[Acetate] [Benzoatel] [Pivaloate] [Levulinate] [Back to Carb. Synthesis]

Ac - (acetate) ester

Standard Protection Procedure

To a solution of the glycoside in dry pyridine (25 eq) under an inert atmosphere at room temperature, acetic anhydride (10 eq) is added and stirred until complete by TLC (usually 16 h).  The reaction mixture is then poured into ice/water and extracted three times with chloroform.  The combined organic layers are extracted with 3% HCl, saturated aqueous sodium bicarbonate, and water.  The organic layer is then dried and concentrated in vacuoThe resulting residue is purified by flash chromatography (SiO2) if necessary.
Removal
The glycoside is dissolved in methanol and a solution of sodium methoxide in methanol (0.1 eq per -OAc) is added drop wise at 0°.  The solution is warmed to room temperature and stirred under an inert atmosphere until complete by TLC (usually within a few hours).  Amberlite cationic exchange resin is then added with vigorous stirring until the pH of the mixture is neutral.  The mixture is then filtered and concentrated.  The resulting residue is purified by flash chromatography (SiO2) if necessary.
OR
The Glycoside is dissolved in methanol and hydrazine hydrate (15 eq per -OAc) is added in two portions over 1.5 hours.  The solution is stirred at room temperature under an inert atmosphere until complete by TLC (usually 6 hours).  The solution is then neutralized with glacial acetic acid.  The mixture is filtered through celite and concentrated.  The resulting residue is purified by flash chromatography (SiO2) if necessary.
References
    J. Org. Chem., 1996, 61, 6442-6445.
    "Synthetic Methods for Carbohydrates" Lemieux, Ch 6, pg. 90-115.
    J. Chem. Soc., Perkin Trans. 1, 1996, 985-993.
[Back to Top] [Back to Carbohydrate Synthesis]

Bz - (benzoate) ester

Standard Protection Procedure

To a solution of the glycoside dissolved in dry pyridine (0.3M), benzoyl chloride (4 eq per -OH) is added at room temperature under an inert atmosphere and stirred until complete by TLC (usually 16 h).  Methanol is added and the solution was concentrated in vacuo.  The residue was dissolved in DCM and washed with water, saturated copper sulfate, and brine.  The organic layer is dried and concentrated in vacuoThe resulting residue is purified by flash chromatography (SiO2) if necessary.
Removal
The glycoside is dissolved in MeOH and a solution of NaOH (2.5 eq per -OH) in MeOH is added slowly.  The mixture is stirred at room temperature under an inert atmosphere till complete by TLC (usually 1 h) and then concentrated in vacuo.  The residue is dissolved in ethyl acetate and the organic solution is washed twice with water, dried (sodium sulfate), and concentrated in vacuo.  The resulting residue is purified by flash chromatography (SiO2) if necessary.
References
    Tetrahedron, 1970, 26, 803-812.
    Carb. Res., 1993, 244, 237-246.
[Back to Top] [Back to Carbohydrate Synthesis]

Piv - (pivaloate) ester

Standard Procedure

The glycoside is dissolved in pyridine:DCM 2:1 and cooled to -20°.  PivCl (1.2 eq per -OH) is then added drop wise under an inert atmosphere and the result is stirred at 0° until complete by TLC (usually 8 h).  The mixture is then diluted with DCM and extracted with 5% HCl twice and once with water.  The organic layer is then dried (sodium sulfate) and concentrated in vacuo.  The resulting residue is purified by flash chromatography (SiO2) if necessary.
Removal
The glycoside is dissolved in MeOH, and a solution of sodium methoxide in methanol (10 eq per -OH) is added drop wise at room temperature under an inert atmosphere and stirred until the reaction is complete by TLC (usually 24 h).  Aqueous HCl (10%) is added to the solution until neutral and the resulting mixture is concentrated in vacuo.  The residue is purified by flash chromatography (SiO2) if necessary.
References
    Tet. Asym. 2000, 295-303.
    J. Am. Chem. Soc. 1990, 112, 3693-3695.
[Back to Top] [Back to Carbohydrate Synthesis]

Lev - (levulinate) ester

Standard Protection Procedure

To a solution of the glycoside in dry pyridine at room temperature under an inert atmosphere, levulinic anhydride (2 eq) is added drop wise.  The reaction was stirred until complete by TLC (usually 24 h) and ice water is added.  The mixture is extracted with chloroform and then the organic layer is back extracted with brine, dried (sodium sulfate), and concentrated in vacuo.  The resulting residue is purified by flash chromatography (SiO2) if necessary.
Removal
The glycoside is dissolved in pyridine and hydrazine hydrate (1M in pyridine:acetic acid 3:2) is added at room temperature under an inert atmosphere.  After addition of hydrazine is sufficient to consume starting material completely by TLC (usually a few minutes), pentane-2,4-dione is added with cooling to quench the reaction.  The mixture is diluted with chloroform and extracted once with water and once with brine.  The organic layer is dried (sodium sulfate) and concentrated in vacuo.  The residue is purified by flash chromatography (SiO2) if necessary.
References
    JACS, 1975, 97, 1614-1615.
    Tet. Lett., 1976, 52, 4875-4878.
[Back to Top] [Back to Carbohydrate Synthesis]