Keteniminium ions

Finally, a carbocyclic ring formation initiated by a keteniminium cyclization is depicted in Scheme 1.8 [6]. In the presence of triflic anhydride and DTBMP, pyrrolidine amide 1-20 was converted into the keteniminium ion 1-22, traversing inter-... 

Due to Random Cleavage of Glycosidic Bonds by Keteniminium Ions. .. 180... 

Reactions of the DMAc-Derived Keteniminium Ions with Cellulose. 183... 

In heated DMAc/LiCl, the keteniminium ions are generated in situ from the solvent. When the degrading agent was added externally in the form of a keteniminium precursor, cellulose degradation was even more pronounced. The thermolabile keteniminium precursor compound 4-(l-chloro-2-methyl-propenyl)morpholine (54) immediately releases keteniminium ions 55 upon thermal treatment (Scheme 22) [71]. Addition of the precursor in very small... 

Scheme 22 Cellulose degradation by keteniminium ions being either generated in the reaction mixture or added externally...

Decarboxylation of a-tertiary amino acids in phosphorus oxychloride results in iminium salt products197,198. Using Polonovski reaction (treatment of the tertiary amine-N-oxide with trifluoracetic acid) various iminium salts have been made199-204. Cycloaddition of keteniminium ions to olefins gives cyclobutyliminium salts205-207. Anodic oxidation208-214 and oxidation of trityl salts of tertiary amines to iminium salts215 216 has been described. 

Shortly after Meerwein s and Eschenmoser s original reports, Ficini disclosed the use of ynamines as suitable precursors of aUyhc ketene N,0-acetals (Scheme 7.8) [10, 17]. AUyhc alcohols add to ynamines (15) either in the presence of catalytic amounts of a Lewis acid, for instance BFj-OEtj, or at elevated temperatures. This addition presumably proceeds through the intermediacy of a keteniminium ion 16. The resulting ketene N,0-acetals then undergo the sigmatropic rearrangement to yield the corresponding amides. 

Scheme 89 Keteniminium ion-initiated cascade cationic polycycUzation...

Keteniminium cations and imines can undergo a formal 2-1-2 thermal cycloaddition to yield 2-azetidinones [j8-lactams (8)] see Scheme 2. A computational study suggests the cycloaddition occurs via a stepwise mechanism, with N—C bond formation occurring first. Stereochemistry is determined in the second step, by torquoelec-tronic effects. However, the nature of the anion can affect the stereochemistry, which appears to explain the change in stereochemistry found when X = Cl, i.e. when chloro-enamines are used as precursors of keteniminium ions. 

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