Protection for 1,2- and 1,3-Diols

The prevalence of diols in synthetic planning and in natural sources (e.g., in carbohydrates and nucleosides) has led to the development of a number of protective groups of valuing stability to a substantial array of reagents. Dioxolanes and diox-anes are the most common protective groups for diols. The ease of formation follows the order  

In some cases the formation of a dioxolane or dioxan -can result in the generation of a new stereogenic center, either with complete selectivity or as a mixture of the two possible isomers. Since the new center is removed on deprotection, it should not seriously complicate the use of those protective groups that generate a new stereogenic center. 

Cyclic carbonates and cyclic boronates have also found considerable use as protective groups. In contrast to most acetals and ketals the carbonates are cleaved with strong base and sterically unencumbered boronates are readily cleaved by water. 

Some of the protective groups for diols are listed in Reactivity Chart 3. 

Methylene acetals are the most stable acetals to acid hydrolysis. Difficulty in their removal is probably the reason that these compounds have not been used much. 

CI3S1H, EtiN, CH2CI2, 4-48 h, 25-80°C, 80-95% yield. Primary, secondary, tertiary, allylic, propargylic, or benzylic derivatives are cleaved by this method. 

BU4NNO2, AC2O, pyridine, 40°C, 79-100% yield. Deprotection proceeds by nitrosation of the amine which facilitates nucleophilic addition to the carbonyl. A similar process is used to hydrolyze some amides. 

This carbamate is prepared from the carbamoyl chloride (CH2CI2, DMAP, TEA or RONa, 88-94% yield). It is cleaved by photolysis at 248-365 nm in EtOH, H2O, (91-100% yield) to afford the alcohol and 2-nitrosoaniline.  

In some cases the formation of a dioxolane or dioxane can result in the generation of a new stereogenic center, either with complete selectivity or as a mixture of the 

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Acetonide formation is the most commonly used protection for 1,2- and 1,3-diols. The acetonide has been used extensively in carbohydrate chemistiy to mask selectively the hydroxyls of the many different sugars. In preparing acetonides of triols, the 1,2-derivative is generally favored over the 1,3-derivative, but the extent to which the 1,2-acetonide is favored is dependent on stmcture. Note that the 1,2-selectivity for the ketal from 3-pentanone is better than that from acetone. ... 

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