Covalent Templated Catenane Synthesis Requiring Hindered Ester Cleavage.

Previously, we developed efficient methodology toward hetero [n]rotaxanes using 2,5-dihydroxyterephthalate as a covalent scaffold in the axis of a 30-membered all-carbon macrocycle. Herein we show that to arrive at the more challenging catenanes, proper preorganization provided by 2,6-dihydroxyterephthalate is required for efficient closure of the second macrocycle. Extreme steric hindrance of the specific terephthalate ester covalently linking both macrocycles prevented disconnection of the rings by saponification. To overcome this, to the endocyclic phenyl rings in the macrocycle spanning the templating terephthalate moiety 4-OMe groups were introduced allowing liberation of the two macrocycles by oxidation to the para-benzoquinones with concomitant cleavage of the esters. However, the strong electron-withdrawing properties of the central terephthalic template completely blocked oxidation. Eventually, aluminum hydride as a small and powerful nucleophile was able to cleave the highly shielded ester liberating the mechanically interlocked catenane architecture.