Galactopyranoside, methyl 3,4-anhydro structure

Anhydro-D-dulcitol (XXXV) has been prepared by a method which demonstrates its structure. Hockett and coworkers11 prepared it from methyl 3,6-anhydro-a-D-galactopyranoside but both the anhydride itself and its tetraacetate were liquid. Carr63 obtained another anhydro derivative of dulcitol by heating it with phosphoric acid. This also was a liquid but gave rise to a crystalline dibenzoate and a dicarbanilate and was a dianhydro derivative, but no constitutional studies have yet been carried out on this substance. 

Some examples are known where the formation of alternative, anhydro-ring structures is possible, and the deformation of the molecule required in the alternative reactions is approximately the same, so that entropy changes may be largely neglected. In such cases the ethylene oxide ring is not favored. The action of bases on methyl 3,4-di-0-acetyl-2,6-di-0-tosyl-/J-D-glucopyranoside and on methyl 2,6-di-0-mesyl- -D-galactopyranoside affords predominantly the 3,6-anhydro-2-0-sulfonyl derivative in each case. 

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