Templating the formation of the metastable cage isomer using tetrahedral guests. Cage C (Figure ) is structurally similar to the tripyridine-based Fujita cage (Figure ), which has been reported to bind and stabilize the otherwise unstable colorless, closed-ring isomers of phenolphthalein and spiropyran, both of which contain a central sp -hybridized quaternary carbon atom. These examples show that the Fujita cage's tetrahedral (Td) symmetry entails its ability to encapsulate like-shaped guest molecules. Here, we turned this reasoning around and hypothesized that stable molecules with tetrahedral geometries might be good guests for-and thus shift the equilibrium towardsthe otherwise unstable Td-symmetric C from the D2h-symmetric T. Therefore, we set out to investigate the possibility that appropriately sized tetrahedral guests could surpass 6's potency in transforming T into C.