Benzoannelated centropolyquinanes. Part 21. Centrohexaindane: three complementary syntheses of the highest member of the centropolyindane family
The syntheses of centrohexaindane 1, a unique hydrocarbon bearing six mutually fused indane units in a centrohexacyclic, Td symmetrical, and topologically nonplanar framework is reported in full detail. Three independent and complementary synthetic routes have been developed making use of derivatives of the readily accessible centrotriindanes 9, 10 and 11(R = CH3) and centrotetraindane 13(fenestrindane). The most efficient synthesis gives the title compound 1 in seven steps starting from indane-1,3-dione 15 in 40% yield, involving the ‘broken’ fenestrane 10(‘broken-fenestrane route’). Also starting from dione 15, an even shorter route furnishes our target molecule 1 in six steps via triptindanetrione 24 in 25% yield (‘propellane route’). Both of these routes surpass our first synthesis reported in 1 988 via fenestrindane 13 which comprises eleven steps and gives a 7.5% overall yield from indanedione 15(‘fenestrane route’). The three routes allow the six benzene rings of compound 1 to be introduced in complementary ways, thus promising the synthesis of centrohexaindane derivatives with various substitution patterns. Particularly remarkable key steps of the syntheses of compound 1 are, inter alia: (i) single and two-fold condensation of benzene by AlBr3-catalysed Friedel–Crafts reaction of cyclic 1,3-dibromides, (ii) unusual three-fold addition of phenyllithium to a 1,3,3′-triketone (compound 24), (iii) two-fold and even three-fold cyclodehydration. Finally, a low-yield formation of compound 1, among other products, has been observed upon cyclodehydrogenation of 10-methyl-1,4,7-triphenyltribenzotriquinacene 32, which is obtained in two steps from 10-methyltribenzotriquinacene 11(R = Me), with Pd/C at 500 °C. The spectroscopic properties of compound 1 are presented along with some unique structural features of its rigid centrohexacyclic framework.