Controlled Ring Opening of N-Sulfinyl- and N-Silyl-N-carboxyanhydrides (NCA): One-Pot Synthesis of Dipeptides and Unsymmetrical Peptidyl Ureas from Unprotected NCA

Abstract

We report herein new labile protecting groups of N-carboxyanhydrides (NCA) useful to prevent polymerization during coupling reactions with nitrogen nucleophiles. Thus, N-sulfinyl-NCA 1 and N-silyl-NCA 2 were prepared in situ and involved, without being isolated, in coupling reactions with various α-amino esters to furnish dipeptides 3 and unsymmetrical peptidyl ureas 4, respectively, in good yields.

Key words

N-protected carboxyanhydrides (NCA) - peptidyl ­ureas - dipeptides

References and Notes

• 1 For an excellent review on the main applications of NCA, see: Kricheldorf HR. Angew. Chem. Int. Ed.  2006,  45:  5752 
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  J. Am. Chem. Soc.  2004,  126:  8134 
• 13 The formation of ureas from the reaction of unprotected NCA with silylated amines has been reported previously and was explained by the formation of an isocyanate as intermediate: Kricheldorf HR. Greber G. Chem. Ber.  1971,  104:  3168 
• 14a Myers AC. Kowalski JA. Lipton MA. Bioorg. Med. Chem. Lett.  2004,  14:  5219 
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6

Val-NCA and Leu-NCA were generously gifted by ISOCHEM company.

8

After elimination of the triethylamine hydrochloride salt by filtration and evaporation of THF, the major diastereomer could be isolated by precipitation in Et₂O. Selected data of the major diastereomer: ^¹H NMR (300 MHz, CDCl₃): δ = 7.69 (m, 5 H), 3.77 (d, 1 H, J = 3.4 Hz), 2.45 (m, 1 H), 1.02 (d, 3 H, J = 6.8 Hz), 0.95 (d, 3 H, J = 7.1 Hz). ^¹³C NMR (75 MHz, CDCl₃): δ = 16.0, 17.9, 31.7, 65.8, 125.3, 130.5, 133.6, 139.7, 150.8, 165.2. IR (KBr): 1853, 1787 cm^-¹.

11

General Procedure for the One-Pot Synthesis of Dipeptides 3a-f To a solution of Val-NCA or Leu-NCA (7 mmol) in dry THF (20 mL) at 0 ˚C was added dropwise Et₃N (1 mL, 7 mmol) followed by tert-butylsulfinyl chloride (0.98g, 7 mmol). The resultant solution was stirred at this temperature for 5 h before adding the amino ester hydrochloride (7 mmol) and Et₃N (7 mmol). After stirring for 5 h at r.t., the solvent was evaporated under reduced pressure. Ethanol (20 mL) and dry HCl (4 M) in dioxane (3.5 mL) were added at 0 ˚C. The solution was stirred for 2 h at 0 ˚C and then concentrated under vacuum. Dipeptides 3a-f were obtained as white solids after precipitation from EtOH with Et₂O.
H-Val-Phe-OEt (3a): ^¹H NMR (300 MHz, CDCl₃): δ = 8.23 (d, 1 H, J = 7.1 Hz), 7.92 (s, 2 H), 6.92 (m, 5 H), 4.38 (m,
1 H), 3.78 (q, 2 H, J = 7.1 Hz), 3.43 (d, 1 H, J = 5.3 Hz), 2.73 (m, 2 H), 1.85 (m, 1 H), 0.86 (t, 3 H, J = 7.1 Hz), 0.66 (m, 6 H). ^¹³C NMR (75 MHz, CDCl₃): δ = 14.2, 17.7, 18.6, 30.4, 37.5, 54.1, 58.2, 61.3, 126.9, 128.6, 129.5, 136.8, 168.7, 171.2. IR (KBr) 1721, 1692 cm^-¹. HRMS: m/z calcd for C₁₆H₂₅N₂O₃ [MH⁺]: 293.1865; found: 293.1864.
[α]_D ^²0 +21.1 (c 0.75, EtOH).
H-Leu-Phe-OEt (3b): ^¹H NMR (300 MHz, CDCl₃): δ = 8.38 (d, 1 H, J = 7.5 Hz), 7.89 (s, 2 H), 6.77 (m, 5 H), 4.17 (m,
1 H), 3.61 (q, 2 H, J = 7.0 Hz), 3.40 (m, 1 H), 2.60 (m, 3.5 H), 1.19 (m, 3 H), 0.89 (t, 1.5 H, J = 7.4 Hz), 0.70 (t, 3 H, J = 7.0 Hz), 0.45 (m, 6 H). ^¹³C NMR (75 MHz, CDCl₃): δ = 8.7, 14.1, 22.2, 22.8, 24.0, 37.3, 45.9, 51.6, 54.2, 61.1, 126.8, 128.4, 129.4, 136.8, 169.4, 171.0. IR (KBr): 1732, 1667 cm^-¹.
H-Val-Thr-OEt (3c): ^¹H NMR (300 MHz, DMSO): δ = 11.12 (s, 1 H), 9.12 (d, 1 H, J = 6.6 Hz), 8.33 (s, 3 H), 7.61 (d, 1 H, J = 7.7 Hz), 7.47 (d, 1 H, J = 7.9 Hz), 7.19 (s, 1 H), 7.13 (m, 2 H), 4.65 (dd, 1 H, J = 6.6, 7.7 Hz), 4.51 (s, 1 H), 4.12 (q,
2 H, J = 7.2 Hz), 3.81 (s, 1 H), 3.27-3.15 (m, 2 H), 2.26 (m, 1 H), 1.18-1.06 (m, 6 H). ^¹³C NMR (75 MHz, DMSO): δ = 14.2, 17.9, 18.6, 27.2, 30.2, 39.0, 40.4, 53.8, 56.4, 57.3, 61.0, 109.2, 111.8, 118.2, 121.3, 124.5, 127.3, 136.4, 168.5, 171.6. IR (KBr): 1728, 1672 cm^-¹. Anal. Calcd for C₁₈H₂₆ClN₃O₃: H, 7.48; N, 10.75. Found: H, 7.46; N, 10.72. Mp 152 ˚C. [α]_D ^²0 -125.1 (c 0.51, EtOH).
H-Val-Pro-OEt (3d): ^¹H NMR (300 MHz, DMSO): δ = 8.34 (s, 1 H), 4.35 (m, 1 H), 4.08 (q, 2 H, J = 7.0 Hz), 3.99 (d,
1 H, J = 5.5 Hz), 3.79 (m, 1 H), 3.50 (m, 1 H), 2.23-2.11 (m, 2 H), 1.89 (m, 3 H), 1.17 (t, 3 H, J = 7.0 Hz), 0.99 (dd, 6 H, J = 6.8, J = 6.6 Hz). ^¹³C NMR (75 MHz, DMSO): δ = 14.3, 17.6, 18.5, 25.1, 29.0, 29.8, 47.6, 55.8, 59.2, 61.0, 167.4, 171.6. IR (KBr): 1739, 1652 cm^-¹. HRMS: m/z calcd for C₁₂H₂₂N₂0₃ [MH⁺]: 243.1705; found: 243.1708. [α]_D ^²0 -47.7 (c 1.01, EtOH).
H-Val-Met-OEt (3e): ^¹H NMR (300 MHz, CDCl₃): δ = 7.89 (d, 1 H, J = 7.7 Hz), 4.68 (m, 1 H), 4.20 (q, 2 H, J = 7.2 Hz), 3.28 (d, 1 H, J = 4.0 Hz), 2.49 (m, 2 H), 2.30-1.95 (m, 6 H), 1.28 (t, 3 H, J = 7.2 Hz), 0.99 (d, 3 H, J = 7.0 Hz), 0.84 (d,
3 H, J = 7.0 Hz). ^¹³C NMR (75 MHz, CDCl₃): δ = 14.6, 15.9, 16.6, 20.0, 30.4, 31.2, 32.4, 51.6, 60.5, 61.9, 172.4, 174.6. IR (KBr): 1720, 1650 cm^-¹. HRMS: m/z calcd for C₁₂H₂₄N₂0₃S [MH⁺]: 277.1508; found: 277.1510.
H-Val-Ala-OEt (3f): ^¹H NMR (300 MHz, CDCl₃): δ = 7.75 (d, 1 H, J = 6.0 Hz), 4.55 (m, 1 H), 4.15 (q, 2 H, J = 7.2 Hz), 3.23 (d, 1 H, J = 4.0 Hz), 2.25 (m, 1 H), 1.39 (d, 3 H, J = 7.2 Hz), 1.26 (t, 3 H, J = 7.0 Hz), 0.97 (d, 3 H, J = 7.0 Hz), 0.81 (d, 3 H, J = 7.0 Hz). ^¹³C NMR (75 MHz, CDCl₃): δ = 14.5, 16.4, 18.5, 20.0, 31.3, 48.1, 60.4, 61.7, 173.5, 174.5. IR (KBr): 1745, 1636 cm^-¹. HRMS: m/z calcd for C₁₀H₂₀N₂0₃ [MH⁺]: 217.1556; found: 217.1552.

12

General Procedure for the Preparation of N-Silylated-NCA 2a-c
To a solution of Val-NCA or Leu-NCA (7 mmol) in dry THF (20 mL) at -30 ˚C, were added dropwise Et₃N (7 mmol) and then Me₃SiCl or TBSCl (10.5 mmol). The resulting solution was stirred at this temperature for 5 h. The reaction mixture was filtered under a nitrogen atmosphere and evaporated in vacuo at r.t. to give N-trialkylsilyl-NCA 2a-c in 95-99% yields as white solids without further purification.
Selected Data for N -TMS-Val-NCA 2a ^¹H NMR (300 MHz, CDCl₃): δ = 4.07 (d, 1 H, J = 3.4 Hz), 2.08 (m, 1 H), 1.19 (d, 3 H, J = 7.1 Hz), 0.94 (d, 3 H, J = 7.1 Hz), 0.39 (s, 9 H). ^¹³C NMR (75 MHz, CDCl₃): δ = 15.0, 16.5, 18.2, 32.4, 66.1, 154.2, 169.7. IR (KBr): 1840, 1785 cm^-¹.
Selected Data for N -TBS-Val-NCA 2b ^¹H NMR (300 MHz, CDCl₃): δ = 3.87 (d, 1 H, J = 3.4 Hz), 2.19 (m, 1 H), 0.99 (d, 3 H, J = 6.8 Hz), 0.90 (s, 9 H), 0.84 (d, 3 H, J = 6.8 Hz), 0.27 (s, 3 H), 0.26 (s, 3 H). ^¹³C NMR (75 MHz, CDCl₃): δ = 3.4, 3.5, 17.9, 18.2, 19.2, 21.5, 26.9, 27.2, 32.4, 33.2, 64.7, 65.8, 128.9, 154.3, 171.3, 172.7. IR (KBr): 2258, 1857, 1786, 1733 cm^-¹.
Selected Data for N -TMS-Leu-NCA 2c
^¹H NMR (300 MHz, CDCl₃): δ = 4.19 (dd, 0.5 H, J = 3.8, 9.8 Hz), 3.96 (dd, 0.5 H, J = 6.8, 7.9 Hz), 1.99 (m, 0.5 H), 1.85-1.61 (m, 3.5 H), 0.96 (m, 6 H), 0.39 (s, 4.5 H), 0.32 (s, 0.5 H). ^¹³C NMR (75 MHz, CDCl₃): δ = 0.01, 0.39, 21.5, 21.8, 23.3, 24.2, 25.5, 42.4, 42.9, 57.4, 59.4, 68.4, 127.0, 154.8, 171.5, 172.5. IR (KBr): 2255, 1845, 1780, 1720 cm^-¹.

16

General Procedure for the One-Pot Preparation of Unsymmetrical Peptidyl Ureas 4a-j
To a solution of Val-NCA or Leu-NCA (7 mmol) in dry THF (20 mL) maintained at -30 ˚C were dropwise added Et₃N
(1 mL, 7 mmol) and TMSCl (1.33 mL, 10.5 mmol). The resultant solution was stirred at this temperature for 4.5 h before adding the amine nucleophile (7 mmol) and Et₃N
(2 mL, 14 mmol). After stirring for 2 h at 0 ˚C, the solution was acidified to pH 4.0 with HCl (4 M) in dioxane. The precipitate was then filtered and dried to afford peptidyl ureas 4d,h,i. Ureas 4a-c,e,f,g,j were isolated as followed. After acidification of the solution to pH 4 with HCl (4 M in dioxane), solvents were evaporated and the resulting residue was dissolved in CH₂Cl₂. The CH₂Cl₂ layer was washed with brine and evaporated in vacuo to provide pure ureas 4a-c,
e,f,g,j.
Urea 4a: ^¹H NMR (300 MHz, CDCl₃): δ = 6.58 (s, 1 H), 3.84 (m, 2 H), 2.22-2.05 (m, 3 H), 1.80 (d, 2 H, J = 12.8 Hz), 1.62 (d, 3 H), 1.35-1.16 (m, 3 H), 1.01 (d, 3 H, J = 6.9 Hz), 0.86 (d, 3 H, J = 6.8 Hz). ^¹³C NMR (75 MHz, CDCl₃): δ = 16.0, 19.1, 25.4, 26.1, 26.2, 29.5, 29.8, 30.7, 51.7, 61.9, 158.9, 174.1. IR (KBr): 1641, 1565 cm^-¹. Anal. Calcd for C₁₈H₂₆N₂O₅: C, 59.48; H, 9.15; N, 11.56. Found: C, 59.64; H, 9.32; N, 11.58. Mp 94 ˚C. [α]_D ^²0 -75.9 (c 0.7, EtOH).
Urea 4b: ^¹H NMR (300 MHz, CDCl₃): δ = 4.45 (m, 1 H), 3.57 (m, 1 H), 1.92-1.55 (m, 5 H), 1.52-1.30 (m, 3 H), 1.35-1.16 (m, 5 H), 1.01 (m, 6 H). ^¹³C NMR (75 MHz, CDCl₃):
δ = 20.1, 21.9, 22.3, 22.5, 22.9, 28.0, 33.3, 33.4, 39.8, 49.5, 54.3, 157.6, 174.8. IR (KBr): 1638, 1565 cm^-¹. Anal. Calcd for C₁₃H₂₄N₂O₃: C, 60.91; H, 9.44; N, 10.93. Found: C, 60.96; H, 9.48; N, 10.94. [α]_D ^²0 -80.0 (c 0.7, EtOH).
Urea 4c: ^¹H NMR (300 MHz, CDCl₃): δ = 7.99 (s, 1 H), 7.66 (s, 1 H), 7.15 (t, 2 H, J = 7.9 Hz), 6.72 (d, 3 H, J = 8.7 Hz), 6.25 (d, 1 H, J = 9.0 Hz), 4.07 (dd, 1 H, J = 4.9, 9.0 Hz), 2.03 (m, 1 H), 0.84 (d, 3 H, J = 6.8 Hz), 0.77 (d, 3 H, J = 6.8 Hz). ^¹³C NMR (75 MHz, CDCl₃): δ = 17.9, 19.5, 30.8, 57.5, 112.7, 129.1, 158.9, 174.0. IR (KBr): 1703, 1621, 1563
cm^-¹. Anal. Calcd for C₁₂H₁₇N₃O₃: C, 57.36; H, 6.82; N, 16.72. Found: C, 57.46; H, 6.90; N, 16.94. [α]_D ^²0 +24.0 (c 0.58, EtOH).
Urea 4d: ^¹H NMR (300 MHz, CDCl₃): δ = 7.28 (m, 5 H), 5.11 (m, 2 H), 4.37 (m, 1 H), 4.25 (m, 1 H), 2.15 (m, 1 H), 1.54 (m, 3 H), 0.85 (m, 12 H). ^¹³C NMR (75 MHz, CDCl₃): δ = 16.4, 17.8, 20.8, 21.6, 23.7, 30.0, 40.5, 51.1, 57.4, 66.5, 127.2, 127.5, 127.6, 134.0, 157.5, 173.7, 175.2. IR (KBr): 1715, 1644, 1566 cm^-¹. HRMS: m/z calcd for C₁₉H₂₈N₂0₅ [MH⁺]: 365.1998; found: 365.2006. [α]_D ^²0 -14.5 (c 1.0, EtOH).
Urea 4e: ^¹H NMR (300 MHz, CDCl₃): δ = 7.09 (m, 2 H), 4.47 (m, 1 H), 4.35 (m, 1 H), 3.77 (s, 3 H), 3.70 (s, 3 H), 2.47 (m, 2 H), 2.20 (m, 2 H), 2.06 (m, 1 H), 0.98 (m, 6 H). ^¹³C NMR (75 MHz, CDCl₃): δ = 17.8, 19.1, 27.8, 30.4, 31.4, 52.7, 53.2, 53.3, 58.8, 158.8, 173.9, 175.1, 176.7. IR (KBr): 1720, 1655, 1561 cm^-¹. HRMS: m/z calcd for C₁₃H₂₂N₂0₇ [MH⁺]: 318.1427; found: 318.1430. [α]_D ^²0 +4.3 (c 0.58, EtOH).
Urea 4f: ^¹H NMR (300 MHz, CDCl₃): δ = 4.33 (m, 2 H), 3.77 (s, 3 H), 2.21 (m, 2 H), 1.87 (m, 1 H), 1.41 (m,1 H), 1.18 (m, 1 H), 0.95 (m, 12 H). ^¹³C NMR (75 MHz, CDCl₃): δ = 11.8, 15.7, 18.0, 19.2, 25.4, 31.5, 38.5, 52.9, 58.8, 68.3, 158.8, 174.7, 175.6. IR (KBr): 1721, 1640, 1566 cm^-¹. HRMS: m/z calcd for C₁₃H₂₄N₂0₅ [MH⁺]: 289.1762; found: 289.1763. [α]_D ^²0 +10.1 (c 0.89, EtOH).
Urea 4g: ^¹H NMR (300 MHz, CDCl₃): δ = 5.98 (m, 2 H), 4.54 (m, 1 H), 4.33 (m, 1 H), 4.19 (q, 2 H, J = 7.1 Hz), 2.50 (m, 2 H), 2.16-1.91 (m, 6 H), 1.26 (t, 3 H, J = 7.1 Hz), 0.94 (d, 3 H, J = 6.8 Hz), 0.88 (d, 3 H, J = 6.8 Hz). ^¹³C NMR (75 MHz, CDCl₃): δ = 14.5, 15.7, 18.1, 19.4, 30.3, 31.5, 32.7, 52.8, 58.6, 62.2, 158.5, 174.1, 176.3. IR (KBr): 1740, 1704, 1634 cm^-¹. Mp 118 ˚C. Anal. Calcd for C₁₃H₂₄N₂O₅S: C, 48.73; H, 7.55; N, 8.74. Found: C, 48.79; H, 7.58; N, 8.65. [α]_D ^²0 +7.6 (c 0.8, EtOH).
Urea 4h: ^¹H NMR (300 MHz, CDCl₃): δ = 7.13 (m, 3 H), 7.00 (m, 2 H), 5.87 (d, 1 H, J = 8.7 Hz), 5.73 (d, 1 H, J = 7.9 Hz), 4.70 (m, 1 H), 4.30 (m, 1 H), 3.98 (m, 2 H), 3.02 (m,
2 H), 2.07 (m, 1 H), 1.09 (t, 3 H, J = 7.1 Hz), 0.87 (d, 3 H, J = 6.8 Hz), 0.79 (d, 3 H, J = 6.8 Hz). ^¹³C NMR (75 MHz, CDCl₃): δ = 14.2, 17.9, 19.2, 31.1, 54.6, 58.5, 81.8, 127.1, 128.6, 129.6, 136.2, 158.0, 176.1. IR (KBr): 1733, 1637, 1562 cm^-¹. Anal. Calcd for C₁₇H₂₄N₂O₅: C, 60.7; H, 7.1; N, 8.3. Found: C, 60.31; H, 7.2; N, 8.3. Mp 102.6 ˚C. [α]_D ^²0 +34.5 (c 0.70, EtOH).
Urea 4i: ^¹H NMR (300 MHz, CDCl₃): δ = 7.28 (m, 5 H), 5.11 (m, 2 H), 4.37 (m, 1 H), 4.25 (m, 1 H), 2.15 (m, 1 H), 1.54 (m, 3 H), 0.85 (m, 12 H). ^¹³C NMR (75 MHz, CDCl₃): δ = 16.4, 17.8, 20.8, 21.6, 23.7, 30.0, 40.5, 51.1, 57.4, 66.5, 127.2, 127.5, 127.6, 134.0, 157.5, 173.7, 175.2. IR (KBr): 1715, 1644, 1566 cm^-¹. HRMS: m/z calcd for C₁₉H₂₅N₃0₅ [MH⁺]: 375.1794; found: 375.1799. [α]_D ^²0 +34.0 (c 0.15, EtOH).
Urea 4j: ^¹H NMR (300 MHz, CDCl₃): δ = 7.11 (m, 3 H), 7.01 (m, 2 H), 5.77 (s, 1 H), 5.66 (d, 1 H, J = 7.5 Hz), 4.69 (m,
1 H), 4.31 (m, 1 H), 3.96 (m, 2 H), 2.98 (m, 2 H), 1.57 (m,
2 H), 1.37 (m, 1 H), 1.08 (t, 3 H, J = 7.1 Hz), 0.81 (d, 6 H, J = 5.7 Hz). ^¹³C NMR (75 MHz, CDCl₃): δ = 14.4, 22.3, 23.3, 25.1, 39.8, 42.1, 52.7, 54.6, 61.8, 127.2, 136.5, 160.5, 173.3. IR (KBr): 1740, 1644, 1557 cm^-¹. Anal. Calcd for C₁₈H₂₆N₂O₅: C, 61.7; H, 7.48; N, 7.99. Found: C, 61.66; H, 7.46; N, 8.04. [α]_D ^²0 +1.2 (c 0.4, EtOH).