THE PAGODANE ROUTE TO DODECAHEDRANES - HIGHLY FUNCTIONALIZED, SATURATED, AND UNSATURATED PENTAGONAL DODECAHEDRANES VIA ALDOL-TYPE CYCLIZATIONS

Pentagonal dodecahedranes with four (69), six (67 and 83), and eight (79) skeletal positions being functionalized are made available from dimethyl 14,19-dioxopagodane-4-syn,9-syn-dicarboxylate 7 as a common precursor. Key steps are the installation of the two carbonyl functions of 7 into the expeditiously available pagodane 4-syn,7-syn-diester, the 2sigma --> 2pi pagodane isomerization into the respective bissecododecahedradiene (46), and two transannular C,C bond formations. The implied oxidation of two unactivated methylene groups is brought about by a Barton reaction of unusual complexity (at least 14 bond breaking/bond forming events), conveniency (one-pot reaction), and performance (nearly quantiative yield). The subsequent cyclobutane opening (2sigma --> 2pi) in 7 and several model systems by bromine addition and bromine elimination is found to be complicated by heavy skeletal substitution but is efficiently effected for 7 by an intriguing detour (isododecahedranes 48, secododecahedradienes 50). Thus, for the 20(21) steps between isodrin and the various dodecahedranes, total yields of 12-16% are achieved. Under acid catalysis the two (exothermic) cyclization steps are kinetically sufficiently differentiated to allow the selective generation of intermediate secododecahedranes (66 and 78). Limitations of this aldol type route are the cyclizations calculated to be endothermic and which could not be executed irreversibly. Dodecahedrenes (67) with their highly bent C=C double bond (psi ca. 46-degrees) are found to be kinetically surprisingly stable; from mass spectra, indications for the existence of even higher unsaturated dodecahedranes and leads for further functional group manipulations are derived. In the X-ray determinations, the doubly epoxyannulated dodecahedrane 79a is found to be slimmer by ca. 0.5 angstrom than the parent dodecahedrane skeleton of 69a.