1-Hydroxybenzotriazole Monohydrate

Used Chemicals

• N-Benzyloxycarbonyl-L-phenylalanine [C0660]
• 1-(2-Methoxyphenyl)piperazine [M0883]
• 1-Hydroxybenzotriazole Monohydrate (= HOBt·H₂O) [H0468]
• 1-(3-Dimethylaminopropyl)-3-ethylcarbodiimide Hydrochloride (= EDC·HCl) [D1601]
• DMF

Procedure

1-(2-methoxyphenyl)piperazine (500 mg, 2.60 mmol) was dissolved in DMF (15 mL). To this solution was added N-benzyloxycarbonyl-L-phenylalanine (770 mg, 2.60 mmol) and HOBt·H₂O (414 mg, 2.71 mmol). The reaction mixture was cooled to 0 °C, then EDC·HCl (515 mg, 2.71 mmol) was added. Reaction was allowed to warm to room temperature, then stirred for 20 h at room temperature. Complete of the reaction was confirmed by TLC. Reaction was quenched by adding H₂O (20 mL). Reaction mixture was extracted by EtOAc (30 mL). Organic phase was washed with H₂O (20 mL, twice) and brine (20 mL), dried over Na₂SO₄, concentrated under reduced pressure to afford the crude product (1.2 g) as pale yellow oil. The crude product was purified by column chromatography on silica gel (hexane : ethyl acetate = 40 : 60) to afford the product 1 as a white powder (1.10 g, 2.32 mmol, 90%).

Experimenter's Comments

The reaction mixture was monitored by TLC (hexane / ethyl acetate = 4 : 6, Rf = 0.40)

Analytical Data（Compound 1）

¹H NMR (400 MHz, CDCl₃); δ 7.39-7.16 (m, 10H), 6.99 (m, 1H), 6.91-6.84 (m, 2H), 6.78 (m, 1H), 5.73 (d, J = 8,2 Hz, 1H), 5.12-5.01 (m, 2H), 4.91 (dd, J= 7.7, 7.3 Hz, 1H), 3.82 (s, 3H), 3.71-3.63 (m, 2H), 3.51 (m, 1H), 3.31 (m, 1H), 3.07-2.78 (m, 5H), 2.51 (m, 1H).

HOBt is widely used in condensing reaction as an activating agent.

Used Chemicals

• N-(tert-Butoxycarbonyl)-L-leucine Monohydrate [B1187]
• L-Phenylalanine Methyl Ester Hydrochloride [P1278]
• N,N'-Dicyclohexylcarbodiimide (= DCC) (Granulated) [D4876]
• N,N-Diisopropylethylamine [D1599]
• 1-Hydroxybenzotriazole Monohydrate (= HOBt·H₂O) [H0468]
• Acetonitrile

Procedure

To a solution of N-(tert-butoxycarbonyl)-L-leucine monohydrate (2.49 g, 10.0 mmol), L-phenylalanine methyl ester hydrochloride (2.16 g, 10.0 mmol), N,N-diisopropylethylamine (2.59 g, 20.0 mmol) and HOBt·H₂O (1.53 g, 10.0 mmol) in acetonitrile (46 mL) was added DCC (granulated, 2.06 g, 10.0 mmol) in acetonitrile (7 mL) at 0 °C and the mixture was stirred at 0 °C for 1 hour. The suspension was warmed at room temperature for 3 hours. The suspension was filtrated and the solvent was removed under reduced pressure. The residue was diluted with ethyl acetate (75 mL) and washed with 0.33 mol/L citric acid solution (50 mL), saturated aqueous sodium bicarbonate solution (50 mL, twice) and water (50 mL), dried over sodium sulfate and filtered. The solvent was removed under reduced pressure and the residue was purified by column chromatography (1 : 3 ethyl acetate / hexane on SiO₂), giving Boc-Leu-Phe-OMe as a white solid (3.42 g, 87%).

Experimenter’s Comments

The reaction mixture was monitored by TLC (ethyl acetate / hexane = 2 : 1, Rf = 0.38).
DCC was weighed promptly not to melt while being weighed.
To avoid a rash by DCC, appropriate protective equipment is used and the work was done in a fume hood.

Analytical Data（Boc-Leu-Phe-OMe）

¹H NMR (400 MHz, CDCl₃); δ 7.28–7.19 (m, 3H), 7.08 (brd, 2H, J = 7.2 Hz), 6.51 (d, 1H,J = 8.0 Hz), 4.85–4.80 (m, 2H), 4.12–4.07 (m, 1H), 3.68 (s, 3H), 3.15–3.03 (m, 2H), 1.66–1.57 (m, 2H), 1.41　(brs, 10H), 0.89 (t, 6H, J = 6.0 Hz).

¹³C NMR (101 MHz, CDCl₃); δ 172.1, 171.6, 155.4, 135.7, 129.2, 128.5, 127.1, 80.0, 53.1, 52.2, 41.2, 37.9, 28.2, 24.6, 22.8, 21.9.

Lead Reference

• 17α-Ethynylestradiol Peptide Labeling by ‘Click’ Chemistry
  • O. I. Bol'shakov, I. O. Lebedyeva, A. R. Katritzky, Synthesis 2012, 44, 2926.

Other References

• Synthesis of Natural and Unnatural Cyclooligomeric Depsipeptides Enabled by Flow Chemistry
  • D. Lücke, T. Dalton, S. V. Ley, Z. E. Wilson, Chem. Eur. J. 2016, 22, 4206.