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Synlett 2011(3): 327-330  
DOI: 10.1055/s-0030-1259510
LETTER
© Georg Thieme Verlag Stuttgart ˙ New York

N β-Fmoc-N β-Methyl-aza-β³-amino Acids

Irène Nicolas, Ksenija Kisseljova, Patrick Bauchat, Michèle Baudy-Floc’h*
Groupe ‘Ciblage et Auto-Assemblages Fonctionnels’, UMR 6226 CNRS, Sciences Chimiques de Rennes, Université de Rennes I, 263 Av. du Général Leclerc, 35042 Rennes Cedex, France
Fax: +33(2)3236738; e-Mail: michele.baudy-Floch@univ-rennes1.fr;
Further Information

Publication History

Received 29 October 2010
Publication Date:
25 January 2011 (online)

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Abstract

The potential of peptides as drug candidates is limited by their poor pharmacokinetic properties. Many peptides have a short half-life, in vivo half-life, and insufficient oral availability. Inspired by the N-methylation of peptides as a promising way to rationally improve key pharmacokinetic characteristics, we report our efforts toward an easy synthesis of new monomers N β-Fmoc-N β-Me-aza-β³-amino acids. These synthetic building blocks will be useful for solid-phase synthesis of new peptidomimetics.

Key words

N-methyl - aza analogues - hydrazine - reductive amination - amino acids

    References and Notes

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17

To a solution of methylhydrazine (0.88 g, 19 mmol) in CH2Cl2 (20 mL) at -78 ˚C and Et3N (2.1 g, 1.1 equiv) was added dropwise a CH2Cl2 (10 mL) solution of FmocCl (3.80 g, 0.8 equiv) over 2 h. The reaction mixture was allowed to stir for 1 h at r.t., diluted with CH2Cl2, and washed with H2O (3×), brine, dried over anhyd Na2SO4, and concentrated
in vacuo. The crude solid was triturated in PE. The FmocMeNNH2 (1) was obtained as a white powder (2.9 g, 73%); mp 84 ˚C. ¹H NMR (200 MHz, CDCl3): δ = 3.17 (s, 3 H), 4.13 (br s, 2 H), 4.30 (t, 1 H, J = 6.8 Hz), 4.48 (d, 2 H, J = 6.8 Hz), 7.29-7.85 (m, 8 H) ppm. ¹³C NMR (75.5 MHz, CDCl3) δ = 157.4, 143.7 141.3, 127.7 127.1 125.0, 120.0, 67.4, 47.2, 38.4 ppm.

18

To a solution of N-Fmoc-N-MeNH2 1 (1 equiv) in CH2Cl2 (or EtOH for R¹ = H; 20 mL) was added the aldehyde or the ketone (1.2 equiv). The mixture was stirred overnight at r.t. (or refluxed for 2 h for R¹ = H). After removal of the solvent under reduced pressure, the crude hydrazone was obtained, which was pure enough to be used in the following step.
Compound 3d [R¹ = CH(CH3)2]: yield 77%; mp 80 ˚C. ¹H NMR (200 MHz, CDCl3): δ = 1.24 (d, 6 H, J = 6.8 Hz), 2.74 (m, 1 H), 3.30 (s, 3 H), 4.35 (t, 1 H, J = 7.7 Hz), 4.49 (d, 2 H, J = 7.7 Hz), 7.03 (d, 1 H, J = 5.2 Hz), 7.29-7.85 (m, 8 H) ppm. ¹³C NMR (75.5 MHz, CDCl3): δ = 154.78, 148.91, 143.90, 141.30, 127.71, 127.09, 125.58, 119.94, 68.34, 46.98, 31.39, 30.5, 20.29 ppm.

19

The hydrazone 3 (1 equiv) was dissolved in MeOH (40 mL). NaBH3CN (1.2 equiv) was added, and the pH value was brought to 3 by slowly adding a solution of 2 N HCl. The mixture was stirred for 1 h, and then adjusted the pH value to 1. After 10 min of stirring, the solution was neutralized with solid NaHCO3, the mixture was filtered, concentrated under vacuum, and the residue was taken up with EtOAc (50 mL) and washed with H2O and brine. The organic layer was dried over Na2SO4, and the solvent was removed to give crude oil, which was purified by chromatography on silica gel (EtOAc-PE, 3:7 and 5:5), or precipitate, which was recrystallized in EtOH. Due to the slow pyramidal inversion of trisubstituted nitrogen atom some NMR spectrum appears as a set of two conformers or are not well resolved.
Compound 4d [R¹ = CH(CH3)2]: yield 95%; oil. ¹H NMR (200 MHz, CDCl3): δ = 0.83 (br s, 3 H), 0.90 (br s, 3 H), 3.12 (s, 3 H), 3.22 (br s, 1 H), 4.28 (t, 1 H, J = 6.1 Hz), 4.52 (br, 2 H), 4.67 (br s, 1 H), 7.30-7.90 (m, 8 H) ppm. ¹³C NMR (75.5 MHz, CDCl3): δ = 154.8, 143.9, 141.3, 127.7, 127.1, 125.6, 119.9, 68.3, 47.0, 31.4, 39.1, 20.8, 20.7 ppm.

20

N-Fmoc-N-methylhydrazine (4, 5 mmol) and glyoxilic acid (1 g, 2 equiv) were dissolved in MeOH-CH2Cl2 (15 mL:10 mL) and NaBH3CN (0.67 g, 2 equiv) was added in portions over 1 h. Reaction was monitored by TLC using Et2O-toluene (1:1) as eluent. After 1.5 h of stirring reaction was quenched with 2 M aq HCl, concentrated in vacuo, diluted with EtOAc, and extracted with H2O (3×), brine, and dried over anhyd Na2SO4. Crude N β-Fmoc-N β-Me-aza-β³-aa-OH was obtained and was purified by column chromatography. Due to the slow pyramidal inversion of trisubstituted nitrogen atoms most of the NMR spectrum appears as a set of two diastereomers or are not well resolved.
N β-Fmoc-N β-Me-aza-β³-Leu-OH (5d, R = CH2CH(CH3)2]: yield 80%. ¹H NMR (200 MHz, CDCl3): δ = 0.85, 0.93 (m, 6 H), 1.39 (m, 1 H), 2.10-3.10 (AB, 2 H), 2.72 (s, 3 H), 3.17, 3.37 (AB, 2 H), 4.24 (t, 1 H, J = 5.7 Hz), 4.63 (br, 2 H), 7.29-7.83 (m, 8 H) ppm. ¹³C NMR (75.5 MHz, CDCl3): δ = 172.4, 158.15, 143.5, 141.7, 127.8, 127.2, 124.95, 120.0, 67.95, 62.5, 57.3, 47.3, 30.4, 26.3, 20.6, 20.3 ppm. ESI-HRMS:
m/z calcd for C22H26N2O4 [M + H]+: 383.19708; found: 383.1971 (0 ppm).

21

A mixture of N-Fmoc-N-MeNH2 (0.766 g, 2.86 mmol), benzaldehyde (0.334 g 1.1 equiv), and MgSO4 (0.1 g) was irradiated (power 30 W) for 15 min. The reaction was carried without solvent. The crude product was diluted in CH2Cl2, filtered, and concentrated under reduced pressure. The crude hydrazone was used in the next step without further purification.
Compound 3 (R¹ = Ph): yield 99%; oil. ¹H NMR (300 MHz, CDCl3): δ = 3.31 (s, 3 H), 4.26 (t, 1 H, J = 10.5 Hz), 4.43 (d, 2 H, J = 10.6 Hz), 7.59 (s, 1 H), 7.18-7.77 (m, 13 H) ppm. ¹³C NMR (75.5 MHz, CDCl3): δ = 154.8, 143.9, 141.4, 139.9, 135.0, 129.7, 128.8, 127.8, 127.2, 127.1, 125.6, 120.0, 68.5, 47.1, 30.8 ppm.

22

N β-Fmoc-N β-Me-aza-β³-Gly-OH (5, R = H; 500 mg, 1.53 mmol) with BrCH2CO2 t-Bu (2 equiv, 597 mg, 3.06 mmol), and DIEA (2.2 equiv, 0.44 g) were mixed at r.t. in DMF (10 mL) for 2 d. Reaction mixture was diluted with EtOAc, extracted with H2O (3×), brine, dried over anhyd Na2SO4, and evaporated in vacuo. Crude product was purified by column chromatography using EtOAc-PE (1:1) as eluent; 4.78 g (91%) of N β-Fmoc-N β-Me-aza-β³-Asp(Ot-Bu)-OH (5h, R = CH2CO2 t-Bu) was obtained. N β-Fmoc-N β-Me-aza-β³-Asp(Ot-Bu)-OH (5h, R = CH2CO2 t-Bu): ¹H NMR (300 MHz, CDCl3): δ = 1.48 (s, 9 H, CH3), 3.12 (s, 3 H, CH3), 3.74 (br s, 2 H, CH2), 4.25 (t, 1 H, J = 6 Hz), 4.49 (br d, 2 H), 4.55 (s, 2 H), 7.27-7.78 (m, 8 H) ppm. ¹³C NMR (75.5 MHz, CDCl3): δ = 169.7, 166.4, 156.4, 143.8, 141.3, 127.8, 127.1, 124.9, 120.0, 82.6, 67.2, 61.3, 51.0, 47.2, 37.5, 28.0. ESI-HRMS: m/z calcd for C24H28N2O6 [M + H]+: 440.194738; found: 440.1951 (3 ppm).

23

MeNHNHMe˙2HCl (0.50 g, 3.76 mmol) was dissolved in MeCN (7 mL), Et3N (2.5 equiv, 2.11 mL) was added and precipitate formed was removed by filtration. To remaining solution of MeNHNHMe solution of FmocCl (0.5 equiv, 0.49 g) was added dropwise over 2.5 h at 0 ˚C. Reaction was monitored by column chromatography using EtOAc-PE (1:1) as eluent. Organic phase was then diluted with EtOAc, extracted with H2O (3×), brine, dried over anhyd Na2SO4, concentrated in vacuo, and purified by column chroma-tography using EtOAc-PE (2:3) as eluent; 0.54 g (39%) of MeFmocNNHMe 4b was obtained.
MeFmocNNHMe (4b): ¹H NMR (300 MHz, CDCl3): δ = 2.55 (br s, 3 H), 3.05 (s, 3 H), 4.27 (t, 1 H, J = 6 Hz), 4.48 (s, 2 H), 7.37-7.75 (m, 9 H) ppm.