[kahlua]

I am having some trouble with one of my assignments. We have to ring-flip a compound. However, I've done it a dozen times, and am not getting an answer that matches up with any of the given answer. I thought I understood how to do this, and have tried it a dozen different times, but am not having any luck. Can anyone help?

Here is the problem, and the options.

[PhDoc]

Before you can expect to flip rings on paper, you must first build models to understand what happens to what after a ring flip. When flipping the ring on paper, be certain not to change the absolute configuration of any chiral centers.

Model kits are important. Even professors use them.

[kahlua]

Thank you - so far, we have only learned to do the ring-flipping on paper. We are starting to use models and this week chiral carbons were introduced, and we did use models for that.

I have done the problem the way we were taught in class, and the answer I got is what I keep coming up with. I have looked over what we were taught many times, and I am doing it correctly. But maybe this problem takes into account things we haven't learned yet.
So far, we learned to number each carbon, and flip/renumber the structure, trading axial for equatorial and vice versa.

I would just like to know if the correct answer is there...this has been driving me nuts!

[orgopete]

Your answer, B and D are the same (though inartfully drawn).

[Dan]

You need to be careful with your chairs. Which bond is in front of which. At the moment none of the structures in the options, or the one you provided are correct:

[kahlua]

Thank you - I did notice B and D are the same. I just figured I would post both...in case I was missing something...

This assignment was before we started to learn about chiral carbons, or enantiomers, but I have asked the professor twice about the question and he tells me to try it a different way - last time was to try flipping each of the answers to see which one matches up with the original...but I can't get it to work that way either.

At this point, I just can't figure out how to do this question correctly, I guess I will either have to guess or tell him the correct answer is not present. I'm not sure how well that will go over. I would love to be able to prove it, but I don't know enough (yet) to be able to do that...

Oh well, I have 2 days to try to figure it out. If anyone knows of any resources on how to learn what I'm missing...that would be WONDERFUL! I have also looked in our textbook, but the info provided in the chapter matches up with what we were given - I guess it is a dumbed-down versoin of ring flipping? I will have to look ahead at later chapters, maybe it will go into more detail elsewhere...

[orgopete]

I am not sure I agree with Dan's drawing of the enantiomers here. I believe the 'bond in front' came from ChemDraw's placing the last drawn bond in front of the chair and not necessarily the upside down chair indicated by Dan (draw on paper to see). If so, then you should be able to identify which are the same. Hint, you can look to see which drawing has the ethyl groups cis and the methyl group trans and in the same order. (You may find it helpful to add hydrogens to be certain which is which.)

[discodermolide]

This is a wonderful question, to which I don't have an answer yet. But I agree with Dan, the compounds he drew are enantiomers, (R)(S)(R) and the (S)(R)(S).
In ChemDraw if the bond is in the front or at the back it does not change the ChemDraw assigned configuration!!
This may be an error in ChemDraw? (stupid program )

[kahlua]

We were given the answer today...it's B or D. Our professor didn't realize that the two were the same - the question was pulled from the test bank, which hasn't been the most reliable...

I haven't had a chance to try to suggestions given by orgopete, but will give that a shot to see if I can get it to work out that way...

[Dan]

I am not sure I agree with Dan's drawing of the enantiomers here. I believe the 'bond in front' came from ChemDraw's placing the last drawn bond in front of the chair.

Yes, ChemDraw places the last drawn bond in front by default, but my point is that it matters which bond is in front because it changes the spatial arrangement of the bonds. It is one of my pet hates when this is ignored and molecules are drawn with incorrect sterochemistry because the ChemDraw default is accepted without giving it a second thought to what it actually means.

Perhaps this is clearer:

[orgopete]

Ah, Dan beat me to it, and has done a better job. Since I did draw it out, I am going to show mine as well.

I am going to choose a different explanation as well. Based upon the drawings, I thought B, D, and the poster's structure were correct, except the poster ignored the bond in front issue. If other orientations were among the answers, then I too would have agreed with Dan. However, this looked like a rather plain and simple change (except for the error). Furthermore, I didn't know whether the structural error was the poster's error or one created in the problem. So I tried to give a hint that I thought the poster could analyze to get the correct answer.

[kahlua]

Thanks so much for taking the time to draw them out.

I posted the question as it was printed on the test, with the bonds in front. I think the question was just meant to be a simple ring flipping. I wasn't aware that more than one structure is correct, but it makes more sense now.

I actually ordered a cheap model kit online, so I could build the molecule and figure it out that way. I guess the kit isn't necessary right now, but I'm sure it will come in handy later.

[digchick21]

Just keep in mind which carbons are which, number them if you have to. Then remember that ring flip's result in axial/equatorial position-so whatever positions your substituents are in originally, do the opposite after your ring flip.