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Author Topic: Carbohydrates: Fischer -> Haworth and Haworth -> Chair?  (Read 10035 times)

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nexisrocks

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Carbohydrates: Fischer -> Haworth and Haworth -> Chair?
« on: December 01, 2007, 07:23:58 PM »

When converting a Haworth Projection with a six-membered ring to a chair conformation, how do you know which groups are axial and which are equatorial?

Also, I was trying to transform a Fischer Projection of L-talose into alpha-L-talopyranose and also into beta-L-talofuranose, but I have no idea whether I did it right.  Could you please take a glance at it and see if it looks alright to you?  Here it is:



Thanks so much!
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Dan

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Re: Carbohydrates: Fischer -> Haworth and Haworth -> Chair?
« Reply #1 on: December 02, 2007, 04:57:42 AM »

When converting a Haworth Projection with a six-membered ring to a chair conformation, how do you know which groups are axial and which are equatorial?

A general rule of thumb is to draw the conformation with the most substituents equatorial

Quote
alpha-L-talopyranose

What you have drawn there is unfortunately alpha-D-allopyranose.
First thing I will say is that:

For Haworths: In the D series the alpha-anomer projects the anomeric OH down, and the beta-anomer up.
In the L sereis it's the other way round, so for L sugars the alpha-anomer goes up and the beta-anomer goes down.

Secondly, watch your stereochemistry at the C before the ring-O (C5 in this example). As you have to rotate the chain to get the OH into the ring it can get a bit tricky to see, but your CH2OH substituent should be down for L-talopyranose.

Quote
and also into beta-L-talofuranose

For this one you've drawn beta-D-gulofuranose

First off, same thing about anomeric position. This is an L sugar so the anomeric OH projects down in the Haworth for a beta-anomer.

Again, you've got mixed up at the C before the ring O (C4 in this example).
You've also inverted at C5 (which you shouldn't have).

Have another shot at it. I'll post some diagrams in a bit.
« Last Edit: December 02, 2007, 10:02:21 PM by Dan »
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Dan

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Re: Carbohydrates: Fischer -> Haworth and Haworth -> Chair?
« Reply #2 on: December 02, 2007, 05:27:46 AM »

Get yourself a model kit if you still have trouble with it
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nexisrocks

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Re: Carbohydrates: Fischer -> Haworth and Haworth -> Chair?
« Reply #3 on: December 02, 2007, 06:24:51 AM »

Thanks so much!  I'm fairly certain I have the alpha-L-pyranose structure correct now, however I am still having trouble rotating about the bond on the beta-L-furanose ring.  I think I have the proper configuration of up/down in the Haworth, but I wasn't sure whether or not to invert the O on C5.  This is what I have now:

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Dan

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Re: Carbohydrates: Fischer -> Haworth and Haworth -> Chair?
« Reply #4 on: December 02, 2007, 11:51:13 AM »

Both correct, nicely done!
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