Synthesis, chiroptical properties and absolute configuration of spiro[1,3-benzodioxole-methanocyclooct[b]indole]

The synthesis of chiral 1′H-spiro[1,3-benzodioxole-2,12′-[6′,10′]methanocyclooct[b]indole] 3, a fused polycyclic structure derived from bicyclo[3.3.1]nonane, was accomplished via the Fisher indolization reaction. Enantiomers of this structure were obtained by chiral HPLC enantiomer separation on a swollen microcrystalline triacetylcellulose column. Chiroptical properties of the resolved enantiomers containing indole and 1,2-methylenedioxybenzene chromophores were studied. Application of the sector rule to the 1,2-methylenedioxybenzene chromophore to establish the absolute configuration of this polycyclic structure did not lead to an unequivocal conclusion which is likely to be due to the transannular interaction of the chromophores. The enantiospecific synthesis from enantiomerically enriched (−)-(1R,5S)-bicyclo[3.3.1]nonane-2,9-dione 1 was performed to prove unequivocally the (+)-(6′R,10′S)-configuration of the title structure. This study demonstrates that semiempirical rules should be applied cautiously to the determination of the absolute configuration of molecules containing several chromophores.