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#1 2010-07-26 11:57:44

orgopete
Administrator

Grignard addition, ketal hydroysis

Yahoo!Answers in chemistry wrote:

Mechanism for this reaction?
Okay, so I'm studying for an exam and I don't get how this works.
     http://www.curvedarrow.com/chem/ZZ661EFBAA.jpg

I understand how the PCC and the H30+ works in more simple reactions, but i don't understand how we went from that oxygen ring on the grignard compound to it being open and losing an oxygen or something like that?

A: There are really only two reactions here. First is the addition of the Grignard reagent to the ketone. This gives a tertiary alcohol, which remains into the product. Then, an acid catalyzed hydrolysis of the cyclic ketal. The result is the carbon which had two oxygen atoms attached becomes the new C=O. That is the final product. PCC will not change the final product because the alcohol is tertiary and so resists oxidation. This is shown diagramatically below. The mechanism for the ketal hydrolysis involves many steps and so is not detailed. The complete mechanism is provided in A Guide to Organic Chemistry Mechanisms, see http://www.curvedarrowpress.com/

     http://www.curvedarrow.com/chem/ZZ6D4ACCA4.jpg

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