Regioselective Preparation of Vinylcyclopentadienes and Selected Cycloadditions

Article abstract of DOI:10.1021/jo00365a015

A variety of vinylcyclopentadienes have been prepared by deprotonation of fulvenes with LDA at -78 deg C and quenching with acid at 0 deg C.The compounds were present as a mixture of valence tautomers, the 1-substituted isomer dominating over the 2-substituted isomer by a ratio of 2:1.In all instances the regioisomer with the 1,1-disubstituted ethylene side chain was formed.No isomer with a trisubsitituted exocyclic double bond was discernible by NMR.Fulvenes are thermodynamically more stable than the less highly substituted vinylcyclopentadienes.Thus, on contact with base vinylcyclopentadienes rearrange to fulvenes.Vinylcyclopentadienes react with acetylenedicarboxylic acid dimethyl ester to give Diels-Alder adducts, e.g., 7a and also bis adduct 8a.Intramolecular Diels-Alder reactions of appropriately substituted vinylcyclopentadienes are feasible in a protic solvent such as 1,2-ethanediol on heating to 190 deg C.In this case, the isomerization of the substituted vinylcyclopentadiene to substituted fulvene is a side reaction only.