Special Methods of Glycoside Synthesis

Synthesis of glycosides of the required structure (and configuration) from the appropriate sugars and alcohols or phenols has been discussed in the preceding Sections. In this Section, the transformation of one glycoside into another, whether by anomerization, substitution of one aglycon for another, or alteration of the sugar residue, will mainly be dealt with. 

In practice, anomerization is almost always applied to the glycoside acetates, and in almost all recorded instances the transformation is from /3-d to a-D. Many of the catalysts employed in the synthesis of acetylated sugars or in their condensation with aglycons (for example, zinc chloride or sulfuric acid) have been employed for the anomerization of the glycoside acetates. This list does not, however, include titanium tetrachloride, the best-known anomerizing agent. The theory of anomerization is dealt with in an earlier review in this Series.  

Methyl tetra-O-acetyh/S-D-glucopyranoside (1 g. dried over phosphorus pent-oxide) in anhydrous chloroform was saturated with gaseous boron trifluoride and the solution was kept for 24 hr. The gelatinous precipitate was decomposed by shaking with sodium bicarbonate solution, and the chloroform layer was washed with water. After drying with calcium chloride, it yielded 0.98 g. of the a anomer m. p. 97-98 [ Id +121°. Recrystallization from ethanol gave material of m. p. 100-101° [q ]d +130°. 

Methyl jS-n-fructopyranoside can be obtained in 80% yield from the corresponding benzyl glycoside by treatment with 0.03 N methanolic hydrogen chloride for 40 min. at 20° benzyl /S-n-fructopyranoside was prepared in 30 % yield by condensing D-fructose with benzyl alcohol in the presence of 0.2 N hydrogen chloride for 60 min. at 20°. 

A variation of the Helferich reaction (see Section III, 2), which might be of practical use under certain circumstances, is illustrated by the preparation of phenyl tetra-O-acetyl-a-D-glucopyranoside from the methyl a-D-glucoside acetate by fusion with phenol and zinc chloride the yield of the phenyl a anomer was 55 %, and some of the phenyl /3 anomer (15 %) was also isolated. 

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