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15.2: Use of protecting groups

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  • A challenge organic chemists face in the laboratory when planning a transformation or modification of a functional group in a polyfunctional compound is the possibility of unintended changes in the other functional groups in lieu of or in addition to the intended change.
    eg:  Terminal alkyne 1 can be converted to internal alkyne 2 by treating 1 with a very strong base, such as NH2, followed by an ethyl substrate.


    However, 3 can not be converted to 4 using the same two-reaction sequence.



    In 3, the most acidic hydrogen atom is not the alkynyl hydrogen but the hydrogen atom in the alcohol group.  Consequently, treatment of 3 with the base results in the base deprotonating the alcohol group in 3 giving an alkoxide ion, which reacts with the substrate yielding 5, not 4, as the organic product.


    In order to convert 3 to 4 using the methodology employed to convert 1 to 2, the alcohol group in 3 must first be removed temporarily.  One way to do so is to convert the alcohol group into a silyl either group.



    In 6, the most acidic hydrogen is the alkynyl hydrogen.  Treatment of 6 with ¯NH2, followed by the substrate results in 7.



    Replacement of the silyl either group in 7 with the alcohol group yields 4.



    In the overall reaction (①+ ②+ ③), the silyl ether is said to act as the protecting group of the alcohol group; reaction ① is known as the protection step and reaction ③ the deprotection step.


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