Aldoses, cyclic acetals

Aldoses and aldosides, cyclic acetals of, 20, 219-302 Alkaline degradation, of polysaccharides, 13, 289-329 Altrose,... 

In this Section, hydrolysis, acetolysis, and isomerization of acetals are considered. Selective deprotection of acetals may also be achieved through halogenation, hydrogenolysis, ozonolysis, and photolysis, but these topics, are covered in an accompanying article in this Series, on the reactivity of cyclic acetals of aldoses and aldosides,3 and will not be discussed here. 

Of all the selective, deprotection procedures that are available to carbohydrate chemists, the partial hydrolysis of polyacetals is probably the most familiar. Articles by de Beider4,5 and Brady6 contained examples of this type of reaction for aldose and ketose derivatives, respectively, and an article by Barker and Bourne7 gave useful information from the early literature on the graded, acid hydrolysis of acetal derivatives of polyols. A discussion of the stereochemistry of cyclic acetals of carbohydrates was included in an article by Mills 70 and in one by Ferrier and Overend,76 and a survey of the formation and migration of carbohydrate cyclic acetals was made by Clode.7c... 

Since 1895, when Emil Fischer1 described the reaction of aldehydes and ketones with glycoses, an impressive part of the chemistry of carbohydrates has dealt with acetals, and especially cyclic acetals (mainly 1,3-dioxolanes and 1,3-dioxanes). There are probably relatively few studies on the synthetic chemistry of monosaccharides that do not describe at least one acetal of a carbohydrate, be it for routine protection, or for use in an original synthesis. At least, in this Series, three articles have appeared on the cyclic acetals of the aldoses and aldosides2,3 and of the ketoses4, one article dealt with acetals of tetri-... 

This article is limited to the reactions involving cyclic acetals of aldoses and aldosides. However, for comprehensiveness, or for their potential usefulness in carbohydrate chemistry, some properties of cyclic acetals not directly relevant to aldoses and aldosides are included. It may be recalled that the chemistry of dithioacetals had already been reviewed in this Series.7... 

Study of this reaction has not been limited to cyclic acetals of aldoses and aldosides, and synthetic applications in the ketose series have been developed,273,276,278,278 particularly for isopropylidene acetals of D-fructose. As an illustration of these reactions, two examples are given here. The action of butyllithium on 2,3 4,5-di-0-isopropyli-dene-l-0-methyl-/3-D-fructopyranose (280) gave273 a 30% yield of the 5-enopyranose 282. Abstraction by the base of the axial hydrogen atom from the 6-methylene group, giving the anion 281, was invoked... 

Several reviews have already been published on the subject, for example, the acetala-tion of alditols [4], of aldoses and aldosides [5,6], and of ketoses [7]. Some aspects of the stereochemistry of cyclic acetals have been discussed in a review dealing with cyclic derivatives of carbohydrates [8], also in a general article [9] and, more recently, in a chapter of a monograph devoted to the stereochemistry and the conformational analysis of sugars [10], Aspects on predicting reactions patterns of alditol-aldehyde reactions are reviewed within a general series of books on carbohydrates [11]. The formation and migration of cyclic acetals of carbohydrates have also been reviewed [12,13],... 

J. Gelas, The reactivity of cyclic acetals of aldoses and aldosides, Adv. Carbohydr. Chem. Biochem. 39 71 (1981). 

Dialkyl dithioacetal derivatives of ketoses, such as D-fiuctose and L-sorbose, me inaccessible directly from the parent sugars, the ketose undergoing extensive decomposition under the conditions employed for mercaptaladon of aldoses. Such derivatives can, however, be prepared by indirect methods. Acetylation of D-fiuctose [40] and L-soibose with acetic adiydride and zinc chloride [41] leads to good yields of acyclic pentaacetates in which foe ketose carbonyl is not involved in a cyclic acetal. Subsequent treatment of these acetylated derivatives with thiols affords foe acetylated dialkyl dithioacetals in satisfactory yields, and conventional deacetylation affords foe unprotected dialkyl dithioacetals [40,41]... 

In base, aldoses and ketoses rapidly equilibrate to mixtures of sugars (Fig. 123) (174). Most sugars react with alcohols under acidic conditions to yield cyclic acetals (glycosides). Glycoside formation, like acetal formation, is catalyzed by acid and involves cation intermediates (Fig. 124). 

This type of reaction has been used by Spath69 and Tschitschibabin60 for the synthesis of a-substituted ethers. Since the fully methylated aldoses have a cyclic acetal structure, Bonner and Craig91 attempted to extend Reaction (6) to the carbohydrate series. 

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