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

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