Acetals and ketals groups

There are many references in the literature to experiments in which acetals and ketals of the polyhydric alcohols (and their derivatives) have been treated with reducing agents without there being any effect on the acetal and ketal groups. For example, 2,4-benzylidene-xylitol,66 2,4 3,5-(56) R. M. Haim, A. T. Ness and C. S. Hudson, J. Am. Chem. Soc., 66,1769 (1946). 

The hemiacetal and hemiketal groups of monosaecharides undergo dehydration with alcohol in the presence of an acid catalyst to form acetal and ketal groups, respectively. For example, reaction of... 

Problem 18.29. Show the formation of sucrose from a-glucose and /3-fructose. Mark each hemiacetal and hemiketal carbon with an asterisk. Label acetal and ketal groups. 

The concept of torsional deactivation was expanded further by Ley and coworkers in their exploration of 1,2-diacetal systems. The acetal and ketal groups, when fused to carbohydrate coupling donors, impart rigidity into the structure, making more difficult the conformational changes required for glycosylation, hence the reactivity of the sugar is diminished. 

There are numerous examples of the use of cyclic acetal and ketal groups for the protection of 1,2- and 1,3-glycol systems in steroid [30], glyceride [128], sugar [126, 127] (including nucleoside) and cyclitol [69] chemistry. The most commonly used protecting groups of this type are isopropylidene ketals [30, 126-8] and benzylidene acetals [126-8]. Methylene and ethylidene acetals have also been used frequently [126, 127]. 

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. 

Catechols can be protected as diethers or diesters by methods that have been described to protect phenols. However, formation of cyclic acetals and ketals (e.g., methylenedioxy, acetonide, cyclohexylidenedioxy, diphenylmethylenedioxy derivatives) or cyclic esters (e.g., borates or carbonates) selectively protects the two adjacent hydroxyl groups in the presence of isolated phenol groups. 

Acyclic monothio acetals and ketals can be prepared directly from a carbonyl compound or by transketalization, a reaction that does not involve a free carbonyl group, from a 1,3-dithiane or 1,3-dithiolane. They are cleaved by acidic hydrolysis or Hg(II) salts. 

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