MECHANISMS IN PYROLYSIS OF POLYSACCHARIDES .3. CYCLOHEPTAAMYLOSE AS A MODEL FOR STARCH IN THE PYROLYSIS OF POLYSACCHARIDES

The pyrolysis of cycloheptaamylose has been studied as a model for starch. 1,6-Anhydro-beta-D-glucopyranose (levoglucosan, LG, 7) and its furanose isomer are major products from vacuum pyrolysis at 280, 300, and 320-degrees, with combined yields ranging from 38 to 50% of the substrate dependent on temperature. Pyrolysis in methyl sulfoxide at 150-degrees produced LG and glucose as well as gluco-oligosaccharides of d.p. up to 7, with both reducing and 1,6-anhydro end-groups. A mechanism is postulated in which the first step is the heterolytic scission of a glucosidic linkage to form a linear, seven-membered oligosaccharide having a glucosyl cation in place of the reducing end-group. The cation is stabilized either by intramolecular attack of O-6 on the C-1 cation or by intermolecular transglycosylation. The former product subsequently yields LG upon scission of a terminal glucosidic linkage.