New Approach for the Efficient Synthesis of Carbamate-Tethered Glycosylated Amino Acids and Their Insertion into Peptides

H. P. Hemantha; Vommina V. Sureshbabu

doi:10.1055/s-2008-1032087

LETTER

New Approach for the Efficient Synthesis of Carbamate-Tethered Glycosylated Amino Acids and Their Insertion into Peptides

H. P. Hemantha, Vommina V. Sureshbabu*
Peptide Research Laboratory, Department of Studies in Chemistry, Central College Campus, Dr. B. R. Ambedkar Veedhi, Bangalore University, Bangalore 560 001, India
Fax: +91(80)22292848; e-Mail: hariccb@rediffmail.com;

Abstract

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Abstract

An efficient two-step route for the synthesis of carbamate-tethered glycosylated amino acids by coupling of oxycarbonyl chlorides derived from side-chain hydroxyl group of N ^α-Fmoc-Ser, Thr, Tyr and homoserine (Hser) with appropriately protected sugar-1-amine has been described. The utility of these neoglycoamino acids as building blocks for the synthesis of neoglycopeptides possessing the carbamate moiety has been demonstrated.

Key words

neoglycopeptides - oxycarbonyl chlorides - carbamates - carbamate-tethered glycosylated amino acids - carbonates

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References

References and Notes

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Alternatively, the reaction was also carried out by purging phosgene in to a solution of Fmoc-Ser-OMe (1.1 mmol) and the resulting Fmoc-Ser(OCOCl)OMe (1a) was isolated by the removal of solvent. But considering the health hazards and handling difficulties associated with phosgene, the use of triphosgene is preferred.

Procedure for the Synthesis of N ^α -Fmoc-Ser(OCOCl)OMe (1a)
To a stirred solution of N ^α-Fmoc-Ser-OMe (1 mmol) in anhyd CH₂Cl₂ (15 mL) at 0 °C was added triphosgene (0.4 mmol) and pyridine (1.5 mmol) and stirring was continued for 30 min or till the completion of reaction (TLC analysis). Then it was washed with sat. sodium meta bisulfate solution (2 × 10 mL), H₂O (2 × 10 mL) and dried over anhyd Na₂SO₄. The solvent was evaporated under reduced pressure and the residue was precipitated in EtOAc-hexane in to a pure solid powder which was filtered and dried.
Selected Spectral Data of Fmoc-Ser(OCOCl)OMe (1a)
Yield 92%; mp 108 °C. ¹H NMR (300 MHz, CDCl₃): δ = 2.40 (m, 1 H), 3.20 (s, 3 H), 3.70 (t, 2 H), 3.90 (d, 2 H), 4.28 (m, 1 H), 5.30 (m, 1 H), 7.10-7.50 (m, 8 H). ¹³C NMR (300 MHz, CDCl₃): δ = 43.1, 50.2, 55.4, 67.1, 70.8, 119.0, 122.2, 125.3, 127.0, 141.0, 149.0, 152.0, 156.0, 170.5. IR (KBr): 1698, 1770 cm^-1. HRMS: m/z calcd for [M + Na]: 426.8028; found :426.8014.

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Preparation of Carbamate-Tethered Glycosylated Amino Acid Ester 2
To a solution 1 or 3 (1 mmol) in THF (15 mL) was added glycosyl-1-amine at 0 °C followed by NMM (1.5 mmol) and the reaction mixture was stirred till the completion of reaction. The solvent was evaporated and the residue was taken in CH₂Cl₂ (15 mL)_. To this, 10% citric acid solution (15 mL) was added and the resulting layers were separated. The organic phase was washed with 10% Na₂CO₃ solution (2 × 10 mL), H₂O (2 × 10 mL), and brine (15 mL). The residue, after the removal of solvent, was triturated with hexane to get the product as white crystalline solid. Spectral Data of N ^α -Fmoc-Thr(OCONH-2,3,4,6-tetra- O -acetyl-β-d-glucopyranosyl)OMe (2d)
Yield 82%. ¹H NMR (300 MHz, CDCl₃): δ = 1.2 (d, 3 H), 1.8 (m, 1 H), 2.0 (s, 12 H), 2.31 (m, 1 H), 3.75 (s, 3 H), 4.12 (d, 2 H), 4.23 (m, 1 H), 4.45 (m, 5 H), 4.98 (d, 2 H), 5.4 (m, 2 H), 7.40-7.63 (m, 8 H). ¹³C NMR (300 MHz, CDCl₃): δ = 17.1, 21.3, 21.8, 22.1, 45.0, 55.7, 62.2, 62.5, 67.0, 68.2, 68.9, 71.1, 71.5, 72.8, 90.5, 118.8, 123.2, 125.9, 127.3, 141.5, 149.0, 152.0, 157.2, 166.0, 168.2, 169.3, 178.0. IR (KBr): 1695, 1762 cm^-1. HRMS: m/z calcd for [M + Na]: 751.6863; found: 751.6870.

Preparation of Fmoc-Ser(OCOOPfp)OMe (3a)
Pentafluorophenol (1.1 mmol) was added to a stirred solution of Fmoc-Ser(OCOCl)OMe (1 mmol) in anhyd CH₂Cl₂ (15 mL) at 0 °C followed by NMM (1.5 mmol). After 30 min the precipitated carbonate was filtered and the residue was washed with H₂O, Et₂O, and dried under suction to afford the product as white solid powder.