Ochem stereochem q's

[chiddler]

Consider isoleucine:

If the side chain chiral center is changed, then the new structure would be a diastereomer or an epimer?

So the image is S,S. If the side chain is changed, it becomes S,R. Which means that they are diastereomers with one stereogenic difference = epimers!

how am i wrong :'(

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[MedPR]

There's another thread about this.. big debate about whether or not epimer refers to hydroxyl groups only. IUPAC says it's an epimer (only one tetrahedral chiral center differs), but there are two examples in TBR that say diastereomer, and even EK 1001 says something like "it isn't a sugar, so epimer can't be right."

 

[chiddler]

oh...damn.

thanks very much.

 

[chiddler]

More question:

I'm trying to brand it with good ol fashioned S/R naming. I'm having difficulty with C3 and 4.

C3 left is OH. 1. Up has carbonyl, 2. Down is lest, 3. Left, up, down makes clockwise which is R.

This is wrong! It's S.

I'm guessing that the adjacent carbon bonds are considered into the plane of the screen which would reverse the configurations that i'm getting. Is this right?

 

[MedPR]

More question:

I'm trying to brand it with good ol fashioned S/R naming. I'm having difficulty with C3 and 4.

C3 left is OH. 1. Up has carbonyl, 2. Down is lest, 3. Left, up, down makes clockwise which is R.

This is wrong! It's S.

I'm guessing that the adjacent carbon bonds are considered into the plane of the screen which would reverse the configurations that i'm getting. Is this right?

You're right about which groups have which priority, but when you have a fischer projection, think "Horribly wrong" for horizontal. Meaning, if the 4th priority is on the horizontal (which, by convention for fischer projections is in front) then must switch whatever direction it is. So in this case, yes, the groups are organized such that you go clockwise and get R, but since the H (lowest priority) is on the horizontal, you switch R to S.

The whole switching R to S since the lowest priority is in front and not in back is consistent with what you would do regardless of how the structure is drawn. You should draw it in a newman and stick model to convince yourself

 

[chiddler]

You're right about which groups have which priority, but when you have a fischer projection, think "Horribly wrong" for horizontal. Meaning, if the 4th priority is on the horizontal (which, by convention for fischer projections is in front) then must switch whatever direction it is. So in this case, yes, the groups are organized such that you go clockwise and get R, but since the H (lowest priority) is on the horizontal, you switch R to S.

The whole switching R to S since the lowest priority is in front and not in back is consistent with what you would do regardless of how the structure is drawn. You should draw it in a newman and stick model to convince yourself

thanks once again. i'm very comfortable with what you have written already. my mistake was thinking that all the carbons are in plane!