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Three conceptual routes (A, B, C) from [1.1.1.1]pagodane (1) to pentagonal dodecahedrane (2) are evaluated by MM2 (MM3) calculations. After limited experimental success with a catalytic one-pot route (A), a more selective transformation along one of two stepwise routes (B/C) is explored. An expeditious entry into route C is achieved by hydrogenolytic cyclobutane opening in 1; secopagodane 7 (100%), however, resists both progression along route C (dehydrogenative C—C bond formation to isododecahedrane 8) and crossover into route B (hydrogenolysis to bissecododecahedrane 5). The first transformation along route B, the 2sigma;2π-isomerization of the highly strained 1 to bissedodecahedra-1,10(11)-diene 3, is not attainable by metal catalysis and cannot productively be brought about by thermal activation: The necessarily very high reaction temperatures (> 700°C) enforce instead a mechanistically interesting fragmentation into two C10H10 halves to give ultimately naphthalene. The very rapid pagodane opening occurring after one-electron oxidation, too, is not a preparatively useful alternative. Highly efficient, on the other hand, is a two-step process affording a high yield of the product and consisting of regiospecific, photochemically induced bromine addition to the central four-membered ring ( dibromosecopagodane 37) followed by reductive bromine elimination ( diene 3). In spite of the necessarily rather severe reaction conditions in both steps, this procedure is applicable to the preparation of various 3,8-difunctionalized bissecodienes (dienedione 11, diene diesters 43, 50, 52, dichlorodiene 56). Limitations of this procedure are met with the 4,4,9,9-tetrachloropagodane 60 (inert) and the [2.2.1.1]pagodane 80 (bridgehead bromination). The lateral half-cages of the (seco)-pagodane structures are explored for preparatively (dis)advantageous steric effects, that might be later exploited on the way towards functionalized dodecahedrane derivatives. |
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