Temporary protection, hydroxyl groups

Dihydro-2f/-pyran (167) is well known in organic chemistry as a reagent employed for the temporary protection of primary and secondary hydroxyl groups. Addition reactions to the double bond of 167 afford compounds that fall into the category of 3,4-dideoxypentoses. 

The chemical transformations of dialkyl dithioacetals have been reviewed in detail [47] and offer routes to a variety of useful carbohydrate derivatives. Dialkyl dithioacetal derivatives of sugars continue to play an important role in modem synthetic carbohydrate chemistry through reactions of die dithioacetal function and manipulation of the sugar hydroxyl groups. Dithioacetals also provide a convenient method for temporary protection of sugar carbonyl groups in the synthesis of noncarbohydrate natural products. 

The triphenylmethyl (trityl) group remains the most frequently used for the temporary protection of primary hydroxyl groups (for review see Refs. [2, 3, 254]) which can be conveniently regenerated from the corresponding ethers by mild acid treatment or by hydrogenolysis. Moreover, good crystallizing properties of trityl ethers, easy... 

In addition to standard syntheses in solution, even the solid-phase version has been developed, using a polymer-bound tritylation reagent [260-263]. With such temporary protection of the primary hydroxyl group, methyl 2,3,4-tri-O-benzoyl-a-D-gluco-pyranoside has been prepared from methyl a-D-glucopyranoside [260, 261]. 

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. 

To increase the scope of available hydroxyl protecting groups substituted benzyl ethers, which can be selectively removed in the presence of unsubstituted benzyl ethers have been developed. These substituted benzyl ethers are generally less stable to different reaction conditions than unsubstituted benzyl ethers and therefore are used as temporary protecting groups. 

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