Galactose 6-sulfonates, displacement reactions

In contrast to the facile reaction of 1,2 3,4-di-0-isopropylidene-6-0-p-tolylsulfonyl-a-D-galactose with ethanethiolate anion, the same displacement reaction on 2,3 4,5-di-0-isopropylidene-l-0-p-tolylsulfonyl-/3-D-fructose, which also has a primary p-tolylsulfonyloxy group, proceeds more slowly, to give l-(S-ethyl-2,3 4,5-di-0-isopropylidene-l-thio-/3-D-fructose. This behavior parallels the reactivities of the two sulfonate ester derivatives with iodide ion. Inspection of models indicates that rear-side approach to C-1 in the D-fructose derivative is subject to considerably more steric hindrance than the approach to C-6 of the D-galactose derivative. 

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