Anomerization acid-catalyzed

This reaction has been used m an imaginative way to determine the ring size of glycosides Once all the free hydroxyl groups of a glycoside have been methylated the glycoside is subjected to acid catalyzed hydrolysis Only the anomeric methoxy group IS hydrolyzed under these conditions—another example of the ease of carbocation for matron at the anomeric position... 

The scope of the acid-catalyzed formation of C-glycosyl compounds has been greatly expanded with the finding that enol ethers and ketene acetals can be used as the carbon source in electrophilic substitution reactions at the anomeric center.126 Treatment of 198 with the trimethylsilyl enol ether derived from cyclohexanone, in the presence of stannic chloride, led to 2-(2,3,5-tri-0-benzoyl-/J-D-ribofuranosyl)cyelohexanone (206), presumably by way of the inter-... 

In the course of their studies of pseudouridine,164 Asbun and S. B. Binkley183 reported the synthesis of 5-/3-D-arabinofuranosyl- and 5-/3-D-xylofuranosyl-uracil (258 and 259) by the acid-catalyzed ring-closure of the corresponding alditol derivatives. The configuration at the anomeric carbon atom was determined on the basis of optical rotatory dispersion studies. 

David and Lubineau191 reported the synthesis of pseudocytidine [5-/3-D-ribofuranosylcytosine (270)] and its a anomer by a procedure analogous to that used in preparing pseudouridine.155-157 Thus, 2,4 3,5-di-O-benzylidene-a/de/iydo-D-ribose (223) was condensed with the dilithio derivative of 2-0,4-N-(trimethylsilyl)cytosine, and the resulting, epimeric, acyclic derivatives were subjected to acid-catalyzed cyclization. The anomeric configuration of the free C-nucleosides was ascertained by spectroscopic methods and by their transformation into a- and /3-pseudouridine in the presence of nitrous acid. The anomeric 5-(/3-D-ribofuranosyl)isocytosines have also been prepared by Fox and coworkers.1913... 

Pikho and co-workers50 found that nonanomeric [6,5]-spiroketals (having a pyranoside moiety with an equatorial CO acetal bond) can be formed under conditions of kinetic control. For instance, 62 undergoes acid catalyzed spiroacetalization giving the anomeric (most stable) acetal 63... 

Furthermore some O- and. S -glycosylated pyropheophorbide-a derivatives have been prepared.75 The O-glycosylated pyropheophorbide derivative 97 was obtained by carbohydrate per-acetate glycosylation acid catalyzed method of derivative 96 (Fig. 10). The reagent proportions and the reaction times have been shown to rule the reaction yield. The best results were found when the reagents ratio was 2 1 2 (glycoside pyropheophorbide Lewis acid). Under these conditions the reaction yield was 36% with 83% of anomeric purity. The glycosylated derivatives 97 are diastereomeric mixtures. 

The Lewis acid catalyzed C-glycosylation method is probably one of the most efficient to prepare C-glycosides of ketoses, the anomeric carbocation intermediate being in this case more stabilized by a further substituent. We applied this procedure to synthesize C-fructosides [15] (Scheme 8).The reaction of methyl... 

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