Question about Carbohydrate stereochemistry.

[ravupadh]

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I'm pretty good with determining the nomenclature and stereochemistry for linear sugars. However, when it comes to determining the stereochemistry of cyclic carbs or dicyclic carbs, determining the stereochemistry of the OH groups becomes a little challenging. I try transforming the molecule into linear form, so I particulary get challenged by determining the stereochemistry of the O (OH on the second to last carbon in the linear form) that has attacked the anomeric carbon. I can't tell what sugar it is I'm looking at or whether it's D or L because of this problem. Any help would be appreciated. The orientation of the rest of the OH groups is easy for me to determine.

 

[coyotelime]

I think that anomers are formed after the nucleophilic attack. In lecture my professor always drew a squiggly line for the hydroxyl group off the anomeric carbon which indicates it can either be alpha or beta.

Since TBR orgo teaches you "downright", as in the groups on the right side of the main chain in linear form translates to down in the cyclic form, this basically means alpha and beta will be R or S. To determine D/L just translate into linear form and look at the terminal carbon. Once you figure out one form, remember the enantiomer will be the non superimposable mirror image, so all the groups in the main chain carbons will be flipped.

In addition, TBR taught something like "Be a DoctoR or a LoSer" to remember that D = R and L = S(this was in the explanations for the nitrogen containing compound section passages I believe).

I hope this helped, sorry if it was confusing