Organic Chemistry 2 · Acetals, imines and related equilibria
Methyl glucoside anomeric carbon is an acetal
Conditions: Glucose + MeOH, HCl
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The question
Glucose with methanol and HCl gives methyl β-D-glucopyranoside. What is the anomeric carbon (C1) of this glycoside?
Reveal the answer and explanation
Answer and explanation
Correct answer An acetal carbon (ring O and OCH3)
Two ether oxygens on the same carbon.
Not this one A hemiacetal carbon
That is free glucose; the OH has been replaced by OCH3.
Not this one An aldehyde carbon
The open-chain aldehyde is locked away in the cyclic acetal.
Not this one A ketal of a ketose
C1 of glucose bears an H (aldose), so it is an acetal from an aldehyde.
Worked solution
In glucose, C1 is a cyclic hemiacetal: it bears the ring oxygen and an OH. Fischer glycosidation replaces that OH with OCH3, so C1 carries two OR groups and an H: an acetal. That is why glycosides are stable to base, do not mutarotate, and are non-reducing until acid hydrolysis.
What you'll practice
Classify the anomeric carbon of methyl β-D-glucopyranoside.
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