A substantial number of molecular rearrangements lead to the incorporation of heteroatoms into carbon–carbon chains. Common mechanism: Curtius, Lossen, Schmidt and Hofmann Curtius rearrangment Lossen rearrangement Schmidt reaction Hofmann degradation Beckmann rearrangement.
The nature of the substituent on the double bond and the utilized solvent were found to be crucial to the yield of pyridines. Although the nitrogen atom of a nitrene has no formal charge, it is electron deficient and serves as a locus for 1,2-rearrangements.
Because of the carbon chain joining the oxime function to the ortho-carbon of the benzene ring, the phenonium ion that normally facilitates phenyl migration may be unable to assume its preferred structure (three-membered ring orthogonal to the phenyl ring). 23.5 Schmidt Reaction . As a rule, these products are not generally isolated but are treated further with either amines or alcohols. The first three reactions illustrate the Hofmann rearrangement, which is a particularly useful method for preparing amines. A variety of functional groups are tolerated in this reaction including vinyl, cyclopropyl, pyridyl, aryl, benzyl, and nitro groups. In the Lossen rearrangement (# 7) butyl amine adds to the isocyanate to give a substituted urea (urea is NH 2 CONH 2). 30.8: Rearrangements of Cationic or Electron Deficient Nitrogen, Rearrangement of Acyl Nitrenes to Isocyanates. Were this to occur, both carbon substituents (1R & 2R) would be candidates for the subsequent 1,2-shift. Pedro Molina, Alberto Tarraga, in Comprehensive Organic Functional Group Transformations, 1995. N-Methylimidazole-catalyzed Synthesis of Carbamates from Hydroxamic Acids via the Lossen Rearrangement. Four examples of these transformations, sometimes called "Stieglitz rearrangements", are shown below. The second example involves an oxime derivative with different carbonyl substituents, which exists as a pair of stereoisomers (syn & anti). The enantioselective desymmetrization of 2-substituted-2-azidopropyl 1,3-hexanediones through an asymmetric intramolecular Schmidt reaction using a chiral Brønsted acid has been developed for the first time. The rate of this base-catalyzed rearrangement depends on the electronic nature of the groups R and R1: the rate increases as the leaving group R1CO−2 becomes more electron-attracting, and as the electron-releasing power of R in (111) increases <74AG(E)376>. The oxime from cyclohexanone has identical carbonyl substituents. A third problem traditionally associated with the rearrangement is the undesired dimerization of any starting hydroxamic acids with the highly reactive isocyanate formed.98, Like other members of this class, the Lossen reaction has been established as a thermal, ionic, concerted decomposition process. The Curtius procedure is particularly useful if the isocyanate is the desired product, since the decomposition of the acyl azide intermediate can be conducted in the absence of hydroxylic solvents that would react with the isocyanate. For a nitrogen cation to initiate rearrangement it must have an unfilled valence shell, and two such azacations are shown in the center of the diagram (pink box). This technology has been applied to a formal total synthesis of lepadiformine A and a total synthesis of lepadiformine C. Synthesis of Amide Isosteres of Schweinfurthin-based Stilbenes, Synthesis and preliminary biological evaluation of furo- and oxazolopyrimidines, An approach to the synthesis of α,α,α - trisubstituted alkylhydrazine derivatives from carboxylic acids, Dioxazolones: Stable Substrates for the Catalytic Transfer of Acyl Nitrenes, Recent Advances in the Applications of the Intramolecular Suzuki Cross-coupling Reaction in Cyclization and Heterocyclization: An Update, Asymmetric Synthesis of Less Accessible α-Tertiary Amines from Alkynyl Z-Ketimines, Asymmetric Synthesis of Less Accessible α‐Tertiary Amines from Alkynyl Z‐Ketimines, Preparation and characterisation of N-(diphenylphosphinoyl)hydroxylamine, and conversion into O-sulphonyl derivatives that undergo Lossen-like rearrangement, Asymmetric Synthesis of α-Alkylated α-Amino Acids: Azocane-2-carboxylic Acids.
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