Unsaturated dodecahedranes - Synthesis of the highly pyramidalized, highly reactive C20H18 and C20H16 olefins

Methodological alternatives for the preparation of highly strained, highly pyramidalized dodecahedrene 2 (E(str) = 87.3 kcal mol(-1); phi = 43.5 degrees, MM2) and 1,16-dodecahedradiene 3 (E(str) = 105.3 kcal mol(-1); phi = 42.9 degrees, MM2) have been explored, protection/deprotection strategies have been tested - with the eye on their utilization for the generation of higher unsaturated dodecahedranes (e.g. 1,4,16-triene 4, 1,4,10(14),16-tetraene 5). For the acquisition of preparative quantities of monoene 2 the ''P2F'' catalyzed cis-beta-elimination in bromododecahedrane, of diene 3 the FVP fragmentation of a ''twofold protected'' precursor (bis-furan adduct) have become the protocols of choice, which both profit from the recent synthetic advances along the pagodane --> dodecahedrane scheme. Because of unusually effective steric protection the highly tilted C = C double bonds of 2 (lambda(max) (CH3CN) = 254 nm, nu(C = C) = 1658 cm(-1), delta(C = C) = 164.4) and 3 (delta(C = C) = 170.5) enter into thermal stabilization pathways (dimerization, oligomerization) only at higher temperatures (for 2 ca. 50% consumption after 5 h at 100 degrees C in a 3 . 10(-3) molar toluene solution); extreme sensitivity to oxygen is primarily attributed to kinetically and thermodynamically promoted allylic hydrogen abstraction.