{(E)‐ and {(Z)‐2‐[α,β‐bis­(methoxy­carbonyl)­vinyl]­cyclo­pentadien‐1‐yl­idene}tri­phenyl­phospho­rane

The Ramirez yl­ide undergoes electrophilic substitution with di­alkyl acetyl­ene­di­carboxyl­ates, yielding a mixture of the Z and E adducts. The crystal structure analyses of the two adducts formed using di­methyl­acetyl­ene, viz. di­methyl (E)‐ and (Z)‐1‐2‐(tri­phenyl­phospho­ranyl­idene)­cyclo­pentadien‐1‐yl]­ethyl­ene­di­carboxyl­ate, both C₂₉H₂₅O₄P, explain an unusual chemical shift observed for the vinyl H atom of the Z adduct, which had previously precluded a definitive assignment of the isomers. In addition, the structures explain why only one of the isomers reacts further with acetyl­ene esters to produce azulenes with a rare substitution pattern.