Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: nexisrocks on December 02, 2007, 01:23:58 AM
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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:
(https://www.chemicalforums.com/proxy.php?request=http%3A%2F%2Fi2.photobucket.com%2Falbums%2Fy33%2Fshaylafitz%2F3401Pathway2.jpg&hash=96fc94fdfb60911c70f72da68bd8456dd92a4268)
Thanks so much!
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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
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.
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.
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Get yourself a model kit if you still have trouble with it
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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:
(https://www.chemicalforums.com/proxy.php?request=http%3A%2F%2Fi2.photobucket.com%2Falbums%2Fy33%2Fshaylafitz%2F3401Haworth.jpg&hash=ee61dceea5f34f6e26a13faebbdda280daa501ad)
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Both correct, nicely done!