1.3- Dioxolane carbohydrate

The prevalence of diols in synthetic planning and in natural sources (e.g., in carbohydrates and nucleosides) has led to the development of a number of protective groups of valuing stability to a substantial array of reagents. Dioxolanes and diox-anes are the most common protective groups for diols. The ease of formation follows the order ... 

O-Isopropylidene derivatives of carbohydrates form structural isomers from carbohydrates which themselves are epimers. Since structural isomers often fragment differently whereas epimers do not, mass spectra of these derivatives may permit interpretation in terms of stereochemistry. Although molecular-ion peaks are not observed, the molecular weight can be determined readily from a relatively intense M-CH/ peak, resulting from loss of a methyl radical from a 1, 3-dioxolane ring (12). 

The stereochemical outcome for addition of r-l,3-dioxolan-4-yl and oxiranyl radicals to phenyl vinyl sulfone has been probed. The results indicated that the symanti selectivity could be altered by changing the group next to the radical in the diox-olanyl case but not in the oxiranyl case (bulky groups had a large xyn-directing effect) (Scheme 39). Several alkenyl-lactones and -lactams have been subjected to hydrosilylation conditions using carbohydrate-derived thiols as homochiral polarity reversal catalysts (yields 25-96% ee 5-95%) " ... 

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-... 

A procedure for bromination of 2-phenyl-l,3-dioxolanes with N,N-dibromobenzenesulfonamide (C6H5S02NBr2) has been described,184 but it has not yet been applied in the carbohydrate field. The behavior of this reagent is essentially identical to that of N-bromosuccinimide, but some differences were noted, and a suggested mechanism was discussed.184... 

Only short comments will be given for other acetal derivatives that are less popular Chart 1 presents a list of formulae of cyclic acetals, mainly, those with five- and six-membered rings (1,3-dioxolanes and 1,3-dioxanes). Seven-membered ring acetals are omitted because they are scarcely represented in carbohydrate chemistry. The special case of spiroacetah and cydohexane-l,2-diacetal-protecting groups, which have been reported recently, will be presented in Part K. 

Heterocycles with two oxygen or sulfur atoms in one of the rings, i.e., dioxane and dioxolanes as well as their thia analogues have not been considered in this chapter many such ring systems are in the carbohydrate series in the form of acetals. 

Sulfuric-fluorosulfonic acid mixtures give complete cyclization of allyl and methallyl esters and thiol esters to give the l,3-dioxolan-2-ylium and l,3-oxathiolan-2-ylium cations (72CRV357) as shown in equation (29). Such cations are frequently involved in the solvolysis of haloacyl derivatives. This neighboring group effect has important mechanistic and stereochemical consequences, and is frequently encountered when working with carbohydrates. 

Although there exist numerous ground state reactions, photochemically induci asymmetric radical additions can be very efficient and even highly stereoselectr [125]. Furthermore, no particular functionalization of the starting material is n< essary prior to the formation of a C-C bond. In this context, the photosensiti addition of alcohols, cyclic acetals, and tertiary amines to electron-deficient kenes has been particularly studied. This will be illustrated by a few exampli First attempts to induce chirality in the photoinduced addition of ket radicals (e.g., U) involved a, 3-usaturated carbonyl compounds such as 208 rived from carbohydrates (Scheme 56) [126]. With benzophenone as sensitizi these radicals could be added stereoselectively, and similar reactions were carri out with dioxolane and a, 3-usaturated nitropyranones [127]. 

Methyl acetals and ketals are rapidly reduced to methyl ethers by sodium cyanoborohydride in methanol with dry HCI at ice temperatures. A dioxolane is completely cleaved to a methyl ether, showing intervention by the solvent at some stage (equation 14), but when an inert solvent such as THF is used only single cleavage occurs this reagent shows interesting selectivity in the reduction of benzylidene acetals in the carbohydrate series (see Section 1.9.3.4). 

Dioxolanes, formation by acetylation of carbohydrates using 2-alk-oxypropenes 81 H(16)l587. 

First attempts to induce chirality in the photoinduced addition of ketyl radicals (e.g., U) involved a,p-usaturated carbonyl compounds such as 208 derived from carbohydrates (Scheme 56) [126]. With benzophenone as sensitizer, these radicals could be added stereoselectively, and similar reactions were carried out with dioxolane and a,p-usaturated nitropyranones [127]. 

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