Question

Please explain how can we differentiate beta and alpha-D-fructopyranose from beta and alpha-L-fructofuranose?? What is the difference in their ring structures?

Answer 1

Here's what I get.

Explanation:

The words pyranose and furanose come from the names of two cyclic ethers, pyran and furan.

A sugar with a six-membered cyclic ether structure is a pyranose.

A sugar with a five-membered cyclic ether structure is a furanose.

Thus, the two top structures in the diagram are fructopyranoses, while the two at the bottom are fructofuranoses.

The terms alpha and beta refer to the configurations of the anomeric `"OH"` group (the `"OH"` on `"C2"` — the ring carbon on the far right of the Haworth structures above.

If the `"OH"` group is pointing "down", it is alpha. If the `"OH"` group points "up", it is beta.

The L-isomers

The L-isomers of fructose are the corresponding mirror images of the D-isomers.

To convert a D-isomer to its enantiomer, you change the configuration of every chiral carbon.

If a group (`"OH"` or `"CH"_2"OH"`) is "up" in one isomer, it is "down" in the other.

For example, we could say that the configurations of carbons 2 to 5 in α-D-fructofuranose are "down-up-down-up".

In α-L-fructofuranose, the configurations of carbons 2 to 5 are "up-down-up-down", as we see in the picture below.

"We leave it as an exercise for the student" to draw the structures of the other isomers of L-fructose.