Q19. The structure of the major product S of the following reaction is
The major product S is option (B).
This reaction represents Fischer glycosidation, where a sugar undergoes acid-catalyzed reaction with methanol to form a methyl glycoside primarily at the anomeric position. The starting material is β-D-glucopyranose, and under these conditions (MeOH, H⁺, 65°C), the anomeric OH is selectively replaced by OCH₃ while preserving other hydroxyl groups.
Reaction Mechanism
The mechanism begins with protonation of the exocyclic anomeric oxygen, leading to ring opening and formation of an oxocarbenium ion intermediate. Methanol then attacks this ion, followed by deprotonation and ring closure to yield the thermodynamically favored α-anomer methyl glycoside, though mixtures occur; longer reaction times favor pyranose forms.
Option Analysis
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Option (A): Shows multiple methoxy groups replacing various OHs, indicating non-selective permethylation rather than anomeric-specific glycosidation.
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Option (B): Displays β-D-glucopyranose with only the anomeric OH converted to OCH₃ (β-methyl glucoside), matching the selective reaction at C1.
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Option (C): Features extensive O-methylation across the ring, resembling full per-methylation, not typical for mild Fischer conditions targeting the anomeric center.
Introduction to Fischer Glycosidation Reaction
Fischer glycosidation of sugar with MeOH HCl at 65°C converts the anomeric hydroxyl of pyranose sugars like β-D-glucose into methyl glycoside. This acid-catalyzed process selectively forms methyl β-D-glucopyranoside as the major product S (option B), crucial for carbohydrate chemistry in CSIR NET Life Sciences.
Keyphrase Focus: Methyl Glycoside Formation
Methyl glycoside formation involves protonation, oxocarbenium ion generation, methanol nucleophilic attack, and ring closure. At 65°C, equilibrium favors the pyranose α-anomer due to the anomeric effect, but β-forms predominate initially from the starting β-sugar.
Why Option B is the Major Product S
Option B retains the β-D-glucopyranose configuration with OCH₃ at C1 only, avoiding over-methylation seen in A and C. This specificity arises because the anomeric position is most reactive in hemiacetal sugars under dry HCl/methanol conditions.
