[2 + 2]‐Cycloaddition of N,N ‐Dimethyl‐ N (2‐methyl‐1‐propenylidene)ammonium (“Tetramethylketeniminium”) Ion to α,β‐Unsaturated Carbonyl Compounds

The title compoud (1) is considerably more reactive than ketenes in [2 + 2]‐cycloadditions and even reacts with α,β‐unsaturated carbonyl compounds due to the presence of extremely low‐lying unoccupied MO's. Mechanisms, which explain the orientation of the subtituents in the cyclobutanones (2) and (3) formed, are discussed.