The Pagodane Route to Dodecahedranes Directed Conversions - The Pagodane → Bissecododecahedradiene Stage

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.