[2 + 2] Cycloaddition Reactions of Butadienyl Ketene with 1, 4‐Diazabuta‐1, 3‐dienes: Synthesis of Functionalized Butadienyl‐4‐iminomethyl‐azetidin‐2‐ones and Butenylidene‐butadienyl‐[2, 2′‐biazetidine]‐4, 4′‐diones. (28th August 2015)