BENZOANNELATED CENTROPOLYQUINANES .9. SYNTHESIS AND CONFORMATIONAL BEHAVIOR OF FENESTRINDANS (TETRABENZO[5.5.5.5]FENESTRANES) WITH 4 BRIDGEHEAD SUBSTITUENTS

all-cis-Tetrabenzo[5.5.5.5]fenestranes (fenestrindans) with four bridgehead substituents (4-10) and two doubly bridged, centrohexacyclic analogues, the bis-endo-peroxide 11 and the bis-disulfide 12, have been synthesized from the tetrabromofenestrindan 3. Pronounced steric interactions between the substituents at opposite bridgehead positions have been revealed by NMR and DNMR spectrometry, showing remarkable size-dependent hindrance of the degenerate interconversion of the two S4 symmetric conformers of the fenestrindans.