[wonka_vision]

Hello. I'm trying to perform the following reaction:

http://i.imgur.com/VGUKS.png

The NMR result I expect to get is:

http://i.imgur.com/YTOHk.png

But the NMR I get after fractional distillation under vacuum is:

http://i.imgur.com/YJypp.png

I simply can't figure it out. I have no idea why I'm getting a doublet of doublets at 4ppm and 1.3ppm, which integrate for 4H and 2H, respectively. I've done this reaction several times and always end up with the same peaks and integrations. The DD seems to indicate the formation of an alkene group, but an elimination isn't favored under these conditions, and even if it were a DD wouldn't result following abstraction of the tertiary carbon's proton. Sn2 also isn't likely given the NMR result. There is definitely a ring structure being formed, however due to the formation of a multiplet at 1.9ppm. I wouldn't ask if I wasn't absolutely stumped, so any help would be awesome!

Thanks

Mat

[discodermolide]

Hello. I'm trying to perform the following reaction:

http://i.imgur.com/VGUKS.png

The NMR result I expect to get is:

http://i.imgur.com/YTOHk.png

But the NMR I get after fractional distillation under vacuum is:

http://i.imgur.com/YJypp.png

I simply can't figure it out. I have no idea why I'm getting a doublet of doublets at 4ppm and 1.3ppm, which integrate for 4H and 2H, respectively. I've done this reaction several times and always end up with the same peaks and integrations. The DD seems to indicate the formation of an alkene group, but an elimination isn't favored under these conditions, and even if it were a DD wouldn't result following abstraction of the tertiary carbon's proton. Sn2 also isn't likely given the NMR result. There is definitely a ring structure being formed, however due to the formation of a multiplet at 1.9ppm. I wouldn't ask if I wasn't absolutely stumped, so any help would be awesome!

Thanks

Mat

Are you removing the methanol by distillation as the reaction proceeds?

[fledarmus]

All you are showing is a computer-generated spectrum - do you have a spectrum of the known product?

If you did have an alkene, it would be further downfield than 4.

Look at your integrations again - I see three at 1H each and 3 at 2H each, accounting for all 9 of your hydrogens. I think you have your product. My guess is that your bromomethylene is sufficiently large to freeze the chair-chair transitions, and the protons in the ring are not equivalent. The axial and equatorial protons will show different chemical shifts. Also, you won't get coupling between two adjacent axial protons. Your doublet at 1.3 is the axial para-ish proton (split only by the equatorial para-ish proton), and the multiplet at 1.9 is the equatorial one. The triplet of doublets at 3.7 would be your equatorial meta-ish protons, and the doublet of doublets at 4 your axial ones.

A good MS would certainly help your confidence in your structure...

[wonka_vision]

Oh Jeez you're absolutely right 

The only literature I've found is:
H-NMR (60 MHz, CDCl3)  1.13-1.53 and 1.67-2.60 (m, 2, -OCH2CH2-), 3.45 (d, 2, J = 4.5 Hz, -CH2Cl), 3.40-4.33 (m,4,2 -OCH2-),4.67 (t, 1,-OCHO-).

The reported multiplet at 3.4 - 4.33 is probably just due to the lower powered magnet. Now I feel silly 

Thanks a bunch!