Fully Substituted Cyclobutenes via [2 + 2] Cycloaddition of Ynimines with Cinnamamides

Cyclobutenes are found in natural products and medicinal chemistry and can be readily functionalized. Herein is presented a straightforward, transition-metal-free [2 + 2] cycloaddition reaction of readily available ynimines with <i>N</i>,<i>N</i>-dimethyl cinnamamides to access fully substituted cyclobutenes. It is proposed that the ynimine is deprotonated, and the resulting allenyl anion undergoes conjugate addition to the cinnamamide followed by ring closure to afford the cyclobutene products. This reaction has good scope, proceeds in 40-83% yield, and is atom economical.