Background: Rigid receptor molecules based on triphenylene ketals exhibit a strong affinity towards some particular guests such as caffeine and trinitrobenzene, thus being good candidates for caffeine or trinitrobenzene sensing applications. It is most important to produce cuppy receptor molecules with special and rigidly oriented functional moieties for the application of molecular recognition. It is still necessary to introduce the bicyclic moiety into the ketal in order to benefit to molecular recognition. Hence, we report a more direct synthesis of triphenylene ketal from commercially available 2,3,6,7,10,11- hexamethoxytriphenylene, which could lead to an easy way to functionalize rigid receptor.
Methods: An optimized synthetic protocol has been developed based on triketalizion of 2,3,6,7,10,11-hexahydroxytriphenylene to prepare the rigid triphenylene ketal having bicyclic moieties. To bring in the bicyclic subunits, we synthesized 1-carbethoxy-bicyclo[3.3.1]nonan-9-one starting from the commercially available 2-carbethoxycyclohexanone.
Results: Reaction of hexahydroxytriphenylene with the bicyclic ketone in toluene under reflux using a Dean-Stark apparatus yielded a statistical mixture of isomers comprising of 22% of all-syn (2a) and 67% of anti,anti,syn (2b). The process of isomerization of 2b to the statistical mixture and subsequent separation can repeat three times to increase the total yield of 2a to ~60%. The structures of triphenylene ketals were confirmed from their spectroscopic data.
Conclusion: Easier and higher-yielding synthesis of rigid triphenylene ketal was developed based on the triketalization of hexahydroxytriphenylene, which eliminates the electrochemical oxidative trimerization step typically required to produce the versatile and rigid receptor molecule. This makes the present route useful for large-scale production.