Selective Reduction of Azido Groups in Monosaccharides with Triphenyl-phosphine

Abstract

The regioselective reduction of one azido group in glycopyranoside and mannitol derivatives containing two azido functions with triphenylphosphine is described. In cyclic substrates with two secondary azides, equatorial azides were reduced in good yields in preference to axial azides. Steric, but not electronic, factors ­appear to determine the regiochemical outcome of the Staudinger reduction.

Key words

selective reduction - azido group - monosaccharides - triphenylphosphine - functional-group manipulation

References and Notes

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24

p -Tolyl 4,6- O -Benzylidene-2-acetamino-3-azido-2,3-dideoxy-1-thio-α-d-allopyranoside ( 1b): To a solution of 1a (64.2 mg, 0.15 mmol) in anhyd THF (4.0 mL) was added PPh₃ (44.0 mg, 0.167 mmol, 1.1 equiv), and the mixture was stirred at 65 °C for 8 h. Then H₂O (2.0 mL) was added, and the mixture was stirred at 65 °C for another 8 h. The reaction mixture was concentrated to dryness. The resulting residue was dissolved in py (3.0 mL) and Ac₂O (1.5 mL) was added dropwise at 0 °C. The mixture was stirred for 6 h at r.t. and the solvent was removed under vacuum. The residue was purified by column chromatography on silica gel (PE-EtOAc, 3:1 to 2:1) yielding the title compound 1b (64.5 mg, 97%) as a colorless oil. ¹H NMR (500 MHz, CDCl₃): δ = 7.52-7.50 (m, 2 H), 7.40-7.36 (m, 3 H), 7.33 (d, 2 H, J = 8.0 Hz), 7.11 (d, 2 H, J = 8.5 Hz), 6.20 (d, 1 H, J = 9.5 Hz), 5.59 (s, 1 H), 5.32 (d, 1 H, J = 6.0 Hz), 4.66-4.59 (m, 2 H), 4.35 (dd, 1 H, J = 5.5, 10.5 Hz), 4.23 (t, 1 H, J = 3.0 Hz), 3.87 (dd, 1 H, J = 3.0, 10.5 Hz), 3.78 (t, 1 H, J = 10.5 Hz), 2.32 (s, 3 H), 2.08 (s, 3 H). ¹³C NMR (125 MHz, CDCl₃): δ = 169.44, 137.86, 136.77, 132.01, 131.71, 129.90, 129.27, 128.34, 126.15, 102.05, 88.94, 78.37, 68.88, 59.93, 59.86, 48.93, 23.28, 21.04. HRMS: m/z calcd for C₂₂H₂₅N₄O₄S [M + H]⁺: 441.1591; found: 441.1595.