Acetals Anhydro sugars

The unusual formation of an anhydro sugar by the hydrolysis of a trityl derivative has been described.76 The treatment of 5-trityl-D-ribofuranose triacetate with hydrogen bromide in acetic acid gives a ribosan diacetate which is presumed to be 1,5-anhydro-D-ribofuranose diacetate. 

The cyclic acetals of sugars constitute an important class of compounds, and their value as synthetic intermediates is well recognized.100 The reaction of methyl /3-maltoside with benzaldehyde in the presence of zinc chloride has been reported to give the crystalline methyl 4, 6 -0-benzylidene-/3-maltoside.52 Similar benzylidenation of l,6-anhydro-/8-maltose affords the corresponding 4, 6 -0-benzylidene derivative in 63% yield.82... 

In the second approach to 1,6-anhydro sugars, anomeric esters, particularly glycosyl acetates, are activated by Lewis acids and displaced by labile ethers at 0-6. Rao has reported an efficient synthesis of tri-0-acetyl-l,6-anhydro-/3-D-glucopyranose by treatment of the trityl ether (4) with titanium chloride (O Scheme 3) [20]. 

Anhydro sugars have proven to be versatile glycosyl donors for the preparation of many glycoconjugates. The 1,2-anhydro bridge is an epoxide that is also part of an acetal and is responsible for the unique reactivity of 1,2-anhydrohexopyranoses. The first well-defined member of this class was prepared by treatment of the partially functionalized pyra-nose (17) with ammonia in benzene, and is commonly referred to as BrigTs anhydride (O Scheme 15) [46]. 

C. O. Obenland,/. Chem. Ed., 41,566 (1964) procedure submitted to Org. Syn. Bromine-Sliver acetate. Lemieux and Fraser-Reid found a route to 2,5-anhydro-sugars in the reaction of methyl 2-desoxy-2-iodo-/3-D-glucopyranoside triacetate with a large excess of bromine and silver acetate to give in high yield an anomeric mixture of 1,3,4,6-tetra-0-acetyl-2,5-anhydro-l-methoxy-D-mannoses. 

As already mentioned, the polymerization of 1,3,5-trioxane and bicyclic acetals (including anhydro sugars) is discussed separately. In this section we shall only give the conditions for preparation of high-molecular-weight polymers from the parent monomer of this class 1,3-dioxolane (DXL). The majority of conclusions applies also to the 7-membered acetal 1,3-dioxepane (DXP). 

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