, as well as of the corresponding heterocyclic base, are described. N-[4-(2,3,5-tri-O-Acetyl-β-d-ribofuranosylmethyl)-2-methoxypyridin-3-yl]acetamide was regiospecifically nitrated and upon reduction and protection of the amino group underwent ring closure to the corresponding pyrazolopyridine derivative. The guanosine analogue was obtained via successive cleavage of the protecting groups.

Key words

C-nucleosides - heterocycles - pyrazolo[3,4-c]pyridine - guanosine - lithiation - ring annulation - proton-deuterium exchange

References and Notes

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10

Preparation of 1-Acetyl-5-pthalimido-7-methoxy-1 H -pyrazolo[3,4- c ]pyridine (7)
Potassium acetate (77 mg, 0.78 mmol) and Ac₂O (0.15 mL, 1.56 mmol) were added under argon to a solution of the acetamide 6 (170 mg, 0.52 mmol) in dry benzene (40 mL). The reaction mixture was heated at 80 ˚C, isoamyl nitrite (0.07 mL, 0.52 mmol) was added, and the resulting mixture was refluxed for 10 h. The insoluble material was then filtered off, the solvent was vacuum evaporated, and the residue was purified by column chromatography (silica gel) using a mixture of cyclohexane-EtOAc (60:40, v/v) as the eluent to give 7 as a white solid (153 mg, 87%); mp >300 ˚C (EtOH). ^¹H NMR (400 MHz, CDCl₃): δ = 2.84 (s, 3 H, COCH₃), 4.13 (s, 3 H, OCH₃), 7.82 (m, 3 H, H-4, H-4′,
H-5′), 7.98 (m, 2 H, H-3′, H-6′), 8.17 (s, 1 H, H-3). ^¹³C NMR (50 MHz, CDCl₃): δ = 23.9 (CH₃CO), 54.8 (OCH₃), 106.3 (C-4), 123.9 (C-3′, C-6′), 124.7 (C-3a), 131.8 (C-2a′, C-6a′), 134.5 (C-4′, C-5′), 135.6 (C-7a), 138.1 (C-3), 145.5 (C-5), 151.2 (C-7), 166.9 [CO(Phth)], 168.5 (COCH₃). Anal. Calcd for C₁₇H₁₂N₄O₄: C, 60.71; H, 3.60; N, 16.66. Found: C, 60.82; H, 3.45; N, 16.88.

11

Preparation of 7-Methoxy-1 H -pyrazolo[3,4- c ]pyridin-5-amine (8)
Compound 7 (120 mg, 0.73 mmol) was dissolved in a sat. solution of NH₃ in MeOH. The solution was stirred at r.t. for 4 h, the solvent was vacuum evaporated, and the residue was purified by column chromatography (silica gel) using a mixture of cyclohexane-EtOAc (20:80, v/v) as the eluent to give 8 (54 mg, 92%) as white crystals; mp 162-164 ˚C (EtOH). ^¹H NMR (400 MHz, CDCl₃): δ = 4.07 (s, 3 H, OCH₃), 5.20 (br s, 2 H, NH₂, D₂O exch.), 6.29 (s, 1 H, H-4), 7.82 (s, 1 H, H-3). ^¹³C NMR (50 MHz, CDCl₃): δ = 53.3 (OCH₃), 86.8 (C-4), 122.9 (C-7a), 132.0 (C-3a), 132.7 (C-3), 149.1 (C-5), 149.6 (C-7). Anal. Calcd for C₇H₈N₄O: C, 51.21; H, 4.91; N, 34.13. Found: C, 51.43; H, 4.80; N, 34.26.

13

Preparation of 5-Amino-1 H -pyrazolo[3,4- c ]pyridin-7 (6H )-one (9)
Sodium iodide (81 mg, 0.54 mmol) and TMSCl (68 µL, 0.54 mmol) were added under argon to a solution of 8 (85 mg, 0.52 mmol) in dry MeCN (5 mL). The resulting mixture was heated at 65 ˚C for 3 h, the precipitate was filtered, washed with EtOAc, and it was purified by column chromatography (silica gel) using a mixture of EtOAc-MeOH (98:2, v/v) as the eluent to give 9 (60 mg, 77%); mp >300 ˚C (EtOH). ^¹H NMR (400 MHz, DMSO-d ₆): δ = 5.09 (br s, 2 H, NH₂, D₂O exch.), 5.40 (s, 1 H, H-4, D₂O exch.), 7.54 (s, 1 H, H-3), 10.50 (br s, 1 H, N⁶H, D₂O exch.), 13.38 (br s, 1 H, N^¹H, D₂O exch.). Anal. Calcd for C₆H₆N₄O: C, 48.00; H, 4.03; N, 37.32. Found: C, 47.83; H, 3.95; N, 37.17.

14

Data for 7-Methoxy-3-(β- d -ribofuranosyl)-1 H -pyrazolo[3,4- c ]pyridin-5-amine (15)
Mp 216-218 ˚C (EtOH). ^¹H NMR (400 MHz, CD₃OD): δ = 3.72 (dd, 1 H, H-5′, J _{4 ′ ,5 ′} = 4.70 Hz, J _{5 ′ ,5 ′} = 12.13 Hz), 3.84 (dd, 1 H, H-5′, J _{4 ′ ,5 ′} = 3.52 Hz, J _{5 ′ ,5 ′} = 12.13 Hz), 4.01 (m, 1 H, H-4′), 4.04 (s, 3 H, OCH₃), 4.18 (m, 1 H, H-3′), 4.31 (m, 1 H, H-2′), 5.04 (d, 1 H, H-1′, J _{1 ′ ,2 ′} = 6.65 Hz), 6.46 (s, 1 H, H-4, D₂O exch.). ^¹³C NMR (50 MHz, CD₃OD): δ = 53.7 (CH₃O), 63.5 (C-5′), 72.7 (C-3′), 76.4 (C-2′), 80.4 (C-1′), 86.6 (C-4′), 87.9 (C-4), 124.0 (C-7a), 131.0 (C-3a), 143.2 (C-3), 150.7 (C-7). Anal. Calcd for C₁₂H₁₆N₄O₅: C, 48.65; H, 5.44; N, 18.91. Found: C, 48.45; H, 5.28; N, 18.83.

15

Data for 5-Amino-3-(β- d -ribofuranosyl)-1 H -pyrazolo[3,4- c ]pyridin-7 (6 H )-one (16)
Mp 158-160 ˚C (EtOH). ^¹H NMR (400 MHz, CD₃OD): δ = 3.72 (dd, 1 H, H-5′, J _{4 ′ ,5 ′} = 4.70 Hz, J _{5 ′ ,5 ′} = 12.13 Hz), 3.82 (dd, 1 H, H-5′, J _{4 ′ ,5 ′} = 3.52 Hz, J _{5 ′ ,5 ′} = 12.13 Hz), 3.99 (m, 1 H, H-4′), 4.16 (m, 1 H, H-3′), 4.26 (m, 1 H, H-2′), 4.97 (d, 1 H, H-1′, J _{1 ′ ,2 ′} = 6.26 Hz), 5.81 (s, 1 H, H-4, D₂O exch.). Anal. Calcd for C₁₁H₁₄N₄O₅: C, 46.81; H, 5.00; N, 19.85. Found: C, 46.75; H, 5.12; N, 19.97.

18

The AM1 calculations were performed in combination with RHF method and a convergence criterion of 0.01 kcal mol^-¹, using the Polak-Ribiere (conjugate gradient) geometry optimization method as implemented in the HyperChem 5.0 software (Hypercube Inc).