Preasperterpenoid A, featuring a 5/7/(3)6/5 pentacyclic structure, is a C25 sesterterpenoid produced by Penicillium verruculosum. The results of density functional calculations on putative biosynthetic carbocation cyclization/rearrangements leading to preasperterpenoid A revealed a highly concerted four-step cyclization mechanism. Interestingly, two secondary carbocation structures were obtained as minima, but appeared almost as shoulders in the energy profile, and may represent essentially transient structures during the highly concerted reaction.