Q5 · CSIR NET Chemistry, June 2017
Paper: CSIR NET June 2017 · Subject: Organic Chemistry · Chapter: Natural Products · Topic: Carbohydrates · Marks: 2 · Difficulty: Hard
The major product formed in the reaction of D-glucose with 2 ZnCl in MeOH is a methyl glucopyranoside (A or B). The structure of this product and the molecular orbital interaction present between ring-oxygen and the anomeric C-O bond responsible for its stability, respectively, are 
A 
B
A B(a)A and $\mathrm{n}\rightarrow\sigma^{*}$
(b)A and $\mathrm{n}\rightarrow\sigma$
(c)B and $\mathrm{n}\rightarrow\sigma^{*}$
(d)B and $\mathrm{n}\rightarrow\sigma$
Answer
Answer: C ✓ checked by 4AB · confidence high
Explanation
Fischer glycosidation under thermodynamic control favours the α-anomer B (axial OMe). It is stabilised by the anomeric effect: donation from a ring-O lone pair into σ*(C1–OMe), n → σ* (c).
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