BR test 1 biological science q 116

[2010premed]

Which of the following is an L-aldohexafuranose?

does any1 have an easy way of doing this?

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[fizzgig]

is it fischer? i dont have BR.

 

[donaldduck]

Well, first off you kno that its going to be a five carbon cyclical structure with an aldehyde functional group. Think of the sugar ribose. The "furanose" means that its a five carbon ring. The aldo means that it has an aldehyde functional group, but in the cyclical form its goin to be an hemi-acetal.

 

[2010premed]

A.

B.

.

The correct answer is A

 

[fizzgig]

I assume D vs L is the issue here.

the last chiral carbon either has no information (no wedges drawn or anything) OR it is drawn as a fischer projection. it seems from what i've seen that the latter is the case.

so A is on the right. BUT, fischer should be drawn with the last carbon at the bottom. rotate the whole drawing in the plane of the paper by 180. now you're looking at the last chiral carbon in the correct orientation, and OH is on the left.

 

[2010premed]

That's what they did in the explanation.. I didn't know that the last C should be on te bottom for Fisher. Ok so after rotating, if OH is on the left, then it is automatically L?

 

[fizzgig]

i think so, yes. drawing out the ring closing reaction might help visualization. i think part of the fischer rules though for sugars is the carbonyl goes on top where you start the carbon count and the highest numbered chiral carbon has to go on the bottom.