ASSIGNMENT OF ALL DIASTEREOMERS OF PERHYDROFLUORENE IN A MIXTURE USING C-13 NMR-SPECTROSCOPY

By hydrogenation of fluorene 1 using different catalysts, mixtures con taining variable amounts of the six possible diastereomers of perhydro fluorene 2 were obtained. The C-13 NMR spectra of all diastereomers ha ve been determined in these mixtures whithout previous separation proc edures. By molecular mechanics calculations the structures of the pref erred conformations were simulated for all diastereomers. The substitu ent induced C-13 NMR chemical shifts in conformers were calculated by using increments for the non-bonding 1,3-synaxial interactions. Three stereo-chemical increments, combined with the shift values found earli er by Beierbeck and Saunders, were sufficient to calculate the chemica l shift differences of conformers empirically. The assignment of relat ive configurations of the diastereomers was possible by comparison of the empirically calculated and experimental C-13 chemical shifts of th e clearly distinguishable bridgehead carbons. The difference between c alculated and experimentally determined shift values was only 1.3 ppm, averaged over all bridgehead carbons.