Synthesis of 1-Cyclobutyl-4-(3-isothiocyanatophenyl)piperazine

The syntheses of final compounds 10–12, 20–23, 34–37 and 43 are described in the Supplementary Materials.
Tert-butyl 4-(3-nitrophenyl)piperazine-1-carboxylate (39). A solution of bromide 38 (808 mg, 4.00 mmol) and NaOtBu (461 mg, 4.80 mmol) in dioxane (15 mL) was degassed with N₂ for 10 min. Next, Pd₂(dba)₃ (183 mg, 0.20 mmol), Xantphos (347 mg, 0.60 mmol) and tert-butyl piperazine-1-carboxylate (1.12 g, 6.00 mmol) were added. The reaction mixture was heated for 1 h at 80 °C under microwave irradiation. The reaction mixture was diluted with water (40 mL) and extracted with DCM (3 × 30 mL). The combined organic layers were dried over Na₂SO₄, filtered and concentrated in vacuo. Purification by flash chromatography (heptane:EtOAc 5:0 to 4:1) gave the title compound as an orange solid (700 mg, 57%). ¹H NMR (500 MHz, CDCl₃) δ 7.72 (t, J. = 2.2 Hz, 1H), 7.71–7.67 (m, 1H), 7.39 (t, J. = 8.2 Hz, 1H), 7.20 (dd, J. = 8.3, 2.1 Hz, 1H), 3.61 (t, J. = 5.2 Hz, 4H), 3.24 (t, J. = 5.1 Hz, 4H), 1.49 (s, 9H). HPLC-MS (acidic mode): t_R = 5.3 min, purity: 98.6%, [M + H]⁺: 308.
1-(3-Nitrophenyl)piperazine (40). To a solution of carbamate 39 (1.20 g, 3.90 mmol) in dioxane (20 mL) was added HCl in dioxane (4N, 9.75 mL, 39.0 mmol). The reaction mixture was stirred for 1 h at rt. The solvent was removed under reduced pressure. The residue was mixed with satd. aq. Na₂CO₃ (40 mL) and extracted with DCM (3 × 30 mL). The combined organic layers were dried over Na₂SO₄, filtered and concentrated in vacuo. The title compound was obtained as a brown solid (633 mg, 78%). ¹H NMR (500 MHz, CDCl₃) δ 7.71 (s, 1H), 7.66 (d, J. = 8.0 Hz, 1H), 7.37 (t, J. = 8.2 Hz, 1H), 7.19 (d, J. = 8.3 Hz, 1H), 3.28–3.21 (m, 4H), 3.10–3.02 (m, 4H). HPLC-MS (acidic mode): t_R = 2.6 min, purity: 97.3%, [M + H]⁺: 208.
1-Cyclobutyl-4-(3-nitrophenyl)piperazine (41). To a solution of amine 40 (630 mg, 3.04 mmol) in DCM (20 mL) was added cyclobutanone (273 μL, 3.66 mmol). After 10 min of stirring at rt, NaBH(OAc)₃ (966 mg, 4.56 mmol) was added and the resulting mixture was stirred at rt overnight. The reaction mixture was quenched with satd. aq. Na₂CO₃ (30 mL) and extracted with DCM (3 × 15 mL). The combined organic phases were dried over Na₂SO₄, filtered and concentrated in vacuo. Purification by flash chromatography (DCM:MeOH 20:0 to 19:1) gave the title compound as a yellow oil (600 mg, 76%). ¹H NMR (500 MHz, CDCl₃) δ 7.71 (t, J. = 2.3 Hz, 1H), 7.65 (dd, J. = 8.1, 1.5 Hz, 1H), 7.37 (t, J. = 8.2 Hz, 1H), 7.18 (dd, J. = 8.3, 2.1 Hz, 1H), 3.38–3.23 (m, 4H), 2.80 (p, J. = 7.9 Hz, 1H), 2.51 (t, J. = 5.1 Hz, 4H), 2.13–2.04 (m, 2H), 2.00–1.87 (m, 2H), 1.81–1.57 (m, 2H, overlaps with residual water). HPLC-MS (acidic mode): t_R = 2.8 min, purity: 97.8%, [M + H]⁺: 262.
3-(4-Cyclobutylpiperazin-1-yl)aniline (42). To a solution of nitrocompound 41 (59 mg, 0.23 mmol) in MeOH (2 mL) was added HCOONH₄ (71 mg, 1.13 mmol) as a solid followed by a suspension of Pd/C (10%, 24 mg) in water (2 mL). The reaction mixture was stirred at rt overnight. The mixture was filtered over Celite and the filtrate was concentrated in vacuo. The residue was diluted with aq. Na₂CO₃ (1.0 M, 10 mL) and extracted with DCM (3 × 5 mL). The combined organic layers were dried over Na₂SO₄, filtered and concentrated in vacuo. The crude product was purified by flash chromatography (DCM:MeOH 20:0 to 19:1). The selected fractions were collected and the solvents were evaporated. The residue was dissolved in aq. HCl (1.0 M, 10 mL), washed with EtOAc (2 × 5 mL) and c-hexane (3 × 10 mL). The pH of the aqueous layer was adjusted to 10 with satd. aq. Na₂CO₃ and extracted with EtOAc (3 × 10 mL). The combined organic phases were dried over Na₂SO₄, filtered and concentrated in vacuo. The title compound was obtained as an off-white solid (13 mg, 25%). Mp: 92.5–92.7 °C. ¹H NMR (500 MHz, CDCl₃) δ 7.04 (t, J. = 8.0 Hz, 1H), 6.36 (dd, J. = 8.2, 2.3 Hz, 1H), 6.25 (t, J. = 2.3 Hz, 1H), 6.21 (dd, J. = 7.7, 2.0 Hz, 1H), 3.59 (br, 2H), 3.26–3.10 (m, 4H), 2.78 (p, J. = 7.9 Hz, 1H), 2.56–2.37 (m, 4H), 2.12–2.02 (m, 2H), 2.00–1.88 (m, 2H), 1.80–1.64 (m, 2H). ¹³C NMR (126 MHz, CDCl₃) δ 152.6, 147.4, 130.0, 107.1, 107.1, 103.0, 60.4, 49.6, 48.8, 27.1, 14.4. HPLC-MS (basic mode): t_R = 4.1 min, purity: 98.4%, [M + H]⁺: 232. HR-MS [M + H]⁺ calcd for C₁₄H₂₂N₃⁺: 232.1808, found 232.1818.
1-Cyclobutyl-4-(3-isothiocyanatophenyl)piperazine (44). To an ice-cold mixture of aniline 42 (46 mg, 0.20 mmol) in DCM (2 mL) and aq. NaHCO₃ (1.0 M, 30 mL) was added dropwise a solution of CSCl₂ (18 μL, 0.24 mmol) in DCM (1 mL). The reaction mixture was stirred for 1 h at rt. The reaction mixture was diluted with water (10 mL) and extracted with DCM (3 × 5 mL). The combined organic layers were dried over Na₂SO₄, filtered and concentrated in vacuo. Purification by flash chromatography (DCM:MeOH 20:0 to 19:1) gave the title compound as a white solid (35 mg, 64%). The compound is stable as a solid in the freezer (−20 °C) over a period of at least 2 years as judged by NMR and LC-MS analysis. Mp: 79.7–80.1°C. ¹H NMR (500 MHz, CDCl₃) δ 7.18 (t, J. = 8.1 Hz, 1H), 6.81 (dd, J. = 8.5, 2.4 Hz, 1H), 6.72–6.65 (m, 2H), 3.23–3.15 (m, 4H), 2.77 (p, J. = 7.9 Hz, 1H), 2.50–2.42 (m, 4H), 2.11–2.02 (m, 2H), 1.96–1.86 (m, 2H), 1.79–1.64 (m, 2H). ¹³C NMR (126 MHz, CDCl₃) δ 152.2, 134.2, 131.9, 130.0, 116.5, 114.9, 112.6, 60.3, 49.3, 48.3, 27.1, 14.4. HPLC-MS (acidic mode): t_R = 3.6 min, purity: 98.9%, [M + H]⁺: 274. HR-MS [M + H]⁺ calcd for C₁₅H₂₀N₃S⁺: 274.1372, found 274.1374.