“…After that, the formal [4 + n] cycloaddition reactions of DÀ A cyclobutanes with various dipolarophiles such as aldehydes, imines, alkenes, nitrones, and indoles, emerged and attracted much attention. [1b,g, h, j, 4] For instance, Christie, [5] Johnson, [6] Pagenkopf, [7] Waser, [8] Matsuo [9] and Tang group [10] have developed effective approaches to construct various formal [4 + n] cyclic compounds, respectively.Dialkyl methylidenemalonates are very reactive and sensitive compounds, [11] have been used in [2 + 2] cycloadditions with electron-rich alkenes to form DÀ A cyclobutanes in the presence of Lewis acid catalysts at À 78 °C (Scheme 1a). [6,7b, 12] Recently, Waser group developed an FeCl 3 ⋅Al 2 O 3 -catalyzed [2 + 2] cycloaddition reactions of enimides and alkylidene malonates, giving aminocyclobutanes in excellent yields (Scheme 1b).…”