A Novel Approach to the Solid-Phase Synthesis of Peptides with a Tetrazole at the C-Terminus

Sarah J. Gunn; Alison Baker; Richard D. Bertram; Stuart L. Warriner

doi:10.1055/s-2007-986661

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Synlett 2007(17): 2643-2646
DOI: 10.1055/s-2007-986661

LETTER

A Novel Approach to the Solid-Phase Synthesis of Peptides with a Tetrazole at the C-Terminus

Sarah J. Gunn^a, Alison Baker^b, Richard D. Bertram^c, Stuart L. Warriner*^a
^a School of Chemistry, University of Leeds, Leeds, LS2 9JT, UK
Fax: +44(113)3436565; e-Mail: s.l.warriner@leeds.ac.uk;
^b Centre for Plant Sciences, University of Leeds, Leeds, LS2 9JT, UK
^c Infineum UK Ltd., PO Box 1, Milton Hill, Abingdon, Oxfordshire, OX13 6BB, UK

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Publication History

Received 17 July 2007
Publication Date:
12 September 2007 (online)

Abstract
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Abstract

Peptidomimetics containing a C-terminal tetrazole can be easily prepared using modifications to traditional peptide synthesis protocols.

Key words

bioorganic chemistry - peptides - peptidomimetics - solid-phase synthesis - tetrazole

References and Notes

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11

Analytical Data of the Fmoc-Protected Tetrazoles Compound 3a: mp 175-177 °C; R _f = 0.8 (EtOAc); [α]_D -32 (c 1, MeOH). IR (solid): ν_max = 3308, 2961, 1682, 1532 cm^-1. ¹H NMR (500 MHz, CD₃OD): δ = 0.85 (3 H, d, J = 6.6 Hz, Lδ₁), 0.87 (3 H, d, J 6.6 Hz, Lδ₂), 1.54 (1 H, m, Lγ), 1.69 (1 H, m, Lβ₁), 1.75 (1 H, m, Lβ₂), 4.10 (1 H, t, J = 6.5 Hz, CH), 4.27 (1 H, dd, J = 10.5, 6.5 Hz CH_2a), 4.38 (1 H, dd, J = 10.4, 6.8 Hz, CH_2b), 4.96 (1 H, dd, J = 5.6, 9.8 Hz, Lα), 7.18 (2 H, t, J = 7.4 Hz, Fmoc_Ar2), 7.28 (2 H, t, J = 7.4 Hz, Fmoc_Ar3), 7.54 (2 H, t, J = 7.6 Hz, Fmoc_Ar1), 7.68 (2 H, d, J = 7.5 Hz, Fmoc_Ar4). ¹³C NMR (75 MHz, CD₃OD): δ = 22.2 (Lδ), 23.5 (Lδ), 26.0 (Lγ), 43.5 (Lα), 43.5 (Lβ), 46.4 (CH₂), 68.1 (CH), 121.2 (Fmoc_Ar1), 126.4 (Fmoc_Ar2), 128.4 (Fmoc_Ar3), 129.1 (Fmoc_Ar4), 142.9 (Fmoc_Ar5), 145.4 (Fmoc_Ar6) 158.6 (CO), 160.5 (L_tetrazole). MS (ES): m/z calcd for C₂₁H₂₂N₅O₂: 376.1779; found [M - H]^-: 376.1773.
Compound 3b: mp 97 °C; R _f = 0.13 (10% MeOH in CH₂Cl₂); [α]_D -32 (c 2, DMF). IR (solid): ν_max = 3308, 2976, 2763, 1679 cm^-1. ¹H NMR (500 MHz, CD₃OD): δ = 1.10 (9 H, s, t-Bu), 3.06 (1 H, m, Yβ₁), 3.22 (1 H, m, Yβ₂), 3.98 (1 H, t, J = 7.0 Hz, CH), 4.10 (1 H, m, CH_2a), 4.20 (1 H, m, CH_2b), 5.11 (1 H, t, J = 6.0 Hz, Yα), 6.72 (2 H, d, J = 8.1 Hz, Y_Ar1), 7.00 (2 H, d, J, = 8.1 Hz, Y_Ar2), 7.18 (2 H, t, J = 7.2 Hz, Fmoc_Ar2), 7.27 (2 H, t, J = 7.3 Hz, Fmoc_Ar3), 7.48 (2 H, t, J 8.1 Hz, Fmoc_Ar1), 7.67 (2 H, d, J = 7.4 Hz, Fmoc_Ar4). ¹³C NMR (75 MHz, CDCl₃): δ = 29.2 (CMe₃), 38.2 (Yβ), 47.5 (CH), 56.3 (Yα), 67.5 (CH₂), 78.9 (OCt-Bu), 121.2 (Fmoc_Ar1), 125.5 (Fmoc_Ar2), 126.5 (Y_Ar1), 128.4 (Y_ArCH2), 129.1 (Fmoc_Ar3), 131.2 (Fmoc_Ar4), 133.3 (Y_Ar2), 142.9 (Fmoc_Ar5), 145.5 (Fmoc_Ar6), 155.7 (Y_ArO _t _-Bu), 158.4 (CO), 159.8 (Y_tetrazole). MS (ES): m/z calcd for C₂₈H₂₈N₅O₃: 482.2198; found [M - H]^-: 482.2180.
Compound 3c: mp 186-190 °C; R _f = 0.43 (20% MeOH in CH₂Cl₂); [α]_D -28 (c 1, DMF). IR (solid): ν_max = 3316, 2899, 2469, 1680 cm^-1. ¹H NMR (500 MHz, DMSO-d ₆): δ = 3.19 (1 H, dd, J = 10.3,13.6 Hz, Fβ₁), 3.30 (1 H, dd, J = 6.0,13.6, Fβ₂), 4.16-4.25 (3 H, m, CH₂, CH), 5.14 (1 H, t, J = 6.3 Hz, Fα), 7.19-7.31 (7 H, m, F_Ar, Fmoc_Ar2), 7.42 (2 H, t, J = 7.4 Hz, Fmoc_Ar3), 7.63 (2 H, d, J = 5.3 Hz, Fmoc_Ar1), 7.89 (2 H, d, J = 7.5 Hz, Fmoc_Ar4). ¹³C NMR (75 MHz, DMSO-d ₆): δ = 38.7 (Fβ), 46.9 (CH), 56.4 (Fα), 66.1 (CH₂), 120.5 (Fmoc_Ar1), 125.7 (Fmoc_Ar2), 126.9 (F_Ar1), 127.4 (FAr₂), 128.0 (F_Ar3), 128.6 (Fmoc_Ar3), 129.6 (Fmoc_Ar4), 137.5 (_FAr-CH2), 141.0 (Fmoc_Ar5), 144.1 (Fmoc_Ar6), 156.0 (CO), 156.6 (F_tetrazole). MS (ES): m/z calcd for C₂₄H₂₀N₅O₂: 410.1622; found [M - H]^-: 410.1633.
Compound 3d: mp 178-181 °C. R _f = 0.32 (20% MeOH in CH₂Cl₂); [α]_D +3 (c 0.5, DMF). IR (solid): ν_max = 3291, 3041, 2924, 2851, 1704 cm^-1. ¹H NMR (500 MHz, DMSO-d ₆): δ = 0.79-1.02 (2 H, m, Cha), 1.07-1.23 (4 H, m, Cha), 1.30 (1 H, m, Chaγ), 1.56-1.87 (6 H, m, Cha, Chaβ), 4.28 (2 H, m, CH₂), 4.39 (1 H, m, CH), 5.00 (1 H, dd, J = 7.5, 15.2 Hz, Chaα), 7.33 (2 H, t, J = 6.8 Hz, Fmoc_Ar2), 7.43 (2 H, t, J = 8.6 Hz, Fmoc_Ar3), 7.72 (2 H, t, J = 8.9 Hz, Fmoc_Ar1), 7.90 (2 H, d, J = 8.1 Hz, Fmoc_Ar4). ¹³C NMR (75 MHz, DMSO-d ₆): δ = 25.9 (Cha), 26.1 (Cha), 26.3 (Cha), 32.0 (Cha), 33.2 (Chaγ), 33.7 (Chaβ), 47.0 (CH), 52.5 (Chaα), 66.0 (CH₂), 120.5 (Fmoc_Ar1), 125.6 (Fmoc_Ar2), 127.4 (Fmoc_Ar3), 128.0 (Fmoc_Ar4), 141.1 (Fmoc_Ar5), 144.0 (Fmoc_Ar6), 156.2 (CO), 158.7 (Cha_tetrazole). MS (ES): m/z calcd for C₂₄H₂₆N₅O₂: 416.2092; found [M - H]^-: 416.2086.

13

A substoichiometric quantity of tetrazole was loaded relative to the theoretical loading of the resin. The success of loading was determined by isolation of the target peptide. Yields are calculated from the starting quantity of 3 employed in the loading reaction.

14

Typical Procedure
2-Chlorotrityl chloride resin (Novabiochem, 100-200 mesh, 100 mg, 1.4 mmol/g loading) was suspended in CH₂Cl₂ (1 mL) and agitated (30 min), then drained. A solution of Fmoc-amino tetrazole (0.08 mmol) and DIPEA (78.2 µL, 0.47 mmol) in DMF (1 mL) was added and the mixture agitated (4 h). The solution was removed and the resin was washed (DMF, 2 × 1 mL, 2 min) and then treated with a solution of CH₂Cl₂-MeOH-DIPEA (80:15:5, 2 × 1 mL, 5 min) and then was washed again (DMF, 3 × 1 mL, 2 min). The Fmoc group was removed (25% piperidine in DMF, 2 × 1 mL, 2 min). Finally, the resin was washed with DMF (6 × 1 mL, 2 min), i-PrOH (3 × 1 mL, 5 min) and hexane (4 × 1 mL, 2 min), sucked dry in air (10 min) then dried in vacuo, over KOH (16 h) and stored at 5 °C. The loaded resin was subject to standard Fmoc solid-phase peptide synthesis conditions: Couplings were performed with a solution of the amino acid to be coupled (5 equiv), HOBt (5 equiv), HCTU (4.9 equiv), and DIPEA (10 equiv) in DMF for 60 min and Fmoc deprotection was performed using a solution of 20% piperidine in DMF. Cleavage from the resin and side-chain deprotection was effected with TFA containing 2.5% triisopropyl silane and 2.5% H₂O. Peptides were purified by preparative HPLC where necessary (with the exception of 2a; all the other peptides were single peaks by HPLC following cleavage from the resin, and could be used without further purification).
Compound 2a: ¹H NMR (500 MHz, D₂O): δ = 0.89 (3 H, d, J = 6.6 Hz, Lδ₁), 0.94 (3 H, d, J = 6.6 Hz, Lδ₂), 1.26-1.41 (2 H, m, Kγ), 1.55-1.68 (3 H, m, Lγ, Kδ), 1.70-1.87 (3 H, m, Kβ₁, Lβ₁, Kβ_2,), 1.94 (1 H, m, Qβ₁), 2.04 (1 H, m, Qβ₂), 2.31 (2 H, at, J = 7.6 Hz, Qγ), 2.94 (2 H, at, J = 7.6 Hz, Kε), 3.11 (1 H, dd, J = 5.6, 14.2 Hz, Yβ₁), 3.13 (1 H, dd, J = 5.0, 14.2 Hz, Yβ₂), 3.83 (1 H, dd, J = 5.8, 11.4 Hz, Sβ₁), 3.88 (1 H, dd, J = 5.9, 11.4 Hz, Sβ₂) 4.22 (1 H, t, J = 7.3 Hz, Yα), 4.34 (1 H, t, J = 7.2 Hz, Kα), 4.38 (2 H, m, Sα, Qα), 5.33 (1 H, dd, J = 6.0, 9.6 Hz, Lα), 6.85 (2 H, d, J = 8.4 Hz, Y_Ar1), 7.12 (2 H, d, J = 8.4 Hz, Y_Ar2). ¹³C NMR (125 MHz, D₂O): δ = 21.2 (Lδ₁), 22.2 (Lδ₂), 22.3 (Kγ), 24.6 (Lγ), 26.6 (Kδ), 27.7 (Qβ), 30.7 (Kβ), 31.3 (Qγ), 36.4 (Yβ), 39.6 (Kε), 41.3 (Lβ), 43.6 (Lα), 53.1 (Qα), 54.0 (Kα), 54.7 (Yα), 56.0 (Sα), 61.5 (Sβ), 116.2 (Y_Ar1), 125.7 (Y_ArCH), 131.2 (Y_Ar2), 155.6 (Y_ΑrOH), 158.6 (L_tetrazole), 169.3 (Yco), 172.0 (Sco), 172.5 (Qco), 173.8 (Kco), 178.2 (Q_CONH2). MS (ES): m/z calcd for C₂₉H₄₆N₁₁O₇: 660.3587; found [M - H]^-: 660.3601.
Compound 2b: ¹H NMR (500 MHz, D₂O): δ = 1.15-1.23 (2 H, m, Kγ), 1.57-1.61 (4 H, m, Kδ, Kβ₁, Κβ₂), 1.90 (1 H, m, Qβ₁), 2.01 (1 H, m, Qβ₂), 2.30 (2 H, t, J = 7.5 Hz, Qγ), 2.91 (2 H, t, J = 7.5 Hz, Kε), 3.11 (2 H, d, J = 7.0 Hz, Y5β), 3.21 (1 H, dd, J = 9.4, 14.0 Hz, Y1β₁), 3.32 (1 H, dd, J = 6.5, 14.0 Hz, Y1β₂), 3.75 (1 H, dd, J = 5.8, 11.4 Hz, Sβ₁), 3.85 (1 H, dd, J = 5.8, 11.3 Hz, Sβ₂), 4.19-4.27 (2 H, m, Y1α, Kα), 4.36 (2 H, m, Sα, Qα), 5.49 (1 H, t, J = 6.5 Hz, Y5α), 6.82 (2 H, d, J = 8.3 Hz, Y1_Ar1), 6.84 (2 H, d, J = 8.3 Hz, Y5_Ar1), 7.08 (2 H, d, J = 8.4 Hz, Y1_Ar1), 7.11 (2 H, d, J = 8.4 Hz, Y5_Ar1). ¹³C NMR (125 MHz, D₂O): δ = 24.7 (Kγ), 29.1 (Kδ), 30.2 (Qβ), 33.2 (Kβ), 33.7 (Qγ), 38.9 (Yβ), 40.1 (Yβ), 42.0 (Kε), 49.0 (Y₅α), 55.5 (Qα), 56.6 (Kα), 57.2 (Y₁α), 58.4 (Sα), 63.9 (Sβ), 118.4 (Y1_Ar1), 118.7 (Y5_Ar1), 128.2 (Y1_Ar-CH), 130.6 (Y5_Ar-CH), 133.5 (Y1_Ar2), 133.7 (Y5_Ar2), 157.4 (Y1_ArOH), 158.0 (Y5_ArOH), 160.6 (Y5_tetrazole) 171.8 (Yco), 174.4 (Sco), 174.9 (Qco), 176.1 (Kco), 180.6 (Q_CONH2). MS (ES): m/z calcd for C₃₂H₄₆N₁₁O₈: 712.3525; found [M + H]⁺: 712.3538.
Compound 2c: ¹H NMR (500 MHz, D₂O): δ = 1.15-1.29 (2 H, m, Kγ), 1.56-1.66 (4 H, m, Kδ, Kβ₁, Κβ₂), 1.94 (1 H, m, Qβ₁), 2.02 (1 H, m, Qβ₂), 2.30 (2 H, t, J = 7.5 Hz, Qγ), 2.91 (2 H, t, J = 7.5 Hz, Kε), 3.13 (2 H, d, J = 6.7 Hz, Yβ), 3.30 (1 H, dd, J = 9.9, 13.6 Hz, Fβ₁), 3.40 (1 H, dd, J = 7.2, 13.6 Hz, Fβ₂), 3.76 (1 H, dd, J = 6.4, 11.2 Hz, Sβ₁), 3.87 (1 H, dd, J = 6.0, 11.2 Hz, Sβ₂), 4.23 (1 H, t, J = 7.3 Hz, Yα), 4.28 (1 H, t, J = 7.0 Hz, Kα), 4.35-4.40 (2 H, m, Sα, Qα), 5.55 (1 H, t, J = 7.9 Hz, Fα), 6.87 (2 H, d, J = 8.2 Hz, Y_Ar1), 7.13 (2 H, d, J = 7.9 Hz, Y_Ar2), 7.23 (2 H, d, J = 6.4 Hz, F_Ar1), 7.30-7.38 (3 H, m, F_Ar2, F_Ar3). ¹³C NMR (125 MHz, D₂O): δ = 24.7 (Kγ), 29.0 (Kδ), 30.1 (Qβ), 33.2 (Kβ), 33.7 (Qγ), 38.8 (Yβ), 41.1 (Fβ), 41.9 (Kε), 49.1 (Fα), 55.5 (Qα), 56.5 (Kα), 57.1 (Yα), 58.3 (Sα), 63.9 (Sβ), 118.7 (Y_Ar2) 128.2 (Y_ArCH2), 130.1 (F_Ar3), 131.6 (F_Ar2), 132.1 (F_Ar1), 133.6 (Y_Ar1), 138.8 (F_ArCH2), 158.0 (Y_ArOH), 161.0 (F_tetrazole), 171.7 (Yco), 174.3 (Sco), 174.9 (Kco), 176.0 (Qco), 180.6 (Q_CONH2). MS (ES): m/z calcd for C₃₂H₄₆N₁₁O₇: 696.3576; found [M + H]⁺: 696.3587.
Compound 2d: ¹H NMR (500 MHz, D₂O): δ = 0.89-1.04 (2 H, m, Cha), 1.16 (4 H, m, Cha), 1.23-1.31 (1 H, m, Cha), 1.32-1.42 (3 H, m, Kγ, Chaγ), 1.57-1.72 (6 H, m, Cha, Kδ), 1.75 (2 H, m, Kβ_1, Chaβ₁), 1.84 (2 H, m, Kβ₂, Chaβ₂), 1.94 (1 H, m, Qβ₁), 2.04 (1 H, m, Qβ₂), 2.32 (2 H, t, J = 7.6 Hz, Qγ), 2.94 (2 H, t, J = 8.1 Hz, Kε), 3.12 (2 H, d, J = 7.3 Hz, Yβ), 3.83 (1 H, dd, J = 5.7, 11.3 Hz, Sβ₁), 3.90 (1 H, dd, J = 5.7, 11.3 Hz, Sβ₂), 4.22 (1 H, t, J = 7.2 Hz, Yα), 4.33 (1 H, dd, J = 6.4, 8.2 Hz, Kα), 4.37-4.41 (2 H, m, Sα, Qα), 5.36 (1 H, dd, J = 6.4, 9.5 Hz, Chaα), 6.85 (2 H, d, J = 8.5 Hz, Y_Ar1), 7.12 (2 H, d, J = 8.2 Hz, Y_Ar2). ¹³C NMR (125 MHz, D₂O): δ = 24.8 (Kγ), 28.5 (Cha), 28.6 (Cha), 28.8 (Cha), 29.1 (Kδ), 30.2 (Qβ), 33.1 (Kβ), 33.7 (Qγ), 34.5 (Cha), 35.6 (Cha), 36.4 (Chaγ), 38.8 (Yβ), 42.0 (Kε), 42.3 (Chaβ), 45.4 (Chaα), 55.5 (Qα), 56.4 (Kα), 57.1 (Yα), 58.5 (Sα), 63.9 (Sβ), 118.7 (Y_Ar2), 128.2 (Y_ArCH2), 133.7 (Y_Ar1), 158.0 (Y_ArOH), 161.3 (Cha_tetrazole), 171.6 (Y_CO), 174.4 (S_CO), 174.9 (K_CO), 176.2 (Q_CO), 180.6 (Q_CONH2). MS (ES): m/z calcd for C₃₂H₅₁N₁₁O₇: 702.4046; found [M + H]⁺: 702.4048.