( 100 ) 



carbon-atom two stereo-isomeric gliicosides ought to bo capable of 

 existence. These two isomers would then be comparable to the two 

 penta-acetates then known. 



About a year afterwards, Alberda van Ekenstein ') succeeded in 

 obtaining this second isomer |?-methjlglucoside. He found that if 

 the reaction between glucose and methylalcohol (with hydrochloric 

 acid as catalyser) was stopped the moment that all the glucose as 

 such had disappeared, the two isomers were both present, the 

 «-form being predominant. They could be separated by fractional 

 cystallisation. He further noticed that the ,?-form passes into the «-form 

 in presence of a solution of hydrochloric acid in methylalcohol; if, 

 therefore, the reaction is continued for a long time, we observe 

 tliat the rotation increases [the ["]ö of the «-isomer is -f- 158", that 

 of the /?-isomcr — 32°] whilst the j?-methylglucoside disappears more 

 and more. 



The /i-isomer therefore appeared to be the metastable and the 

 «-isomer the stable form. 



The observations of Albekda lead to the couclusion that, as in so 

 many analogous cases, the so-called metastable form is here the first 

 ])roduct of the reaction and that the isomer is ))roduced from this 

 afterwards. 



It now became important to fuither investigate the transformation 

 of the one isomer into the other with a view both to its velocity 

 and to the influencing factors. The view {)ropounded by Emil Fischer*) 

 that glucosedimethylacetal CII, OH- (CHOH), CH (OCR.y may be 

 the intermediate j)roduct in the formation of the two glucosides might 

 be tested by an investigation of this kind. This acetal is a syrupy 

 liquid which occurs as the first product of the action of methyl- 

 alcoholic hydrochloric acid on glucose ; it does not react with phe- 

 nylhydrazine or Fehling's solution and is very readily reconverted 

 into glucose by the aqueous acids; it was however not obtained pure 

 and not analysed. As tliis substance, supposed to be the dimethyl- 

 acetal, was converted into the two glucosides on warming with metiiyl- 

 alcoholic hydrochloric acid, the transformation being however not 

 complete and as moreover the two other substances were obtained 

 when starting from one of the two glucosides, Fischer concluded: 

 ,/dass der Vorgang welcher vom Acetal zum Glucosid führt, umkehr- 

 bar ist, dass ferner die Verwandlung der Glucoside in einander über 

 das Acetal führt and dass mithin die drei Verbinduno:en als Factoren 



1) Recueil 13. 183. 



2) Ber. 28, 1146. 



