A Mild and Environmentally Benign Synthetic Protocol for Catalytic Hydrolysis of Thioglycosides

Abstract

A wide variety of thioglycosides 1 are selectively hydrolyzed to the corresponding 1-hydroxy sugars 2 in good yields at 0-5 °C, by employing V₂O₅-H₂O₂ catalyzed oxidation of ammonium bromide in CH₂Cl₂-H₂O solvent system. The methodology is very mild, environmentally benign, efficient and highly chemoselective. No side reactions such as bromination either at the anomeric position or double bond or oxidation at the sulfur are encountered.

Key words

thioglycosides - 1-hydroxy sugars - hydrolysis - vanadium pentoxide - hydrogen peroxide - ammonium bromide

References

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16

Khan, A. T.; Schmidt, R. R. unpublished results.

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A Typical Procedure: To a suspension of V₂O₅ (0.018 g, 0.1 mmol) in water (0.5 mL) is added 30% H₂O₂ solution (0.6 mL, 5 mmol) at ice-bath temperature and stirring is continued. After 25 min, the solution changes into a clear brown-red in colour and ammonium bromide (0.098 g, 1 mmol) is added into it. The substrate ethyl-2,3,4,6-tetra-O-acetyl-1-thio-β-d-gluco-pyranoside (0.392 g, 1 mmol) is added after 10 min by dissolving in CH₂Cl₂ (5 mL) and stirring is continued further. After 30 min of stirring, H₂O₂ (0.6 mL, 5 mmol) is added again and the reaction is completed with additional stirring of 3.5 h as monitored by TLC. The reaction mixture is then extracted with CH₂Cl₂ (2 × 15 mL), washed with water (1 × 15 mL). The organic layer is dried over anhyd Na₂SO₄ and concentrated in vacuo. Evaporation of the solvent gives the crude residue, which is purified by column chromatography on silica gel (eluent: hexane/EtOAc, 9:1). The product is obtained as colourless liquid 0.289 g (83%).

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Spectroscopic Data for Compound 1b: White solid, mp: 73-74 °C, lit. mp: 74-75 °C; [α]_D -8.5 (c 1, CHCl₃), lit.: [α]_D -9.0. ¹H NMR (400 MHz, CDCl₃): δ = 5.43 (d, 1 H, J = 2.87 Hz), 5.28 (m, 1 H), 5.05 (dd, 1 H, J = 3.27 and J = 9.94 Hz), 4.49 (d, 1 H, J = 9.9 Hz, H-1), 4.14 (m, 2 H), 3.94 (m, 1 H), 2.73 (m, 2 H, SCH₂CH₃), 2.16 (s, 3 H, OAc), 2.07 (s, 3 H, OAc), 2.05 (s, 3 H, OAc), 1.99 (s, 3 H, OAc), 1.29 (t, 3 H, J = 7.4 Hz, SCH₂CH₃); ¹³C NMR (100 MHz, CDCl₃): δ = 170.43, 170.26, 170.11, 169.61, 84.10, 74.41, 71.95, 67.30, 67.23, 61.50, 24.40, 20.84, 20.69 (2 C), 20.62, 14.88. Anal. Calcd for C₁₆H₂₄O₉S: C, 48.97; H, 6.16. Found: C, 48.42; H, 5.97. Spectroscopic Data for Compound 2b: mp:144-145 °C, lit. mp: 143-146 °C; [α]_D = +105.5 (c 1, CHCl₃), lit.: [α]_D = +135-69. ¹H NMR (400 MHz, CDCl₃): δ = 6.30 (d, 1 H, J = 3.64 Hz), 5.45 (d, 1 H, J = 2.2 Hz), 5.18 (dd, 1 H, J = 3.40 and J = 10.72 Hz), 4.28 (m, 1 H), 4.16 (m, 1 H), 4.13-4.04 (m, 3 H), 2.19 (s, 3 H, OAc), 2.15 (s, 3 H, OAc), 2.07 (s, 3 H, OAc), 2.04 (s, 3 H, OAc). ¹³C NMR (100 MHz, CDCl₃): δ = 170.77, 170.39, 170.04, 169.39, 91.90, 70.36, 68.67, 67.50, 66.03, 61.25, 20.94, 20.74, 20.62, 20.56. Anal. Calcd for C₁₄H₂₀O₁₀: C, 48.28; H, 5.78. Found: C, 48.07; H, 5.62.