Dear Ngoc,

You can protect the three hydroxyl groups of galic acid by making the corresponding tribenzylic derivative via reacting galic acid with benzyl halide. Deprotection of the tribenzylic derivative can be done by a reduction with H2/Pd-C.

For more on the protection and deprotection, please use the following link:

http://www.tcichemicals.com/en/us/support-download/tcimail/application/117-30a.html

Hoping this will be helpful,

Rafik

From Oxidation by Dioxygen? 

pKa of gallic acid is 4.4 (carboxylic group) ~9 (phenolic). Phenolate is extremely easily oxidized by O2. Fully protonated gallic acid is badly soluble in water. Therefore, gallic acid is the most stable at pH range 5-6. Addition of ascorbic acid will also protect gallic acid from oxidation until complete consumption (by oxidation) of ascorbic acid

Thank you for your suggestions, I will try :)