Efficient Formation and Cleavage of Benzylidene Acetals by Sodium ­Hydrogen Sulfate Supported on Silica Gel

Abstract

NaHSO₄·SiO₂ was used as an efficient heterogeneous catalyst for both the formation and the cleavage of benzylidene ­acetals. This catalyst is compatible with many functional or protective groups. Under different solvent systems, either the formation or the cleavage of benzylidene acetals was carried out smoothly in ­excellent yields and with good chemoselectivity.

sodium hydrogen sulfate supported on silica gel - protection - deprotection - benzylidene acetals - carbohydrate

References and Notes

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Method for Preparation of NaHSO ₄ ·SiO ₂
To a solution of 4.14 g (0.03 mol) of NaHSO₄·H₂O in 20 mL of H₂O in a 100 mL round-bottomed flask was added 10.0 g of SiO₂ (column chromatographic grade, 60 Å, 200-300 mesh). Then, H₂O was removed under reduced pressure, and a free-flowing white solid was obtained. This reagent was further dried by placing the beaker in an oven maintained at 105 °C for at least 10 h prior to use.

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General Procedure for the Formation of Benzylidene Acetals To a stirred solution of the glycoside substrate (100 mg) and benzaldehyde dimethyl acetal (1.5 mmol) in MeCN (10 mL) was added activated NaHSO₄·SiO₂ (200 mg, dried at 105 °C for 10 h prior to use) at r.t. After completion of the reaction (monitored by TLC), the reaction mixture was quenched with Et₃N (0.2 mL). The catalyst was filtered off through a plug of Celite, and the filtrate was removed under reduced pressure to yield the product. Although the product was pure enough, analytical samples were prepared by passing the crude reaction product through a short column of silica gel using PE-EtOAc as eluent.
Compound 16: colorless solids, mp 108-109 °C. ¹H NMR (500 MHz, CDCl₃): δ = 8.10-8.08 (m, 2 H), 7.61-7.57 (m, 1 H), 7.49-7.44 (m, 4 H), 7.37-7.33 (m, 3 H), 6.02-5.94 (m, 1 H), 5.53 (s, 1 H), 5.39-5.35 (m, 1 H), 5.26-5.23 (m, 1 H), 4.98 (dd, J = 3.5, 10.5 Hz, 1 H), 4.52-4.46 (m, 3 H), 4.38-4.35 (dd, J = 2.0, 12.5 Hz, 1 H), 4.22-4.17 (m, 1 H), 4.13-4.08 (m, 2 H), 3.55 (d, J = 1.0 Hz, 1 H). ¹³C NMR (125 MHz, CDCl₃): δ = 166.0, 137.5, 133.50, 133.45, 130.0, 129.3, 128.9, 128.5, 128.1, 126.1, 117.9, 101.0, 100.7, 72.8, 72.7, 70.3, 68.9, 66.4, 60.6. LRMS-FAB: m/z calcd for C₂₃H₂₇N₄O₆ [M + NH₄ ⁺]: 455; found: 455. Anal. Calcd for C₂₃H₂₃N₃O₆: 63.15; H, 5.30; N, 9.61, Found: C, 62.80; H, 5.47; N, 9.57.

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General Procedure for the Cleavage of Benzylidene Acetals To a stirred solution of the 4,6-O-benzylidene-glycopyrano-side (100 mg) in a mixed solvent of CH₂Cl₂ and MeOH (or i-PrOH; v/v 4:1, 10 mL) was added activated NaHSO₄·SiO₂ (200 mg, dried at 105 °C for 10 h prior to use) at r.t. After completion of the reaction (monitored by TLC), the mixture was filtered through a Celite bed and the filtrate was con-centrated. The residue was purified by column chromatog-raphy on silica gel using PE-EtOAc as eluent to afford the pure product. If the acid-sensitive functional groups exist, it is necessary to quench the reaction with Et₃N (0.2 mL) after completion of the reaction.
Compound 19: white solids, mp 135-136 °C. ¹H NMR (300 MHz, CDCl₃): δ = 7.45 (d, J = 8.1 Hz, 2 H), 7.35 (d, J = 8.7 Hz, 2 H), 7.27-7.24 (m, 2 H), 7.10 (d, J = 8.1 Hz, 2 H), 6.91-6.85 (m, 4 H), 4.76 (d, J = 10.2 Hz, 1 H), 4.65 (d, J = 12.6 Hz, 3 H), 4.56 (d, J = 12.9 Hz, 1 H), 3.99-3.93 (m, 2 H), 3.81 (s, 3 H), 3.80 (s, 3 H), 3.79-3.72 (m, 1 H), 3.67 (t, J = 9.3 Hz, 1 H), 3.54 (dd, J = 3.3, 9.3 Hz, 1 H), 3.46-3.44 (m, 1 H), 2.58 (br s, 1 H), 2.32 (s, 3 H), 2.01 (br s, 1 H). ¹³C NMR (75 MHz, CDCl₃): δ = 137.8, 132.6, 130.3, 129.9, 129.7, 129.6, 113.9, 113.8, 87.8, 82.1, 77.9, 75.4, 72.0, 69.4, 67.4, 62.8, 55.3. 21.1. ESI-HRMS: m/z calcd for C₂₉H₃₅O₇S [M + H⁺]: 527.2098; found: 527.2104.

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