Figure 3: Formation of highly substituted cyclobutenes.

The reaction is believed to proceed through an α-bromination/cyclization pathway. The structure of the product was assigned by X-ray. The products in the box were synthesized using 0.05 mmol dienes and 0.1 mmol N-bromosuccinimide (NBS) in 1 ml dichloromethane for 2 h; isolated yield.