Cycloaddition with keteniminium ions

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. 

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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