A short, formal synthesis is reported for pentalenene. The key steps are copper-catalyzed 1,4-addition of isobutylmagnesium bromide to a cyclooctenone derivative, enolate alkylation with the thiocarbene complex [(η5-C5H5)(CO)2Fe=CHSPh]+ PF6−, and cyclopentane ring formation by means of intramolecular C-H insertion, leading to a late intermediate in a previously reported synthesis of pentalene.