Correct Answer - D
`alpha - D- (+) -` glucopyranose and `beta - D- (+) -` glucopyranose, the two cyclic forms of naturally occurring `D - (+) -` glucose, are diastereomers as they are not mirror images of each other. These diastereomers differ only in the configuration of `C1`. In carbohydrate chemistry diastereomers of this types are called anomers, and the hemiacctal carbon atom is called the anomeric carbon atom.
When we draw the cyclic forms of a `D` sugar in the Haworth formula, the `alpha` anomer has the `-OH` trans to the `-CH_(2)OH` group while the `beta` anomer has the `-OH` cis to the `-CH_(2) OH`, group, `X`-ray analysis has shown that the actual conformations or the rings are the chair forms. In the `beta`,anomer of `D-` glucose, all of the large substituents, `-OH` and `-CH_(2)OH`, are equatorial, In the `alpha`- anomer, the only bulky axial substituent is the `-OH` at `C_(1)`.
Note that `beta-D`- glucopyranose is the most abundant naturally occuring aldohexose because all ring. substituents in the chair conformation are equatorial.
Diastereomers with more then one sterecocenter that differ in the configuration about only one stercocenter are called epimers. `alpha - D -(+) -` glucopyranose and `beta- D - (+) -` glucopyranose are epimers because their configuration about `C1` and differs. Similarly `D`- threose and `D`- erythrose are epimers because their configuration about `C2` differs.
