Synthesis and chiroptical properties of methanocycloocta[b] indoles

The synthesis of chiral methanocyclocta[b]indoles, the fused structures obtained from enantiomeric bicyclo[3.3.1]nonanones via Fisher indolization reaction, is reported. The starting optically active bicyclo[3.3.1]nonane-2,6-dione (1) was obtained by a chiral HPBC enantiomer separation on a swollen microcrystalline triacetylcellulose column and by the enzymatic resolution of the racemic dione. The circular dichroism (CD) spectra of the chiral structures 4, 5, and 7 were recorded, and the absolute configuration for the indole compounds was assigned. The theoretical calculations of the CD spectrum of diindole 4 reproduce the B-1(b) couplet at 229 nm but predict wrong signs for the L-1(a) and L-1(b) bands using standard polarization directions. The CD spectrum of indole ketone 5 is reproduced correctly.