The renaissance of strained azabicyclo[1.1.0]butanes as useful reagents for the synthesis of functionalized azetidines

Since their discovery in the late 1960s, 1-azabicyclo[1.1.0]butanes have demonstrated to be interesting precursors of azetidines, because of the peculiar reactivity of the C3-N bond that allows the double functionalization in 1,3 positions. In particular, the recent advances reported by Baran, Lopchuck, Aggarwal, and others witnesses the synthetic relevance of such strained azabicycles in the synthesis of highly functionalized azetidines. However, synthesis and reactivity of 1-azabicyclo[1.1.0]butanes remain a poorly explored topic in organic chemistry. This review aims to furnish a comprehensive knowledge either on the preparation of 1-azabicyclo[1.1.0]butanes or the transformation into functionalized saturated four-membered azacycles.