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Topic: Why do we make acylation in Julia classical olefination reaction?  (Read 5466 times)

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Offline chimiste

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The detailed reaciton is here:

http://www.organic-chemistry.org/namedreactions/julia-olefination.shtm

Why do we make acylation, I know AcO groups are used for protecting the alcohol functional group, but here against what? Or we don't use them for protecting at all here??

Offline sjb

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Re: Why do we make acylation in Julia classical olefination reaction?
« Reply #1 on: October 14, 2007, 06:13:03 AM »
The detailed reaciton is here:

http://www.organic-chemistry.org/namedreactions/julia-olefination.shtm

Why do we make acylation, I know AcO groups are used for protecting the alcohol functional group, but here against what? Or we don't use them for protecting at all here??

One suggestion might be that in the absence of an acetate trap, there is no real driving force for the reaction to proceed as written, if you're waiting around for a proton to form the alcohol it may quite readily decompose back to aldeyde and lithiosulfone. So you may be able to at least do the first step with some other protecting group precursor, like a silyl chloride. Lithium acetate is also produced in the scheme that you have described, which I don't think is that soluble in e.g. THF, so this may also push the equilibrium across to the RHS.

Alternatively, you may need the acetate to form a decent leaving group in the elimination, in which case silyl groups would not be the best bet out there.

Just a few thoughts - have not really done much Julia olefination work so have not really read (or more importantly, understood) the literature. clarifications from others welcome.

S

Offline kiwi

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Re: Why do we make acylation in Julia classical olefination reaction?
« Reply #2 on: October 24, 2007, 06:53:37 AM »
The detailed reaciton is here:

http://www.organic-chemistry.org/namedreactions/julia-olefination.shtm

Why do we make acylation, I know AcO groups are used for protecting the alcohol functional group, but here against what? Or we don't use them for protecting at all here??
...
Alternatively, you may need the acetate to form a decent leaving group in the elimination, in which case silyl groups would not be the best bet out there.
...

Got it, you need the acetate for the E1cb elimination that follows. That alcohol ain't going anywhere by itself under those conditions.

Offline faust

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Re: Why do we make acylation in Julia classical olefination reaction?
« Reply #3 on: October 28, 2007, 01:53:14 PM »
the OH- could be ejected in the julia olefination.

You need to acetylate for displacing the first equilibrium, if not you will return back to the aldehyde!

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