[a student]

what does orthogonal protection exactly means?
I have read this topic at wikipedia but I didn't get what does it exactly means?

[azmanam]

orthogonal protection means you can protect two functional groups with two different protecting groups and later in the synthesis one can be removed in the presence of the other, without deprotecting the second group.

For example, take 1,4-butanediol.  You do a monoprotection of one primary alcohol with NaH, BnBr to give the monobenzyl diol.  Then you monoprotect the other side with, say a TBS group (TBSCl, imidazole).  Now you have 1,4-butanediol with one alcohol protected as a benzyl group, and the other protected as a TBS group.

If you want to take off the silyl group, you can use TBAF and the TBS alcohol will be deprotected, but the benzyl alcohol will remain protected.  Or, if you want to take off the benzyl group, you can use H2, Pd and the benzyl alcohol will be deprotected, but the silyl alcohol will remain protected.  The two protecting groups are orthogonal to each other.