Acetals Derived from Aldehydo-sugars

The dimethyl acetals of D-glucose, D-xylose, and L-idose were obtained in 15— 25% yield by Fischer glycosidation of the aldose in the presence of strontium or calcium chloride, whereas they were not detected in the absence of these salts. This increase must be due to complex formation with these cations whereas the ring form of the sugars lacks the steric arrangement (a c., eq., ax.) required for complex formation with the cations, the acyclic form is able to adopt a suitable conformation for complexation. 

6- and 3,4 5,6-di-0-isopropylidene derivatives of D-glucose dimethyl acetal were obtained when D-glucose was treated at room temperature first with acidified methanol and then with 2,2-dimethoxypropane (or acetone).  

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MI1 74MI7). The acetates of these acydo C-nucleosides (539, R = Ac) were obtained by oxidative cyclization of the Schiff bases 540, derived from aldehydo-sugar acetates 180 and 4,5-diimino-l,3-dimethyluracil, with mercury(II) chloride in dimethyl sulfoxide [79CPB1094, 79H(12)359] (Scheme 144). 

The first example of methoxyl participation reported in carbohydrate chemistry was the migration454 of a methoxyl group from C-l to C-4 during an attempted benzoate displacement reaction with 2,3,5-tri-O-benzyl-4-O-p-tolylsulfonyl-D-ribose dimethyl acetal (185). Instead of the 4-O-benzoyl-L-lyxose derivative expected, the isomeric l-0-benzoyl-2,3,5-tri-0-benzyl-4-0-methyl-L-lyxose methyl hemi-acetal (187) was obtained, presumably by way of the cyclic, oxonium ion 186. Solvolysis of the related aldehydo sugar, namely, 2,3,5-tri-O-benzyl-4-O-p-tolylsulfonyl-D-ribose, readily gives 2,3,5-tri-O-benzyl-L-lyxofuranose, and the reaction probably involves participation by the free aldehyde group.455... 

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