Although, in particular cases and under specific conditions, it is possible to obtain aldofuranose peresters by direct acylation of the free sugars', it is more common to make them by strategies which restrict the products to the five-membered ring forms. Acetylation of 5-0-triphenylmethylpentoses* and the use of 3,5di-0-acetyl-1 ,2-0-isopropylidene-cY-D-xylose3 exemplify such routes, but these methods are limited and the more general procedures available utilise glycofuranosides or thioglycofuranosides.
A useful method, which gives anomeric mixtures of products, is based on the esterification of alkyl furanosides followed by acetolysis of the glycosidic bonds4. However, for the preparation of benzoylated glycofuranosyl l-acetates, which were required for conversion into their 4-halogenated derivatives', we preferred to use 1-thiofuranosides, from which 1,2-truns-related products have now been made with high efficiency.
Thio sugar derivatives can be used in several ways as synthetic precursors of furanosyl esters, for example, in the preparation of 5-substituted aldopentoses (which on esterification can only give furanose products) from aldose dithioacetals*. Otherwise, 1-thiofuranosides, which, in some cases, are readily available from these dithioacetals, can be acylated and then converted into esterified glycofuranosyl halides and hence l-esters by way of halogenosulphonium intermediate@. However, since 1-thioglycosides can be used to give glycosides directly by activation with mercury(I1) salts', it appeared that they might also afford direct access to the l-esters, Such conversions have been reported previously, but yields have been moderate; thus, for example, ethyl 5-0-benzoyl-1-thio-P-Larabinofuranoside, when treated at room temperature with mercury(I1) acetate in acetonitrile, gave' the corresponding a-l-acetate (34%), and similar yields were obtained in related cases and with other mercury(I1) carboxylates'. We now report that treatment of l,Zcis-related 1-thioglycofuranoside esters6 in acetic acid in the presence of mercury(I1) acetate gives the l,Ztrans-related l-acetates with high efficiency. The 1-thioglucofuranoside tetrabenzoate 2, easily obtained from the dithioacetal 1, on heating with mercury(I1) acetate in acetic acid, gave the p-acetate 3 in almost quantitative yield, and the p-D-gukzcto isomer was obtained with similar