Common diols protecting groups

Diols (1,2 and 1,3) are commonly protected as their 0,0-acetals and ketals. Acetals are compounds with general structure RR C (OR ) (OR ), where R and R may be H (but not necessary), but R and R H. Ketals are subclass of acetals where neither R nor R is H. 

In a similar manner, ethylidene acetals, cyclopentylidene acetals, cyclohexylidene acetals, arylidene acetals and cyclic carbonates can be prepared. 

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A benzylidene acetal is a commonly used protective group for 1,2- and 1,3-diols. In the case of a 1,2,3-triol the 1,3-acetal is the preferred product. It has the advantage that it can be removed under neutral conditions by hydrogenolysis or by acid hydrolysis. Benzyl groups and isolated olefins have been hydrogenated in the presence of 1,3-benzylidene acetals. Benzylidene acetals of 1,2-diols are more susceptible to hydrogenolysis than are those of 1,3-diols. In fact, the former can be removed in the presence of the latter. A polymer-bound benzylidene acetal has also been prepared." ... 

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 ... 

Protective Groups for Diols. Diols represent a special case in terms of applicable protecting groups. 1,2- and 1,3-diols easily form cyclic acetals with aldehydes and ketones, unless cyclization is precluded by molecular geometry. The isopropylidene derivatives (also called acetonides) formed by reaction with acetone are a common example. 

Cyclic acetals are useful and common protecting groups for aldehydes and ketones, especially during the course of a total synthesis [8]. The successful synthesis of acetals frequently relies on the removal of water, a by-product of the reaction between the carbonyl compound and the corresponding diol. A Dean-Stark trap is often used for the removal of water as an azeotrope with benzene, but this method is not suitable for small-scale reactions. In addition, the highly carcinogenic nature of benzene makes it an undesirable solvent. Many of the reported catalysts for acetal synthesis such as p-toluenesulfonic acid and boron trifluoride etherate are toxic and corrosive. 

Silane N2 can be used as a silylene-type protecting group for diol compounds. The introduction can be easily achieved by using 1-hydroxybenzotriazole (HOBt) as catalyst and triethylamine as base in acetonitrile at ambient temperature [7]. Protection can be achieved under milder conditions compared to the di-tert-butyl silylene group (heating is necessary). The additional preparation of a highly reactive (and sensitive) ditriflate, which is commonly used for the di-tert-butylsilyl derivative, is not necessary. This method is suitable for aliphatic and aromatic 1,2-diols as well as for 1,3-diols. 

Cyclic Acetals. 2,2-Dimethyl-1,3-propanediol (1) has been used effectively as a protecting group for ketones and aldehydes. The formation of the cyclic acetal can be accomplished in many ways. The most common is to reflux the ketone and diol... 

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