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Altius FLE4 COVID19 Passage 2 Question 6
Emma_5484
#1 Posted : Tuesday, July 28, 2020 1:07:48 PM
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Hi there,

I am having trouble understanding the explanation they give for this answer as to why it is an invertible epimer because it mentions something about aldehydes and ketones which are not in the structure of figure 3. I chose that it was a fixed stereocentre because it is an sp3 carbon attached to four different groups and it is a part of a cyclic structure and therefore fixed.

I would appreciate some clarification.
Thanks,
Emma
INSTR_Katerina_102
#2 Posted : Tuesday, August 04, 2020 9:03:01 PM
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Hi,

I don't love this question, but the idea is that this is an anomeric carbon with a phenol group attached to it, where the phenol group is a good leaving group.

As a result, this stereocentre can be inverted or retained when you react at the anomeric carbon (as shown in the figure 3 reaction scheme), and because of this can be defined as an invertible epimer.

However, I agree that this is not a particularly well worded question.

I hope this helps,

Katt
Isabel_6367
#3 Posted : Monday, June 14, 2021 10:52:42 PM
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Hi I had a follow-up question to this question as well.

From figure 3, I thought this anomeric carbon was in acetal form, and therefore could not be in equilibrium between cyclic and open-chain forms (I got this from my CC page 248). This is why I also thought it was a fixed stereocenter. Similar to how non-reducing sugars can't go back to the open form. Right?

Thanks,
Isabel


INSTR_Molly_129
#4 Posted : Saturday, July 10, 2021 6:17:40 PM
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Hi Isabel,

This question was definitely tricky, and I second Katrina's sentiment that this question is not one of my all-time faves.

In general, acetalated anomeric carbons are incapable of interchanging between open and closed-ring forms. The groups that we use to acetalate are usually -CH3 groups, which are terrible leaving groups.

In the case of this particular leaving group, however, there is a lot of resonance structure that helps to stabilize and negative charges built up from the loss of the entire group. Therefore, it is possible for acetalation to become reversed and thereby allowing for interconversion between ring and chain form.

Therefore, this is an example of an irrevertible epimer, vs. a fixed stereocenter.

Hope that helps,

Molly
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