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Protecting Groups in Organic Chemistry

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Termini in questo insieme (19)

  • Define orthogonality in the context of protecting groups.
    Orthogonality means only one functional group reacts or is cleaved selectively in the presence of several similar groups.
  • List key requirements for an ideal protecting group.
    Cheap, stable, easy to introduce/remove selectively, stable under various conditions, no stereogenic centers, minimal functional groups, non-toxic cleavage reagents, and easy by-product removal.
  • What are the main classes of protecting group cleavage methods according to Kocienski?
    1) Basic hydrolysis, 2) Acid cleavage, 3) Heavy metal cleavage, 4) Fluoride ion cleavage, 5) Reductive elimination, 6) β-elimination, 7) Hydrogenolysis, 8) SET reduction, 9) Oxidation, 10) Nucleophilic substitution, 11) Transition metal catalysis, 12) Light-induced cleavage.
  • Why are silyl ethers commonly used as protecting groups for alcohols?
    They are easy to introduce, have tunable stability to acid/base, and can be selectively cleaved by fluoride ions.
  • What is the driving force for cleavage of silyl ether protecting groups by fluoride ions?
    The high bond energy of Si-F (~590 kJ/mol) compared to Si-O (~440 kJ/mol) drives cleavage.
  • Describe the stability trend of silyl ethers to acid hydrolysis based on substituents.
    Bulkier substituents increase stability; e.g., C6H13OSiMe3 cleaves in <1 min, while C6H13OSiBu'Ph2 has a half-life of 225 min under acid.
  • What is the typical cleavage condition for acid-labile protecting groups like tBu ethers?
    Cleavage is usually done under acidic conditions such as TFA/DCM at room temperature via carbocation intermediates.
  • How are benzyl-type protecting groups typically cleaved?
    By catalytic hydrogenolysis (H2/Pd), acid hydrolysis, or oxidative cleavage depending on substituents.
  • What is the advantage of the Fmoc protecting group in peptide synthesis?
    Exceptional stability under acidic conditions and selective cleavage by secondary amines via β-elimination.
  • What is the main disadvantage of methyl ethers as protecting groups?
    They are very resistant and difficult to cleave, requiring harsh Lewis or Brønsted acid conditions.
  • What is the role of heavy metals like Ag+ or Hg2+ in protecting group chemistry?
    They selectively cleave S,S- and O,S-acetals which are stable to acid hydrolysis.
  • Why is the levulinyl (Lev) group used as a protecting group?
    It is an ester protecting group that can be selectively cleaved under mild hydrazinolysis conditions.
  • What is the 'safety-catch' principle in protecting group chemistry?
    A protecting group is transformed into a more labile form before cleavage to improve selectivity and ease of deprotection.
  • How are allyl (All) protecting groups typically removed?
    By transition metal catalysis using Pd(0) or Rh(I) complexes causing isomerization and cleavage.
  • What is the typical method for protecting carboxylic acids as esters?
    Acid-catalyzed esterification with alcohols or SN2 reactions with alkyl halides under basic conditions.
  • What is the advantage of tert-butyl esters as carboxyl protecting groups?
    They are stable under basic conditions and cleaved selectively by acid to form tert-butyl cations.
  • Describe the formation and cleavage of cyclic acetals as carbonyl protecting groups.
    Formed by reaction of aldehydes/ketones with diols under acid catalysis; cleaved by acid hydrolysis.
  • What is the main challenge with acyl migration in ester protecting groups?
    Intramolecular transesterification can cause migration of acyl groups to neighboring OH groups, complicating synthesis.
  • Why are carbamates widely used as amine protecting groups?
    They reduce nucleophilicity and acidity of amines, are stable under many conditions, and can be selectively cleaved.