Preparation of Methyl -Glucopyranosides by Fischer Glycosidation

Preparation of Methyl o-Glucopyranosides by Fischer Glycosidation THE REACTION  

Steps 1-3 Acid-catalyzed nucleophilic addition of methanol to the carbonyl group of o-glucose. (See Mechanisms 17.2 and 17.4 for details of acid-catalyzed addition to aldehydes and ketones.) 

Step 4 Protonation of the —OH group of the hemiacetal unit. The proton donor is shown as the conjugate acid of methanol. It was formed by proton transfer from the acid catalyst to methanol. 

Step 5 Loss of water from the protonated hemiacetal to give an oxonium ion. 

Step 6 Cyclization of the oxonium ion. An unshared electron pair of the C-5 oxygen is used to form a bond to C-1, forming the six-membered ring of the glycopyranoside. Both the a and (3 stereoisomers are formed in this reaction with the a stereoisomer (axial OCH3) predominating. 

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