A polyanionic supramolecular assembly (1) is shown to catalytically cyclize the monoterpene citronellal and two homologues. In contrast to cyclization in acidic aqueous solution, the hydrophobic interior of 1 prevents the capture of reactive intermediates by water. This effect was also observed in the gold-catalyzed cycloisomerization of an enyne. Due to the steric confinement of the catalyst's interior, Prins cyclizations in 1 proceed deanly both for substrates containing and lacking gem-dimethyl substitution. Encapsulation in 1 consequently imposes a degree of mechanistic control that, similar to enzyme catalysis, is not observed in bulk aqueous solution.