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16.9: Protection of Carbonyl Groups

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    There are few reactions of aldehydes and ketones that do not in some way affect the carbonyl function. For this reason, it may be necessary to protect the carbonyl function when it is desirable to avoid reaction at this function. For example, you may plan to synthesize 4-cyclohexylidene-2-butanone by way of a Wittig reaction (Section 16-4A), which would involve the following sequence:

    Roberts and Caserio Screenshot 16-8-1.png​​​​​​

    This synthesis would fail in the second step because the phenyllithium would add irreversibly to the carbonyl group. To avoid this, the carbonyl group would have to be protected or blocked, and the most generally useful method of blocking is to convert the carbonyl group to a ketal, usually a cyclic ketal:

    Roberts and Caserio Screenshot 16-8-2.png

    With the carbonyl group suitably protected, the proposed synthesis would have a much better chance of success:

    Roberts and Caserio Screenshot 16-8-3.png

    Notice that the carbonyl group is regenerated by acid hydrolysis in the last step.

    Contributors and Attributions

    John D. Robert and Marjorie C. Caserio (1977) Basic Principles of Organic Chemistry, second edition. W. A. Benjamin, Inc. , Menlo Park, CA. ISBN 0-8053-8329-8. This content is copyrighted under the following conditions, "You are granted permission for individual, educational, research and non-commercial reproduction, distribution, display and performance of this work in any format."

    This page titled 16.9: Protection of Carbonyl Groups is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by John D. Roberts and Marjorie C. Caserio.