PUB - Publikationen an der Universität Bielefeld

Tribenzofenestranes possessing the strained cis,cis,cis.trans[5.5.5.6]-fenestrane skeleton have been synthesized from cis-2.6-diphenylspiro[cyclohexane-1.2 ' -indane]-1 ' .3 ' -diols by twofold cyclodehydration, in striking analogy to the strategy used previously to construct the stereoisomeric all-cis-tribenzo[5.5.5.6]fenestranes from the corresponding trans-diphenylspirodiols. In this manner, both of the parent hydrocarbons, all-cis-tribenzo[5.5.5.6]fenestrane 3 and cis.cis.cis.trans-tribenzo[5.5.5.6]fenestrane 4, have been made accessible from the spirodiketones 5 and 6, respectively. The C6-functionalized derivatives of 4-cis.cis.cis.trans-fenestranol 9 and cis.cis.cis.trans-fenestranone 12-were prepared through cis-diphenylspirotriol 8 and cis-diphenyldispiroacetaldiol 11. by using the same strategy. The cis,cis, cis.trans-[5.5.5.6]fenestrane framework readily epimerizes to the more stable all-cis isomers under basic conditions, but is stable under neutral or acidic conditions. For example, cis,cis.cis.trans-fenestranone 12 yielded all-cis fenestrane 3 under Wolff-Kishner conditions, but cis,cis,cis,trans-isomer 4 under Clemmensen conditions. Epimerization was also circumvented by radical-induced desulfurization of fenestrane dithiolane 15 with nBu(3)SnH/AIBN. producing 4 in excellent yields. A single-crystal X-ray structure analysis of 4 revealed that, in accordance with force field and semi-empirical MO calculations. the extra strain of the benzoannelated cis,cis,cis,trans-[5.5.5.6]fenestratriene framework [E-strain(4)- E-strain(3) = 46 kJ mol(-1)] is due both to the almost perfect boat conformation of the six-membered ring and to considerable bond angle widening at the central non-bridged C4b-Cl5d-C11b unit (121). H/D exchange experiments with the cis cis,cis,trans hydrocarbon 4 under basic conditions demonstrated that the strain-induced epimerization to 3 occurs through direct deprotonation of the "epimeric" benzylic bridgehead C7a-H bond, which was found to be more acidic than the two C-H bonds at the benzhydrylic bridgeheads.