N²-tert-Butoxycarbonyl-N⁵-[N-(9-fluorenylmethyloxycarbonyl)-2-aminoethyl]-(S)-2,5-diaminopentanoic Acid

Abstract

N²-tert-Butoxycarbonyl-N⁵-[N-(9-fluorenylmethyloxycarbonyl)-2-aminoethyl]-(S)-2,5-diaminopentanoic acid (5) has been synthesized by the reaction of N²-tert-butoxycarbonyl-L-2,5-diaminopentanoic acid (Boc-L-ornithine, 3) and N-Fmoc-2-aminoacetaldehyde (N-Fmoc-glycinal, 4) in the presence of sodium cyanoborohydride in methanol containing 1% acetic acid at room temperature.

Introduction

Ornithine (1, Figure 1) is a non proteinogenic amino acid, which plays an important role in ammonia metabolism via the urea cycle. Its intake has been recommended as a nutritional supplement for its antifatigue effect [1,2]. N⁵-Ornithine derivatives showed their potential as orally active non-peptide fibrinogen receptor antagonists [3] and selective prostaglandin EP₄ receptor antagonists [4]. L-N⁵-(1-Iminoethyl)-ornithine dihydrochloride (2, Figure 1) was used for studies of bactericidal activity of human eosinophilic granulocytes against Escherichia coli [5]. The capability of the N⁵-ornithine derivatives as biologically active agents inspired us to synthesize its new-fangled derivatives.

Reductive amination has become a versatile and practical method for the preparation of amines in organic synthesis. New catalysts for reductive amination exemplify organocatalysts, complexes of transition metals (Ru, Rh, Ir), reagents involving boron, tin and silicon [6]. This reaction has found its application for the synthesis of natural products, drug molecules and chiral ligands. In this short note, we describe a reductive amination between N²-tert-butoxycarbonyl-L-2,5-diaminopropionic acid (3) and N-Fmoc-2-aminoacetaldehyde (N-Fmoc-glycinal, 4) to derivatize the ornithine moiety by converting its N⁵-primary amine into a secondary amine and extending it with an aminoethyl group; thus producing the building block for the new triamino acid ^δN-(2-aminoethyl)ornithine (Scheme 1). As judged by thin layer chromatography (TLC), the Fmoc seems to survive the conditions, but it is possible that some of the aldehyde was consumed by direct reaction with the borohydride, resulting in a moderate yield. Although a change of reagent perhaps could have improved the outcome, we found the isolated yield after chromatography quite acceptable, considering that only 0.9 equivalents of aldehyde was used (for simplicity of purification).

Experimental

N²-tert-Butoxycarbonyl-N⁵-[N-(9-fluorenylmethyloxycarbonyl)-2-aminoethyl]-(S)-2,5-diaminopentanoic acid (5)

N²-tert-Butoxycarbonyl-L-2,5-diaminopropionic acid (3, Boc-L-ornithine, 0.116 g, 0.5 mmol) was dissolved in a solvent mixture (acetic acid/methanol, 1:99, v/v, 10 mL) at rt under stirring. N-Fmoc-2-aminoacetaldehyde (4, N-Fmoc-glycinal, 0.129 g, 0.46 mmol) [7] was added into the reaction mixture slowly followed by addition of NaBH₃CN (0.072 g, 1.14 mmol) in a single lot. The reaction mixture was stirred at rt and the progress of the reaction was monitored by thin layer chromatography. After 18 h, the reaction mixture was evaporated to dryness under reduced pressure and was dissolved in ethyl acetate (15 mL). Organic layer was washed with water (10 mL) and brine (10 mL × 2), dried over Na₂SO₄ and evaporated to dryness under reduced pressure to get crude compound. Pure compound was obtained by purification of the crude using column chromatography (2%–4% methanol in dichloromethane containing 1% acetic acid) to afford pure compound 5 (0.104 g, 42%).

Physical characteristics: White sticky solid.

Yield: 0.104 g, 42%.

R_f = 0.17 (Methanol/dichloromethane/acetic acid = 1:8.9:0.1, v/v/v).

¹H-NMR (400 MHz, CD₃OD) (δ/ppm): 7.70 (2H, d, J = 7.6 Hz, Ar-H), 7.55 (2H, d, J = 7.6 Hz, Ar-H), 7.29 (2H, d, J = 7.6 Hz, Ar-H), 7.21 (2H, d, J = 7.6 Hz, Ar-H), 4.32 (2H, d, J = 6.8 Hz, NCOOCH₂CH), 4.11 (1H, t, J = 6.8 Hz, NCOOCH₂CH), 3.86 (1H, br s, 2-CH), 3.31 (2H, t, J = 5.6 Hz, NCH₂CH₂NHFmoc), 2.98 (2H, t, J = 5.6 Hz, NCH₂CH₂NHFmoc), 2.90 (2H, t, J = 5.6 Hz, 5-CH₂), 1.74–1.61 (4H, m, 3-CH₂, 4-CH₂), 1.32 (9H, s, C(CH₃)₃).

¹³C-NMR (100 MHz, CD₃OD) (δ/ppm), methine and methyl carbons were distinguished from methylene carbons by ¹³C-DEPT: 179.5 (COOH), 159.2, 157.6 (2 × NCOO), 145.2, 142.6 (C-Ar), 128.8, 128.1, 126.1, 120.9 (CH-Ar), 80.2 (C(CH₃)₃), 68.0 (NCOOCH₂CH), 56.2 (C-2), 48.8, 48.6 (C-5, NCH₂CH₂NHFmoc), 48.4 (NCOOCH₂CH), 38.5 (NCH₂CH₂NHFmoc), 31.3 (C-3), 28.8 (C(CH₃)₃), 23.5 (C-4).

IR (KBr) ν_max 3433, 1700, 1570, 1415, 1355, 1250, 1160, 1050, 957, 815, 725, 692 and 620 cm⁻¹.

Optical rotation: ${\left[\mathsf{\alpha }\right]}_D^{27}$ +8.2 (c 1.0, MeOH).

HRMS (ESI-Tof) (m/z) calcd for C₂₇H₃₄N₃O₆⁻ [M-H]⁻ 496.2453; found 496.2463.