title: [(E)- and [(Z)-2-[alpha,beta-bis(methoxycarbonyl)vinyl]cyclopentadien-1-ylidene]triphenylphosphorane.
abstract: 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 C(29)H(25)O(4)P, 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.
authors.affiliation: Department of Chemistry, University College Dublin, Belfield, Dublin 4, Ireland.
authors.forename: Lee J
authors.initials: LJ
authors.lastname: Higham
authors.forename: P Gabriel
authors.initials: PG
authors.lastname: Kelly
authors.forename: Helge
authors.initials: H
authors.lastname: Müller-Bunz
authors.forename: Declan G
authors.initials: DG
authors.lastname: Gilheany