Solid-state [2+2] photodimerization of eniminium salts: stereoselective syntheses of 1,3-diacetylcyclobutanes.

Solid-state [2+2] photodimerization of eniminium ions oriented in a head-to-tail manner controlled by cation-π interactions produced syn HT dimers in high yields. As the resulting dimer is readily converted to 1,3-diacetylcyclobutane, the iminium serves as a removable orientation-controlling group for the conjugated ketones.