[may4ik_l]

Someone knows how to remove it ? What is the chemical mechanism for that ?

[DavidW]

Are you referring to alcohol protecting group, 3,4-dimethoxybenzyl ether, and the removal of it to regenerate the alcohol?  If you are, DDQ should do the trick.

[lavoisier]

For 3,4-dimethoxybenzyl-protected alcohols, Greene suggests DDQ (as DavidW wrote), and the overall mechanism is oxidative (you get your alcohol + 3,4-dimethoxybenzaldehyde). The detailed mechanism is probably via free-radicals, as you have a quinone around.

For 3,4-dimethoxybenzyl-protected amines, Greene gives "H2SO4/anhydrous TFA, anisole". I think it means either acid + anisole. In this case, the mechanism is probably via carbocation, i.e. you get N-protonation followed by departure of 3,4-dimethoxybenzyl cation, which is scavenged by anisole. Or maybe it's anisole which attacks the N-protonated substrate in the transition state.

[movies]

Another common oxidative removal uses cerric ammonium nitrate (CAN) as the oxidizing agent.

Like all the other benzyl protecting groups, it can also be cleaved by reduction with hydrogen and Pd/C.