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    Protection Groups in Organic PDF

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    Toàn Minh
    The document thanks several people who provided support and guidance for a project on protecting groups in organic synthesis. It provides an introduction explaining that protecting groups are used to selectively modify less reactive functional groups in the presence of more reactive groups. Key factors in choosing a protecting group and examples of protecting groups for common functional groups like alcohols, carbonyls, carboxylic acids and amines are summarized. References used in the project are also listed.

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    Protection Groups in Organic PDF

    Uploaded by

    Toàn Minh
    212 views67 pages
    The document thanks several people who provided support and guidance for a project on protecting groups in organic synthesis. It provides an introduction explaining that protecting groups are used to selectively modify less reactive functional groups in the presence of more reactive groups. Key factors in choosing a protecting group and examples of protecting groups for common functional groups like alcohols, carbonyls, carboxylic acids and amines are summarized. References used in the project are also listed.

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    The document thanks several people who provided support and guidance for a project on protecting groups in organic synthesis. It provides an introduction explaining that protecting groups are used to selectively modify less reactive functional groups in the presence of more reactive groups. Key factors in choosing a protecting group and examples of protecting groups for common functional groups like alcohols, carbonyls, carboxylic acids and amines are summarized. References used in the project are also listed.

    Copyright:

    © All Rights Reserved
    Download as PDF, TXT or read online from Scribd
    Download as pdf or txt
    212 views67 pages

    Protection Groups in Organic PDF

    Uploaded by

    Toàn Minh
    The document thanks several people who provided support and guidance for a project on protecting groups in organic synthesis. It provides an introduction explaining that protecting groups are used to selectively modify less reactive functional groups in the presence of more reactive groups. Key factors in choosing a protecting group and examples of protecting groups for common functional groups like alcohols, carbonyls, carboxylic acids and amines are summarized. References used in the project are also listed.

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    Download as pdf or txt
    of 67

    I wish to convey my sincere regards and thanks to Dr.

    Renu Gupta, Head Department of Chemistry Lucknow


    Christian P.G College, Lucknow for her keen interest,
    encouragement and fruitful suggestions and for providing
    library facilities.
    I would like to place on record my deep gratitude to Mr.
    Mayank Joseph under whose guidance this project entitled
    “Protecting Groups In Organic Synthesis ” is being
    submitted, but for the constant support and encouragement
    at every stage, it would not have been possible complete this
    study.
    My sincere thanks are to Dr. Stuti Gupta & Dr. Abhas
    Asthana for their co-operation during the completion of the
    project.
    Apoorva Rastogi
    Introduction
     When there are two functional groups of unequal reactivity
    within a molecule, the more reactive group can be made to
    react alone, but it may not be possible to react the less
    reactive functional group selectively.

     A group the use of which makes possible to react a less


    reactive functional group selectively in presence of a more
    reactive group is known as protecting group.

     A protecting group blocks the reactivity of a functional group


    by converting it into a different group which is inert to the
    conditions of some reaction(s) that is to be carried out as part
    of a synthetic route
    When we choosing a suitable protecting group, the
    following features must be considered :-

    ◦ Cheap and commercially available.


    ◦ Simple to put in high yield.
    ◦ Stable to reaction conditions.
    ◦ Easy to remove in high yield
    ◦ Protection of Alcohols.

    ◦ Protection of Carbonyl Groups in Aldehyde And Ketones.

    ◦ Protection of Carboxylic acid.

    ◦ Protection of amines.
    Protection of Alcohols
     By putting a bulky protecting group on the hydroxyl
    oxygen which can sterically prevent it from
    competing effectively for electrophiles.
     By delocalising the lone pairs of electrons by

    conjugation
     Acetals Or Ketals As Protecting Groups For Alcohols

     Ether As Protecting Groups For Alcohals

     Ester As Protecting Groups For Alcohals


     Acelals or ketals not only serve as useful protecting groups
    for aldehydes and ketones but they also find widespread
    use in the protection of alcohols.

     Commonly used ketals include Tetrahydropyranyl (THP)


    derivatives, methoxyethoxymethyl (MEM) derivative
    and methoxymethyl (MOM) derivative.

     Acetals / ketals are simply removed by treatment with


    aqueous acid.
     Alcohols may be simply converted to ethers by
    nucleophilic attack on a suitable alkyl halide.

     Ethers are stable to basic and mildly acidic


    conditions; they do not react with oxidizing /
    reducing agents and are inert to organo-metallic
    reagents. However, this stability means that many
    others are not easily cleaved to their parent alcohol
    under mild conditions. Therefore, only certain
    ethers, which are easily cleaved under mild
    conditions, are commonly used as protecting groups
    for alcohols.
     Benzyl ether which are converted to alcohols under
    neutral conditions by catalytic hydrogenolysis.
     t-butyl ethers which are readily hydrolyzed with
    dilute acid.
     Tri-phenylmethyl group is the most important
    for the protection of 1°-alcoholic group. Only 1°-
    alcohols react at a useful rate with trityl-chloride in
    pyridine so selective protection of primary hydroxyl
    groups is possible.
     Trichloroacetate esters [Cl3CCOOR]:

     Pivoloate ester [Me3CCOOR]:-


     Cis 1, 2 diols and cis and trans-1,3 diols can be
    protected as cyclic acetals and ketals (e.g., dioxlanes
    and dioxanes) or cylic ortho esters that are cleaved
    by acidic hydrolysis or as cylic esters (e.g,
    carbonates) that are cleaved by basic hydrolysis.

     Cyclic ortho esters are more readily cleaved by acidc


    hydrolysis than cyclic acetals or ketals.

    Isopropylidene (acetonides):-

     1, 2 acetanoid formation is usually favored over 1, 3


    acetonidess
     Cleavage:-mild aqueous acid
     The acetal / ketals protective group is introduced by
    treating the carbonyl compounds with an alcohol, an
    ortho-ester, or a diol in the presence of a Lewis acid as
    catalyst.

     Acetals / ketals are stable to strong aqueous bases,


    nucleophilic reducing agents, organometallic reagents,
    oxidation under non-acidic conditions, Na or Li/NH3
    reductions.

     Acetals / ketals are cleaved by acid-catalyzed hydrolysis


     General order of reactivity of various carbonyl
    groups (probably due to the steric effect).

     1,3-dioxanes (six-membered ring) hydrolyze faster


    than the corres-ponding 1,3-dioxolanes (five-
    membered ring acetal)
     Acyclic acetals is prepared by the reaction of
    aldehyde with monohydric alcohal in the presence
    of dry HCl.
     Acylic ketones cannot be prepared with monohydric
    alcohols under the same condition of acetals
    formation because of the unfavourable entropy,
    which the equillbrium towards the ketones So
    acyclic ketals are prepared by the reaction of
    ketones with ethylorthoformate in presence of the
    NH4Cl.
     Thioacetals / thioketals are quite stable toward
    hydrolysis; there is no special need to remove the
    H2O formed during the reaction.
     Protecting groups for carboxylic acids are used to

    avoid reaction of the acidic -COOH hydrogen with

    bases and nucleophiles or to prevent nucleophilic

    additions at the carbonyl carbon.


     Most common group for the protection of
    acid is ester.
     The basic problem of peptide synthesis is one of
    protecting the amino group. In bringing about
    interaction between the carboxyl group of one
    amino acid and the amino group of a different amino
    acid, one must prevent interaction between the
    carboxyl group and the amino group of the same
    amino acid. In preparing glycylalanine, for example,
    one must prevent the simultaneous formation of
    glycylglycine. Reaction can be forced to take place in
    the desired way by attaching to one amino acid a
    group that renders the NH2 unreactive.
     Amides are readily prepared from an amine and an

    acid chloride or anhydride. These are relatively

    more stable compounds that, classically, are cleaved

    by heating in strongly acidic or basic solutions.


    [1] Singh, J ; Yadav, L.D.S; Organic Synthesis;Pragati
    Publicaton; thirteenth edition- 2017; pp. 696-
    721.

    [2] Kar, K.R.; Application of Redox and Reagents


    in Organic Synthesis; New Central Book
    Agency (P) Ltd; volume-1; pp 376-385.

    [3] Clayden, J; Greeves, N; Warren, S; Wothers, P;


    Organic Chemistry; pp 633-637 & 657
    [4] Hanson, J.R.; Protecting Groups in Organic
    Synthesis; Sheffield Academic Press; pp 8- 80.

    [5] Morrison, R.T.; Boyd R.N.; Organic Chemistry;


    Prentice-Hall of India Private Limited; Sixth Edition-
    2002; pp. 1147

    [6] March, J; Smith, M.B.; March’s Advanced


    Organic Chemistry: Reactions, Mechanisms, and
    Structure; Wiley Publication; Sixth Edition; pp
    1271-1279

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