THE PAGODANE ROUTE TO DODECAHEDRANES - HIGHLY FUNCTIONALIZED, SATURATED AND (BIS-)UNSATURATED PENTAGONAL DODECAHEDRANES VIA SN2 CYCLIZATIONS

Through S(N)2 substitutive transannular cyclizations in either bisseco (21 and 30-34) or seco precursor substrates (25 and 35-39), (bis-)uns aturated, epoxy-annulated, and saturated dodecahedranes featuring nove l substitution patterns-two (45), four (42 and 44), and six (40, 41, a nd 43) skeletal positions being pairwise functionalized-are synthesize d starting from 14,19-dioxopagodane-4-syn,9-syn-dicarbonitrile (4). Cr ucial stages on the way from 4 to the key 1,16-dodecahedradiene 40 are the installation of anti-positioned leaving groups at C14(19) via pag odane bislactone (4 --> 5b --> 7b --> 11 --> 3, overall average 75%) a nd the pagodane --> bisseco diene isomerization (3 --> 18 --> 17 --> 2 1, overall average 80%). A shorter but less expeditious route to 40 is utilized for the preparation of secododecahedradiene 25 (3 --> 18 --> 25, derivatives 35-39). X-ray analyses provide definite structural da ta on highly functionalized secopagodanes (17), isododecahedranes (23) , and secododecahedradienes (25). The phi angle of 35.3-degrees measur ed in 25 for C4(12) represents the strongest pyramidalization of olefi nic carbon experimentally determined to date. The enormously bent C=C double bonds (psi ca. 45-degrees) in the unsaturated dodecahedranes (4 0-42) confer high chemical reactivity (e.g., toward oxygen, electrophi les, and 1,3-dienes) but do not impair the isolability of these olefin s; intermolecular thermal reactions occur only under very forcing reac tion conditions. Direct (sensitized) excitation of dienes 21, 25, and 40 with their perfectly synperiplanar pi bonds (pi,pi distances ca. 2. 6, 2.8/3.0, and 3.5 angstrom (MM2)) does induce transannular bond (cyc lobutane) formation in 21 but not, however, in 25 and 40 (slow polymer ization instead). Bromine addition does proceed in the homoconjugate f ashion in 21 and 25 but not, however, in 40. There is ample MS evidenc e for the existence of higher unsaturated dodecahedranes and for the o peration of C20 --> C-11 + C-9 and C20 --> C-15 + C5 fragmentation pat hways.