Permanent protection, hydroxyl groups

The classical permanent protecting group of carbohydrate hydroxyl functions is probably the benzyl ether. It is very stable and can be readily removed under essentially neutral conditions. For this reason, numerous benzylation and 0-debenzylation procedures have been described. Benzyl ether formation is usually achieved by the reaction of alcohols and benzyl halides in the presence of a base such as sodium hydride in anhydrous DMF (O Scheme 2) [11], or a mild base (Ag20) in THF using a phase-transfer catalyst [12]. Benzylation can also be accomplished by the use of an acidic catalyst with benzyltrichloroacetimidate as the reagent [13]. A method using the reductive etherification of TMS ethers under non-basic conditions has also been reported [14]. 

For the acceptor 2, the other partner in the reaction in Scheme 1, all hydroxyl groups, except the targeted one(s) are usually protected. Benzyl is the traditional permanent protecting group, for it must survive until the final step where it is removed to provide the free glycan . 

Below is a list of the most commonly used hydroxyl protecting groups including conditions for their introduction and removal. For a discussion of their regioselective introduction, see Section 3.3. A rough distinction between permanent and temporary protecting groups is also made. 

---