Carbohydrates protection with boronic acids

Although boronates are quite susceptible to hydrolysis, they have been useful for the protection of carbohydrates. Note that as the steric demands of the diol increase, the rate of hydrolysis decreases. For example, pinacol boronates are rather difficult to hydrolyze in fact, they can be isolated from aqueous systems with no hydrolysis. The section on the protection of boronic acids should be consulted. 

Acylated carbohydrate derivatives. Treatment of glycosides with polymer-supported boronic acid (1) with azeotropic removal of water aSords the more stable five- or six-membered boronate. The unprotected hydroxyl groups of the protected glycosides can be acylated, often in yields higher than those realized by classical methods. 

Probably unjustly these have been somewhat neglected as protective groups, except perhaps in the carbohydrate field. They are prepared by acid-catalyzed reaction of carbonyl compounds with alkyl thiols or with dithiols. The conditions which have been used include ethanedithiol in acetic acid-boron trifluoride [73], ethane-thiol or -dithiol in the presence of hydrogen chloride [81, 82, 83], or in dioxan in the presence of zinc chloride [84]. An interesting alternative is offered by the reaction of carbonyl compounds with ortho-thioboric esters at room temperature without catalyst [85]. 

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