[Hurat]

Hi Fellow Members,

I have been reading these forums for a long time and have finally made an account.

I was wondering if my proposed mechanism for the intramolecular reaction shown below is correct. I think it is, but I just wanted to confirm.

Also, as you can see, the intramolecular reaction happened with the cis isomer. If I had the trans-isomer instead I wouldn't be getting the same product because my product would be cis (cycloheptene rings are usually always cis). However, in order to get my 6-hydroxy-6-methylcyclohept-2-en-1-one from trans-hept-3-ene-2,6-dione, could I indirectly get there by first doing the intramolecular reaction then doing a simple a double-bond rotation by heating the daylights out of the trans-cycloheptene ring to get over the activation barrier to get to the cis-cycloheptene ring?

I hope someone can confirm this for me please!

[fledarmus]

I'm not sure I understand the question. Are you asking:

a) Can the trans form of your starting material (which would be 3E-hept-3-ene-2,6-dione) be converted into the cis form (which would be 3Z-hept-3-ene-2,6-dione), then used in this reaction? or...

b) If you pull the proton on the trans form of your starting material, could you get it to isomerize to the cis form and then cyclize? or...

c) If you start with the trans form of your starting material, can you cylize to form the trans cyclohexene and then isomerize that to the cis form?

[Hurat]

I'm not sure I understand the question. Are you asking:

a) Can the trans form of your starting material (which would be 3E-hept-3-ene-2,6-dione) be converted into the cis form (which would be 3Z-hept-3-ene-2,6-dione), then used in this reaction? or...

b) If you pull the proton on the trans form of your starting material, could you get it to isomerize to the cis form and then cyclize? or...

c) If you start with the trans form of your starting material, can you cylize to form the trans cyclohexene and then isomerize that to the cis form?

Very sorry I wasn't too specific.

I really had two questions: 1) Was the above reaction correct (I'm about 95% sure it is but always nice to have confirmation)

and

2) If you start with the trans form of your starting material, can you cylize to form the trans cyclohexene and then isomerize that to the cis form?

I read on the interenet about the trans cyclohexene being a "theoretical" compound so I wasn't too sure!

[fledarmus]

The reaction mechanism looks good to me.

As far as I know, nobody has been able to form a monocyclic compound with a trans alkene any smaller than cyclooctene. There is too much strain involved in trying to get the two sides to come together in a smaller ring. Play with some molecular models and you can feel the strain.

EDIT - let me rephrase that first part - the reaction mechanism looks good as long as your instructor is only concerned about how you got the product. If s/he is a stickler for electron movement, arrows, and proton transfers, you've still got some cleaning up to do...

[Hurat]

As far as I know, nobody has been able to form a monocyclic compound with a trans alkene any smaller than cyclooctene. There is too much strain involved in trying to get the two sides to come together in a smaller ring. Play with some molecular models and you can feel the strain.

Yah, the strain looks pretty bad! But... how could I form the 6-hydroxy-6-methylcyclohept-2-en-1-one from the 3E-hept-3-ene-2,6-dione without going through the the cyclooctene if it is impossible to do so?!

EDIT - let me rephrase that first part - the reaction mechanism looks good as long as your instructor is only concerned about how you got the product. If s/he is a stickler for electron movement, arrows, and proton transfers, you've still got some cleaning up to do...

Yah, I just drew the mechanism very fast, all I had to say was what the final product was, but I wanted to draw a quick free-hand mechanism. Thanks though for the concern, appreciate it!

[fledarmus]

Ooops, sorry, I mis-counted earlier - you have an extra carbon in your product.

If you are starting with the trans isomer, you would need to run the reaction under conditions that would allow cis-trans isomerization of your starting material.

[Hurat]

Ooops, sorry, I mis-counted earlier - you have an extra carbon in your product.

Wow you're right! So I'd have the cyclohexene conformation! Wow, I can't believe we both missed that the first time around! 5-hydroxy-5-methylcyclohex-2-en-1-one it would be then! Thanks a lot, that would have been very embarrassing

If you are starting with the trans isomer, you would need to run the reaction under conditions that would allow cis-trans isomerization of your starting material.

Hmmm, I guess to allow cis-trans isomerization I'll heat the heck out of the of the trans isomer in the presence of light!