Glucopyranose, 1,2-anhydro 3 anomer

The zinc chloride (ZnCli) catalysed glycosidation of ara-substituted phenols with 1,2-anhydro-3,4,6-tri-(9-methyl-a-D-glucopyranose gives predominantly the corresponding a-anomer [69]. Addition of TMG (1) enhances the (3-selectivity, even to practical completion under the conditions of potassium carbonate (K2CO3) and 18-Crown-6, in THF (Table 4.7). 

The mechanism of the reaction has been studied by McCloskey and Coleman 121). It appears that a 1,2-anhydride may be an intermediate in the formation of 1,6-anhydro-jS-D-glucopyranose because the reaction is blocked by the presence of a methoxyl group at carbon 2 whereas the presence of methoxyl groups at carbons 3 and 4 does not prevent the reaction. This reaction course does not explain the formation of 1,6-anhydro-jS-D-mannopyranose from the phenyl /3-D-anomer and the corresponding... 

Lewis acid catalysed thioacetolysis of the benzyl protected l,6-anhydro-j8-D-glucopyranose derivatives (4) and (5) gave 9 1 mixtures (7) and (8), respectively, of or- and -1-thioacetates in reasonable yields (Scheme 2). From the peracetylated starting compound (6), on the other hand, the a- and )8-products (9) and (10) were formed in a high yield and in 1 9 ratio. The ot-anomer (9) has also been prepared by butylhydroperoxide catalysed addition of thioacetic acid to tetra-0-acetyl-l,5-anhydro-D-flmhi/to-hex-l-enitol, ... 

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