Ketals diol protection

The reduction of ketals to protected secondary alcohols is readily accomplished in high yield with borane-dimethyl sulphide upon activation with TMSOTf at —78 °C (eq 85). Other Lewis acids require higher temperatures, and 1 equiv of TMSOTf is essential for complete conversion of the ketal. The solvent has a decisive role for controlling the site selectivity in the case of unsymmetrical diols. ... 

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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