A Simple Synthesis of Imide-Dipeptides

Abstract

We report a simple approach to synthesize a new class of imide-dipeptides. This approach is mild, and the starting materials are the easily accessible amino acid derivatives. The imide-dipeptides were synthesized from the coupling of an amide and a 4-nitrophenyl activating ester of an amino acid. The acidic proton of the first amide was removed by 1 equivalent of n-butyl lithium to afford the corresponding amide anion which then reacted with the activating ester to give the desired imide derivatives.

Key words

imides - dipeptides - amino acids - amides - coupling

References and Notes

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13

A Typical Procedure to Synthesize the Imide-dipeptides
The amide (1.1 mmol) of an N-Boc-l-amino acid was dissolved in distilled THF (10 mL). Then, n-BuLi (1.1 equiv, 1.5 mM in hexane) was added at -76 ˚C. One hour later, the reaction mixture was injected into 4-nitrophenyl N-Boc-l-amino acid ester (1.0 mmol) in distilled THF (10 mL) and stirred overnight at r.t. The mixture was then refluxed for another 3 h. After removal of solvents, the residue was applied to chromatography to yield desired imide-dipeptides as a white solid.
Imide-dipeptide (1a)
After removal of solvents, the residue was applied to chromatography by using PE-EtOAc (4:1) as eluents to yield 1a as a white solid (200 mg, 0.56 mmol, 56%, R _f = 0.2). ^¹H NMR (400 MHz, CDCl₃, 16.7 mM): δ = 9.21 (s, 1 H, imide NH), 5.02 (d, 2 H, carbamate NH, ^³ J = 6.7 Hz), 4.57-4.52 (m, 2 H, α-proton, ^³ J _{HN α}= 8.0 Hz), 1.45 (s, 18 H, Boc H), 1.27 (t, 6 H, β-H of Ala, ^³ J = 6.8 Hz) ppm. ^¹³C NMR (100 MHz, CDCl₃): δ = 171.9, 154.5, 98.6, 79.6, 27.2, 16.5 ppm. ESI-MS: m/z = 359, 382 [+ Na⁺].
Imide-dipeptide (1b) After removal of solvents, the residue was applied to chromatography by using PE-EtOAc (3:1) as eluents to yield 1b as a white solid (300 mg, 0.68 mmol, 68%, R _f = 0.2). ^¹H NMR (400 MHz, CDCl₃, 10 mM): δ = 9.02 (s, 1 H, imide NH), 4.89 (d, 2 H, carbamate NH, ^³ J = 7.7 Hz), 4.57 (br s, 2 H, α-protons), 1.74-1.60 (m, 4 H, β-H of Leu, ^³ J = 5-7 Hz), 1.47-1.40 (m, 2 H, γ-H of Leu, ^³ J = 4.8 Hz), 1.45 (s, 18 H, Boc H), 0.98-0.94 (q, 12 H, δ-H of Leu, ^³ J = 6.5, 5.2 Hz) ppm. ^¹³C NMR (100 MHz, CDCl₃): δ = 173.3, 156.0, 80.6, 54.0, 40.7, 28.4, 24.9, 23.3, 21.5 ppm. ESI-MS: m/z = 443, 466 [+ Na⁺].
Imide-dipeptide (1c) After removal of solvents, the residue was applied to chromatography by using PE-EtOAc (5:1) as eluents to yield 1c as a white solid (290 mg, 0.65 mmol, 65%, R _f = 0.2). ^¹H NMR (400 MHz, CDCl₃): δ = 9.13 (s, 1 H, imide NH), 7.19-7.16 (m, 6 H, phenyl H, ^³ J = 6.0 Hz), 7.01-7.00 (d, 4 H, phenyl H, ^³ J = 6.4 Hz), 4.94 (d, 2 H, carbamate NH, ^³ J = 8.0 Hz), 4.57 (q, 2 H, α-protons, ^³ J = 8.2 Hz), 3.01 (d, 4 H, BnCH₂, ^³ J = 6.0 Hz), 1.45 (s, 18 H, Boc H) ppm. ^¹³C NMR (100 MHz, CDCl₃): δ = 174.0, 156.0, 139.5, 128.7, 127.6, 126.2, 80.6, 54.5, 37.8, 28.4 ppm. ESI-MS: m/z = 511, 534 [+ Na⁺].
Imide-dipeptide (1d) After removal of solvents, the residue was applied to chromatography by using PE-EtOAc (3:1) as eluents to yield 1d as a white solid (400 mg, 0.54 mmol, 54%, R _f = 0.2). ^¹H NMR (400 MHz, CDCl₃): δ = 8.96 (s, 1 H, imide NH), 7.35-7.26 (m, 10 H, phenyl H, ^³ J = 6.0 Hz), 5.15 (s, 2 H, Cbz NH), 5.06 (s, 4 H, BnCH₂), 4.95 (s, 2 H, carbamate NH), 4.30 (m, 2 H, α-protons, ^³ J = 8.0 Hz), 3.11 (m, 4 H, CH₂, ^³ J = 6.0 Hz), 1.79 (br s, 2 H, CH₂), 1.65 (m, 2 H, CH₂, ^³ J = 7.9 Hz), 1.47-1.32 (m, 8 H, CH₂, ^³ J = 7.4 Hz), 1.45 (s, 18 H, Boc H) ppm. ^¹³C NMR (100 MHz, CDCl₃): δ = 173.9, 156.0, 141.1, 128.6, 128.2, 80.6, 66.6, 53.4, 41.0, 32.0, 29.4, 28.4, 22.5 ppm. ESI-MS: m/z = 741, 764 [+ Na⁺].
Imide-dipeptide (1e) After removal of solvents, the residue was applied to chromatography by using PE-EtOAc (4:1) as eluents to yield 1e as a white solid (430 mg, 0.68 mmol, 68%, R _f = 0.2). ^¹H NMR (400 MHz, CDCl₃, 10 mM): δ = 9.09 (s, 1 H, imide NH), 7.29-7.13 (m, 10 H, phenyl H), 5.22 (s, 1 H, Cbz NH), 5.06 (m, 1 H, carbamate NH, overlapping, ^³ J = 7.2 Hz), 5.03 (s, 2 H, BnCH₂), 4.91 (s, 1 H, carbamate NH), 4.73 and 4.67 (br s, 1 H, α-protons), 4.46 (br s, 1 H, α-protons), 3.11 (s, 2 H, CH₂), 3.11 (br s, 1 H, BnCH₂, overlapping), 2.85 (br s, 1 H, BnCH₂), 1.74 (br s, 2 H, CH₂), 1.47 (m, 4 H, CH₂, ^³ J = 6.4 Hz), 1.29 and 1.20 (s, 18 H, Boc H) ppm.^¹³C NMR (100 MHz, CDCl₃): δ = 172.7, 171.9, 156.7, 155.9, 155.4, 136.5, 135.8, 129.4, 128.6, 128.5, 128.1, 127.1, 80.4, 66.7, 56.3, 55.1, 40.1, 37.5, 31.0, 29.2, 28.3, 22.4 ppm. ESI-MS: m/z = 626, 749 [+ Na⁺].
Imide-dipeptide (1f) After removal of solvents, the residue was applied to chromatography by using PE-EtOAc (5:1) as eluents to yield 1f as a white solid (290 mg and 310 mg, 0.56 mmol and 0.60 mmol, 56% and 60%, R _f = 0.2). ^¹H NMR (400 MHz, CDCl₃): δ = 8.89-8.82 (d, 1 H, imide NH), 7.35-7.17 (m, 10 H, phenyl H), 5.29 (s, 1 H, Cbz NH), 5.05 (s, 2 H, BnCH₂), 5.00 (d, 1 H, carbamate NH, ^³ J = 8.4 Hz), 5.00 (m, 1 H, α-protons, overlapping), 4.57 (m, 1 H, α-protons, ^³ J = 8.2 Hz), 3.11 (m, 1 H, BnCH₂), 2.85 (m, 1 H, BnCH₂), 1.74-1.60 (m, 2 H, β-H of Leu, ^³ J = 5-7 Hz), 1.47-1.40 (m, 1 H, γ-H of Leu, ^³ J = 4.8 Hz), 1.45 (s, 9 H, Boc H), 0.98-0.88 (q, 6 H,
δ-H of Leu, ^³ J = 6.5, 5.2 Hz) ppm. ^¹³C NMR (100 MHz, CDCl₃): δ = 174.0, 173.3, 156.2, 156.0, 141.1, 140.3, 129.5, 128.7, 128.6, 128.5, 128.2, 127.1, 80.6, 66.0, 54.9, 54.0, 40.7, 37.8, 28.4, 24.9, 23.3, 22.2 ppm. ESI-MS: m/z = 511, 534 [+ Na⁺].

14

Typical Procedure for the Synthesis of 4-Nitrophenyl N -Boc- l -alanine Ester and N -Boc- l -alanine Amide
N -Boc- l -alanine Acid
l-Alanine (4.5g, 50 mmol) and KOH (3.8 g, 55 mmol) were dissolved in a mixture of H₂O (200 mL) and THF (20 mL). Then, di-tert-butyl-dicarbonate (13.0 g, 55 mmol) was added. The resultant solution was allowed to react at 50 ˚C for 2-3 h and at r.t. overnight. Then, 1 M HCl (55 mL) was added dropwise to adjust pH value of the solution to about 5. The solution was extracted with EtOAc (3 × 200 mL) and CH₂Cl₂ (3 × 200 mL), respectively. The organic phases were mixed and dried with anhyd Na₂SO₄. The reaction afforded N-Boc-l-alanine acid as an oil-like solid (9 g, 47 mmol, 95%). ^¹H NMR (400 MHz, CDCl₃): δ = 10.66 (br s, 1 H, acid H), 5.08 (s, 1 H, carbamate H), 4.35 (s, 1 H, α-proton), 1.53 (d, 3 H, CH₃, ^³ J = 7.6 Hz), 1.45 (s, 9 H, Boc H) ppm. ESI-MS: 188 [- H⁺], 189, 211 [- H⁺ and + Na⁺], 227 [- H⁺ and
+ K⁺].
4-Nitrophenyl N -Boc- l -alanine Ester N-Boc-l-alanine acid (1.9 g, 10 mmol), 4-nitrophenol (1.7 g, 12 mmol), and DCC (2.8 g, 12 mmol) were mixed with CH₂Cl₂ (100 mL). After stirred overnight at r.t., the precipitate was filtered out, and the solvents of the filtration were removed. The resultant residue was applied to flash chromatography with PE-CH₂Cl₂-EtOAc (10:2:1) as eluents to afford 4-nitrophenyl N-Boc-l-alanine ester as a light yellow solid (2.6 g, 8.4 mmol, 85%, R _f = 0.2). ^¹H NMR (400 MHz, CDCl₃): δ = 8.30 (d, 2 H, phenyl H, ^³ J = 9.0 Hz), 7.32 (d, 2 H, phenyl H, ^³ J = 9.0 Hz), 5.11 (s, 1 H, carbamate H), 4.54 (t, 1 H, α-proton, ^³ J = 7.1, 6.5 Hz), 1.58 (d, 3 H, CH₃, ^³ J = 7.4 Hz), 1.47 (s, 9 H, Boc H). ^¹³C NMR (100 MHz, CDCl₃): δ = 171.4, 155.3, 155.2, 145.6, 125.4, 122.4, 80.5, 49.7, 28.4, 18.1. ESI-MS: m/z = 333 [+ Na⁺].
N - tert -Butoxycarbonyl- l -alanine Amide 4-Nitrophenyl N-Boc-l-alanine ester (2.40 g, 7.7 mmol) was dissolved in MeCN (30 mL) and bubbled by NH₃ flow for
4-5 h. After removal of MeCN, the reacted residue was purified with chromatography using PE-CH₂Cl₂-EtOAc (1:1:1) as eluents to afford N-tert-butoxycarbonyl-l-alanine amide as white solid (1.4 g, 7.4 mmol, 96%, R _f = 0.1). ^¹H NMR (400 MHz, CDCl₃): δ = 6.04 (s, 1 H, amide H), 5.32 (s, 1 H, amide H), 4.89 (s, 1 H, carbamate H), 4.10 (s, 1 H, α-proton), 1.36 (s, 9 H, Boc H), 1.30 (d, 3 H, CH₃, ^³ J = 7.2) ppm. ^¹³C NMR (100 MHz, CDCl₃): δ = 171.7, 155.5, 80.3, 49.9, 28.3, 18.2 ppm. ESI-MS: m/z = 211 [+ Na⁺], 227 [+ K⁺].