Biomimetic Synthesis of Phenazine-1,6-dicarboxylic Acid (PDC)

Abstract

The biomimetic synthesis of the natural product phenazine-1,6-dicarboxylic acid (PDC) is reported. A 2-aminocyclohexanone resembling the proposed biosynthetic substrate underwent facile dimerization and oxidation in air to a tetrasubstituted pyrazine. Oxidation and saponification delivered the natural product PDC.

Key words

biomimetic synthesis - phenazine - alkaloid - natural product

• References and Notes

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The intermolecular dimerization of 2-aminocyclohexanone is known to yield octahydrophenazine, see:
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• 13 Diethyl Phenazine-1,6-dicarboxylate (11) To a solution of 6 (70 mg, 0.25 mmol) in CH₂Cl₂ (7.8 mL) was added TFA (0.8 mL), and the reaction mixture was stirred at r.t. for 3 h. The reaction mixture was concentrated in vacuo, the crude residue taken up in CH₂Cl₂ (9 mL), and the reaction mixture was stirred under air for a further 16 h. Concentration in vacuo gave the pyrazine 10 (ca. 30 mg) as a yellow gum which was used immediately in the next step. A small amount was purified by flash chromatography on silica gel eluting with hexanes–EtOAc (1:1) for characterization purposes. IR (neat): ν_max = 3296, 2934, 2870, 1730, 1248 cm^–1. ¹H NMR (400 MHz, CDCl₃): δ = 4.20 (4 H, q, J = 7.3 Hz, 2 × CH₂), 3.95–3.87 (2 H, m, 2 × CH), 3.02–2.81 (4 H, m, 2 × CH₂), 2.27–2.07 (4 H, m, 2 × CH₂), 2.07–1.80 (4 H, m, 2 × CH₂), 1.27 (6 H, t, J = 7.3 Hz, 2 × Me). ¹³C NMR (100 MHz, CDCl₃): δ = 173.7 (2 × C=O), 149.9 (2 × C), 147.6 (2 × C), 61.0 (2 × CH₂), 47.7 (2 × CH), 31.1 (2 × CH₂), 26.7 (2 × CH₂), 20.2 (2 × CH₂), 14.2 (2 × Me). ESI-MS: m/z (%) = 355 (100) [M + Na]⁺, 333 (16). ESI-HRMS: m/z calcd for [C₁₈H₂₄N₂O₄ + Na]⁺: 355.1628 [M + Na]⁺; found: 355.1639. The crude pyrazine 10 (ca. 30 mg) was taken up in xylenes (15 mL). Pd/C (10%; 500 mg) was added, and the reaction mixture heated to 200 °C for 70 h. The reaction mixture was filtered through Celite^®, concentrated in vacuo, and purified by flash chromatography on silica gel eluting with hexanes–EtOAc (3:2) to yield the title compound 11 (21 mg, 0.06 mmol, 53% from 6) as a yellow solid; mp 96.2–98.1 °C. IR (neat): ν_max = 2987, 2926, 2853, 1728, 1620, 1194 cm^–1. ¹H NMR (400 MHz, CDCl₃): δ = 8.45 (2 H, d, J = 8.8 Hz, 2 × H-2), 8.26 (2 H, d, J = 7.0 Hz, 2 × H-4), 7.88 (2 H, t, J = 8.0 Hz, 2 × H-3), 4.59 (4 H, q, J = 7.3 Hz, 2 × CH₂), 1.51 (6 H, t, J = 7.1 Hz, 2 × Me). ¹³C NMR (100 MHz, CDCl₃): δ = 166.4 (2 × C=O), 143.0 (2 × C), 141.0 (2 × C), 134.1 (2 × CH), 132.5 (2 × CH), 131.6 (2 × C), 129.5 (2 × CH), 61.7 (2 × CH₂), 14.4 (2 × Me). ESI-MS: m/z (%) = 347 (100) [M + Na]⁺, 325 (8). ESI-HRMS: m/z calcd for [C₁₈H₁₆N₂O₄ + Na]⁺: 347.1002 [M + Na]⁺; found: 347.0995.
• 14 Phenazine-1,6-dicarboxylic Acid (PDC, 1)^12b To a solution of KOt-Bu (532 mg, 4.74 mmol) in anhyd Et₂O (10 mL) at 0 °C was added deionized H₂O (0.025 mL, 1.39 mmol), and the slurry was stirred for 5 min. Phenazine 11 (26 mg, 0.08 mmol) was added, and the reaction mixture was stirred at r.t. for 20 h. Ice cold H₂O (50 mL) and Et₂O (50 mL) were added, and the aqueous layer was separated, acidified to pH 2 with HCl, and the resulting solid was filtered and kept. The aqueous layer was repeatedly extracted with CH₂Cl₂ (5×), and the combined organic extracts were dried (MgSO₄), filtered, and concentrated in vacuo. The crude solid was combined with the solid obtained from filtration of the acidified aqueous layer, then washed with H₂O (5 mL), MeOH (5 mL), and Et₂O (5 mL) to give the title compound 1 (18 mg, 0.07 mmol, 83%) as a green solid; mp >330 °C [lit.^12b >290 °C]. IR (neat): ν_max = 2917, 2624, 1727, 1614, 1389 cm^–1. ¹H NMR (400 MHz, CF₃CO₂D): δ = 9.34 (2 H, d, J = 7.3 Hz, 2 × CH), 9.03 (2 H, d, J = 9.0 Hz, 2 × CH), 8.56 (2 H, dd, J = 7.3, 9.0 Hz, 2 × CH), CO₂H not observed. ¹³C NMR (100 MHz, CF₃CO₂D): δ =170.8 (2 × C=O), 144.5 (2 × CH), 140.4 (2 × C), 140.4 (2 × C), 138.5 (2 × CH), 134.3 (2 × CH), 124.2 (2 × C). ESI-MS: m/z (%) = 267 (70) [M – H]^–, 241 (50), 223 (100), 178 (40). ESI-HRMS: m/z calcd for [C₁₄H₈N₂O₄ – H]^–: 267.0411 [M – H]^–; found: 267.0418.