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Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: zzblue on October 06, 2011, 10:31:38 PM

Title: Robinson Ring Annulation
Post by: zzblue on October 06, 2011, 10:31:38 PM

Hello, my question is concerning Robinson ring annulations.

From my understanding the robinson ring annulation consists of a micheal addition followed by an aldol condensation reaction. My question is why is it neccessary that the micheal addition occur before the aldol condensation, and not the other way around.

I.E. Why cant a robinson ring annulation be an aldol condensation followed by a micheal addition? Would that not give the same product? I have tried going through the mechanisms and I do not see why this is not possible.

Any help is greatly appreciated. Thanks.

Title: Re: Robinson Ring Annulation
Post by: discodermolide on October 07, 2011, 12:43:50 AM

Quote from: zzblue on October 06, 2011, 10:31:38 PM

Hello, my question is concerning Robinson ring annulations.

From my understanding the robinson ring annulation consists of a micheal addition followed by an aldol condensation reaction. My question is why is it neccessary that the micheal addition occur before the aldol condensation, and not the other way around.

I.E. Why cant a robinson ring annulation be an aldol condensation followed by a micheal addition? Would that not give the same product? I have tried going through the mechanisms and I do not see why this is not possible.

Any help is greatly appreciated. Thanks.

Check out the differences in reactivity between the various functional groups involved.

Title: Re: Robinson Ring Annulation
Post by: orgopete on October 10, 2011, 03:23:30 PM

I am going to try this without drawing anything out. Let us use the example of 2-methylcyclohexanone and methyl vinyl ketone. If the aldol preceded the Michael addition, then you must perform the equivalent of the Michael addition (or a new mechanism) to complete the reaction. I think if you draw that aldol product, two challenges should be found. One, the hydrogens are slightly less acidic because they are now delta to the carbonyl group. Secondly, once enolization takes place, the carbonyl oxygen accepting electrons will have a negative charge in the greater resonance contributor.

If the conjugate addition takes place first, then the aldol condensation can take place normally between the two ketones.