Biomolecules and Chemistry in Everyday Life 1 Question 24
26. The Fischer presentation of $D$-glucose is given below.
The correct structure(s) of $\beta$-L-glucopyranose is (are)
(c)
(d)
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Solution:
- (d) A pyranose ring is a 6 membered ring having 5 carbon atoms and one oxygen atom. In glucose, it is formed by the reaction between $>\mathrm{C}=\mathrm{O}$ group at position 1 and $-\mathrm{OH}$ group at 5 th carbon atom. In general reaction between $>\mathrm{C}=\mathrm{O}$ group and $>\mathrm{C}-\mathrm{OH}$ looks like
The product formed in called hemiacetal (if $>\mathrm{C}=\mathrm{O}$ group belongs to an aldehyde) or hemiketal (if $>\mathrm{C}=\mathrm{O}$ group belongs to a ketone). $\mathrm{L}$ - glucose has the mirror image configuration of D-glucose i.e.,
So, $\beta$ - $\mathrm{L}$ glucopyranose is formed as
The $\alpha$-L-glucopyranose has configurational change at $\mathrm{C}_{1}$ only and looks like
