[bluestar2012]

       COOH
H               OH
H               OH
HO             H
HO             H
     COOH

Say you have the above sugar (fischer projection form) and want to assign it with D or L.  I though the answer was L, since the OH on the last stereocenter is on the left side.  However, the answer to this question is D, and the key said that because the configuration of the last stereocenter is R, we call it a D sugar.  If there was a CH2-OH group at the bottom instead of the -COOH, it would be an L sugar, since the stereocenter would have an S configuration since the priorities would be changed. 

Does this sound correct? Does R/S relate to D/L in sugars? Or more like, does oxidizing a sugar change its D/L assignment? thanks!

[discodermolide]

       COOH
H               OH
H               OH
HO             H
HO             H
     COOH

Say you have the above sugar (fischer projection form) and want to assign it with D or L.  I though the answer was L, since the OH on the last stereocenter is on the left side.  However, the answer to this question is D, and the key said that because the configuration of the last stereocenter is R, we call it a D sugar.  If there was a CH2-OH group at the bottom instead of the -COOH, it would be an L sugar, since the stereocenter would have an S configuration since the priorities would be changed. 

Does this sound correct? Does R/S relate to D/L in sugars? Or more like, does oxidizing a sugar change its D/L assignment? thanks!

R/S has nothing to do with D/L, the latter were arbitrarily chosen by Fischer in 1900 or so. I don't think oxidation should not change the D/L unless it is that OH which has been oxidized.

[Dan]

Aahhh. I get a fuzzy, warm nostalgia from carbohydrate nomnenclature. In short, you are correct. Can you give the source of the question?

Does this sound correct? Does R/S relate to D/L in sugars? Or more like, does oxidizing a sugar change its D/L assignment? thanks!

1. No, it does not sound correct (at least it goes against IUPAC).
2. R/S and D/L are unrelated.

You are correct, it is an L-sugar. Specifically, it is L-mannaric acid.

3. There are occasions where oxidation might change the D/L prefix because it changes the parent monosaccharide name.

For example, if you took D-gulose and oxidised both C1 and C6 to the acid level (forming the aldaric acid), you now have two choices of name - D-gularic acid or L-glucaric acid. Gluco wins over gulo because it comes first alphabetically.

This naming decision has to be made if you have a sugar derivative where the termini are at the same oxidation level (polyols, dialdoses and aldaric acids). In the case of the manno configuration (as in this example), the C₂ symmetry of the molecule means that manno is the only appropriate descriptor. This is also true for ido for the same reason. In the cases of allo and galacto, the molecule becomes achiral if the termini are identical.

So, taking aldaric acids (more specifically hexaric acids) as an example, the possible structures are:

Allaric acid (achiral)
D-Altraric acid (= D-Talaric acid)*
D-Glucaric acid (= L-Gularic acid)*
D-Mannaric acid
D-Idaric acid
Galactaric acid (achiral)
L-Altraric acid (= L-Talaric acid)*
L-Glucaric acid (= D-Gularic acid)*
L-Mannaric acid
L-Idaric acid
*The name in parentheses is equivalent but not preferred (IUPAC)

There are 10 in total. Similarly there are 10 possible hexitols and 10 possible dialdohexoses.

Oxidation at a non-terminal position of a sugar (forming an ketoaldose, for example) will change the configuration prefix completely and often the D/L descriptor as well. That is more complex and I won't go into it unless you have specific queries about it.

For full details regarding carbohydrate nomenclature, see the IUPAC recommendations, which you can find here.