Assignment of all Diastereomers of Perhydrofluorene in a mixture using 13C NMR spectroscopy

By hydrogenation of fluorene1 using different catalysts, mixtures containing variable amounts of the six possible diastereomersof perhydrofluorene 2 were obtained. The 13C NMR spectra of all diastereomershave been determined in these mixtures whithout previous separation procedures. By molecular mechanics calculationsthe structures of the preferred conformations were simulated for all diastereomers. The substituent induced 13C NMR chemical shiftsin conformers were calculated by using increments for the non-bonding 1,3-synaxial interactions. Three stereochemical increments, combined with the shift values found earlier by Beierbeck and Saunders, were sufficient to calculate the chemical shiftdifferences of conformers empirically. The assignment of relative configurations of the diastereomerswas possible by comparison of the empirically calculated and experimental 13C chemical shiftsof the clearly distinguishable bridgehead carbons. The difference between calculated and experimentally determined shift values was only 1.3 ppm, averaged over all bridgehead carbons.