[noda]

In the scheme, lactonic esters formed in about a 1:1 ratio, after the separation on column chromatography, lactone 7 converted into its stereoisomer 8 almost quantitatively (1:9).
1.my question is why and how?
2.why C-NMR spectra is superimposable for lactones 7 & 8?

[noda]

I'M SORRY I have a mistake before separation on column chromatography DBU WAS ADDED to the mixture and that caused a change in the ratio.
I know that DBU IS non-nucleophilic base.
sorry for my mistake still looking for an answers, I THINK I HAVE THE ANSWER FOR my first question but i would like to hear your answer to be sure.
don't forget to answer the second question
thank u

[Dan]

I THINK I HAVE THE ANSWER FOR my first question but i would like to hear your answer to be sure.

You first, forum rules and all...

I'm puzzled by the second question too. I don't see why these diastereomers would have superimposable ¹³C spectra.

[noda]

I THINK but I'm not sure that DBU neutralize PTSA and that caused (by Le Chatelier law) the opening of the lactone and after that we get the trans reclose of the trans lactone because of the steric substituents is minimal.
the second question I don't why the diastereomers would have superimposable 13C spectra. but I think he meant that it would be superimposable to distinguish between the two lactones, because of that in the article was written that they do NOE measurements I would like to hear your opinions about my answers
thank you.

[Dan]

I THINK but I'm not sure that DBU neutralize PTSA and that caused (by Le Chatelier law) the opening of the lactone and after that we get the trans reclose of the trans lactone because of the steric substituents is minimal.

I don't see how opening and reclosing the lactone could change the stereochemistry of the C3 position. could you draw it out?
My idea is that the DBU is causing enolate formation, which then can reprotonate to form the less hindered trans isomer in preference to the cis (the H at C3 will be relatively acidic).

[noda]

You are right my idea doesn't explain the change of the abs. config.
thank you
any idea about the second question

[Yggdrasil]

I'll take a shot at #2.  You can only really distinguish between cis and trans diastereomers by looking at the coupling constants in a proton NMR.  Since you can't detect coupling in ¹³C NMRs, you can't distinguish between cis and trans isomers.

[DaveD]

Hi Noda -

I have an idea for #2 - it may be simply a coincidence that your cis and trans lactones show superimposable 13C spectra.  Were the 1H spectra of the same sample also superimposable??

I'm wondering if you used CDCl3 - and if the acid present in the solvent caused an equilibration of the two lactone isomers into the same species (I have had this happen to me before).  If you did use CDCl3 - you could rerun the NMRs using C6D6; then you wouldn't have to worry about residual acid catalyzing any isomerization.  I think it unlikely you would have identical spectra in two different solvents.
You can also analyze the two original NMR samples to make sure they are still the cis and trans lactones; this would answer the question about possible isomerization in the NMR tube.

[movies]

I agree, it's probably just coincidental unless something is changing in the NMR tube (like the lactone opening with traces of water).  In principle diastereomers should look different in the NMR, even the carbon NMR.  I've been able to see diastereomeric alcohols in carbon NMR, but the chemical shifts only differed by about 0.1 or 0.2 ppm.  As I recall, I could only resolve the carbon signals on a 500 MHz NMR.