Rebek Jr and co-workers reported several cases of iteroselective reactions on di-functional molecules via a supramolecular protection of one reactive site within a deep cavitand. Unlike conventional covalent protecting groups modifying chemical functions, supramolecular protecting groups modify the environment and, consequently, the reactivity of the functional groups, which is the origin of the iteroselectivity. One remarkable example is the Staudinger mono-reduction of diazido alkanes in water (Figure ). The diazido alkane guest is included in a resorcinarene-based deep cavitand with one of the two azides nesting in the cavity. The other azide protrudes from the cavity and can readily reacts with an excess of trimethylphosphine, affording the mono-amine product in 99% yield (the ite is equal to the yield). The further reaction of the unreacted azide is inhibited as this group is less polar than the amine and is thus preferentially hidden from the water. Such a strategy based on host-guest chemistry can be used to achieve regio-and iteroselective reactions on either the guest or the host, but also reactions of the host with the guest. Figure . Iteroselective Staudinger reduction on a diazido alkane with one azide protected inside a deep cavitand.