Preparation of Glycofuranosides from Dithioacetals

Preparation of Glycofuranosides from Dithioacetals 1. Isolation of Products 

As the glycofuranosides are the final thermodynamic products, there is no need to control the reaction conditions closely, unlike the isolation of kinetic products in the Fischer method. Constant agitation is needed, to ensure reaction of the insoluble mercuric oxide, and a desiccant (Drierite) is used for removing the water as it is formed. In most of the examples cited in this review, the diethyl dithioacetal is used as the starting material. 

In Table VII is given a list of products obtained by this method the preponderant anomer obtained is the trans-l, 2-glycofuranoside (18), an effect similar to that noted for products formed by the Fischer method (see Table II on p. 105). The data in Table VII are limited in scope the yields are mostly preparative, and do not represent quantitative recoveries. In only four of the examples cited are the yields above 50%. The 91% yield of the anomers of ethyl L-rhamnofuranoside was obtained by separation on a chromatographic column, and represents the only quantitative information on this method. In all other examples, the products were isolated by crystallization techniques, and it has often been assumed that the crystalline product isolated, often in low yield, had been the preponder- 

Glycofuranoside Anomcr Yield, % Predicted anomer References 

The formation of the tranu-l, 2-glycofuranosides is effected in neutral solution, where anomerization cannot occur. Treatment of 3,4-O-isopropyl-idene-2,5-di-O-methyl-L-rhamnose diethyl dithioacetal with mercuric chloride in boiling methanol gave a 61% yield of the -L-furanoside no trace of the 0-l anomer was found. In this experiment, with an acidic solution (no mercuric oxide present), anomerization could have led to the formation of the more stable anomer. In the normal reactions cited, with maintenance of a neutral solution, no anomerization can occur, and yet the more stable anomer, presumably the kinetic product, is still the major anomer formed. 

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