Stannic chloride, anomerization

C-Glycosylation in the D-ribofuranose series can be effected under very mild conditions in the presence of stannic chloride as the catalyst, but the product may consist of anomeric mixtures. Thus, 2-j3-D-ribofuranosylnaphthalene (202), 2,4,6-trimethoxy-1-/3-D-... 

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-... 

Lemieux and Brice33 have studied the rearrangement under the conditions employed by Pacsu,80 involving the use of stannic chloride as catalyst, with pure chloroform as solvent. The reaction was shown to be specific for the anomeric center, and the following observations appear to establish definitely the main features of the mechanism for this reaction. A quantitative yield of silver chloride was obtained on treating silver acetate with an excess of stannic chloride in chloroform, and it seemed probable that there results a chloroform solution of stannic chloride and stannic trichloride acetate (LVIII). The solution was able to catalyze the anomerization. 

Assuming that the mechanism involves dissociation of the Cl-to-acetoxy-group bond, the Lewis acid, stannic chloride, undoubtedly would catalyze the anomerization of /3-D-glucopyranose pentaacetate by complexing with the Cl-acetoxy group, to lead to the equilibrium represented by equation 4, where G represents /3-D-glucopyranose. 

Note In a recent paper the solvent used for the condensation of 293 and 294 with stannic chloride as catalyst was changed from 1,2-dichloromethane to nitromethane. This improved the yield of the anomeric mixture 295 and 296 by at least 22% (93JHC509). 

Thioglycosides. These glycosides are prepared in high yield by reaction of an alkyl trlbutylstannyl sulfide with peracetyl pyranosyl or furanosyl halides in the presence of stannic chloride. An anomeric mixtufe is usually formed. 

Anomeric Rearrangement The transformation of certain glycosides from the to the a-configuration by the action of stannic chloride or titanium tetrachloride was observed by Pacsu 15), Recently this transformation has been applied to the oligosaccharides, effecting the synthesis of isomaltose from gentiobiose (see Isomaltose). 

The nucleophilicity of an hydroxy-group to be linked to the anomeric centre of sugars can be enhanced by trialkylstannylation. Thus treatment of 2,3,4,6-tetra-O-acetyl-a-D-glucopyranosyl bromide with one equivalent each of tributyl-stannyl methoxide and stannic chloride in dichloromethane at 20 °C furnished an 86% yield of methyl 2,3,4,6-tetra-0-acetyl-/3-D-glucopyranoside. Orthoesters were obtained in the reactions with tributylstannyl alkoxides when the glycosyl halide was epimerized (with tetraethylammonium bromide) in situ (see p. 40). t-Butyl ethers have also been used to enhance the nucleophilicity... 

O-acetyl-jS-D-glucopyranose reacts with phenol in the presence of stannic chloride does not appear to arise by anomerization of the /S-glycoside. An inter-molecular process seems to be involved since stannic chloride did not anomerize the j8-glycoside in the absence of phenol, whereas added 2-chlorophenol was incorporated into the a-glycoside produced. The process is apparently catalysed by diphenoxytin dichloride - which was isolated from the reaction mixture -and stannic chloride. 

Niedballa and Vorbriiggen have shown that the use of this a-chloro sugar (4) in the condensation with 5-ethyluracil in the presence of stannic chloride produced approximately a 1 1 (a P) anomeric mixture of 10 and 9 under a variety of reaction conditions (Scheme 3). This was apparently due to the use of stannic chloride which either causes anomerization of the chloro sugar or more likely formation of a carbocation that has an equal chance of a or p attack. 

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