Fulgides Synthesis and Photochromism

Many fulgides , compounds which undergo facile valence bond tautomerism on irradiation, are 

Condensation of dicyclopropyl ketone with diethyl succinate using sodium hydride as base gave 

C13H18O4, after acidic work-up. Esterification of A with EtOH/HCl and condensation of the 

Z) mixture. Hydrolysis of B with KOH/EtOH, acidification, and treatment of the product with 

Elucidate the synthesis of the fulgide and give a mechanistic explanation for its photochromic behaviour. 

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