Functionalized Cyclobutenes via Multicomponent Thermal [2 + 2] Cycloaddition Reactions

Enamine [2 + 2] cycloadditions can be achieved in useful yields simply by stirring a mixture of an aldehyde, diethylamine, a dialkyl fumarate,
and potassium carbonate in acetonitrile at 25 °C, conditions that are compatible with the presence of a potential leaving group on the β-position
of the intermediate enamine. Methylation and elimination of the product cyclobutanes completes a mild nonphotochemical route to functionalized
cyclobutenes.