A ring expansion with

Dimethyl 1,4-cyclohexadiene-1,2-dicarboxylate has been traditionally prepared by three methods (i) dimethyl 3-vinyl-1,2-dichlorocyclobutane-1,2-dicarboxylate undergoes a ring expansion, with concomitant loss of the chlorine atoms to give the cyclohexadiene in 52% yield, upon treatment with Ni(CO)4 in refluxing benzene-dimethylformamide [68], (ii) butadiene sulfone and dimethyl acetylenedicarboxylate have been allowed to reflux in xylene for 150 min, and (iii) butadiene and dimethyl acetylendicarboxylate have been mixed in dioxane in a sealed tube, and allowed to stand at room temperature for 5 days [69]. 

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