Diols as Building Blocks in Solid-Phase Synthesis of Polyamine Toxins by Fukuyama-Mitsunobu Alkylation

Abstract

Philanthotoxin-433 (PhTX-433) is a polyamine wasp toxin that antagonizes certain ionotropic receptors. In the present work three analogues of PhTX-433 were synthesized in good overall yields (27-31%) employing a Fukuyama-Mitsunobu strategy on solid phase. Primary diols were used as building blocks in solid-phase synthesis of polyamines for the first time. Furthermore, experiments with sterically congested alcohols indicated that incorporation of such structures by the solid-phase Fukuyama-Mitsunobu approach is not possible.

polyamines - philanthotoxins - Fukuyama-Mitsunobu alkylation - diols - solid-phase synthesis

References

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Experimental procedures and characterization of philanthotoxins (9a-c, Scheme [3] ): Functionalized trityl resin 4 (170 mg, 0.176 mmol) was suspended in THF (1 mL). Diol 3 (277 mg, 0.88 mmol, 5 equiv) in CH₂Cl₂ (1 mL), PMe₃ (0.95 mL, 0.95 mmol, 6 equiv; 1.0 M in THF) and ADDP (222 mg, 0.88 mmol, 5 equiv) in CH₂Cl₂ (1 mL) were added, and the mixture was shaken at r.t. under N₂ for 3 h. The resin was drained and washed with DMF, CH₂Cl₂, MeOH and CH₂Cl₂ (3 ×), and then the procedure was repeated two times, shaking for 16 h during the last repetition. The resin was then treated with TBAF (278 mg, 0.88 mmol, 5.0 equiv) in THF (3 mL) for 1 h at 50 °C, and washed with DMF, CH₂Cl₂, MeOH and CH₂Cl₂ (3 ×) to give resin 6. The resin 6 was then suspended in THF (1 mL), diamine 2 (368 mg, 0.88 mmol, 5 equiv) in CH₂Cl₂ (1 mL), PBu₃ (0.34 mL, 0.95 mmol, 6 equiv) in THF (1 mL) and ADDP (222 mg, 0.88 mmol, 5 equiv) in CH₂Cl₂ (1 mL) were added, and the mixture was shaken at r.t. under N₂ for 3 h. The resin was drained and washed with DMF, CH₂Cl₂, MeOH and CH₂Cl₂ (3 ×). The above procedure was repeated two times, shaking for 16 h during the last repetition. After washing, the resin was treated with TBAF (278 mg, 0.88 mmol, 5.0 equiv) in THF (3 mL) for 1 h at 50 °C, cooled to r.t., and washed with DMF, CH₂Cl₂, MeOH and CH₂Cl₂
(3 ×) to give resin 7. Attachment of the (S)-cyclohexyl-alanine residue was accomplished by suspending the resin in DMF (1 mL), and adding DIEA (80 µL, 0.46 mmol, 2.6 equiv), HODhbt (29 mg, 0.177 mmol, 1 equiv) in DMF (1 mL), and (S)-N ^α-Fmoc-Cha-OPfp (295 mg, 0.528 mmol, 3 equiv) in DMF (2 mL). The mixture was agitated at r.t. under N₂ for 2 h. The resin was drained and washed with DMF, CH₂Cl₂, MeOH and CH₂Cl₂ (3 ×), and then the coupling procedure was repeated. The resulting resin was treated with 20% piperidine in DMF (2 mL) for 10 min at r.t., washed with DMF (3 ×), treated again with 20% piperidine in DMF (2 mL) for 10 min, and washed with DMF, CH₂Cl₂, MeOH and CH₂Cl₂ (3 ×). After removal of the Fmoc group, the resin was suspended in DMF (1 mL) and acylated by treatment with DIEA (80 µL, 0.46 mmol, 2.6 equiv) and HODhbt (29 mg, 0.177 mmol, 1 equiv) in DMF (1 mL), and a carboxylic acid Pfp ester (3 equiv) in DMF (2 mL) under N₂ for 2 h. The resin was drained and washed with DMF, CH₂Cl₂, MeOH and CH₂Cl₂ (3 ×). The acylation procedure was repeated to give resin 8a, 8b or 8c. The resin was then treated with DBU (131 µL, 0.88 mmol, 5 equiv) in DMF (1 mL) and 2-mercaptoethanol (124 µL, 1.76 mmol, 10 equiv) in DMF (1 mL) for 2.5 h, and was then drained and washed with DMF (3 ×). This treatment was repeated for 10 min, and a colorless drain confirmed that deprotection was complete. The resin was washed with DMF, CH₂Cl₂, MeOH and CH₂Cl₂ (3 ×), and the product was cleaved off with TFA-CH₂Cl₂-triisopropylsilane-H₂O (47.5:47.5:2.5:2.5; 2 mL) at r.t. for 2 h. The filtrate was combined with the MeOH (2 × 5 mL) and CH₂Cl₂ (2 × 5 mL) washings, and concentrated in vacuo. The residue was purified by RP-18 VLC [H₂O-MeCN-TFA (95:5:0.1, 90:10:0.1, 85:15:0.1 and 80:20:0.1)], to give 9a-c.
Philanthotoxin 9a: Yield: 44 mg (31%). ¹H NMR (400 MHz, CD₃OD): δ = 0.77-1.32 (m, 6 H), 1.52-1.70 (m, 11 H), 2.08 (br p, J = 7.5 Hz, 4 H, 2 × CH₂), 2.90-3.28 (m, 12 H, CH₂N), 3.56 (d, J _gem = 14.2 Hz, 1 H, CH₂Ph), 3.59 (d, J _gem = 14.2 Hz, 1 H, CH₂Ph), 4.28 (m, 1 H, H-α), 7.24-7.36 (m, 5 H, Ar-H). ¹³C NMR (100 MHz, CD₃OD): δ = 26.6, 26.7, 27.9, 29.7, 29.8, 29.9, 30.0, 35.6, 37.4, 37.9, 40.3, 41.7, 42.8, 46.1, 48.0, 48.5 (2 × C), 51.0, 55.6, 130.5, 132.2
(2 × C), 132.7 (2 × C), 139.6, 176.9, 177.9. HRMS: m/z calcd [C₂₇H₄₈N₅O₂]⁺: 474.38025; found: 474.38006.
Philanthotoxin 9b: Yield: 41 mg (28%). ¹H NMR (400 MHz, CD₃OD): δ = 0.86-1.39 (m, 10 H), 1.54-1.73 (m, 18 H), 2.11 (m, 6 H, 3 × CH₂), 2.89-3.24 (m, 12 H, CH₂N), 4.31 (m, 1 H, H-α). ¹³C NMR (100 MHz, CD₃OD): δ = 24.4, 24.6, 25.7, 27.5 (2 × C), 27.6 (4 × C), 27.8 (2 × C), 33.5, 34.3, 34.5, 35.2, 35.7, 37.3, 38.0, 39.5, 40.5, 45.1, 45.9, 46.3 (2 × C), 53.0, 175.8, 176.0. HRMS: m/z calcd [C₂₇H₅₄N₅O₂]⁺: 480.42720; found: 480.42686.
Philanthotoxin 9c: Yield: 37 mg (27%). ¹H NMR (400 MHz, CD₃OD): δ = 0.95 (t, J = 7.4 Hz, 3 H, CH₃), 0.86-1.43 (m, 6 H), 1.52-1.87 (m, 13 H), 2.05-2.19 (br m, 4 H), 2.24 (m, 2 H, CH₂CO), 2.85-3.45 (m, 12 H), 4.24 (t, J = 7.7 Hz, 1 H, H-α). ¹³C NMR (100 MHz, CD₃OD): δ = 15.0, 21.3, 25.2 (2 × C), 26.4, 28.2, 28.4, 28.5, 34.3 (2 × C), 35.9 (2 × C), 36.5, 37.8, 38.8, 39.7, 40.2, 41.3, 46.6, 47.0, 53.9, 176.6, 177.4. HRMS: m/z calcd [C₂₃H₄₈N₅O₂]⁺: 426.38025; found: 426.37984.

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