Formal (4 + 1)-Cycloadditions of Donor–Acceptor Cyclobutanes and Isocyanides: Access to β-Enamino-diesters and β-Enaminoesters

In this work, a synthetic protocol for accessing substituted five-membered carbocyclic β-enamino-diesters from isocyanides and donor-acceptor (D-A) cyclobutanes is described. A formal (4 + 1)-cycloaddition furnishes β-enamino-diesters in up to 89% yield from various aliphatic and aromatic isocyanides. In addition, a tandem formal (4 + 1)-cycloaddition followed by decarboxylative tautomerization is demonstrated in a preliminary example, enabling the selective formation of a β-enaminoester.