An Efficient Synthetic Route to N-Glycosylamino Acids Using N ^α-Fmoc-Asp/Glu-5-oxazolidinone as Internal Protection

Abstract

A new facile synthetic route to prepare N-glycosylamino acids in fewer steps is reported. 5-Oxazolidinone served as an effective protecting moiety for N ^α-Fmoc-Asp/Glu and after glycosylation, the ring opening resulted in free α-carboxylic acid, which can be directly used to extend the chain to obtain N-glycopeptides. Both the minimum number of steps as well as circumvention of ortho­gonal protection strategy leads to a cost-effective route for their synthesis. This protocol can be easily scaled up to prepare N-glyco­sylated Asn/Gln acids in large quantities in fairly good yields.

5-oxazolidinone - glycosyl amine - protecting group - N-glycosylamino acid - coupling agent - building-block strategy

References and Notes

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17

Typical Procedure for the Synthesis of Glycosylated N ^α -Fmoc-Asp/Glu-5-oxazolidinones
To a solution of Fmoc-Asp/Glu-5-oxazolidinone (10 mmol) in 10 mL of THF was added 1-amino β-glucose (10 mmol) in THF (40 mL), HBTU (10 mmol), and DIPEA (11 mmol), and stirred at r.t. for about 30 min. The solvent was evaporated under reduced pressure and the residue was diluted with H₂O (15 mL) and extracted with EtOAc (3 × 20 mL). The combined organic layer was washed with 10% citric acid (3 × 10 mL), 5% Na₂CO₃ solution (3 × 15 mL), brine, and dried over anhyd Na₂SO₄. The solution was concentrated and purified by column chromatography using EtOAc-hexane (2:8).
Selected Spectral Data Compound Fmoc-Asn(2,3,4,6-tetra-O-acetyl-β-d-gluco-pyranosyl)oxazolidinone (3a: n = 1): white solid. IR (KBr): ν_max = 1700, 1735, 1801 cm^-1. ¹H NMR (300 MHz, CDCl₃): δ = 2.03-2.13 (4 s, 12 H), 2.81 (m, 2 H), 3.80 (m, 1 H), 4.03 (t, 1 H), 4.24 (m, 2 H), 4.42-4.52 (m, 2 H), 4.98 (t, 1 H), 5.05 (t, 1 H), 5.11 (s, 2 H), 5.21-5.35 (m, 2 H), 5.92 (br d, 1 H), 7. 33 (t, 2 H), 7.41 (t, 2 H), 7.52 (d, 2 H), 7.77 (d, 2 H). HRMS (ES): m/z calcd for C₃₄H₃₆N₂NaO₁₄ [M + Na]⁺: 719.2064; found: 719.2081.
Compound Fmoc-Gln(2,3,4,6-tetra-O-acetyl-β-d-gluco-pyranosyl)oxazolidinone (3b: n = 2): white solid. IR (KBr): ν_max = 1698, 1730, 1800 cm^-1. ¹H NMR (300 MHz, CDCl₃): δ = 1.86 (m, 2 H), 2.03-2.13 (4 s, 12 H), 2.22 (m, 2 H), 3.80 (m, 1 H), 4.03 (d, 1 H), 4.21-4.32 (m, 3 H), 4.48 (m, 2 H), 4.88 (t, 1 H), 5.06 (t, 1 H), 5.12 (s, 2 H), 5.27 (m, 2 H), 5.50 (br d, 1 H), 7. 33 (t, 2 H), 7.41 (t, 2 H), 7.52 (d, 2 H), 7.77 (d, 2 H). HRMS (ES): m/z calcd for C₃₅H₃₈N₂NaO₁₄ [M + Na]⁺: 733.2221; found: 733.2254.

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Typical Procedure for the Preparation of Glycosylated N ^α -Fmoc-Asn/Gln-OH To the glycosylated N ^α-Fmoc-Asn/Gln-5-oxazolidinone (10 mmol) dissolved in THF was added 1 N LiOH solution (1 equiv) and stirred at r.t. for 30 min. The reaction was monitored by TLC. After the completion of reaction, the reaction mixture was acidified with 10% citric acid solution and extracted with EtOAc. The organic layer was washed with brine and dried over anhyd Na₂SO₄. The solvent was removed in vacuo, and the resulted residue was purified by column chromatography using CHCl₃, MeOH and AcOH (40:2:1).
Compound 4a: white solid. IR (KBr): ν_max = 1705, 1733 cm^-1. ¹H NMR (300 MHz, CDCl₃): δ = 2.03-2.13 (4 s, 12 H), 2.81 (m, 2 H), 3.80 (m, 1 H), 4.03 (t, 1 H), 4.24 (m, 2 H), 4.42-4.52 (m, 2 H), 4.98 (t, 1 H), 5.05 (t, 1 H), 5.21-5.35 (m, 2 H), 5.92 (br d, 1 H), 7. 33 (t, 2 H), 7.41 (t, 2 H), 7.52 (d, 2 H), 7.77 (d, 2 H). HRMS (ES): m/z calcd for C₃₃H₃₆N₂NaO₁₄ [M + Na]⁺: 707.2064; found: 707.2080.
Compound 4e: white solid. IR (KBr): ν_max = 1698, 1732 cm^-1. ¹H NMR (300 MHz, CDCl₃): δ = 1.86 (m, 2 H), 2.03-2.13 (4 s, 12 H), 2.22 (m, 2 H), 3.80 (m, 1 H), 4.03 (d, 1 H), 4.21-4.32 (m, 3 H), 4.48 (m, 2 H), 4.88 (t, 1 H), 5.06 (t, 1 H), 5.27 (2 H, m), 5.50 (br d, 1 H), 7. 33 (t, 2 H), 7.41 (t, 2 H), 7.52 (d, 2 H), 7.77 (d, 2 H). HRMS (ES): m/z calcd for C₃₄H₃₈N₂NaO₁₄ [M + Na]⁺: 721.2221; found: 721.2413.