Two independent syntheses of the new centrotetraindan 2, a "trifuso", C,-symmetrical isomer of fenestrindan 1, are described. The first approach is based on the "endo"-phenyldiindanone 7, which is converted into 2 by benzylation to ketone 8 and subsequent cyclodehydration with polyphoshoric acid. The second, more efficient approach is based on the diindandione 17, which is converted into ketol 19 in two steps, which, in turn, is subjected to a three-step cyclodehydration-reduction-cyclodehydration sequence via 26 and 27 to give 2 in 8% overall yield. Some limits of the cyclodehydration of diindan alcohols and ketones are demonstrated with regard to effects of steric crowding and fragmentation of the carbon skeleton. The steric hindrance of "endo"-phenyl-substituted diindans is demonstrated in the case of ketone 26.Among the centropolyindans, the Dzh-symmetrical congener, fenestrindan (1)[*], has gained particular interest because of its well-known [5.5.5.5]fenestrane framework. A closely related, isomeric centrotetraindan with C, molecular symmetry, trifuso-centrotetraindan (2), may appear less attractive from an aesthetic point of view, but in fact it represents another polycyclic hydrocarbon with a particularly interesting carbon skeleton.

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In memory of Klaus HafnerThe synthesis of several centrotriindanes bearing ortho-phenyl groups in the sterically constricted bay regions is described. The classical three-step sequence including twofold cyclodehydration of the corresponding 2,2-dibenzyl-1,3-indanediols afforded an angular difuso-triidane and several all-cis-tribenzo[5.5.5.6] fenestranols equipped with two or four ortho-phenyl groups. In contrast to the construction of these conformationally flexible centrotriindanes, attempts to synthesize ortho,ortho-diphenyl-substituted tribenzotriquinacenes failed due to the increased rigidity of the TBTQ skeleton. The high steric load imposed by the para-terphenyl moieties affects the spirocyclization between 1,3-indanedione and 1,5-di-(para-2'-terphenylyl)pentadienone, kinetic control of which is critical to access the all-cis-tribenzo [5.5.5.6]fenestrane framework. In contrast, the corresponding tetraphenyl-cis,cis,cis,trans-tribenzo[5.5.5.6]fenestranol is easily accessible.

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Naphtho-Anellated (5.6.5)-and (6.5.5.5)Fenestranes.-Several naphtho-anellated (5.6.5)-and (6.5.5.5)fenestranes, e.g. (V) and (XI), are prepared for the first time. This work is presented in context with studies towareds the synthesis of fenestranes which contain more than one six-membered ring. -(SEIFERT, M.; KUCK, D.; Tetrahedron 52 (1996)