Sugar acetalation

B. Mukhopadhyay, D. A. Russell, andR. A. Field, One-pot acetalation-acetylation of sugar derivatives employing perchloric acid immobilised on silica, Carbohydr. Res., 340 (2005) 1075-1080. 

B. Mukhopadhyay, Sulfuric acid immobilized on silica An efficient promoter for one-pot acetalation-acetylation of sugar derivatives, Tetrahedron Lett., 47 (2006) 4337-4341. 

The search for new catalysts that would be effective for acetalations still stimulates interest. Better yields, and increased rates, have been claimed for the reaction of monosaccharides with acetone in the presence of ferric chloride.48 (Ferric chloride was already known to catalyze acetalation in other series.47) Not yet applied in the carbohydrate field, but potentially useful for sugars, is the separate use of two different catalysts. The first one recommended is pyridinium p-toluenesul-fonate as a mild and efficient catalyst for the tetrahydropyranylation of alcohols.48 The main interest in this catalyst lies in the excellent yields... 

Organofluorine Compounds and Fluorinating Agents. Part 21 Perfluoroalkylidene Sugars by Non-Classical Acetalation of Pyranoses ... 

Application of the Wittig reaction in the carbohydrate field is accompanied by certain difficulties. A correct choice of the initial sugar components is the main problem, owing to the basicity of phosphoranes and, especially, to the drastically basic conditions employed with phosphonium ylides (2a). It is not surprising, therefore, that protected (acetalated and aeetylated) aldehydo sugars and resonance-stabilized phosphoranes were used at first,3-5 although partially protected, and even unprotected, aldoses were shown to be amenable to the reaction with various resonance-stabilized phosphoranes, thanks to the presence of the carbonyl form in the mobile equilibrium. The latter reactions, however, are extremely complicated (see Section IV, p. 284). 

Fig. 15 Grafting of an unprotected sugar on an acetal-functionalized polysiloxane by acetalation...

Since the formation of cyclic pyruvate acetals from alkyl pyruvates and diols under classical conditions (i.e. catalysis by acids) is expected to be unfavoured due to the necessity of the intermediate formation of a destabilized carbocation, early attempts to prepare 1-carboxyethylidene sugars used indirect procedures. Later, it was shown that under carefully controlled conditions the direct acetalation of a sugar diol with methyl pyruvate can also be used for preparative purposes. 

Acetals are constructed using a pair of oxygens from hydroxyl groups. In the acetalation of a sugar derivative which has more than two free hydroxyl groups, several acetals are, therefore, possible. Most of the time, an acid catalyst is used to accelerate the equilibrium between possible acetals, and the most stable one is finally isolated as the major product. 

The reader will have perhaps wondered why, in the Fischer glycosidation reaction, there is no acetalation by two alcohol functions of the sugar molecule, leading to a completely internal and bicyclic mixed acetal. As a general rule, an intramolecular reaction is faster than the intermolecular analogue and this type... 

The availability of convenient routes to many rare sugars has prompted studies on their acetalation. Often, the products had previously only been prepared indirectly. Thus, acetonation of D-allose was found to give 2,3 5,6-di-0-isopropylidene-D-allofuranose as the main product.13,14 The corresponding 1,2 5,6-diacetal is also formed, but in very low yield. Likewise, the condensation of acetaldehyde (as... 

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