AND POLYHYDRIC ALCOHOLS 167 



of laevo-rotation. This is at variance with all other optical 

 values obtained for the glucoses and admits of no simple 

 explanation. The abnormality can hardly be due to a highly 

 pronounced laevo-rotatory effect attending methylation of 

 the primary alcohol group in the sugar chain, as inspection 

 of the available optical data will show. 



APPLICATIONS OF PARTIALLY METHYLATED SUGARS 



The various applications of these compounds have already 

 been briefly referred to in the introduction, and it will be seen 

 that they afford considerable scope for investigation. An 

 additional question of theoretical interest was raised in the 

 course of the work, this being the varying capacity of members 

 of the sugar group to enter into condensation with aldehydes 

 or ketones. Thus methylmannoside gives both mono- and 

 dibenzylidene derivatives, while methylglucoside and methyl- 

 galactoside only condense with one molecule of benzaldehyde. 

 Several other examples are known in which, although the 

 necessary hydroxyl groups are present in a sugar derivative, 

 and are moreover situated in the spatial proximity apparently 

 favourable to condensation, are nevertheless incapable of 

 participating in condensation reactions. 



This behaviour seems to be due to the stereochemical 

 arrangement of the hydroxyl groups, and consideration of 

 the available data points to the idea that condensation proceeds 

 readily when the reacting hydroxyl groups are in the cis 

 position with reference to the internal ring of the sugar 

 molecule, but not when they are in the trans position. These 

 considerations permit of the configuration of the a- and /8-forms 

 of reducing sugars being definitely established, and obviously 

 the study of partially methylated sugars will prove useful in 

 developing this line of research, as compounds of this nature 

 may be obtained in which only the trans positions in the 

 molecule are methylated. 



